Author Topic: SMPS Blowing the MOSFET  (Read 782 times)

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Offline Andersounds

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SMPS Blowing the MOSFET
« on: March 17, 2019, 09:59:24 am »
Hi,

I'm fairly used to linear power supplies, but someone just threw me a switch mode, d&b audio E-PAC power amp, with a blown MOSFET, this is right after the bridge rectifier that is putting out the expected 320vdc, tried powering it up with a new mosfet installed it blew in a few seconds.. looking at the PFC and other components nothing is obviously burnt, any ideas what commonly causes this fault would be very interesting.

Have checked out caps and other ICs for anything that is shorted out, but I would assume the amp would shutdown rather than blow the mosfet if there were down stream issues, making me think it is something near the power transistor, but my experience with switchmode is limited.

the power transistor is an infineon 20N60C2

new to the forum, so thanks for any help!  :-+
 

Offline JFJ

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Re: SMPS Blowing the MOSFET
« Reply #1 on: March 17, 2019, 11:33:29 am »
.. I would assume the amp would shutdown rather than blow the mosfet if there were down stream issues ...

If a transformer secondary winding is being shorted by a failed rectifier, for example, then the amp may not have the opportunity to shut down. Putting an incandescent lamp in series with the mains input could prevent another MOSFET from blowing (by limiting the current), while giving you some time to check if the MOSFET is switching or is just turned on continuously.
 

Offline PKTKS

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Re: SMPS Blowing the MOSFET
« Reply #2 on: March 17, 2019, 11:34:33 pm »
Yes as said above the VARIAC + BULB is an essential tool in these cases.

*IF**  your SMPS  does have the PFC thing .. things are more complicated.
in this case you need a clue about the PFC supervisor with typical apppl circuit

if not things are better you go directly to the SMPS topology and take
note the supervisor IC and brand of transformer (it helps to fit wiring)

Come back w/more info..

Paul
 

Offline kripton2035

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Re: SMPS Blowing the MOSFET
« Reply #3 on: March 18, 2019, 12:53:47 am »
a capacitor with too high esr could lead to a blown mosfet. did you check that ?

Offline Andersounds

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Re: SMPS Blowing the MOSFET
« Reply #4 on: March 18, 2019, 08:01:59 am »
I powered it up with a variac but didn't get far before it was too late.. have been meaning to setup the current limiting bulb just haven't needed to until now. so yes this will push me to get that setup, I have all the bits for that.

 

Offline Andersounds

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Re: SMPS Blowing the MOSFET
« Reply #5 on: March 18, 2019, 08:05:52 am »
Checked the ESR and DC resistance all look pretty typical, no bulging, gunk or heat marks, I have an old 60 to 600v cap leakage tester I could use to check them under load, but they aren't obviously bad.
 

Offline Andersounds

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Re: SMPS Blowing the MOSFET
« Reply #6 on: March 18, 2019, 08:36:41 am »
The power factor corrector is an L4981AD, I powered the sub board it is on extrenally with refernece to the datasheet, it isn't drawing excessive current, the other ic's on there seem happy too, there's a 5v rail that comes on line also.

As JFJ said, I was also thinking the PFC might be turning on the MOSFET constantly, trying to trick it in to running might be a good angle, just not familiar with these circuits.

The mosfet isn't cheap, otherwise I would have got a few, so will setup the bulb limiter and give it another go.

Have attached a couple of pics, the board looks very high quality to me at least. have looked at a few different SMPS's now, interesting stuff, not sure why I mostly avoided them for so long.
 

Offline Andersounds

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Re: SMPS Blowing the MOSFET
« Reply #7 on: March 18, 2019, 08:38:06 am »
Overall pic  8)
 

Offline Chris56000

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Re: SMPS Blowing the MOSFET
« Reply #8 on: March 18, 2019, 11:25:13 am »
Hi!

What model no. is your amplifier?

Typical krauts, not an inch of silkscreen or other legends on the pcb to assist you with!!

Never mind, all is not lost!

Instantaneous power MOSFET failure in this type of switching supply very soon after switching on can be caused by the various low–value small blue WIMA rectangular film capacitors dotted around the large switch–mode power transformer going low in value or open–circuit, I recommend you remove and test all of these, and those pale green electrolytics don't look the best quality on the sticky–up L4981AD FC control module either – I'd turf 'em all out and put good–quality Panasonic FC or similar dark blue/gold ones of reputable manufacture in!

Be aware that another cause of instantaneous power MOSFET failure is floating or high resistances in the gate (left–hand lead looking at the device with the type markings towards you) drive circuits – this will leave unwanted charges in the gate which will prevent it turning off correctly during the flyback–part of the cycle, resulting in immediate failure!

Catastrophic MOSFET failure can also damage the L4981AD PFC controller, any signs of burnt or discoloured resistors adjacent to it should prompt you to throw it out – you can't rely on the safe operation of these devices if there's been a power–stage blowout!

Remove the MOSFET from the pcb and look at all the resistors, any diodes or smaller driver transistors near it for obvious damage or faults (please post again with more pics if you get stuck) – they may be the surface mount type so do please look at your main board carefully!

Finally, please be aware that power MOSFETs, like many of the older obsolete bipolar audio devices, DO get "faked" by unscrupulous F.E. vendors, a typical warning sign is also sudden and catastrophic failure immediately power is applied or very soon afterwards – the best way of gaurding against this is to only order from RS, Farnell, Mouser or Digi–Key!

I have had a look to see if there's any sort of service manual, schematic, etc., and there doesn't seem to be a sausage!

Please go through your board carefully (a good DMM and one of those cheap Chinese LCR T4 testers will do for this!), paying attention to the components & points I made above, and please do come back if you're still stuck!

Chris Williams
« Last Edit: March 18, 2019, 11:45:30 am by Chris56000 »
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 
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Offline Andersounds

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Re: SMPS Blowing the MOSFET
« Reply #9 on: March 20, 2019, 10:58:41 am »
Found at least one of the culprits, checked all the caps for ESR and DC resistance but missed this one, as one of the other 2200uf 63v caps measured around 100 \$\Omega\$ in circuit but seemed ok when taken out, typical the one I took out before was ok the other dead, anyway thanks all for interesting ideas and leads to follow, will recap the whole amp and give it another go.

 

Offline kripton2035

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Re: SMPS Blowing the MOSFET
« Reply #10 on: March 20, 2019, 05:58:45 pm »
fluke 87 has an integrated esr meter ?!?!?

Offline PKTKS

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Re: SMPS Blowing the MOSFET
« Reply #11 on: March 21, 2019, 01:04:27 am »

100 ohms on that cap..  not ESR

 that thing is shorted

 

Offline Andersounds

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Re: SMPS Blowing the MOSFET
« Reply #12 on: March 28, 2019, 11:20:09 am »
usually check for DC resistance and AC (ESR), The esr meter has paid for itself, slowly, but can also be good for checking if coils are shorted, as DC resistance be close to zero either way, with the ESR it is usually reads quite high or OL over a coil if good.
 

Offline mslr

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Re: SMPS Blowing the MOSFET
« Reply #13 on: April 03, 2019, 06:54:04 am »
Hi, I repaired several SMPS of this Power amplifier. There is a known issue on two capacitors (1000uF 16V in the edge) which fail over time. Had also some shorted power diodes (2 diodes in one TO220 package) at the output of the transformer. Had a 100% success rate by repairing it. Check also the diodes and resistors nearby the mosfet linked to the gate. d&b suggests to replace the mosfet by an IRFIBC40G.


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