When you close a switch you're stopping current from flowing through an inductance (in the context of this thread, a parasitic inductance). If you sit down and draw your power bridge's schematics of the power paths, including the (parasitic) inductors on all connections, and do the exercise of simulating the current flow for both PWM input signal states, you'll see where you are cutting the currents.
trialspower2 wrote:Currently I have high side pwm (with synchronous rectification) with the low side switched on for the duration of the cycle. Am I not cutting the current when the high side switches off?
I guess you mean with the bottom side switched OFF, or that would be a short. Synchronous rectification means top FET on & bottom FET off -> top FET off & bottom FET on and so on. When you close the top FET, current stops flowing through it and starts flowing through the bottom FET's body diode, for the duration of the dead time, then the bottom FET opens and current moves from the diode to the channel. This change in path cuts the current through the parasitic inductance between the exit point to the motor's phase and the top FET's source, causing it to spike the voltage (then the ringing comes from resonance with FET's parasitic capacitances).
trialspower2 wrote:and the synchronous rectification switches on to allow the current to flow without it going through the low side diode?
The red part makes no sense, "synchronous rectification" is a technique's name and not some kind of switch.
You'll gain a big understanding of all this by doing the exercise I mention above. This stuff is much easier to see/explain with a drawing than by words. A thread to help: https://endless-sphere.com/forums/viewt ... 30&t=55641