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Old 11-07-2015, 04:23 PM   #2306 (permalink)
e*clipse
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Quote:
Originally Posted by MPaulHolmes View Post
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.
Ok, I think I got it; I'll look into the ripple current issue. It's kind of like spec'ing the big one for the main power bus.

I did look into the transistor; the Dpak package is about as small as reasonable. The problem is I can't find the transistor you spec'd in a Dpak package - it only comes in a TO-220 package. (that's the DV45VH10) It's rated for 12A and the best I can find is rated for 8 or 10A VERY intermittantly. Also, I have no idea how to match the specifications for a BJT Time to drag out my "art of electronics" book and hopefully learn something.
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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.
On your solar inverter - the panels really don't like the heat. It's pretty ironic, but the solar panel's output drops dramatically as it heats up. My solar system's best output is on those strange days when there's full sun and it's cold. The middle of summer is not nearly as good. Perhaps you could sense the panels temperature and alter the MPPT accordingly?
Regarding capacitors - I really don't like electrolytic caps - I've seen too many things - from LED lights to inverters die because of the electrolytic caps. And that was easy stuff, compared to the automotive environment. I'm going to avoid them if possible.
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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.
I'm trying to design the board to be as multi purpose as possible - so it's going to have some capability that won't be used in all cases. Believe it or not, I've got a double DC motor drive that needs a double H-bridge control and this controller could do that.

So, because SiC switches are pretty expensive right now, I'm leaving the option for other technology, like standard Mosfets or maybe IGBT's ....The place for the buffer stage will be there; maybe I can jumper around it as a simplicity/cost benefit of using SiC switches.

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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.
LOL! - my first experience with all that was in Oregon. Of course here in NorCal there's an awful lot of HiPot testing as well.
The transformer I found is a toroid with two windings; the windings are connected magnetically and don't overlap at all. They don't mention capacitance in the spec sheet. The inductance is 130uH and the DC resistance is 6.75mOhms. The AC impedance peaks at about 2Mhz @ 500 Ohms.

I thought the power circuit just had to supply a fairly constant power - say 3-4A at 24V. The gate drive would be drawing something like 10 or 12A at a very intermittant duty cycle. The poor capacitors would have to buffer this, but oh well - that's their job. However, if the power matches, the transformer and gate drive would just putt putt along happily at a much lower output. - Or am I missing something??

Oh - this brings up the question of power use for the whole inverter!! Do you have numbers for that? Maybe thingstodo could measure that??

The reason I'm asking is that I'd like to run the 24V inverter power in with all the other stuff in that big 35pin connector. The pins are rated at 16A under perfect conditions with gold plated contacts. It's probably much more realistic to think 10A or 12A. Waving my hands around, I'd say each gate drive runs at 4A - potentially 8 gate drives. Say the "brains" of the operation requires much less - 50W - 2A. That's about 800+ watts or 34A!! That would require 3 pins for just the power supply...

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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.

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
LOL! High School is probably the worst from that perspective. I like teaching if the students are interested and want to learn - then it can be fun.
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MPaulHolmes (11-07-2015)