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Old 11-07-2015, 11:26 AM   #2303 (permalink)
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Originally Posted by e*clipse View Post
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?
The 22uF caps are rated for 3 amps of ripple current each. So, I was trying to design for 12amp of turnon current and 12amp of turnoff current. But that is only because I was intending to drive a 1200v 600amp monstrocity in the worst case scenario. A typical 66uF electrolytic cap might have 1A of ripple current for its rating. Higher uF MLCC caps usually have worse ripple ratings. I am fairly certain that 40uF would be enough, and that 10uF 1206 caps would work fine. And those are like $0.05 if you buy a bunch of them, but I was using my philosophy of "stuff as much as you can into the available space, and if it still doesn't work, go home and cry, because there's nothing else you can do" TM/CIRCLE R. What powerex does is use 1000uF eletrolytic caps instead. But those are rated for 3000 hours, and the MLCC caps are rated for like 300 years at 125 degC. haha. But I know for a fact that you can use the 1000uF caps for more than 10,000 hours, since they are running my house right now. I made an inverter/maximum power point tracker charger thing that uses the vla500-01 driver with the 1000uF caps, and it's been running 24-7 for over 2 years. Well, the maximum power point tracker thing now has an issue but is working better now that it's cooling down, and I bet it's related to the electrolytic caps drying out. But it's 150 degrees in my shop during the summer, so what should I expect.

If you are using SiC mosfets, you could probably dump the whole current buffer stage, and just use an optocoupler + gate driver. I bet one 4.7uF cap would be OK. Well, maybe one from +20v to "emitter", and one from "emitter" to -5v, and maybe one from +20v to -5v.

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.
The gate driver is actually rated for 4amp peak I think. I'm not sure about that, but I've always interpreted it to be "no peak gate current over 4 amps". I might be wrong though. The line filters I've been using I tested at my job to over 7000 volts from primary to secondary on a high (on) pot test. It was oregon/washington after all. What better place for a high pot test. haha. So that is probably overkill, but it has very low coupling capacitance. You would have to make sure that the transformers you found also have very low coupling capacitance. I think as long as secondary isn't on top of primary, it might be good.

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.
If the stupid fod8316 or the like would allow you to change the 6-7v desat trigger point to maybe 3-4v, it would be probably better. But they hide that inside the chip.
Thanks again, Paul - I'd like to add that you are an excellent teacher.
Well, I did go to school to be a teacher! I still have nightmares about teaching at Timberline High School. "Why do you even come to school if you are just going to mess up the class the whole time?" "My P.O. says I have to." umm... I had to go home and look up P.O. haha
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