Steve, I started off with a 555 timer chip as my pwm generator I used a 2n3904 and a 2n3906 to create a differential time constant in the feedback loop of the chip and I controled the duty cycle via the current to the bases of the transistors... this was kind of cool except as the PW was modulated the frequency would vary... I ended up burning up all my chips trying to get 0...100% duty cycle

it would of been neat to see it work...
So I grabbed your 7404 oscillator and started building an integrator and comparitor out of the parts I had in the junk box. Basically I came up with a dual voltage supply using 2 7805 regulators so I have 0,5,10,12 volts available to me. I used a single TL074 as not only an integrator but also a comparitor Vcc is 10v Vdd is 0 volts and the non inverting input of the integrator is connected to 5v. I get a good approx 4.5v +/- triangle out and I feed that into the other side of the TL074 acting as a comparitor. so the error voltage range is +.5v to +9.5 0...100%. To buffer and invert I use another 7404 Vcc +10v and Vdd +5v so it floats along on the 5v supply. The inverter drives a single 2n3904 dc bypassed with a 10uf cap... 510ohm to the collector to +12v, emiter grounded and 10k feedback to the base from the collector. This boosts up the TTL to 0v/+12v and I use that amp to drive a 2n3904/3906 half bridge. witch poorly drives a single IRFP260N. See bob's comments.
Well it was fun to play around and get something somewhat working.
I had a nice regulated 40 volts across a cap at a few mA from a +12v batt...
Once I feel confident I'll play around with that UCC25701 that you use in your single chip PWM and use a real MOSFET driver etc, etc, etc...[/code]