Substituting Power MOSFETS, buying from AliExpress, obsolete parts

[tango17]

I enjoy electronics as a hobby. I bought a "parts" 60 amp 13.8 vdc converter/charger from eBay for $10 knowing it probably had a blown power mosfet, which it did - an IRFP32N50K, plus other misc components.

That mosfet is obsolete, but I can buy it from Digikey, where I usually buy parts, for $8.25 plus shipping. Mouser is about the same price.  I can buy 5 of them from AliExpress for that price, but ... we all know there are risks about fakes, etc.  LCSC has them for $4.75, but I've never dealt with them.

My skills are such that there's a serious risk I'll need 2-3 and I may not succeed in the repair.  There's no circuit diagram and some traces and components were completely blown away, but I have some vaguely similar circuit diagrams

I don't need to have this device, but I'd like to succeed in the repair, but not if it costs me twice or three times what I paid for the parts unit just for a single component.  I'm more interested in learning something from the repair attempt.  I realize that this mosfet (32A, 500V) must carry a big load, so a fake part would be bad.

I'd love some guidance or suggestions from those who have been down this road.  Is the Digikey price that high because it's obsolete?  Can I substitute a cheaper modern component?  It's. 500v rating seems high to me (it runs in a single mosfet forward converter design, 120vac only where it switches from the bridge rectifier's 170vdc caps). Am I crazy to consider buying from AliExpress?

The control chip is a UC3845B if it helps. I don't care about long shipping times.  I'd appreciate any comments from those of you with more experience.  Thanks!

[radiolistener]

I can buy 5 of them from AliExpress for that price, but ... we all know there are risks about fakes, etc.

I don't think this is a risk, you can consider it as a guarantee that it will be fake. 

[exmadscientist]

Don't buy from AliExpress. Only run that risk once you're experienced enough to spot fakes and know what to do with them (and when they might be OK anyway). Not a beginner's move.

LCSC is legit, no hesitations there.

I recognize you might not want to spend a lot of money on this, but how else are you going to learn? It's cheaper than a course and a whole lot more instructive.

[wraper]

Buying western brand power MOSFETs on aliexpress is like 99% chance getting counterfeits (of variable crappiness) if you buy cheap, and 90% if you buy expensive. Simply don't, absolutely not worth it. If you want cheap parts with real known specs, find cheap Chinese brand substitute at LCSC. The only parts that are worth buying on aliexpress are those which are hard to substitute with something else that will likely be operational on a first glance.

[PlainName]

I can buy 5 of them from AliExpress for that price, but ... we all know there are risks about fakes, etc.

Do it and buy them.

Either you hit the one in a million and it works, or you learn a valuable lesson relatively cheaply. What's not to like?

[wraper]

If you want to save money you can buy this for example:
https://www.lcsc.com/product-detail/MOSFETs_YFW-YFW28N50AP_C5444245.html

[wraper]

The control chip is a UC3845B if it helps.

If MOSFET is bad, it's much safer to replace PWM IC as well. Chance of it going bad is very high. Also you should check source resistor and if nothing in gate circuit is bad.

[Phil1977]

Only buy Power MOSFETs from AliExpress or ebay if they are that cheap that you wanna gamble.

They may work, but often you get de-soldered parts that were handled and shipped without any ESD protection. Even if they do not fail at once in operation they easily may be pre-damaged and may fail when it hurts most.

It´s much better to get a new part with similar datasheet parameters e.g. from LCSC.


If the FETs of a power supply have been blown you should carefully check the whole primary side. Check the rectifier with a diode tester, apply a safe low voltage to the buffer cap to check for shorts. Look for blown diodes in the snubber network. A blown power FET very often electrically scorches the parts around it.

[PCB.Wiz]

There's no circuit diagram and some traces and components were completely blown away, but I have some vaguely similar circuit diagrams

That puts it in the high risk category, as other parts likely also were 'damaged' in the copper vapor explosion !

