Dying mosfet - why, oh why? - Page 1
Forum section: electronics / beginners

author: florian-x
author_rank: Contributor
timestamp: October 11, 2021, 07:34:29 pm
content: Dear all,I have a problem with a circuit I’m building. A small n-channel mosfet gets destroyed - it becomes conductive between base and source (although no smoke is released…) and I can’t think of any reason for this. I have simulated the circuit in LTSpice, with no problems whatsoever, and it behaves as I intended.I’m pretty beginner at electronics, and self-taught at that, so I may be missing something obvious here...The circuit is the following (actually, part of a bigger thing, but the rest is not relevant). The mosfet that fails is Q205 (schema below).And the PCB (the white arrow shows the problematic transistor).This is a load switch, driven by a microcontroller, with a few “features”:- The switched voltage (LDSW+) is 12V, same as SW_12V. But LDSW+ is present all the time, while SW_12V can be turned on and off (a microcontroller shuts down this voltage, and with that it shuts down itself too).- An objective is to have very low current when SW_12V is turned off (i.e., only leakage current, less than 1uA).- When SW_12V is off, I don’t want the LDSW+ voltage to leak back on the left part of the circuit through R206 and R205, so I added Q205. The idea being that if SW_12V is at ground, the mosfet will be off and no current can flow backwards. And when SW_12V is on, at +12V, the push-pull transistors Q203 and Q204 will be able to drive the gate of the switching p-mosfet, Q210, first via the body diode of Q205 then directly when Q205 turns on, as the voltage goes from 12V to 0V.- The Q211 is not relevant to the problem, its role is to stop reverse current if load is powered directly, SW_12V is shut down and LDSW+ is connected to ground.For my tests, the load is just the LED (but it’s expected to be a load of a few amps, driving a 25W motor under 12V (and a freewheel diode exists, but not shown here)).So, when I power this circuit, I find that Q205 leaks current from gate to source -- the voltage at the source of Q205 never goes close to 0 when I turn the switch on, but remains at 3-4 volts. To make sure this is really the problem, I tested with a 2n7000 mosfet. When new, not in circuit, I can apply 15V in all directions: gate to source, source to gate, gate to drain, drain to gate, and there is no current passing. Then I put it in the circuit, remove it and measure again, raising the voltage slowly. Once I get over 2-3 volts gate to source, it starts conducting 1-2 milliamps (and goes quickly higher if I increase the voltage). So basically the mosfet dies. I tried this 2-3 times, always with a similar result.I also tried with a 1k resistor at the gate of Q205 (which would have been a good idea anyways), with the same result.If I remove Q205, the push-pull transistors work fine, the voltage goes from almost 0V to almost 12V.So, I’m a bit out of hypotheses on why this happens. I haven’t seen anything strange with an oscilloscope. Here is a screenshot with the Q205 removed, the blue trace measuring the net at the source of (the removed) Q205, showing the two transitions: going low (switching on) and going back high (switching off). (The former being much slower than the latter, but that is normal.) The yellow trace is just the power, SW_12V. When Q205 is present, the trace does not go to 0V, but remains at 3-4V minimum.I'd appreciate your opinions, and anything I can test. Thanks!

author: mikerj
author_rank: Super Contributor
timestamp: October 11, 2021, 09:20:48 pm
content: Take a look at your SW_12V rail whilst it's being switched on, it's possible you are getting an inductive spike that's causing the gate to breakdown. Are C201 and C202 ceramic parts? Is there a significant length of cable between the circuit and the power source?

author: Jwillis
author_rank: Super Contributor
timestamp: October 12, 2021, 02:34:45 am
content: The Mosfet 205 is backwards .

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 12, 2021, 04:21:57 am
content: Also q211...

author: magic
author_rank: Super Contributor
timestamp: October 12, 2021, 06:05:51 am
content: No idea what's wrong.But the circuit looks extremely complex for what it's doingYou could power the whole gate drive circuit from the permanent 12V rail and this would eliminate problems with leakage to the MCU side as Q201 would be the only connection remaining.You could ditch the Q202 inverter and invert polarity of MCU signal.Given that the circuit probably doesn't need to switch at high frequency, you could even remove Q203,Q204, reduce Q206 to 1kΩ and connect it straight to Q201.

author: mikerj
author_rank: Super Contributor
timestamp: October 14, 2021, 11:36:47 am
content: Quote from: Jwillis on October 12, 2021, 02:34:45 amI'm probably missing something, but I don't think it is? If you put it in the other way the body diode would conduct when SW_12V was turned off, defeating it's purpose. In normal operation the gate is held at 12v and the source is switched between 12v (MOSFET off) and 0v (MOSFET on). When SW_12V is removed both the gate and the source will be at 0v, so the MOSFET will be off. This seems like an entirely reasonable way of doing things. This does assume that the SW_12V does actually get pulled down to ~0v when switched off, if not then all bets are off.

author: florian-x
author_rank: Contributor
timestamp: October 27, 2021, 10:04:01 pm
content: Thank you all for the comments.I gave up trying to understand what goes wrong with the circuit -- after using up like 10 or so mosfets.... I looked at the rail while being switched on, no spikes or anything. The caps are ceramic, yes. Cables from the power source - like 1m leads from a DC power supply, nothing exceptional.It's unfortunate, because I lost an occasion to learn something...Anyways, I simplified things, as "magic" was saying. I used a push-pull mosfet pair, and counted on their gate insulation to provide the isolation I wanted. And two gate resistors to control turn on and off speed. Works nicely.Here is the schematics, a scope trace (blue is the voltage at the power transistor, yellow the output, showing both turn on and off). And my heavily patched PBC!

author: langwadt
author_rank: Super Contributor
timestamp: October 27, 2021, 10:26:02 pm
content: Quote from: magic on October 12, 2021, 06:05:51 amcould probably do with just Q210,Q211,Q206 and R207https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/2742.Switch_2D00_Pch.jpg

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2021, 10:35:41 pm
content: Does it always happen to fail after plugging it in (namely hot-plugging the SW_12V)?Try a TVS from GND to SW_12V.Tim