Introduction

The gate drive and the snubber are the two circuits that decide whether a power switch survives. The gate drive controls how the device turns off, but the commutation-loop inductance always stores energy, and without a snubber that energy appears as a voltage spike at the device terminals that can exceed the blocking rating and destroy an expensive module. Faratronic snubber capacitors clamp that spike, damp the ringing and reduce the electromagnetic interference the converter radiates. This application note explains how to size and place them for a real IGBT converter.

The Turn-Off Event in Brief

When the switch turns off, the load current commutates to the freewheeling path and the rate of change of current interacts with the loop inductance to produce a voltage, added to the bus voltage, across the device. The faster the turn-off and the larger the loop inductance, the higher the spike. A capacitor placed directly across the device provides a local path for that current, absorbing the energy before it can raise the voltage beyond the rating. The lower the equivalent series inductance of the capacitor and its connection, the sooner it takes effect.

Sizing the Snubber

Estimate the energy stored in the loop at the peak current, then choose a capacitance that clamps the spike within a safe margin below the module rating. The voltage class must exceed the worst-case clamp level with margin, which is why high-voltage converters use snubbers rated to 3000 V or more. Too little capacitance fails to clamp; too much adds switching loss and slows the transition, so the value is a balance, checked on the bench.

Layout and Mounting

The snubber is only as good as its connection. Mount the capacitor at the module terminals, keep the snubber loop and the commutation loop as small as the layout allows and use the shortest, widest conductors. A snubber on a long lead adds its own inductance and clamps far less effectively, which is the most common reason a correctly sized snubber still fails to control the spike. Measure the waveform at the module, because the bus measurement hides the peak the device actually sees.

EMI Benefit

The ringing that a snubber damps is a source of both radiated and conducted noise, so controlling the spike also lowers high-frequency EMI. That can reduce the burden on the input filter, the shielding and the grounding, and it often turns a marginal EMC result into a comfortable one. A quiet switching waveform is the natural result of a well-designed snubber and a tight layout.

Choosing the Part

A PCB snubber such as the C3H suits a driver or power board, while a lug-terminal snubber such as the C38 suits a power stack where it bolts across the module busbars. Both use the same low-ESL, self-healing film construction. The BeiLuo FAE team helps size the capacitance and the voltage class and reviews the layout so the snubber does its job.