Author Topic: Mosfet Keeps blowing  (Read 5514 times)

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Offline timenutgoblin

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Re: Mosfet Keeps blowing
« Reply #25 on: October 08, 2021, 04:29:27 am »
On schematic, PSU2 capacitor C71 is 33µF / 100V, PSU1 capacitor C72 is 330µF / 100V
On PCB, both capacitors look like they're the same physical size (330µF ?)

Q3 MOSFET:
Gate terminal:
Solder Gate terminal to top of PCB. The track on the top of the PCB goes to the gate resistor. Remove red-insulated wire.

Drain terminal:
Scrape solder mask from adjacent copper trace and solder Drain terminal directly to the exposed PCB. This should reduce connection resistance and inductance. Remove black-insulated wire.

Is it easier to remove and install Q3 MOSFET with the jumper wires in place? Also, are the PCB tracks connecting to Q3 actually damaged? Seems like a stupid question to ask.
« Last Edit: October 08, 2021, 04:33:30 am by timenutgoblin »
 
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Offline dark_hawkTopic starter

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Re: Mosfet Keeps blowing
« Reply #26 on: October 08, 2021, 06:16:22 am »
On schematic, PSU2 capacitor C71 is 33µF / 100V, PSU1 capacitor C72 is 330µF / 100V
On PCB, both capacitors look like they're the same physical size (330µF ?)

Q3 MOSFET:
Gate terminal:
Solder Gate terminal to top of PCB. The track on the top of the PCB goes to the gate resistor. Remove red-insulated wire.

Drain terminal:
Scrape solder mask from adjacent copper trace and solder Drain terminal directly to the exposed PCB. This should reduce connection resistance and inductance. Remove black-insulated wire.

Is it easier to remove and install Q3 MOSFET with the jumper wires in place? Also, are the PCB tracks connecting to Q3 actually damaged? Seems like a stupid question to ask.

Again, you are correct, another typo in the schematic, C71 and C72 are identical.

Not a stupid question at all. Most if not all the tracks are fine, the pads are the ones that's gone. I just don't want to deal extra damage to the tracks at this stage with all the soldering and desoldering, if I got it working I will remove most of the wires and solder directly to the tracks. Desoldering from a wire is easier that from a track.
 
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Offline Dacke

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Re: Mosfet Keeps blowing
« Reply #27 on: October 09, 2021, 03:46:02 pm »
It's interesting to note that initially Q3 kept failing until you swapped the replacement for Q3 to Q1,  then it was Q1 that failed.  Why did the failure follow the mosfet?  International Rectifier components are very commonly counterfeited.  Did you get them from a reputable source?
 
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Online wraper

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Re: Mosfet Keeps blowing
« Reply #28 on: October 11, 2021, 08:40:54 pm »
Those vishay branded mosfets that I have are reading 0.43 ohm which is even better than the 0.55 ohm on the datasheet, and the original which reads around 0.5. Compared to some other mosfets I have from another source which reads 1.1 ohm.
It's probably not better and may be a sign of counterfeit. They may have a lower RDS(on) because they are actually rated for lower voltage. Counterfeiters may use not only less beefy parts with the same voltage rating, they may use cheaper parts with lower voltage rating and smaller die. Which at the same time may have low enough RDS(on). Post a picture (front and back) of the MOSFETs you have.
« Last Edit: October 11, 2021, 08:42:32 pm by wraper »
 

Offline dark_hawkTopic starter

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Re: Mosfet Keeps blowing
« Reply #29 on: October 15, 2021, 10:38:41 pm »
I was able to solve it. I desoldered everything I replaced in the primary side, and inspected every pad under the microscope. I then resoldered them making sure that the pad or the terminal was making a good connection. I don’t know exactly which pad or connection was the culprit here, but If I had to guess I think it would be the positive terminal of C70, it beeped in continuity before, but maybe it was hanging there by a thread.
The unit while in standby with no load draws around 200ma from mains, and while working with medium sound levels goes to about 250ma. So, I think that even the bad fake FETs would’ve worked for a while and not blown right away.
But I’m interested in how you would differentiate fake from original FETs by how they look.
Here are some photos of the IRF740 I have from 6 different sources:



 

Online Audiorepair

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Re: Mosfet Keeps blowing
« Reply #30 on: October 15, 2021, 10:52:32 pm »
I once bought fake transistors on Ebay from a supposed reputable seller. 

It was soon confirmed  they were fake by weighing them against the originals.
 

Online wraper

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Re: Mosfet Keeps blowing
« Reply #31 on: October 16, 2021, 06:45:58 am »
5 and 6 should be genuine, 2, 3, 4 look counterfeit. If taken out of an old equipment, 1 should be genuine.
 

Offline dark_hawkTopic starter

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Re: Mosfet Keeps blowing
« Reply #32 on: October 17, 2021, 08:25:11 pm »
Like this guy says, it's not about being fake or original, it's more of a "fit for purpose" approach:


VgsOn/Ciss(gate capacity)/RdsOn/Weight
1 - The original disassembled from the unit: 3.xxv/3.xxuF/0.5R/1.98g
2 - 3.42v/1.53nF/0.43R/1.95g (Maybe fake but fits the purpose)
3- 3.46v/760pF/2.58R/1.92g (Completely fake, and a very bad fake too. 2.58 ohm RdsOn is way high)
4- 4.53v/744pF/1.08R/2.02g (Also fake, and a bad one with Rdson double of that on the datasheet)
5- 3.29v/2.93nF/0.43R/1.98g (The lowest VgsOn, the highest Ciss and among the lowest RdsOn, maybe original)
6- 3.54v/2.71nF/0.45R/1.97g (The second best I think)

Weight does not determine much here I think, the TO220 is already light enough, maybe weighing some TO-247 could show a difference.
« Last Edit: October 19, 2021, 04:07:48 pm by dark_hawk »
 

Online wraper

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Re: Mosfet Keeps blowing
« Reply #33 on: October 17, 2021, 08:40:57 pm »
2 - 3.42v/1.53uF/0.43R/1.95g (Maybe fake but fits the purpose)
Or it does not. You don't know its VDS.
 

