Detailed explanation of the application of MOV, NTC and PTC in power module protection scheme.Linear lens wholesaler

　　Common special resistors include varistor and thermistor, which play a key role in the design and application of AC-DC switching power supplies. Learn about the characteristics and specific functions of these two resistors:

　　The varistor MOV is one of the most commonly used devices in circuit electromagnetic compatibility (EMC), and is widely used in electronic circuits. Its characteristics are commonly understood as the breakdown of the varistor when the front-end voltage is higher than the turn-on voltage of the varistor. , the resistance of the varistor is reduced and the current is shunted to prevent the subsequent stage from being damaged or interfered by excessive instantaneous voltage, thus protecting the sensitive electronic components. The nonlinear characteristic of the varistor is used. When an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.

　　Thermistor is a temperature-related device, generally divided into two types, NTC is a negative temperature coefficient thermistor, that is, the higher the temperature, the smaller the impedance TC is a positive temperature coefficient thermistor, that is, the higher the temperature, the higher the impedance. big. The use of impedance's sensitivity to temperature plays an important role in circuit design.

　　In the circuit, NTC mainly suppresses the startup current during the startup process of the circuit. During the startup process of the system, due to the existence of power circuits, capacitive and inductive loads in the system, a very large inrush current will appear at the moment of startup. If the instantaneous anti-current capability of the device is not considered in the selection process of the circuit device, then the system will easily cause the device to be broken down during the operation of multiple startups, and adding NTC to the circuit is equivalent to the input loop startup. Increasing the input impedance reduces the inrush current, and when the system is in a stable state, due to the heating of the NTC, according to its negative temperature characteristics, the impedance is reduced, and the loss on the NTC is also reduced, reducing the overall loss of the system.

　　PTC can act as a fuse in the circuit, so it has another name as a resettable fuse. During the operation of the system, when the circuit is abnormal, resulting in a large current, if there is a PTC in series in this part of the circuit, it means that there is a large current flowing in the PTC, and the PTC generates heat. According to its positive temperature characteristics, its The impedance will become very large, making the impedance of the entire loop larger, so that the current in the loop will become smaller, which acts as a fuse. According to its positive temperature characteristics, another role of PTC is to realize over-temperature protection in the circuit.

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