How to choose FPGA for converter control?

If you insist on using an FPGA, you need to learn some points. Use websites that provide simple tutorials ranging from design languages to simulation & synthesis tools, like: http://asic-world.com (http://asic-world.com). There are many video tutorials also available on YouTube, etc.

If you can run a simple counter on a development board and observe how it counts, you may be able to design your power electronics controller.

So:

1. Learn a little Verilog (simpler than VHDL).

2. Learn how to simulate a simple design using a simulator (e.g. ModelSim).

3. Learn how to synthesize & implement your design on FPGA using Quartus.

4. Run it on a development board.

Now start designing your comparator/PWM. You can copy/paste on-line Verilog codes. Just google "PWM Verilog".