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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Ok, this is why I believe against your approach:
Your IGBT's have 30nF gate capacitance max. You inevitably need to charge and dump ( so we don't divide by 2) their energy each cycle, which for say 15 volts is 15^2*30*10^-6 = 0,00000675J, times frequency is 0,3375W.
This gives current of 22.5mA, not much, but wait. You also need to include driver quiescent current which is 11mA max in this case.
So you need to provide at least like 35mA to power a single driver.
With your 560V bus voltage, losses get colossal.
P = 545*0.035 = 19,075W, times 4 IGBT's!
You need to choose right dropper resistor for this, if you put 100k voltage will sag and IGBT won't turn on completely.
R = 545/I = 15.5k min. You would need some additional margin there.
Terry got out of this because he used small IGBT's with only 3.3nF gate capacitance, lower bus voltage, and only 1-10% duty cycle so his losses were tolerable.
Slow in what way? The scope shots showed that they can deliver the current, and thay are part of the supply I use at this moment, so speed is no problem.
Slow in way of 1,5us switching time, 500ns propagation delay. But since you are slowing your IGBT's down anyway it might not matter, you may also use some slower bricks you couldn't for DRSSTC or something else.
Can you give a good reason why I should need a negative supply?
I would just hate to have the IGBT turn on due to ringing when it shouldn't. Everybody drives their IGBT's negative and I always considered it a good habit.
GDT which you are trying so hard to get rid off gives that for free (OK, only during conduction of another IGBT, but that's where it's important) among with excellent speed and ruggedness. And it's probably least overkill of all solutions.
Your IGBT's aren't so big not to be driven by 2 pairs of paralleled UCC's. I've driven 15nF gates to 30V with them in tens of ns rise and fall without problems, you shouldn't have trouble with these at 15V.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Marko, the losses aren't really that colossal, 10-15 watts per IGBT is nothing if you have fan cooling in the chassis. (I guess you guys use commas instead of decimal points and you didn't actually mean 15kW.) But you are right, Terry's source follower supply is just a linear voltage regulator running off the DC bus.
I found that my Odin drivers would drive those bricks CW up to 50kHz, and the power supply drew 0.6A at 20V when powering two drivers hooked up to one halfbridge brick at 25kHz. So it's only twice as efficient as Finn's approach. :/ My drivers waste a lot of current at idle because the rails are regulated by zeners.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Steve:
I don't know for you guys, but I would hate to have a gate driver circuit which dissipates like 50% of all dissipation of entire machine.
20W per IGBT is actually lower limit, I can hardly see how could he make it better; maybe lower to 12V but not less (driver limit). I narrowed down the margin he would need to use with zeners a bit too much, I think he 'd need few more miliamps for good regulation.
80W is a lot of power to get rid off. I would avoid that at all costs...
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Steve Conner wrote ...
Terry's source follower supply is just a linear voltage regulator running off the DC bus.
and
Marko wrote ...
Steve:
I don't know for you guys, but I would hate to have a gate driver circuit which dissipates like 50% of all dissipation of entire machine.
80W is a lot of power to get rid off. I would avoid that at all costs...
Ok, Marco is right....again.
I thought that the sourse follower could regulate without dissipating heat, switch instead of linear, but of course: no free lunch.
So for now, the notion of writing the cookbook of CCPS is off. Come to think of it, what a preposterous idea! No, why change something that is working perfectly, at least at this stage. Let`s focus on getting a functional design, then, perhaps, we can slim it down.
So the present gate driver stays, and we will install a cute little CCPS which Daniel put together because he had to, I know it`s overkill, but that`s the way to fly:
I`l probably even build another low inductance buss just for you, Marco
If this CCPS doesn`t like the load it`s going to work into, smoothing caps loaded by voltage regulators, then I`l just wind that 6-section isolation transformer which I should have done in the first place.
Finally, I`d like to extend my warmest thanks to you all for helping out so far. This project has taught Daniel and myself so much more than any thing else we have been soldering together. So much fun, So much more to learn.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
So the present gate driver stays, and we will install a cute little CCPS which Daniel put together because he had to, I know it`s overkill, but that`s the way to fly:
Finn he is adorable! And looks quite powerful for it's size. Is it a scale model of sorts? I don't know how much windings are you going to fit into that form.. but still very neat. :)
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Marko wrote ...
Finn he is adorable! And looks quite powerful for it's size. Is it a scale model of sorts? I don't know how much windings are you going to fit into that form.. but still very neat. :)
Yes, a flyback transformer core would probably be more appropriate, but this is what we had handy. The core is from a series choke from a welder. I think it gives 4 volts per turn, but Daniel knows this.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Finn, I just love it.
You can increase frequency somewhat more since it's ZCS, especially if you are using smaller IGBT's. As I think you can go to few hundred kHz, as long as you can push enough power through the cap.
If this CCPS doesn`t like the load it`s going to work into, smoothing caps loaded by voltage regulators, then I`l just wind that 6-section isolation transformer which I should have done in the first place.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Steve Conner wrote ...
I get the impression that it's the gate drive power supply
"With gate drive like this, who needs a DC bus"
O_________O
PS. Why in the world that? One small royer and flyback core are more than enough.. or you want to try something else? Weren't we talking about reducing overkill :p
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Hi Steve & Marko.
Yes, it’s the gate drive power supply
I liked the Idea of having a small CCPS driving a monster. The little thing delivers 50J/sec, and the core was used for my first CCPS that was able to deliver 1700J/sec!!!
I use this core because I needed 80 primary turns to get a decent turn ratio, so we have 4 volts per turn. And I think that a flyback core was too small, I needed some wire with decent insulation because it has 320V on the primary.
Here are some scope shots of the primary current:
Wee, 1Amp peak in the primary at 320V, I guess IRFP460 Mosfets was a overkill too
And of course it can charge a capacitor
Cheers, Daniel
BTW.
Finn,
About the load to the CCPS, I still have the 4 Mylar caps you gave me a couple of years ago. They are rated 0.25uF 25kV each, so in parallel we have a 1uf cap to play with.
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CCPS (Capacitor Charging Power Supply)
