Mosfet failure - all pins shorted - Page 1
Forum section: electronics / projects

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 14, 2021, 07:26:19 pm
content: I've made a circuit to PWM a 20 ohm load at 325VDC. The switching frequency goes from 2kHz to 500kHz ( it's for a research project). I already tested the circuit with a smaller load (10Kohm load and 200W light bulb) and everything was working fine. But as soon as i use the circuit with the 20 ohm load ( it's a 5kW braking resistor), the mosfet brakes down. Not with smoke or anything. The mosfet just doesn't turn of any more. After taking out the mosfet and measuring it, all 3 terminals are shorted together. So i guess there was a large voltage spike that coupled into the gate but i'm not sure about this. I was thinking to protect the mosfet gate with a zener diode but i'm not sure if this would solve the problem. The mosfet is driven with a gate driver IC ( VO3120). The 20A shouldn't be a problem for the mosfet, it can handle a peak of max 60A. Could somebody help me understand what's the cause of this failure and what would be the best way to solve this issue?

author: Refrigerator
author_rank: Super Contributor
timestamp: January 14, 2021, 07:30:46 pm
content: 33seems a bit high for a gate resistor.

author: retrolefty
author_rank: Super Contributor
timestamp: January 14, 2021, 07:34:58 pm
content: Most likely your driving the device past it's safe operating area, SOA. Device is melting down. What is your heat management precautions?

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 14, 2021, 07:37:15 pm
content: Quote from: Refrigerator on January 14, 2021, 07:30:46 pmI have another circuit with same gate driver and mosfet. The 33 ohm resistor is fine, the goal is to turn the mosfet on or off as fast as possible. The gate driver can handle 2.5A

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 14, 2021, 07:38:53 pm
content: Quote from: retrolefty on January 14, 2021, 07:34:58 pmA big active cooled heatsink is attached. It doesn't get hot at all. The circuit works perfect with a small load. The 20 ohm load destroys the mosfet immediately. I have another circuit with two of those mosfets and same cooling solution to chop AC. In this case the mosfet is working without any issues.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 14, 2021, 07:50:06 pm
content: Add a freewheeling diode across the load resistor. Most resistors of this size have significant inductance, the kickback will drive the mosfet to avalanche, if the frequency is high enough, this pumps enough energy into the MOSFET to kill it. Other cause might be driving it to slow, 33R appears on the high side, I'd rather expect 3R3 here.

author: Refrigerator
author_rank: Super Contributor
timestamp: January 14, 2021, 09:06:36 pm
content: Did you buy your MOSFETs from aliexpress or a reputable source?Would be good to confirm that the MOSFETs themselves are not inherently bad.

author: jkostb
author_rank: Regular Contributor
timestamp: January 14, 2021, 09:14:08 pm
content: You need a gate resistor. Gate resistor of 33R or mrore is not unusual. Problem is that without gate resistor or too low the lead inductance combined with the total capacitance forms a undamped LC circuit. This could destroy your mosfet, because it fails to turn off. Have you measured the gate voltage and are you sure that there are no oscillations? If you increase the gate resistance then this also slow downs the turn off of the mosfet. You could ensure fast turn off by placing a schottky diode parallel over the resistor . What is the frequency and duty cycle of your control signal? Have you scope pictures of gate signal?

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 14, 2021, 09:50:09 pm
content: Quote from: Refrigerator on January 14, 2021, 09:06:36 pmMosfet's are original Rohm semiconductor purchased from DigiKey.Quote from: capt bullshot on January 14, 2021, 07:50:06 pmI know it would be good practice to add a flywheel diode. But i don't think this is the issue. I have a similar circuit but it's chopping AC( 2 mosfet's combined). With exactly the same load and driving frequency's, it works without any issues. Cooling setup is the same.Quote from: jkostb on January 14, 2021, 09:14:08 pmAs mentioned above, i have a similar circuit with the same mosfets. I don't have any issues here with turning off the mosfet. Also the mosfet can't turn off since it broke down. All leads are shorted. The frequency we used to test initially with the big load was 10Hz. It turned on but never turned off again. For some reason the mosfet broke instantly. I think for some reason an over voltage occurred on the gate...