Offline dark_hawkTopic starter

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Re: Mosfet Keeps blowing
« Reply #34 on: October 17, 2021, 08:49:45 pm »
2 - 3.42v/1.53uF/0.43R/1.95g (Maybe fake but fits the purpose)
Or it does not. You don't know its VDS.

I got 2 of those in the unit right now, the voltage being applied to them is about 120V, and they seem to work fine. Their operating temperature is almost the same as the original IR next to them. So they do fit this purpose.
Desoldering them and replacing them with number 5 will be a hassle, so I'm leaving them for some time
But I see your point.
 

Online wraper

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Re: Mosfet Keeps blowing
« Reply #35 on: October 18, 2021, 06:00:12 am »
Lower gate capacitance at the same RDS(ON) suggests either more modern manufacturing technology or lower VDS rating. And considering that it's a counterfeit (TO220 MOSFETS from Vishay Siliconix do not look like this), the latter is more probable, as it would be a cheaper part.
 

Offline dark_hawkTopic starter

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Re: Mosfet Keeps blowing
« Reply #36 on: October 18, 2021, 09:26:49 pm »
Lower gate capacitance at the same RDS(ON) suggests either more modern manufacturing technology or lower VDS rating. And considering that it's a counterfeit (TO220 MOSFETS from Vishay Siliconix do not look like this), the latter is more probable, as it would be a cheaper part.

So a lower Input Capacitance (Ciss) is considered a good thing?
Following that logic, when choosing a mosfet from this list, the best one would be the cheapest one:IRF740BPBF with the lowest Input capacitance.
https://www.digikey.ca/en/products/compare?s=N4IgzCBcDaIBwEYBMAGMB2EAaeyUBYVtckE1j98MBOahY5MagNioh0ZavwbAFZCfanxABdHD0ggA9AFMAdtIAOAJwD2AEwCuAYwAuAZ2kAzAJYAbPbJXS9KgIbyDpg3rUqDAWmOzDngLZqBj5%2BzvIA5uay0kjocAD8BgC8AHL4AJLhAFYAwgBCOkrhAEYAYmoAIgDiAKp5-nkVAIIp4QBaAO4A0gCaeWoAinnoXU3FAEpNeT0d9QCyOuEVABZNpSg5TQASawCeq7VITfJNAMqzRzoAHlPGTeGlPU3oTU09AKLpbQBSWwAqbTg4xqphyKlOfD4TW%2BVxSAHUtqcUgAvPgVcbvADWABlSvIrmpTAhvuYOqVTJi5uYVABHZgABTymIA8gQ4Hl0nABukVHlzAZzKUahoEKUrnklE0cj84QM2v5irs5n9jGA5jk5qUEGA8n8clt4TpdrJTqdjD1kXBkQNvm9mFccpjZAgavIKgANb5tWVSpoVGoAN3ekqaADUaqUAKRIPKtPJ8UpTNpbCoDAxrDSyLZbd5XOanJogAC%2BQA

Also, why are you so sure that number 2 is a counterfeit? It's readings are close enough to number 5 and 6?
 

Online wraper

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Re: Mosfet Keeps blowing
« Reply #37 on: October 19, 2021, 11:14:24 am »
So a lower Input Capacitance (Ciss) is considered a good thing?
Yes it's a good thing since it's easier to drive and will have faster rise/fall times with the same gate drive. However using IRF740B as a drop-in replacement for IRF740 might be not the best idea (if original part is available). 99% chance it will work just fine, however undesirable consequences like increased EMI are also possible.
Quote
Also, why are you so sure that number 2 is a counterfeit? It's readings are close enough to number 5 and 6?
Yes, 100% counterfeit, a very obvious one. It's package is wrong. And marking is applied too close to the bottom as indent is located on top. Not to say gate capacitance is too low compared to genuine parts.
BTW you typed capacitances as uF but they should be nF.
« Last Edit: October 19, 2021, 11:26:26 am by wraper »
 
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Offline Haenk

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Re: Mosfet Keeps blowing
« Reply #38 on: October 19, 2021, 02:20:36 pm »
I'd vote for number 6 (only; besides 1) being a genuine one.
Number 5 looks like an IR case being coated, the Siliconix logo looks off. Should be a fat "S" with an "i" embedded.
(Edit: Number 5 might be the correct part in itself, just not from the stated manufacturer, so maybe old stock being "refreshed". Maybe try rubbing it with some acetone to check for coating...)
« Last Edit: October 19, 2021, 02:22:50 pm by Haenk »
 

Online wraper

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Re: Mosfet Keeps blowing
« Reply #39 on: October 19, 2021, 03:51:53 pm »
I'd vote for number 6 (only; besides 1) being a genuine one.
Number 5 looks like an IR case being coated, the Siliconix logo looks off. Should be a fat "S" with an "i" embedded.
(Edit: Number 5 might be the correct part in itself, just not from the stated manufacturer, so maybe old stock being "refreshed". Maybe try rubbing it with some acetone to check for coating...)
Number 5 and 6 look exactly like examples in the datasheet from 2 different factories.
« Last Edit: October 19, 2021, 03:53:53 pm by wraper »
 


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