A question about Mosfets....

Scott · 2010-06-25T21:59:33+00:00

I've attempted to build the circuit depicted here:It's a flyback driver. I understand the basic idea behind the circuit,with the TL$94 operating in a push-pull configuration driving themosfets. I have the dead-time adjusted to a value that approximates a50% duty cycle square wave. Pin #9 of the TL494 outputs the squarewave, and pin #10 outputs the inverse of the square wave. That's a bitof a simplification, but I think it's close enough for discussion.The problem is when I attach my scope to pins #9 and #10. Pin #10looks fine, but pin #9's square wave is truncated. The leading edge isthere, but it immediately drops back to ground. Thus what I'm seeingif momentary spikes instead of the "on" portion of the square wave.The scope looks sort of like this:* * * ** * * ** * * ** * * ***********************When it should look like this:**** ***** ****** * * * * * * * * * * * * * . * * ***** ****** *I hope the crappy ascii art comes out somewhat meaningful.If I disconnect pin #9 from its mosfet, then the output looks fine.Reconnect it and the square wave will appear truncated again.Strangely enough, pin #10 looks fine. If I switch the TL494 over toparallel mode instead of push-pull mode, it'll also work fine.My only guess is that what's happening is that perhaps the initialGate-Source current of the mosfet is so high that it's causing theoutput transistor in the TL494 to shut down? I've tried substituting alarger resistor for the 10ohm R5. I have to go to at least a fewhundred ohms before the effect will clear up, and this seems so faroutside the design of the circuit.I'm using IRF810 as the mosfets.This happens regardless of whether or not the flyback is connected (soit's definitely not inductive kickback from the flyback), and whetheror not the +30V supply is connected.Any thoughts?Thanks,Scott

Jun 25, 2010 20 Replies

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I'm starting to see why the square wave business looked more attractive at the beginning. There's none of this root 2 stuff to worry about. With that square wave output, I'd be getting almost 120 volts out, which I don't want to use.

It's not that I hate square waves, you understand..I just don't like the noise a connected motor makes. I don't want my furnace motor disintegrating when I need it the most.

I have a cheaper model modified sine wave 3000 watt inverter. I connected it to a 5kw transformer and the noise made me disconnect it NOW. The frightening thing was that it was an epoxy potted core in a metal case and it still made a racket.

I will continue looking for a solution. The only thing I will NOT consider is a brushed motor. Maybe I can find a transformer with a wider range of taps.

mike