trialspower2 wrote: ↑
Nov 04 2018 6:02pm
I am thinking this might be due to the low amount of capacitance in the FET's, and may be fixed by adding addition capacitance across the gate/ source to stabilise the voltage difference? And then go for a lower gate resistor value?
The ringing is likely not real; the FET isn't seeing it. You are seeing it due to poor probe connection. If you doubt this, keep your current connections and probe a ground plane somewhere else on the circuit, or a signal line known to be at ground potential. I bet you see a similar ringing.
1) The probe/scope should be isolated to reduce the amount of ground current you are returning.
2) Differential probes are ideal for applications like this.
3) If you are using regular probes DO NOT use the grounding leads that come with them. You must have the probe and its ground ring touching the device directly. They make sockets to accept the probe tip that helps with this.
4) When working with designs like this you need a current probe. They are expensive but are critical. Remember, a motor doesn't care about voltage; it cares about current, and switching current is the #1 job of the controller.
I assume that blue is gate and yellow is phase voltage. If so your gate drive is now way too slow.
One of the reasons I know that that ringing isn't real is that the gate voltage changes and the phase voltage changes are in phase. If they were real (i.e. if the gate were really going on and off that fast) the phase voltage would show the opposite response - phase voltage would get closer to zero as gate voltage increases.
The other thing I was thinking about was making a two stage driver which ramps upto 10v slowly then switches on hard. I think this would reduce switching losses and make ringing more difficult??
You are trying to solve the wrong problem. Get good measurements first.