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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I've gone through a couple of design iterations for a big boost converter, and it looks like this one will finally be a winner. The main design goals are to get 300-1000V variable output, decent power factor, and 10kW sustained with bursts up to 20-30kW. My inductor is wound for ~250uH.
Originally, I was using a TL494 design which kept burning out my brick. One thing I really don't like about the TL494 is that setting the max duty cycle for a 3x input to output boost so that it is in discontinuous mode means it could be highly continuous at 1.5x input with hundreds of amps in the inductor. Additionally, I did some diagnostics and found the TL494 was exhibiting a very odd behavior. It was not usually properly changing pulse width to get a stable voltage output, and was rather turning the whole system on and off at about 200hz. At each of these whole-system turn-on's, the gate signal was oscillating all over the place causing the brick to be in a sad state.
So, I set out to design a boost converter that could solve both these issues. Forced discontinuous operation at all operating points, and a clean gate signal no matter what the situation is. What better way than a microcontroller?
So I ended up putting a hall current sensor on the rectified input from mains which feeds into a comparator set at a threshold that will trip at about 10A current. Also, a comparator on a voltage-set potentiometer comparing to the actual output voltage. These two digital signals are fed into an Atmega168. First, the script checks that voltage is lower than voltage-set. Next, it makes sure input current is below 10A (easy way to provide what should be rock solid short circuit protection; if the input through-current with no boost is over 10A, the boost will never even start). If these conditions both indicate "go", the controller puts out a fixed 100uS pulse into the IGBT ramping up inductor current. When the current gets back down to 10A, an interrupt fires telling the controller to check conditions again and proceed with another pulse if everything is a go, or otherwise wait until it should again go.
One important thing to note is that this system would not work if the input is not 3-phase (unless an input bus-cap were used sacrificing power factor). It would still technically work, but there would be 60hz noise since the controller wouldn't register a falling current for times when the input voltage is near zero.
Testing has shown the boost to work great at 1kV output, 8KW into a large resistive test load, with 208 3-phase input. I'm very satisfied with how it's working, and feel that it is far more reliable than a TL494 system was ever getting me. I'll test it out on a big coil the next night it's not raining.
You may notice that I tried to get Steve Ward's clever active-snubber working, which is currently disconnected. I kept burning out the switching transistor on that so I just took it out for now. Efficiency seems fine though since I'm running at a max of 7khz (although not measured, the load gets super hot and the heatsink doesn't get warm). Although 7khz seems awful, it is remarkably quiet.
Update: Ran it on BigCoil, and it works great! Maintained through 850VDC no problem where my old TL494 would sag to 500V under coil load. Also, short circuit protection worked perfect. Coil blew out at the end to a short circuit condition, and the only thing that was lost in the boost was the input bridge rectifier.
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