harrimansat

If this is with the IGBTs in place, then it's very hard to decide whether a spike on the scope is real or not.
If it's without the IGBTs in place, it's still challenging, with fast changing signals everywhere. I find I get the least worst results when using the spring earth contacts to get a really good local earth for the scope probe. Something like this:

The spring part attaches to the scope probe earth area, and you have to carefully manoeuvre the point of the spring to a good local earth / reference point. It can be a real pain, and of course, so few times will you be able to earth the emitter of the IGBT for the measurement. There is a bit of skill to this, almost an art. I'm no artisan here.

Ah. I'd say that's D40 on this schematic:

HFPW+ (High Frequency PoWer +) is a square(ish) wave power connection from the main power supply. Rectifying and filtering it produces +12 V. The main power supply is a flyback design; these are intended to provide power in only one "stroke". The "intake stroke" ("induction stroke"?) fluxes the multi-winding inductor; during the "power stroke", the inductor de-fluxes and pushes current into the loads. D20/D40 is intended to ensure that power only flows during that "power stroke".
TX7 provides isolated power for the IGBT drivers: the two high-side full-bridge IGBTs, and the third drives the three BUS- referenced transistors: two low-side full-bridge transistors, and the buck transistor. Each winding provides ≈+15 V and -5.6 V for the gate driver output side.