Hi Neil -

You already have 12 V in your box - connect a diode between your MOSFET D a=
nd the 12 V supply to the gate drive. Then make sure you have a heavy condu=
ctor supplying the circuit. Use an ordinary power diode with current rating=
 sufficient for the full load current (4 A?).

There is much that is less than ideal with this but it will serve.

The bottom line is that you have stored energy that MUST be dissipated.

When you use non-Zener diodes across your MOSFET the MOSFET will enter aval=
anche breakdown and conduct the current, getting hot. When you use a Zener,=
 it conducts (as long as Vz < Vbd) and it gets hot. Neither of these is a p=
roblem IF the components are suitably rated AND appropriate precautions tak=
en to dissipate the heat.

With a diode across the coil, most of the dissipation will be in the coil R=
.. A Schottky will be faster to turn on but so long as the diode turns on be=
fore Vd rises above Vbd, the MOSFET will not conduct. Fast turn on can brin=
g EMI issues (as indeed can the big current loop of my suggestion but you a=
lready have a BIG loop). The only question then to resolve is "Is it fast e=
nough?".=20

For understanding these issues, SPICE is really helpful - LTSpice is free a=
nd easy(-ish)...

hth
Stephen




-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 Brent Brown
Sent: Friday, 27 November 2015 9:23 AM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] MOSFET heat

On 26 Nov 2015 at 16:45, Neil wrote:

> I've been avoiding tying anything to the positive side of the load as=20
> that would be outside of the circuit.

The flyback diode can only work (in the mode where it maintains inductor cu=
rrent and minimses energy loss during PWM off periods) if it is across the =
inductive load.=20
Appreciate that it may be inconvenient to place it there, but that's where =
it must go.



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