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In a DC/DC converter, MOS tubes Q1 and Q2 are connected in parallel and controlled by the driving signal of the common point A. Why might MOS tube Q2 break down when point A is disconnected?

During normal operation, point A is connected to an appropriate driving voltage (high or low level, depending on the type of MOS tube), and Q1 and Q2 will be turned on or off synchronously.

 Once point A is disconnected, if the drive signals of Q1 and Q2 are not isolated, a floating node will be formed between them. Since there is capacitance between the gate and source of the MOS tube, after point A is disconnected, the capacitance will discharge through other paths (such as the resistance from the drain to the source of Q1 or Q2).

If the drive circuits of Q1 and Q2 are not completely isolated and the gate capacitances are different, the voltage will be unevenly distributed between the two MOS tubes when point A is disconnected. This may cause one MOS tube (such as Q2) to withstand an overvoltage exceeding its rated voltage.

When the voltage MOS tube bears exceeds the rated value, it will enter the avalanche breakdown region, the current will increase sharply, and eventually cause the MOS tube to be damaged.

To avoid such situations, the following measures can be taken: First, ensure that the driving circuits of each MOS tube are isolated from each other to prevent mutual interference; second, select MOS tubes of the same model and specifications to reduce the risks caused by parameter differences; third, add resistors or capacitors between the gate and source of the MOS tube to reduce the possibility of uneven voltage distribution; fourth, monitor the key parameters of the DC/DC converter such as voltage and current in real time to deal with potential problems in a timely manner.

In short, when MOS tubes are used in parallel in DC/DC converters, correct driving and isolation are crucial to effectively avoid breakdown and other faults.