[kreutz]

Try this thread: http://www.cnczone.com/forums/showthread.php?t=25361 on that thread, We designed a fixed frequency chopper, and it is already tested two ways: first, by using one LM555 as pwm oscillator, used also to set the blanking time for the current compare circuit, and second, using a pwm channel on a microcontroller to do the same thing with adjustable blanking time.