Why the power MOS tubes in the power supply are always burned out

The power MOS tubes in the power supply are always burned out. Could it be due to the following reasons?

I. Overvoltage Issues

Input Voltage Surges

The input side of the power supply may experience voltage surges caused by lightning strikes or the start-up and shutdown of large equipment in the power grid. When the instantaneous input voltage far exceeds the breakdown voltage of the MOS tube, the MOS tube will be broken down and burned out.

For example, without an appropriate surge protection circuit, the lightning-induced voltage may be directly applied to the power circuit, causing the MOS tube to withstand an excessively high voltage.

Spike Voltages

Inductive elements such as inductors and transformers in the circuit will generate spike voltages when the current changes abruptly. If these spike voltages are not effectively clamped, they will be superimposed on the normal working voltage of the MOS tube, resulting in its damage.

For instance, in a switching power supply, when the switching tube (MOS tube) is turned off, the primary winding of the transformer will generate a reverse electromotive force, forming a spike voltage.

Voltage Overshoots

When the rising or falling edge of the driving signal of the MOS tube is too steep, a voltage overshoot phenomenon will occur between the drain and source of the MOS tube. This is mainly caused by the interaction between the parasitic capacitance and inductance in the circuit, and an excessively high voltage overshoot may exceed the breakdown voltage limit of the MOS tube.

II. Overcurrent Issues

Load Short Circuits

When a short circuit occurs at the output end of the power supply, the MOS tube will bear an extremely large current. If there is no effective short-circuit protection measure, such as the overcurrent protection circuit not responding in a timely manner, the MOS tube will be burned out due to overheating.

For example, in a computer power supply, if an element on the motherboard is short-circuited, the MOS tube in the power supply will attempt to provide a large current to maintain the output voltage, thereby causing damage to itself.

Overload Currents

If the MOS tube is made to work in a state where the current exceeds its rated current for a long time, even if it does not reach the level of a short circuit, it will also cause the MOS tube to generate excessive heat. This is because the power consumption of the MOS tube (mainly the conduction resistance loss) is proportional to the square of the current, and the accumulation of excessive heat will cause the temperature of the MOS tube to rise and eventually burn out.

For example, in a power amplifier power supply, if the output power exceeds the design value for a long time, the current in the MOS tube will continue to be too large.

III. Heat Dissipation Issues

Ineffective Heat Sink Design

The power MOS tube will generate heat during operation. If the area of the heat sink is too small, the thermal conductivity is low, or the contact between the heat sink and the MOS tube is poor, the heat cannot be effectively dissipated. This will cause the junction temperature of the MOS tube to exceed its allowable maximum junction temperature, thereby damaging the MOS tube.

For example, if the thermal grease between the heat sink and the MOS tube is not evenly applied or has dried up, it will form a thermal resistance and affect the heat dissipation effect.

High Ambient Temperature

When the ambient temperature where the power supply works is already high, such as in some high-temperature industrial environments or poorly ventilated computer cases, the difficulty of heat dissipation for the MOS tube will increase. If this environmental factor affecting heat dissipation is not considered, it is easy for the MOS tube to be overheated and burned out.

IV. Driving Issues

Abnormal Driving Voltages

If the driving voltage of the MOS tube is too high or too low, it will affect the normal operation of the MOS tube. If the driving voltage is too high, the insulating layer between the gate and source of the MOS tube may be broken down; if the driving voltage is too low, the MOS tube may not be able to be fully turned on or off, causing it to work in an abnormal state and generate excessive power consumption.

For example, if the voltage-stabilizing element in the driving circuit is damaged, the driving voltage will exceed the normal working range of the MOS tube.

Interference of Driving Signals

Electromagnetic interference (EMI) or radio frequency interference (RFI) in the circuit may affect the driving signal of the MOS tube. When the driving signal is interfered with and abnormal pulses or jitters appear, the on and off times of the MOS tube may change, thereby increasing its power consumption and even causing abnormal situations such as mis-triggering, and finally burning out.

For example, in a high-frequency switching power supply, nearby high-frequency equipment may emit interference signals that affect the driving signal of the MOS tube.