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High side Vgs oscillating back to 0V in miller region
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uer166:
Trying to design a 3-phase BLDC motor controller. Tinkering in simulations with one half-bridge switching leg, it seems that the high side Vgs, during turn on, turns on, blows past the "normal" miller region that I would expect to see, and then goes to -8V, and recovers with some oscillation. I'd like to see a textbook, flat miller region in this design, and I'm not sure what is causing this.

I thought about:

* The reverse recovery time of low-side body diode causes large current to shoot-through, somehow reducing high-side  FET's Vgs?
* dV/dt induced turn on of lower FET? But during the high dV/dt region, the lower FET's Vgs also goes negative

I'm not sure if either of those effects can explain the issue. I've tried adding anti-parallel diodes to gate resistors for harder turn-off. Tried slowing switching times via larger gate resistors, adding a snubber, using both logic-level and standard level FETs. Fundamentally though, I want to understand what's happening and not make it work with  trial-and-error. The current out of the half-bridge into inductor is about 20A in the simulation. Switching frequency is 25KHz, although that wouldn't matter for the edges.
uer166:
On the Nth look, looks like a hard low-side body diode recovery is causing the oscillation. But still no idea what would fix this, apart from a FET with softer recovery.
mvs:

--- Quote from: uer166 on January 24, 2019, 08:24:54 pm ---On the Nth look, looks like a hard low-side body diode recovery is causing the oscillation. But still no idea what would fix this, apart from a FET with softer recovery.

--- End quote ---
No, its bounce from inductor L2. As long as low side switch U2 is conducting, there is current flowing throught L2 (V5 - U2 - GND - U1 - L2 - V5). If you close U2, SW node gets bounced. Since high side MOSFET U3 has some gate-drain capacitance this bounce influence gate-source voltage.
You can leave U3 closed and it should not change anything.
uer166:
I don't think so, V5 isn't supplying any current, 20A is flowing *into* V5, not out of it. It's there to simulate BEMF of a motor. In the scope trace, the captured moment is when:
* U2 turns off
* Some dead-time: current flowing though U2's body out of GND into V5 and the inductor
* U3 turns on
* High current spike into U2 from U3's turn on, because body diode needs to recover
* U2's body diode recovers hard, (100A->0A in single-digit ns)
* Such fast dI/dt does weird things
....Or at least that's my theory, unless I'm woefully misunderstanding it.

Can you explain what makes you think it's L2's fault? L2 is a very very large inductor simulating motor winding that can't really cause such high frequency oscillation I think.

edit: wrong refdes..
mvs:

--- Quote from: uer166 on January 24, 2019, 10:55:10 pm ---Can you explain what makes you think it's L2's fault? L2 is a very very large inductor simulating motor winding that can't really cause such high frequency oscillation I think.

--- End quote ---
Voltage at SW node gets higher then you supply voltage, so there might be some inductance in play. It might be not L2, but some parasitic inductance.

I have tried to simulate your circuit in LTSpice now, and have found no issues. It works just fine. The only difference was another MOSFET type.
Where you have this BUK7Y1R7 model from? It may include some lead inductance, that another models do not have.
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