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Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
Hello:
I was just going to post this as a question to Steve McConner, based on a legitimate good answer that he gave in one of his responses to something I wrote. Then I thought however, that this may be worthy of its own thread since IGBT's have applications all over the place, for so many members. Now that we can find huge cheap IGBT's easy on ebay etc, but can we use them, this is the million dollar question? So my question, or maybe better said: My humble request is:
Could Steve, or someone else please post something reference the switching losses on IGBT's to better quantify this? I mentally tackled this subject briefly and what I found was all very technical, and above my head to be honest. I found some notes on the Fuji semiconductor site with references to things like Trench Design etc? I did understand the part about the tail effect, but I have a difficult time putting it all into perspective in the real world? So how about a real possible example, with a real big IGBT:
Lets say for example that you set up a half bridge inverter with this Fuji 1200V 450A 2 pack, and you drive it at 20KHz with 680V at 4 amps. So you would be shooting for an output 340V at 20KHz. If you drive this IGBT with a well designed driver with a nice fast hard on and off what would the ballpark switching losses be?
Are we talking about a significant percentage? or Just a couple hundred watts?
Sorry if this seems like a stupid question, but I cannot seem to be able to figure out how to calculate this. Thank you for your time and wisdom, the sharing is appreciated :) ]fujiigbtexample.pdf[/file]
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
You can use the following equation to approximate switching losses:
Vpulse = Voltage you are switching Ipulse = Peak current you are switching freq = frequency of switching ton = switching risetime C = capacitance load you are switching duty = duty cycle of operation
Switching Loss = [((Vpulse*(Ipulse/2))/6)*freq*ton] + [0.5*C*Vpulse^2*freq]
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The switching loss chart is right there on page 11. Unfortunately 4 amps is right off the bottom of it (I guess Fuji never considered anyone would buy their device for a power level that can be handled by a couple of TO-220s) but we can guesstimate.
It's about 7mJ total at 50 amps, so let's call it 1mJ at 4 amps, since the lines look a little less than linear. Then we have 1m * 20k * 2 (switching events per cycle) = 40 watts switching loss. You're delivering about 2800W, so you're over 95% efficient which is great.
Assuming that the switching losses really do scale down linearly-ish with current. They might not because of the energy stored in the device capacitances (6nF in this case) which depends only on voltage and is lost at every switching event.
20kHz is a low enough frequency that these bricks may work fine. When I talk about switching losses, I'm trying to discourage guys who want to use them at 200kHz in tabletop SSTCs. (400W switching losses now, and that's before we even start on current tailing and shoot-through.)
Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
Thank you so much!!!
You have done it, I am not questioning my own sanity, or pondering early onset Alzheimer's anymore!!! I remembered the mJ figure and this way of calculating the switching loss seemed right, but without an example I was just not sure, and with my limited knowlege base here, I had no way of confirming my instincts. Thanks Folks :)
So this is very nice for people who stick to the lower frequency part of the HF range, these big ebay IGBT's may be of use then. Wow just think about the induction heater you could build etc. Man this has some real potential in many areas.
If you are looking for these on ebay I have found one little trick, the expensive ones are often listed as IGBT's because obviously the seller knows what they are. If instead you do a search under "Transistor Module" or "Rectifier Module", you will find many cheap IGBT's. I was just looking at some new Toshiba 1200V 100amp 2packs for 14 bucks for 2 of them !
Anyway if we observe the limitations these could be the source of some good clean fun :)
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BIG IGBT Half-Bridge Switching Losses?
