With the three phase outputs coming off the bottom of the board (top in this picture) rather than the side, they don't all pass over the Phase A current sensor, messing with its reading. With this new arrangement, the coupling between phases is much less. Here's the before and after:
This quick fix brings the coupling down to tolerable levels for moving on to other testing. I'm amazed at how close the actual gains are (~15LSB/A) to what the first coupling matrix came up with. I guess linear algebra does work. With these gains, the maximum phase current that can be sensed is about 100A, so if I make it past that mark I will have to reconfigure the traces for a less sensitive current measurement. At that point, I will need bigger wires anyway.
I'm still leaning towards a return to the ACS714 integrated-conductor Hall effect current sensors, but before I give up on the through-the-board sensors, I will also try shielding the wires and/or sensors in such a way that they can only pick up the field from the traces passing under them. Then, I may be able to re-route the wires the way I want, which is off the side of the board.
The next milestone was a light load test on a single phase with a current-limited bench supply. Using the SepEx motor field as a giant thermal sink, I ran one phase for 10 minutes at about 350W (48V/7.5A in, 24V/15A out) with passive heat sinking only. No problems passing this test. The temperature reading was clean and the rate of temperature rise on the sink was a mere 0.5ºC/min. Ballpark thermal estimates (the only type I can do) suggest this is reasonable for such a light load.
At this point I'm past the bench supply stage and the next thing to do is run off a battery. If destruction is to occur, this is the point at which it will happen. Shoot-through on a power supply is survivable, but with a large battery pack ready to shove in hundreds of amps, almost any failure is a catastrophic one for the controller. I will also need a suitable load, since the SepEx was already pretty unhappy after sinking 350W for 10 minutes. The next target to hit will be about 2.5kW, in full three-phase operation. Perhaps I need something that cools itself..........