Inverter mosfets getting very hot - Page 1
Forum section: electronics / beginners

author: piku
author_rank: Contributor
timestamp: October 05, 2020, 12:25:58 am
content: Hello all. I am trying to repair an inverter power supply as an educational advanture. No schematics available. It is out of a Norcold MRFT-460 TEK-II refrigerator/freezer and aside from the thermostat control circuitry the board here is primarily a 12vdc to ~20vac inverter. I found the 120vac to 12vdc board exploded (separate board) but I don't really need that functionality. I also found on this board at least one mosfet looked melted (and didn't work under test) and a dual schottky diode in to-220 package one side was shorted.For completeness the diode that was there was SF30SC6https://www.digchip.com/datasheets/parts/datasheet/425/SF30SC6.phpand I replaced it with VF30100SG-E3/4WGI-ND. I don't think that is the source of my current issues but who knows!The mosfets were K2232https://datasheetspdf.com/pdf/635943/ToshibaSemiconductor/K2232/1The first attempt to replace them was with IPP060N06NAKSA1-ND but after reassembly the power and ground of the power supply was a dead short! Turns out the heat sink of these non fullpak mosfets is connected to drain. In the same order I also got part IPA032N06N3GXKSA1-ND which is fullpak. Everything looked to work fine. I plugged the refrigerator into a car battery and ran it for several days with a charger on the battery. Once I was satisfied I took it back to the motorhome and it blew the circuit breaker within 5 minutes. The power and ground of the power supply were completely shorted. One of the mosfets failed shorted.I thought maybe this could be crib death so I put in IPP060N06NAKSA1-ND I still had but did not let the back touch the heat sink. I was surprised that this mosfet got VERY hot VERY quickly. I know it needs to be on the heatsink but the other mosfets attached to the heat sink stay relatively cold. This got too hot to touch within seconds. So I believe there is something still wrong but I can't imagine what.I got out my USB oscilloscope and made traces of what things look like after turning it on. The power supply is about 13v and drawing 5.5amps. The first screenshot is the drain at the start, followed by a slightly later shot of the waveform changing. It's a part of the same sample so it's obviously not a lengthy period of time. Same there are 2 shots of the gate, early and later. The source I took 3 shots at different zoom levels.There are also pics of the board. The two mosfets that get hot are on the left side of the heatsink (top down view) and the schottky is just to the right.At first I thought these mosfets were acting as a simple solid state relay. The fridge has no relays that the thermostat uses to turn the thing on or off. I was surprised to find them actually switching at all. Can anyone tell based on these traces why these mosfets would be getting so hot so quickly and more interestingly to me, what the purpose of these mosfets might be? The drain of the mosfets connects to the inputs of the schottky and then goes further down into the circuit.I realize this might be difficult to make heads or tails of based on a simple post, no schematics available.I don't understand the function of these mosfets. It looks to me like the source is ground (as expected) the gate goes off through some resistors to the little pcb with the control logic (expected). But it looks like the source connects directly to +12 and also the schottky diode. I noticed that even if I supply 13v to the fridge the drain reads 12v. Are these mosfets shorting to ground to bring the input voltage to 12v? Based on that theory I hooked them up without the charger and the mosfets still got very hot very fast. I notice that the gate voltage is averaging out to 5v. I see the 2SK2232 says the full gate voltage is 4v. But I can't tell the equivalent on either of my replacements. I'm hoping that I just improperly specified the replacement mosfets.ANY help is appreciated!

author: piku
author_rank: Contributor
timestamp: October 05, 2020, 12:27:29 am
content: And here are the overall pics.

