Hey,
I'm building a 3 phase variable frequency drive for small (less than 2kW) motors. I've pretty much finished building a prototype circuit and everything is just dandy, except...
My IGBT's keep exploding. Badly exploding...
When you say exploded what do you mean?
cracked case?
ded?
There are some simple DMM tests you can do on an IGBT (module) to give an idea of how it died
Also if you can open the module up and look at the die you can actually tell if it was over-voltage or over-current
To try and isolate the problem I built an independant circuit that consisted of a 3 phase rectifier with my IGBT's across the DC bus (as in the picture attached that I found on wikipedia). I had no motor connected, and I had nothing connected to the gate terminal of the IGBT (so they were not being turned on). The IGBT's were rated for 600V 10A, and I was rectifying 415V AC (so it ended up 580VDC). When I flicked the switch they exploded. I had assumed that 2 of these IGBT's in series gave me a voltage rating of 1200V. Maybe not...
While two series 600V IGBT will provide you will a static blocking of 1200V, it will not under dynamic/switching cases.
When you turn on an UPPER IGBT its emitter will be essentially at +VDC voltage, which mean the LOWER IGBT must be able to support the full DClink potential
For 415Vac 3ph this is 540V.
This in itself is bad, you really need to provide twice the blocking voltage for your nominal DClink. 600V IGBT's are fine for a 3phase 115Vac system (gives a DClink of 270V) but for 230Vac 3phase with a nominal DClink of 540Vdc its faar to close!
Once you actually start switching these devices you will get voltage overshoots on your IGBT's at turn-off, you can easily see 900Vdc peak voltage from a 540V DClink inverter. The thing about voltage and silicon is it is VERY unforgiving. While you can over-current silicon quite happily (and as long as you keep within the I^2t or the rule of 10 you are fine) BUT you a fraction over the voltage that the device will support (and a 600V IGBT may actually support 602V) them BAM dead!
I don't think this killed it (see next) BUT the use of 600V silicon in this cct and this voltage is a concern
So annoyed and with a burnt hand I decided to try a lower voltage. This time I rectified single phase 240VAC (so it ended up 340VDC). This time I tested between the rails of the DC bus to make sure I didn't have a short, and the multimeter read 0.8Mohms. When I flicked the switch the replaced IGBT's exploded again.
I've tested the entire circuit using a 14VDC power supply as the DC bus and it worked well. Anything useful seems to make the IGBT's explode.
Does someone know what's going on here?
Two things that could cause the issue
#1 do you have any deadtime between turning an UPPER IGBT {on,off} and turning the equiv LOWER IGBT {off,on} A safe starting position would be 500ns (I use 1us for 1200V, 800Amp IGBT's and they are actually ON/OFF in ~ 500ns)
#2 what is your gatedrive circuit (this is my specialty at work
). The uppers need to be floating and your isolation method need to be able to handle the common-mode voltage you will see as well as your isolating drive signal being able to cope with being bounced by 1200V/us. A good OPTO can easily cope with that.