I have a problem with Mosfet failure - Page 1
Forum section: electronics / projects

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 04:45:53 am
content: First post. Hello everyone. I'm a huge fan of David Jones!I have a problem with Mosfet failure.I'm pulsing an inductive load ~10mH, ~15ohms (both measurements @ 100Hz).I'm pulsing the load at 20-60Hz, ~4% duty cycle with an 80-100Vdc source provided by batteries.I'm using a SDUR16Q20CT flyback diode across the load. It's a TO-220 package w the two anodes tied togetherfor greater current handling.The mosfet I'm using is an FDL100N50F that I bought on ebay from a reputable seller (possibly ancient stock though?).I'm driving the gate at 10V with a TC4420 (with 3 caps on it) and I have a TVS diode from gate to groundas well as a 500mA FB fuse right on the gate pin.Scoping the gate waveform it looks absolutely perfect. So I'm assuming everything before the gate of the FET is working fine.The FETs continue to fail.Before they fail I see a ton of shoot-through (or an inductive spike) when scoping Vds.They work correctly sometimes (when they do the scope shows 100V across the load with a fairly nice waveform considering all the inductance).At other times the drop across the load is only 50-60V and the waveform across Vds has a diagonalpiece to it (see jpg below).The 1st two JPGs are with an 80V source.The 2nd two JPGs are with 100V source.The 1st FET that I used got to where its waveform was always bad (per above) with bad shoot through (or inductive spike).The 2nd FET that I tried stopped working completely after an hour.When I say "shoot-through" or "inductive spike" I'm talking about a a needle thin 20-40V spike on one of the edges.Not really sure of the correct nomenclature for this phenomenon (see jpg).I'm assuming that pulsing a load w so much inductance is a chore for a FET and I'm thinking I need to buy a $50chassis mounted FET instead of the FDL100N50F, but I though I'd check here before I spend the money.Thanks!

author: jonpaul
author_rank: Super Contributor
timestamp: April 04, 2023, 05:00:44 am
content: Schematic ? load current wave? details on gate, FET voltage probing?jon

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 05:08:51 am
content: I can add a hand drawn schematic.I don't have a current probe.

author: daisizhou
author_rank: Super Contributor
timestamp: April 04, 2023, 05:13:56 am
content: Have you added a current limiting resistor to your MOSFET?It is possible that the impact current caused the breakdown of the MOS transistor,Or you bought a counterfeit MOS transistor.If you want to confirm it, perhaps you can refer to it by measuring the conduction resistance value

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 05:32:32 am
content: @daisizhouThanks!Yes anything on ebay is suspect although it looks like the real thing. The FDL100N50F is backordered everywhere else....like a year out.I have a 500 ohm trimmer on the gate set to negligible resistance (~5-10ohms). There's no resistance added to the drain or sourcebecause I want high current.

author: jwet
author_rank: Frequent Contributor
timestamp: April 04, 2023, 05:50:32 am
content: Your specs are pretty broad but first order, everything is undersized by at least a factor of 2. if you charge up a 10 mH inductance for 4% of a 30 hz waveform you're turning it on for about 1.3 mS. I don't know if this is your worst case, I just picked some values in your range. If you have the 80V, you will charge that inductance up to about 10.6 amps. (I=VT/L). You can put in your worst case values- probably even worse. Your diode's ratings say 8 amps continuous and 100 amps once- I don't know what continuous means- DC but if you're hitting this thing at 30 Hz and getting 10 amp peaks, you're probably stressing it. I think what's happening is that in some test condition, that diode is getting overstressed, burn up open (why you see the tall spikes) and shortly after, your FET is getting killed. That is a big inductance to charge up. The energy in a 10 mH inductor, charged up to 10 amp is 5 joules which is the max rating of your FET. (its listed at 5000 mJ to sound better). I haven't considered secondary breakdown but this is likely an issue at these voltages and currents. Driving big inductive loads with high voltage takes a lot of grunt. Look at some Siliconix app notes. You're just overstressing parts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2023, 10:21:39 am
content: Your probe compensation seems to be a bit off. But it's hard to tell at this large scale.Speaking of scale, why not zoom in on the transient in question? How high is it peaking to? How wide is it? What's the rise and fall time (of the peak or of the transistor switching)?Note that you will most likely need scope and probe bandwidth above 20MHz to properly resolve the peak. Depending on gate resistor value of course. Note that 1x probes have very low bandwidth (~6MHz). I'm guessing you have 10x probes and the scales are just set incorrectly (it says 1x), since you say the supply is 80/100V.The diode should be fine if it's wired both halves in parallel, and adequately heatsunk. Needless to say, anything getting excessively hot is cause for concern. Note that fingers are not a good thermometer: 60°C is uncomfortably hot while semiconductors can be fine at 125°C and up.You have a clamp diode in the circuit but a local bypass capacitor is not shown. How long is the power supply loop? For the clamp diode to do its job, there must be a capacitor nearby, shortening this loop to just the transistor, diode and capacitor, all close together with short lead/trace lengths. A few cm should be feasible with these components. (Probably a few 100uF electrolytic would be adequate bypass for this.)Tim

author: daisizhou
author_rank: Super Contributor
timestamp: April 04, 2023, 10:37:23 am
content: If you want to confirm its authenticity, please break it and check the internal crystal size to confirm it

author: jonpaul
author_rank: Super Contributor
timestamp: April 04, 2023, 12:22:32 pm
content: quik:>>500 ohm trimmer on the gate set to negligible resistance (~5-10ohms).NG, UHF parasitics! Y 10..100 Ohm discrete R exactly AT the gate.>>FDL100N50F ... counterfeit .... confirm...From epay/Ali/internet, ANY part /IC/FET is counterfeit ChiCom, until proven otherwise or bought from authorized USA or UK/EU distrib.With no schematic not further help possilbej

author: coppercone2
author_rank: Super Contributor
timestamp: April 04, 2023, 12:36:52 pm
content: looks like you need a resistor kit

