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DRV8328: How to understand: IGS(Total average gate-drive current)

Eason Tian
Prodigy 630 points
Part Number: DRV8328

Hi Expert,

In datasheet, the Igs is 30mA(max): 

How to understand this parameter? 

What is the difference between Igs and source current? 

In the real application, I observe the GVDD dropped when running BLDC. Could you list the possible reason?

Thank you!

Eason

  • 0
    TI__Genius 17631 points

    Hello Eason,

    Hope you are doing well!

    I will be happy to assist you.

    The term IGS refers to the average current that is used to activate the highside MOSFET. And for DRV8328 it is 30mA. To turn on the MOSFET, the source current (gate drive current) is applied at the gate of the MOSFET, but since the MOSFET is constantly being turned off and on in BLDC application, we use the term IGS, to account for the switching frequency and number of MOSFETs switching. Equation 7 on the following application note shows how to calculate the average current or IGS.

    App Note: www.ti.com/.../slva714

    To clarify, the source current is the instantaneous current needed for the gate-to-source voltage (Vgs) to reach the Miller plateau of the MOSFET. It is dependent on the specific charge of the MOSFET used (Qg) and can be calculated using Equation 5 of the above app note.

    Lastly, the GVDD drops for a brief period when the device is powered on because it is recharging the Booststrap capacitor while the lowside MOSFET is on, and is expected to have a small “ripple” because of this behavior. However, if the GVDD remains low for an extended period of time it might be due to the GVDD capacitor being too far away from the device which leads to inductance being added through length of the trace. Another possible reason might be that the GVDD capacitor is of a low capacitance which prevents it from being able to regulate the voltage at GVDD as a bulk capacitor.

    If you need more assistance on the GVDD, please provide us a waveform output so that we could better assist your specific need.

     

    Hope this answers your question! Please let me know if you have any more questions.

    Best Regards,

    Akshay

  • 0 in reply to Akshay Rajeev Menon
    TI__Prodigy 630 points

    Hello Akshay,

    Thank you for your great support.

    About the calculation of I_avg:  in the APN 

     I_AVG = 44 nC * 6 * 45 kHz = 11.88 mA

    This "6" means there are 6 MOS in system. But I think there are only 2 MOS(Trap) or 3 MOS(FOC) switching at the same time. 

    Those switching MOS need current from GVDD, and other MOS don't need supply current from GVDD. 

    So  I think this "# MOSFETs Switching" should be 2(Trap) or 3(FOC). 

    looking forward to your comments

    Thanks

    Eason

  • +1 in reply to Eason Tian
    TI__Genius 17631 points

    Hey Eason,

    You are correct, the equation refers to the number of MOSFETs switching. For the DRV8328, the MOSFETs get their gate charge from GVDD.

    For Trap control we have 1 high side and 1 low side MOSFET synchronously switching for a particular phase. (Note: there is an additional low side MOSFET of one of the phases that is on, but it is only switched on once and then left on for 2 commutation states and therefore has negligible impact on the IAVG so it can be ignored.) Thus, we say that number of MOSFETS being switched on is 2 as you had mentioned.

    In FOC all 6 MOSFETs will be switching based on the PWM frequency, so the IAVG equation will have 6 for the number of MOSFETs. There will be 3 high side MOSFETS which rely on GVDD to charge the bootstrap capacitors, and 3 low side MOSFETS which also rely on GVDD to switch on. Even if a MOSFET will be turned on for a brief amount of time, the demand on GVDD is based on how much charge is needed to turn on the MOSFET and therefore it will get accounted in the equation for IAVG.

    I hope this provides some explanation. Please feel free to ask any questions you might have.

    Best Regards,

    Akshay