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MCT8316Z: Issue with VM pins

Patryk Szczesny
Intellectual 290 points
Part Number: MCT8316Z

Hi TI Team,

Recently I have found a problem with the custom boards based on the MCT chip.

Previously we had a motor running without any issues using the same layout and software. It was a max 3000 RPM motor, using 24V as a reference input voltage to the system. The issue we have found was that when we assembled two new boards and attached both Hall effects and the motor phases and successfully uploading code. When we powered the system, all outputs from the MCT chip were current, both  the reference voltage and the buck regulator voltage were at 3.3V. 

When a trigger input was applied the motor spun, seemed like it was working without any issue, spinning in the right direction and speeding up due to a PID controller. When it ran for about half a minute more the area near the VM pins (MCT input pins) burnt/got damaged. Rendering the MCT chip useless, but only that one component had damage on it, the rest of components onboard where fine. Although the pads where the VM input pins were had been permanently damaged. 

Edit: I was informed that this happened during load testing on the motor. When a load was applied on the motor, the motor had difficulties speeding up and eventually it lead to the VM pin damage. FYI the application of this is a power tool.

Did this issue ever happen to anyone else you know off? What might the possible cause for this? Let me know if you need any more details from my side.

Regards,

Patryk Szczesny.

  • 0
    TI__Mastermind 18981 points

    Hi Patryk, 

    Thanks for posting to the motor drivers e2e forum - 

    I've assigned this thread to a member from our team, and we'll aim to provide a response before end of the week. 

    Best Regards, 
    Andrew 

  • 0
    TI__Mastermind 24415 points

    Patryk,

    Can you do a visual inspection on the burnt PCB and locate the pins on the IC that got damaged? I suspect high voltage event on the IC. What is the VM voltage applied to the device?

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Intellectual 290 points

    All three VM pins got damaged while the event occurred. The supplied voltage to the circuit was 24V.

  • 0 in reply to Patryk Szczesny
    TI__Mastermind 24415 points

    Patryk,

    Overloading the motor can cause the motor to draw high current but we have ILIMIT and OCP protection in MCT8316Z which should eventually shut down the device. A high voltage event can occur when the motor is decelerated at very high deceleration rate. Can you try to re-create this issue and monitor the voltage on VM pin?

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Intellectual 290 points

    I'll haver to assamble one more board for that to happen. I'll do that in the coming days for sure. Also I'd like to update you on another thing I was told when the test was carried out. 

    The motor was started when it was against the surface of the test station, being slightly pressed in with give or take 4 pounds of pressure, then the motor was turned on. The power supply that we used gave an average current reading of around 7A being used although I belive the duty cycle was too small to even move the motor since the tester mentioned that it tried to catch RPMs.

    Another detail to add, I have found just a while ago that when testing for any shorts, I found out that the GND and all three motor phase wires are shorting.

    Does that tell you anything else? 

  • 0 in reply to Patryk Szczesny
    TI__Mastermind 24415 points

    Patryk,

    I believe the 7A is drawn due to the short across GND and motor phases. When the IC receives speed command, it starts to commutate by turning on the internal FETs. When there is a short across motor phases and GND, huge current will flow from the VM to GND and this explains why 7A is being drawn from the supply. 

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Intellectual 290 points

    Hi Vishnu,

    The short between the GND and the motor phases wasn't there before we began testing the motor. In fact, without the load the motor was spinning fine and well, without using more than 1A. It's when the load was applied to the motor the board got damaged and that short has occured. 

    We tested a random ESC we had laying around in the office too, when the motor was put under the same load the current went up to 7A aswell and continued to operate around that current level. 

  • +1 in reply to Patryk Szczesny
    TI__Mastermind 24415 points

    Patryk,

    Thanks for the details. 7A is very close to the abs max rating of this device. I would not recommend using this part with 7A load current.

    Regards,

    Vishnu

  • 0 in reply to Vishnu Balaraj
    Intellectual 290 points

    Hi Vishnu, 

    Thank you for the quick responce. I'm already working on a design based on the DRV832 family of smart gate drivers, giving us more room when it comes to current flow.