Varistor type EMI countermeasure component
Transient voltages and surge voltages are a form of electromagnetic interference that often occurs. Transient voltage and surge voltage are different. Generally, the transient voltage has a short duration, and the voltage range is wide, from millivolts to 10,000 volts. The surge voltage lasts for a long time, and its amplitude is only several times of the normal value. In most cases, no strict distinction is made, collectively referred to as transient surge voltage. Switch start, short circuit fault, spark discharge, electromagnetic pulse, electrostatic discharge, lightning strike, etc. may cause different levels of transient surge voltage, causing great harm to electronic and electrical equipment. In 2000, 21 of the 42 switching power supply products sampled by the Beijing Technical Supervision Bureau were unqualified. The number of products in the base of the electrostatic discharge and transient surge voltage projects that did not meet the electromagnetic compatibility requirements was as high as 16 kinds. Anti-transient surge voltage has become one of the main contents of electromagnetic compatibility, and IEC, EU, and US FCC have established corresponding standards.
In recent years, electronic products have developed toward high-speed digitalization, and the operating voltages of ICs and various semiconductor devices used in circuits have become lower and lower, which greatly increases the hazard of transient surge voltages. Transient surge voltages often cause burnt-out of PN junctions of ICs and semiconductor devices, breakdown of oxide layers, destruction of plated metal films, etc., causing them to fail or malfunction. In order to prevent these hazards, various protective components such as gas discharge tubes, Zener diodes, silicon carbide varistors, zinc oxide ceramic varistors, barium titanate ceramic varistors, etc. have been used. Practice has shown that ceramic varistors are the best transient surge protection components and have been widely used.
1. Lead type ceramic varistor is composed of zinc oxide (or barium titanate) ceramic body and lead wire (electrode). According to its characteristics and application, it can be divided into low voltage varistor, high voltage varistor, and large flow. Varistor, lightning protection varistor and other types. They are all old products that have been produced for many years and will not be described in detail. The main manufacturers of domestic ceramic varistors are Xi'an Radio 2 Factory, Xi'an Huaxing Varistor Factory, and Beijing 798 Factory.
2. Laminated Chip Ceramic Varistor With the development of SMT, some manufacturers in the world have used ceramic lamination co-firing technology to slice ceramic varistor to produce laminates of various sizes and performances. A chip ceramic varistor, Figs. 18 and 19 are schematic views of its structure and volt-ampere characteristic curve. The laminated chip varistor has many advantages such as low varistor voltage, fast response, large flow rate, good temperature characteristics, small size, and is suitable for SMT circuit requirements. It has been widely used in various ICs and CMOS. Transient surge protection of semiconductor devices such as MOSFETs is very effective.
For a varistor of this structure, the varistor voltage depends only on the distance between two adjacent electrodes, that is, the thickness of the semiconductor ceramic film, which has little to do with the number of layers and the electrode area; The number of layers increases as the electrode area increases. Each layer is equivalent to a varistor connected in parallel by terminal electrodes. Although its structure and manufacturing process are very similar to those of MLCC, the process requirements are much more stringent, requiring high-precision equipment and strict process control. Therefore, the industrialization of laminated chip varistors is relatively difficult. The Xinjiang Institute of Physics of the Chinese Academy of Sciences, in cooperation with the Chengdu 715 Factory, has successfully researched and developed this new chip electronic component and has already put into production. Several companies in Japan, the United States, Europe, and Korea have produced in large quantities. South Korea's CTC produces 1005, 1608, 2012, 3216 and 4532 products. Table 6 lists the product performance comparisons between Chengdu 715 Factory and Siemens and CTC.
Table 7 lists the performance comparison of a stacked chip ceramic varistor with a semiconductor diode transient voltage suppressor (TVS) currently on the market. It is not difficult to see the superiority of the laminated chip varistor.
3. The laminated chip varistor array is juxtaposed with two varistors in a 2012 chip component; four varistors are juxtaposed in a 3216 chip component, and the performance is substantially equivalent to the corresponding single varistor The performance of the device is equivalent.
4. Laminated Chip Surge Absorber Some manufacturers use ceramic laminated co-firing technology to make a semiconductor wafer material based on Zn0 into a laminated chip surge absorber. The size series are 1005, 1608, 2012, 3216. Operating voltage 100 ~ 500V; maximum leakage current is less than 50μA; varistor voltage (41mA.DC) between 100 ~ 700V; capacitance 1 ~ 10pF, this product is especially suitable for electrostatic discharge (ESD) protection.
5. Three-lead wafer pressure-sensitive capacitor Some manufacturers use a ceramic material with good pressure-sensitive characteristics and high dielectric constant to make a three-wire varistor-capacitor. The structure and equivalent circuit are shown in Figure 20. . It is a monolithic structure with two leads connected in series on the signal line and the other lead grounded. In this way, the bypass capacitor can be used to filter out high-frequency interference and transient voltage protection. For example, the VRF303 series produced by Murata. It is used to protect semiconductor devices with a varistor voltage of about 50V and a capacitance of 130pF. The suppression of electromagnetic interference above 100MHz is above 20dB.
6. Three-lead wafer pressure-sensitive capacitor with ferrite bead Figure 21 shows the structure and equivalent circuit of a three-lead wafer pressure-sensitive capacitor with two ferrite beads on the input/output lead. In the circuit, two inductors are connected in series, and Fig. 22 shows the suppression characteristics and pressure-sensitive characteristics of the EMI element against electromagnetic interference. Murata's DSS710 series, varistor voltage 22V, capacitance up to 22000pF, coupled with the role of ferrite beads, its suppression of electromagnetic interference can be reduced to more than 3MHz, attenuation greater than 20dB, and the suppression of the frequency range is significantly broadened .
7. The laminated chip type three-terminal pressure-sensitive capacitor uses a ceramic laminated co-firing technology to produce a laminated chip type three-terminal pressure-sensitive one capacitor, and its structure, shape and equivalent circuit are shown in FIG. Both ends are terminal electrodes with a grounding electrode in the middle. This EMI chip component has both capacitive characteristics that bypass high frequency interference to ground and voltage sensitive to prevent transient voltage damage. Murata's laminated chip three-terminal pressure-sensitive capacitor products have a varistor voltage of 27V, a capacitance of 2200pF, and attenuation of more than 20dB above 20-1000MHz. This chip EMI component is suitable for automotive electronics, portable electronics, communication terminals, OA and various automatic control systems.
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