ESD selection method of SMD varistor
1, the maximum DC working voltage (Vdc) DC voltage: maximum DC voltage varistor must be greater than the signal line (Vn), Vdc = Vn.
2, capacitance Cp (Capacitance): for high frequency transmission signal, capacitor Cp should be smaller, and vice versa. Such as the USB3.0, HDMI interface using less than 1pF varistor.
3, resistance matching (Resistance Match): protected components (line) resistance R (R = 2) and varistor transient resistance Rv: R = 5 Rv; for smaller internal resistance protected components, without affecting the signal transmission rate, as far as possible the use of large capacitor varistor.
4, place (Layout): close to the ESD source, that is, the closer the interface, the better.
5, grounding design (Ground Design): good grounding.
ESD application error of SMD varistors and TVS tubes
Zinc Oxide varistors and TVS tubes are commonly used in ESD protection devices, to enhance the overall performance of the ESD has a very important role. However, the mechanism and structure of the varistor and the TVS tube are different, so the performance is not the same. Compare the characteristics of both, as shown in table 2.

Table 2 Comparison of chip varistor and TVS tube
By comparison, can be seen MLCV advantages are mainly embodied in the low cost, fast response time, energy absorption capacity and excellent temperature characteristics, so part of the overvoltage protection varistors for large transient energy, and the price advantage. While the price of TVS is slightly higher, the temperature characteristics of the attenuation problem from 25 degrees, but the clamp voltage is low, fast response, so that it is more appropriate in the case of fine protection circuit.
At present, the most common types of errors, mainly reflected in the response time and energy absorption capacity of two aspects, and then focus on its analysis, so as to enable readers to better understand the varistor and TVS tube.
1, response time
Overvoltage protection element response time is determined by the element material and structure, when the parasitic inductance and capacitance in the product structure, in addition to the response time effect on the protective element, will influence instantaneous line voltage oscillation process.
At present, there is a misunderstanding that the response time of the varistor is slower than that of TVS, and the response time of the device is determined by the material and product structure.
The conductive mechanism of Zinc Oxide varistor ceramics is tunnel breakdown, so the material response time is the time of the tunnel electron breakdown, which is 0.3ns. The principle of TVS tube motor is avalanche breakdown, and its response time is the avalanche breakdown time, usually between 0.5~1ns.
Zinc Oxide chip varistors with multilayer monolithic structure, the parasitic inductance is very small, the response time of the varistor have little effect, some designers about time mainly refers to the plug-in varistor surge AC side response, because of the long lead students to send into the inductance of the slow response time (25ns). However, in order to meet the requirements of SMT, the electrode lead is designed at both ends of the TVS tube, and parasitic inductance can also be generated.
2, energy dissipation contrast and the impact on Performance
When TVS type high energy shock tube ends subjected to moment, it can in the nanosecond time the PN junction impedance is suddenly reduced, the voltage between the ends of the clamp in a predetermined value, so as to ensure that the circuit element behind from transient high energy impact damage. Due to the internal TVS tube is only an avalanche PN junction structure, conduction time in both ends of TVS showed low resistance conductor characteristics, thus limiting the varistor voltage is lower, in the TVS branch flow is larger, the properties for use in high voltage ESD is particularly poor or protected parts of impedance small parts (such as the receiver. MIC, audio). But TVS can not absorb the instantaneous pulse energy, can only be transmitted to the single direction of the energy of the common ground line, it is possible to connect to other ESD sensitive devices in the public cause two damage.
Varistor microstructure is a PN junction and grain size of the string and connection fit, can absorb and transfer energy, when the line in any over-voltage, varistors rapidly from megohm insulation resistance level to the resistance in ohms, the overvoltage suppression to the low water level and absorbs some energy, so the energy absorbing ability of varistor TVS stronger, and can prevent transient EMI caused by ESD and two damage.

Notes on the application of SMD Varistors
1, the problem: do not read card

2, the problem: bad video output

3, the problem: short circuit