Function

1. Act as a "bias voltage"

2. Serve as a pull-down resistor to drive current release

First, the first one, acting as a "bias voltage":

We know that the gate voltage of a MOSFET is generally higher than the source voltage by a certain voltage in order to turn it on, and this voltage difference is called the gate-source voltage.

If there is no resistor acting as a bias voltage here, the source voltage of the MOSFET will vary with the change of the load current, leading to a change in the gate-source voltage and affecting the working state of the MOSFET.

When a resistor is added, the resistor provides a stable bias voltage to the MOSFET through voltage drop, stabilizing the working state of the MOSFET.

So, when does the pull-down resistor between the MOSFET gate (G) and source (S) become particularly important?

This is the second point we want to discuss.

There is a large impedance between the G-S of the MOSFET. Once there is a small amount of static electricity between G-S, a large resistance value will generate a high voltage between the gate (G) and source (S). In this high-voltage state, coupled with the original current, the voltage will be even higher.

At this time, we need to discharge the small amount of static electricity to prevent the high voltage at both ends from causing MOSFET misoperation or G-S breakdown risk, and the resistor plays a role in protecting the MOSFET.

Pull-down low-resistance circuit for current release （Image source: Internet）

Specific explanation:

We know that the MOSFET is a high-impedance device, and there is an insulator between the gate (G) and the source (S), which is silicon dioxide (SiO2). The MOSFET has a Miller effect. To avoid the transistor staying on a Miller plateau for a long time, we will choose to accelerate the conduction and turn-off time of the MOSFET to reduce the switch loss.

Once the MOSFET driver is abnormal, the Miller capacitance may charge the gate (G) and source (S) through current, and then a small current impedance corresponds to a high voltage, and the gate voltage is charged. If it exceeds the threshold voltage "Vgs (th)", it is easy to cause the MOSFET to re-open, which is very dangerous.

Pull-down resistor for charge discharge path 

 In addition, when the MOSFET is used in a switching circuit, if the gate voltage does not drop in time during the switching process, it is easy to cause the MOSFET to be in a partially conductive state, and then generate a large amount of heat, causing damage to the MOSFET. Adding an appropriate pull-down resistor between the gate (G) and source (S) can accelerate the decrease of the gate voltage, thus protecting the MOSFET.