A question about Mosfets.... [Design]

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From: Scott on 25 Jun 2010 17:59

I've attempted to build the circuit depicted here:

http://www.instructables.com/files/orig/FVE/I3SC/FR123O01/FVEI3SCFR123O01.png

It's a flyback driver. I understand the basic idea behind the circuit,
with the TL$94 operating in a push-pull configuration driving the
mosfets. I have the dead-time adjusted to a value that approximates a
50% duty cycle square wave. Pin #9 of the TL494 outputs the square
wave, and pin #10 outputs the inverse of the square wave. That's a bit
of 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 #10
looks fine, but pin #9's square wave is truncated. The leading edge is
there, but it immediately drops back to ground. Thus what I'm seeing
if 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 to
parallel mode instead of push-pull mode, it'll also work fine.

My only guess is that what's happening is that perhaps the initial
Gate-Source current of the mosfet is so high that it's causing the
output transistor in the TL494 to shut down? I've tried substituting a
larger resistor for the 10ohm R5. I have to go to at least a few
hundred ohms before the effect will clear up, and this seems so far
outside the design of the circuit.

I'm using IRF810 as the mosfets.

This happens regardless of whether or not the flyback is connected (so
it's definitely not inductive kickback from the flyback), and whether
or not the +30V supply is connected.

Any thoughts?

Thanks,
Scott

From: Scott on 25 Jun 2010 18:00

> I'm using IRF810 as the mosfets.

Correction -- IRF840

From: George Jefferson on 25 Jun 2010 18:18

"Scott" <smbaker(a)gmail.com> wrote in message
news:7d05afef-7f77-4e8f-a7a3-25e16ed9177f(a)h10g2000prh.googlegroups.com...
> I've attempted to build the circuit depicted here:
>
> http://www.instructables.com/files/orig/FVE/I3SC/FR123O01/FVEI3SCFR123O01.png
>
> It's a flyback driver. I understand the basic idea behind the circuit,
> with the TL$94 operating in a push-pull configuration driving the
> mosfets. I have the dead-time adjusted to a value that approximates a
> 50% duty cycle square wave. Pin #9 of the TL494 outputs the square
> wave, and pin #10 outputs the inverse of the square wave. That's a bit
> of 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 #10
> looks fine, but pin #9's square wave is truncated. The leading edge is
> there, but it immediately drops back to ground. Thus what I'm seeing
> if 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 to
> parallel mode instead of push-pull mode, it'll also work fine.
>
> My only guess is that what's happening is that perhaps the initial
> Gate-Source current of the mosfet is so high that it's causing the
> output transistor in the TL494 to shut down? I've tried substituting a
> larger resistor for the 10ohm R5. I have to go to at least a few
> hundred ohms before the effect will clear up, and this seems so far
> outside the design of the circuit.
>
> I'm using IRF810 as the mosfets.
>
> This happens regardless of whether or not the flyback is connected (so
> it's definitely not inductive kickback from the flyback), and whether
> or not the +30V supply is connected.
>
> Any thoughts?
>

Seems like a pretty sorry circuit IMO...

From: Scott on 25 Jun 2010 18:31

> Seems like a pretty sorry circuit IMO...

Yes, I should have taken more time to figure out how it worked (and
whether it worked!) before diving in and building the thing...
Sometimes one puts too much faith in things found randomly on the
Internet.

From: Martin Riddle on 25 Jun 2010 18:55

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