[tango17]

I appreciate the comments and suggestions.  Perhaps I wasn't very clear about my goals.  I've traced much of the circuit, and identified about 6 components that are bad.  Most I can scavenge from my parts bin.

  I spent a lot of time studying the different types of switching converter smps designs and identified this as a forward converter using a single mosfet.  It has a custom transformer with 3 windings, using one as a reset winding.

I am trying to learn more about things like under what circumstances people buy from AliExpress (never?), how best to choose a substitute mosfet (what parameters do I need to pay the most attention to), etc.

The original mosfet is rated at 500V and 32A continuous, but the input is only 120vac with the input filter caps are only 250v.  Is it common to have twice the rating for the switching mosfet? How do designers choose the mosfet?

Also, since the output is only 60A max at less than 15vdc, is 32A on the primary side for the mosfet truly necessary (20A at 100°C)? I would think the current on the primary side would be less than that.  It's rated at 130A pulsed repetitive.

I'd like to try getting it working with a cheaper mosfet with lower ratings that I can afford to blow up if I do something stupid.  Yes, I can current limit the input and try to protect it, but you know how easy it is to be stupid.

Once I get it working, I don't mind spending for the correct replacemement, but much of this whole game is to try to learn how to substitute.  How to choose something suitable, when an exact replacement isn't available.

Is there anything you guys feel comfortable buying from AliExpress?  SMD resistor or capacitor assortments?

Thanks for your comments.  I really do appreciate them.

[wraper]

^You should be primarily be concerned about R_DS(ON) being not worse than original part and similar gate charge when finding substitute (somewhat lower gate charge is OK in most cases, but but vastly lower may have negative consequences due to increased switching speed). Current rating barely matters as generally limitation is heat produced rather than mosfet not being able to handle the current.

[T3sl4co1l]

Yeah, oddball parts like that, fell out of production as new and better came along.  A few still remain, like IRF460, but you'd need at least two in parallel to do the same job.

A modern replacement is possible, but requires knowledge of the circuit (schematic and layout), particularly what snubbing if any is nearby, and how nearby it is.

It will change the EMI profile of the thing of course, and ideally one would test and confirm nominal behavior.  This requires special equipment that repairpersons don't usually have.  Alternately, the new transistor can be slowed down (with the use of snubbers, e.g. source ferrite bead, R+C from D to G) comparable to the original, and performance can be confirmed with oscilloscope.

Cascade failures are likely, and the controller, gate resistor, and maybe some nearby zeners (supply clamping?), need to be checked.  The initial failure might be transient overvoltage (transistor blows), or caused by inappropriate load (rectifier shorted?).

Tim

[lilJohn]

That mosfet is obsolete, but I can buy it from Digikey, where I usually buy parts, for $8.25 plus shipping. Mouser is about the same price.  I can buy 5 of them from AliExpress for that price, but ... we all know there are risks about fakes, etc.  LCSC has them for $4.75, but I've never dealt with them.

LCSC is good -- I think they are the under the same company as JLCPCB and easyEDA; well that information holds more weight if you have used those before. And they have been my first stop for parts since I found them while doing hobby electronics in 2019. The only issue I have with LCSC is that shipping is a bit expensive, usually starts at about $6, depending on your cart, for the cheapest option (7+ business days?) and they tell you they will add a $3 fee if your merchandise total isn't above $15. So usually my workflow has been that I get at least 2 or more projects going, determine all the components I need, make my components order, and then sketch up my PCBs.

I'd love some guidance or suggestions from those who have been down this road. [...] Am I crazy to consider buying from AliExpress?

In my experience, AliExpress is really good for WYSIWYG (What-You-See-is-What-You-Get) parts without too complicated specs, e.g. resistors, capacitors, connectors and headers, ICs designed by Asian companies and modules based on those, and legacy ICs that there are generics for (e.g. CD4049). For things like power inductors where the saturation current matters, I'd typically go for LCSC because they provide a datasheet. Like the others have been saying, AliExpress is pretty risky for Western manufacturer ICs; the idea "if it is too good to be true, it probably is" holds true most of the time. The only time I tried AliExpress for ICs was out of desperation; it was back in the 2020s when there was a ST32 supply crunch. I guess I was lucky they turned out fine but it was because the listing price was already higher than that of market prices before ST announced production suspensions and probably because the seller wasn't aware of the incoming supply crunch / some other reason they hadn't adjusted the price to reflect latest market prices.

