Hi Neil,

It's all about the diode.

With zener as shown in schematic this a typical "turn off and get the curre=
nt to zero=20
as quick as possible" circuit. The stored energy in the inductor is dumped =
as quickly=20
as possible... the higher the zener voltage the quicker this will happen, P=
 =3D VI.=20
Exactly what you want for a solenoid/injector driver when mechanical speed =
is=20
paramount. Yes, the zener dissipates some power.

If you replace the zener with a regular diode (was you say you've done) wha=
t will it=20
do? Nothing. Current flowing through an inductor resists change. At switch =
off of the=20
MOSFET the current through inductor continues to flow in the same direction=
=20
(downward in the schematic) and looks for an alternate path. The diode is r=
everse=20
biased and blocks it. The voltage across the inductor increases, the MOSFET=
 Vds=20
is exceeded and goes into avalanche breakdown. The MOSFET gets hot. In your=
=20
case perhaps it's rugged enough and the stored energy low enough that the=20
MOSFET survives.

The place for a "regular diode" in your schematic is across the inductor, c=
athode to=20
+12V. In this mode it is a free-wheeling/re-circulating/fly-back diode. At =
MOSFET=20
switch off the current through the inductor is the same as when it was on, =
the diode=20
is now forward biased and current circulates through the diode and indcutor=
.. The=20
current decays slower than the above example and likewise the stored energy=
 is=20
dissipated at a slower rate, still P=3DVI but now the V is minimised (=3D t=
he forward=20
voltage of the diode, ~0.7V). Exactly what you want when PWM'ing to reduce =
and=20
regulate the inductor current for "hold" mode.

You may notice that the two requirements for your circuit are diametrically=
 opposed:=20
Fast decay for quick solenoid/injector switch off, slow decay for current r=
egulation in=20
"hold" mode. A simple circuit won't solve both problems at the same time.

Brent

On 26 Nov 2015 at 11:59, Neil wrote:

> Hi all,
>=20
> Back again, and I have time today to get back to this.  First, I'll add=20
> more detail...
>=20
> Schematic...
> http://bit.ly/1Q11zbg
> The PIC controls the MOSFET, even during the "PWM" (current-controlled=20
> hold) phase, using an interrupt at ~20kHz.  It checks the comparator to=20
> see if the voltage across the sense resistor is above or below the 50mV=20
> reference and toggles the MOSFET on/off accordingly.  In these, diode Df=
=20
> was previously a ~30V 5W zener but got very hot, and I didn't have a=20
> lower-voltage zener that could handle any decent power, so I swapped it=20
> to a S2M 10A general-purpose rectifier.
>=20
> Voltage across Rs (50mOhm), with different levels of detail...
> http://bit.ly/1OtFNes
> http://bit.ly/1OiCVmt
> http://bit.ly/1T7FgA5
>=20
> Vds...
> http://bit.ly/1SmFymu
>=20
> I've not seen any indication of ringing (though I see random spikes=20
> occasionally that must be noise being picked up by the scope). However,=20
> I have not considered the body diode causing the heat.
>=20
> Cheers
> -Neil.
>=20
>=20
>=20
>=20
>=20
> On 11/25/2015 1:29 AM, Neil wrote:
> > Hi all,
> >
> > I got back to my solenoid/injector driver design this past weekend and
> > built and coded it, and it works nicely, except that the MOSFET is
> > getting very hot... though the calculations say it should run really co=
ol.
> >
> > Quick overview -- PIC18F driving a TC4427A MOSFET driver (running off
> > 12V), which drives this MOSFET ...
> > http://www.nxp.com/documents/data_sheet/PSMN3R0-60PS.pdf
> > It's ~3mOhm RdsOn at Vgs=3D10V.  There's a 0.05-ohm resistor on the MOS=
FET
> > source pin for current sensing.  And for the inductive snubber I was
> > using a 30V 5W zener across the drain & source, but that was getting
> > quite hot so for now I swapped it for a generic rectifier I had laying
> > around (1.2Vf).  Much cooler and I notice the current drops off quite
> > quickly still.
> >
> > it's currently setup to do 4.1A peak (approx 2.1ms), then 1.1A hold for
> > up to ~22ms, before going into idle/off phase and then starting another
> > cycle. The hold cycle is PWMed (I'm still calling it PWM, but in realit=
y
> > I'm current limiting by rapidly monitoring the output of a comparator
> > (~10 khz) and toggling the MOSFET on/off accordingly).
> >
> > I'm sure I'm missing something here, but can't figure out what. Calcs
> > say that if I run the peak hold cycle less frequently, I should get
> > slightly less power consumption through the mosfet, but not significant
> > enough to make a difference.  I'll run some tests tomorrow to see if I'=
m
> > getting RdsOn in the range I expect, etc. but any thoughts on what may
> > be causing this?
> >
> > Cheers,
> > -Neil.
> >
>=20
> --=20
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>=20



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