[MPaulHolmes]

Originally Posted by PStechPaul

The Open Revolt website has some schematics, but they are for their MOSFET version. It does not actually show the MOSFETs - or the freewheeling diodes

Here you go:

Capacitor bank + ------------------------- Freewheel diode Cathode
Freewheel diode anode ------------------- MOSFET drain
Mosfet Source ---------------------------- Capacitor bank minus.

-Paul

[subcooledheatpump]

This is what he's doing


This is what PSTechPaul says he should do



Only difference is, a higher voltage is required to drive the upper IGBT, thats the positive IGBT (Collector on positive, emitter to the motor negative)

And the motor wouldn't be connected to the high voltage battery positive at all times.

[PStechPaul]

Yes, I see now. I just ASSumed that the B- was at chassis potential as it is for ICE cars. If it is isolated and floating, then the IGBT drive must also be isolated and floating, and it does not matter if it is on the top rail or the bottom. The gate reference can be connected to either E1 or E2, and the isolated PWM drive will still provide the same +15V and -8V as specified for IGBTs.

How is reverse handled in such DC drives? A full H-bridge would take care of this nicely, and it could also short out both motor terminals for dynamic braking (although for series wound motors I don't think this would apply). And an H-bridge requires four IGBTs and drivers for each, so complexity is increased.

Thanks for teaching me this fact about EVs. I still don't know why the waveforms are screwed up and the IGBTs are blowing. Hopefully the OP can find out why and fix it!

[cpate]

Well yesterday we got some fancy new snubbers and a big electrolytic cap! Per Jack Bauer's advice I installed a 33 ohm gate resistor on the IGBT. With a resistive load and no snubber, there's basically no spiking and the miller plateaus are very clear. I guess that is a good indication that this is working ok.

Then we hooked the starter motor back up and added the new snubber. Unfortunately it still spikes to 20-30 volts upon turn off. As Jack Bauer pointed out, the IGBT did get warm. We were doing about 30 amps peak, with the motor.. We then hooked up 24v to see how big the spikes got and they jumped to 50-70 volts. It seems that the freewheel diodes are not doing their jobs very fast, or at all. However, the spikes are much smaller and we can now see the oscillations in the voltage, instead of the turn off and turn on being a huge spike up followed by a curve down. We will post oscilloscope pics soon to illustrate what I'm talking about.

I guess I expected the new snubber and high gate resistor to solve our problem, but at least we're making progress.

[MPaulHolmes]

Sometimes the spikes can actually be noise being picked up on the oscilloscope. A good way to see what the spikes really are is to put a diode from B+ to another capacitor + (like 330uF or whatever you feel like using), then connect B- on the controller to B- on the new capacitor. It will act like a peak detector. Or a crab trap! Little electron crabs go into the capacitor, and they want to leave, but they can't! The stupid diode won't let them. Then you boil them for 20 minutes in ocean water, and eat them.

[PStechPaul]

One easy way to see if the scope is picking up noise rather than a real signal is to connect the probe to the same point as its ground reference on the device. It should not show anything significant. Otherwise, what you see is probably inductively coupled or transients in the grounds. Sometimes a full differential reading is better, using two identical probes, and using the scope's A-B mode. Sometimes you can adjust one of the gains for minimum noise.

Sounds like good progress. Turn-off transients can be a bear!

[cpate]

Thanks guys I will definitely try those things! That would be nice if it turned out to be interference. Oh and Paul your crab metaphor made my day

[cpate]

Today we put on a 2200uF electrolytic capacitor and used that in conjunction with the new 5uF snubber. Here is the result



This is with one IGBT, one electrolytic and one snubber cap, inductive load, 33R gate resistor, 8khz switching circuit. Purple line is ground and green is signal. 10 volts per vertical division and 20us per horizontal division. The battery pack that it's switching is about 24v.

Does anyone have any idea why the IGBT is switching off twice in the time it's supposed to switch on once??? and we have these huge capacitors and gate resistor but the spikes are still large and there is this big curve down after the turn off? I connected the oscilloscope probes to the same point on the ground and there were only small ripples in the signal. I don't think this is enough to create the voltage spikes that we see. I will do the diode/cap test next, but even if it's reading too high this waveform makes no sense

Perhaps there is something very wrong with the Freewheel diodes... I guess I know what I should do now
a. augment/replace freewheel diodes??
b. do diode/cap test to verify oscilloscope results.

Any input is appreciated as we are stumped

[PStechPaul]

I assume the waveform is the voltage from the bottom IGBT collector to GND? What is the gate signal under the same conditions? It appears that there are two fairly solid turn-ons, but one is about 20 uSec while the other is about 30 uSec. And these have 5 volts drop, so if you are running 50 amps that's 250 watts! Then there are the second turn-ons 6-12 uSec later, but there is a much higher voltage drop of 20-15 volts, which is an even higher amount of power.

You may be exceeding the safe operating area (SOA) of the IGBT, which can happen when there is a very high power dissipation for even a short time. It is generally destructive, but maybe you've been lucky so far.
http://en.wikipedia.org/wiki/Safe_operating_area
http://en.wikipedia.org/wiki/IGBT
http://www.electronics.dit.ie/staff/...-Thyristor.pdf

Apparently older devices were more susceptible to second breakdown, so if you have IGBTs from old drives that may be a problem

It would be very helpful to show your complete schematic, especially the base drive components, FWDs, and any snubbers or capacitors or inductors you have in the circuit.

If you have a very high snubber capacitance across the IGBT, the turn-off may cause a transient due to the motor inductance which could result in a breakdown of the IGBT, especially if the gate drive is too "soft". Thus the IGBT will act as part of the snubber and it will dissipate the inductive energy rather than the FWD. And the FWD also might be too slow, or defective.

So if you can show exactly what you have, I might be able to help.

Oh, and another clue is that the saturation voltage seems to be decreasing with time, which also hints at slow gate drive. If the IGBT is turned on full, the voltage should start very low and then rise as the inductance saturates. And it should be no more than 1 or 2 volts. If this is to be used for a motor that will draw 500 amps, you want to limit the peak dissipation to maybe 500W. If the IGBT has a 5V drop on a little starter motor, it will surely blow up on an automotive traction motor.

[swoozle]

Let's see if i can get this right.
It looks sooo simple...

Controller 1.gif