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Application Analysis of Varistor in Circuit Surge and Transient Protection

Varistor is a voltage-limiting protection device with transient voltage upset function. Utilizing the particularly sensitive non-linear characteristics of the device, when an overvoltage occurs between the two stages of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent stage circuit. It can be used to replace the combination of transient boost diode, Zener diode and capacitor. Varistors commonly used for overvoltage protection include metal oxide varistors (MOV) and silicon carbide (SiC). Both the forward and reverse directions of the varistor have volt-ampere characteristics similar to the reverse breakdown of a diode. When the voltage applied to both ends reaches a certain value, the resistance is very sensitive to voltage.

The most notable feature of varistors is that they have good nonlinear characteristics, a wide voltage range, from several volts to several thousand volts, and the absorption of surge current from tens of amperes to thousands of amperes. The response speed is fast and the nonlinear index is large. No polarity, no freewheeling, long service life and low cost. It is mostly used for DC power supply, AC power supply, low frequency signal line and feeder line. Surface mount varistors are the most widely used in mobile phones, laptop computers, PDAs, digital cameras, medical instruments and other equipment.

The application of varistor in circuit surge and transient protection can be roughly divided into four types:

1) Connect a varistor between the power lines and between the ground

The use of this varistor is the most representative. It protects electronic products when power lines and long-distance transmission signal lines encounter lightning strikes and lead to surge pulses. Generally, the varistor connected between the lines is effective for the induction pulse between the lines; and the varistor connected between the line and the ground is effective for the induction pulse between the transmission line and the earth. If the two forms of line-to-line connection and line-to-ground connection are combined, the surge pulse can be better absorbed.

2) Protection in load

The varistor is connected in parallel to the two ends of the inductive load, which is mainly used to absorb the inductive pulse caused by the sudden opening and closing of the inductive load to prevent the component from being damaged. Generally speaking, it is enough to connect the varistor to the inductive load in parallel. If the current type and energy size are also considered, it is more ideal to use it together with the R-C series absorption circuit.

3) Connection protection between contacts

Connecting the protective varistor in parallel to the two ends of the protected contact can prevent the switch contact from being burned out by the induced charge.

4) Protect semiconductor devices

Connect both ends of the varistor to the high-power collector and emitter ends, or both ends of the thyristor anode and cathode to limit the voltage below the withstand voltage level of the protected device. This is a very important feature for semiconductors. Effective protection.

In the specific use of varistors, if the electrical equipment withstand voltage level Vo is low, but the surge energy is relatively large, you can choose a varistor with a lower V1mA and a larger chip diameter; for example, a higher Vo The varistor with higher varistor voltage V1mA can be selected, which can protect the electrical equipment and prolong the service life of the varistor.

In addition, the varistor can also be combined with air discharge tube and TVS transient voltage suppressor to form a comprehensive surge protector to get the best protection effect. The above-mentioned devices can be composed of secondary protection or tertiary protection. The gas discharge tube is generally placed at the input end of the line as a primary surge protection device, which can withstand large surge currents; the secondary protection device uses a varistor at the μs ( Respond faster within a time range of microseconds; for high-sensitivity electronic circuits, a third-level TVS protection can be added to respond to surge voltages within a time range of ps (picoseconds).

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