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INA185: Problems measuring negative currents with INA185

Carl Jensen
Prodigy 30 points

Part Number: INA185
Other Parts Discussed in Thread: DRV8313

Hi there,

 

BACKGROUND:

I am having some trouble with the INA185 sensor that I use to sense the currents through a PMSM phases (phase A, B and C) as well as to sense the dc bus voltage. I have configured the sensors with a reference voltage Vref of 1.65V and VS of 3.3V and they are placed in a low-side topology (see top figure). Along with sensors, I use a DRV8313 that manages the PWM switching (see bottom figure). In a previous post (link) we discussed the need for a bidirectional sensor. Vm and the currents are quite low (Vm = 9V and currents is below 300mA) on a quite small motor wye-wound PMSM .

 

PROBLEM:

When I PWM the motor with a fix frequency with a duty cycle of 50% on phase B and phase C and 10% on phase A (just to test) I can measure positive currents through the motor. However, when I instead, on phase A, apply a duty cycle above 50%, the INA185 will not output anything lower than 1.544.  

 

WHAT I HAVE TESTED:

I have checked that the motor does in fact experience a negative current in the oscilloscope by measuring the current through the DRVs output to the motor. I have also tested with various switching frequencies on the PWMs (2kHz to 10kHz). The lowside topo

 

Do You have any ideas on what the reason behind this problem?

 

Best regards,

Carl

  • +1
    TI__Mastermind 35620 points

    Hi Carl,

    It is likely that the motor has a short decay constant (L/R). The largest negative current happens right after PWM switching but this will not be captured by INA185, or any current sensor for that matter. Only after some time will INA185 be able to catch up with the actual current, but by this time the current has already decayed exponentially to a level that is much smaller. The situation is opposite for positive current where the largest current happens toward the end therefore INA185 has time to track.

    To check the circuit is able to respond to negative current, you could apply a load to the motor. You should see the INA185 output low level decreases somewhat. A changing output indicates that the circuit is functioning fine.   

    Regards, Guang