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TVS vs Thyristor to Protect FET

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Lumina

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I am trying to protect a FET from an Ldi/dt voltage spike and would really appreciate any advice. I am trying to protect in the event of a short circuited load. When I short out the load ie. connect Lead A to Lead B in the diagram below the FET blows, not from over current but over voltage. The leads are about 2 foot long 16 AWG. The supply is current limited at 25 Amps. i have some 1500 watt bidirectional TVS diodes with a clamping voltage of 55 Volts.( It's an 80 volt FET ) When I put 1 TVS across the output of the FET the TVS went up in smoke when I created the short. I tried putting 3 TVS in parallel and that worked at a 50% duty cycle however as I tried adjusting the duty cycle they failed. Is it just a matter of adding a higher power TVS. Should I try a thyristor based crowbar instead? The FET will see probably 8 Amps max when working. The 25 Amp supply will be shared across other channels. Should I put a PTC in line as well so that once the over voltage is clamped the PTC will go after some time? As you can tell I am a newbie when it comes to FET protection. Thanks for any advice.

FETcircuit.png
 

SCR might stay latched on, probably a bad choice.

An "avalanche rated" MOSFET might solve the problem
cleanly, depending on the stored energy. Should be
app notes / selection info for any such parts.

A "catch diode" that returns overshoot energy to the
supply could also do the job, less dissipatively than
the TVS (TVS will have a Joule energy per-pulse
limit, and a continuous-pulse-train power limit, along
with steady state power limit, and since it drops the
full voltage @ full current for a period, this is worse
in terms of applied power than a catch-diode.

I find it unlikely that shorting drain to supply causes
an overvoltage failure mode. Overcurrent or over-
power, yes. I would not be so quick to assign the
failure mode without some inspection of the die (and
this wants more / better equipment and experience
than the hobbyist is likely to have).

If it is overvoltage then the fail mode should produce
a gate leakage signature and occur on a single pulse.
Overcurrent and over-power will take multiple pulses
to blow open any source bond wires or local interconnect
(at which point full drain voltage D-G is applied and gate
rupture could occur, without the body / neck shielding
the gate).

Rooting out the true failure series-of-events and
mechanisms might help you decide but I kind of doubt
it's necessary. If everything on the page is on one
board then I suggest the catch diode approach (like
a fat 50-100V Schottky), anode to LEAD B, cathode
to LEAD A, and forget the TVS or call it a backstop.
 

SCR might stay latched on, probably a bad choice.
An "avalanche rated" MOSFET might solve the problem
cleanly, depending on the stored energy. Should be
app notes / selection info for any such parts.
It is this so called "rugged FET" rated for avalanche situations so I am a bit surprised it's failing in this scenario.
I find it unlikely that shorting drain to supply causes
an overvoltage failure mode. Overcurrent or over-
power, yes. I would not be so quick to assign the
failure mode without some inspection of the die (and
this wants more / better equipment and experience
than the hobbyist is likely to have).
If it isn't overvoltage why do you think the TVS would fail? Seems to me it must be absorbing energy for it to become damaged and the only way I can see it conducting is if it goes above it's breakdown voltage which is much higher than my supply voltage.
Rooting out the true failure series-of-events and
mechanisms might help you decide but I kind of doubt
it's necessary. If everything on the page is on one
board then I suggest the catch diode approach (like
a fat 50-100V Schottky), anode to LEAD B, cathode
to LEAD A, and forget the TVS or call it a backstop.
OK thanks I can add that in as well. I have always used a flyback diode when driving a motor or relay but didn't think my load was inductive enough in this case to warrant it. Any thought on what current rating I should use for the catch diode?
Thanks again. I'm really lost on this one and really appreciate the support :)
 

Have you put a 'scope to the FET drain (single sweep) to
try and catch the real waveform?

Perhaps the source power supply has some kind of nasty
behavior when forced into current limit (like, the loop
winds up and when released, you get a harsh voltage
surge?). Seen stuff like that on some older ATE when
you bury the pin electronic into compliance limit, and
then release. Many supplies act inductive, maybe much
more so than your load (and definitely more than your
couple of feet of loose wire).

I'd go for a diode that can handle the full supply
current limit, maybe 2X derate it for luck.

If the supply has no real ability to sink current and
a light capacitance at the output, it's possible that
the catch ("flyback") diode would just pump it up to
higher voltage than set. And maybe this is part of
the problem. 'Scope time. I've had several supplies
that would source plenty and sink nothing, have had
to burden them with a low value resistor to give the
test setup the ability to hold down voltage as well
as hold up.

TVS will fail for overcurrent or overtemperature at
some point. Again, maybe you instrument it up (like
put a high current 0.1 ohm sense resistor in the
ground leg, and see what's up when you pulse and
release the FET gate one time.

You might try substituting a more well behaved
power source (like 2 series car batteries) and see
if the failure chain of events includes some power
supply action (which then you could chase down).

The avalanche rated FETs always have a Joules
per pulse rating. Do you know the energy that is
being put on the FET? 'Scope again (with I and V
simultaneously recorded).
 

Thanks @dick_freebird .
Have you put a 'scope to the FET drain (single sweep) to
try and catch the real waveform?
Yes I am going to do this. I am a bit concerned about blowing the input of the scope considering how the circuit blows up TVS diodes so easily. I will take a look at the output of the power supply first and see what is going on there where the voltage levels should be reasonable.
I'd go for a diode that can handle the full supply
current limit, maybe 2X derate it for luck.
Thanks, I will try putting a flyback diode in as well as the TVS on the drain and see how that works out.
TVS will fail for overcurrent or overtemperature at
some point. Again, maybe you instrument it up (like
put a high current 0.1 ohm sense resistor in the
ground leg, and see what's up when you pulse and
release the FET gate one time.
One of the issues here is I am trying to apply a protection circuit to the output of a product another manufacturer makes. So it means cutting into the circuit to do something like this on the input. Not a big deal. It would be a lot easier if I had designed the input circuit :) . Something I am considering is using the current flowing through a TVS at the output of the drain to turn on a circuit that along with a high side switch turn off the 24 volt signal going to the FET. Since significant current flowing through the TVS would indicate an overvoltage condition it could be used to indicate a short on the load, though I suppose I could also use a more conventional high side current sense circuit as well. Anyway, thanks again for all the great suggestions and advice.
 

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