Saturday, February 21, 2009
Despite the recent snowfall and subfreezing temperatures, I have managed to stay relatively productive. The most recent developments (as of last night) are a little more progress on my motor controller design. So far, I've completed the isolation circuit for the IGBT driver. I'm using an optocoupler between my PIC's output and then input of my driver IC, and a 5v to 15v isolated DC/DC converter to power it. I'm using a resistor to return the driver to a logical low after the PWM pulse from the optocoupler. All things considered, I'm pretty proud of the circuit. It allows for full isolation (a few kv, need to review the datasheets to be sure) between the high voltage and low voltage systems, while still requiring only one low voltage input and without tapping the high voltage to power the driver circuitry. This keeps the parts count minimal (6 ICs including voltage regulators and stuff) and the price down. The most expensive part on the driver board cost me $8.
Right now I'm trying to come up with an internal precharge circuit. I want to avoid a high voltage tap across the contactor if at all possible. Right now what I have in mind is a circuit that disconnects the caps from the high voltage while the contactor closes, and then reconnects them through a MOSFET or IGBT once it's juiced. That way, the solid-state part of the circuit would take the brunt of the charge current and minimize arcing. Still working out the details.
After that's all taken care of, I'll be working on designing the PCB. I'm hoping to have only one board for the whole controller. This should be possible if I use the same board (with adequate gaps for isolation, of course) for the high voltage cap circuit as well as the low voltage driver circuit. If I do this right, I'm pretty sure I'll only need one internal connection outside of the PCB- the one that goes to the gate of the IGBT. This means less opportunity for failure, more vibration resistance, etc.
Anyway, I'll be doing my best to get this thing ready to test while I'm home over spring break in about 3 weeks, so I'll be staying busy. Stay tuned!