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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 07 Jul 2004 10:53:34 -0400
From: Bruce Bowling <bbowling@xxxxxxxxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)

I have screwed around with different FET gate drives for the MegaSquirt EFI
controller FET drive section, and came to the conclusion that it is easiest
to use a dedicated gate driver device. I used a MC34151P from On-semi (and
there are others that drop-in). I also added gate resistance in order to
limit gate surge current, and adjusted to eliminate oscillation, like
suggested below.

Link for schematic: www.megasquirt.info

- Bruce


At 07:50 PM 7/5/2004 -0600, you wrote:
>Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
>---------- Forwarded message ----------
>Date: Mon, 5 Jul 2004 20:00:07 +0200
>From: gtyler <gtyler@xxxxxxxxxxxxxxx>
>To: High Voltage list <hvlist@xxxxxxxxxx>
>Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)
>
>ok, looking at your waveforms I see some parasitic oscillations as
>well.you may need to increase the drive resistor value, maybe 68R for
>each fet. Another thing to look at is the gate voltage in the "off"
>state. A totem pole does not pull down to 0v, maybe a resistor from gate
>to source? The thing I don't understand is that I use totem pole drive a
>lot and don't have a problem with it pulling down, the 0.7V low level
>drive is usually enough, so just corrent the turn on voltage and the
>parasitic problem first and try again.
>
>----- Original Message -----
>From: "High Voltage list" <hvlist@xxxxxxxxxx>
>To: <hvlist@xxxxxxxxxx>
>Sent: Monday, July 05, 2004 4:19 PM
>Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)
>
>
> > Original poster: Steven Roys <sroys@xxxxxxxx>
> >
> >
> >
> > ---------- Forwarded message ----------
> > Date: Mon, 05 Jul 2004 07:35:46 +0930
> > From: Matthew Smith <matt@xxxxxxxxxx>
> > To: High Voltage list <hvlist@xxxxxxxxxx>
> > Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)
> >
> > George Tyler wrote:
> > > Are you driving the gates of the fets with 5V? If you are that is
>too
> > > low, you will not turn them fully on and they will get hot. There
>could
> > > be other factors as well.
> >
> > Matthew replies:
> >
> > Guilty as charged!  I'll put some extra "juice" onto the gate-drive
>totem-pole
> > and report back.
> >
> > Spot the guy who's spent too long working with logic levels ;-)
> >
> > Cheers
> >
> > M
> >
> > --
> > Matthew Smith
> > Kadina Business Consultancy
> > South Australia
> >
> >
> >