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I'm a VHDL guy for work, and play with power converters on the weekends so please forgive if this question is dumb.

When using MOSFETs, IGBTs, or SCRs in power converters, it is typical to see snubbers in parallel with the switch in order to manage dV/dt. The higher the voltage being switched, the more likely you are to see snubbers. (The reason I've seen for transistors are to avoid latchup; however that cannot be the only reason because SCR's work because of latchup. But I digress.)

I have occasionally seen snubbers on diodes, but I haven't looked at 1000's of power converter schematics.

I've never heard of a snubber in parallel with a switch hurting anything; however since some of my diode networks are large, this could get needlessly expensive.

Here is my question:

When a diode repeatedly sees > 80% of its rated voltage (i.e. the voltage is "high"), and that voltage is in the hundreds of volts, should I put snubbers in parallel with it by default? Is that a good practice that makes life easier, or is it usually overkill?

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  • \$\begingroup\$ You add snubbers when you want to solve a problem. Commonly for fast diodes that are used to commutate high dV/dT signals to keep them quiet in an EMC perspective. The fast switching can cause oscillations that create EMC issues. \$\endgroup\$
    – Kartman
    Jul 29, 2023 at 13:59

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Snubbers are required for two different reasons:

  • To dampen high-frequency oscillations, so the device emits less radiation. Snubbers make it easier to pass EMC regulation if there are high dV/dT transients over the diode.
  • To dissipate energy coming from inductive kickback of the device switched by a relay or transistor. The snubber ensures that the switch isn't destroyed by voltage spikes from inductive kickback.

If the switching device is a diode, and it switches fast enough (how fast is "fast enough" depends on the dV/dT of the signal), it will always switch off when the voltage and current across the diode is zero (as the diode itself is primarily not acting as a capacitor or inductor, so voltage and current are in-phase). So there will be no inductive kickback. So for a diode that switches on and off due to its rectification action, a snubber might be required for conformance reasons, but is very unlikely to be required to protect against component destruction.

In fact, diodes do have a capacity, and diode leads do have an inductance. If you are switching at very high frequencies (>100kHz, considerable power), sometimes the approximation that "you always switch at zero current" is not good enough anymore, but if that's your application, you likely need to consider the implications of switching high power at high frequencies in way more detail than just adding a snubber.

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