This article details the common methods of protection against lightning and overvoltage: shunt, voltage equalization, shielding, grounding, and protection. The internal components that make up the surge protector such as discharge tube, varistor, TVS transient voltage suppressor, common mode coil, etc. are also described in detail, and the ideal surge protector is designed according to the respective characteristics of the device.
Microelectronic equipment suffers from lightning
It is not uncommon for microelectronic devices to cause failure or damage to the device due to lightning discharge or switching operation of electrical equipment, which causes huge economic losses. Direct loss usually reflects the hardware loss of the device user and can be repaired or replaced. However, software losses and losses caused by equipment downtime are irreparable. Effective protection measures for microelectronic devices to achieve reliable protection of electronic systems (equipment) with higher integration and lower overvoltage capability, and minimize interference with lightning or shock overvoltage And damage has become an urgent problem to be solved in the reliability of microelectronic equipment.
Microelectronic devices typically operate in low-voltage power grids. There are four types of overvoltages in low-voltage networks: overvoltages caused by lightning, electrostatic discharges, operating overvoltages, and power-frequency overvoltages. Overvoltage is usually interfered with by the common mode (overvoltage is generated between the live conductor or the neutral line and the earth) and the differential mode (the overvoltage is generated between the live conductors). The lightning overvoltage is most destructive. .
Lightning protection and overvoltage protection mechanism
In the lightning protection and overvoltage protection of electronic equipment, shunting, voltage equalization, shielding, grounding and protection are usually adopted. This kind of electronic equipment is an indispensable device in lightning protection, and it was also called "overvoltage protector (SPD)" in the past. Its function is to limit the instantaneous overvoltage of the incoming power line and signal transmission line to the voltage range that the equipment or system can withstand, or to discharge a powerful lightning current into the earth, so that the protected equipment or system is not affected.
Common lightning protection and overvoltage devices
At present, lightning protection and overvoltage lightning protection devices commonly used have discharge tubes (inflatable discharge tubes), varistor and transient voltage suppressors.
1, gas discharge tube
The gas discharge tube is a low-sensitivity protection device, and its working part is usually packaged in a glass package or a ceramic, and the inside is a pair of mutually separated cold cathode electrodes, and is filled with a certain pressure of inert gas (mostly argon gas). In order to increase the trigger probability of the discharge tube, there is also a triggering agent in the discharge tube, which is structurally divided into a diode or a three-pole type.
Common overvoltage arresters can discharge transient currents below 10KA (8/20μs). The reaction time of the gas discharge tube refers to the time from the applied voltage exceeding the breakdown voltage to the occurrence of the breakdown phenomenon, and the gas discharge tube is generally on the order of μm microseconds. Gas discharge tubes have a variety of DC breakdown voltages of different specifications, the value of which depends on factors such as the type of gas and the distance between the electrodes.
The gas discharge tube has a small capacitance, generally ≤1~5pF. Its working principle means that when the voltage between the two electrodes of the gas discharge tube reaches the electrode breakdown voltage Ubr, the discharge gap immediately ignites and discharges, and a large current flows. The voltage across the gas discharge tube drops to the arc voltage between the electrodes, presenting a low resistance. The gas discharge tube can be used under DC and AC conditions. The selected DC discharge voltage Udc≥Uo (Uo is the DC voltage for normal operation of the line); when used under AC conditions, Udc≥1.44Un (Un is the normal working AC line) Voltage rms).
The operating time of the gas discharge tube is in the range of milliseconds, and is widely used in the field of remote communication. The advantage is that the current resistance is large and the static capacitance is small. The disadvantage is that the ignition voltage is high and the ignition performance is limited by time. Another disadvantage of gas discharge tubes is the potential for power flow. After the gas discharge tube is ignited, in a low-impedance circuit with a voltage exceeding 24V, it is particularly easy to maintain the short circuit caused by the gas discharge tube after only a few microseconds, and as a result, the gas discharge tube will burst in an instant. Therefore, in an overvoltage protection circuit using a gas discharge tube, a circuit breaker must be preset to cut the circuit in a very short time.
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