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  #61  
Old 07-05-2012, 08:27 AM
swoozle swoozle is offline
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Default Re: Open Revolt with IGBT driver blew IGBTs

OK, we got the double-tap taken care of. It was a couple of things, one of which is a weird interaction between the netgain hall throttle and the revolt board. Whenever the revolt 5V power is connected to the throttle, the 5V line takes on a 8khz roughly-sinusoidal 4-6V pattern which is then superimposed on the throttle output. Needless to say the VLA500 doesn't like that. Couldn't figure out the root cause so I took care of it with an off-board 5V regulator to supply the throttle. Problem solved.

The one that remains and is giving us fits is this long tail on turn-off. With a 30R gate resistor the switch time should be around 2us. The FWD reverse recovery time is supposed to be .3us. And yet the tail looks like the FWD isn't working at all over 40 or 50 us. What the heck? This is only 12V and 60 or 70 amps.

The behavior is the same using another IGBT of the same model (Fuji 2MBI300L) as well as another different IGBT (2MBI200N, same as in the EMW charger). I can't help but think it must be something obvious but we aren't finding it. Ideas are very welcome.
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  #62  
Old 07-05-2012, 11:05 AM
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jackbauer jackbauer is offline
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Default Re: Open Revolt with IGBT driver blew IGBTs

My original controller used a pair of 1mbi800 fuji parts and worked fine even with a unipolar 12v gate drive signal. Next thing to do is start dropping down the gate resistor and see if the the spike magnitude increases with switching speed.
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  #63  
Old 07-05-2012, 11:42 AM
swoozle swoozle is offline
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Default Re: Open Revolt with IGBT driver blew IGBTs

Quote:
Originally Posted by jackbauer View Post
Next thing to do is start dropping down the gate resistor and see if the the spike magnitude increases with switching speed.
Yes, it does. I dropped back to 10R with the same setup and the spikes got worse.

The gate on the FWD is just grounded. If that switch is turning on (as has been mentioned in other threads), would it exhibit this behavior? Doesn't seem like it, but I'm barely a novice here. Would a negative applied voltage on the FWD gate help by making sure the FWD switch stays off?
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  #64  
Old 07-05-2012, 02:12 PM
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Lightbulb Re: Open Revolt with IGBT driver blew IGBTs

Trying to analyze this... If you are getting faster, higher spikes at turn-off, that shows that the IGBT is working properly. The magnitude of the spike should not be much higher than the bus voltage if the upper IGBT as FWD is working properly. A properly designed RC snubber across the motor (or across the lower IGBT) should reduce that spike and the ringing, although it may eat some power.

But your scope trace awhile ago also shows a decaying voltage that seems to start at a voltage substantially higher than the nominal DC bus voltage, and this cannot be if the upper FWD is working. The motor connection between the two IGBTs cannot go any higher than one diode drop above B+. So, either the diode is defective, or your bus voltage is not solid. If you are isolating the B+ from the battery with a diode, then it could rise to a much higher voltage, which would then bleed off through the bus capacitors. This could also be caused if the motor was generating during the OFF cycle. I don't think a series DC motor will do this, but if your starter motor is a PM type, it may do so. In that case, you may need much larger capacitors from B+ to B-. And you should be able to see this effect by scoping the B+ line.
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  #65  
Old 07-05-2012, 02:27 PM
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Default Re: Open Revolt with IGBT driver blew IGBTs

I have not heard of anyone putting snubbers across the load or freewheel diodes. Are you sure that this is a common practice? I think that the next step may be to attach some external diodes directly to the motor and try to stamp out the spike at the source.
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  #66  
Old 07-05-2012, 07:53 PM
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Exclamation Re: Open Revolt with IGBT driver blew IGBTs

The snubber is used to absorb the transient that happens when current through an inductance is interrupted. Once the freewheeling diode conducts, it is no longer needed, but without an RC snubber there can be a very high, fast spike that can damage the IGBTs. I found a lot of information and posted some links on Stiive's thread:
http://www.diyelectriccar.com/forums...-74151p18.html

Particularly useful might be the following:
http://www.cde.com/tech/design.pdf

It does show snubbers across the DC voltage supply but only when there is series inductance from the original source (batteries). Even lengths of wire have inductance, and the insulation of the windings of the motor have distributed capacitance which can "ring", which is shown by the high frequency damped oscillations. If you do not have a really good scope, it is possible that you are not seeing all of the transients, and what you don't see can damage the semiconductors or degrade the insulation.