As for your IRF640, chances are your AliExpress seller is selling one that they purchased in bulk from LCSC (https://www.lcsc.com/search?q=IRF460) or one of the manufacturers LCSC lists. I have seen AliExpress sellers straight up rip LCSC images for their AliExpress item listing. If the seller isn't claiming what the manufacturer is, it is probably from LCSC. If the seller is claiming who the manufacturer is, say TI or Infineon, I'd say "go for it" only if the seller has 95%+ reputation and been open for at least 2 years, and/or there are actual ratings with a decent quantity sold for the listing.

And no, you aren't crazy for considering buying from AliExpress. I, too, know the temptation of a good deal and how I do a sanity check whether or not the it is "too good to be true" is through broader awareness how ridiculous the margins are for retailers and Amazon FBA. Overall my experience on AliExpress has been decent for more than just electronics: mechanical hardware (beats the hell out of McMaster except for shipping time and unknown mechanical stress specs, but if you need a "m3 knurled hand screw" just for an enclosure, you probably don't need to know whether or not its tensile strength is >60k psi) and replacement car parts (e.g. vanity mirror light / nice aftermarket parts / car detailing chemicals).

[ironcurtain]

Only buy Power MOSFETs from AliExpress or ebay if they are that cheap that you wanna gamble.

They may work, but often you get de-soldered parts that were handled and shipped without any ESD protection. Even if they do not fail at once in operation they easily may be pre-damaged and may fail when it hurts most.

It´s much better to get a new part with similar datasheet parameters e.g. from LCSC.


If the FETs of a power supply have been blown you should carefully check the whole primary side. Check the rectifier with a diode tester, apply a safe low voltage to the buffer cap to check for shorts. Look for blown diodes in the snubber network. A blown power FET very often electrically scorches the parts around it.

This reminds me of the 60V power supply I have in a box, which blew up through the first thermistor. Replacing it did not solve the issue, as the replacement was heating up immediately upon connection to mains power. I'm convinced the FETs are gone, and possibly other things. Never got around to diagnosing it, as it took more time than throwing it in a bin and grabbing a replacement from a reputable brand. It was an Aliexpress PS sold for the Ruideng "benchtop" series.

TL;DR Wear eye protection during repair work, especially with AC-DC power supplies.

[wraper]

Only buy Power MOSFETs from AliExpress or ebay if they are that cheap that you wanna gamble.

Such suggestion only works if one is a connoisseur of shit.

[Phil1977]

As an example:

- Buy 25 LMC662 opamps for €3,- at Aliexpress:

With maybe 75% probability you get some surplus from someone with an acceptable quality. Then you have a stock of these parts you can use for tinkering where you anyhow risk the life of the ICs. Especially the LMC662 is a nice part for electrometer experiments, and chances are high you destroy some of them when combining super high impedance inputs with high voltage.

- Buy 1 LMC662 for €3,- at Aliexpress:

To say it in a provocative way: Now you pay the counterfeiter for relabelling some fake ICs. The more expensive the parts are, the more criminal some people get

- Buy 1 LMC662 for €3,- at Reichelt, Mouser or LCSC:

You get what you order and pay for. With 99% probability or more you get a good device.

That´s why I only consider option 1 and option 3. And if I need something for more than tinkering, then it´s only option 3.

[wraper]

As an example:

- Buy 25 LMC662 opamps for €3,- at Aliexpress:

With maybe 75% probability you get some surplus from someone with an acceptable quality. Then you have a stock of these parts you can use for tinkering where you anyhow risk the life of the ICs. Especially the LMC662 is a nice part for electrometer experiments, and chances are high you destroy some of them when combining super high impedance inputs with high voltage.

