The most effective trouble-shooting/repair tool available for computers or microprocessor-controlled systems is turning power off, waiting a period of time, and turning power back on.
Here's why it works:
Computers and microprocessors are control systems which are generally not fully controllable. This means that either the hardware or software can put them into a state where normal control inputs have no effect on the system. This topic is called "Controllability" in formal Control Theory jargon.
An analogy would be an Interstate off-ramp with no returning on-ramp. Once you get off, you have to do something abnormal, like back-track several miles on surface roads, to get back on.
From now on, I will use the term "computer" to mean computer or any microprocessor-controlled system. Your microwave, VCR, and fancy coffee pot are non-computer examples.
One state that all functioning computers can recover from is the power-off state. Hardware and software engineers work diligently to make sure a computer can turn on into a known controllable state. Imagine how upset customers would be if, regularly, they took their electronic devices out of the box, flipped the on switch, and nothing happened. You can be sure the manufacturer would hear from the customer, and that the hardware or software engineer at fault would also hear about it in no uncertain terms.
So, how does a computer get into an uncontrollable state?
In hardware, there are many causes for what is called a Single Event Upset (SEU). A power glitch, a cosmic ray passing through an integrated circuit (IC), or an alpha ray from the plastic IC package, can all cause an SEU, possibly changing a logic state (1 to 0 or vice versa), or triggering latchup in the pnpn layer most ICs have. In software, the computer can get caught in an infinite loop.
How do you turn your computer off, and how long do you keep it off?
Using the off/on switch or normal software shutdown will cure more than 90 percent of the problems, but not all of them. After turning off the computer, you need to pull the plug from the wall and make sure anything the computer interfaces with (modem, printer, etc.) is also turned off and unplugged. (Power strips are great for this.) If your computer has a battery, such as a laptop does, or a built-in UPS battery, you also need to remove this power source. The reason is that even if you turn off your computer, it still draws vampire power to keep certain monitoring and startup circuits alive -- which may be causing the problem.
Now that you've turned it off, how long do you keep it off?
Usually, but not always, 30 seconds is enough. This is because bleeder resistors across capacitors used to be designed to discharge logic, memory, and interface voltages to less than five percent of normal voltage in about this amount of time. It was considered good design practice. These discharge paths were also often included in ICs to remove charge from junctions and internal nodes when un-powered.
Today, the discharge resistor is often not included in the design. Cost-savings is one reason, but also because real estate in ICs is so valuable. Active pull-down and pull-up devices, which take less real estate than resistors, are used instead. These work when there is power, but they can be high impedance with power removed. What this means is that a charge on system components and IC capacitance can keep the computer in an uncontrollable state for a longer time. As computers get "better" you need to leave them off longer.
What's my personal approach?
I turn the computer off (and remove the battery in a laptop), wait 30 seconds, replace the battery if necessary, and turn it on. If this does not cure the problem, I turn it off, unplug everything, remove battery backup, wait several minutes (up to 30 minutes), and try again. If this does not work, then I leave it off overnight. If these steps aren't successful, re-cycling power will not solve the problem.
As those of you who have visited my website know, I include a personnel anecdote for every problem/solution description I provide. Here is my anecdote for this problem:
When I was newly married, my wife would ask me to fix this or that electronic device that went out. Confidently, I would instruct her to pull the plug, reverse it (before plugs were polarized), wait 30 seconds, and plug it back in. She would first complain that this would not help, and then was amazed when it solved the problem.
Now, 47 years later, she just reported that the microwave went out. Without a word from me, she reached back behind the cabinet, pulled the plug for 30 seconds, plugged it back in, and reset the clock. She now accepts this as the best way to trouble-shoot/repair any electronic equipment. From her own practical experience, she knows it works most of the time.
My daughter-in-law, who edited this, just had her computer system hang-up. She tried the technique and much to her amazement it worked.
The bottom line: One of the most effective methods for trouble-shooting electronics is unplugging power. The time-tested technique is to turn it off, pull the plug, remove the battery, wait, replace the battery, plug it back in, and turn it back on. And best of all…this powerful method of trouble-shooting electronics is free!
Acknowledgement: My daughter-in-law, Jonelle Foutz, is a freelance copywriter and editor. Sometimes, when she is not too busy, I ask her to edit something I have written (so I can learn to write better from a professional). She was kind enough to edit this blog. If you are looking for a writer or editor, you can check out her website at www.writemindonline.com.
If for some reason you thought that power sequencing is a trivial problem, reading this paper will give you a good idea of the complexities. The authors of Sequencing Power Supplies in Multiple Voltage Rail Environments in the Texas Instruments 2004/05 Power Supply Design Seminar provide an excellent explanation of why turn-on and turn-off sequencing is necessary, the types of sequencing, and a variety of solution examples. This information is in 31 compact pages.
The paper shows many examples of power sequencing implemented with:
The paper does an outstanding job of explaining problems and current solutions. Never-the-less early into reading this paper a sickness struck me in the pit of the stomach and stayed with me to the end. This had nothing to do with the excellence of the paper - which is outstanding. However, the paper brought home the fact that the digital designers appear to be winning the decades-long battle of pushing difficult design problems out of their integrated circuits and into the hands of everybody else. In this case, the need to sequence power supplies for reliability purposes and for a variety of non-critical purposes.
Now don't get me wrong. Much of my career has been in designing power supplies for digital computers and I am well aware of the need to sequence certain critical circuits to cleanly turn the computer on into a known initial state and the need to sequence power off to prevent wild things happening to the I/O circuits. But things have gone too far when sequencing is required by circuits that may not be critical to these tasks or to prevent reliability damage or failure of circuits if not sequenced properly.
My early experience was designing power supplies to survive and recover a nuclear bomb blast. Now that is a severe environment! The air around the circuits is ionized into a conductive gas for a brief period of time. Imagine dipping your operating circuit into a pool of mercury and pulling it out and being required to have it still work like nothing happened. In addition, primary photo-current generators inherent in each semiconductor junction generate currents approaching hundreds of amperes in circuits normally operating in the sub milliampere range. Every junction becomes a short for a brief period of time and then things return to normal in a totally unsequenced way. And that unsequenced recovery could not degrade the reliability of the circuit. Harsh? Yes, but harsh things also happen when a fuse is blown or an output shorted. You really don't want a momentary short and clearing to require you to replace some or all of your logic and analog circuits because reliability has been degraded.
So how did we solve this problem? For one thing, every digital and analog circuit had to be simulated with all combinations of power on/off sequencing and no part could be overstressed beyond limits set by reliability degradation. Then it had to be tested in the lab. For 24 hours in an oven cycled between minus 55 C and plus 125 C, the circuit had to survive all combination of sequencing, spending 24-hours in each partial sequenced state. It should come as no surprise that initially most circuits failed this requirement, but over time, circuit designers learned to design their circuits to pass this analysis and test criteria. It can be done, and when done, these circuits remained reliable even for more mundane events such as the momentary shorting of an IC across a bus with subsequent clearing, or a solder splash, or dropping a screwdriver into the computer circuit enclosure.
Another technique was the use of "housekeeping" power supplies that came on first and went off last and allowed the main power to be controlled by sensors and logic circuits.
Then, for the couple of critical circuits that required sequencing (less than one watt of circuits in a 300 W computer), the extraordinary circuit techniques needed to operate in this environment could be invoked and CRITICAL sequencing needs were met.
When you allow many circuits to require sequencing, you get into a catch-22 situation where one set of designers require system power to be sequenced in one combination and another set of designers require a different and incompatible sequencing. Often this is not discovered until the system is in final test or in the field. In the last year I have had calls from at least two engineers who were facing this problem only after shipping several units to the customer. The only solution to this besides requiring the circuit designers to redesign and do what they should have done in the first place (that is probably not going to happen) is to provide multiple power busses that can be individually sequenced. This results in a band-aid system design and complicates the recovery from latchup. When latchup occurs the normal (only?) remedy is to remove all power from the latched circuit immediately, often less than one or a few microseconds. When the carriers have cleared, you can reapply power. If this has to be done on multiple buses with sequencing requirements, things get difficult in a hurry.
