Trying to figure out why MOSFET is shorting

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strelok
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Trying to figure out why MOSFET is shorting

Post by strelok »

So I'm trying to figure out why this particular MOSFET is prone to failure. Its a DN2540, depletion mode N-channel. Its used in a constant current source of a tube pre-amp. It works great in circuit, however the pre-amp has an external power supply, if you don't wait long enough after switching the unit off for the supply to discharge and disconnect it, it can cause the device to short out.

I setup a test circuit to try and see what exactly was going on and if I could repeat the results independent of the CCS circuit itself. Just a simple common source amplifier. Indeed if the supply has any residual voltage left on it (say just half a volt.) the device will short out when the source is disconnected. If the gate is first shorted to the source , it seems to survive, or if the supply has zero volts across it. Now the device is rated at some 400v if I remember right. I'm looking at the specs and I just can't see why it should fail, especially with such low voltage present after the supply has been turned off. I'm thinking I should just redesign the circuit with different FET's or maybe even BJT's as they seem much more resilient, but It'd be nice to see if I can't figure out why its failing and if there's something simple that can be done to protect it before going to such lengths. Thoughts?
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JMFahey
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Re: Trying to figure out why MOSFET is shorting

Post by JMFahey »

Impossible to answer if you don´t post the schematic :?:
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tubeswell
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Re: Trying to figure out why MOSFET is shorting

Post by tubeswell »

What Juan said. Need a schematic with voltages.

Where did you buy the MOSFET? Taiwan? China? Some stuff on fleabay from sources in those countries is fake and will more than likely blow whatever circuit you put it into.
He who dies with the most tubes... wins
R.G.
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Re: Trying to figure out why MOSFET is shorting

Post by R.G. »

The two replies already are the first replies you needed. Without a schematic, we can't figure out what likely stresses the FET is under, so it's not possible to sort out real circuit causes from "gremlins are breaking it"

The third reply you need is that once we know it's genuine MOSFETs in a respectable circuit, we can opine about what is overstressing the FET. I'd guess at a gate transient breaking the gate, but there is nothing you've given us so far that would support that, nor offer any kind of fix.
strelok
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Re: Trying to figure out why MOSFET is shorting

Post by strelok »

Well like I said it fails even in a simple common source circuit. I can put up a schematic for that if you really need it but its just a DN2540 with the drain connected to a +15v supply through a 4k75 resistor. The gate is tied to ground through a 10k resistor and the source is also tied to ground through a resistor, though I forget the value. Was probably a few hundred ohms just to bias it pretty close to being off. For the test the circuit is switched on, a signal generator hooked up to the gate and a scope probe at the drain just to verify its working. Then the supply and signal are shut off. The source is then disconnected from ground, 90% of the time this causes the device to short. The residual supply voltages are below one volt by the time I disconnect it. I've experimented with plenty of other FET's and BJT's in the past when doing bread boarding stuff and I've never had one fail because the source/emitter was disconnected (say from changing the source resistor out while experimenting). The only way I've found to keep the DN2540 from shorting reliably is to connect the gate directly to the source before disconnecting the source from ground.

The devices were sourced from a reputable place, either mouser or digikey. I can't remember which. They're rated at 400v for Vdg and Vds. Vgs is +/- 20v. I'm going nowhere near those limits, which is why it has me puzzled.
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TUBEDUDE
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Re: Trying to figure out why MOSFET is shorting

Post by TUBEDUDE »

Schematic.
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R.G.
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Re: Trying to figure out why MOSFET is shorting

Post by R.G. »

strelok wrote: Sun Sep 17, 2017 6:24 am They're rated at 400v for Vdg and Vds. Vgs is +/- 20v. I'm going nowhere near those limits, which is why it has me puzzled.
You're almost certainly exceeding the gate-source voltage and puncturing the gate. It may be that your power supply feeding the circuit does odd things at turn off, it may be some set of parasitic capacitances get added up to more than 20V under the turn off conditions. A MOSFET gate only has to be over-volted for microseconds, perhaps nanoseconds, to puncture.

The DN2450 does not have gate protection diodes. My next move if it were mine would be to put a pair of zeners from gate to source, connected cathode to cathode, with the two anodes connected one to gate, one to source. Because this is a low frequency linear setup, it would make sense to put a 100R to 1K resistor as close to the gate as you can physically get it to help damp RF oscillations at frequencies so high your scope can't see it.
strelok
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Re: Trying to figure out why MOSFET is shorting

Post by strelok »

Oh of course! Man I can't believe I overlooked that. Most of the MOSFET's I've used in the past had them built in internally. I knew I'd overlooked something. I'll give it a try tomorrow and report back. Thanks!
strelok
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Re: Trying to figure out why MOSFET is shorting

Post by strelok »

Well it seems like that solved the problem. I finally got around to trying it in the preamp today, and after disconnecting and reconnecting the power supply at least a dozen times, even with the unit switched on I didn't see any device failures. Here's to it staying that way lol.
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