The selection of varistor or TVS tube, two - conduction mechanism and application analysis and comparison
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. This will be the two in the conductive mechanism, the pulse energy dissipation mechanism, the response time of ESD protection when different analysis, correct understanding errors in response time may exist, so as to make the reader a better understanding of varistors and TVS.
First, the conductive mechanism
1 Zinc Oxide pressure sensitive ceramics
Zinc Oxide ceramics is a polycrystalline semiconductor ceramic element to Zinc Oxide as the main body, add a variety of metal oxides, the electronic ceramic workmanship, with nonlinear conductive properties, is a main component of material to inhibit the over voltage and surge energy absorption and protection of ESD.
The microstructure of Zinc Oxide varistor ceramics is shown in figure 1. Zinc Oxide is composed of grain and grain boundary material, which Zinc Oxide grain mixed with donor impurity is N type semiconductor, the formation of a large number of interface states containing a large amount of metal oxide grain material, two grain and grain boundary (i.e. a micro unit) to form a similar back back bidirectional NPN junction, the ceramic is composed of many two back-to-back series parallel combination of NPN junction.
Because Zinc Oxide varistor ceramics is very thin, only the order of Angstrom, when the applied voltage is less than the reverse breakdown voltage of PN junction, which belongs to the Schottky barrier thermal electron emission conductance, the conduction current and junction barrier and temperature; when the applied voltage is greater than the PN junction reverse breakdown voltage (3.2V), which belongs to the electron tunneling breakdown conductivity. The conduction current is only related to the applied voltage, electron tunneling breakdown time is tens to hundreds of picoseconds.
Fig. 2 equivalent circuit of the micro structure chart 1
Figure 2 is the equivalent circuit of the varistor. Of which: when the applied voltage is less than the reverse breakdown voltage of PN junction, Rb far more than Rg, and almost all of the applied voltage on the grain boundary, Rb > 10M;
When the applied voltage is greater than the reverse breakdown voltage of PN junction, grain boundary tunnel breakdown electronic conductive Rb, far less than Rg, and voltage is applied in the grain and grain boundary resistance, only Rg+Rb ohm;
So when the applied voltage is less than the Zinc Oxide varistor ceramics breakdown voltage (i.e. varistor voltage), the varistor exhibits insulator of high resistance, the leakage current only when applied voltage microampere level; more than Zinc Oxide varistor ceramics breakdown voltage (i.e. varistor voltage), varistor has low resistance conductor, current through dozens of ampere. And the applied voltage increased slightly, by the current rapid growth.
The typical V-I characteristic curve of Zinc Oxide varistor is shown in figure 3:
Fig. 3 volt ampere characteristic curve of varistor
Zinc Oxide is the Zinc Oxide chip varistor varistor material, after a polycrystalline semiconductor ceramic element made of electronic ceramic casting process.
As the chip Zinc Oxide varistors can be used in electronic power lines and data transmission lines, so the protection circuit has a wide range of operating voltage, while the data line has special requirements for its capacitance. Based on the structural design and process adjustment, such as the number of layers and the thickness of the film, we can get the different requirements of line protection. Its structure and circuit as shown in figure 4:
Figure 4: structure and circuit diagram of chip varistor
Two, TVS tube
TVS (TRANSIENT VOLTAGE SUPPRESSOR) or transient voltage suppression diode is a new product developed in the regulator based on the technology, the circuit symbol and ordinary zener diode, the shape is no different from ordinary diode, divided into unidirectional and bidirectional, conductive property is non linear, for line overvoltage suppression, ESD protection.
The reverse breakdown voltage at TVS tube junction is generally greater than 6V, when the applied voltage is less than the reverse breakdown voltage of PN junction, which belongs to the Schottky barrier thermal electron emission conductance, the conduction current and junction barrier and temperature; when the applied voltage is greater than the PN junction reverse breakdown voltage (6V), which belongs to the avalanche breakdown conduction, the guide the current is only related to the applied voltage, electron avalanche breakdown time is 1 ~ 2ns.
Because the TVS pipe is PN junction micro structure, the equivalent circuit is similar to figure 2. TVS is a p-n junction, no grain resistance, namely Rg=0. So when the applied voltage is greater than the reverse breakdown voltage of PN, PN junction avalanche breakdown voltage applied mainly in conductive, PN junction resistance Rb, Rb ohm resistance only, so when the applied voltage is greater than the breakdown voltage of TVS (i.e., the varistor voltage) has low resistance conductor, current through tens of amperes, and with the TVS the current rapid growth, and the voltage of TVS is still low (relative chip varistors for Zinc Oxide). The I-V characteristic curve is shown in figure 5.
Figure 5 volt ampere characteristic curve of TVS tube
Three, energy dissipation contrast and the impact on Performance
The conduction mechanism analysis based on TVS chip, when the tube ends subjected to high energy impact 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. The characteristic of the varistor is especially suitable for overvoltage protection of the power supply part and the larger transient energy.
Four, response time
As the overvoltage protection element, when overvoltage occurs, the peak voltage protection element into a low resistance conductor, the overvoltage from high resistance insulator to greatly reduce the time, called the response time of voltage protection element. Only when the response time of over-voltage protection element is less than the rise time of over-voltage, the overvoltage suppression function can be achieved.
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 are many designers in the sense of the existence of the response time is slower than the TVS error, the response time of the device is generally determined by the material and product structure, the following analysis of these two factors.
1, material intrinsic response time
According to the above analysis of the conduction mechanism, it is known that the conductive mechanism of the Zinc Oxide varistor ceramics is the breakdown of the tunnel, so the response time of the material is the electronic breakdown time of the tunnel, 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.
2, the impact of product structure on response time
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.
Figure 6 ESD waveform
ESD in general 1nS peak discharge waveform (Figure 6), which requires an overvoltage protection device of rapid response in the range of 1nS, suppress overvoltage, protection of IC and ESD sensitive line. From the response time, the chip varistor and TVS response time to meet the needs of ESD protection, which plays a good protective effect.
Based on the above analysis and comparison, the Zinc Oxide chip ceramic varistors and TVS tube are effective devices to inhibit ESD, TVS tube lower limit voltage transient, smaller internal resistance, suitable for high voltage ESD is particularly poor or protected parts of impedance small parts (such as the receiver, MIC audio, etc.). The ability to absorb the energy of the varistor is better than the TVS tube, in addition to the general ESD protection, but also particularly suitable for the power supply parts and a large transient energy over pressure protection. In response time, to avoid falling into the chip Zinc Oxide varistor ceramic response time is slow. Because of the difference of the technology, the price of the chip varistor is much lower than TVS, showing a good price performance ratio, the designer can choose the chip varistor or TVS according to the practical application of the circuit.
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