Sneak paths are always a system problem and sequencing is often used as a solution but if used, it can conflict with other incompatible sequencing requirements and can result in system failure by failure of maintaining the proper sequencing. Sneak paths are such a problem that the military requires sneak path analysis along with reliability analysis before a system is approved.
The bottom line is to design your circuits and systems so they do not fail or degrade for any order of sequencing and use sequencing only for those function critical to the operation of your system.
The structure of the paper is:
The paper also discusses the special problem with synchronous rectifiers and the impact on power sequencing.
Here is the detailed bibliography information and abstract. After the 2004/05 seminar is complete it will probably appear on the Texas Instrument website. Until then you might be able to request a copy from the authors or a TI field engineer.
Reference: Daniels, David, Dirk Gehrke, and Mike Segal, Sequencing Power Supplies in Multiple Voltage Rail Environments, Texas Instruments 2004/05 Power Supply Design Seminar, SEM1600, pp. 2-1 to 2-31. 31 pages, 39 figures, 0 tables, 20 references, 4 appendices.
Author Abstract: Designers must consider timing and voltage differences during power up and power down in systems where multiple power rails are involved. A simple example would be a single DSP with its core and I/O voltages, requiring power supply sequencing. The possibility for a latch-up failure or excessive current draw exists when power supply sequencing is not designed properly. The trigger for latch-up may occur if power supplies are applied at different potentials of the core and I/O interfaces. This paper addresses some of the more common sequencing requirements of digital signal processing (DSPs), field programmable gate arrays (FPGAs), application-specific integrated circuits (ASICSs) and microprocessors, and proposes a variety of practical solutions implemented with power management devices. These techniques take advantage of the reset, power good, enable and soft-start features available on many types of power management devices ranging from low drop out (LDO) regulators to plug-in power modules.
]]>I know of no source of statistics that collects the number of power supply designers, technicians, or assembly workers killed in designing power supplies or the number of users killed by design faults -- but I suspect it is non-trivial. This is because I know of one statistic. Over 80 sailors electrocuted aboard U.S. Navy ships over a period of several years. Often these were experienced electricians who were familiar with electrical practice on grounded commercial systems, but not familiar with the safety requirements of the ungrounded shipboard power systems. Safety needs to be taken seriously.
There are two important aspects of power supply safety. First is to keep the power supply user and their maintenance people safe. The second is to keep yourself, your technicians, and manufacturing personnel safe during the design and manufacturing of your power supply.
Safety Considerations in Power Supply Design in the Texas Instruments 2004/05 Power Supply Design Seminar is concerned with the first. The second is outside the scope of the reviewed paper, but I have added a few personal anecdotes on this important subject.
Bob Mammano of Texas Instruments and Lal Bahra of Underwriters Laboratories pretty much followed the problem solving approach used in this website. First they make you aware of safety problems, cover the relevance of design categories and specifications, present a method of solving the problems, and then write about solution details. Safety problems to be aware of include:
Relevance is covered in a discussion of the various safety standards and the equipment to which they apply. The solvability method is to partition the power supply into elements and then analyze each element for safety issues. Finally details are given such as required insulation thickness and layering, clearance and creepage distances, etc.
The structure of the paper is:
All in all, the paper is excellent in making the designer aware of safety issues related to the power supply operator and maintainer.
An important subject not discussed is how you keep yourself and fellow workers from harm during the design and manufacturer of the power supply. Here are some personal anectodes that illustrate some of these pitfalls.
The first occurred when I was eight years old. My brother was trying to learn about electrical circuits by wiring bells and lights in various circuits and then energizing them with 115 V ac. The circuit was on a large round oak table in our dining room and power came through two long wires from the chandelier above the table and ending in non-insulated alligator clips.
The thing I did not notice was that my brother always put on rubber gloves before picking up the alligator clips. After showing me what fun it was, he cautioned me not to touch anything when he left the room. I immediately picked up the clips to try my hand with the lights and bells. The first shock involuntarily caused a spasm in my hands that would not allow me to let go of the electrodes. 115Vac went from one hand through my arm, upper torso, arm, and out the other hand. Unable to let go and shouting at the top of my lungs, I made two quick involuntary circles of the table and was then thrown deep into the fire place where I was pressed between the back wall and grate -- still unable to let go or even move in my tight fire-place-cage. It was probably less than 15 seconds before my brother responded to my yelling and pulled the wires out of the chandelier, but it seemed to me more like a lifetime.
According to the National Institute for Occupational Safety and Health (NIOSH), a total of 5,348 workers were electrocuted from 1980 through 1992, an average of 411/year. NIOSH considers this figure is under reported. Also it does not include non-worker electrocutions in households, etc. I consdider myself lucky to have lived beyond my eighth year. (Ray Ridley in his seminars always warns to touch a circuit with the back of your hand, so the involuntary spasm due to shock takes you away from the circuit instead of forcing you to grab it. I recognize the worth of this advice from personal experience.)
Another incidence occurred when I was older and supposedly wiser. I was taking a rectifier and filter circuit to its input voltage limits to determine its mode of failure. I had the circuit in an oven and my head in the oven as I slowly turned up a variac while I monitored the voltage and current meters and the circuit. All at once the hermetically sealed electrolytic tantalum foil capacitor on the rectifier output blew its end plug, sounding like a 45 caliber pistol being fired -- right in my ear in a confined space. I involuntarily jerked my head up away from the circuit and hit my head hard on the top of the oven and brought it down over the circuit just in time to take a deep breath of carcinogenic electrolyte fumes being released by the capacitor. The reflex action also pushed my face into the burning circuit. A deep stabbing pain in my lungs and burnt eyebrows sent me back away from the oven onto my back on the laboratory floor -- where I lay stunned with my ears ringing, my lungs hurting, my head aching, and my nostrils filled with the stench of my burnt eyebrows. Alone in the lab during late evening hours, I sat quietly for a half hour before packing up and driving home.
Another story concerns being hit by aluminum spalling when a hermetically sealed nickel-cadmium D cell was inserted backwards into the circuit. It held up for about 6 minutes before it exploded, bending and spalling the 1/16 inch aluminum frame in its circuit drawer enclosure. I just happened to be in the lab doing other things, but the spalling hit me from 12 feet away.
One interesting accident I observed was when an engineer placed a cup of hot coffee on his bench while testing a high current circuit. The banana plug wiring was neatly dressed in place but not tied. The force induced by the current caused the wires to jump apart when current was applied, knocking the coffee off into the engineer's lap. This was before Stella's time, so their was no million dollar settlement. (For more about Stella, see StellaAwards.com .)
I personally know of only one death caused by unsafe power supply design practice, but one is one too many.
There is a point to be made from these anecdotes. The circuit design engineer usually knows the most about the dangers associated with the circuits he works with. Often times from personal experience. It is his responsibility to communicate this information through all the sources available to him. There is often a safety boiler plate in functional tests, manuals, and other documents associated with a power supply. Make sure that it is more than the standard boiler plate -- make sure it contains all your knowledge of what can be unsafe about your circuit.
Here is the detailed bibliography information and abstract. After the 2004/05 seminar is complete it will probably appear on the Texas Instrument website. Until then you might be able to request a copy from the authors or a TI field engineer.
Reference: Mammano, Bob, and Lal Bahra, Safety Considerations in Power Supply Design, Texas Instruments 2004/05 Power Supply Design Seminar, SEM1600, pp. 1-1 to 1-13. 13 pages, 9 figures, 3 tables, 0 references (several specifications are mentioned in the paper), 0 appendix.
Author Abstract: Increasingly the responsibilities of a power supply designer extend beyond merely meeting of functional specification, with designing to meet safety standards an important collateral task. Since all commercial and home-use supplies must eventually be certified as to safety, knowledge of the requirements should be a part of every designers repertoire. This simplified overview has been prepared with the collaboration of Underwriters Laboratory Inc. to provide a basic introduction to the issues and design solutions implicit in assuring the safety for both the user and service personnel of your power supply products, as well as easing the certification process.
What I said in How To Design a Power Supply applies to how to layout a power supply. There is an near infinite number of ways and if left to their own devices a new designer will usually do a poor job of it. For these designers, Papers like Topic 4, Constructing Your Power Supply - Layout Considerations, in the Texas Instruments 2004/05 Power Supply Design Seminar is highly useful.
My favorite quote from the author, Robert Kollman, is actually the summary at the end of the paper.
