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It's just that I was warned about soldering the mosfets to the M- bar. The source legs are going to be going crazy up and down, but the new driver has 6 isolated supplies, each of which won't care about what the neighbor supplies are doing, one supply babysitting 2 mosfets. They were suggested to me by Fran specifically for high side drive. It's actually 6 homemade dc-dcs.
I think your idea about including an M+/B+ bar is a really good one, but I'm just worried that part of the mosfets failure and horribly dirty gate turn-on was because of the noise on the M- bar. I think with the gate to source having to stay in -30v to 30v, the mosfets will have a longer life if the gate turn on doesn't look like it's going from -5v to being clamped at 27v from the P6KE20A-T TVS diode at 200 MHz.
This is a guess, but I think the bad part about the capacitance on M- was noise the gate was picking up from it. the source capacitance might not be the same problem because the mosfets' gate was really really close to the radiating M- bus bar, since the mosfet back was soldered directly to it. So we are talking 1mm or however thick the isolation is between the gate inside the TO-247 package to the copper back of the package. I didn't see noise like that on the 500amp version, and I'm guessing that it's because the M- bar was "far away" from the inside of the mosfet case. There were instead solder joints and PCB tracks, and then some silver conductive epoxy, and finally the M- bus bar.
So, the only component to suffer being soldered directly to a wildly changing bar would be the diodes, but they don't have any control signals for them, and I think they are a little more robust because of it. I of course don't really know what I'm talking about, but I really really wanted to clean up that horrible gate turn on.
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