Issues converting [12V to 5V] Buck converter to Forward conveter

[cmo]

Hello,

Just discovered these forums. Seems I've been missing out.

I've got a 12V to 5V buck conveter which I am to now convert to an isolated design. Obvious progression was a forward converter.

Using the TL5001 for PWM/Control and a P-Channel MOSFET (FQP17P06/7) with basic totem pole driver. 47uH ind/100uF cap.

Wound the transformer 4:2:4 (Primaryemag:Secondary), chucked another diode in on the secondary side but there's no output. Seems there is a short somewhere, as the short circuit protection on the TL5001 is high.

Is there any reason that a forward converter would not work with a P-Channel MOSFET?
I initially tried the converter with an N-Channel but switched to the P-Ch due to simpler gate drive circuitry. As I understand it the N-Ch needs Vin + Vgs(thresh) to switch the FET. True? How to overcome?
Is there a way I can drive the N-Ch without too much hassle?

For the isolated feedback I was going to go for an opto/431(programmable zener/shunt reg) combo. That's the best bet?


Any help/pointers to resources would be much appreciated


Cheers,
simon.

 

[kchriste]

One issue with going from a buck to a forward converter would be that the max FET On time for the forward is WAY less than the max FET ON time for the buck. Posting a diagram of both circuits would help alot!