"Power supply layout is as important as any other design consideration. The power supply engineer must be involved in parts placement and routing. An understanding of AC and DC parasitics, grounding and cooling makes a successful design."
I added the emphasis on the second sentence because of its high importance. The power supply engineer needs to either sit elbow-to-elbow with the person doing the layout or spent time with him every few hours. Failure to do so can either ruin an otherwise good design or result in a schedule slip or cost over-run if the layout needs to be redone to avoid ruin. The reason is that there are innumerable ways a circuit can be layout and all of them have some compromises between performance, EMI, and thermal. Only the circuit designer who did the design and spent time in the lab getting to know their circuit has the detailed knowledge needed to make these tradeoffs and compromises. Both the designer and layout person need a crib sheet on the tradeoffs on this important task. This paper provides it by cramming an incredible amount of useful information in its 23 pages.
The papers organization is:
"There have been numerous articles written on this subject... because of its importance in ensuring a successful design. This article gathers useful guidelines and calculations to enable the neophyte as well as the experienced engineer to understand issues in physically realizing the electrical schematic."
Each of the following sections contains the necessary equations, figures, and tables to explain and solve the problems discussed.
"In high current power supplies, resistance of components is always an issue as it degrades efficiency, can create cooling problems, and may also impact regulation. Even with it being a problem, the resistance of the PWB traces is overlooked and adds to the issue."
Discussed are the impact of resistance on performance. The important topics of the dc resistance of traces and vias, the fact the copper plating is higher resistivity than pure copper, and the effect of temperature, skin effect, and proximity on effective resistance are discussed.
My own worst experiences with dc resistance affecting layout was when I have had logic layout designers with no power supply experience layout the PWB, applying their knowledge of layout gleaned from logic boards. Following their training and experience, they usually pick the wrong weight copper for each layer, never make the traces wide enough for the current or to minimize inductance, use a ground-plane blindly, often adding capacitance to ground where you do not want it, and seldom consider the current density and voltage drop in the vias. I've had the vias actually get too hot to touch. This experience led me to the ground rule of sitting elbow-to-elbow with the designer during layout to avoid constantly ripping out the layout.
"Just as PWB traces add unseen resistors to schematics, they can also add inductors, capacitors,and transformers."
I like the approach taken in this paper. Kollman first discussed how the device parasitics degrade the performance of capacitors, inductors, and transformers (including common-mode transformers). He then shows how PWB traces add to these parasitics and further degrade the device. He states that the performance of a capacitor can be ruined by the PWB even before it is mounted. There is a good discussion on parasitic inductance and the most effective approach of reducing it. There is also a good discussion of the layout of control circuits and minimizing the interaction with power components, including parasitic magnetic coupling. He ends with the following, which should be self-evident, but often is not.
"In summary, the layout of the power supply is crucial to maintaining the high frequency characteristics of the power components, this providing a satisfactory design.
"As with layout, grounding is one of those things that must be done correctly for a functioning circuit."
I think the greatest disservice inflicted on the circuit designer was the invention of the ground symbol -- because it propagates the fiction that an unvarying ground actually exists. In reality, there are only signals and their returns. This paper does give you an introduction to the problems with series, parallel, and ground-plane connections of return circuits. It points out the dangers of the ground plane and gives some ground rules to avoid them.
Personally, I prefer Lenz' law to ground planes. Make sure the nearest piece of metal is the intended return path. Since you make the signal and trace wide and close together to minimize inductance, my designs often look like they have a ground plane, but I never think that way. The advantage is that I don't have to remember all the ground-rules for removing metal from a ground-plane to avoid circuit problems due to excessive capacitance, undesirable common returns paths, etc.
One thing discussed in the paper that I had never seen as clearly before is the impact of the control IC grounding plan on layout of the PWB. Some ICs use a right-left separation of analog and power pins, others ICs split signal and power lengthwise. Which way the IC splits it can affect the PWB layout significantly.
"One of the key layout considerations in a power supply is removing the heat from components. Historically, that meant figuring out which components generated significant heat and mounting them to a heatsink. But as the power supply becomes integrated with the system, mounting components to a heatsink is becoming less attractive and there is a move to have the PWB act as the heatsink."
The lead-off quote is important because thermal design has greatly increased in difficulty. This paper does a good job of describing the reasons for the increased difficulty and provides some design solutions.
Design Examples
You can never have enough examples to clarify general discussions. There are some good ones here.
The authors summary has already been quoted in full.
Here is my summary.
This is an exceptional paper on layout. It has packed a tremendous amount of information in a relative small number of pages. If you understand everything that is in these 23 pages you are definitely on your way to be successful with the design of your power supply. One of the importantly things that I am glad he mentioned is the necessity of the power supply designer to work closely with the layout designer. Almost everything in layout is a compromise between conflicting requirements. If the layout designer is left to his own devices, he will probably make some poor choices. Only the circuit design who has made all the design tradeoffs and worked with his design in the lab has the background details necessary for the best decision.
I consider this a key paper to have in your personal library.
In the introduction, Topic 2 of SEM-1500 has nothing to do with layout. In Figure 28 and 29, the captions are the same saying both are the right way. Figure 29 shows the wrong way to connect output capacitors. In Figure 30 and 31 the captions are the same saying both are the right way. Figure 31 is the wrong way to connect sense leads.
Reference: Coleman, Robert, Constructing Your Power Supply - Layout Considerations, Texas Instruments 2004/05 Power Supply Design Seminar SEM1600, pp 4-1 to 4-23, 23 pages, 35 figures, 6 tables, 7 references, 2 appendices.
Author Abstract: Laying out a power supply design is crucial for its proper operation; there are many issues to consider when translating the schematic into a physical product. This topic addresses methods to keep circuit parasitic components from degrading the operation of your designs. Techniques to minimize the impact of parasitic conductance and capacitance of filter components and printed wiring boards (PWB) traces is discussed, together with the description of the impact that PWB trace resistance can have on power supply regulation and current capacity. A general overview of thermal design is also included as well as sample temperature rise calculations in a natural and forced air environment. Finally, some practical examples of Power stage and control device layouts are reviewed.
]]>My favorite quote from the author, Ed Walker, is the understatement taken from his introduction:
"And if the power supply is not designed properly, it may well attract too much attention by not working."
A power supply is primarily a buffer or interface between a power source and a load, which, without the power supply would be incompatible. It also has to interface with an environment. I like that the paper recognized this by discussing load, power source, safety, EMI, mechanical, and thermal design right at the beginning in the requirements section of the overview.
Then comes defining the power train consisting of: Topology, Input Configuration, Transformer Design, Voltage-Mode or Current-Mode Control, and Output Filter Design.
The paper throws the control mode (voltage-mode or current-mode control) in the middle of this section, which implies you should mix control in with the power train. My own approach gets the power train working first, and then goes on to design the control. However, no harm done, and in truth, design is an iterative process, not a linear flow. Since you need to sense current for current-mode control and this may affect the power train details, perhaps it does belong here.
What the paper does not do, thank goodness, is to throw everything into a SPICE circuit schematic and hope for the best. I have seen enough new designers do this that I think it is the approach taught in school. Disaster usually awaits those that take the SPICE total-schematic approach for initial power supply design.
Next the paper describes the detailed design, called the plan of record in the paper. Each topic is filled with the expected design equations, part trade-offs including cost, tables, and figures.
The sequence is logical:
The paper ends with the test results and the references.
Ok, well and good. But how do experienced power supply designers really design? Some continue to do it as in the paper, but the process gets modified over time as it is influenced by company policies, the designer's preferences, and other constraints.
I define a good designer as one whose designs go into the field and meet their performance requirements and don't fail, even in abnormal use. After performance requirements are met, field reliability rules. No two designers have the same approach.
One designer I knew who met this criteria started with a drawer filled with schematics that had been proved reliable in the field. When assigned a circuit, he found the closest schematic, had a breadboard built, modified the breadboard until it met the performance requirements, and then did the most complete and prolonged testing of it I ever witnessed. (Those that start with vendor reference designs follow a similar path, although the design usually has not been proved reliable in the field.) His designs did not fail in the field.
