PWM 5 - 15V, 35A schematic

Hello everyone.

I have to develop a PWM that will supply fuel cell (or something like that). This is the customer's concept: Transformer (220VAC to 15VAC) - rectifier - PWM - fuel cell.
The PWM module should regulate the average value of the output voltage. Note that the output voltage should be rectified, but not filtered.The transformer, rectifier and the MCU that will produce the pulses are familiar to me and have no troubles with creating that part of the ware. I will program PIC MCU to produce the square pulses. I am rather worried about the power part/driver that will "amplify" the PWM signal. Shall I have inductance, reverse diode, what kind of transistors should be used. I consider the frequency of the PWM to be greater than 20KHz in order to avoid hearing noise. I do have certain electronic knowledge, but I have not much experience in creating power electronics. If you need more information, don't hesitate to ask. Any help would be much appreciated. Thank you.

- - - Updated - - -

https://drive.google.com/file/d/0B_EYtdS7J5eKcXI3dm96N3pvQVk/view?usp=sharing This image is just a demo of PWM driver I just drew to make the question clearer. The enclosed part of the schematic is the one I am not sure how to implement so to withstand power of 5-15V/35A.

What you have is probably essentially OK. You would just need to size the power drivers accordingly to handle the current/voltage/power required. You showed two, but may only need one. There are plenty of FETs that can handle 35A or more. I would probably pick something at least double the current rating to handle switching transients, etc., and probably also at least double the max expected voltage. From there, the power rating and any heatsinking considerations would be made. Since this is PWM-driven, the power loss when switched on will be minimal (just Rdson*I), but there will be switching losses as the FET turns on and off (and the Rds value increases). So the longer the switching time, the higher the loss. But you also don't want it switching too quickly which may cause unacceptable electrical and EM noise. So you probably want to include a series gate resistor and you can adjust that value to compromise between smooth switching and power dissipation.

I would add a good cap after the bridge rectifier if you weren't already planning that. I'm not sure what type of load you are driving, but I think a fuel cell could be modeled as a resistive load, so you probably don't need to worry about snubbers, flyback diodes, etc.