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DRV8432 General Question on Heat Buildup

John Cole
Intellectual 340 points
Other Parts Discussed in Thread: DRV8412, DRV8840

I have a general question on the DRV8432.  We're using this in a dual full-bridge setup and our team is wondering if lowering the PWM frequency will somehow reduce heat generation.  Wild shot, I know, but right now we're running at about 10kHz on the PWM frequency and we can lower it significantly to 1,000 or maybe even 200 Hz.

Thank you for your help!  

John

  • 0
    TI__Mastermind 21320 points

    Hi John,

    Yes it is possible that by lowering your PWM frequency you could improve your thermal performance. This would be due to a decrease in the switching loses of the device. The dangers of this is that in the <20kHz range you can sometimes end up with an audible whine/noise from the motor. 

    -Nick

  • 0 in reply to Nicholas Oborny
    TI__Guru 55615 points

    John,

    Please be cautious on the DRV8432 going below 10kHz as this device uses a bootstrap architecture and needs to be periodically replenished in order to maintain a proper Vgs on the high-side transistor.

    This is discussed in the datasheet on the top right of page 11.

     

  • 0 in reply to Ryan Kehr
    Intellectual 340 points

    Ryan,

    Thanks.  I actually struggled with PWM problems with the same device a few months ago, and I was told the following on the forums:  

    "For applications with lower than 10kHz switching frequency, and not to trigger BST_UVP protection, a larger bootstrap capacitor can bused (eg 1 uF cap for 800 Hz operation).  When using a bootstrap cap larger than 220nF, it is recommended to add 5 ohms resistors between 12V GVDD power supply and GVDD_X pins to limit the inrush current on the internal boostrap circuitry. "

    We increased the capacitor size to 1uF, but did not add a 5 ohm capacitor, and I just burned out the board operating at about 500 hz.  Could this have caused the burnout?  (Kicking myself now for not doing that).  

  • 0 in reply to John Cole
    TI__Guru 55615 points

    John,

    Yes...that is basically paraphrasized from the datasheet (top of page 11).

    Damage to the DRV8412 can result without this series resistor to limit the inrush current to the cap.  The internal metal structure on the bootstrap lines will be damaged. 

    Just FYI, we do offer charge pump based devices like the DRV8840/42 that can handle lower frequency operation and higher currents.

    http://www.ti.com/lit/ds/symlink/drv8840.pdf

    http://www.ti.com/lit/ds/symlink/drv8842.pdf

     

  • 0 in reply to Ryan Kehr
    Intellectual 340 points

    Ryan,

    I think we'll stick to a higher frequency.  It's probably too late in the game to change the hardware around much.  

    I think I could get some extra resistors in place though, and I'll be re-designing the footprint in a day or so.  What wattage resistor should be used; ie what's the magnitude of the inrush current?  We're operating at around 12V or less.  

    Thanks Ryan!  Best,

    John

  • 0 in reply to John Cole
    TI__Guru 55615 points

    John,

    It is <50mA of current, so a standard 1/10W 0603 or 0805 surface mount component is acceptable. 

     

  • 0 in reply to Ryan Kehr
    Intellectual 340 points

    Ryan, thanks!  Is this the correct configuration?