When the MOS tube is working in the switching state, Q1 and Q2 are turned on in turn, and the gate of the MOS tube is in a state of repeated charging and discharging. If the power is turned off at this time, the gate of the MOS tube will have two states, which may cause charge accumulation problems.

One state is the discharge state, in which the gate equivalent capacitor has no charge storage, and the other state is the charge state, in which the gate equivalent capacitor is just in a fully charged state, as shown in FIG1 .

 Since Q1 and Q2 are both disconnected after the power is turned off, the gate charge cannot be released through the circuit, resulting in the gate electric field still existing. When the power is turned on again, the excitation signal has not yet been established, and the drain power supply (VDS) of the MOS tube will randomly provide voltage. If the gate electric field still exists, a conductive channel will be formed immediately, causing the MOS tube to generate an uncontrolled huge drain current (ID), which may burn out the MOS tube.

So how to avoid MOS tube damage?

We can connect a discharge resistor R1 in parallel between the gate and source of the MOS tube, as shown in Figure 2. After shutdown, the charge stored in the gate is quickly released through R1. The resistance of this resistor should not be too large (usually around 5KΩ to several 10KΩ) to ensure rapid release of charge.

 In addition to solving the problem of gate charge accumulation, the MOS tube may also cause switching action lag due to capacitive input characteristics in switching applications. In this case, a perfusion circuit can be introduced to reduce the delay of MOS tube switching action by optimizing the charging and discharging process of the gate charge.

It should also be noted that the perfusion circuit is mainly used in scenarios where MOS tubes are used as switching tubes. When MOS tubes are used for other purposes (such as linear amplification), due to their different working characteristics, it is usually not necessary to set up a perfusion circuit.