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Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Goodchild wrote ...
I mite also point out that 800nS is just the thing in there dumb test setup using standard drive value. They can be switched much much faster with a dedicated low impedance gate driver circuit and a higher gate drive voltage like 20 or 24V.
An easy way to get your "real" switching speed is to look at the gate charge graph and get the gate charge for the voltage you plan to drive the gate at. Then use ohms law and the output impedance along with the capacitor current voltage equation and you will find how quick you will be switching.
You could switch at 1MHz but chances are you will need an insane gate drive capable of delivering 10's of amps of drive.
What IGBT are you using? if you link the data sheet I will do an example on how to calculate this for all on 4hv that wish to know how.
That's only (partially) true for MOSFETs, IGBTs have inherent delays that limit the switching frequency no matter how fast you drive the gate. You can't switch MOSFETs as fast as you want either, but you can switch them a lot faster than IGBTs, MOSFET switching speed is limited by the internal gate resistance which is usually a few ohms I think.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Goodchild: But the bipolar transistor part of the IGBT has its delay and fall time, and you can't alter that. What you wrote might be true for MOSFETs. Also, with FETs when the internal resistance starts to become a problem, you're asking for trouble, because the center of the die turns off faster but the edges take more time as the current passes through the resistance. This way the edges of the die are stressed more.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Obviously...
Your not going to go switching at 10MHz or something silly like this. Because also just the device capacities as well would make it impossible. But my point being they will switch much faster than what the data sheet specs. I know this is true because the speeds I get are always faster than what is specked in the datasheet, regardless if it's IGBT or MOSFET.
A perfect example of this is the IGBT's I use in my smaller DR's (CM300DY-12H). Datasheet specs rise and fall times that suggest at 109Khz I would be spending ~35% of my time switching. In reality these things switch way faster taking up about 5% of the switching cycle. This is because I switch them softly and also because I drive them with a higher voltage and a low impedance gate drive.
I was just going to show some equation that show the basics of getting in the ballpark.
As one of the beginners in the world of power electronics, I know I would love to see those formulas (and a valid example of them in use), Goodchild. So if you would be so kind, I would be much appreciative.
I figured out how to determine the amount of current needed to drive a FET gate at a certain frequency, but not how to determine the max frequency the FET can safely run at. I was just using I = C * dv/dt where dt = 1/10 of a cycle at the chosen frequency, and C = gate capacitance.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Goodchild wrote ...
Datasheet specs rise and fall times that suggest at 109Khz I would be spending ~35% of my time switching. In reality these things switch way faster taking up about 5% of the switching cycle.
You do realise that the datasheet states the switching times for the *current* waveform? Voltage rise/fall times are indeed a lot shorter.
Registered Member #4497
Joined: Thu Apr 19 2012, 12:53PM
Location: Behind you
Posts: 62
Goodchild wrote ...
I mite also point out that 800nS is just the thing in there dumb test setup using standard drive value. They can be switched much much faster with a dedicated low impedance gate driver circuit and a higher gate drive voltage like 20 or 24V.
An easy way to get your "real" switching speed is to look at the gate charge graph and get the gate charge for the voltage you plan to drive the gate at. Then use ohms law and the output impedance along with the capacitor current voltage equation and you will find how quick you will be switching.
You could switch at 1MHz but chances are you will need an insane gate drive capable of delivering 10's of amps of drive.
What IGBT are you using? if you link the data sheet I will do an example on how to calculate this for all on 4hv that wish to know how.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Most solid state coils are driven at the resonant frequency. If you want to build a solid state alternative of the SGTC, then look up on OLTC. Even for a small coil, the peak current will most likely be several hundred Amps, so you will need a quite bit bigger IGBT.
Registered Member #4497
Joined: Thu Apr 19 2012, 12:53PM
Location: Behind you
Posts: 62
so, i ordered this IGBTs andsome 1200V 6A diodes (3 in parallel will be instead of D6 and D7) 663ns maximal time is pretty fast, i can wind tesla coil 50cm high 9cm thick with 2500 turns of 0.2mm wire, f = 200kHz, are IGBTs fast enough? i can buy 0.125mm wire and wind 2.5k turns on 10cm tube, which will be 130kHz, but the first one will be cheaper because i have wire and perfect clean piece of thick PVC (1cm thick walls) tube, so it's better choice for me :)
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How mad (possible) is this idea?
