when i design using PI expert suit i found that the clamp diode is across primary of the transformer but in case of sample design or in application note i do not found clamping diodes across primary but across dpa switch.

what should i consider?
or what is preffered?

Dear Utkanth.
Thank you very much for contacting us.


Are you trying to design a flyback or forward converter application?
Generally speaking, for a flyback converter , placing the Zener clamp (Zener and diode) across the primary winding of the flyback transformer results in higher efficiency and possibly lower cost.
By instead placing the Zener in parallel with the DPA Switch, surely the maximum Switch voltage is clamped to the Zener voltage, but at each Turn-ON of the primary switch, the energy which is stored in the parasitic capacitance of the Zener is discharged through the DPA switch and therefore wasted. This does not happen with the first configuration. Also, note that the Zener diode placed across the DPA switch would need to have a much higher Zener voltage (VDC+VOR+Vring) which translates in higher cost and also higher energy loss.


Hope this may help.
If you have further question, do not hesitate to write on this forum again.


Best regards
PI-Start Lord