author: nuclearcat
author_rank: Unknown
timestamp: October 05, 2020, 05:37:26 am
content: I am not experienced in this stuff enough to give any advice, but i will try, just to warm up topic a bit.Usually when mosfet heats up in any kind of switching application with PWM, it means it stays too long in linear modeWhen i compared both, i noticed several important things:1)C iss (capacitance of gate), on IPA it is significantly higher, 10000pf vs 1000pv on 2SK2232. This means very likely mosfet gate driver in this circuit was calculated for 1000pf, it wont bring gate to open state fast enough, and under question if it can keep it open long enough. On first oscillogram you provided it is visible, that gate voltage is too low on high duty cycle (mosfet_gate_early.png), this means mosfet have higher resistance and will heat up considerably. It is barely above 5V.2)On first graph, gate strangely not reaching zero as well, thats subject to worry as well. Incompletely closed gate might cause mosfet to be in linear mode. Maybe it is also result of miller effect(?)3)I noticed negative spike on gate, it might cause latch-up as well. Probably because of too different capacitance again?Very interesting to hear opinion of professionalsDatasheet to both, so others wont google:https://www.infineon.com/dgdl/IPA032N06N3_Rev2+0.pdfAnother one trying to attach.

author: piku
author_rank: Contributor
timestamp: October 05, 2020, 12:02:41 pm
content: Thank you so much for this. Is it possible the K2232 is a special kind of mosfet then? At the top of the datasheet it says "4-V gate drive". Not knowing any better that would suggest to me that the K2232 is fully driven at 5V. Your theory about the gate capacitance certainly sounds good. It's amazing how complicated something that seems very simple on the surface can be

author: nuclearcat
author_rank: Unknown
timestamp: October 05, 2020, 12:27:34 pm
content: If i am reading datasheet properly, Rds of K2232 on 4V/12A is 57 mOhm, and IPA032N06N3, according to chart "6 Typ. drain-source on resistance" is on 5V ~25A approx 5.5 mOhm. This means Infineon should be way better than K2232 on 5V.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2020, 12:32:04 pm
content: MOSFET substitution is an art and careful attention need be made to match as close as possible to the originals.Gate thresholds and particularly total gate capacitance need be matched closely so the available gate drive can do its job as the circuit designer calculated. Miller capacitance isrealand will trip you up if you ignore it.Even the value of the gate resistor can impact MOSFET performance.As NC ^ mentions RdsON need be matched or bettered with any substitution.Often gate measurements are an art themselves requiring the use of a differential probe or a 2 probe Math A-B measurement.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 05, 2020, 12:32:53 pm
content: The infineon is 10 times the Ciss and has almost double the gate charge. There may be ringing now.

author: piku
author_rank: Contributor
timestamp: October 05, 2020, 01:07:52 pm
content: I didn't really consider the Ciss parameter. Since it wasn't in the search parameters for the big electronics site I thought it wasn't a very important parameter. Clearly I need to read more guides to mosfet selectionAnyway, I am actually having trouble finding a suitable part with 1000pf Ciss. I found 1900 but not 1000. The only 1000 part I found has a drain power dissipation of only 25W. I think the 35W power handling is probably pretty important as this refrigerator runs right around 60-70W.This is the closest I can find so farhttps://www.mouser.com/datasheet/2/408/TK8R2A06PL_datasheet_en_20170906-1075499.pdfEDIT: As it turns out the exact part is still available:https://www.ebay.com/itm/2SK2232-K2232-FET-TO220-2-PIECES-qzty/283736638275?hash=item4210058343:g:wTMAAOSw7oBeFl5sMaybe I should just go that route.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2020, 01:16:33 pm
content: 25W power dissipation parameter is marketing speech; it's a parameter seldom looked at. Datasheet saying that actually means you can dissipate maybe some 10W depending on how well you heatsink it. In reality, you calculate/approximate power dissipation first (conduction + switching loss, the former is way easier), then calculate thermal performance from Rth parameters, deciding some Tambient and Tj_max first (such as 50 degC and 110 degC, respectively).This being said, if this thing is even remotely properly designed, obviously the transistor isn't going to dissipate anything near 25W. That would be horribly inefficient inverter for a 60-70W load! Unless it's a very old-school linear sine wave inverter but I guess not?

author: nuclearcat
author_rank: Unknown
timestamp: October 05, 2020, 01:27:47 pm
content: There is also such parameters such as thermal impedance and transient thermal impedance. Even how fast mosfet heating up - matter.

