DRV8323: Reading the BEMF

Nick,

Most BLDC motor systems have a Hi-z period before and after the each phase is driven where the BEMF of the motor can be seen if you monitor the phase switch node using a scope while the motor is running.

Some systems use this hi-z period to measure current or BEMF, some systems measure this at other times.

I can send you a traditional waveform for a sensored trapeziodal BLDC Switch node if that will help.

Regards,

-Adam

Yes, thanks Adam,

if you could that would be a starting point. 

not sure how to put the one phase into a HiZ state whilst still driving the other two phases..

Nick,

Naturally when you drive current out of one phase (A for example driven HIGH) and into another phase (B for example driven LOW), the third phase (C in this case) will not be driven high or low and will therefore be in Hi-Z.

This is a standard practice for driving three phase brushless motors using trapezoidal commutation. 

We implement this in one of our hall sensor based design, the motor phase waveforms look like this:

As you can see above, there is a period before and after the phase is being driven when you can see the BEMF increasing and then decreasing respectively.

The ramp up and ramp down surrounding the squared region are the Hi-Z periods for that phase. we drive each phase for 120 degrees of the cycle, 3 phases makes the full 360.

Does your motor have hall sensors? If so the DRV8323 can do this for you automatically using the hall sensors as input to the DRV.

Regards,

-Adam