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InstaSPIN FOC, F28027F: Possible to compensate for Miller Plateau in the inverter MOSFETS?

stomp
Genius 4675 points


Hi,

I've buit a BLDC drive using InstaSPIN FOC on a F28027F device.

I've built the inverter stage using a combination of an Infineon 6EDL04I06PT 600V Gate Driver and 6x Fairchild FCMT299N60 12A MOSFETS.

While the design is ultimately for a 0.5KW BLDC motor, I am bringing up the board at 32V with the appropriate software changes.

Now to the problem.

I have noticed that the HIGH SIDE MOSFETS on each phase do not turn off quickly. Indeed there is a 2.6uS delay between when InstaSPIN's PWM commands the FET off and when the FET actually switches off.  This is related to the gate charge (MIller Plateau?) and this causes the smoke to exit the FETs.

The top trace in the image below shows what I am seeing. The first falling edge at about time 6us is when the InstaSPIN switches off the PWM signal into the gate driver, the second falling edge is when the FET actually switches off. As the DC bus voltage increases, the pulse width from InstaSPIN decreases until the time both FET's overlap and smoke appears.

The bottom trace shows the phase to the motor, indicating that power is still applied until the gate voltage is about 0.

Question:

Is it possible to adjust one side of the PWM's in InstaSPIN to compensate for this?. I know the charge changes with electrical conditions, but I am trying to evaluate the hardware I have without having to make massive modifications at this stage.

Thanks

Stomp!

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