author: Someone
author_rank: Super Contributor
timestamp: January 14, 2021, 10:26:07 pm
content: Quote from: reyntjensm on January 14, 2021, 09:50:09 pmSo far you have only shown theory/simulation (and missed many of the important details, that you may not think are important but those of us with experience recognise as critical parameters). How can you be sure that it never turned off? "instantly" is indistinguishable from one switching cycle, unless you capture the behaviour with a scope.Scope the gate drive (at the mosfet), and the output (at the mosfet). It won't be anything like the simulations/ideal assumptions.If you first capture that with the lower power loads then you can estimate the switching losses, and extrapolate that up for the larger load. Just because the heatsink isn't hot doesn't mean the silicon die is ok.

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 14, 2021, 10:34:41 pm
content: Quote from: Someone on January 14, 2021, 10:26:07 pmI have some scope images from the output and it looks indeed different from the real world. I'm aware of this fact. I tested with a 200W lightbulb and the 20 ohm resistor in parallel with a 10Hz frequency. The light turned on but never turned off... I can't keep trying this and measure the gate signal since i'm running out of mosfets.

author: SF
author_rank: Newbie
timestamp: January 14, 2021, 10:43:02 pm
content: Maybe I'm wrong but, aren't you using it outside max operating area?

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 14, 2021, 10:55:40 pm
content: Quote from: SF on January 14, 2021, 10:43:02 pmThis graph suggests i could only switch 0.2A on 300VDC for 10ms right? Strange i don't have any other issues with the same mosfets switching AC... Also the light bulb i first tested draw 1.3A...

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 15, 2021, 01:04:04 am
content: I have measured the gate voltage. Since i don't have the 20ohm load with me i've used a 10Kohm power resistor. I don't see anything special on the gate?

author: jmelson
author_rank: Super Contributor
timestamp: January 15, 2021, 01:51:26 am
content: What provides the PWM in your circuit? This transistor needs to conduct over 16 A with the 20 Ohm load. So, it will have a lot of gate capacitance, and the Miller effect will fight the gate driver both turning on and off. You need a high-current MOSFET driver chip to drive it, I hope you are not using a microprocessor pin.Jon

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 15, 2021, 01:57:12 am
content: Quote from: jmelson on January 15, 2021, 01:51:26 amThe signal is generated by a signal generator. It's amplified with an opamp and this signal drives a gate driver IC ( VO3120 from vishay). This gate driver is capable of sourcing/sinking 2.5A. I don't see any problems with the gate voltage. So the only thing i can think about is a flywheel diode who is not there. Although the 20ohm resistance has almost no inductance especially on this low frequency. Or the lack of a zener diode to protect the mosfet gate for over voltage. Could a high voltage spike couple trough the mosfet into the gate?

author: uer166
author_rank: Super Contributor
timestamp: January 15, 2021, 02:13:05 am
content: Quote from: reyntjensm on January 15, 2021, 01:57:12 amThe inductance has nothing to do with operating frequency. It could damage the fet with one pulse, if enough energy is stored in the inductance. Vds and Vgs traces at full load are needed to figure it out, they will look very different than with the 10k load.

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 15, 2021, 02:17:36 am
content: Quote from: uer166 on January 15, 2021, 02:13:05 amSo a flywheel diode could solve this issue? But this diode should also be capable of handling a lot of current right? I can't measure the gate under full load since i'm running out of mosfet's...

author: uer166
author_rank: Super Contributor
timestamp: January 15, 2021, 02:21:55 am
content: Quote from: reyntjensm on January 15, 2021, 02:17:36 amIf the issue is avalanching due to the inductance, a diode will solve the issue. You won't know until you look at the Vds at full load and see if it exceeds the FET Vds rating. The clamp diode needs to handle the full load current but only for a few microseconds, so it can be small-ish, a >5A rated diode will probably do (with correct voltage rating).

