Hi Brent,

> Hi Jim,
>
> When the load is shorted, yes the power supply is now directly connected =
across
> Drain/Source of the FET, but no, VDS will not be 24V. The FET is in the o=
n state
> (around 9 milli Ohms) and the power supply will hit it's current limit of=
 25A.
> Therefore VDS is around 0.009 x 25 =3D 225mV. Power dissipation in the FE=
T is
> 0.225 x 25 =3D 5.6W, well within the FETs safe operating area - for the m=
oment.
>
> Just to check that VDS can not be 24V, that would require 24 / 0.009 =3D =
2667A.
> While brief duration high current peaks might occur if there were suffici=
ent
> capacitance on the output of the supply, and the FET would be destroyed, =
that's
> likely not the case here given the supply is characterised for use in con=
stant current
> mode.
>
> The failure mechanism is probably what occurs a short time later. As the =
supply has
> been shorted (0.225V) then whatever circuitry is driving the FET gate has=
 also lost
> power and the gate voltage begins to drop (relatively slowly as there'll =
be some
> capacitance in the drive electronics eg. could be some seconds or so, and
> assuming there are no effective under voltage lock out schemes in place a=
nd
> seems likely in this case there are none). The FET begins to come out of =
saturation
> (into the linear region ie the state between on and off) and RDS increase=
s. IDS
> stays the same (external supply still happily current limits at 25A) so p=
ower
> dissipation in the FET increases proportionally with RDS, junction temper=
ature rises
> at an exponential rate and very soon the FET melts and is destroyed. Can =
do some
> I2R at a few data points to illustrate:
>
> 25^2 x 0.009 =3D 5.6W
> 25^2 x 0.05 =3D 31W
> 25^2 x 0.1 =3D 63W
> 25^2 x 0.2 =3D 125W
> 25^2 x 0.5 =3D 313W
> 24^2 x 1.0 =3D 576W
> 12^2 x 2.0 =3D 288W
>
> Note: Supply comes out of current limiting above max power point of  R =
=3D 0.96
> Ohms (24V/25A) and power declines beyond that, but dissipation still very=
 high and
> depending on time-frame SOA likely well and truly exceeded and meltdown a=
lready
> acheived.
>
> Ideally: Under voltage lockout circuit to keep FET off when there insuffi=
cient gate
> voltage - but you're unable to modify the drive circuitry in your case.
>
> Plan B: Separate external supply for driver circuit to keep it working wh=
en main
> supply drops due to short. RDS stays low, current limits at 25A, lights g=
o out, short
> is removed, lights back on again - everyone is happy~!
>
> Hope this helps.
Super helpful, so awesome of you to take the time to answer this so=20
completely. It had slipped my mind that the current limiting would kick=20
in forcing the voltage to drop on the power supply. Hopefully next week=20
I will get my hands on this unit and see what happens if the driver has=20
a secondary supply.
Cheers,
Jim
>
>

--=20
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