Fixed it for you: With 90% probability you'll get something not better than LM358. With remaining 10% probability you'll get counterfeit that is better than LM358.

[Phil1977]

As an example:

- Buy 25 LMC662 opamps for €3,- at Aliexpress:

With maybe 75% probability you get some surplus from someone with an acceptable quality. Then you have a stock of these parts you can use for tinkering where you anyhow risk the life of the ICs. Especially the LMC662 is a nice part for electrometer experiments, and chances are high you destroy some of them when combining super high impedance inputs with high voltage.

Fixed it for you: With 90% probability you'll get something not better than LM358. With remaining 10% probability you'll get counterfeit that is better than LM358.

Do you speak out of experience or prejudice?

I think a "fake-ratio" of 25% is quite realistic for Aliexpress semiconductors. Opamps maybe a little higher, but definitely not 90%.

[wraper]

Do you speak out of experience or prejudice?

I think a "fake-ratio" of 25% is quite realistic for Aliexpress semiconductors. Opamps maybe a little higher, but definitely not 90%.

For particular example of 25 LMC662 opamps for €3 chance of being counterfeit is 100%. 75% chance of being genuine is for something that is hard to substitute or costs not that much less than from reputable distributor.

[Phil1977]

Please tell me some test I should perform with the cheap LMC662 - and please don't say bias current, I tested it already to be <0.1pA and that´s the best I could do. I also have a few LMC662 from digikey, if you have a good idea what to compare then please let´s test it.

I don't recommend anyone to buy this cheap stuff if you need if for something serious. But some good bargains are real.

[Wolfram]

The original mosfet is rated at 500V and 32A continuous, but the input is only 120vac with the input filter caps are only 250v.  Is it common to have twice the rating for the switching mosfet? How do designers choose the mosfet?

Also, since the output is only 60A max at less than 15vdc, is 32A on the primary side for the mosfet truly necessary (20A at 100°C)? I would think the current on the primary side would be less than that.  It's rated at 130A pulsed repetitive.

If the original MOSFET blew up, I would argue that it's not overdimensioned

Datasheet current ratings have very little to do with real world current handling, for many reasons. It completely ignores switching losses for example, and it assumes a heatsink that can hold that temperature at the case given the dissipation at that operating point. The 20 A at 100°C rating still assumes that the case is held at that temperature. Here it would be dissipating over 100 watts still, and assuming 0.25 k/W of case-heatsink thermal resistance and an ambient temperature of 40°C, that would require a 0.3 k/W heatsink which is a pretty big one, and that's ignoring switching losses. It's safe to assume that the designers chose a MOSFET of this size for a reason, and going with something smaller might suffice for low power testing but I would be careful if the real load is close to the rated power.

[wraper]

Please tell me some test I should perform with the cheap LMC662 - and please don't say bias current, I tested it already to be <0.1pA and that´s the best I could do.

Does not prove anything other than you are incapable to actually verify it's genuine. The only thing it means is that it has FET input. Spec is 2 orders of magnitude less.
For starters you could post a photo, I'm pretty good at finding issues with marking/packaging. Also you could try rubbing IC surface with strong solvent, like acetone.
Spec you can easily check:

ISS = 400μA/amplifier; Independent of V+
Slew rate: 1.1V/μs

And short circuit current (I_SC spec).

[Phil1977]

Spec is 1-4pA max. - and that´s for a good reason because just some humidity on the package can destroy the "TYP"-value of 2fA. And by far not all other FET-opamps have that high input impedance.

Pictures of the Digikey-part and the AliExpress-part will follow - let´s see if you can distinguish them. I never said I can proof that they are genuine - but you can trust I can distinguish them from the obvious LM358 fakes that you sometimes get delivered.

[wraper]

Spec is 1-4pA max. - and that´s for a good reason because just some humidity on the package can destroy the "TYP"-value of 2fA.

Of course I meant it at room temperature, not full temperature range.

[Phil1977]

Slew rate and short circuit current measurements will follow...

PS: Both pictures are after thoroughly wiping with acetone.