There are two types of MOS tubes: N-channel and P-channel. Today we take the N-channel enhancement MOS tube as an example. It consists of a source (S), a drain (D) and a gate (G). In normal operation, when the gate-source voltage VGs is greater than the turn-on voltage Vth, a conductive channel is formed under the oxide layer under the gate, allowing current to flow between the drain and the source.

 So why should the gate-source voltage be negative in the off state?

When the MOS tube is to be completely turned off, it is necessary to ensure that the conductive channel disappears. Ideally, as long as VGs ≤ Vth, the MOS tube will be turned off. However, in actual circuits, in order to more reliably turn off the MOS tube, VGs is made negative.

 This is because in some application scenarios, there may be interference signals or parasitic capacitance of the MOS tube itself. When the MOS tube is in the off state, there is a parasitic capacitance CGs between its gate and source. If there are interference signals in the surrounding environment or noise from other parts of the circuit, the gate voltage may increase. If VGs is positive and close to the turn-on voltage Vth at this time, it may cause the MOS tube to turn on by mistake.

To avoid this situation, VGs is set to a negative value so that the direction of the electric field between the gate and the source is opposite to that when it is turned on, which can more effectively suppress the formation of the conductive channel and ensure that the MOS tube is stably in the off state. In addition, the negative Vgs can quickly release the charge accumulated in the gate through an appropriate circuit (such as a gate drive circuit) to prevent mis-conduction caused by charge accumulation.

Therefore, we can set the voltage at the source of the MOS tube to +5V without using a dedicated SiC-MOSfet driver chip, and the gate voltage changes in the range of +23V~-5V with the PWM signal. When the drive signal is positive, the gate-source voltage difference is 18V, and the MOS tube is turned on; when the drive signal is negative, Vgs=-5V, and the MOS tube is turned off.