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January 04, 2005

How To Design a Power Supply

"And if the power supply is not designed properly, it may well attract too much attention by not working." -- Ed Walker

One of the problems most new circuit designers encounter when they design their first power supply is how to go about it. As it turns out, there are a near infinite ways to do it. Left to their own devices a new designer will usually do a poor job of it. Those in luck, will have an experienced and vigilant mentor to help them through the process. But for those that don't, papers like Topic 3, Design Review: A Step-By-Step Approach to AC Line-Powered Converters, in the "Texas Instuments 2004/05 Power Supply Design Seminar are invaluable.

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:

  • Define Requirements
  • Define Topology and Control Method
  • Define Input Section
  • Transformer Turns Ratio
  • Output Inductor
  • Output Capacitors
  • Output Rectifier Snubbers
  • Output Rectifiers
  • Power Switch FETs
  • IC PWM Controller
  • Setting Up the Loop
  • A Few Optional Ideas

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."

Posted by Jerrold Foutz at January 4, 2005 02:58 PM