Quote:
Originally Posted by MPaulHolmes
The purpose of the isolated supply is to power an isolated gate driver, so it has to have tiny capacitance since it's going to also be the supply for high side igbts.
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Paul, what is the power budget that you have for the output side of the isolation supply? I'm guessing since the IGBT's are off board from the driver that you have quite a bit of inductance? Any idea what power you need to overcome it? Also, are you going to to make it a dual output so you get a negative voltage to help turn off the IGBTs?
The output side of those MIC4451 (assuming that is what you are still using) have 12A output stages, I'm assuming to help aggressively drive the IGBT inductance/capacitance.
Its amazing that you are going this way. I was reverse engineering a Mitsubishi Air Cond which has an ACIM compressor. The isolation was done with a transformer with 4 isolated secondary windings (one transformer, 4 outputs). I thought AH HA, but then realized we didn't have AC to drive it. I like Fran's idea of driving it with a oscillator, but its going to be noisy and inefficient. The MAX 250/251 use something like what Fran mentions. The primary side switches at 200khz. The transformer for that one is something like the 470-1005-ND @ Digikey.
Sorry, didn't mean to design over your shoulder, just trying to help fill in some data. The key to designing a good secondary stage is going to be developing a power budget for the opto-isolators and the IGBT driver element, including the cap/inductance budget. I'm here to help.