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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 12 Jul 2004 08:49:11 +0100
From: colin.heath4 <colin.heath4@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)

hi all,
i would second that. a much smaller resitor will improve things greatly. if
you still have trouble with unwanted turn on run the mosfet -6 +6 to ensure
gate is pulled down and held there while off although i suspect just the
resistor will do the job as i run a half bridge of irfp460 on a tesla coil
right next to it and unwanted turn on is not a prob there.
cheers
colin

----- Original Message -----
From: High Voltage list <hvlist@xxxxxxxxxx>
To: <hvlist@xxxxxxxxxx>
Sent: Monday, July 12, 2004 5:39 AM
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)


> Original poster: 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
>
>
>