...there is something mystically attractive in power electronics...
so after getting my 10kW charger design to a reasonably good place, I wanted to live dangerously and have wired some 1200V, 600A IGBTs together for a quick DC controller setup. Would love to get your feedback.
The idea is more about learning advanced power electronics rather than building any kind of production version (which we got covered quite well by the EVnetics guys ;-)
Having read a bunch of docs on how to design these things without having them blow up right away, I am using, among other things:
1. Laminated DC input bus with 1/4" polycarbonate between 2 large copper plates.
2. Lots of parallel caps mounted with studs on one line with IGBT leads to reduce current loop area
3. On-IGBT control boards with SMD 8A drivers (all to minimize inductance between driver and gate)
4. +/- 15V driver to ensure turn-off
5. Emitter current balancing resistors (0.3Ohm) to sync turn on and turn off of the parallel IGBTs
6. Output cables connected to the opposite sides of the controller to avoid concentration of current in one pair of devices
You can see some pics of the setup below - including my very sophisticated 'load system' - a bucket of cold water with a 5" iron powder toroid with ~80 turns of 8x14 gauge copper wire (15 mOhm, 800 microHenry at zero current; according to micrometals.com software, saturating to ~1/3 of the inductance at 100A etc).
In my first test run, I've used a 30V battery (I know, not super-impressive but hey, I didn't want to have to catch plasma balls in case something went wrong ;-), switched at 10kHz, and went to 250A 'motor' current.
Couple of scope captures attached. You can see (images numbered in order they appear - some images in the next post due to limit of 10 per post):
1.1. no ringing on gates. good thing
1.2. Turn-on/off takes ~1us. ok for 100us cycle.
2. Peak gate current of 4A both ways. Rg=5Ohm. I drive both SMD drivers by one A3120 isolated 2.5A driver.
3. There is some ringing on the +/-15V DC rail for SMB driver boards - to the tune of ~2V at ~0.5Mhz. I thought it's passable - let me know what you think.
4. Now this is where I'd REALLY love to hear your input
: at 250A, I can see negative IGBT emitter spikes of -80V at turn-off (ringing back to zero within 400ns or so at ringing frequency of ~10Mhz). At the same time, I can see positive IGBT collector spikes of +40V. So the total spike swing is ~120V, or ~4x the input DC voltage - for the total voltage stress of 5x the input DC voltage. That is not cool. I made a conclusion that I am driving these IGBTs too fast for freewheeling diodes (which are same model IGBTs with shorted G and E) to catch the negative inductive undershoot from the load. So I am going to try 10Ohm Rg (now at 5Ohm) and lower the switching frequency to 5kHz. What do you guys think????
PS. I am using the same control board I've developed for my charger (http://www.emotorwerks.com/cgi-bin/VMcharger.pl
). Just finished adapting code to the controller duty, as well. Surprising amount of stuff seems to be reusable (current limiters, precharge circuits/logic, etc).