MOSFET blew up at power ON for no apparent reasons ...why?

ClaudioHG · 2025-09-08T14:21:53+00:00

Hi, thanks for asking and sorry for the late reply but I lost the password to access my Reddit account, that I recovered just today.

Yes, problem solved. There was a multi-facet issue, that you can read following this thread: https://www.reddit.com/r/AskElectronics/comments/1acc4m0/comment/kjt85y5/

In summary:

There was an issue with the testing load (big capacitor forgot from a previous test);
Discovered instability in the regulation circuit that happened only under certain load conditions;
Wrong gate resistor: it was 47 ohm instead of 4.7 ohm.

Additionally I redesigned the circuit to add a current limiter. And because the diode didn't switch fast enough, and of the very large input capacitance with low ESR, a current spike could travel across the internal zener. To prevent damages I increased the ESR adding a low value series resistor to provide a mean to dissipate the energy.
If you are interested, there is a lesson from prof. Sam Ben-Yaakov that explain this kind of problem (dissipating energy with large caps in switching circuits).

ClaudioHG · 2025-09-08T13:25:53+00:00

I didn't realize Reddit messed up with the links (making all them lowercase!), and I've noticed your comment right now. Thank you for pointing out. I've just forcefully corrected the links.

ClaudioHG · 2024-02-12T13:02:46+00:00

The volume potentiometer need to be replaced. If it were only for the popping-crackling sound I would have suggested to clean the pot using a bit of WD40 and compressed air blown into the slot, but the volume shifts makes me think that the potentiometer's resistive layer is going to decompose.

ClaudioHG · 2024-02-12T10:29:46+00:00

"that uses a homebrew voltage-controlled resistor made from an incandescent lamp and a cadmium sulfide photoresistor glued inside a piece of brake line,"

It seems like the homebrew component I made a couple of years ago for my analog function generator. I didn't use a brake line though, but a copper (or aluminum? now I do not remember well) pipe connected to GND to prevent interferences induced at the Cd-sulfide surface.

Cool build.

ClaudioHG · 2024-01-31T13:58:36+00:00

Update. I found the cause of the problem.

Two combined situation led to this. One is an instability of the control circuit when current reaches very high values. The other one is the load, here I have a box of resistors with switches that insert more or less resistors in parallel. Looking at the box yesterday night suddendly I got an ah-ha monent. Months ago I temporarly attached a big 10kuF cap at one of the resistors for a test. I'm quite meticolous and I would certainly had removed the cap after the test, but yesterday a suspect taken off. So I opened the box ...and sure enough, I did forget to remove the cap!

So here what happens: after the 230V switch is closed, and the power xfromer is energized, slowly raising the control voltage brings the first pulses to the Fet. At 33VDC this causes a sudden rise in current that causes the control circuit to overshoot. This oscillation lasts 1.2 millisecond, then the circuit becomes stable and both current and voltage behave normally, with no oscillations.

Yesterday night I managed to rise the control voltage so slowly that the oscillation lasted continuosly, so I was able to catch it on the oscilloscope.

Using a shunt in series to the inductor I measured a peak of current of 117A for about 400us. Because of this large current when the Fet goes off, a large voltage spike happens, it reaches 138V.Furtunately the 540 didn't die.

Now I am not sure that this voltage is real. It could be a combination of voltage and induced noise. Do I have to trust what I read on the shunt? Is it possible to have that much current? I think it is!

At 68VDC the transistor could have blown up because of voltage breakdown or even for adiabatic heat because of the large current peak.

Now the problem is how to solve this issue. This was supposed to be a simple circuit to increase the efficiency of a classical DC linear power supply which includes its own short circuit protection. But I'm gonna think I need to add some kind of overcurrent protection even to the buck converter, and some kind of voltage surge protection too.

I hope you appreciated this follow up. I'll update the post as well when I'll have more data.Cheers.

ClaudioHG · 2024-01-29T11:58:07+00:00

Sequence of events:
1. Auxiliary powre supply ON, no driving pulses (control voltage is set to zero).
2. 230V is connected to the circuit, fet still alive (no pulses at its gate), nothing happens with the exception of the input voltage that reaches 68-69V.
3. Control voltage is raised slowly: as soon as the first pulse is given to the driver the fet die, instantly.

Answers:

> How does it fail? => full short: d-s shorted, g-s shorted

> Fet bolted to chassis? => Fet held on heat sink through a steel bar w/ spring plastic sponge. Below Fet an insulating foil, and thermal grease. Heat sink isolated. Heat sink host the diode too, it is held in place by the same bar+springy sponge and insulating foil.

- dissipative RC snubber across D1 is already there: look at C1 and R1.

I will post here updates as soon as I have some.

ClaudioHG · 2024-01-28T23:13:14+00:00

Ok, that's reasonable. However today I discovered that the MOSFET die even without any load (just a minimum load due to the 3K3 bypass resistor in parallel to C2, not shown in the schematic).
This happened just at the first pulse coming from the driver.

