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Original question:

DRV8711: Pre-driver fault occurs everytime

DRV8711: Pre-driver fault occurs everytime with CSD88539ND

Ilya Maximov
Prodigy 10 points
Part Number: DRV8711
Other Parts Discussed in Thread: CSD88539ND, CSD88537ND

I use DRV8711 with IRF7351in custom board (motor current up to 5A, supply voltage up to 48V). Then i try change mosfet to CSD88539ND in same board, but now I am constantly getting Predriver Fault when the supply voltage is greater than 30V. 

No recommendations from this forum have yielded results. The transistors are very similar in characteristics. Which of their parameters can affect this error?

  • 0
    TI__Mastermind 37683 points

    Hi IIya,

    Thank you for posting to the forum.

    Can you provide a waveform of the V_GATE, V_Source, and nFAULT on one of the FETs experiencing the pre-fault? Make sure to trigger on the nFAULT edge fall.

    Have you tried using different Idrive setting or T_drive settings? What Idrive and T_drive are you currently using?

    Have you tried replacing the MOSFETs with new ones?

    This app-note has useful information about gate drive. Section 2.2.2 describes the gate-fault detection. In summary, predriver fault will trigger when the Gate voltage does not rise after T_drive. In most cases, this occurs due to the a short or a faulty MOSFET. In some other cases, the fault may trigger if the I_drive or T_drive is not configured properly. Section 7.3.8 of the DRV8711 datasheet explains the process for configuring the predrivers. Comparing the gate charge of both FETs, the Q_G of the CDS MOSFET is much smaller. I suggest increasing the T_drive and I_drive.

  • 0
    TI__Guru 56650 points

    ,

    There is a big difference between IRF7351's 24nC Qg and CSD88539ND's 7.2nC Qg. 

    Please reduce the predrive peak source current setting 3 or 4 time less than IRF7351.

    Regards,

    Wang Li

  • 0 in reply to Pablo Armet
    Genius 3590 points

    Hi Ilya,

    I would just add a few things to all that Pablo and Wang said.

    CSD88539ND has very low Qgd for DRV8711, I would try to set the lowest possible Idrive settings ie. 50/100mA. For me LS gate resistors of 47Ohm fixed most of problems, in your case 100Ohm may be a better choice + dead time 850ns. 

    CSD88539ND seems to have a bit high Rdson for 5A unless for short duty cycle, CSD88537ND might be a better choice.

    Regards,

    Grzegorz

  • 0 in reply to Grzegorz Pelikan
    Prodigy 10 points

    Hello.

    Thanks for the help.

    I have studied Gate-Fault Detection conditions. And i don't understand why low Qgd is bad? Why FETs with 40nC will show no nFAULT while FETs with 10nC assert the nFAULT more easily?

  • +1 in reply to Ilya Maximov
    Genius 3590 points

    Ilya,

    It's all about switching speed ie. slope of output voltage or current, usually it's measured as a time from 10% to 90% or vice versa and for average circuit desirable switching time is 100-300ns. For Mosfet with Qgd = 1.1nC and for 100mA of gate current you will get switching time around  Tswitch = 1.1 / 0.1 = 11ns (usually it's slightly longer than that) what will bring very high dv/dt and di/dt. High di/dt causes high voltage undershoots/overshoots and ringing which can cause all kinds of errors and blowing DRV8711 or/and Mosfets as well. You can read a bit more about IDRIVE setting in the link below.

    https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/796378/faq-selecting-the-best-idrive-setting-and-why-this-is-essential

    Regards,

    Grzegorz

  • 0 in reply to Grzegorz Pelikan
    Prodigy 10 points

    thank you very much