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The role of varistor in switching power supply

source:Industry News     Popular:adapter     release time:2022-05-10 10:03:14     Article author:yu    

  The role of varistor in switching power supply(图1)

  Varistor is a component with transient voltage suppression function, generally used in surge and transient protection circuits of switching power supply circuits, and can be used to replace the combination of transient suppression diodes, zener diodes and capacitors . Varistors protect important components such as integrated circuits and other circuits and equipment from damage caused by electrostatic discharge, surges, and other transient currents such as lightning strikes. When using, only need to connect the varistor in parallel to the protected circuit. When the voltage is momentarily higher than a certain value, the resistance of the varistor drops rapidly, conducts a large current, and prevents instantaneous overvoltage for protection. When the voltage is lower than the working voltage of the varistor, the resistance of the varistor is extremely high, almost open circuit, so it will not affect the normal operation of the device or electrical equipment.

  A varistor (Voltaga Sensor Resistor, VSR) is a new type of overvoltage protection component. The varistor is a metal-oxide-semiconductor ceramic component made of zinc oxide as the main material. The core material of the varistor is zinc oxide, which includes zinc oxide grains and the grain boundary layer around the grains. The resistivity of zinc oxide grains is very low, while the resistivity of the grain boundary layer is very high. A potential barrier equivalent to a Zener diode is formed between the two grains in contact, becoming a varistor unit, and many units The varistor substrate is formed by series and parallel connection. When the varistor is working, each varistor unit bears the surge energy, and these varistor units are generally evenly distributed in the entire resistor body, and the entire resistor body bears the energy, unlike the Zener secondary stabilizer. Like the pressure tube, only the junction area bears the electric power, and its resistance changes with the change of the terminal voltage.

  Structural characteristics and working principle of varistor

  The main features of the varistor are the wide operating voltage range (6~3000V, divided into several grades), fast response to overvoltage pulses (nanosecond level), strong impulse current resistance (100~2000A), and small leakage current ( Microampere level), small resistance temperature coefficient, high performance and low price, small size, it is an ideal protection component, which can constitute an overvoltage protection circuit, a noise elimination circuit, a spark elimination circuit, and an absorption circuit. When the overvoltage pulse is superimposed on the power supply network, after connecting the varistor, the overvoltage peak waveform is flattened and limited to a certain range. When the load circuit with inductive and capacitive load is turned on or off, there will be switching spikes in the DC waveform, and the varistor can absorb this back electric potential in the circuit, thereby effectively protecting the switching circuit from damage.

  The commonly used varistors include silicon carbide varistors and zinc oxide varistors. Among them, zinc oxide varistors are more widely used. It uses zinc oxide as the main raw material and adds a variety of trace metal oxides. A new type of ideal overvoltage protection device assembled by sintering, its conductivity value changes nonlinearly with the change of the applied voltage, it is called a varistor or a surge absorber.

  Varistors are often connected in parallel between the live wire and the live wire, between the live wire and the neutral wire at the input end of the switching power supply, and between the inductive device end as a surge and peak voltage absorbing element. The working voltage is usually half the breakdown voltage.


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