The Dirty DRSSTC

I finally got started on a Dual Resonant SSTC. This is just a little one with TO-247s, so I can get a feel for how it works before I try anything big. I'll use the primary and secondary coils, and the tank cap, from my Tesla-2 spark-gap coil, and the PLL driver board from my DW-SSTC.

So far, I have only made the H-bridge. I used Fairchild HGTG30N60A4D IGBTs. These are rated at 600V, 75A continuous, 240A pulsed (but can only turn off 150A without latching up/dying) and have a turn-off time of 200ns. Fairchild market them as energy-saving (and hopefully spark-enhancing) replacements for the IRFP460.

If Antonio's calculations are right, the Tesla-2 should only draw 70A peak from the inverter, so these devices should loaf along. In terms of current at least. The 270kHz resonant frequency is uncomfortably high.

I decided to try out a special low inductance layout for this H-bridge. The pictures below explain it better than I could. Basically it's a 4-layer laminated bus structure, with the DC bus + and - on the outside (made from thin copper sheet) and the HF output on the inside (made from double sided copper-clad PCB)

I fired it up at reduced DC bus voltage (30V) and frequency (120kHz) for testing. The current drawn from the DC link was only about 10mA so I suppose shoot-through was not a problem. However, the UCC gate driver chips got very hot. You can see a scope trace of the gate waveform measured at the terminals of one IGBT. (Does anyone know why the top is rounded.. I don't)

Then, I loaded it with a 12V 50W halogen lamp, in series with a 0.47uF capacitor to prove that the output was really HF :) At 16V input the lamp was lit fully, and the heatsink (from Colin's laser power unit) got warm. Hardly surprising since each IGBT has a 2V volt drop. I noticed quite a lot of ringing on the DC bus, this seemed to be due to the ESL of the big RIFA electrolytic cap. Adding two 0.33uF poly caps in parallel with it improved things.


More construction pics Return to Tesla page

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