Inverter circuit shoot-thru HELP

[famousken]

Hey everyone, I am working on a project right now and am having some problems with it. It is an inverter consisting of a tl494 PWM IC running at 50 to 200Khz, discrete gate driver, gate drive transformer made with 6 turns of cat5 cable, and an H-bridge of mosfets, irfp840s I think. Anyways, my problem is that I can't PWM the output, I scoped the output of my gate driver and its good, nice square wave output, same thru the GDT, but when I hook the GDT to my mosfets, The waveform looks more like a sine wave than a square wave. I am using 33ohm resistors on the gates, with +/- 15V gate voltage. The output of the H bridge is a clean square wave, but I can't PWM it, Ill turn the duty cycle down on the tl494 and the output will still be 100%, If I turn the duty cycle up past around 30%, I get shoot-thru, even though I am using a +/- gate drive set up, anyone have any suggestions?

 

[vk6kro]

You would have to put a load across the centre of the H-Bridge and observe the waveform across it with a differential oscilloscope.

Maybe a load of about 100 ohms.

Are you already doing that?

 

[famousken]

Yes, but I am able to see a waveform with or without a load. I have been using a 60W light bulb as a test load and a 10:1 probe