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Old 11-06-2015, 04:17 PM   #2299 (permalink)
e*clipse
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Awesome - thanks a bunch Paul! This actually goes into some questions I thought of later. I'm getting close to fitting the driver on the board with individual gate resistors and back to back protection diodes. It looks possible to put each gate drive on the top of a 2 layer board, which will help make soldering easier.

On the subject of saving some space, what's the reason for the two banks of four 22uF capacitors - the ones tied to +15V , IGBTemitter, -8.2V? Are these buffer capacitors or filter capacitors? Why not just have one capacitor each at about 48uF? Or if these are filtering capacitors, one 22uF and one 66uF capacitor each?

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Originally Posted by MPaulHolmes View Post
turning the IGBTs off and on only takes the 8 or 12 amps for less than a microsecond with a duty of maybe 0.001%. Those pnp and npn transistors are rated for 8 amps continuous, so I think you could do just about any old current you want for very short bursts. Probably 10 Ohm resistors would be fine, and that would be around 7.5amp peak for 3 of them. But I doubt 4.7Ohm gate resistors would cause a problem.
This brings up the question about the gate drive's power consumption. I checked into the driver that push-pulls the transformer, and it's rated at 4 amps. So, if everything were matched perfectly 24V * 4A = 96watts - - - Close enough?
This would bring some opportunities to downsize certain components a bit - for example, it sounds like the gate drive transistors could be smaller.
and I've found a transformer that is much smaller, but a bit more expensive. It's made by Pulse and can take 5A, with something like a 1500V Hi-Pot test. This actually is a deal because many of the transformers I looked at could only handle < 100V. There's also a similar sized one my Eaton that can handle 4A and is "rated" for 300V for about $1 more.

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I don't think you need local checking for desat detection. It was just mechanically convenient for me to have 3 local spots connected to 3 separate B+'s.
That's good to know about not needing a local desat reference. That will save 4 connections because I can use the 10mm pins that are already there.

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I'm not sure how well the desat detection converts to mosfets though. IT's designed for IGBTs. Do mosfets have a 5 or 10uS short circuit capability ever? And would a drain to source on voltage of like 6 or 7 volts destroy the part before it had a chance to shut off? Now that I think about it, I bet it would work OK. A lot of these parts can dissipate several hundred watts. So, 600 or 700 watts (7volts * 100amp) probably would be OK for 1 or 2 microseconds. crap... Let's say the RdsON was 10mOhm. I don't know what it would really be. Then, to get to the required 7v desat on voltage, it would take 700amps. That would be 4900 watts. haha. Maybe desat isn't practical in that case. I don't know. I do know I've tested it on accident with IGBTs, and it works very nicely. It shuts off before the hardware overcurrent circuit from the LEM Hass current sensor. So, it's quicker than 3uS.
About needing desat detection at all - VERY interesting point. Since mosfets are breaking into the high voltage realm ( with SiC Mosfets ) maybe just don't bother with IGBT's? The current is limited to what the legs of the TO-247 can handle, which is covered by Mosfets.

So desat really relies on a big RdsON to work? Very interesting - because I'm trying to get away from as many losses as possible with this design. Maybe I should just commit to Mosfets... Wow, that's got my head spinning.

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Yes, if you want -5v, just use a 5v zener. But if you are push-pulling with 24v, it will only be around 19v down to -5v. And yes, I think you could reduce the duty so that you could control the output voltage. Right now it is 50%.
Very cool!
Thanks again, Paul - I'd like to add that you are an excellent teacher.

- E*clipse
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