What should I do if the MOS tube is misconnected?

There are usually two situations that cause misconductance, one is the Miller effect, and the other is caused by inductance.

Miller Effect

When the upper tube is turned on and the lower tube is turned off, the high dv/dt generated by the rapid turn-on will form a current on the Miller capacitance of the lower tube, and flow through the lower tube drive impedance to produce a voltage drop in the gate resistor, thereby increasing the gate voltage and generating a large voltage spike.

 Parasitic inductance

When the parasitic inductance of the lower tube source is large, during the rapid shutdown of the MOS tube, as the current decreases, the parasitic inductance will form a negative voltage at the top and positive voltage at the bottom, resulting in false conduction.

In a half-bridge circuit application, when the upper MOSFET is turned on, the lower MOSFET must be turned off to avoid shoot-through or overcurrent.

What happens if there is miscommunication?

The upper and lower tubes may be turned on at the same time, causing damage to the MOS tube. In addition, when the peak voltage of the gate exceeds the maximum voltage allowed between the gate and the source, the gate oxide layer of the MOS tube will be broken down and damaged.

Measures to avoid miscommunication include:

1. Increasing the gate drive resistor or adding a GS capacitor at the gate can reduce the MOSFET switching speed. Disadvantage: It will cause greater losses.

2. When the lower tube is turned off, a low impedance path is provided for the current of the Miller capacitor to avoid interference caused by high dv/dt when the upper tube is turned on.

 3. Reduce the gate voltage when it is turned off, that is, to achieve negative voltage drive, so that the gate voltage will not exceed Vgs-th when it rises. There are two ways to drive negative voltage: 1. Use a voltage regulator 2. Use a DC blocking capacitor to generate negative voltage

Misconduction can also be avoided when selecting MOSFET, so how should we choose it?

To avoid parasitic conduction, the UGS/UDS ratio must be as small as possible. Conversely, the UDS/UGS ratio must be as large as possible. The CGS/CGD ratio must also be as large as possible, greater than 15 (or at least greater than 10) .