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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, lets say I have a standard SSTC (no DR) which will run at 100 kHz and I want to use a very fast IGBT in it, such as this one: It is a 64amp@25C, 1200V, 110/40ns toff device. If you look at Figure 4. Load Current vs. Frequency on page 4, you can see the IGBT can only switch ~9 amp peak square wave at 100kHz. My question is, should I go with this figure for SSTC design, or does the resonant nature of a Tesla coil decrease switching losses which means I can safely push more current?
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
I get the feeling that the 9A estimate assumes fairly ideal conditions which are typically not achieved in Tesla Coils. The graph suggests 100 watts of dissipation at 9A/100kHz. That's some serious heat, and you can multiply it by however many switches you have.
I think it's risky. Sure, if you have a very closely coupled coil that is loaded down with a decent arc there will be softer switching than if you just had an inductor, but can you guarantee that your coil will always be in tune?
But really, the only way to know for sure would be to observe the primary current while the coil is operating... easier said than done.
I would personally stick with the figure in the datasheet as an absolute maximum and would see how it behaves before ramping the power up.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I think that MOSFETs are going to be your switch of choice with a SSTC. This is because in a SSTC losses are mainly from switching while with a DR losses are dominated by conduction because of the very high pulsed currents. IGBTs works best for slower switching and very low conduction losses. The MOSFETs would probably be a better choice because they will have much lower switching losses which is mainly what you are going to see in most SSTC designs.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, I wanted to make a 7 kW SSTC supplied from 400 volts AC (halfwave rectified, unfiltered). For this I would need to use 800V FETs, which usually have just a few A rating and if I put a lot of them in parallel, the gate capacitances would be huge.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Definitely use MOSFETs for a hard-switched CW SSTC. Without a resonant primary, and with significant CW breakout from the resonator lowering it's Q factor, there will be a lot of magnetising current for the inverter to switch. It is very hard to tune the driver to get zero current switching under these conditions. MOSFETs can quite happily switch off tens of amps in a few hundred nano-seconds without breaking sweat. Just keep the layout tight so you don't get voltage spikes across the devices from Ldi/dt.
IGBTs prefer soft turn off. More specifically in the DRSSTC the resonant load current smoothly passes through zero as it reverses direction. The load current then automatically commutates from the active IGBT to it's co-packaged free-wheel diode. The IGBT gate drive can then be turned off any time after this without experiencing current tail losses at turn-off.
In general, MOSFETs prefer a net inductive load where the current naturally lags the applied voltage and they achieve ZVS to minimise capacitive switching losses. On the other hand, IGBTs prefer the current to lead, and change direction before the switching instant so that they exhibit ZCS and no tail losses at turn-off.
Of course, if you application uses gated RF pulses with an overall low duty ratio, you can probably just about get away with anything. Even with a high switching loss during the burst, device thermal mass, and duty ratio can work in your favour and keep the die from overheating in the long term.
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IGBTs in SSTC, resonant vs. hard switching
