RC Snubbers: Taming dv/dt Across an SCR

Lesson notes

Key points

• Every SCR has a dv/dt rating, the maximum rate of rise of forward voltage it tolerates without self-triggering.
• An RC snubber placed in parallel with the SCR slows the voltage rise, because the capacitor must charge first.
• The resistor limits the capacitor's discharge current into the SCR at turn-on, protecting it from a damaging di/dt spike.
• For di/dt protection itself, designers add a small series inductor, so snubbers and inductors work as a pair.

Definitions

dv/dt rating

Maximum rate of rise of anode voltage an off SCR withstands without unwanted turn-on, in V/µs.

Takeaway: RC snubbers limit dv/dt; series inductors limit di/dt.

dv/dt ratingRC snubberSeries resistorSeries inductor