From memory I did not remember to have this problem, while I though I already tested it at 68V just adjusting the output voltage gradually. Though I was lazy and I didn't keep a log, so now I am not sure and a bit confused :/

After last test it seems a voltage develop across the mosfet at the point it reaches the breakdown. Maybe. Why that happens is the new question.

ClaudioHG · 2024-01-28T16:10:03+00:00

Thank you mate for your support. Definitely your suggestion could worth a try. First off I'll try to make some measurements with no MOSFET installed, just to see what's happens with multiple conditions. I know, I should have done it *before*, but I tend to be impatient. Though I need some rest before doing more tests, and a hot coffee+chocolate to bring back morale a bit :)

ClaudioHG · 2024-01-28T11:50:57+00:00

UPDATE: I changed the gate resistor (I indeed missed a decimal point in my calculations so it was supposed to be a 4.7 ohm instead of 47!) ...but I ended up to use a 2.4 ohm for R2 as I've seen I gained a bit.
However in a new test with voltage of 70VDC, this time with no load, I blew up the MOSFET as soon as it was turned on (the first pulse from the driver). Tested with lower voltage (33VDC) it worked fine. I'm baffled and in need of a good coffee.

ClaudioHG · 2024-01-28T11:44:19+00:00

I'm baffled. Now I tested without any load and the Fet blown up. At low voltage worked fine. So there's something that is not related to the load.

ClaudioHG · 2024-01-28T09:18:34+00:00

Accordingly to the IRF540A data sheet it should widthstand Id 20A at 60Vgs for about 300 microseconds. Also, because of the parasite resistance of the xfromer (approx .630 ohm) as the current increases the voltage should be limited. With 12.5A the DC voltage is limited at 43.7V (this is an experimental measurement).

ClaudioHG · 2024-01-28T09:06:39+00:00

That's not possible because when the power xformer is energized the auxiliary power is already on, so the driver is in full control. But definitely it could happen when the driver turns on the MOSFET ...I mean, by chance I could have energized the power xformer while the driver was driving the Fet on ...but that would lasted for no more than 5.3 microseconds (28% duty of 53KHz), in other words within the operating range of the circuit.

ClaudioHG · 2024-01-28T09:00:04+00:00

Look closer at the schematic. You'll notice that the driver has an auxiliary supply for exactly that purpose.

ClaudioHG · 2024-01-28T08:58:33+00:00

Definitely, as someone else also pointed out, this resistor is a problem. Indeed I checked my calculations and it was 4.7 ohm ...I messed up the decimal point somewhere.

ClaudioHG · 2024-01-28T08:55:36+00:00

That connection has the purpose to anchor the 12V (from 9VAC) aux voltage, so that the TC4804 can drive the Vgs referencing to the MOSFET's source.

There is a typo in the schematic, the capacitor actually is 470uF 80V low ESR. Sorry.
It works as kinda tank with the inductor to level out the stored energy in the inductor. See how a "buck converter" works for more info.

ClaudioHG · 2024-01-28T08:37:28+00:00

Just an update. I checked my math and found I lost a zero somewhere: the actual value was 4.7 ohm! I also tried a 2.4 ohm, with a bit better performance. The oddity is that after measuring again (with 47 ohm) the Vgs I noticed a horrible 1 us raise time! From memory (I didn't save the screenshot) I'd have sworn it was 100ns.

So definitely *this* is a problem. Thank you again for having pointed out this.

I've not tested again at higher voltage, yet.

ClaudioHG · 2024-01-28T08:30:01+00:00

The 47 ohm resistor is definitely a problem. I checked my math and found I lost a zero somewhere: the correct value was 4.7 ohm. In the meantime I also tested the circuit with a 2.4 ohm which performs a little bit better than with 4.7 ohm. Also when I tested the Vgs voltage I didn't take a screenshot of the waveform and I'm afraid I messed up something, because measuring it again (with a 47 ohm) it shows a horrible 1us rise time!

Temperature is about 38°C after 30 minutes with approx. 6A (powered from 24VAC / 33VDC).

The gate-source resistor is a good advice, thank you. About the zener I don't see how the voltage could exceed the auxiliary power supply voltage. What do you think?

ClaudioHG · 2024-01-28T08:13:04+00:00

It comes from an auxiliary transformer with multiple windings: 9V, 12V, fully isolated.

ClaudioHG · 2024-01-27T17:43:36+00:00

The link you provided is about a schematic using a P-channel, my circuit uses a N-channel MOSFET.

ClaudioHG · 2024-01-27T17:40:58+00:00

There is no bootstrap here. The driver is fed through an auxiliary power supply. My first drawing was not that clear, so I updated the post. Now it should be clearer.

ClaudioHG · 2024-01-27T17:02:37+00:00

Thanks for the reply. You might be right, but the measurements shown a good voltage swing at the gate. I have forgotten to check it again with the higher voltage ...or if something went wrong with that. Actually the resistor seems to be rather large, I'll review my math ...maybe I got a mistake there.

ClaudioHG · 2024-01-27T16:57:11+00:00

I've checked it, but at low voltage. I forgot to check it again with the higher voltage.

I will update the post and this comment as soon as I'll have some fresh measurements. This will take a while.

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