Flyback snubber....purpose?

[eem2am]

In a flyback SMPS. We may put an RCD snubber across the FET.

This is usually configured as a turn-off snubber since the turn-off is usually considered to be the “worst case”.

But what is the true function of such an RCD snubber, across the FET?

Is it…..

1. To delay the rise of FET drain voltage and hence reduce EMC problems.

2. To reduce switching losses in the FET.

3. To provide a temporary conduction path for the primary leakage inductance at FET turn-off, and hence prevent the FET drain voltage from going ruinously high at turn-off.

 

[biff44]

When the fet turns off, the current in the coil drops to zero, which really p.o.'s the coil, which will do anything possible to keep the current flowing. It will convert the collapsing magnetic field into a big reverse voltage spike of voltage, which will wipe out the fet in no time at all. The diode clamps the voltage spike to something the fet can tolerate, the capacitor slows the rise time down a little to let the diode work.

 

[eem2am]

thans biff44,

i see your point, but in a flyback, the output diode will be turned on, and the core will thus be de-magnetized.

the output diode will take some time to turn ON, -but so would the diode in the RCD snubber.

I am afraid i am still unsure as to the real purpose of the RCD snubber that appears across the FET.

Also, to an extent the transformer capacitances and fet drain-source capacitance will absorb some of the flyback energy, up until the point when the secondary diode turns ON.