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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 12 Jul 2004 08:22:15 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)

On 9 Jul 2004, at 8:51, High Voltage list wrote:

> Original poster: 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>.

It would be a good idea to lower the value of that gate resistor to
somewhere around 10 - 20 Ohms. That should speed switching up no end.
And I still maintain that a single MOSFET is adequate for your design
given what I know about the transformer.

Malcolm