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DRV8304: Backwards EMF + Damaging the battery

Spencer Stock
Prodigy 30 points
Part Number: DRV8304

Hi.

We have a product for an electric wheelchair.

We do not necessarily want to do regenerative breaking, but when going downhill, the chair will make the motor turn... so if we go fast enough, the motor will generate a voltage higher than the battery... or even higher than all circuit in the system. We need something to protect to safeguard against that.

Would the DRV 8304 have any safeguards for this? Or would we need something external?

What would you recommend here?

  • 0
    TI__Mastermind 21805 points

    Hi Spencer,

    The DRV8304 is not going to take any action to regulate the motor (it doesn't have any intelligence built in). However this is a common problem for motor systems and there are common ways to solve this:

    • Adding more bulk capacitance at the motor will cause the voltage to rise slower during supply pumping. Obviously if you continue back driving the motor the voltage will continue to increase, just at a slower rate.
    • Monitor the supply voltage from the MCU and apply a brake state (all low-sides ON) to prevent further voltage spiking.
    • Use a comparator circuit on the supply voltage to force ON the low-side FETs through the DRV8304
    • Add a resistive braking or MOSFET braking circuit to burn the power. This circuit can be employed to “short” the motor externally and dissipate the motor power. 

    Thanks,

    Matt

  • 0 in reply to Matt Hein
    TI__Prodigy 30 points

    Hi Matt,

    Thanks for the answer here. We are specifically interested in the following system design requirements:

    * Power consumption must be minimal.

    * Size does not really matter

    Is there a solution that would have the lowest Iq/power consumption?

    Do you have a recommendation given these requirements?

  • +1 in reply to Spencer Stock
    TI__Mastermind 21805 points

    Hi Spencer,

    The optimal solution from a power perspective is monitoring the supply voltage in the MCU and enabling the DRV8304 from sleep mode in the event that the power supply increases beyond a set threshold. This way DRV8304 does not need to be ON the whole time. In regeneration mode the DRV8304 actually gets powered by the motor (generator) and not the battery itself.

    • Monitor the supply voltage from the MCU and apply a brake state (all low-sides ON) to prevent further voltage spiking.

    Resistive or MOSFET braking with an external component does not require additional power consumption per se, but you do need to size the external brake to be able to dissipate all of the motor power.

    Thanks,

    Matt