How to prevent the self-conduction phenomenon of MOS tube?

We usually see the following three methods:

1. Add a capacitor between the gate and source

2. Add Miller clamp circuit

3. Use negative voltage to provide gate drive voltage

 We use a circuit to simulate a self-conduction phenomenon.

 During reverse recovery, Q2 turns on when the inductor load current flows back to Q1's diode, and the inductor current flows through Q2, causing the diode to turn off.

So what happens when a high dv/dt voltage is applied to the MOS tube in the off state ?

we adjust the resistance value of R4 , we will see the change.

 a capacitor is added between the gate and source of the MOS tube (Q1) to absorb the gate current. This is to reduce the gate voltage generated by the gate resistance and reduce the self-conduction voltage.

 This is the waveform after adding capacitance . The increase of gate-source capacitance also changes the switching time of the MOS tube. At this time, the capacitance and gate resistance need to be adjusted together.

 So what is the function of R1 in the gate drive circuit?

 This is a pull-down resistor. Obviously, one of its functions is to provide a discharge loop for the circuit so that the MOS tube is only in the two states of switch. Another function is that it can prevent lightning strikes and static electricity to a certain extent.

Back to the beginning, there are three ways to solve the problem of self-conduction:

1. Add a capacitor between the gate and source

The capacitor between the gate and source is used to absorb the drain-gate current caused by dv/dt. The capacitor is connected in parallel with Cgs inside the MOS tube, and the gate charge will increase.

 If the gate voltage is fixed, the MOS switch can be kept unchanged by changing the gate resistor value, but this will lead to an increase in driving power.

2. Add Miller clamp circuit

This can be achieved by adding another MOS tube between the gate and source. When the voltage is lower than the preset Miller voltage, the comparator will provide a logic high level to turn on the MOS tube between the gate and source, short-circuiting the gate and source , and increasing the gate voltage through the current of the Miller capacitor and the gate resistor.

 3. Use negative voltage to provide gate drive voltage so that it does not exceed Vth. Generally, a negative power supply is required .