Surge protection principle of regulated power supply
Surge Protective Device (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 appliances with low withstand voltage levels or Electronic systems are protected from lightning strikes and lightning electromagnetic pulses or operating overvoltage damage. 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.
The surge protector is suitable for 220/380V low-voltage power supply protection. It is a non-linear component. According to the IEC standard, the surge protector is a device that mainly suppresses the overvoltage and overcurrent of the conducted line. The surge protector plays a protective role. The basic requirement is that it must withstand the lightning current that is expected to pass through, and through the maximum clamping voltage of the surge, it can effectively extinguish the power-frequency continuous flow generated after the lightning current passes through, and prevent the instantaneous surge into the power line and signal transmission line. The overvoltage is limited within the voltage range that the equipment or system can withstand, or the strong lightning current is leaked to the ground to protect the protected equipment or system from damage due to impact.
The type and structure of surge protectors vary according to different purposes, but at least one nonlinear voltage limiting element is included. Commonly used surge protectors include MOV (Metal Oxide Varistor) and gas discharge tubes. Power surges contain a lot of energy and cannot be stopped. For this reason, the strategy for protecting sensitive electrical equipment from damage from electrical surges is to shunt the surge away from the equipment and into the ground.
The surge protector MOV consists of three parts: the middle is a metal oxide material, and two semiconductors are connected to the power and ground. When a surge occurs, the MOV acts immediately, and the response time is 1 to 3 nanoseconds. The "V" in the MOV is a rheostat. At the moment of response, the resistance of the MOV drops from the maximum value to almost zero ohms, and the overcurrent flows into the ground through the MOV. The protected electrical equipment continues to operate under normal working voltage. Its semiconductor elements have the property of changing resistance as voltage changes. When the voltage is below a certain value, the movement of electrons in the semiconductor creates a high resistance. Conversely, when the voltage exceeds that specific value, the movement of electrons changes and the resistance of the semiconductor decreases close to zero ohms. The voltage is normal, and the surge protector MOV is idle, which does not affect the power line.
The indicators of the pros and cons of surge protector MOV: (1) Clamping voltage: Indicates the voltage value that will cause the MOV to connect to the ground. The lower the clamping voltage, the better the protection performance. (2) Energy absorption/dissipation capacity: This nominal value indicates how much energy the surge protector can absorb before it burns out, in joules. The higher the value, the better the protection performance. (3) Response time: Surge protectors do not disconnect immediately, they have a slight delay in responding to the surge.
Another common surge protection device is a gas discharge tube. These gas discharge tubes do the same thing as MOVs, they move excess current from live to ground by using an inert gas as a conductor between the two wires. When the voltage is in a certain range, the composition of the gas determines that it is a poor conductor. If the voltage surges beyond this range, the current will be strong enough to ionize the gas, making the gas discharge tube a very good conductor. It conducts current to ground until the voltage returns to normal levels, then becomes a poor conductor again.
