I am in process of converting my DC TIG welder into AC for aluminum welding. I've got H-bridge on the output of the welder and to drive it I am using 2 IR2104 Half-Bridge drivers. For testing I've been using simple NE555 timer to drive H-bridge, but when I change the frequency it also changes the duty cycle, so I have hooked it up to the oscilloscope constantly to monitor the output. I tried using schmitt trigger to generate square wave with variable frequency, then converting it to triangle wave and feeding it into comparator to get variable duty cycle. It works but when the frequency changes so does the triangle wave voltage from almost 0-3Vp-p to as low as 0,5Vp-p or even less. So I've got 2 options:
a) do something about fluctuating triangle wave voltage or
b) build another circuit.
IMO the way of generating the triangle wave and then converting it into square PWM isn't that bad. Other things I tried was using 2 NE555, one driving the other through its trigger input. And second try - generating the triangle wave using op-amps but both methods didn't work out. In summary I need a square wave generator with:
-variable duty cycle 50-95%,
-variable frequency 20-400Hz.
Any clues on how to achieve that are really appreciated.