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| MOSFET: RC snubber vs. slower gate drive |
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| TimNJ:
Hi all, For a MOSFET switching a load with some inductance, voltage overshoot appears on the drain (AKA switch node) when the circuit is opened up. With the right (wrong?) parasitic elements present, this overshoot can lead to switch node ringing. In my experience, there are two basic ways to cut down overshoot and ringing, without changing the MOSFET. 1.) Add RC snubber from drain to source 2.) Increase gate drive resistance to slow down turn off In my understanding, these two methods sort of do the same thing from different angles. The RC snubber is sort of a tuned circuit intended to have a low impedance at the problem frequency. Energy at that frequency will be dissipated in the snubber, cutting down on overshoot/ringing. A higher gate drive resistance slows down the turn off, reducing the peak overshoot voltage. Now, to me, it seems like both these things kind of cut down on overshoot/ringing by slowing things down. The snubber must also slow down the transitions too, right? My question is: When is a snubber more appropriate? When is bumping up the gate drive resistance more appropriate? Thanks! Tim |
| dzseki:
My limited experience in the field tells me usualy it is the best to use both. For switch off ringing the drain RC snubber is the best. But proper gate resistance is also needed, because too fast ramp up on gate will cause overshoot at turn-on. Sometimes the gate resistance needs to be asymmetrical, ie with series diode shunt. |
| station240:
The way to think about it, is the spike itself is a waveform, and the RC snubber is tuned for that frequency. Lets say the switching is 100kHz squarewave 50% duty cycle, 10us p-p. The spike appears on the scope as 0.25us, or 1/20th of the positive part of the main waveform. f = 1/ time; = 1 / 0.25us = 4Mhz So the spike is 4Mhz, and you design the snubber for that. Anyway the trade off between faster risetime, and not needing a snubber comes down to: a. Mosfet is most efficient when either fully on, or fully off, in-between states produce heat as Drain-Source is higher resistance. b. Slower rise time means less EMI issues, and less expense in snubber components. c. The voltage overshoot is also an issue for the poor mosfet, as you need to over specify it's voltage to cover any spikes no matter how short. The practical RC snubber networks I've been looking at lately (flyback SMPS), all have a diode as well, presumably to reduce ringing. |
| TimNJ:
Thank you both. Dzeski, that's usually what I've noticed too. Typically, we tune both the gate resistance and the RC snubber. Station240, that's basically how I look at it too. The overshoot is basically half of a cycle at some frequency. Maybe a couple of MHz. Design such that the snubber will present a sufficiently low impedance at that frequency. Both the RC snubber and gate turn on/off speed affect efficiency. In an RC snubber, there is always power dissipated in the resistor. And slowing down the transition speed (increasing gate drive resistance) drives up switching losses...Why spread the inefficiency between the MOSFET and the RC snubber? Why not just slow down the MOSFET sufficiently and eat the efficiency loss in the MOSFET alone? Why add extra two parts for the snubber? I guess those are my big questions. I know my understanding is incomplete/wrong, but I'm curious to know why we usually use a combination of the two. Thanks. |
| ogden:
--- Quote from: TimNJ on February 13, 2019, 09:32:29 pm ---Why spread the inefficiency between the MOSFET and the RC snubber? Why not just slow down the MOSFET sufficiently and eat the efficiency loss in the MOSFET alone? Why add extra two parts for the snubber? --- End quote --- Snubber is more power-efficient than slow switching. Power controller IC manufacturers happen to have appnotes regarding subject. First one in the search result list: https://www.maximintegrated.com/content/dam/files/design/technical-documents/design-solutions/DS32-Correct-Snubber-Power-Loss-Estimate-Saves-the-Day.pdf |
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