How did he come by this approach? He came from a company who first gave the design to one engineer and when he was finished, gave the laboratory notebook, schematic and breadboard to a second engineer to perform a design review. If the circuit proved itself in the field, both got credit, if it failed in the field, the reviewer could expect a period of poor performance reports and no raises. Strong motivation to make sure your fellow engineer's circuit worked. His approach was logical in his initial design environment and worked very well in other design environments.
My own approach was formed by a design environment where my power supplies had to go into a computer on circuit boards the same form factor as the logic boards, which changed form factor and slot pitch for each computer. The only variable was how many logic slots the power supply occupied. You could not get a power supply in a single logic board slot, but keeping the slots used to the minimum was critical. The capacitor tubes controlled the minimum slot consumption and their diameter in turn determined the maximum height the magnetics could take without increasing the number of slots taken by the power supply.
I started each design with a tool kit of balsa wood, wooden dowels, and other modeling material and tools that let me mechanically do the layout of the number of capacitor tubes and magnetics. The envelope of the maximum magnetics height controlled the cores that could be used. The power train was laid out on the board like a jig saw puzzle. Then a design was created that fit the layout. Later I replaced the tool kit with a desk top PC CAD program. The input EMI filter was designed and lain out first (since doing it last often resulted in an EMI filter bigger than the rest of the power supply). Then the Middlebrook Criteria was used to fit and design the output filter. The rest of the design was then juggled to make every thing else work. This unusual design method worked, producing designs that beat the best watts/per/cubic-inch state-of-the-art at the time.
Other designers start with equations and solve the equations with circuit elements.
And many use an approach similar to that in the paper.
The point is that you need a starting point such as a reference design, a previous design you have done, or a design procedure as given in the paper. With experience, you will develop your own methods. In my experience, no two experienced designers reach the goal of performance with field reliability in the same way. What counts is not how you reach it but that you reach it in a way that works efficiently for you. But you have to start some place, and papers like this serve the need.
Here is the detailed bibliography information and abstract. After the 2004/05 seminar is complete it will probably appear on the Texas Instrument website. Until then you might be able to request a copy from the author or a TI field engineer.
Reference: Walker, Ed, Design Review: A Step-By-Step Approach to AC Line-Powered Converters, Texas Instruments 2004/05 Power Supply Design Seminar, SEM1600, pp. 3-1 to 3-25. 25 pages, 19 figures, 10 tables, 7 references, 1 appendix.
Author Abstract: "An offline, three-output, 150-W forward converter is used as an example to illustrate the design process for typical isolated converters. This example emphasizes the basics with a double-ended forward topology using coupled inductors for output accuracy. Design issues and trade-off decisions to optimize power efficiency while keeping costs to a minimum are highlighted. Finally, the presentation of measured performance results confirms the design process."
]]>I try to never miss the Texas Instruments Power Supply Design Seminars (the old Unitrode seminars) when they come to my local area. My preferred location is Orange County, California, because I usually meet several design engineers I have worked with over the years. If not Orange County, Los Angeles County is my second choice, again because I get to visit with power supply designers I know. This year I had conflicts for both of these April 2004 seminars and had to attend the seminar in San Diego County on 7 October 2004, the last seminar given in the USA. I still got to see two engineers from my past and make some new acquaintances.
The seminars continue in April and May 2005 in Europe. I think these seminars are one of the best values around and encourage European designer to attend if they can.
All seven of the seminar topics were presented at the seminar I attended in addition to an introduction to new TI power supply components. I will discuss the topics in later blogs.
Although the papers were written by various authors, they were all presented by Ed Walker, a TI/Unitrode staff engineers since 1997, or Lloyd Dixon, Unitrode's longest-term employee, now retired.
The highlight of the seminar was Lloyd Dixon singing Physics Utopia (Faraday's Law Song). Not gifted vocally, only someone with the total self-confidence -- developed by being at the top of the power supply design field for a life time and being among friends -- could have carried this off. I'm glad I witnessed Lloyd Dixon's performance. Happenings like this are just one of the many reasons you get much more from a live seminar than just reading the papers on your own.
The seminars are not free. The one I attended was $95 US, but this probably all went to the hotel for food in a breakfast spread, lunch, and breaks. All excellent. These breaks over food give you a chance to talk to your fellow engineers. At my lunch table, the major topic was power supplies for the medical industry. I used to know something about this, but my knowledge got a considerable update.
I've said it before, and say it again. Don't miss these seminars when they get to your area, even if you have to take a day vacation and pay for them yourself. They are well worth it.
Buried deep in the Texas Instruments Website are the archives of all the previous Power Supply Design Seminars and this one will probably be archived when it completes its European tour.
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At first, I edited and moved the old blogs into MT and placed them into categories. Watch the category list and contents grow. As I update I am reading each entry, checking links, and adding new content to the answers. So even if the date is old, the content is current.
Now that the old updates are complete, I am adding new content, publishing about once a week.
Here is what I said when I first started blogging in May of 2001.
Today starts my web log, usually called a Blog by the Bloggers who engage in this type of exhibitionist activity. Why?
- I'm looking for a better way to document the question and answer dialogs I have with readers.
- There are a lot of things I want to say that do not fit into other formats.
- It lets me experiment with this latest fad in personal communication.
Hence this weB log or Blog on switching-mode power design. We shall see how it goes. Let me know what you think.
I am still interested in what you think.
]]>Question: Can you recommend some good articles on current mode DC-DC converters?
Answer: Current-mode control, also called current-programmed control and current-injected control, has existed since at least 1978 (earlier in U.S. Government reports). I have annotated some papers with abstracts that might be of interest.
C. W. Deisch, Simple Switching Control Method Changes Power Converter Into A Current Source, IEEE Power Electronics Specialists Conference - 1978 Record, pp. 300-306.
A switching converter with an LC output filter behaves as a loose-tolerance voltage-controlled current source if each switch closure is ended when switch current reaches an adjustable threshold. This converter is then combined with an external feedback to produce a precise output voltage. By generating a fixed voltage with a current source in this manner, the converter has many advantages including continuous protection of the switches, stable and equal load sharing when several converters are operated in parallel, inherent overload protection, automatic switch symmetry correction, and fast system response. (AUTHOR ABSTRACT) Bell Laboratories, Naperville, IL. 7 pages, 3 references.
Key authors since then are Redl/Sokal, Ridley, and Middlebrook/Erickson. I have left out a host of authors who have made major contributions, but you can search the above authors names for additional papers and find others by their co-authors and references. Also, there are many vendor application notes on current-mode controller ICs. The references below should get you started.
R. Redl and N. O. Sokal
Redl/Sokal published one of the early reviews of current-mode control in 1985.
R. Redl, and N. O. Sokal, Current-Mode Control, Five Different Types, Used With the Three Basic Classes of Power Converters: Small-Signal AC and Large-Signal DC Characterization, Stability Requirements, and Implementation of Practical Circuits, IEEE Power Electronics Specialists Conference - 1985 Record, pp. 771-785.
Current-mode control effectively eliminates the phase lag of the control function, associated with the output filter inductor or the energy-storage inductor. The several types of current-mode controllers differ slightly in their phase-lag characteristics, but they are all far superior to the commonly used PWM duty-ratio controllers. Unfortunately, industry has been slow to appreciate and exploit the considerable advantages of current-mode control. There seem to be two reasons for that: (a) the unavailability of integrated circuits which are well-suited to implementing current-mode controllers and (b) the lack of detailed, systematically organized, theoretical and practical information on current-mode controllers. The objective of this paper is to fill this gap by providing a well organized compendium of useful information for the design engineer who wants to understand the operation of a current-mode-controlled power converter and to accomplish an optimal design. Five different control methods are considered, used with the three basic types of regulated rectangular-wave power converters. (AUTHOR ABSTRACT) Design Automation, Inc., Lexington, MA 02173-3992. 15 pages, 18 figures, 10 tables, design equations in table form, 16 references.
Nathan Sokal has provided more information on this that is provided his comments near the end of this page.
R. B. Ridley
Ray Ridley published a Ph.D thesis in 1991 that contains a review of current-mode control up to that point in time.
Ridley, R. B., A New Small-Signal Model for Current-Mode Control, Ph.D dissertation, Virginia Polytechnic University and State University, Blacksburg, VA.
