Monthly Archives: October 2011
I finished making big-boost which now is controlled by a micro-controller instead of a TL494. By feeding a hall-effect current sensor signal into the ATMEGA168 controller, I can sense zero-current returns in the boost inductor to assure that the system never goes into continuous mode. This is critical because at low output voltages (<600V), the TL494 was going into continuous mode and the current of many hundreds of amps was burning out transistors. I now haven’t had any issues with burn-outs. The new system also allows me to detect the absolute input current, and shut off the boost if it exceeds a safety threshold (it worked! A 1000V hard-short output didn’t damage the boost beyond the input three-phase rectifiers).
Shiny things! Controller
So, let’s hook it up to big coil and see if it really works. Does great, boosts to 850V while under coil load (previous TL494 boost sagged to 500V under coil load).
But, the coil burned out :(. The exact cause is unknown, but half of the bridge had destroyed transistors. The TVS string attached to those IGBT’s had one set that was sheared apart (or maybe exploded off, but it appeared to be a previous mechanical failure point – I had just hauled it to new york so I probably didn’t notice that it got cut off). But who needs TVS diodes, right? A perfect phase-lead compensator on the current feedback should completely take away the need for them (the idea of the phase leader being that the signal coming back from the current transformer is slightly leading the actual primary tank current, so the delay in the logic and IGBT’s can be taken into account while still switching before the freewheeling diode goes into reverse recovery which creates ringing). Right now I use an RL phase compensator on the current transformer signal, but the problem with this is that the first phase is not corrected for since the inductor takes time to get a phase lead started (if anyone can explain this to me in a mathematically rigorous manner, I would be very happy! The mathematics and simulations say it should compensate instantly).
So to fix this problem, I replicated the phase leader, except using op-amps instead of an inductor. This seems to work great on a breadboard with a test input. At the start of the sine signal, the compensator adjusts practically instantly, and is fully adjusting the signal long before the first zero crossing.
Here is a video of it operating on a test signal:
The idea is invert the input (IC1A), take the derivative times a negative constant A (IC1B + IC2A), and add that to the original inverted input times constant B (IC2B). Invert this output, and you get the input scaled with an angular offset of tan-1(wA/B).
I will etch and test this in the coil this weekend (yay 4 day weekend! Thanks Christopher Columbus for finding this wonderful huge hunk of land and society still caring 500 years later).