Hmm negative thermal coefficient, That probably explains why IGBT's can be put in parallel without instabilities. This water heater board was designed for two in parallel but they were too close together for proper heat dissipation. I think that is why he left the second one out of the final product.
The vast majority of the parts on this board are for the MPPT. It derives its logic voltage from the PV voltage so there are also some parts for the low voltage power supply. The high side is just an IGBT, nothing else. There isn't even a dedicated gate driver, just a resistor fed by two transistors (in push-pull I think) for turning the gate off and on. The collector goes straight to the output lug. The seller advises keeping the wire between the board and heater under five feet. I'm guessing this is to reduce radiated noise.
Looks like a perfect application for an Arduino. I think I would splurge an extra three bucks for an optically coupled gate driver though, like the HCPL3120. Logic and high voltage/power should live in separate rooms.
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