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DRV8711: Regarding predriver fault

Part Number: DRV8711
Other Parts Discussed in Thread: CSD18543Q3A, , CSD18531Q5A, , DRV8770

Hi support team.

We have received a request from our customer to investigate the cause of the PDF (predriver fault) occurring in the market, albeit infrequently.
I have obtained the waveform data from the customer and have confirmed that noise is superimposed on the gate signal.

Could you please comment on whether this noise could be the cause of the PDF?

Best regards,

Higa

  • Hi Higa,

    I think it may be possible that this noise is the cause of the PDF fault. Recently there was a thread with links describing this problem very well

    https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1129643/drv8711-predriver-fault-in-indexer-mode

    In my case 56 Ohm LS gate resistors has solved this problem for good.

    Best Regards,

    Grzegorz

  • Hi Higa,

    I agree with Grzegorz reply. Let's start by adding the 56OHM LS gate resistors. This should help with the PDF fault.

    Regards,

    Pablo Armet

  • Hi Pablo-san

    Thank you for your advice.

    I proposed your advice to my customer and got the feedback.

    They evaluated varying gate resistances between 30Ω and 120Ω. However, it did not improve.

    Additionally, they made the following checks:

    - Increasing tDRIVE: 2us -> no improve 

    -  IDRIVEEP=50mA,IDRIVEN=100mA -> no improve

    - Check Q_G -> OK

    TBLANK : 2us

    TOFF: 50.5us

    DTIME : 450ns

    PWM frequency : 18.7kHz

    Q<267nF

    FET : CSD18543Q3A

    - PDF errors tend to occur when the load torque increases.

    Could you give me your advice?

    Best regards,

    Higa

  • Hi Higa-san,

    There are settings below that I use for my drivers, there is little chance that they would help but it might be worth of trying.

    TOFF = 31us

    TBLANK = 3,56 us

    Auto mixed decay at all times

    DTIME = 850ns

    TDRIVE = 500ns

    IDRIVE = 300/150 mA -  I use CSD18531Q5A (the same as in DRV8711EVM) which have 3.5x higher Qgd than CSD18543Q3A. I think "IDRIVEP=50mA, IDRIVEN=100mA" would be optimum or even slightly to high for CSD18543Q3A (unfortunately DRV8711 does not have lower settings).

    If I had a problem like this I would:

    1. Try to find any anomalies in voltages (LS and HS gates), H-bridge outputs, VM, VCP during increased load torque. I would check which H-bridge is causing the fault, set nFault signal as a trigger and check voltages in that bridge some 10us back. I would also check phases currents for any anomalies.

    2. Try to slow Mosfets even more by:

    - placing series resistors also for HS gates

    - increasing their value (HS or/and LS) even more (no more than lets say 220 Ohm) - that and point above may cause damage to Mosfets and DRV8711 (remember about DTIME = 850ns)

    - replacing Mosfets with ones that have higher Qgd.

    3. Review PCB layout.

    I you could tell at what current level (phase) the fault happens (rms/peak), VM voltage, motor parameters like inductance, resistance, size and speed before the fault? 

    Regards,

    Grzegorz

  • Daisuke,

    - PDF errors tend to occur when the load torque increases.

    Could you give me your advice?

    It may be possible that noise from the motor is getting coupled to the gate of the FET through the internal capacitance. The schematic below is from the DRV8770 gate driver. In this circuit, a 100kΩ resistor is placed between the gate and source of each FET to prevent accidental turn-on of the FET during switching. Try adding this resistor along with Grzegorz's recommendations.

    Regards,

    Pablo Armet

  • Hi Pelikan-san

    Thank you for your advice.

    I will propose your advice to my customer.

    Thanks!

    Higa

  • Hi Pablo-san

    Thank you for your reply.

    I will proposed you advice.

    I have a question about the motor noise that you mentioned.

    Is it BEMF? or ringing noise that caused by L(cable or layout)?

    If yes,  in case of the motor drives with constant current and rotation is constant speed at heavy load torque, I think motor noise is no change.

    Is my understanding correct?

    It would be greatly appreciated if you could provide a little more detail about the motor noise you mentioned.

    Best regards,

    Higa

  • Hi Higa-san,

    The motor noise I'm referring to is due to the fast voltage switching happening at the switching node (source of High side FET). The fast dV/dt and dI/dt occurring at the switching node can couple to the gate of the FET via the internal parasitic capacitance inside the FET. 

    Regards,

    Pablo Armet 

  • Hi pable-san

    Thank you for answering

    I will let you know as soon as I get feedback from my customer. 

    Thanks,

    Regards,

    Higa

  • Higa-san,

    Let me know if they require further assistance.

    Regards,

    Pablo A.

  • Hi Pablo-san

    I got feedback from my customer.

    They confirmed no improvement at 100k and improvement at 220k. On the other hand, going to 51k made it worse.

    Do you know anything about the reason for this?

    The high side series resistance is 1Ω and the low side series resistance is 47Ω.

    Best regards,

    Higa

  • Higa-san,

    Please see this section in the datasheet:

    8.1.2 Optional Series Gate Resistor
    In high current or high voltage applications, the low side predriver fault may assert due to noise in the system. In
    this application, TI recommends placing a 47 to 120-Ω resistor in series with the low side output and the gate of
    the low side FET. TI also recommends setting the dead time to 850 ns when adding a series resistor.

    Have they tried a larger series resistor on the low-side?  Also, they need to increase the dead time when doing this.

    Regards,

    Ryan

  • Hi Higa-san,

    Any updates from your side? I will close this thread due to inactivity. You can still reply if further assistance is needed. Ask a new related question if thread is locked.

    Regards,

    Pablo Armet