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Capacitor across transformer primary

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Hi,

Does anyone have an opinion on why C19 is present in the attached DCM flyback schematic.

Its a bit from this TI demo board.

https://www.ti.com/lit/an/snva465a/snva465a.pdf

I was thinking its just a snubber but why do that instead of an RCD snubber. Surely its going to be more lossy?
Unless its for some parellel resonance effect?

LM5021 Flyback.PNG
 
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Snubber capacitor
it's not for resonant, because of D3,D4
it's valuse about 1/10 of reosnant frequency for L,C

regards,
 

Hi,

I don't like the direct connection of a capacitor on a mosfet switching signal.
I think a 10 Ohms series resistor is useful.

Klaus
 

Thanks for the replies guys.

Anyone know what the losses associated with this would be.

Off the bat I'm thinking that C19 is charged to have Vin across it whilst The FET is on. When the FET is turned off the capacitor will be rapidly discharged to the difference between Vin and the zener clamp (dissipation in the zener clamp there).
After that at some point (when Isec falls to 0) C19 will be fully discharged.
So do I just assume that the loss could be 0.5CV^2 multiplied by the switching frequency. Where C is the C19 value and V is Vin?

Also in a DCM flyback the FET on loss is assumed to be zero as there is no change in current. Will C19 change that?

Cheers
 

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