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AliExpress MOSFETs Are They Real or Fake? Why Did My Repaired PSU Blow Up AGAIN?

Learn Electronics Repair

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Sometimes you can replace the faulty components in a switch mode PSU only to plug it in and watch your shiny new parts self destruct. I recently have this problem with a power supply that blew up even though it was connected to a current limiter (light bulb). In the previous video I reverse engineered the PSU and eliminated just about every possible reason for it to fail again. What I was left with were either fake Replacement Mosfets or a couple of hard to test fault conditions.

Previous videos on this repair
How To Repair A Switch Mode Power Supply W...
How To Repair A Switch Mode Power Supply W...
How To Repair A Switch Mode Power Supply W...

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Equipment used in my videos. These are affiliate links, you pay the normal price and I make a small commission.

DM40C
https://s.click.aliexpress.com/e/_c4q...
Aneng AN870 (Same as Zoyi ZT219)
https://s.click.aliexpress.com/e/_c2w...
Zoyi ZT-DQ02
https://s.click.aliexpress.com/e/_oB2...
WinAPEX 268B Clamp Meter
https://s.click.aliexpress.com/e/_opJ...
VC480C+ MILLIOHM METER
https://s.click.aliexpress.com/e/_onE...
MESR-100 ESR METER
https://s.click.aliexpress.com/e/_oom...
XC6013L CAPACITOR METER
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FNB58 USB ANALYZER
https://s.click.aliexpress.com/e/_c3h...
4KV Breakdown Tester
https://s.click.aliexpress.com/e/_omK...
LED Backlight Tester
https://s.click.aliexpress.com/e/_oCR...
NI-210SC Curve Tracer
https://s.click.aliexpress.com/e/_o2e...
NI210SJ (cheaper option but watch my review)
https://s.click.aliexpress.com/e/_oD6...
PCI POST ANALYZER
https://s.click.aliexpress.com/e/_c44...
FNIRSI 1014D SCOPE
https://s.click.aliexpress.com/e/_c2x...
NPS3010W 30V 10A PSU
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Nice-Power Dual 120V 3A PSU
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HR1520 Pro Short Killer
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KSGER T12 Soldering Station
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Richard

20 Comments

@learnelectronicsrepair Creator 2 years ago Pinned

It seems I'm featured on Hackaday again
https://hackaday.com/2024/08/24/comparing-aliexpress-vs-lcsc-sourced-mosfets/

For anyone interested we also tested the RDSon of these mosfets on the live stream
https://www.youtube.com/watch?v=UmdEpzmz6e4

@jackipiegg 2 years ago

LCSC and JLCPCB are sister companies.
When you order a PCB from them with BOM (bill of materials) soldered, it sources the parts from LCSC. 100% reliable.

@copernicofelinis 2 years ago

Y'all need to learn unit conversion: one Chinese ampere is 0.25 SI amperes. One Chinese volt is 0.5 SI volt.
Also, the chemical symbol of copper is "Al".

@dunk_law 2 years ago

Vgs varies between 2V & 3V nominally dependant on manufacturer & suffix.

@gastube22 2 years ago

Good video - thanks. There's shed-loads of fake semiconductors out there. I had a similar problem of blowing mosfets (genuine ones) in a high-power amplifier PSU, and reluctantly suspected the transformer. Thankfully I was in the happy (and unusual) situation of having another nearly identical amp to compare with. A shorted-turn transformer will have much less primary inductive reactance (compared with the resistive part of the impedance), so, as an alternative to the ring method, I checked this by disconnecting one end of the primary on the good PSU and injecting a sine wave at 20kHz (the highest I can get from my audio generator - nearer to the operating frequency of this PSU would have been better). The sine wave is injected via a series resistor selected to show an approx 90 degree phase difference between the voltage dropped across the resistor and that across the primary, seen on a basic 2-channel scope. Something a bit less than 90 was obtained with a 4.7 ohm resistor. Then I tried the same on the faulty amplifier and got something around 15 degrees with the same resistor. No need for any expensive LCR analysis equipment, and no need to remove the transformer as this is done at very low signal levels that are insufficient to turn on the secondary rectifier diodes (I had checked these were not shorted). This seemed very conclusive, and was proved by removing and dismantling the transformer (destructively) - it looked fine on the outside but the primary winding was badly temperature-damaged. [Without the good amp to compare with, I'm not sure this method would have been so practical - but an approx resistor value to use could perhaps have been calculated.]

@weerobot 2 years ago

17:09 Checks Mosfets...

@randomthingschannel2692 7 days ago

This was a wonderful journey and educational!! Thank you.

@Manticore1960 2 years ago

Datasheet shows that the gate threshold voltage should be 2v to 4v.
I haven't heard of LCSC before. I'll add them to my favourites.

@JasonBurchell76 2 years ago (edited)

To test transformer, remove from board. Hook up a function generator to primary and scope to secondary, tertiary, etc. Also, you mentioned not knowing the frequency at which the Mosfets should be operating. All be it crude, the resonant frequency of the primary and secondary windings can be characterized using a function generator and scope, easily. If I can figure it out, anyone can. I learned how to do it by trying to match resonance of the primary and secondary coils of a Tesla Coil(slayer exciter, really) that I’m perpetually building.

@Chupacabras222 2 years ago (edited)

If they cost a fraction of price of reputable distributor, than it is 99% fake. My own experience. I burnt my fingers several times, and I no longer buy chips nor mosfets from aliexpress. Never again. In comparison LCSC is a reliable source.

@mattsan70 2 years ago

Consider shorted transformer windings when this happens. Use a DIM BULB to test if all ok will barely light up. Those Active speakers are common for transformers to short out

@dugannagud6254 2 years ago

Always good videos.

@Pelican_t 2 years ago (edited)

Useful Tip: Salvaging from E-Waste electronics gives you a higher chance to obtain a genuine component but may come with the risk that salvaged components can be damaged (mainly capacitors in my experience).

@WillJad 2 years ago (edited)

That is a very good example showing why working with preassumtions can make you fail no matter how much experience you have, interesting case!

@watchmakerful 2 years ago

Check also the gate capacitance, it must perfectly (+/- 5..10%) match the datasheet. If the capacitance is way lower, the transistor is fake.

@Mcfreddo 2 years ago

I've learnt something important! (Thanks)

@romancharak3675 2 years ago

After getting burned many years ago by buying components from Ebay, I only EVER buy components from reputable Parts Houses. It is definitely worth the price difference to have peace of mind, and a good result in my DIY circuits.

@warwickbunn1250 2 years ago

Brilliant Brilliant Brilliant
If you hadn't got those, substandard 😉😅, mosfets you wouldn't have needed to go through drawing out the workings again and your various possible causes and methods of testing.
But because you did I now know so much more and with your excellent drawings I now understand the shorted turn issue so much better.😊
So, bad parts are not good for fixing stuff, but great for this channel as they are so good to learn from 😁
What a dilemma 😅

@johnmcdaniel3471 2 years ago

Thanks!

@Andrew-rc3vh 2 years ago

You can't just assume if the drive is OK then it will work. The circuit uses inductors that can break down at high voltage , or another suspect thing with a power supply like that is dodgy capacitors. Because of the nature of how it works it make sit difficult to test. It's times like this it is great if you ever have an identical board for spares you can swap components. Test by substitution.