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Grounding

Neil Warnock
Prodigy 40 points
Other Parts Discussed in Thread: DRV8824

Hello TI motor experts!

I was doing some testing on the DRV8824 evaluation board where the logic is supplied via the USB lead and the motors from a 24V PSU with its 0V connected to mains ground. I noticed that the current in the +24V motor supply line did not match that in the motor supply return. It seems that quite a lot of the motor return current is flowing back down the USB lead and presumably then goes through the PC mains ground connection back to the PSU. Isolating the PSU 0V from mains ground balances the current correctly.

If I design a motor driver board which is supplied with a logic 5V and a motor 24V from the same PSU (with a common, grounded 0V) should I keep the two 0V rails separate on the circuit board (two ground planes)? If I don't do that then I think that there will some motor current flowing back down my logic 0V to the power supply.

Any advice on this?

Many thanks.

Neil Warnock

  • 0
    TI__Expert 8885 points

    Hello Neil,

    In the first experiment with DRV8824 EVM, what you observed  can happen.  Since PSU 24V-return signal and PC (which supplied VDD for MCU and logic inside EVM) were connected to mains ground.  Due to this, a parallel path is created for motor current to be return through PC GND and back to PSU, which you observed.   So in this case, isolating either PSU or PC would solve the issue.

    Coming back to actual motor drive board, it’s good to keep Logic GND plane and Motor Power GND plane separate to avoid motor drive noise corrupting Logic signals. However since DRV8824 has only one GND for both logic and motor power, you will have to short both GNDs eventually. If logic 5Volt supply is derived from 24V PSU input than both 5V VDD and 24V grounds planes can be connected together at DRV8824 GND point to form star grounding. In this case, connecting 24V return to mains ground will not be an issue because mains ground is connected only to 24V PSU.

    If 5V logic supply is derived from separate source other than 24V PSU and by chance if this source is also connected to mains ground than you can end-up in similar situation which occurred with EVM. In this case, I would recommend either isolate 5V logic supply GND or isolate PSU GND from mains.

    Regards

    Milan Anand Rajne

    System Application Lead-Motor control

     

  • 0 in reply to Milan Rajne
    Prodigy 40 points

    Hi Milan,

    Thanks for that helpful reply.

    In fact in the application where this is to go the PSU has 5V and 24V outputs but both the 0V returns are already connected together and connected to mains ground at the PSU. This design is for a replacement board so I am unable to change this :-(

    Do you think that if I isolate the grounds on my pcb - connect the DRV8824 GND to the 24V return and the microcontroller / driving logic's GND to the 5V return - that this will be ok? I am worried that the return path for the control signals from the microcontroller to the motor driver is then via the 24V return back to the PSU and then back up to the logic GND and this may cause motor noise on the control signals.

    Best regards,

    Neil

  • 0 in reply to Neil Warnock
    TI__Expert 8885 points

    Neil

    There is no separate signal and power GND for DRV8824. The two GNDs pins on DRV8824 serve for both power and signal grounds. In the scenario you mentioned, there is only one GND connection can come out from DRV8824 PCB and this will carry both power and logic signal currents. You can connect it to 24V ground at PSU. As long as you provide good copper area for ground at DRV8824 PCB and make sure minimum ground impedance drop between DRV8824 PCB and PCU, you won’t face any issue. Reducing the GND impedance between two boards will make sure that motor current  noise does not corrupt logic sigals.

    Best Regards

    Milan