# P&S Controller/IGBT Tuning, a new beginning



## PStechPaul (May 1, 2012)

Ah, nice traces! I don't see any huge problems, but I don't have direct experience driving a series wound motor (or even a large PM brush motor) so I'm guessing a bit.

First, it seems that the C-E trace shows a quick return to 24VDC and then a slow and steady rise to about 32 VDC. This can only mean that the motor is generating voltage and either the flyback diode is open, or the battery pack and connections have high ESR. You should be able to tell by reading the pack voltage directly (should vary only a couple volts), and by reading across the connecting cables. You can also read across the high side of the half-bridge, which should be across the motor. And also read directly across the motor and along its leads as well. Something's fishy...

The other thing I see, at turn-off, is a slight drop in the base-emitter voltage which corresponds to the voltage spike. I think this can be caused by the Miller capacitance from collector to base (gate), which usually just causes a slower response or frequency roll-off, but in this case I think the increase in voltage at the collector as the IGBT turns off is driving the base higher, causing it to turn back on, and the inductance of the load may be oscillating to cause the spike until the gate drive goes even lower. It seems to occur right at the threshold voltage of the gate, about 3-5 volts. This might be improved by using a stiffer gate drive or even by adding capacitance from gate to emitter.

See also (some of these are also about emitter resistors and general IGBT on MOSFET drive):
http://www.igbt-driver.com/fileadmi...T_and_MOSFET_Drivers_Correctly_Calculated.pdf
http://www.infineon.com/dgdl/IFX+-+...b00a1&fileId=db3a304320896aa201208a819a5f0003
http://www.irf.com/technical-info/appnotes/an-944.pdf
http://www.pwrx.com/pwrx/app/PCIM99.pdf
http://www.vincotech.com/fileadmin/...BTs/Paralleling_of_IGBTs_in_Power_Modules.pdf
http://www.vishay.com/docs/81227/81227.pdf
http://www.pcti.com/App_Data_Sheet_Opto_IGBT_Drive.pdf
http://www.irf.com/technical-info/designtp/dt04-4.pdf
It could also, possibly, be due to the emitter resistor. With a 0R8 resistor, 100 amps is 80 volts, so I think it must be much lower, perhaps 8 mOhms? Even that is 800 mV. So as the current drops, this voltage drops, which increases the voltage on the gate. And if there is any inductance in this resistor, it can have a ringing effect. You should read the waveform on this resistor (and verify its value and if it is inductive), so you can determine the current. If your gate drive is isolated, maybe you can connect it directly from gate to emitter, if it is not already.

A minor point: The scope vertical for the C-E trace seems to be set for 2V rather than 20V, although it might just be your 10x probe. There may be a scope setup that will adjust this. Some do it automatically.

I wanted to add this link as well. In one of the pages it shows a voltage overshoot and transient similar to what you show, and caused by stray inductance in the DC link and other factors. If nothing else, it is nicely illustrated and colorful. Warning, though, it's a large file for download:
http://www.evbmw.com/igbt.pdf


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## swoozle (Nov 13, 2011)

6.2 ohm main gate resistor plus 1ohm on each igbt gate, 10ohm parallel with the 6.2 with a diode, which gives:
Rg off: 7.2 ohm
Rg on: 4.8 ohm
Pack voltage is 53V
5uF snubbers on each IGBT

GE top, CE bottom............................................. Turn off at 1us per div. The turn-off spike is 75-80V (20-25V over nominal).














Turn on








Here's the same with a 1uf snubber across the switch (CE).





















Totally gets rid of the spike but sets up some weird ringing on both turn off and turn on.
Is that a problem?


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## PStechPaul (May 1, 2012)

AFAIK a little bit of ringing like that is no problem, unless it causes RFI emissions. You can probably clean it up a little more by adding a little series resistance. I'm happy to see that 75V spike removed. Those are excellent scope traces. Isn't it great to have good instrumentation?


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