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 04:45:53 pm
content: Quote from: jwet on April 04, 2023, 05:50:32 amI think you're spot on. The energy in a 10mH inductor can become huge under the right conditions.Are you speaking about the FET body diode or the flyback diode?Regardless I think that both are stressed....... but the flyback diode doesn't fail like the FET does.Ironically the flyback diode gets warm (but not hot) and the FET doesn't.The FET remains at room temperature the entire time, which leads me to believe that it's the dv/dt that's killing it and not the current.Thanks!

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 04:50:35 pm
content: Quote from: jonpaul on April 04, 2023, 12:22:32 pmYes! I strongly suspected this also. It looks identical to real FDL100 but it really shouldn't be so fragile.This combined with the fact that NONE are available from legit sources but LOTS are available on ebay LOL.Sorry about the vague specs. I really don't want to take the time to put this into spice because for me it's not necessary.Edit: Missed the part about trimmer parasitics vs lumped and having it directly on the gate. I wasn't aware of that but it makes perfect sensethat a trimmer would have much higher stray L and C vs a discrete resistor. I'll add that to to do list.Thanks!

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 05:21:21 pm
content: Quote from: T3sl4co1l on April 04, 2023, 10:21:39 amThe compensation is good. I checked it recently. FYI I'm using a cheap Siglent scope.Yes, I am using 10x and yes I did not set the scope scale to show it.I have an IR thermometer, but as I said above nothing really gets warm at all except for the flyback diode and even that is minimal.I haven't bothered to zoom the detail of the transients because I suspect that the problems is either:* Fake FDL100* dv/dt of pulse is too large for FDL100 (assuming it's a real FDL100)* or a problem w the flyback diodeI had assumed that everything up to the gate is working fine based on the fact that the gate pulses always look perfect.I haven't seen any similar designs with a capacitor to ground next to a TVS diode but your comment sounds very interestingand yes the power supply loop is relatively long. I'll definitely try adding the cap. Is electrolytic ok?Thanks!

author: johansen
author_rank: Super Contributor
timestamp: April 04, 2023, 06:30:16 pm
content: The energy stored in the inductance of the power supply cables is being dumped into your mosfet at turn off. this is why the voltage rises. you would not normally use a 500 volt fet on a 100v system, but that's why the spike is so short, you've made a 600 volt pulse generator.You might only need 1uf, film cap. Suppose your ~10amps in 2uH inductance of your supply are dumped into the already charged up 100 volt 1uf capacitor. how much higher does the voltage get? its not 10 volts.. far less, because you have to think in terms of energy. 0.1mJ dumped into a capacitor already at 100 volts storing 5mJ

author: Gabe Kron
author_rank: Newbie
timestamp: April 04, 2023, 11:19:04 pm
content: Quote from: johansen on April 04, 2023, 06:30:16 pmI think the spike is simply caused by inductive flyback voltage from a large inductor dumped into the mosfet and not the power supply/cables.It's easy enough to test though. I'm assuming that you mean add a ~300uF cap to ground at the power supply (battery) positive terminal.It does sound like an interesting idea and it would smooth out the battery voltage while taking some power away from the load (10mH inductor).Only caveat is if it also distorts the pulse. At worst it would dump the majority of the pulse right to ground.

author: johansen
author_rank: Super Contributor
timestamp: April 05, 2023, 02:44:35 am
content: The diode across the coil completes the circuit for the inductance of the load but not the supply. In this case, OP could also use an rc snubber across the mosfet to dissipate the energy, but this is not an option for every application which is why they make igbt snubber capacitors that bolt directly onto your half bridge igbt blocks. Typically 10uF on a system for 100 amps. My cheap chinease 3hp vfd had a 1uf film cap on the ibgt block, i added a second one.I have watched people blow up hundreds of dollars of mosfets switching a resistor on thier diy wind turbine stuff because they were too stubborn to belive me when i told them they have to put a capacitor as close as possible to complete the loop and prevent the mosfet from avalanch failure. One guy knew why they failed but kept tring to find fets that could surviveAnd no the capacitor doesnt affect thr load at all . It is connected from the top of the coil (at the + supply) to the mosfet source.The inductance between the diode, mosfet, and capacitor needs to be as little as possible.

author: Gabe Kron
author_rank: Newbie
timestamp: April 05, 2023, 03:33:37 am
content: Quote from: johansen on April 05, 2023, 02:44:35 amIn this case mosfet source is system ground, so it's a bypass cap across batteries. I'll try it. You sound very knowledgeable about this.