Part of which became the paper:
Ridley, R. B., A New Continuous-Time Model for Current-Mode Control, IEEE Transactions on Power Electronics, April, 1991, pp. 271-280.
The accuracy of sampled-data modeling is combined with the simplicity of pole-zero representation to give a new current-mode control model, accurate to half the switching frequency. All of the small signal characteristics of current-mode control are predicted, including high-frequency subharmonic oscillation which can occur even at duty cycles of less than 0.5. The best representation for the control-to-output transfer function is shown to be third-order. Model predictions are confirmed with measurements on a buck converter. (AUTHOR ABSTRACT) Virginia Power Electronics Center at VPI&SU, Blacksburg, VA. 10 pages, 18 figures, 2 tables, 31 equations, 23 appendices, 15 references.
Ray Ridley has also published a recent article on the subject, Current Mode or Voltage Mode? in his magazine Switching Power Magazine, Vol. 1, Issue 2, October 2000, pp 4,5,9. You can subscribe to the magazine. at no cost if you qualify.
If you visit the Ridley Engineering website, look at the logo. It is actually a family of bode plots of a current-mode controller as the compensating ramp is increased. The upper trace is with no ramp compensation and shows the peaking near the switching frequency. Lower traces showing the effect of increasing ramp compensation until you see the characteristics of a voltage-mode controller, the double slope of the LC output filter.
You can find all of Ray Ridley's papers on his website at www.ridleyengineering.com/papers.html.
Ray Ridley's company, Ridley Engineering has been a past sponsor of this website.
R. D. Middlebrook and R. W. Erickson
Robert Erickson pretty much covers the Middlebrook contributions and greatly expands them with his own contributions and those of others in the chapter Current Programmed Control, pp. 439-487, which contains 15 references on the subject, in his text book:
Erickson, Robert W. and Dragan Maksimovic, Fundamentals of Power Electronics, 2nd ed.; Norwell, Mass.: Kluwer Academic, c 2001. xxi, 883 p. ISBN: 0792372700 (alk. paper); LC Control Number: 00052569
This is one of the books I recommend as part of a personal power supply design library.
Current-mode control should also be covered in most current books on power supply design and current-mode controller applications notes from IC vendors are a rich source of information and practical advice.
Speaking of advice, Ridley and many application notes emphasize keeping noise off the current sensing ramp. From my own experience and interaction with others, this advice can not be emphasized enough. Noise on the current sense ramp is nothing but design grief.
Others may have a different perspective on this topic. Comments are always welcome.
Comments
In answering the original email that had this question, I sent a copy to Robert Erickson, Nathan Sokal, and Ray Ridley.
Comments by Robert Erickson
"I certainly agree with the comment about noise. I think that, contrary to the discussions in many papers about selecting the artificial ramp slope to null the audiosusceptibility or to obtain deadbeat control, the most important criterion is to select the ramp magnitude large enough that good noise immunity is obtained."
[Robert Erickson suggested several other key papers with comments on the subject. Where I could, I have expanded the references and added an abstract to the papers - JF].
1. "Professor R. D. Middlebrook gave a series of very substantial seminars and papers in the late 1980's. These are notable for how they make physical sense out of this complex subject."
2. "In the second edition of my textbook, Dragan Maksimovic and I have substantially revised and expanded the coverage of current mode control, including extensive tables of results for basic converters, and simulation in PSPICE using averaged models.
3. "There is a chapter on current mode control in Dan Mitchell's book, DC-DC Switching Mode Regulator Analysis, McGraw Hill, 1988, ISBN 0-07-042597-3. This book is now out of print, but you might be able to order one from ejbloom.com, or to at least find a copy in the library."
4. "Here is a list of basic papers on the modeling subject:"
Middlebrook and the Caltech group
R. D. Middlebrook, Modelling a Current-Programmed Buck Regulator, IEEE Applied Power Electronics Conference, pp. 3-13, Mar 1987
A general small-signal model for current-programmed switching power stages is used for design-oriented analysis of a 150W buck regulator.
The model, into which the current-programming minor feedback loop is absorbed, exposes the desired tendency towards "constant" output current. The regulator voltage loop remains the only explicit feedback loop, allowing the regulator closed loop properties to be easily obtained from those of the open-loop current-programmed power stage.
The design-oriented analytic results allow easy inference of the effects of element changes on the regulator performance functions. Results are obtained for the regulator line-to-output transfer functions (audio susceptibility) and output impedance. (AUTHOR ABSTRACT) California Institute of Technology, Pasadena, California. 11 pages, 8 figures, 45 equations, 11 references.
R. D. Middlebrook, Modelling a Current-Programmed Boost Regulator, Proc. The Power Electronics Show & Conference, San Jose CA, Oct 1986, pp. 273-285.
A general small-signal model for current-programmed switching power stages is used for design-oriented analysis of a 280W boost regulator.
The model, into which the current-programming minor feedback loop is absorbed, exposes the desired tendency towards "constant" output current. The regulator voltage loop remains the only explicit feedback loop, allowing the regulator closed-loop properties to be easily obtained from those of the open-loop current-programmed power stage.
The design-oriented analytic results allow easy inference of the effects of element changes on the regulator performance functions. Results are obtained for regulator line-to-output transfer function (audio susceptibility) and output impedance, for both maximum and minimum load. (AUTHOR ABSTRACT) California Institute of Technology, Pasadena, California. 13 pages, 11 figures, 64 equations, 9
references.
R. D. Middlebrook, Topics in Multiple-Loop Regulators and Current-Mode Programming, IEEE Power Electronics Specialists Conference - 1985 Record, pp. 716-732.
Some general considerations about multiple-loop feedback are discussed, and it is concluded that incorporation of a current-programmed power stage into a "new" power stage model is both justified and useful. A new circuit-oriented model of the current feedback path is derived which augments the well-known power stage canonical circuit model. The current loop gain, though wideband is always stable if the conventional stabilizing ramp is employed, but has a relatively small low-frequency value. Consequently, the "new" power stage is more usefully modeled by a y parameter model in which the current loop is not explicit. Expressions for the y parameters are given that are extensions of those previously derived. Although current-programming tends to make the power stage output behave as a current source, the control to output voltage transfer function exhibits, in addition to the familiar dominant pole, a second pole at the current loop gain crossover frequency, which may lie from one-sixth to two-thirds of the switching frequency. (AUTHOR ABSTRACT) California Institute of Technology, Pasadena, California. 17 pages, 21 figures, 1 table, 51 equations, 12 references.
Verghese, G. C., C. A. Bruzos, and K. N. Mahabir, Averaged and Sampled-Data Models for Current Mode Control: A Reexamination, IEEE Power Electronics Specialist Conference, 1989 pp. 484-491
A correction to the usual derivation of state-space averaged models for constant frequency, current mode controlled converters produces new averaged models. Using a buck-boost example, we compare frequency responses of the new model, the usual model, and an exact sampled-data model. Approximate sampled-data models, often written down but seldom exploited, are also highlighted. (AUTHOR ABSTRACT) MIT Cambridge, MA. 8 pages, 8 figures, 31 equations, 12 references.
Ridley -- 1991 VPEC PhD thesis and papers [see above]
Tan, F. D., and R. D. Middlebrook, Unified Modeling and Measurement of Current-Programmed Converters, IEEE Power Electronics Specialist Conference, 1993, pp. 380-387.
A Unified model is established for a current-programmed converter, which is both a modification and an extension of familiar models. Inclusion of the sampling effect allows the presence of an additional pole in the current-loop gain to be inferred. The resulting final double-slope asymptote is fixed in position, and the crossover frequency cannot exceed half the switching frequency. A new "stability parameter" Q, determines the additional pole and describes the degree of peaking in the closed-loop transfer functions. Experimental verification employs an analog signal injection technique. (AUTHOR ABSTRACT) Caltech. 8 pages, 12 figures, 25 equations, 16 references.
Tan, F. D., and R. D. Middlebrook, A Unified Model for Current-Programmed Converters, IEEE Transactions on Power Electronics, Vol. 10, No. 4, July 1995, pp. 397-408.