author: piku
author_rank: Contributor
timestamp: October 05, 2020, 01:31:22 pm
content: I guess that is a good point. I am thinking of it as power handling and it is a power dissipation figure. Two different things.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2020, 01:40:13 pm
content: MOSFET doesn't "know" how much power the load is using. Power dissipation is voltage over MOSFET times current over MOSFET at all times, integrated over time.During on-time, voltage over MOSFET is Rds_on * I (close to 0, but meaningful). Dissipation is I^2 * R. This is called conduction loss. (Do note that Rds_on at realistic ~100degC-ish temperature is approx. 1.5 times the datasheet 25 degC figure)During off-time, voltage over MOSFET is full Vsupply but I is 0. No loss.During switching, current ramps between zero and full "final" current, and voltage ramps between Vsupply and ~0. The actual switching waveform and time is crucial, and can be handwaved, approximated with several different levels of complexity, simulated, or prototyped. Very initial handwaving approximation is that switching losses are same as conduction losses so double the conduction loss figure for total, but this is only close to the truth whenever the MOSFET choice is well balanced (i.e., you don't pick an excessively large MOSFET with ridiculously small Rds_on, or an excessively small MOSFET with ridiculously large Rds_on), and gate driver circuitry has enough oomph for the MOSFET.

author: nuclearcat
author_rank: Unknown
timestamp: October 05, 2020, 01:46:35 pm
content: You need to calculate power handling at designed Rds + add some losses due switching if you are using mosfet in switching application.If your FET is operating in ohmic region due low voltage on gate, for example if gate is not closed as it should be, in off state, like on your first oscillogram, and you have significant current - nothing will save it from burning.

author: piku
author_rank: Contributor
timestamp: October 05, 2020, 04:44:00 pm
content: It looks like Rds of the part I used is 6.0mohm while the K2232 is 36mohm. And the Ciss is way off as well. I think I may try this one as the other parameters we are discussing are much much closer.https://www.mouser.com/datasheet/2/308/FQPF30N06L-D-1809816.pdfDrain current is 22A instead of 25A but perhaps it is enough.I still don't understand the ohmic region but I am searching for everything I can find.Can anyone comment on whether or not the FQPF30N06L specified above is a good replacement forhttps://datasheetspdf.com/pdf/635943/ToshibaSemiconductor/K2232/1?Thanks!

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2020, 05:00:24 pm
content: Yes, if you try to reduce conduction lossestoo farby choosing excessively low Rds_on, you end up with a huge, and SLOW MOSFET. Beefing up gate driving may solve the issue, but better yet, try to choose a more balanced MOSFET. By balanced, I mean your total MOSFET losses should be like some 50%-50% conduction and switching losses.I tend to look at Qg(tot) instead of Ciss but they do relate. One simple way to choose is to actually calculate the maximum Rds_on you can accept - because conduction losses are so easy to calculate; then just sort for Qg(tot) and pick one with lowest value.

author: piku
author_rank: Contributor
timestamp: October 10, 2020, 12:26:22 am
content: Well thank you all for your help so far. I ordered FQPF30N06L to try. It does take longer to warm up but it still seems pretty hot. Perhaps it is supposed to run hot? I don't know.I've made new oscillograms of these to compare. That only thing I can think of is perhaps something is broken further down the line but overall the refrigerator is drawing roughly the rated power. Do you guys see anything wrong with these? Certainly the dips and spikes look strange. The source and the gate seem to oscillate between the two states pictured.

author: nuclearcat
author_rank: Unknown
timestamp: October 10, 2020, 02:36:15 am
content: Maybe worth to check electrolytic caps if they lost their capacity or ESR increased.

author: piku
author_rank: Contributor
timestamp: October 10, 2020, 02:49:28 am
content: I checked all the big ones and they were above spec. Don't have an ESR meter.

author: tautech
author_rank: Super Contributor
timestamp: October 10, 2020, 05:29:20 am
content: These suggest the VCC supplying whatever IC is driving the MOSFET gate is not as clean as it should be OR affected by interference generated by the circuit itself:Suspects are the IC VCC cap yet if the oscillations are insufficient amplitude WRT the gate threshold maybe they can be dismissed however I would investigate more to find the cause.

