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HV Kit over current protection

RLetizia
Intellectual 990 points


The HV kit uses negative current feedback as described in the InstaSPIN user’s guide section 5.2.2.   This means that with increasing negative sense resistor current the op amp output goes more positive; and therefore over-current protection is based on negative sense resistor current.

 Negative sense resistor current corresponds with motor recirculating current, so over-current protection is based on motor recirculating current.

It seems like positive feedback and over-current protection based on positive current through the sense resistor would have advantages protecting the output stage.   Motor, wiring, or output stage faults that caused excessive current from the HV bus through the low side to common would be detected.

The user’s guide gives examples of using positive and negative feedback but doesn’t compare the two methods.

Are there disadvantages to using positive feedback and over current protection based on positive current through the sense resistors?  What do you think about this better protecting the output stage?

Thanks.

  • 0
    TI__Expert 5180 points

    Possitive or negative feedback only determines the sign correction you need to do in software, but for fault protection purposes, we need to protect a bidirectional current. In our HVkit we only have one side protected though. If you design your own board, I strongly suggest protecting both sides, possitive and negative. I will suggest that improvement to our HVkit when we revise it.

    For example, if transistor 1H and 2L and 3L are turned ON, the current present in the motor will be possitive Ia and negative Ib and Ic. In this scenario the fault needs to be present so that a high possitive current in Ib or Ic causes a fault. since the signs are inverted, this would cause a negative value on the output of the OPAMPS for Ib and Ic current sense, so the comparator that causes a fault needs to compare for a negative threshold.

    Now within the same PWM period, all transistors are swapped providing recirculation of currents, now the current is visible in Ia as a negative current through that shunt. Since we have the same OPAMP circuit, this would create a possitive voltage on the output of the OPAMP for Ia feedback, so the fault circuit needs to compare that output vs a possitive threshold.

    To make a long story short:

    1. The signs described in section 5.2.2 only affect how you adjust the current feedback in software

    2. Fault protection needs to current feedback vs possitive and negative values of all three phases.

    -Jorge