A unified model is established for a current-programmed converter, which is both a modification and an extension of familiar models. Inclusion of the sampling effect allows the presence of an additional pole, Wp, in the current-loop gain to be derived. The resulting final double-slope asymptote is fixed in position, and the crossover frequency cannot exceed half the switching frequency. A stability parameter, Qs, determines the additional pole and describes the degree of peaking in the closed-loop transfer function. Experimental verification employs an analog signal injection technique. (AUTHOR ABSTRACT) QSC Audio Products, Costa Mesa, CA (Tan), Caltech (Middlebrook). 12 pages, 15 figures, 41 equations, 23 references.
Comments by Nathan Sokal
Nathan Sokal recommended other publications co-authored by R. Redl and N. O. Sokal that could be useful. Reprints of technical papers listed below are available via postal mail on request to NathanSokalATcompuserveDOTcomRemoveThis. Also, additional papers on current-mode control were published by R. Redl without Sokal; bibliographic reference citations probably are available from RichardRedlATcompuserveDOTcomRemoveThis. (Email addresses coded for spam protection, change and remove obvious parts to decode.)
Where I had the reference, I added the abstract and other information to Mr. Sokal's list.
Kislovski, Andre S., Richard Redl, and Nathan O. Sokal, Dynamic Analysis of Switching-Mode DC/DC Converters, Van Nostrand Reinhold, New York, 1991. 404p. (Includes detailed analyses of several types of current-mode control. No longer available from Van Nostrand Reinhold. Reprinted with additions and corrections by Design Automation, Inc., 4 Tyler Road, Lexington, MA 02420-2404, U.S.A.; cost USD66 prepaid by check payable on a U.S.A. bank or charged to a VISA or MasterCard credit card, includes shipping via surface book mail. If desired, add USD7 for shipping via AO Air Mail.)
SMPS Technology booklist abstract of above.
Redl, Richard, and Nathan O. Sokal, Using Current Mode Control, Powertechnics Magazine, vol. 5, no. 7, July 1989. pp. 23-28.
What do constant-frequency peak-current commanding, constant-frequency valley-current commanding, constant off-time, constant on-time, hysteretic and PWM-conductance control schemes all have in common? They are all forms of current-mode control, but that is about all they do have in common. (TABLE OF CONTENTS DESCRIPTION) Although often thought of as a single method of controlling switching power supplies, current-mode control rally is an umbrella under which resides several quite different regulation schemes, each with its own unique set of advantages and disadvantages. (ARTICLE LEAD HEADLINE) 4 pages, 7 figures, no equations, 7 references.
Redl, Richard, and Nathan O. Sokal, Superiority of Current-Mode Control, for Stability Over Wide Ranges of Output Capacitance and ESR, Power Electronics Conference, Anaheim, CA, Feb. 1988.
General-purpose power supplies must maintain stability over wide ranges of output capacitance and capacitor ESR, as a consequence of the ways that different systems are configured. Voltage-mode controllers and current-mode controllers differ greatly in their ability to meet that requirement. Because those facts had not been generally appreciated, this important aspect of power-supply design and application had usually been overlooked. After investigating this subject in detail, we found that current-mode control is far superior to the traditional voltage-mode control in this respect, too. (AUTHOR ABSTRACT) Design Automation, Inc. 17 pages, 21 figures, 35 equations, 8 references, 2 appendices with 10 equations.
Sokal, N. O. and R. Redl, Current-Mode Control of Capacitively Coupled Power Converters, U.S. Patent 4,719,559, Jan. 1988. [legal version of next reference]
U.S. Patent Office full text search by patent number.
Redl, R, and N. O. Sokal, How to Use Current-Mode Control with Capacitively Coupled Half-Bridge Converters, IEEE Applied Power Electronics Conference, San Diego, CA, March 1987. pp. 257-265
Switch-peak-current commanding current-mode control produces asymmetrical operation when used in capacitively coupled symmetrical dc/dc converters. This paper discusses the causes and effects of asymmetry, gives an analytical model, and proposes a negative-feedback corrector to eliminate the asymmetry. Test results on a 150-W off-line half-bridge converter prove the effectiveness of the recommended solutions. (AUTHOR ABSTRACT) 9 pages, 15 figures, 11 equations, 4 references, 1 appendix with 10 equations.
Redl, R, and N. O. Sokal, Near-Optimum Dynamic Regulation of DC/DC Converters, Using Feed-Forward of Output Current and Input Voltage, with Current-Mode Control, IEEE Trans. Power Electronics, vol. PE-1, no. 3, July 1986, pp. 181-192. [additional unpublished information available from Sokal on request]
Near-optimum dynamic regulation of a dc-dc converter is obtained by adding feed-forward of output current and input voltage to a current-mode controller. The results are a) near zero output impedance and audio susceptibility, from dc to nearly the switching frequency, b) much reduced magnitude, duration , and energy content of the output-voltage transient after a transient change of output current or input voltage, and c) smaller size and lower cost for the output filter capacitor. Feed-forward is applicable to both forward and flyback types of converters and to all types of current-mode control. The cost of feed-forward for a forward-type converter is a low-power resistor and a current sensor; a flyback-type converter needs also an analog multiplier-divider integrated circuit (IC). A description is given of the control loop, conditions to achieve extremely good transient response, calculation of the peak deviation of the output voltage for a step load change, practical methods for current feed-forward, and experimental results. The theoretical predictions are in excellent agreement with the experimental results. In the experiments, adding output current feed-forward reduced the transient deviations of output voltage by factors of 6.7 in magnitude, 50 in duration, and 335 in energy content. The added components were a 1/4-W resistor and a 12-mm ferrite toroid with a 10-turn winding. (AUTHOR ABSTRACT) 12 pages, 20 figures, 23 equations, 15 references.
Redl, R., and N. O. Sokal, Frequency Stabilization and Synchronization of Free-Running Current-Mode-Controlled Converters, IEEE Power Electronics Specialists Conference, June 1986, Vancouver, BC, Canada. pp. 519-530.
Power converters with free-running current-mode control (hysteretic, constant-"off"-time, or constant-"on"-time) can be frequency stabilized or can be synchronized to a reference frequency. Those can be accomplished by feed forward, feedback, or phase-locking techniques. With hysteretic control, the frequency is controlled by varying the current hysteresis. With constant-"off"-time or constant-"on"-time control, the frequency is controlled by varying the "off" time or the "on" time, respectively. We consider all possible methods of frequency stabilization and synchronization for the three types of free-running current-mode control, used with the three basic types of power converters (buck, boost, and buck-boost). We give analyses and practical design information for the most-important combinations, and experimental data for a buck converter with constant-"off"-time control and feed-forward frequency stabilization. (AUTHOR ABSTRACT) Design Automation, Inc., Lexington, MA 02173-3992, U.S.A., 12 pages, 16 figures, 13 equations, 4 tables, 6 references.
Redl, Richard, and Nathan O. Sokal, What a Design Engineer Should Know About Current-Mode Control, Power Electronics Design Conference, Oct. 1985, Anaheim, CA. pp. 18-33. [update of PESC 1985 paper by same authors.]
In a power converter with a conventional PWM controller, the high-frequency phase lag of the control transfer function approaches 180º in the L-C part of the power circuit, plus more than 90º in the voltage-error amplifier and additional phase lag in some duty-ration modulators. High-gain feedback-control systems with so much phase lag require very careful design to ensure both stability and adequate response speed under all operating conditions.
Current-mode control effectively eliminates the 90º phase lag associated with the filter inductor (or the energy-storage inductor in a flyback converter). The several types of current-mode controllers differ slightly in their phase-lag (and other) characteristics, but all of them are far superior to PWM duty-ratio controllers. Unfortunately, industry has been slow to appreciate and exploit the considerable advantages of current-mode control. There seem to be two reasons: (a) the lack of detailed, systematically organized practical information on current-mode controllers, with theoretical back-up, and (b) the unavailability of integrated circuits which are well-suited to implementing current-mode controllers.
The objective of this paper is to fill the "information gap." We provide a swell-organized compendium of useful design information for the engineer who wants to understand the operation of a current-mode-controlled power converter and wants to accomplish an optimal design. Five different control methods are included, used with the three basic types of regulated rectangular-wave power converters. First we establish the technical basis for an informed choice among the five control methods. Them we recommend which types of current-mode control to use in each of the common applications. We point out special precautions for transformer-coupled converters, to avoid catastrophic disasters. Appropriate control ICs are scheduled to be available in early 1986, from Cherry Semiconductor and at least one alternate source. 16 pages, 13 figures, 10 tables, 16 references.
