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Reasons for MOS tube heating in switch mode power supply

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.23
MOSFET is a type of FET that can be made into either enhanced or depleted types, either P-type or N-type. Regardless of which type, its working principle is essentially the same. There are many types and functions of MOS transistors, and their switching function is mainly used in the use of power supplies.



MOSFET TUBE

MOSFETs often apply voltage to the input gate to control the current of the output drain. By controlling the characteristics of the device through the voltage applied to the gate, there will be no charge storage effect caused by the base current when a transistor is used as a switch. The open drain circuit of MOSFETs is commonly used in switch mode power supplies, where the drain is connected to the load without moving, which is called an open drain. In an open drain circuit, no matter how high the voltage is applied to the load, the load current can be turned on and off, making it an ideal analog switching device.

The working state of MOSFETs can be divided into on process, on state, off process, and off state. Losses can be divided into switch losses, conduction losses, cutoff losses, and avalanche energy losses. Common causes of damage include overcurrent, overvoltage, and static electricity.

There are four main causes of fever:

1. Circuit design issues. MOSFETs operate in a linear operating state, rather than in a switching state. If NMOS is used as a switch, its G-level voltage needs to be several volts higher than the power supply to fully conduct, while P-MOS is the opposite. Failure to fully open and excessive voltage drop leads to high success rate consumption, equivalent DC impedance is relatively high, voltage drop increases, so U * I also increases, and loss means heating.

2. The usage frequency is too high. If the excessive pursuit of volume leads to an increase in frequency, the loss on the MOS transistor increases, and the heat generation also increases.

3. Insufficient heat dissipation design, high current, MOS transistor nominal current value, generally requires good heat dissipation to achieve. So if the ID is less than the maximum current, it is also possible to generate severe heat, so sufficient auxiliary heat sinks are needed.

4. The selection of MOS transistor is incorrect. If there is an error in power judgment, the internal resistance of the MOS transistor is not fully considered, resulting in an increase in switch impedance.

The above discussed the causes of MOS tube heating and damage, and below we will explain the main reasons for the damage of switch power supply switch tubes:

1. Caused by high or low AC voltage input from the power supply;

2. Caused by the failure of the soft start circuit;

3. The failure of the anti peak absorption circuit in the switch tube integrated circuit board;

4. Failure of decoupling capacitor in voltage stabilizing circuit;

5. Negative feedback open-loop of voltage stabilizing circuit;

6. Excessive positive feedback causes;

7. Damaged rectifier diode;

8. Damaged switch mode power transformer;

9. Damaged rectifier bridge of switch mode power supply;

10. The emission limit current resistance of the switching tube is too small, resulting in;

11. Poor performance or low power of switch tubes;

12. Poor insulation between the switch tube and the heat sink.
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