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The role of special resistors in the peripheral protection circuit of the power module
Release time:
Aug 02,2022
The most common special resistors are varistors and thermistors, 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 to protect the circuit from possible damage to the circuit due to sudden voltage changes in the power supply system. Its characteristics are generally understood that when the front-end voltage is higher than the turn-on voltage of the varistor, the varistor is broken down, the resistance of the varistor is reduced, and the current is shunted to prevent the subsequent stage from being damaged or disturbed by excessive instantaneous voltage. Thus protecting sensitive electronic components. Circuit protection is to use the nonlinear characteristics of the varistor. 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.
However, don't think too much about the role of the varistor. The varistor cannot provide complete voltage protection. The energy or power that the varistor can withstand is limited and cannot provide continuous overvoltage protection. . A sustained overvoltage can destroy the protective device (varistor) and cause damage to the equipment. The parts that the varistor cannot provide protection include: inrush current during startup, overcurrent during short circuit, voltage sag, etc. These situations require other means of protection.
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; PTC is a positive temperature coefficient thermistor, that is, the higher the temperature, the smaller the impedance. bigger. The use of impedance's sensitivity to temperature plays an important role in circuit design.
In the circuit, NTC mainly suppresses the starting current during the starting process of the circuit. During the starting process of the system, since there are power circuits, capacitive and inductive loads inside the system, a very large inrush current will appear at the instant of starting. 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 equal 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 play the role of a fuse in the circuit, so it has another name called 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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