Repeating: Others may have a different perspective on this topic. Comments are always welcome.
]]>Original Question: How can the Electromagnetic Interference (EMI) due reverse recovery of output rectifiers be effectively suppressed?
Answer: This has been a problem in power supply design since the days when alloy-junction diodes rectifiers were replaced by "improved" diffused-junction diodes in 50 Hz, 60 Hz, and 400 Hz rectifier circuits.
Things only got worse when switching-mode power supplies came along and the reverse recover of the inductor catch diode became a major EMI source.
Over time, there have been many solutions. First let's look at containing EMI by good layout, then line-frequency rectifier diodes, and then the inductor catch diode in switching-mode power supplies.
Layout: Good layout is simple in concept but not practiced as well it could be. Here is the concept. When a current is established in a conductor, it does not know its return path, so the field reaches out and finds the nearest conductor and establishes an equal and opposite current in it. (Lenz's law: An induced electric current always flows in such a direction that it opposes the change producing it.) If this is your intended return and the loop area is small, then EMI caused by a change in current is minimized.
Another way of looking at this is that a conductor establishes an external field around it and when you change conductors, you want that external field disturbed as little as possible. In rectifier circuits, this often means placing three or more traces as close to occupying the same space as possible -- the diode traces and returns. You also want to distance other conductors, including chassis, heatsinks, ground planes, etc. so they are not possible transient return paths. (Remember, when the electron starts its journey, it has no idea what the desired return path is and reaches out to the nearest conductors to establish the equal and opposite current required by Lenz's law. If this is not the intended return path, then you have EMI.)
For example, in a center tapped transformer secondary with two diode traces, the field transfers from one diode trace to the other diode trace with each sharing the same return. These three traces should come as close to occupying the same space as possible, with the diodes slipped in with as little disturbance as possible to the trace geometry. The effect is that as the current switches from trace to trace, there is little change in the external field, minimizing EMI. Most designers think of laying out parts and then connecting them. A powerful alternate approach is to layout signal and power flow conductors in the best geometry and then slip the parts in with as little disturbance to the geometry as possible. Usually you have to go back and forth between the two layout concepts to get the best layout.
Line Frequency Rectifiers: Back in the 1960's when I designed my first three-phase 400 Hz transformer rectifier sets I used alloy junction 1N538 diodes for the low power outputs. The modules passed EMI tests with no problem. A couple of years later, I got a call from the factory that all my modules were failing EMI tests.
What I found was that the manufacturer had "improved" the diode by using faster diffused junctions.
The initial solution was to add a RC snubber across each diode. Later it was found that by using a poor quality capacitor with low Q at the ringing frequency, the resistor could be eliminated. The final solution was to use the newer diodes with controlled recovery characteristics, but I continued to layout the RC pads across the diodes (with a shorted trace that could be cut for the resistor) in case they were needed. The never were.
Others had similar problems, the initial US military specification for a family of high density avionics 400 Hz 3-phase power supplies contained both switching-mode power supplies and series dissipative regulators. All of the switchers passed EMI tests and all of the series dissipative regulators failed EMI tests. The reason? Everyone knew from the beginning switchers were an EMI problem and this was taken care of in the design, but everyone also knew series dissipative regulators did not generate EMI and no one looked for the EMI problems during design. The reverse recovery spikes of the rectifiers caused them to fail EMI tests.
The solution for the problem now is usually good layout and selection of the rectifier diodes for a soft recovery spike, although snubbers are always a backup solution.
Inductor Catch Diodes: The major reverse recovery problem now is usually the inductor catch diode in switching-mode power supplies and these have been a problem from the beginning.
One of the early solutions, when power transistors were much faster than power diodes, was to place an inductor in series with the power diode with a small fast diode across the inductor with the anode pointing the same direction as the power diode. When the power diode voltage reversed to the blocking state, the inductor would keep reverse current from flowing through the power diode until the diode recovered. In the forward state, the small fast diode would carry the current until the inductor current caught up (the small, fast diode had to have an outstanding surge rating). Games could be played with saturation characteristics of the inductor. Later, lossy ferrites were developed for this purpose.
The next solution was for the semiconductor vendor to match the transistor and diode in the same package. At least one vendor had such a product in the early 1970's and it saved a lot of design grief. An alternate was to match them yourself, a real pain, but it worked. (Remember, the current for the diode spike comes through the switching transistor and this spike also generates EMI.)
Others just slowed down the switch. Just because it switches fast doesn't mean you have to use it that way. This was always part of my design approach.
Now the diode manufacturers shape the reverse recovery to make it softer and generate less EMI. I suspect that this, with good layout, is now the most common solution.
If the above don't work, you can always put an EMI suppression circuit across the diode. I always used to lay out pads across every recovery diode for a 10 ohm carbon resistor and a small ceramic tubular capacitor. If you pick a bad enough capacitor, you don't need the resistor. I heard a lecture from a Bell Labs (remember them?) engineer who said how they used lossy Mylar capacitors for this purpose. You can also use lossy ferrites. The key point here is that you do not want high Q parts for this application, you want them VERY lossy at the EMI frequencies. The Bell Labs engineer put it "you want to find the worse possible parts available."
Others may have a different perspective on this topic. Comments are always welcome.
]]>Power supply failures. Over a period of about a month I have been discussing a problem with a reader through email and telephone conversations concerning failure of hot-swappable switching-mode power supply units in equipment at several locations in a large computer facility.
Reader: The failures are independent of power supply manufacturer and type of load (router, server, etc.), but seem to occur in certain locations. At these locations there seems to be oscillations on the interface between the power supplies and the Uninterruptable power supplies (UPS) providing equipment power.
Answer: The first series of exchanges were about filter interactions, including the effects of cables, discussed on my website. Also discussed were possible consultants and others who might help with the problem.
No joy, since after more effort on the readers part I got the email.
Reader: "I am still struggling with the power supply failure problem that I introduce to you about 2 weeks ago and have some new information that may lead a solution." The email went on to provide some data which was discussed and more questions asked.
Answer: I have not been ignoring your email but did not have any immediate answer except the one given below. I have reread your email several times in the interim, but no new ideas occurred.
Reader: At this point I believe that the power supplies are not compatible with the local power distribution units (UPS's) that are installed.
Answer: I know that you have probably done this, but it is the only thing that comes to mind. I would expect that somewhere in the UPS manufacturer's corporate memory they have experienced something similar and may have some solutions, or at least ideas. The problem is, of course, tapping into the corporate memory -- some times it is nearly impossible to get to the person with the knowledge. However, the manufacturer's website mission statement is ... 100 percent customer satisfaction... I would start with the top engineering manager of the product you are using. He/she probably has the title of VP of Engineering, Director of Engineer, etc, or someone on the corporate engineering staff. They probably do not have the answer, but know the engineers in the company most likely to have the answer and may be able to facilitate a contact -- although they try to protect their engineers and have their field people do the support. Some field engineers are very good and they may have the answers, but I suspect you have tried them already with no success. When you have to go into a corporation, I have found that the higher you go, the more courteous the people are and the more helpful information you get. I have no personal knowledge of the manufacturer at this time, but I know many years ago they had some sharp engineering people. That is who you want working your problem, or at least on the team.
Reader: Would it be advisable to remove the capacitors on the power distribution units? I can think of no reason why they need to be there in an environment that is UPS fed and where all of the loads have input filters.
Answer: I'm torn on this one. I'm an experimentalist, so I am always willing to try anything. But I know from experience that without knowledge of, or a model of the system dynamics and what adding or removing capacitance does to the model, you are working totally in the dark. What seems like common sense may be the worse thing you can do when you plug it into the model or system.
Which leads to another thought. Both the UPS and the power supply people should have a simple model of their systems that you can integrate and take a look at. This is something I always try to do and it often leads to the cause of the problem and solution. As R. David Middlebrook says, start with the simplest possible model you can and only add complexity as you need it to explain something that the simple model doesn't. He revived something called the extra-element theorem to make this easier than conventional analysis. The extra-element theorem is described in Erickson, Robert W., and Dragan Maksimovic, "Fundamentals of Power Electronics, Second Edition," Kluwer Academic Publishers, 2001. The examples given are somewhat relevant to your problem.
