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DRV8701: MOSFET getting hot at higher PWM frequency

Alexandru Berghian
Prodigy 10 points
Part Number: DRV8701

Tool/software:

Hello 

I have developed motor driver board using DRV8701, everything works, however the MOSFETs are getting too hot. I want to understand why and how i can optimize this.

Application details: 

- VCC = 24V 

- VREF = 1.6V 

- I don't use R2, i only use R1 with a 68K to AVDD, so my iDrive config is 150/300mA

- Rsense = 15mOhm

- iChop is set to around 5Amps

- The MOSFET i use is WSD60N10GDN56, datasheet here. The reason for this choice is because the current product already uses this MOSFET.

- PWM frequency = 20kHz

If i use a lower PWM frequency the MOSFET does not get that hot, acceptable performance at around 5kHZ but the problem is that this is in the audible range and it's not an option for our application.

If you think that replacing the MOSFET should solve this issue, do you think BSC098N10NS5(datasheet) would be a good choice?

I suspect one of the issues could be the gate charge on the WSD60N10GDN56 or maybe my iDrive is not set correctly?

Any help is appreciated

Some oscilloscope observations:

As you can see in the image below, our motor load changes constantly as we are driving a piston. I don't know if this affects the driver in any way.

 

  • 0
    Prodigy 10 points
    Alexandru Berghian said:
    datasheet

    Figured out a part of the issue. I was driving the motor the wrong way. This message helped me to understand how you should use the PWM signal to drive it.

    To be honest, this was not clear to me from reading the datasheet.

    With the improved driving control, the MOSFET is not getting that hot anymore but i still think it can be improved, it reaches about 50-60C and stays there. Before it was triggering the thermal runway of the MOSFET.

    Any suggestions are appreciated.

  • 0
    Prodigy 10 points

    Now i'm more certain it's a MOSFET switching issue as i'm able to confirm the MOSFET works perfectly at 90-100% PWM or low PWM when very little switching happens.

    Running the calculations described in the datasheet in the guide on how to choose the external FETs i don't see an issue with the gate charge at 49.9 nC when using PWM at 20KHZ. Maybe there is an issue with a longer Td(on) and Td(off)?  

  • 0 in reply to Alexandru Berghian
    TI__Genius 10501 points

    Hello,

    Thanks for your question.

    The device expert from my team mate will review the test conditions  and get back to you.

    Thanks,

    Ibinu

  • 0
    Prodigy 10 points

    The problem is solved, it seems it was a bad solder joint and the PWM control logic.

    The other question i have is if i need the flyback diodes. I control a high torque gear motor and my microcontroller can be powered off on a switch so i can't control the stopping sequence of the motor and the motor driver would stop before the motor stops. 

  • 0 in reply to Alexandru Berghian
    TI__Genius 17680 points

    Hi Alexandru,

    I'm glad to hear the issue was resolved. 

    I would check your FET datasheet and see if they have any flyback protection in them. If they don't, then yes you will need to include diodes for some safe path for the recirculation current. 

    Best,

    David