strange MOSFET failure mode

The subject MOSFET is a C2M0280120D, a serious 1.2kV SiC TO-247 MOSFET. The scope pic shows driving it with a -3.6 to +16.5V gate pulse, 100ns long in this case, and it's switching a 50-ohm resistor from +200V, or 4A (part of my fast HV pulser project.) I previously had it making a burst of five 15ns-long 250V 5A pulses, and everything was fine, perfect.

But after fooling around a bit, it stopped making 15ns pulses. Yes, it still turned on in under 10ns, as programmed, but it turned off with a 50us time constant. Scope traces:

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(Ignore the ringing, sloppy probe grounding.)

Carefully replacing the MOSFET with a new one (from a different batch, as it happened), I started the signalling pulse generator, and observed the exact same failure mode on the very first pulse.

There are two issues, A. how did I damage the part, and B. what is its failure mode: able to turn on in

Reply to
Winfield Hill
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dV/dT ? I see the spec sheet has parameters for Vds of 800v.

Cheers

Reply to
Martin Riddle

Are you sure the 50 ohm resistor is OK?

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

=0

NASA has specifically characterized those CREE SiC as fairly high reliabili ty. Looks like you've damaged the gate in some weird way so that the gate c harge doesn't have a low impedance path for being removed at turn off. Mayb e there are limitations on peak gate current you're not observing.

Reply to
bloggs.fredbloggs.fred

[ snip ]

S##t, John, brilliant! Why didn't I think of that? I just measured it and the 50-ohm resistor is toast. Damn, that's the second expensive 100-watt resistor I've blown out on this project. Idea A could have caused that as well. Both MOSFETs are likely fine.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I was also going to ask if half the gate driver might've given up. But load resistor, yeah.

Wait, so you're not testing into a termination (dummy load)? :o

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

I had a strange case about a year ago. A fet, standard 2n7002, would turnot on at 0V, and off at negative 3V ish

Turned out it was damaged by a leaking current from a year cap

I am not saying your case is the same, just that a damaged fet can act funny

Cheers

Klaus

Reply to
klaus.kragelund

What kind of resistor is (was) that? What's your average power dissipation?

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

The resistor is a Caddock MP9100 in a TO-247 package, rated at 100 watts. It doesn't have much thermal mass; momentary overload rating is only x1.5 for 5 seconds. 20nH inductance gives my 50-ohm version a 400MHz bandwidth.

I was dissipating 1.25kW five times, 100ns each; 0.6mJ per trial. The "fooling around" afterwards must have destroyed the resistor.

My present circuit board provides for two 100W resistors, and it has a HV shutoff lasting 150ms after the LV is established, to prevent mishaps. I'm going to solder in those parts! A future version of the board has other safeguards. These mishaps are so easy and so deadly.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Hi, Win -

Is it possible that the internal construction can't take the 5A current pulse? The data sheet says it can survive 150W for 5 seconds which is about 1.7A in your application. We have no idea about the internals. Since the resistor appears to be open, I would suspect this failure mode. Call Caddock?

Reply to
John S

Thanks, that's a good suggestion.

The 1.5x over-power rating is much lower than the usual 5x rating we see for other power resistors. The MP9100 resistor must have some thermal mass for short higher-current pulses, but apparently it's quite fragile.

My immediate application uses this resistor to back terminate a 50-ohm line at 250V, and I'm using two 25-ohm parts in series. That's much more benign. The maximum current would be 2.5A and the power 156 watts per resistor with a much-less-than-100% duty cycle.

The design is intended for use to 800V, where the current would be 8A and the instantaneous power 1.6kW each. This may be an impossible goal with poor struggling Caddock resistors. I'd have to severely limit pulse repetition frequency, duration, etc.

There are other chassis-mount wideband 50-ohm resistors I could research and consider.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

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