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DRV8312 Control and Heat Sink Questions

Gregory Powanda
Prodigy 145 points
Other Parts Discussed in Thread: DRV8312, DRV8332

1/ I am using the DRV8312 with a Delfino F28335 to drive a high speed bldc motor (> 60.000rpm). My application needs to stop the motor as fast as possible. I know there are several techniques to stop a BLDC motor but I am not sure the one to use without damaging the power stage of this driver. Is simply disabling the output by pulling up the RESET_x is OK and will not damage the chip ?

2/ By reading the datasheet of the driver DRV8312, it is not clear if I need to add a dead-band on my PWMs when I use a block commuted control or if the dead time is already added to the driver (Ref SLES256B, electrical characteristics, tDT = 5.5ns)

 
3/ Is there any application note / design guideline for a heat-sink of the DRV8332 ?

  • 0
    TI__Mastermind 21305 points

    Hi Gregory,

    1.   As you mentioned there are several techniques to stop a BLDC. The main source of damage to the power stage is an increase in the supply voltage that can exceed max ratings while braking. I would not recommend a dead stop without protection circuitry.

    Some techniques include:

    • Dynamic braking circuit to stabilize the voltage supply
    • Voltage supply clamping circuit
    • Deceleration limits (what we generally utilize for our EVMs)

    2.   The DRV8312 has a built in minimum dead time to prevent shoot through. Additional dead time can be implemented in software.

    3.   See the following links and you can search the forums for additional posts.

     

  • 0 in reply to Nicholas Oborny
    Expert 1580 points

    Hi Nick,

    I have couple concern to follow Greg's question. In my current design I am using a DRV8312 and I also want to implement a faster stop. 

    As datapoint, I have taken couple precautions to protect my driver against overvoltage :

    1. I have protected the regulated GVDD against overvoltage by using a schottky and a coil in line, it seems to do its job.
    2. I have installed on each motor phase a bidirectional TVS diode rated at my working voltage PVDD (24V). 
    3. I use OC latching shutdown mode on my driver..

    I can stop my motor pretty fast by using a controlled stop with the PWM or by setting Hi-z on MOSFET output (RESET-x pulled high) but I would like to stop it faster.

    The techniques listed in your answer mentions a dynamic braking circuit which if I understand correctly would pull PVDD down through a braking resistor ?  I have attached a hand written schematic to show the concept I have in mind. Am I on the right track ?

    Thanks for your help

  • 0 in reply to Cedric 40627
    TI__Mastermind 21305 points

    Hi Cedric,

    You are close. Here is section of the schematic from our Stellaris Motor RDK board that has such a braking circuit. Appropriate sizing of the power resistor and power FET are key or you will end up with a melted mess.

  • 0 in reply to Nicholas Oborny
    Expert 1580 points

    Hi Nick,

    First of all, thanks for the detailed answer. It's always a pleasure to work with you guys from TI. 

    I will give a shot with a similar approach to see if it has a better braking performance. I will re-evaluate the values of the MOSFET/resistor based on my application.

    If I understand correctly, the braking is only active when the back emf increases "significantly" VMOTOR. In that case the MCU monitors VMOTOR and detects when to trigger the brake. The role of the resistor is to pull down the power supply for a short period of time, correct?

    Thanks

  • 0 in reply to Cedric 40627
    TI__Mastermind 21305 points

    Correct, you can monitor VMOTOR wih a simple resistor divider on it. Generally, we add a small capacitor to the low side of the resistor divider for some filtering.

    The power resistor acts as a dump for all of this energy that is suddenly being pushed back into the system (your motor acts as a generator). Without proper braking techniques you will see this energy rush back into the bulk capacitance or power supply pushing up the voltage rail.

    Happy to help! 

  • 0 in reply to Nicholas Oborny
    Expert 1580 points

    Hello Nick,

    I am about to evaluate a variant of the schematics you send me. I had one more concern regarding the driver when using dynamic braking.

    Basically I implement a deceleration  "ramp" stop by controlling my PWMs to a 50% duty cycle until the motor reaches a complete stop. I achieve good results and observe a b-emf of ~3V above 24V but as I mentioned earlier I would like to evaluate performance with a dynamic brake added to it.

    When engaging the braking resistor, when I detect b-emf above threshold, should I make sure that  the outputs of the driver are first off  (hi-Z) ? Or it doesn't really matter (that's my guess) ?

    I also plan to use dynamic brake in case of e-stop (driver fault, oc,ov, etc.) and I know in that case outputs are hi-z.

    Thanks again for your help.

  • 0 in reply to Cedric 40627
    TI__Mastermind 21305 points

    Hi Cedric,

    Since this is a 3-phase motor, I believe the best case is to Hi-Z the outputs when engaging the braking resistor. I would experiment as I am not completely sure here as to what gives the best performance. Let me see if I can't find out anything.