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DRV8873H-Q1EVM: DRV8873H is entering thermal shutdown long before overcurrent is tripped when motor is under load

Chris Crosby
Prodigy 50 points
Part Number: DRV8873H-Q1EVM

With the default 20KHz PWM frequency, I have a motor that is turning a wheel fine at no load (about 0.4A nominally) in one direction.

When we apply resistance to the wheel to test for our 3.5A overcurrent threshold, the motor driver gets very hot very fast, and stops from a temperature fault at less than 2A (peak current, as measured with a current clamp and oscilloscope. I would expect with basically the whole eval board as a heatsink that the part would be able to easily handle 2A without overheating.

We will try a lower PWM frequency and a heatsink, but it still seems to me that according the spec sheet and RDS on resistances, the driver should not be getting that hot at 2A.

The whole point of using a small part was to save space, and the need for a large heatsink defeats that purpose. 

Would appreciate some guidance or advice on resolving on this behavior.

Chris-

 

  • 0
    TI__Genius 13455 points

    Hi Chris,

    You're right, this does not sound normal.

    Can you confirm your input voltage and jumper settings?

    I can run some quick tests here using your settings.

    If your configuration checks out, one thought would be a solder reflow issue causing poor contact between the thermal pad on the device and the exposed copper on the PCB.

    Regards,
    Mike

  • 0 in reply to Michael Erdahl
    Prodigy 50 points

    Hi Mike,

    Thank you for the speedy response.

    So, yesterday, we had the issue with a dozen or so experiments, but today with exactly the same setup, we are unable to recreate the fault with a dozen more experiments. I rechecked it today so I could provide accurate information for you.

    We are using the DRV8873SEVM kit straight out of the box, in PWM mode with all other settings at default values. We are using a 24V linear DC supply capable of up to 7A to provide the motor voltage, and are using the USB and dev GUI for adjusting the PWM. We had it set at 80% PWM for IN1 and 0% for IN2.

    I put my finger on the part to make sure it was a real temperature problem, and yesterday it got too hot to touch in under a second when we loaded the motor, but was only slightly warm when the motor was unloaded and we were at 0.4A.

    Today, we can load the motor down much harder, to about 5-6A (measued via the current clamp and oscope)  before it gets too hot to touch, which is more inline with how I expected it to behave based on I2R calculations for Rds on and current. We are not seeing the temp fault today since we are removing the load when it gets too hot for my index finger on top of the IC. It takes it 3-4 seconds before it reaches the pain threshold on my finger.

    The only thing I can postulate is that maybe it soldered itself down better on the thermal pad with yesterday's experiments? We'll keep experimenting to make sure it's not intermittent or a fluke, but today, it's working great!

    I'll get back on this thread if the problem creeps back up.

    Thanks!