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Non-obvious mosfet failure cause

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forrestc:
I'm hoping someone can point me to a resource to help me out of the "I don't understand how this could be failing" rut I'm in.   Note that this is a problem I sat aside for a couple months but need to pick it back up.  So I need to figure out which tests I need to do from here and adjust a test PCB layout accordingly.

I have an application where I tie two N channel mosfets (in the same package) together back to back - that is tie the gate and source of both together and then use the resulting circuit as a bidirectional switch to turn on and off DC of unknown polarity.  Think "AC" solid state relay but used for DC.

The gates are driven by a single photodiode output optocoupler (TLP3906) across the gate and source so the drive circuit is isolated from the mosfet.

After a supply-chain forced part change, I was having failures of this arrangement where the mosfet will turn on and will never turn off.   The failure results in one or both of the mosfets being shorted out.   Not sure if the failure occurs during turn on or turn off, but I believe it is likely during turn-on but I can't be sure.   I can reliably cause this failure by putting a relatively large (2000uF) cap across the output with a reasonable bleeder resistor across the capacitor.  I've seen this failure in various mosfets, but the SQJB80EP-T1_GE3 is known to easily exhibit this issue, whereas the original mosfet (SQJ974EP) did not show this issue.   Note that I've tried various mosfets after discovering this issue and some fail reliably and some do not.   There doesn't seem to be any parameter that I'm looking at (Rds(on), Vds(max), Vgs(max), Id, Vgs(th), etc), that predicts the failure.

Now, here's where the unknown/non-obvious issue comes from:

In an attempt to understand/prevent this issue I've:

1) Added fast "zener/tvss" diodes around the FET - (i.e. to protect from Vgs and Vds spikes), soldered directly across the part pins.
2) Added gate resistors and/or capacitors to slow turnon and turnoff (in case it was a surge current)
3) measured (with a scope and 70Mhz differential probe) the voltages across the device during switchon/off to verify ratings were not exceeded.
4) measured peak current (using shunt resistor) through the circuit to verify that Id and Idm wasn't exceed. 

With the caveat that 3) and 4) were bandwidth limited by the probe, I did not see a single measurement which explains why these should be failing.   The Voltages are never exceeded, at least external to the die, and the peak currents seem to be within limits.   

Note that this is a single turn on and then attempted turn off event, and not a PWM or any other pulsed event that causes this.  Also, these failures can be induced on the first switch of a brand new part so it isn't wear-out.

I believe what I might be seeing are internal di/dt or dv/dt failures or some other similar on-die interactions.   I've read various discussions about these failures, but I'm struggling to see how the circuit described is likely to cause any of the failure modes that are discussed in the various documents.   Especially with a primarily capacitive load. 

As an example, a Toshiba app note describes dv/dt failures at turnoff, but these all seem to be related to situations where the Vds would drop rapidly such as when driving a inductive load or in a very high speed hbridge application.   Other documents describe similar failures, but none seem to be able to be applied to my circumstances.

Any ideas here?

Wolfram:
A potential candidate is junction over temperature caused by dissipation during the capacitor charging. As much energy ends up dissipated in the MOSFET as ends up stored in the capacitor, independent of how fast it turns on. You have to look at the precharge duration and transient thermal impedance for that time scale to figure out if it's fine.

The mentioned part has an Idm of some 80 A for 300 us, or 24 mC. This is enough to charge 2000 uF to 12 V. What is your supply voltage?

forrestc:
 
--- Quote from: Wolfram on March 28, 2023, 12:26:27 am ---A potential candidate is junction over temperature caused by dissipation during the capacitor charging. As much energy ends up dissipated in the MOSFET as ends up stored in the capacitor, independent of how fast it turns on. You have to look at the precharge duration and transient thermal impedance for that time scale to figure out if it's fine.

The mentioned part has an Idm of some 80 A for 300 us, or 24 mC. This is enough to charge 2000 uF to 12 V. What is your supply voltage?

--- End quote ---


I'll have to rattle this around in my head a bit mainly because I need to understand this principle better since I want to make sure I understand this before fully discounting it.  It's either 24 or 48V typically, so on the face, that's a possibility that this is the issue.   Going to have to do more research about how the heat gets dissipated over time.

However, I sort of ruled this out for the following the reasons:

The part which doesn't fail (SQJ974EP) has a lower Idm rating (65A), and so does it's successful replacement (SQJ980, 68A).    In fact the SQJB80EP is a better part than the SQJ974 and SQJ980 in almost every way (higher Id, higher Idm, lower Rds(on)).   The Vds is lower than the original SQJ974, but higher than the SQJ980 which doesn't fail.   Admittedly the lower Rds(on) may account for a higher peak current into a capacitor.

Oddly I have other alternative parts with higher and lower Idm ratings and also higher and lower Rds(on) which failed testing.

I also added enough r/c network at the gate to slow the turnon to the point where the drain current never exceeded the Id rating and still got failures.

jonpaul:
VHF, UHF parasitic oscillatons between the two FETS.

Use single ended Probe and scope BW 200..500 MHz

Jon

Wolfram:
48 V and 2000 uF is 96 mc of charge, if you limit the current to be constant within the Icm of the part, then the pulse would last for just over a millisecond. The part is specified for 300 us at Icm, with an ambient temperature of 25 C and a maximum junction temperature of 175 C. If Icm is thermally defined, which is not unrealistic, this corresponds to 150 degrees delta T at 300 us, it's left up to the imagination what happens if the pulse is extended to 1200 us. Temperature rise won't be linear for a given dissipation, given that the heat has time to spread out the longer the pulse is, but this still suggests that in case I_inrush is limited to be just within Icm, then you will massively overheat the MOSFET when charging 2000 uF from 48 V.
 
Which MOSFETs fail and which ones survive under these conditions is hard to generalize from datasheet ratings. Instantaneous failure usually means your junction is a lot hotter than the maximum temperature, possibly in the 350 - 500 degree range. It's impossible to say whether a MOSFET with an Icm rating of 60 A will always fail before one with an Icm rating of 80 A, because that's not what the Icm rating means.

Extending the pulse helps, but not linearly. The energy deposited in the MOSFET doesn't change with pulse length, it's fundamentally always the same energy that ends up in the capacitor (U^2*C/2 = 2.30 J at 48 V). If the load draws any current during the precharge event, then the energy dissipation in the MOSFET actually increases with longer pulses. Extending the pulse lowers the peak dissipation, which lowers the delta-t, but the device transient thermal impedance is also higher for longer pulses.

As an example, let's say you extend the precharge to 10 ms, and the power dissipated in the MOSFET is constant (it's easier to correct for this afterwards rather than doing the full calculation), then the power dissipated during the event is 2.3J / 10 ms = 230 W. Transient thermal impedance for the SQJB80EP for a 10 ms single pulse is 0.65 k/W, giving a temperature rise of 150 degrees, marginal given the fact that ambient could be higher than 25 degrees and the fact that dissipation during precharge is not constant. This would suggest that you need to extend the precharge time into the hundreds of milliseconds to have sufficient margins, or ideally chose a part in a larger package.

Id and Idm are just simplified limits with a lot of assumptions. In an application like this, the main thing to look at is the junction temperature against Tj_max. Simulation can be a valuable tool here, especially if you can find a device model that includes thermal dynamics.

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