I. Performance Characteristics of Lightning Protection Components Switching elements are normally open when in operation; when a lightning surge occurs, the switching element turns on, discharging the surge current to the ground, thus protecting electronic equipment from surge damage. Switching elements come in three types: ceramic gas discharge tubes, glass discharge tubes (high-efficiency discharge tubes), and semiconductor overvoltage protectors (semiconductor discharge tubes, solid-state discharge tubes). Their advantages are: ① Before breakdown (conduction), they are equivalent to an open circuit with high resistance and almost no leakage current; ② After breakdown (conduction), they are equivalent to a short circuit, allowing a large current to pass through with a very small voltage drop; ③ They have large pulse current capacity (peak current): the 8/20μs peak current of ceramic gas discharge tubes is commonly 5kA, 10kA, 20kA, etc. (of course, there are also larger ones, reaching over 100kA), and the 10/1000μs peak current is between tens and hundreds of A; the 8/20μs peak current of glass discharge tubes is currently available in three types: 500A, 1kA, and 3kA; the 10/1000μs peak current of semiconductor overvoltage protectors is between tens and hundreds of A. ④ Except for a few semiconductor overvoltage protectors, they all have bidirectional symmetrical characteristics. ⑤ The capacitance of both ceramic gas discharge tubes and glass discharge tubes is very small, below 3pF. ⑥ The response speed of both glass discharge tubes and semiconductor overvoltage protectors is very fast, on the order of nanoseconds. ⑦ The breakdown voltage of glass discharge tubes can be made very high, reaching up to 5kV. ⑧ The breakdown voltage of semiconductor overvoltage protectors can be made very accurate. Their disadvantages are as follows: Ceramic gas discharge tubes: ① Because gas ionization requires a certain amount of time, the response speed is slow, generally 0.2～0.3μs (200～300ns), with the fastest being around 0.1μs (100ns). Before it conducts, a large spike pulse will leak through. ② The breakdown voltage consistency is poor, with a large dispersion, generally ±20%. ③ The breakdown voltage has only a few specific values. Glass discharge tubes and semiconductor overvoltage protectors: ① The current carrying capacity is much smaller than that of ceramic gas discharge tubes. ② The breakdown voltage has not yet formed a series of values. ③ The breakdown voltage of glass discharge tubes has a large dispersion, ±20%. ④ Semiconductor overvoltage protectors have large capacitances, ranging from tens to hundreds of pF. Voltage limiting elements include varistors and TVS diodes (transient voltage suppressor diodes). They have voltage limiting characteristics like Zener diodes. When the applied voltage is less than its turn-on voltage, it has a large internal resistance and a very small leakage current. When the applied voltage is greater than its turn-on voltage, its internal resistance decreases sharply, allowing a large current to flow, while the voltage across its terminals only increases slightly. Both varistors and TVS diodes have a series of turn-on voltages from low to high, making them suitable for use in circuits with various voltage levels. Additionally, both have relatively large capacitances (TVS diodes also have low-capacitance products), making them unsuitable for high-frequency circuits. The difference between varistors and TVS diodes lies in their surge current handling capabilities. Varistors can withstand larger surge currents, and the larger their size, the greater the surge current they can withstand, reaching tens to hundreds of kA. However, varistors have a larger leakage current, poorer nonlinear characteristics (larger dynamic resistance), higher voltage limit at high currents, and the magnitude of the surge current they can withstand decreases with the number of surges (derating characteristic), making them more prone to aging. TVS diodes have the same nonlinear characteristics as Zener diodes, with lower dynamic resistance, lower limiting voltage, and are less prone to aging, resulting in a longer lifespan. However, their current-carrying capacity is relatively low (peak current of a 10/1000μs waveform ranges from a few A to several hundred A). Another difference lies in their response speed: TVS diodes have extremely fast response times, in the ps (ps) range, while varistors have slightly slower response times, in the nanosecond (ns) range. Overcurrent and overheat protection components include resettable fuses, current fuses, and resistors. Overheat protection and detection components include thermal fuses and thermal fuses. A resettable fuse is a positive temperature coefficient thermistor. When the current flowing through it is less than its holding current (lower temperature), its resistance