You can get by with a PWM made from only 3 op-amps on a quad general purpose Jfet op-amp,( comparable to a TL-084 from Texas Instruments), for only $1.50 or so from Radio Shack plus the resistors and capacitors needed.They also have a little engineer's handbook on op-amp circuits that show you how to make the circuits.You will get more duty cycle from an op-amp based PWM,(0-to 100%), instead of using a 555 timer based PWM,(30 to 90%),and it is a simpler configuration than using counters and weighted voltage summators for a very precise staircase wave generator for the comparator.PWM's get even more sophisticated with micro-processor based systems where the counters are programmable. The first op-amp will produce a square wave,the second op-amp will produce a sawtooth wave,and the third op-amp is a positive ongoing comparator.By adjusting the DC reference voltage going into the comparator,the output goes high to +VC when the sawtooth wave equals the reference voltage and stays there until the sawtooth wave goes down to the reference voltage,giving you a nice square wave of varying width.For DC motor control it is best to go with a set 20khz oscillator frequency depending on the RC time constant to get fine control of the duty cycle and to avoid audio noise from the motor.This PWM works good for uni-directional DC motor control where the PWM goes into one gate driver for your IGBT or MOSFET. As you stated,bi-directional motor control incorporates an "H" bridge configuration and 2 gate drivers where one gate driver turns on 2 diagonally positioned MOSFETS and the other gate driver turns on the other two MOSFETS..You can use one PWM with a 3 contact switch where the center connection is common to both gate drivers.This way you cannot turn on both MOSFETS at the same time,as this produces a dead short, roasting your MOSFETS. I hope this helps you!ammoniaman.