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 15, 2021, 02:39:05 am
content: Quote from: uer166 on January 15, 2021, 02:21:55 amJust across the mosfet right? Parallel with the body diode?

author: magic
author_rank: Super Contributor
timestamp: January 15, 2021, 02:44:20 am
content: Quote from: reyntjensm on January 14, 2021, 10:34:41 pmIf it fails immediately at 10Hz then I think some transient condition is more likely than overheating.Any chance you are exceeding the maximum dV/dt on the drain?What's the iductance of the load and the energy it can deliver in one avalanche?

author: Someone
author_rank: Super Contributor
timestamp: January 15, 2021, 03:41:07 am
content: Quote from: reyntjensm on January 14, 2021, 10:34:41 pmBoth the gateandload need to be scoped (ideally the load current too). If there is any significant inductance in the setup it would be seen on the load waveforms. Once that is out of the way then its:Switching Losses...being proportional to the load, and inflated by the large gate resistor.

author: xavier60
author_rank: Super Contributor
timestamp: January 15, 2021, 07:13:19 am
content: Quote from: reyntjensm on January 15, 2021, 02:39:05 amNo, it needs to go from the Drain to a very close bypass capacitor on the HVDC rail.Do you have the means to reduce the HVDC for testing?The MOSFET has an Avalanche rating. There would need to be a lot of wiring inductance to damage it.It would be useful to see your layout.

author: SF
author_rank: Newbie
timestamp: January 15, 2021, 07:48:52 am
content: Quote from: reyntjensm on January 15, 2021, 01:04:04 amNo. It says something different. It presentsvoltage between drain and sourceat the Mosfet.Do you have the same Vds with the bulb?

author: AndersJ
author_rank: Frequent Contributor
timestamp: January 15, 2021, 08:13:48 am
content: Perhaps the 20 Ohm resistor is NOT 20 Ohm.Maybe you’re reading it wrong, and it is instead 2.0 Ohm.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2021, 08:58:04 am
content: I wonder if this is a layout/construction issue. Are there long leads/traces? Please post a picture of the actual circuit.One note about the schematic: LTSpice has a MOSFET symbol, it's called "nmos" so use that, rather than the box. It would make the schematic much easier to read.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 15, 2021, 09:44:27 am
content: Quote from: reyntjensm on January 15, 2021, 01:04:04 amYour gate drive voltage is quite high. Chances you're having some transient overvoltage here.Gate rise / fall time looks rather slow for a MOSFET. I'd try lower gate resistance and lower drive voltage.

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 15, 2021, 06:13:44 pm
content: Quote from: Zero999 on January 15, 2021, 08:58:04 amThe leads between mosfet and load are around 3meter. I will add flywheel diode an try again. I know about the symbol but i don't know how to use it with a spice file supplied from the manufacturer. This symbol is auto generated.Quote from: capt bullshot on January 15, 2021, 09:44:27 amMax drive voltage is 30V. I will add an extra zener diode to protect the gate from over voltage.

author: jkostb
author_rank: Regular Contributor
timestamp: January 15, 2021, 09:15:59 pm
content: I propose that you also add a schottky diode parallel over the 33R resistor. This will speed up the turn off of your mosfet (current flows through the diode during turn off) and during turn on if flows through 33R resistor. This will reduce power dissipation also due to switching losses.

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 16, 2021, 12:35:39 am
content: Quote from: jkostb on January 15, 2021, 09:15:59 pmWhat specs should this schottky have?I did some measurements again with the 200W light bulb ( i don't trust it with the 20ohm load since i'm using my last mosfet... a new order is on it's way). The rise time is significant faster than the fall time. A flyback diode doesn't makes any difference. I only see a lot of overshoot on the turn on signal. This overshoot might be too high with the 20 ohm load. What could be the reason for this overshoot? The 230V AC is rectified with a bridge rectifier. 2200uF caps are installed to keep it as flat as possible. The gate driver is has an internal push pull configuration. I thought the rise and fall time would be equal?

