September 11, 2000
Question: How do you design snubber circuits?
Original Question: In clamp and snubber circuits for flybacks, to reduce voltage spikes at switch-off in the MOSFET drain, the common practice consists of a diode and a resistor paralleled to a capacitor. Such capacitor must keep a constant voltage while the resistor "burns-out" the spike. Which would be the practical considerations (equations) to compute the value of the resistor and capacitor? To reduce the dissipation in the MOSFET at turn off, almost all designers use a RCD snubber. If fall time is so small in MOSFETs (typical less than 50 ns for the IRF830) the current will reach zero quickly, then the improvement achieved with the snubber seems to be negligible. Even under such assumption, it is still used. Which is the reason?
Answer: Snubbers are a complex subject. Rudy Severns gave a full one day seminar on them, Snubber Circuits for Power Electronics. Needless to say, only a couple of ideas will be discussed here. Ray Ridley, a former sponsor of the SMPS Technology website, has a Snubber Design procedure in the design tips section of his Ridley Engineering website, which probably answers your question better than what follows.
Since I first published this blog, I have initiated a website that is dedicated exclusively to Snubber Design with its own mailing list. Central to it is Rudy Severns' ebook on the subject. You may find this website at Snubber Design.
By way of background, I was influenced in the early sixties by one of my peer designers, who always used a faster transistor than he needed and then looked at the voltage and current waveforms at turn-on and turn-off, shaping them as he wanted with external components to make the switching characteristics dependent more on the external parts than the transistor. None of us had ever heard the word snubber -- it was just wave shaping and EMI control, and we used as many lossy ferrite beads as we did lossy capacitors for this purpose (lossy to damp high-frequency ringing) and made our cores lossy above the switching frequency. (Note that the use of lossy components, including magnetic cores above the switching frequency to suppress ringing and EMI is a very useful design technique.) What I mainly learned from this was never to switch faster than you need to, trading some power to avoid exciting high frequency resonances and to use lossy components to damp these resonances. The Electro-Magnetic Interference (EMI) that the resonances cause is easier to kill at the circuit than with an added EMI filter. I also learned that you should always look at voltage and current waveforms -- many designers fail to look at current waveforms, losing half the insights into their circuits performance.
I tend to use transient suppressor diodes and then augment them with snubbers only if needed. The first environments I designed for included Electro-Magnetic Pulses (EMP) and lightning, and I always placed a transient suppression diode across each input capacitor and across the power transistor for reliability purposes. The spike you are trying to snub is caused by leakage inductance and I try to minimize the leakage inductance causing the spike. For adequate design margin, most snubbers dissipate more energy than stored in the leakage inductance. The transient protection diode has an advantage in that it absorbs just the amount of energy needed to protect the transistor, never more. I use a transient suppression diode as much for later failure investigation reasons as engineering reasons. When a failure report comes back on a transistor or capacitor with the usual electrical voltage overstress reason, you can show the circuit schematic with the transient suppression diode and force a more competent failure analysis of the part. You need to use a fast-acting transient suppression diode, not your normal zener diode for this application, whose clamping action is too slow. This approach may be too expensive for some commercial designs, but in my design career, reliability was always the first consideration, not cost. Reliability has a major impact on life-time costs, and if these are considered, the cost impact of using transient-suppression-diodes changes for the better.
Finally, there is the question of lossless snubbers. One way of looking at them is that the lossless snubber circuits can be viewed as miniature switching-mode powers supplies and much of the some thinking applies.
Notice I did not answer your questions, but used it as a sounding board for some ideas. You will also want to look at my other website, Snubber Design.
Others may have a different perspective on this topic. Comments are always welcome.
Posted by Jerrold Foutz at September 11, 2000 08:48 AM