What is the function of the MOSFET gate-source pull-down resistor?
Miller capacitance: A parasitic capacitance between the gate (g) and drain (d).
When the MOSFET is turned off, the presence of Miller capacitance will cause the VDS of the MOS transistor to produce a voltage change process from close to 0 (saturation voltage drop) to the bus, and this voltage change rate is "DV DT", but the capacitor is the device where the voltage change occurs, and the voltage changes at both ends of the capacitor, that is, the current "I" is generated.
There is a layer of insulator between the Gs, that is, silicon dioxide (SiO2), so the G-S is a high impedance (ranging from tens to hundreds of megabytes), once the drive is abnormal, it may charge the G-S through the current of the Miller capacitor, the small current and high impedance may correspond to the high voltage, the gate voltage is charged, when the threshold voltage "VGS(TH)" is exceeded, it will cause the MOSFET to re-open, which is a very dangerous situation.
You can look at the MOSFET driver of this flyback power topology, which is that the Miller capacitor current is released through the internal pull-down low-resistance loop of the driver chip, so as to avoid the gate being charged high and misleading.
Here we know that there is already a pull-down resistor inside the driver chip, but if the drive resistor RG is open or not connected in an external situation, then the pull-down resistor (R8) can provide a pull-down path for the Miller capacitor, so that the MOS transistor G-S can maintain low impedance and a stable safety state. This is where the pull-down resistor comes in.
The pull-down resistor has another purpose: the pre-protection resistor.
We know that the MOS transistor G-S is a high impedance, which is why it is an ESD-sensitive device. Its high voltage is applied to the gate pole and is not easily discharged, and the accumulation process can damage the layer of silica between the G-S poles, causing the device to fail.
As a result, the pull-down resistor also takes into account both power dissipation and actual bleed-down. Generally, the value of this resistance is selected in the small and medium-power power supply (0 500W) is about 10k-20k, and the high-power power supply is selected 47k~10k。
Well, that's all for this issue! Thank you all for your support!