Full Bridge SMPS...snubbers across secondary diodes , or acorss primary?

[treez]

In a Full Bridge SMPS, 400v=vin, 50v=vout, 250W, is it best to snub in the primary or the secondary?
I am talking of snubbing out the resonance due to the leakage inductance.
Surely you would concentrate snubbing in the primary as the current is lower there?

The primary also contains the inductance of the primary of the primary current sense transformer...which also obviously needs snubbing or else it will ring.

 

[goldsmith]

In a Full Bridge SMPS, 400v=vin, 50v=vout, 250W, is it best to snub in the primary or the secondary?

Hi treez
Don't forget that you're not forced to use snubber , if your transformer is well wounded . but primary seems better for snubber .

Best Wishes
Goldsmith

 

[treez]

ok, I see, but thinking more about it, I wouldn't bother snubbering the primary, as the diodes across the switching fets will clamp the voltage across the fets to maximum of vin.
It is the secondary where the snubbering is needed, as they stop overvoltages on the secondary diodes.

So surely, no snubbers across primary, and protection snubbers across the secondary diodes to prevent overvoltages across the secondary diodes.
-simulation seems to bear this out as being a good idea.

Having a snubber across the primary doesn't seem to do much to quell overvoltage ringing on the secondary diodes.

An offline transformer will have isolation requirements, and it will be impossible to wind it with coupling factor better than 0.99....so there is going to be a significant leakage term.