The purpose of book mini-reviews is to explore what some popular texts say about snubber design. Of the hundred or so power supply design books I own, I keep about ten of them on my desk so I can refer to them quickly when I answer questions asked by readers and when I prepare new content for my websites. Here is what some of those books say about snubber design.
What it Says About Snubber Design
Switches: Chapter 4, provides an excellent background to switch characteristics including forward biased and reverse biased characteristics that determine the safe operating areas of switches. However, Safe Operating Area (SOA) is not listed in the index, and there is no example of using the SOA curves, which semiconductor manufactures provide, to help the designer design snubbers for a switch. However, this is not a major shortcoming since all power switches data sheets specify this information, and it is the actual SOA for the device you are using that matters.
Snubber Design: There are seven instances in the index under snubber networks. The first (p. 85) points to a description of forward biased and reverse bias device failure and that snubbers may be required to prevent this. The second, (p. 92) discusses the need for snubbers in MOS-controlled Thyristors (MCT). The third, (p. 97) points out the need to use RC snubbers to control the overshoot and ringing in diodes. None of these instances gives any information on how to design the suggested snubbers. Chapter 7, Soft Switching, (p. 764-765) explains the need and shows the circuit for an RC snubber to protect a diode from excessive voltage caused by ringing. But no advice on how to design it. P. 766 explains the need for a snubber for a flyback converter to avoid excessive ringing and gives a simple rule-of-thumb for getting the first estimate of C and R. This is the only design advice I found in the book for actual snubber design, -- but there is a substantial material in the book on soft switching, a technique to minimize or eliminate the need for snubbers.
Example of Rule-of-Thumb: The snubber for the flyback above is a parallel RC circuit connected between the power source and the cathode of a diode whose anode is connected to the MOSFET drain. The rule-of-thumb for the capacitor is to make it large enough so the ripple voltage across it contains negligible switching ripple. The resistance is then set so its power consumption is equal to the switching loss in the transformer leakage inductance. The equation and conditions to make this calculation are given.
Summary: The book gives solid information on understanding switches and on why you need snubbers, but virtually no information on how to design them.
What it Says About Snubber Design
Switches: In the preface, Dr. Ray Ridley says "Pay close attention to his comments on semiconductor deratings and advice for safe operating regions. If just this part of the book were absorbed into our industry, the number of power supply failures would drop dramatically." But if you go to the index and look up the term switch and follow each instance, you don't get to the switch information you need for snubber design. If you look in the index for safe operating "anything", it is not there. Using other terms that might work lead nowhere also. You might make the erroneous conclusion that the book does not cover the switch characteristics you need for snubber design. So where is it? Look up the converter topology you are working with. There you will find rich descriptions of what you need to know including protecting switches, snubbers, clamps, etc. What this means from a practical approach is that, if you are designing a specific circuit, which you always are, you will probably get the information you need for that circuit. But if you are looking for snubber generalities that apply across many circuits, you may have to dig this out and construct them yourself.
Snubber Design: The index does better in listing snubbers.
Page 529 discusses the need for snubbers to reduce power dissipation in both the power switch and boost diode in boost converters operating in the Continuous-Current Mode (CCM) used for Power Factor Correction (PFC) applications above 1 kW, however there is no information on how to design them.
Pages 835-838, in a section labeled Installing a Snubber, is the major pointer to snubbers in the index. The first example in this section builds up to an RCD snubber by starting with a capacitive snubber across a power MOSFET. The equations are derived for the turn-off conditions showing the snubber reduces power at turn off but increases the turn on losses. The mathematical derivations are rigorous and the complex resulting equations are solved for these conditions in Mathcad to show that there is an optimum value of capacitance that results in the minimum power dissipation in the MOSFET -- but all of the power is dissipated in the MOSFET. This snubber is then turned into a RC snubber off-loading power from the MOSFET to the snubber resistor but a diode is needed to overcome a problem with the RC snubber and regain the power lose achieved with the C snubber. Equation are use to calculate value of R that does this. The index under snubber capacitors points to the part of this section that discusses the selection of the capacitor
Example of Rule-of-Thumb: This text primarily uses equations, not rules-of-thumb.
Summary: The only snubber design found through the index is the RCD snubber, which is used to reduce the power dissipation in a MOSFET, not to fit it into the full safe-operation area. Is there more on snubbers in the individual topologies discussed in the book? I suspect there is, but it would take a careful reading of the whole text to find out.
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