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Original question:

LMG1205: HS Pulse Collapse When external FET is Loaded

LMG1205: Calculation of the bootstrap capacitor

Thomas Tutschka
Prodigy 130 points
Part Number: LMG1205

Hello,

in our application we use the LMG1205 for driving a halfbridge consisting of two EPC2053 GaN-FETs.

Our minimum switching frequency is 20kHz and the maximum duty cycle is nearly 100% (motor drive application).

In the datasheet of the FET a maximum Gate to Source leakage current of 9mA (!) is given.

This has a major impact in calculating the necessary bootstrap capacitance!

Why is the Gate to Source leakage not considered in your formula in the datasheet (chapter 8.2.2.2)?

With your formula i get a value of about:

Qg,max = 14.8nC

IHB = 0.12mA

tON = 45µs (90% duty cycle)

Qrr = 4nC

dV = 0.2V

CBST > (14.8nC+0.12mA*45µs+4nC)/0,2V = 121nF

When putting the GS leakage current of 9mA into account the value is significantly higher!

CBST > (14.8nC+9.12mA*45µs+4nC)/0,2V = 2.146µF

Is it possible to update your formular to avoid false calculation?

--> normal silicon MOSFETs has much lower values of GS leakage, therefore the effect is only present when using GaN FETs.

Best regards

Thomas

  • 0
    TI__Genius 12990 points

    Hi Thomas,

    Thanks for your great and thorough question on lmg1205.

    Gate to source leakage is higher for GaN vs Si since gan needs a metal gate as opposed to oxide to help lower the gate resistance. This creates more leakage on the gate of about 1mA typical. Even this much higher leakage affects the bootstrap calculation especially with very long on times (since this drops the HB voltage lower and needs more off time to replenish).

    Thanks for bringing the datasheet discrepancy to my attention. I will go ahead and update the equation in the datasheet to reflect the additional leakage current that's needed from IHB for normal operation. Please let me know if you have any other questions.

    Thanks,

  • 0 in reply to Jeffrey Mueller
    Prodigy 130 points

    Hello Jeff,

    thank you for your explanation! I highly appreciate your quick response!

    No, there are no more questions regarding this issue. An update of the datasheet will be very good in my opinion.

    Merry Christmas and have a beautiful holiday!

    Thomas