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Why Max Vgs rating is there not Vgd in datasheet

Biprangshu Saha23
Prodigy 10 points

Hi ,

I am designing a motor driver with mosfets in this regard I am in bit confusion with the below question 

1. Why there is max Vgs rating ?

2. Does it indicate the gate oxide breakdown voltage?

3. If the 2 is correct , then the rating should be applicable to the Vgd also .

4. if 3 is correct i.e. the max Vgs applies to Vgd also : then considering a NMOS with gate and source sorted and Vds at its max level, Vgd will be very high compare to max Vgs rating ( max Vds >> max Vgs  generally). So for this case also capacitor breakdown is expected.

Thanks is advance .

-biprangshu saha

  • 0
    TI__Mastermind 18200 points
    Biprangshu,
    You are correct that the max Vgs rating is assigned to prevent breakdown of the gate oxide. Basically, when the gate to source is biased, some current is tunnelled through the the oxide. However, even at the max Vgs, the amount of charge injected by this current is still much less that the Qbd (breakdown charge) such that the long time reliability of the gate is not in any danger.

    But when the Vgs exceeds the max rated value, the tunnelled current increases exponentially and the robustness of the gate can no longer be guaranteed.

    The internal geometry of the gate's position with respect to both the drain and source dictate that the most stress will be absorbed from gate to source, not gate to drain. In other words, the max Vgd is not the same as the max Vgs, and is much greater such that the max Vgs or max Vds will always be exceeded first.