MOSFET is a switch controlled by the gate voltage. When the gate voltage exceeds the turn-on threshold, the MOSFET turns on; when the gate voltage is below the turn-on threshold, the MOSFET turns off.

In practical applications, factors such as device and parasitic parameters of peripheral circuits can lead to unintentional turn-on of the originally turned-off power device.

Today, we will discuss the false turn-on of MOSFETs in drive circuits and their countermeasures.

There are two situations of false turn-on: false turn-on caused by the Miller effect and false turn-on caused by parasitic inductance.

False Turn-On Caused by the Miller Effect

When the MOSFET turns off and then conducts, the Vds voltage (maximum voltage that can be applied between the drain and source) rapidly rises, generating a high dv/dt (rate of change of drain-source voltage during the switch transient process), thereby generating displacement current (igd) in the capacitance Cgd (Miller capacitance).

This displacement current will generate a voltage spike. If this voltage spike exceeds the MOSFET's turn-on threshold, the MOSFET will turn on, causing a direct circuit or even damage.

Another type of false turn-on is caused by parasitic inductance in the circuit, as shown in the figure below, where Ls is the parasitic inductance on the MOSFET source.

When the MOSFET rapidly turns off, the current decreases rapidly, generating a high di/dt, and then a negative voltage (VLS) is generated across the parasitic inductance. If this VLS voltage exceeds the gate threshold of the MOSFET, the MOSFET will be falsely turned on.

So, what methods do we have to deal with the phenomenon of MOSFET false turn-on?

1. Adjust Gate Drive Resistance and Capacitance

Adjusting the size of the gate drive resistance and capacitance can adjust the turn-on/turn-off speed of the MOSFET: increase the gate drive resistance and capacitance to slow down the turn-on/turn-off speed of the MOSFET, reduce dv/dt (di/dt), and thereby reduce the gate voltage spike.

2. Add a Transistor

A transistor can be placed near the gate of the power transistor to prevent false turn-on during turn-off, effectively suppressing false turn-on of the gate due to the Miller effect.

3. Use Anti-Parallel Diodes

Allowing the current in the inductor to dissipate through a diode loop to avoid generating reverse potential, thereby preventing false turn-on.