Replacing blown IGBTs on DRSSTC

[profdc9]

I have a full-bridge of IGBTs with 2 IGBTs paralleled in each part of the bridge.

While running the coil, it seems like only two of the eight transistors is damaged.  The other six seem like they're ok, which I established by the crude method of testing to see if the emitter-collector connection is shorted.

Is it safe to reuse the six transistors, or should I toss the six good transistors and get eight new ones?  I think the two transistors that were blown are the two on each side of the half bridge that the bus power shorted through.

Also, can I use a heat gun to remove the supposedly still good transistors from the bottom side of the board?  They are soldered in the TO-247 sockets, or will this damage the transistors too much?

Thanks,

Dan

[oneKone]

I've never had a fullbridge with parallel igbt/mosfets, but multiple times i have just replaced the damaged switching device in the bridge and have left the existing ones in there. For me it's less of a gamble because i use chinese devices. 

[Hydron]

I'd try and avoid using a heat-gun to remove parts unless you have no choice - even if it doesn't cook the silicon it could still de-laminate the PCB.

As for keeping the rest of the 6 transistors, I am a little confused as to which two you think are blown, but if you still have one of the 2 parallel half-bridges intact on that side of the primary circuit, once you remove the failed devices you could run the coil at low power to check that everything is still OK. A low-voltage (bench PSU) test without a primary circuit and using a signal-generator to supply fake feedback is also a good way to check what's running and whether the IGBTs seem to still work.

[Max]

Good evening,


In my DRSSTC I also use a paralleled fullbridge, made of 4 SKM200GB128D halfbridge modules. It failed once - severely (ripping the enclosure of one brick in parts). That IGBT Module failed entirely; all of its IGBT and diode dies were black. As the coil has been used successfully after replacing only that module, I'm pretty sure the other three modules aren't damaged.

However ... No crash is like another, so this is absolutely no assurance for you that your other IGBTs are still fine. Even worse: the test you made only works in one direction. If there is a measurable resistance between C and E - and also between the gate and any other pin, then you're sure, that it's damaged. Although an IGBT might pass the test, this unfortunately doesn't mean that it's fine. When I stupidly blew up my driver board, some of the MOSFETs (IRF530 and IRF9530) passed the test (not all) and still didn't work correctly anymore.

As far as I know, there is no better way to measure IGBTs or MOSFETs. So the only thing you can do, is replacing the obviously damaged ones and slowly test your coil as suggested.


Kind regards,
Max

[alan sailer]

I have not had your direct experience but have replaced one IGBJT is a paralleled pair and been good to go.

Also as far as removing the blown devices the best way is some stuff called ChipQuick (or a generic replacement). It is great, a low temperature (indium based) alloy that you melt into the existing solder. It forms a new alloy that melt at ~100C making it easy to unsolder multiple leads.

It is a good idea to use solder wick to remove the remaining alloy after you are done or else you will have a low temperature melting solder joint.

Cheers.

[Mads Barnkob]

The rest of the MOSFETs / IGBTs might still be working, but have taken some damage, so its hard to say.

Be sure to also check all gate resistors, ZVS, diodes etc. that is on the secondary side of the GDT, often some of those components will die in a inverter explosion and without checking them you can end of changing good IGBTs out again because you think they failed once again.

[profdc9]

Here's what I ended up doing.  It's ugly, but it seems to work.

The pads on the blown IGBTs were actually vaporized.  So instead of replacing those, I tried to remove the other IGBTs and place them into a new board.

Unfortunately, desoldering IGBTs ended up being very difficult.  Instead, I cut off the leads of the IGBTs at board level and soldered short 18 gauge wires on the IGBTs as new leads.  This seemed to work ok, and is somewhat more beefy than the original thin leads.  The older thin leads on the IGBTs bent very easily.

Because I have 10 copies of the board I had made, I just soldered the IGBTs into a new board and moved all of the other components, and added 2 more new IGBTs to replace the two that had blown.  Seems to work ok so far, as you can see from the videos.

Dan

[Teravolt]

hi profdc9, what I do is use a 3 position 200 mil spacing terminal block with screws on the board and just bend the legs of the device. it makes it way easier to replace it