But the main problem appears to be the decaying voltage with a rather long time constant that occurs at turn-off. This indicates a much larger source of stored energy, which could be a large inductance, or, more likely, the rotational inertia of the motor armature which is acting as a generator. However, if the voltage as shown on the scope is more than a couple volts above the B+ voltage, either the FWD is open, or there is little capacitance on the B+ line. That is easy to check with a scope.
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Old 07-05-2012, 11:49 PM
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Default Re: Open Revolt with IGBT driver blew IGBTs

How to check B+ line capacitance? And yes we seem to be sure that the FWD is simply not working. So I believe the next step is to put an oscilloscope across the FWD and see what is going on with that, and if it's not working, put some external diodes directly across the motor.
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Old 07-06-2012, 12:49 AM
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Exclamation Re: Open Revolt with IGBT driver blew IGBTs

The first thing I would do is put the scope from B- to B+, right at the IGBTs. It should be just about rock solid, with maybe just one or two volts of ripple depending on the drive for the motor. If you see variations similar to what you see when the scope is from the motor (-) to B-, then the capacitance is too small. How much capacitance do you have? IMHO it should be at least 1000uF, and of course it needs to be rated for the maximum B+ voltage.

If you want to measure the actual capacitance, you can just connect the batteries to charge it up to the pack voltage. Then you can disconnect the batteries and put a resistor across the capacitor (it should have a bleeder anyway), and measure the time for the voltage to drop to 37% of the full charge. This is the RC time constant:
http://en.wikipedia.org/wiki/RC_time_constant

So since T=RC, C=T/R. If you have 100 volts and with a 10,000 ohm resistor it drops to 37 volts in 10 seconds, the capacitance is 1,000 uF.
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Old 07-06-2012, 12:53 AM
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Default Re: Open Revolt with IGBT driver blew IGBTs

I remember now that we have done a measurement of B+ to B-. We only did it once and I don't remember under what circumstances. I do remember that it was very wavy, with a sinusoidal thing going on starting at each turn-off.

The thing with the increased capacitance is - we tried a 400v 2200uf cap and it didn't damp much better than our 380uf 400v cap. Installing a small snubber close to the IGBT terminal made more of a difference than using a larger main bus capacitor.
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  #70  
Old 07-06-2012, 02:54 AM
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Arrow Re: Open Revolt with IGBT driver blew IGBTs

To be really effective, the capacitor must have a very low ESR at the frequencies involved. The capacitors I have which are from a VFD are 3300 uF 400V with an ESR of 10mOhm ot 100kHz. They are Kemet PEH200 series. A Vishay/Sprague 36DY series capacitor of similar size is about 40 mOhms at 100 Hz, and not specified at 100kHz. You can get effectively lower total ESR by adding smaller film type capacitors in parallel.

Also, make sure your scope isn't picking up noise. Try shorting the probe and touching it to the point you are trying to measure. Anything you see is noise and not valid signal. Don't make any assumptions. If you saw any significant ripple on the DC bus you should take a snapshot or at least draw it, with measurement of time and amplitude.

Here is a good reference of capacitor terms and properties:
http://wiki.xtronics.com/index.php/Capacitors_and_ESR

Do you have a really good, complete schematic for the entire circuit? You should include any capacitors, resistors, diodes, snubbers, and even long runs of wire and connectors. Also the driver circuit, especially the changes you have done for the IGBTs.
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