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• Inverter: half bridge IGBT IXGN60N60C2D1 600V 300A • Bus capacitor: 2x 1000uF 250V in series • Snubber: 4,7uF 400VDC • Driver: UD1.0 with primary feedback and OCD • MMC: 1.5uF 3kV, 2 in series for a total of 0.75uF 6kV • Primary coil: Flat spiral coil 5mm Ø • Secondary coil: 8x40 cm, 32AWG, ~1800 turns • Topload: 10x40cm aluminum • Resonant frequency: 180kHZ
A picture of my entire coil
I be able to produce a maximum of 70cm spark lenght at 300A ipk ( around 150us)
Here the spark
I have some question about the coil , if someone more experienced could answer and help me, it would be nice
My first question is about the bus capacitance, it seem that i don't have enough bus capacitance for this coil (2x 1000uf in series). Because after running the coil, the bus caps are quite warm. So I measure the voltage across it; at low Bps the bus voltage is relatively continue (bus is fully reacharged after each pulse), but at high bps the voltage is clearly not continue, more like rectified 50hz. I planned to add a capacitor across the 2 in series but how much capacitance ? I run the coil at ~150uS and 900Bps By adding extra capacitance I'm afraid to add more parasitic inductance to the bridge.. do i need more snubber to thwart that ? (4.7UF already here)
My second question is about the primary circuit impedance. My mmc is 0.75uF and primary coil around 1.1uH, this is a realtively high impedence (~12) i achieve 300A in 9 cycles , but if i lower the impedance (more mmc) could I transfer the energy faster? I do not know if it could improve performance..
An other question is how far can i push the pwm? , I often see that drsstc are driven aound 10% max pwm or 20% but i don't know precisely for my coil. My supposition is that Pc the average power dissipation of igbt should not exceed their maximum according to datasheet. (for my igbt it's around 480w) The power dissipation depend of: the average current (300A x PWM%) and Rg the internal resistance of igbt (the higher the gate voltage, the lower the resistance) by following the formula P=RI^2 we know the power dissipation ? but how to determine Rg ?
I have a last question (again ) about breakout point I noticed that when i put a long breakout point (20cm) the spark is more easly produced at low on-time/ low bps , than with a short output point (5cm). But with a short breakout, when I'm increasing the On-time / Bps, the spark suddenly growing and seems to be slightly larger.. why?
During the burst you're drawing a peak current of 300A. Assuming a linear rampup of current that is an average of 150A. The average of a sine wave is 2/pi of this so about a 100A. For a burst length of 150us, that would amount to 15mC of charge, i.e. a voltage loss on your bus cap of 15mC/500uF = 30V for a single burst. At 900bps you have 9 bursts (=270V loss) before a full wave rectifier recharges the caps. So yes, the bus caps are too small.
The numbers for your primary tank seem wrong. Primary impedance is sqrt(L/C), i.e. about 1.2 ohms. 300A through a 0.75uF cap at 180kHz would just cause a voltage drop of 350V (?). That wouldn't require a 6kV MMC.
Instead of looking at the emitter-collector resistance use the datasheet for the emitter-collector voltage drop for the currents you are using. From there you can calculate the power dissipation.
Depends on how much voltage drop between the recharging periods from the line voltage you want to tolerate. I'd go for at least 3 times the present capacitance.
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My first DRSSTC
I made a mistake I meant a mmc of 0.075uF (2x 0.15uF)