author: magic
author_rank: Super Contributor
timestamp: January 16, 2021, 09:32:31 am
content: MOSFETs don't like tens of V/ns rising edges. Again, what is the dV/dt rating of your part?The rising edge is fast because you have an inductive load dumping 20A into drain capacitance of the FET until it demagnetizes and slows down.Not sure what's up with the falling edge.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 16, 2021, 11:35:45 am
content: Quote from: reyntjensm on January 16, 2021, 12:35:39 amwhat are you measuring? drain voltage? or gate voltage? if its Vdrain... turning on will show from hi to 0V... if its Vgate, you really toasting it because i (barely) see the voltage is 200+ something? your capture is so confusing, text is so small... 40.5V/div? what the heck? why dont just round it to 40V/ or 50V/? have you considered or put special care to the heatsink contact the 60W mosfet dissipated? have you tried with a little bit lower, like 10-30W dissipation and see what happened to the mosfet temperature?

author: jkostb
author_rank: Regular Contributor
timestamp: January 16, 2021, 12:43:26 pm
content: I see ringing on your scope picture. This is caused by parasitic capacitance and inductance. The ringing can be a root cause for the failure. A flyback diode will not solve this problem. At my work we had many years ago a similar failure. We solved it by installing a snubber between drain-source. This will dampen the spikes. Please also note that there is capacitance between drain and gate of a mosfet so fast spikes will cause current to flow from drain to gate and if this current flows through your gate resistor it can cause voltage spikes at your gate. Problem is that most mosfet can only handle limited voltage at gate. So I would therefore definitely install schottky diode parallel over the gate resistor and add a zener to clamp the gate voltage as extra protection. Please also install a snubber at drain source.

author: magic
author_rank: Super Contributor
timestamp: January 16, 2021, 02:39:41 pm
content: Quote from: jkostb on January 16, 2021, 12:43:26 pmIsn't Miller feedback a self-correcting problem, in that rising gate voltage slightly increases conductance, limiting the rate of rise of drain voltage?

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2021, 10:18:18 pm
content: Quote from: reyntjensm on January 15, 2021, 06:13:44 pmThere will be a lot of inductance in 3m of cable. A diode should help. Make sure it's a fast diode.QuoteRight click on the MOSFET symbol, where it says NMOS and enter the name of the model into the textbox.

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 17, 2021, 01:14:37 am
content: Quote from: Mechatrommer on January 16, 2021, 11:35:45 amThis is a voltage measurement with a differential probe across the load. The heatsink doesn't get hot since it brakes down as soon as i want to switch it off. Since it was working with a 200W ligt bulb. I tried to add some inductance to it and see what happends ( a flyback diode was installed also at this point). Both diode and mosfet broke donw. So now i'm pretty sure it has something to do with the inductance at turn off. What i don't understand about my measurement is. When the mosfet turns on i should see all my voltage across the load right? This is where the ringing occurs? And when the mosfet turns off i should see no voltage across the load right? So why is there ringing at the turn on slope? Or am i making a big mistake here? I've made a LTspice simulation with some inductance added to my load and i clearly see what happens. But this doesn't makes any sense with what i'm measuring...Quote from: magic on January 16, 2021, 09:32:31 amI could not find any dV/dt rating in the datasheet (https://fscdn.rohm.com/en/products/databook/datasheet/discrete/transistor/mosfet/r6020jnz4c13-e.pdf).The measurement is made with a differential probe across the load. So at turn on all voltage will be over the load. The turn off slope is quite slow. Or i'm making a huge mistake here.

author: magic
author_rank: Super Contributor
timestamp: January 17, 2021, 11:21:13 am
content: I thought it's the drain waveform and got the edges wrong. You probably should test drain waveform, it's the drain voltage that the FET cares about, not voltage across some stupid resistorTurn on ringing: could be anything, like a resonance between ESL of the power supply (capacitors, PCB, cables) and parasitic capacitance of the load or even probe.Turn off: that's indeed rather slow, but you are measuring supply voltage at the load (at some unspecified point) minus drain voltage. There is a variety of reasons why this difference can be high and slow to fall down, for example because drain voltage is high and the supply voltage is high too, due to supply ESL. I would see what happens to drain voltage alone, under some load that doesn't kill it yet.

