What is the purpose of pull-down resistance between the gate and source of a MOSFET?

Miller Capacitance: A parasitic capacitance between the gate (G) and drain (D).

When the MOSFET is turned off, the presence of the Miller capacitance causes a voltage change from near 0 (saturation voltage drop) to the bus across the MOSFET's Vds, and this voltage change rate is "dv/dt". However, the capacitor is the component where voltage change occurs, and current "i" is generated as the voltage changes across the capacitor.

There is an insulating layer between G and S, which is silicon dioxide (SiO2), so the impedance between G-S is high (tens to hundreds of megohms). Once the drive is abnormal, it may charge the G-S through the current of the Miller capacitance, and a small current with high impedance may correspond to a high voltage. When the gate voltage exceeds the threshold voltage "Vgs(th)", it will cause the MOSFET to turn on again, which is a very dangerous situation.

Take a look at the MOSFET drive in the flyback power supply topology. The Miller capacitance current is discharged through the internal pull-down low-resistance circuit of the drive chip to avoid the gate being charged high and falsely turned on.

Here we know that there is already a discharge pull-down resistance inside the drive chip, but if the drive resistance Rg is open or not connected under external conditions, then the pull-down resistance (R8) can provide a discharge path for the Miller capacitance, keeping the impedance between the MOSFET's G-S low, ensuring a stable and safe state. This is the important role of pull-down resistance.

Pull-down resistance also serves another purpose: pre-protection resistance.

We know that the G-S of the MOSFET is high impedance, which is why it is an ESD-sensitive device. Its high voltage applied to the gate is not easily discharged, and the accumulation process can damage the layer of silicon dioxide between the G-S, leading to device failure.

Therefore, the pull-down resistance also takes into account both power consumption and actual discharge effects. Generally, the resistance value is selected around 10K-20K for medium and small power supplies (0~500W），and 4.7K-10K for high-power supplies.

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