Mov varistor

It is a nonlinear resistor that varies with voltage. The sintered metal oxide forms a structure similar to two zener diodes, which are arranged behind. Under normal operating conditions, the typical mov leakage current is 10 Ma and the shunt capacity is about 45 PF. When the voltage rises above the MOV threshold, one of the decentralized central diodes will avalanche, enabling the device to clamp the protected node. The increased current to the rear will increase the voltage across the equipment, which is a limiting factor for most loose materials.

As an installation device, MOV can attract a large amount of transition energy, while the gas discharge tube dissipates the transition energy in the impedance of the transition source and the resistance between the transition source and the protected node. Mov can work with GDT to provide good secondary protection and prevent transient lightning, because mov startup speed is one order of magnitude faster than plasma gas switch. The accumulation of repeated overheat stress will cause the MOV to overheat and reduce its efficiency. Therefore, you should carefully analyze the transition specifications you intend to support, determine the total energy required to absorb MOVs and the transient repetition rate in very bad cases, and conservatively formulate the specifications of the equipment.

TVs transient voltage suppressor

The mobile capacity of TVs is usually only a few dozen pipines, but the mobile capacity of some new TVs is less than 10 PF. The leakage current of low peak voltage TV is generally greater than 100 Ma, while the leakage current of TV with operating voltage greater than 12 V is less than 5 mA.

At present, the trend of TV development is to improve the integration and support of high-density portable devices. Many devices are included in the chip size package to better match the node gap with the protected IC connector or interface. Integrated TV and EMI filters can perform two critical tasks in one package and facilitate bus routing through ports and / or. Many TV packages have become common protection devices in high-density components due to their small size.

Comparison of GDT / mov / TVs

The reaction time of the varistor is ns, which is faster than the air discharge tube and slightly slower than the TV tube. Generally speaking, its reaction speed can meet the requirements of electronic circuit overvoltage protection. The capacity of the varistor connector is usually hundreds to thousands of PF. In many cases, it is not suitable for directly protecting high-frequency signal lines. When used for AC circuit protection, the connector capacity will increase the leakage current, so it should be fully considered when designing the protection circuit. The current capacity of the rheostat is larger, but smaller than that of the gas discharge tube. Specifically, it can be divided into the following four points:

In terms of reaction time, the varistor is located between the television tube and the gas discharge tube. The TVS tube is picosecond, and the varistor is slightly slower, which is nanosecond; The gas discharge pipe has a low peak, usually tens of nanoseconds or longer.

In terms of current capacity, the pressure-sensitive electricity is also located between the TV tube and the gas discharge tube, and the TV tube is usually only a few hundred; According to different specifications μ S overvoltage current, the varactor can pass a few Ka to several tens Ka of a single 8 / 20; For discharge tubes, 10kA class is usually 8 / 20 μ Inrush current can be transmitted hundreds of times.

Usually, TV lamps are based on diode avalanche effect; The varactor is based on the barrier effect between ZnO grains; The gas discharge pipe is based on gas discharge.

In terms of voltage range, the TV tube is usually 5.5V to 550V; Varystor voltage range from 10V to 9000v; The voltage range of the gas discharge pipe is 75V to 3500v.

These three devices have their unique functions. How you choose depends on the damage you want to prevent. In addition, it is expected that only one protection device can be selected in a specific protection system design. FAE engineers can combine the three main circuit protection devices according to the actual protection application and consider them. For example, in the lightning protection circuit of the power supply system, there are many schemes using ceramic gas discharge tube varistors