One of the other people I recommended as a source to query on this problem was Dr. Kasemsan Siri because of talks on similar problems he has given at the IEEE Power Electronics Society Los Angeles Council Chapter. He was kind enough to include me in his replies. They are an order-of-magnitude more professional and useful than my response above. Basically, a plan for taking network analyzer measurements and using the results. He also including pit-falls to be aware of in looking at the dynamics and loops. He also wisely counseled not to touch the system experimentally until you know what is going on from analysis.
One of the things I like about this profession is how helpful people are when you have a problem. And like my experience in going high up in a corporation to get help, often the most knowledgeable, prestigious, and busy technical people in this field are the most generous with their time and help -- if it appears you have given it your best shot and really need some help.
]]>Original Question: The email described a full-bridge topology with MOSFET switches and their drive, then commented: The drive is not working because the MOSFET is not fully on and off. In our view the driving current for turning on the MOSFET seems to be OK So can you tell us how much drive current is required to turn on and off the MOSFET or suggest another drive circuit?
Answer: MOSFET drive circuits are complicated and beyond what I can answer in an email. At the 2001 Series TI/Unitrode World Power Supply Seminar, Laszlo Balogh gave a paper, "Design and Application Guide for High-Speed MOSFET Gate Drive Circuits". I highly recommend you get this paper from TI (it is available at no cost as a pdf download on their website). This paper should be in the personal library of every power supply designer. The main thing is that since the gate drive must provide the charge needed by the power MOSFET, you must know its requirements and monitor the current into the gate versus time to determine if the gate charge requirements are being met. This is the only way you can tell what is actually happening in your circuit. In trouble shooting a circuit you must use both voltage and current probes. In my opinion not using current probes and synchronizing voltage and current waveforms is the most common mistake made by power supply designers in troubleshooting circuits.
]]>Original Question: I am designing a two-switch forward converter rated for 500 Watts, which has a PFC front end. I am facing a problem with the Vcc derived from the PWM transformer. Its regulation is bad. The supply voltage to the PWM IC varies too much with load on the converter output. If the turns are kept low, then the converter won't start at low loads. If the turns are sufficient, then at high load Vcc goes very high, damaging the IC or zener. Can you suggest some solution for this problem?
Answer: I'm not sure I fully understand the problem so let's talk about it.
First, I don't see how the power factor correction (PFC) circuit gets into the act. The forward converter should just see a dc voltage across its input capacitor that varies with line voltage as modified by the PFC. So let's eliminate it from the discussion.
Next, a forward, or any buck-derived converter, usually has fairly good load regulation even when open loop. The DC transfer function to the first order is simple Vout = n*D*Vin, where D is the duty ratio of the switch and n is the transformer turns-ratio and load does not enter into it.
Now we are to the heart of a very common problem. We can get a semi-regulated voltage for Vcc just by adding a winding, rectifier, filter to the main power transformer (or even an auxiliary winding on the inductor). This works fine after the converter is up and running, but where does the power come during start-up? Usually from a special startup circuit. If you just brute-force it from the voltage across the input capacitor, you run into the problem you describe. If you design it to work at low-line voltage you get problems at the high-line voltage. Either the high-line voltage is high enough to damage parts or the power loss either damages parts or raises havoc with efficiency.
The solution usually results in a non-trivial startup circuit that gets power from the input during the first several milliseconds of startup and then gets power from the semi-regulated winding discussed earlier when converter operation starts and voltages come up. When Vcc starts getting its semi-regulated power within acceptable limits, the original starting circuit is turned-off or disabled so it no longer draws power. There are dozens of ways to do this and you can examine a few application notes or reference designs to get some ideas. It does not have to be a forward converter. All converters have this problem and need a solution, either in terms of parts selection or circuitry.
]]>Original Question: How do I define "duty cycle" and what does it actually means. Please don't give me the formula.
Answer: The duty ratio D is the ratio of the on-time (ton) to the period (T). The on-time(ton) is the time that the energy transfer inductor is taking energy from the source and is usually controlled by the on-time of a power transistor switch. The off-time (toff) is the time that the energy transfer inductor is idle or delivering energy to the load which is usually controlled by the off-time of a power transistor switch. The sum of the on-time and off-time is the period (T), which is also the reciprocal of the switching frequency (f). The dimensions of ton, toff, and T is seconds. D is a dimensionless ratio that can have values between zero and one. Frequency (f) has the dimensions Hertz (preferred) or cycles-per-second.
In terms of a formula you did not request:
D = ton/T = ton/(ton+toff) = ton*f
D is often used to describe the DC Gain of a switching-mode power supply.
DC Gain in switching-mode power supplies is defined as the (output voltage)/(input voltage) and is often described in terms of the duty cycle D and 1-D. D' is sometimes used for 1-D. For example, the DC Gain of a buck-boost converter would be expressed as D/(1-D) or D/D'. It is a dimensionless parameter and can have the values of zero to infinity. It is often non-linear.
This information is mostly embedded in an expert system on my website, which identifies the seven single-inductor two-state converters and gives their characteristics.
]]>Original Question: I am requesting you to guide me in designing a 5 V, 1 A step-down regulator with an input voltage of 18 V using a PWM. I'll be very thankful if you help me out.
Answer: This type of question, that basically asks me to design a power supply that meets some requirement, is one that I frequently receive. The answer is always the same, a modified no, -- so don't ask. There is really no way I can design your power supply for you in a short email reply. The best I can do is to find a reference design or application note close to what you want so you can study it, and then modify it to your design. But this takes time, and it is something you can do for yourself. Your best sources of information on the web are the application notes and reference designs of the vendors who make the parts you need --- the makers of power semiconductors and controllers. You can find a partial list of these vendors on my vendor page at www.smpstech.com/vendors.htm. Do a page search on the key word semiconductors and dig what you need out of their websites. An alternate is to use a good power supply design book as a guide. This approach takes a little time to search for a design close to what you want, but is one of your best approaches to beginning a design.
The reason for this response is simple. If you are a student, you are not going to learn much if I do your design for you. If you are a working engineer, you have to take responsibility for your own design, and you can't ethically do this if you don't know in detail how it was done. So what good am I? If you have given it a good shot and are stuck, I can sometime help you over a rough spot. Therefore, ask a questions if you think I can help in a ten minute email answer, but don't ask me to do the research or design your power supply for you.
]]>Original Question: We service a fire alarm panel that uses a switching power supply. Every time the building experiences a power outage (which is at least twice a month) the power supply fails to restore commercial power to the panel without going through a "warm" start procedure which consists of disconnecting the backup batteries then disconnecting 110 V, waiting 30 seconds or so, then reconnecting 110 V followed by reconnecting the backup batteries. Instead the panel continues to run off the backup batteries until they either go dead or until we arrive and "warm" start the system. Since the batteries and charging circuit are obviously good since they "hold" the system up in the absence of commercial power why doesn't the system switch back to 110 V when it's restored? In addition we noticed the LED indicating the power supply is "on" was oscillating rapidly while the problem was present. After "warm" starting the panel it resumed normal operation.
Answer: This is what I think is happening. Input power is sensed at the bulk energy capacitor on the input of the switching-mode power supply. It is sensed here to minimize short-time transients that the power supply can operate through. When this voltage drops below a certain threshold for a period of time (such as the decay when ac power is lost) the battery is switched in either with an SCR, or some latching switch that acts like an SCR, which can only be reset by removing power, both ac and dc. The 30 seconds is needed to make sure all capacitors holding energy in the circuit are discharged. The oscillating LED is probably meant to indicate ac power was lost and the system in running on battery power. The circuit is probably working as designed. It could have been designed that way for two reasons. One reason is that turning off an SCR that is carrying current complicates the circuitry and it is cheaper to design it as it is now working. Another reason is that in some applications you want to know that an event took place and want to require a manual reset so the system can be checked for proper operation. Hence you might design it that way with no regard for cost. If you want automatic reset, you might open a dialog with the manufacturer. It may be as simple as changing a jumper to get the operation you want. I suspect they have a solution for automatic reset, but what is involved in swapping or modifying equipment and the costs involved is the open question.
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