Crap Diodes! The truth about the 150EBU04 eBay special.

I recently finished making an inverter MIG welder but as soon as I tested it out the damn thing blew apart. Now why is that? One could say that I'm dumb and unable to make a proper welder but it's actually not that (not this time at least).

The welder had previously underwent testing and worked flawlessly but all the testing was done on 12V. As soon as I tried pulling the trigger on 230V it blew three of the four primary switching IGBTs.

At first I assumed the IGBT's were the problem, but when I rebuilt the switching stage with MOSFETs I still had nothing on the output.

This made me question the secondary rectifier diodes, a quick diode test with the DMM revealed that they were all shorted, seems weird since the welder was unloaded so no current should have been flowing through the diodes. 

The diodes were marked 150EBU04 and they were supposed to be 150A, 400V diodes but as it turned out they were far from that. Considering I got them from eBay this isn't entirely surprising. But the problem is that there is very little info about these ones in particular so I'll spill it all out in this post.

Basically there are two types of these diodes, the ones that sort of resemble the PowerTab package of the original 150EBU04 and the other type in a TO-247AC package.

The TO-247AC ones are absolutely bonkers considering the back plate of the package is the cathode and the only way to get a good contact on it is to solder it, but as it turns out the back doesn't take solder very well unless you sand it down to bare copper and the TO-247AC package isn't even meant to be conducting through the back plate anyways.

When ordering the diodes from one seller I forgot to type in 4 in the quantity so only one was shipped, but instead of ordering another three from the same seller I decided to order from a different seller with the same listing. The pictures were identical but I received the TO-247AC version which there was no mention of in the listing. So I ended up with one of the PowerTab fakes and three TO-247AC ones. I later ordered three more from the seller who shipped the PowerTab fake but this time I received three more of the TO-247AC fakes which is kinda ridiculous.

Now, back to the issue at hand, the diodes shorted out and I have the answer to why. Since I have the other three TO-247AC diodes I can do some tests on them, which is exactly what I did.

With the DMM the forward voltage comes in close to zero, but with about 6A going through it it's closer to 0.55V. Nothing out of the ordinary yet. 

Next was the reverse breakdown voltage and this is where the ugly truth comes out. My method of measuring the reverse breakdown voltage was crude but it was enough to prove my suspicions. The diode passed about 135µA at 35V and 400µA at 65V so this would make it a 30V diode. MIG welders output about 22V so in a half bridge configuration the diode would see at least double that. So no wonder it failed.

Next i removed the blown diodes from my welder and cracked them open. Before the test, while fumbling around in the welder I managed to break the PowerTab fake so I couldn't test it out but I did do one measurement when I received it and it measured about 0.35V with the diode test on my DMM, if I remember correctly.

Cracking them open revealed significant differences inside, the PowerTab fake had a 5x7mm size die, while the TO-247AC version had a 6x6mm size die. There were also differences between their connection to the die. The PowerTab version had four bond wires while the TO-247AC version had a large copper tab.

So in terms of current handling the TO-247AC version would be better since it not only has a bigger die but also the better connection to it. For low voltage applications I think this diode would be good for about 100-ish amps.

I've recently cracked open a 20A MOSFET and it only had one bond wire so, assuming the ones in the PowerTab lookalike are the same, the diode could perhaps handle about 80A, too bad I don't know the reverse breakdown voltage of it.

That's the problem with these weird franknstein diodes, there's no good info about them, but if I ever get another PowerTab fake I'll be sure to test all aspects of it. Of course I won't be ordering them anytime soon, actually I'll be ordering other cheapo high current rectifier diodes to see how they hold up.

But that's it for now, thanks for reading.