author: IDEngineer
author_rank: Super Contributor
timestamp: January 17, 2021, 06:31:35 pm
content: You should spend some time optimizing your gate drive before exposing the FET to full load. This will be an iterative process, since the parasitic capacitances (and thus the total gate charge) change with Vds. Start with no load and make sure you understand what's happening at the gate. Then add some load and tweak the gate circuit again.Setting the proper gate resistance is not just doing a quick Ohm's Law calculation. If you calculations suggest 33R is the "correct" value to achieve the gate charge/discharge behavior you seek, did you account for the FET's own gate resistance? Its typical value is 2R all by itself (straight from the spec sheet). What about your gate driver's output resistance? It's not zero, either, and in some gate drivers I've seen values approaching 10R. Those two numbers alone are ~33% of your 33R value. Do you need steering diodes to generate/account for asymmetrical charge/discharge behavior?There's a reason gate driving is almost a subdiscipline of its own. If you're dealing with small signals and plenty of margin you can often get away with hand-waving, but you're working with lots of voltage here. Back off of your end-goal specs and take some time to understand how you're actually turning that FET on and off. You'll pop far fewer devices and have more confidence in your end result.Hope this helps!

author: IDEngineer
author_rank: Super Contributor
timestamp: January 17, 2021, 06:35:41 pm
content: Quote from: magic on January 17, 2021, 11:21:13 amEmphasis mine. If you're not already, I strongly suggest getting rid of the "flying" ground lead of your scope probe and using one of the little tip-mount grounding attachments so your ground is ultra-short and immediately next to the thing you're probing. Very often ringing like this isn't what the FET sees, it's what the scope introduces with its additional capacitance and inductance.

author: poorchava
author_rank: Super Contributor
timestamp: January 17, 2021, 07:36:39 pm
content: I would bet on switching too slow combined with transients. When switching on, the fet conducts almost full load current, but with significant resistance. Have you calculated the total turn on and turnoff energy loss in the mosfet?You could also go for SiC. It can typically take more abuse than silicon in terms of energy and switching losses are lower too. 1200V part would do the job in case your problem is with inductive spikes.

author: filssavi
author_rank: Frequent Contributor
timestamp: January 17, 2021, 08:20:22 pm
content: Quote from: poorchava on January 17, 2021, 07:36:39 pmIf it is (as I strongly suspect) a dv/dt related problem, I would strongly advise against SiC or god forbids GaN, as they will make it even worse by switching much faster.I would not bother measuring switching losses, as it is exceedingly difficult to do it correctly ( basically the only way to get it really right is with a calorimeter, any other method is more or less guessing) calculate/simulate them instead it should get you close enoughWhen you go to probe this kind of circuits be aware that the probing technique will have orders of magnitude more impact on what you see on the scope than both your circuit and the instrument itself. The ground clip is useless throw it in the trash, only the ground spring can be used, regular high voltage differential probes are fine to confirm that the thing is indeed switching, nothing more (forget to capture the transients as theantennaeehm leads will pick up all manners of crap, also they have such a high parasitic inductance that any edge will have artifacts due to interactions with both the circuit under test and the probe capacitance.The only probes that can really show what is going on there are the optically isolated ones (like the ten IsoVu or lecroy HVDF) but they are kind of expensive (multiple tens of thousands € each) thus I don’t expect you to have access to thoseWas for what to do:1) buy a lot of those mosfets (at least 10/20) as you will blow up quite a few2) go slow; once you verified that the circuit works in principle, with low loads increase it slowly (if you can’t vary the load easily vary the DC supply in order to have lower voltage and thus current)3)don’t assume the problem is on the transistor/gate driver but evaluate the whole circuit, It took me two months once to find the source of the exploding mosfet syndrome, it turned out to be the TI digital insulator that while rated for 100kV/us of CMTI was latching up and turning on the high side shorting the supply; the solution? Use an optocoupler rated for 25kV/us whose datasheet was not blatantly lyingP.S 33 ohms seems a tad high for a gate resistor