why we use a high value resistor across VGS?

 

A high-value resistor is often placed across the gate-source (V$_{\text{GS}}$) of a MOSFET for the following reasons:

1. Gate Discharge Path: The MOSFET gate has high impedance and acts like a capacitor. Once charged, it can hold the gate voltage for a long time. The high-value resistor provides a discharge path for this gate charge, ensuring that the gate voltage drops to zero when the driving signal is removed, turning off the MOSFET completely.

2. Preventing False Turn-On: In high-impedance circuits or circuits with floating gates, noise or leakage currents can cause the gate voltage to rise unintentionally, turning the MOSFET on when it shouldn’t. The high-value resistor ensures that the gate is pulled to the source (ground) in the absence of a control signal, preventing false triggering.

3. Leakage Current Handling: In some applications, the MOSFET driver may have a small amount of leakage current. The high-value resistor helps to bleed off this leakage current, ensuring that the gate voltage does not rise due to these small currents.

4. Stabilizing the Gate Voltage: The resistor helps to stabilize the gate voltage in case of any floating conditions, ensuring consistent and reliable MOSFET operation.

The value of the resistor is chosen to be high (often in the range of 100kΩ to 1MΩ) so that it does not significantly affect the switching speed or the performance of the MOSFET.