is very small; when the current flowing through it exceeds its trigger current (higher temperature), its resistance increases sharply, thus blocking the continued intrusion of lightning current or the freewheeling current in the circuit. It recovers automatically after the temperature drops. However, due to thermal inertia, its response speed is very slow, typically in the seconds range (the higher the current or the higher the temperature, the faster the response). Self-resetting fuses can replace current fuses, eliminating the hassle of frequent replacements. Temperature fuses and temperature control fuses are temperature switching elements; they are short-circuited during normal operation and open when the temperature exceeds their tripping temperature (non-resettable). They are commonly used for overheat protection and overheat detection. II. General Usage and Precautions for Lightning Protection Components General Usage of Lightning Protection Components: 1. Switching elements are mainly used for common-mode protection and are also commonly used for differential-mode protection in passive circuits. 2. Voltage limiting elements are mainly used for differential-mode protection and are also commonly used in common-mode circuits in series with switching elements to prevent short circuits between the line and ground after the switching element is turned on; or as current limiting elements for switching elements to block freewheeling current and reset the switching element. Precautions: 1. Switching elements cannot be connected alone across an active circuit for differential-mode protection. To avoid short circuits, voltage limiting elements must be connected in series. 2. The surge current flowing through the lightning protection component must be less than its pulse peak current. Varistors should be selected according to their derating characteristics. 3. The minimum breakdown voltage of surge protection components used for differential mode protection must be greater than the maximum operating voltage of the line. 4. The limiting voltage of surge protection components used for differential mode protection must be less than the maximum safe voltage that the protected equipment can withstand. III. Overview of Surge Protector Circuit Design Surge protector circuit design aims to achieve two objectives: first, to discharge the induced lightning surge current in the line to the ground; and second, to limit the surge voltage at the protected equipment to below the permissible safe voltage. The power surge protector circuit design first focuses on two of the most commonly used circuits for single-phase AC power surge protectors: the composite symmetrical circuit and the "1+1" circuit. It describes the circuit's working principle, component selection principles, circuit failure protection methods, and fault alarms. Then, it introduces typical circuits and component selection principles for three-phase AC power surge protectors and DC power surge protectors. Finally, based on pointing out the shortcomings of parallel surge protectors, the concept of series (two-port) surge protectors is proposed. The principle of reducing the limiting voltage of series (two-port) surge protectors is analyzed, and related technical issues in the design of series (two-port) surge protectors are explained. Common signal transmission systems used in signal surge protector circuit design include two-wire transmission lines, ordinary multi-core cables, twisted-pair multi-core cables, and coaxial cables. Signal surge protectors can be categorized into two types: two-wire ungrounded transmission circuits and two-wire or multi-wire transmission circuits with grounding wires. Note: Here, "ground" refers to "signal ground," not earth ground (protective earth PE). Depending on the magnitude of the potential lightning surge current, two-stage or single-stage signal surge protectors can be used for protection. Regarding overvoltage and overcurrent protection in unit circuits, many of our customers add surge protection components to their self-designed circuit boards to achieve equipment protection. Therefore, it is essential to introduce some overvoltage and overcurrent protection methods in unit circuits. This section only introduces overvoltage and overcurrent protection methods for two of the most commonly used unit circuits (power transformers or switching power supplies, integrated amplifiers). IV. Price and Technology in Lightning Protection Scheme Design Many clients request us to design lightning protection schemes for them. This reflects their trust in us and is also our strength. We will definitely provide users with the most satisfactory results by offering the most reasonable protection scheme and the lowest cost, based on the environmental conditions of the protected equipment and relevant domestic and international standards.