A varistor is a voltage-limited protection device. Utilizing the non-linear 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 achieving protection of the subsequent circuit. The main parameters of the varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.
The response time of the varistor is ns level, which is faster than the air discharge tube and slightly slower than the TVS tube. Generally, the response speed of overvoltage protection for electronic circuits can meet the requirements. The varistor's junction capacitance is generally in the order of several hundred to several thousand pF. In many cases, it should not be directly applied to the protection of high-frequency signal lines. When applied to the protection of AC circuits, the large junction capacitance will increase the leakage. Current needs to be fully considered when designing the protection circuit. The varistor has a larger flow capacity, but is smaller than a gas discharge tube.
The varistor voltage (min (U1mA)) and current-carrying capacity of the varistor are important considerations in circuit design. In the DC circuit, there should be: min (U1mA) ≥ (1.8 ～ 2) Udc, where Udc is the rated DC operating voltage in the circuit. In the AC circuit, there should be: min (U1mA) ≥ (2.2 ～ 2.5) Uac, where Uac is the effective value of the AC working voltage in the circuit. The above principle of value selection is mainly to ensure that the varistor has a proper safety margin when it is applied in a power circuit. In the signal loop, there should be: min (U1mA) ≥ (1.2 ～ 1.5) Umax, where Umax is the peak voltage of the signal loop. The current capacity of the varistor should be determined according to the design index of the lightning protection circuit. Generally speaking, the current capacity of the varistor must be greater than or equal to the current capacity of the lightning protection circuit design.
Varistors are mainly used for DC power, AC power, low-frequency signal lines, and antenna-fed lines with power.
The failure mode of the varistor is mainly short circuit. When the overcurrent passed is too large, it may also cause the valve disc to be cracked and open. The varistor has a short service life, and its performance will decrease after multiple shocks. Therefore, the lightning arrester composed of varistor has problems of maintenance and replacement after long-term use.
In consumer electronics, in order to pursue a smaller mounting area, the varistor is made into a multilayer type, called Multi-layer Varistor (MLV), which has the same structure as a multilayer ceramic capacitor (MLCC). The material between the interdigitated electrodes is not a common ceramic dielectric, but a ZnO pressure-sensitive material. Because of this, MLVs all have certain capacitance characteristics, and even MLVs with a certain capacity can be customized according to needs, which is very beneficial for taking EMI design into consideration in protection design.
Because the parasitic inductance of the MLV electrode is very small after the laminated structure is made, its response speed is comparable to that of the TVS, and even faster than some TVS using the bonding structure.
In terms of current capacity, thanks to the laminated structure, the current-carrying capacity of MLV is also much greater than that of TVS with the same volume.
The clamping characteristic curve of MLV is not as steep as TVS, and accurate clamping cannot be achieved. After several large current impacts, the performance of MLV will be degraded to a certain extent, mainly due to the increase of leakage current and the change of clamping voltage. However, if MLV is only used for ESD protection, the above two disadvantages have little effect on the protection effect. This is why MLV can be popular in mobile phones, digital cameras and other fields.
Zinc oxide varistor is used in parallel with the protected electrical equipment or components. When a lightning overvoltage or transient operating voltage VS appears in the circuit, the varistor and the protected equipment and components are subjected to VS at the same time. Because the varistor responds quickly, it quickly presents excellent performance in nanoseconds. Non-linear conductive characteristics. At this time, the voltage across the varistor drops rapidly, which is much smaller than VS. In this way, the actual voltage on the protected equipment and components is much lower than the over-voltage VS, so that the equipment and components are free of Under the impact of overvoltage.