Square wave generator with variable frequency and duty cycle

Question

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.

Answer 1

This circuit works along the same lines as your schmitt trigger and triangle-wave generator approach except that in this design the two sections are enclosed in a feedback loop.

The feedback loop means that the Schmitt trigger changes states at the same voltage threshold levels irrespective of changes in frequency. Therefore the triangle-wave output from the integrator remains at a constant amplitude as frequency is varied.

It will probably require some effort to adjust the resistor values and time constant of the integrator to get the right frequency range, trigger levels and PWM percentage.

Answer 2

There are way more modern ways to make a variable pulse width wave than a 555 these days. Also invest on some serious gate drivers unless you want to burn your bridge: the 555 is limited to about 200mA source/sink. The smaller gate drivers can do 1A peak and there are some with at least 8A peak current (depending on how big your MOSFETs/IGBTs are!)

I have no idea on the criteria for choosing the PWM ratio for a welder… if it's manually chosen you could use a tiny MCU or something like the LTC6992 (in fact the whole timerblox series is suspiciously similar to the various 555 applications). If there is some current or voltage to be kept constant the market is almost overflowing with PWM controllers (some of these with integrated gate drivers)