RC Snubber (Ring Killer)
Size R and C from the measured ringing frequency and estimated parasitic capacitance.
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The engineering
The ring is a parasitic LC tank (switch capacitance against layout inductance) getting kicked every transition. The classic bench recipe: read f_ring off the scope, estimate or measure C_par (add a known test cap until the frequency drops by √2 — that means you doubled C), then set R to the tank's characteristic impedance so it damps critically, with C_snub a few times C_par so the resistor stays connected during the ring but not all cycle.
The power row is the bill: the snubber cap charges and discharges through R every switching cycle, dissipating C·V²·f_sw regardless of how clean the waveform gets — at 400 V and 65 kHz even 200 pF costs 2 W. If that's unacceptable, shrink the loop inductance (the real culprit, see the nH row) before buying a bigger resistor.
Where this math comes from
Snubbers grew up with the thyristor: GE's 1957 commercial SCR switched hard enough to kill itself with its own dV/dt, and RC networks across the device became standard armor. The design lore was scattered rules of thumb until William McMurray's 1972 IEEE paper 'Optimum Snubbers for Power Semiconductors' put the damping-factor math on record.
Power MOSFETs (International Rectifier's HEXFET era, 1978 onward) and now GaN pushed switching edges from microseconds to nanoseconds, moving the ringing from audible-snubber territory to EMI-lab territory — but the bench procedure on this card, from scope reading to resistor, is unchanged.
- 1957General ElectricCommercial SCR — hard switching, and the snubber's job, arrives.
- 1972William McMurray'Optimum Snubbers for Power Semiconductors' formalizes the design.
- 1978International RectifierHEXFET power MOSFETs — faster edges, same RC medicine.
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