IRF540 failed G-S - Page 1
Forum section: electronics / repair

author: mc172
author_rank: Frequent Contributor
timestamp: April 23, 2020, 01:08:21 am
content: I've got a pair of identical amplifiers in for repair with some funny symptoms.The amplifiers have got an IRF540/IRF5210 output pair. The IRF540 has gone short fromgateto source in both of them and both of the IRF5210s are good. I've never seen this before - I've only ever seen drain to source shorts.I've got some IRF540s knocking around so I got one out and blew it up by forcing it to dissipate a bit more power than it should be able to handle while attached to a heatsink, and thus overheat, over a period of about 2 hours and it eventually failed drain to source short.The amplifier has got 12 V Zener diodes from gate to source, and they're all working correctly when I take them out of circuit to test them, so I doubt that excess VGShas caused it.Has anyone seen this before or got any ideas why this device would fail G-S short please?

author: Whales
author_rank: Super Contributor
timestamp: April 23, 2020, 03:55:13 am
content: IRF540 is a common part number used on greymarket sites. I think the ones I bought years back didn't meet datasheet specs.Too high of a G-S voltage could damage the part. Any chance this could be happening?

author: floobydust
author_rank: Super Contributor
timestamp: April 23, 2020, 04:32:15 am
content: The only time I've seen MOSFET gate punch-through (assuming the circuit's zeners are working) is when the parts got damaged by ESD.It seems to be about 2 weeks and then the parts fail. I know that sounds weird but I noticed all the techs who were careless about ESD had recalls after a couple weeks as the parts failed. I read ion migration happens across the damaged oxide layer and then it gets worse.

author: magic
author_rank: Super Contributor
timestamp: April 23, 2020, 03:38:12 pm
content: Does the D-S diode still work?

author: M0HZH
author_rank: Regular Contributor
timestamp: April 23, 2020, 03:59:01 pm
content: I've had plenty of laterally-diffused MOSFETS (aka LDMOS) fail with low (a few ohms) G-S resistance due to Drain overcurrent / overvoltage or even overheating; as a matter of fact, that's the only failure mode on that type of FET I've ever heard of. The Source almost surrounds the Gate on LDMOS, it's quite sensitive.

author: duak
author_rank: Super Contributor
timestamp: April 23, 2020, 10:52:13 pm
content: I'll second M0HZH attributing overheating as one cause. If you put enough power into the device, you can melt enough of the die to connect the gate and source.I learned this while developing a 1 kW BLDC driver. If memory serves, I used a fuse in each phase's DC link + connection for overcurrent protection. If the fuse ever opened, the recirculating current between the phases would cause the now isolated phase to pump up its voltage until something broke down, usually a MOSFET. The other two phases would continue to switch, driving current until a load imbalance was detected. The MOSFETs were in metal cases and one struck an internal arc between the source pin and the case, burning a 1mm dia hole in it.

author: mc172
author_rank: Frequent Contributor
timestamp: April 24, 2020, 12:43:06 am
content: Thanks for the comments chaps.I think I may have misguided myself by not testing everything the same way when I had the opportunity to. It didn't occur to me at the time but hindsight is brilliant as they say.It turns out that both the FETs I received in the amplifiers and the one I deliberately blew up are short betweenall pinsbut to varying degrees, which gave some strange looking results on the meter in diode mode, when compared with the next device.For example, one of the FETs from the amplifiers gives the following in diode mode:30 mV G-S30 mV S-G501 mV S-D1192 mV D-S555 mV G-D1202 mV D-GHowever, when I put this FET on a power supply, it will happily draw as much current as you can give it S-D or D-S, and will draw about 600 mA S-G, G-S at 10 V.For comparison, the one I blew up deliberately, shows:102 mV G-S103 mV S-G22 mV S-D23 mV D-S124 mV G-D125 mV D-GThis FET will again draw as much current as you can throw at it between D-S or S-D, but will draw only 250 mA S-G, G-S at 10 V.Magic, your suggestion to test the body diode is what tipped me towards sticking the devices on a power supply and just brute force them G-S to see what is shorted and what isn't, at an arbitrary 10-ish V. I had done this D-S before but for some reason didn't think to do it between all pins.Thanks everyone for your input, I really appreciate it. I think the conclusion then is overheating.

author: magic
author_rank: Super Contributor
timestamp: April 24, 2020, 05:40:42 am
content: Quote from: mc172 on April 24, 2020, 12:43:06 amThat's the favorite failure mode of MOSFETs, so that the driving circuitry also gets a chance to receive some of that juicy capacitor bus discharge

author: JohnPi
author_rank: Regular Contributor
timestamp: April 24, 2020, 04:41:12 pm
content: It's not clear what your voltage measurements are, but generally power MOSFETs fail from overvoltage or overheating.With 12 V zener, you are unlikely to have exceeded the 20 VGS rating. If the VDS rating is exceeded (IRF540 is rated at 100 V, and it is avalanche rated), you could get D-S breakdown to begin and this will lead to overheating and subsequent permanent failure (not because the silicon melts, but the internal aluminium interconnects melt and get absorbed by the silicon creating a short). But if the power supply is < 100 V, this is unlikely. Actual FETs are quite rugged and don't fail (melt) until the internal temperature rises above 350 deg. C.Defective devices often fail with D-G shorts internally which prevent the external gate from being able to actually switch the device off. This leads to overheating and massive failure of the device.Therefore likely your failure is simple overheating -- too much DC power dissipation. Is the load shorted ?

author: CaptDon
author_rank: Super Contributor
timestamp: April 24, 2020, 05:23:33 pm
content: There is one particular model of Ampeg bass guitar amplifierthat is known to eat FETs on a regular basis. First off, they runso hot they'll burn off your finger prints, second, when youturn the amp off there is such a hellish thump that it drainsall of the energy from the capacitor bank. I find the fets haveall sorts of weird failure modes and indeed they have thegate to source protect zeners. BTW, zeners are somewhatslow to hit their regulation voltage and you would be surprisedhow high the voltage of a fast transient can be and the zenernever reacts to it. The most outrageous spikes pass throughon startup when the zener is not already 'on'.

author: JohnPi
author_rank: Regular Contributor
timestamp: April 25, 2020, 01:19:27 am
content: No, zener are not slow to regulate -- they regulate within nanoseconds of applying a voltage. Conversely gate-source oxide on a FET can withstand 1.5x the rated voltage for many milliseconds.

author: CaptDon
author_rank: Super Contributor
timestamp: April 25, 2020, 05:29:05 pm
content: We tried using 'run-of-the-mill' 1/2 and 1 watt Zeners as transient suppressors.We never saw 'nanosecond' response time, at best maybe tens of nanoseconds.This measurement was taken going from zero volts / zero bias to an overvoltagebut with resistance to limit current to a safe peak. We gave up on a zener-onlytype answer and went with zener plus two stages of R/C filtering. There may befaster zeners today but the 1n7XX stuff and the 1n3xxx/1n5xxx stuff just wasn'tdoing the trick. For that matter the transorbs were even slower 1KPxx stuff andtheir clamp voltage drifted like crazy downward with heat.