Switching Power Supply Surge Protection Principle
SurgeProtectiveDevice, SPD, also known as surge protector, is a non-linear protective device used to limit transient overvoltage and guide discharge surge current in live systems, and is used to protect electrical or electronic systems with low withstand voltage levels from lightning strikes and Damage from lightning electromagnetic pulse or operating overvoltage. In recent years, electronic information systems (such as television, telephone, communication, computer network, etc.) have developed rapidly, and a large number of electronic information devices have emerged and become popular. Such systems and equipment are often expensive and important, and their working voltage and withstand voltage level are very low, so they are extremely vulnerable to lightning electromagnetic pulses. Therefore, SPD is required for voltage protection.
Due to the different standards followed by various countries, the product specifications are not unified, and the parameter identification also has its own emphasis, which is far inferior to other electrical product specifications, which brings great inconvenience to design and selection. In engineering design, common brands can be divided into domestic products, European products and American products according to the place of origin. The parameter settings of domestic products are chaotic, with various specifications and high residual pressure. Some of the model settings of standardized products imitate European products, and some follow the national calibration parameters. Most of the products are marked with In and Imax. Since domestic products have relatively low requirements for application sites, low building grades, and large equipment withstand voltage values, some parameter requirements can be appropriately relaxed.
European products generally mark the maximum discharge current, and the product model is also set according to this parameter. For example, XXX65 and XXX40 of a famous European brand, the values 65 and 40 are Imax. However, the Chinese standard clearly stipulates that the nominal discharge current In should be used for type selection, which is an embarrassing situation encountered in engineering design at present. After checking the product information, the In value of XX65 does not exceed 20kA, and the In value of XX40 does not exceed 15kA. According to the recommended value of GB50343, these two products can only be used for the third-level protection of the equipment terminal, but in the actual design, they are installed on the first and second levels, which is obviously inconsistent with the selection parameters of the national standard, and the residual voltage Higher, ordinary models generally exceed 1200V, once the wiring environment is not good, it is easy to exceed the withstand voltage value of the equipment. Generally, the Uc value of European products is small, and the line voltage is opportunistically marked, so it is easy to be misleading when selecting a model.
Classification of surge protectors
SPD is an indispensable device in the lightning protection of electronic equipment. Its function is to limit the instantaneous overvoltage that enters the power line and signal transmission line within the voltage range that the equipment or system can withstand, or to leak the strong lightning current into the ground. , to protect the protected equipment or system from impact.
Classified by working principle
According to its working principle, SPD can be divided into voltage switch type, voltage limiting type and combination type.
(1) Voltage switch type SPD. It presents high impedance when there is no instantaneous overvoltage, and once it responds to lightning instantaneous overvoltage, its impedance suddenly changes to low impedance, allowing lightning current to pass through, also known as "short-circuit switch type SPD".
(2) Pressure-limiting SPD. When there is no instantaneous overvoltage, it is high impedance, but with the increase of surge current and voltage, its impedance will continue to decrease, and its current-voltage characteristics are strongly nonlinear, sometimes called "clamp SPD".
(3) Combined SPD. A combination of voltage-switching and voltage-limiting components that can exhibit either voltage-switching or voltage-limiting or both characteristics, depending on the characteristics of the applied voltage.
Classification by purpose
According to its use, SPD can be divided into two types: power line SPD and signal line SPD.
(1) Power line SPD
Since the energy of lightning strikes is very large, it is necessary to release the energy of lightning strikes to the ground step by step through the method of hierarchical discharge. In the direct lightning non-protection zone (LPZ0A) or at the junction of the direct lightning protection zone (LPZ0B) and the first protection zone (LPZ1), install a surge protector or a voltage-limiting surge protector that has passed the Class I classification test as the first The first level of protection is to discharge the direct lightning current, or when the power transmission line is directly struck by lightning, it will discharge the huge energy conducted. Install a voltage-limiting surge protector at the junction of each division (including LPZ1 area) after the first protection zone, as a second, third or higher level of protection. The second-level protector is a protection device for the residual voltage of the previous-level protector and the induced lightning strike in the area. When a large lightning strike energy absorption occurs in the front-level, there is still a part that is quite large for the equipment or the third-level protector. The energy will be transmitted and needs to be further absorbed by the second-level protector. At the same time, the transmission line passing through the first-level lightning protection device will also induce lightning electromagnetic pulse radiation. When the line is long enough, the energy of the induced lightning becomes large enough, requiring a second-level protector to further discharge the lightning energy. The third-level protector protects the residual lightning energy passing through the second-level protector. According to the withstand voltage level of the protected equipment, if the two-level lightning protection can limit the voltage lower than the withstand voltage level of the equipment, only two levels of protection are required; if the withstand voltage level of the equipment is low, four levels or even More levels of protection.
(2) Signal line SPD
With the wide application of information systems, due to the large number of network lines and the low withstand voltage level of electronic equipment, lightning strikes are more and more harmful to information systems. The harm of lightning to the information system is mainly caused by the lightning electromagnetic pulse, including the lightning overvoltage wave conducted along the line, the high potential counterattack generated by the lightning current on the ground wire, the electrostatic induction and electromagnetic induction of the lightning electromagnetic field. The protective measures for electromagnetic pulses include interception, shunting, equipotential bonding, shielding, grounding, and reasonable wiring. Installing SPD on the signal line is an important measure for the information system to prevent electromagnetic pulses. It can simultaneously play the role of interception, shunting, and equipotential bonding. The signal line SPD should be connected to the signal port of the protected equipment. Its output terminal is connected to the port of the protected equipment, which can be divided into serial connection and parallel connection, and is generally installed in series on the signal line. Therefore, when selecting a signal SPD, an SPD with a smaller insertion loss should be selected.
