any help there with snubber design? i wnt a snubber cct for MOSFET 2N 6766 for a 8A constant current source. input s 70V . i am getting ringing in the waveform across MOSFET. this MOSFET is used in push pull configuration with a transformer primary.

 

It depends on the operating frequency.

A typical 50Hz contact snubber uses 0.1uF + 100 Ohm.

The capacitor should be of a suitable value so fast edges are passed to the resistor, but not too much of the base AC waveform.

 

I think the 2N6766 N chan Mosfet has a free wheeling diode across D-S, so that will protect the semiconductor, so a suitable capacitor across the transformer winding will stop the ringing. The 0.1/100 RC may do the job, as Jenkins suggests. I would observe the damping with a scope, however, and add a MOV if any HV spikes are left over.

Cheers, DPW [ Spent years making heaters out of op-amps.]

 

How about an RCD snubber! They perform very well when designed properly.

http://www.cde.com/tech/design.pdf

http://focus.ti.com.cn/cn/lit/an/slup100/slup100.pdf

 

for a push pull circuit an rc snubber across each switching fet works alright for me.

as a rule of thumb i think usually the capacitor should be a 4 times larger than the output capacitance of the switching mosfet so try 2nf or so and start with a 100 ohm resistor and try smaller resistors too.