"Flyback" Driver - Why are my MOSFETS hot? (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 09 Jul 2004 15:41:57 +0930
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: "Flyback" Driver - Why are my MOSFETS hot?

Malcolm wrote:

>  BTW - have you got yur flyback circuit running correctly yet? It is
> always useful to get feedback so we know which suggestions if any
> proved to be correct or wrong. I too noted the parasitic oscillations
> when the MOSFETs were supposed to have been off and I note that one
> contributor suggested a series gate resistor but I seem to remember
> you saying that you already had one. I think that excessive impedance
> in the source lead is the main cause of those oscillations myself.
> Having said that, current-mode SMPS designs *do* require primary
> current monitoring and do use a source/emitter sense resistor to do
> the job. Perhaps there is too much impedance in the gate lead (as
> opposed to too little). What is the value of your gate resistor?
>

I haven't really had a chance to get back to this yet - I don't get a lot of
free time during the week.  Hopefully this weekend...  What I have done so far is:
* Driven the gate with 12V
* Removed one of the MOSFETs from the array - it was getting significantly
hotter than the others.

All still gets hot in about the same amount of time.

Gate resistor is 100R.  Have got my scanner working, so have posted a (revised)
schematic at <http://www.mss.cx/tmp/sch1.jpg>.

When I get a chance, I'll scope it all up again to see if I'm still getting the
same waveforms with the higher gate voltage.

Thanks for your help to all others for suggestions so far.

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy
South Australia