author: piku
author_rank: Contributor
timestamp: October 10, 2020, 01:55:37 pm
content: The caps are ~ 20 years old and running a hot environment so it makes a lot of sense. I rechecked the datasheet and the "fully open" gate voltage is right around 6.5v so the gate is definitely getting fully opened. I'll try to follow the circuit to figure out what the IC is using for VCC. There is a 12v linear regulator but I'm assuming it's probably getting dropped to 5v somewhere.The peak of those oscillations on the gate is 1.18v and the threshold is 1.0-2.5v so that sounds to me like it's not the culprit in the mosfet heating.Looking over these this really does look a lot healthier than the first oscillograms I posted with the other parts. It does seem like I fixed the gate fully closing issue. I wonder if I was just being a bit too sensitive and I should let it run and see how hot they get steady state. I did buy spares

author: piku
author_rank: Contributor
timestamp: October 11, 2020, 09:40:54 pm
content: I am now wondering if perhaps I am just not aware of the properties of mosfets in a switching application and I overreacted. Possibly I simply had a bad replacement device straight out of the box. I set everything up hooked up to the heat sink. I applied some thermal grease on the front of the case and used a digital meat thermometer to find that the device hovered right around 110C. Searching around I am finding that a reasonable case temperature measurement for a 175C max part is around 120C. The part is attached to a u shaped heatsink which is then screwed to a steel enclosure which has a fan blowing on the other side so clearly the designers planned for heat. I am considering simply drilling a few holes in the power supply case and putting an additional fan through the internal area and seeing if it works in the long run. Note that ambient was 29C while max ambient would be about 37-42C worst case. This does seem likely too warm for the 150C max part I pulled out so I imagine probably something is still wrong. But maybe it's good enough to roll with it if I just add some cooling.The other alternative is a 12v buck boost converter, W1209 temperature controller, a small pure sine wave inverter and a 120vac to 24vac transformer. All relatively inexpensive. I feel like that's cheating though

author: nuclearcat
author_rank: Unknown
timestamp: October 12, 2020, 02:10:23 am
content: I think 100C is way too much for semiconductors and capacitors for long-term operation.And yes, it might be just bad design. Maybe worth to search owners of similar ones and ask them about temperatures?

author: piku
author_rank: Contributor
timestamp: October 12, 2020, 11:17:09 pm
content: I've been reading over various things about mosfets and I recalled at some point that I forgot to replace the ferrite beads. The original mosfets had a ferrite bead on the source line and the schottky had ferrite beads on the two inputs. I'm not sure how much difference that made but when I reassembled I put some arctic silver thermal grease on the back of the mosfets and that made a large difference. The entire heatsink now got uncomfortably hot to touch (which it didn't before) and now the case temperature of the mosfets never got above 70C. So at this point I am tempted to just go with it until....I decided to measure the actual output and I was a little surprised by what it looks like. This was with the probe in 10x because the scope can't handle more than 10v. If you hook a voltmeter up to this it reads 23.4volts in ac mode. Does this look at all like a sane power supply for a portable refrigerator compressor? It definitely sounds pretty much the same when it is on this supply vs a pure sine wave 60hz supply.Also I think this power supply is so interesting because the source voltage is allowed to be 10.5v-35v.Also according to the service manual the compressor is supposed to have a resistance of 3 +/-.5. I actually measured it to be 1.9ohm. Perhaps one of the windings shorted.

author: piku
author_rank: Contributor
timestamp: October 25, 2020, 04:03:02 pm
content: I thought I would post back here. I believe my issues are resolved - at least resolved enough to live with. I added an additional fan that recycles the air inside the power supply in addition to the thermal paste and the mosfets now stay well within an acceptable temperature. I ran the freezer for a week in the very hot enclosed storage compartment of my motorhome (and also for one day sitting out in the hot florida sun) and it has survived my torture tests so I'm hoping it will run for a while.If I ever make another digikey order, I'll throw in all the caps and replace them all which may really help it. Thanks for all your help everyone!