With the continuous advancement of society and the rapid development of the Internet of Things, the outdoor application scenarios of electronic products have seen sustained growth. As a result, electronic devices are now widely used across various fields, including public utilities, commercial sectors, and consumer applications. This widespread adoption has led to a diversification of product functions and an increase in the complexity of their operating environments. As more features are added to these devices, the number of functional interfaces has also grown significantly. These include network interfaces (with PoE functionality), analog video interfaces, audio interfaces, alarm interfaces, RS485 interfaces, and RS232 interfaces. Despite the increasing functionality, there is a growing demand for smaller device sizes. This presents challenges in design, as products face greater threats such as lightning strikes during the rainy season or damage due to static electricity during winter installation.
This article focuses on the basic application of common protective devices in electronic products. By implementing protection circuits, the anti-static and anti-surge interference capabilities of the devices can be improved, thereby enhancing their overall stability. In practical applications, overvoltage and overcurrent caused by lightning strikes can potentially damage device ports. Therefore, appropriate protection circuits must be designed. Each port should be considered based on its product family, network status, target market, and application environment. The type of protection circuit required depends on factors such as signal type and implementation cost.
1. Gas Discharge Tube
A gas discharge tube is a switching-type protection device that operates through gas discharge. When the voltage between its two poles reaches a certain threshold, the interelectrode gap discharges and transitions from an insulating state to a conductive state, acting like a short circuit. The voltage maintained between the two poles in the conductive state is typically low, ranging from 20 to 50V, which helps protect the subsequent circuitry. Key parameters of a gas discharge tube include response time, DC breakdown voltage, impact breakdown voltage, flow capacity, insulation resistance, interelectrode capacitance, and freewheeling interruption time.
Gas discharge tubes have relatively slow response times, ranging from hundreds of nanoseconds to several milliseconds. They are often used as the first stage of a protection circuit, with the second stage typically consisting of a varistor or TVS diode. These latter components offer faster response times and better protection performance. Gas discharge tubes have high insulation resistance, reaching the order of gigaohms, and very low interelectrode capacitance, usually below 5pF. This makes them suitable for use without affecting the line.
However, the freewheeling interruption capability of gas discharge tubes is a critical consideration. In DC power supply circuits, if the voltage between the two lines exceeds 15V, the gas discharge tube cannot be directly applied between the lines. In AC power supply circuits, although the AC voltage has a zero-crossing point, repeated conduction and breakdowns can reduce the freewheeling interruption capability of the gas discharge tube. It is not advisable to use a gas discharge tube alone between the phase and neutral lines in an AC power supply circuit. If the power equipment uses single-phase power, it is important to ensure that the neutral line is not connected to the gas discharge tube separately.
In the design of lightning protection circuits, it is essential to select appropriate parameters such as the DC breakdown voltage, impact breakdown voltage, and flow capacity of the gas discharge tube. For a standard AC line, the gas discharge tube should remain inactive under normal operating conditions and within the allowable voltage fluctuations. Its DC breakdown voltage should satisfy: min(ufdc) ≥ 1.8UP, where ufdc is the DC breakdown voltage and UP is the peak value of the normal operating voltage of the line.
Gas discharge tubes are primarily used for protecting the phase and neutral lines of AC power supply ports, the DC RTN to ground, signal lines to ground, and the shield layer of antenna feed ports.
The failure mode of a gas discharge tube is typically an open circuit, although it may also fail in a short-circuit condition if the discharge tube remains shorted for an extended period due to design issues or other factors. The lifespan of a gas discharge tube is relatively short, and its performance degrades after multiple impacts. Additionally, some gas discharge tubes may experience natural failures over time due to gas loss. As a result, lightning arresters composed of gas discharge tubes may require maintenance and replacement after prolonged use.
2. Varistor
A varistor is a voltage-limiting type of protection device that utilizes the nonlinear characteristics of the material. When an overvoltage occurs across the two terminals of the varistor, it clamps the voltage to a relatively fixed level, thus protecting the subsequent circuit. Key parameters of a varistor include varistor voltage, current capacity, junction capacitance, and response time.
Varistors have a response time in the range of nanoseconds, making them faster than gas discharge tubes but slightly slower than TVS diodes. Under normal circumstances, this response speed is sufficient for most electronic circuits. However, the junction capacitance of a varistor is generally in the range of several hundred to several thousand pF, which makes it unsuitable for direct use in high-frequency signal lines. When used in AC circuits, the large junction capacitance may increase leakage current, which needs to be carefully considered during the design process. The current capacity of a varistor is relatively large but less than that of a gas discharge tube.
When designing a circuit with a varistor, the varistor voltage (min(U1mA)) and the through-current capacity should be taken into account. In a DC loop, min(U1mA) ≥ (1.8 ~ 2)Udc, where Udc is the DC rated operating voltage. In an AC loop, min(U1mA) ≥ (2.2 ~ 2.5)Uac, where Uac is the effective value of the AC operating voltage. These values ensure a proper safety margin when the varistor is used in power supply circuits. In signal loops, min(U1mA) ≥ (1.2 ~ 1.5)Umax, where Umax is the peak voltage of the signal loop. The current capacity of the varistor should be determined based on the design specifications of the lightning protection circuit, ensuring that it meets or exceeds the required capacity.
Varistors are mainly used for DC power supply, AC power supply, low-frequency signal lines, and antenna feed lines.
The failure mode of a varistor is primarily a short circuit. If the overcurrent passing through is too large, the valve may break and open. Varistors have a short service life and degrade after multiple impacts. Therefore, lightning arresters composed of varistors may require maintenance and replacement after long-term use.
3. Voltage Clamped Transient Suppression Diode (TVS)
TVS (Transient Voltage Suppression) is a voltage-limiting protection device that functions similarly to a varistor. It clamps the overvoltage to a lower voltage using the nonlinear characteristics of the device, thereby protecting the subsequent circuit. Key parameters of a TVS diode include reverse breakdown voltage, maximum clamping voltage, instantaneous power, junction capacitance, and response time.
TVS diodes have a response time that can reach the picosecond level, making them the fastest among voltage-limiting surge protection devices. Their response speed is sufficient for overvoltage protection in electronic circuits. The junction capacitance of a TVS diode can be divided into two types based on the manufacturing process: high junction capacitance and low junction capacitance. High junction capacitance TVS devices are typically in the range of several hundred to several thousand pF, while low junction capacitance TVS devices are in the range of several pF to tens of pF. Generally, discrete TVS devices have higher junction capacitance, and both surface-mounted and through-hole types are available. In the protection of high-frequency signal lines, TVS devices with low junction capacitance should be primarily used.
The nonlinear characteristics of a TVS diode are better than those of a varistor. When the overcurrent through the TVS diode increases, the clamping voltage rises faster than that of a varistor, resulting in a more reliable residual voltage output. In many electronic circuits requiring fine protection, TVS diodes are a good choice. The current capacity of a TVS diode is the smallest among voltage-limiting surge protectors. It is generally used for fine protection at the final stage. Due to its small current capacity, it is typically not used for the protection of AC power lines, but rather for the lightning protection of DC power supplies. When using a TVS diode, it is generally recommended to use a device with a larger current capacity, such as a varistor. TVS diodes are easy to integrate and are ideal for use on a single board.
Another advantage of TVS diodes is their flexibility in using one-way or two-way protection devices. In unipolar signal circuits and DC power supply circuits, unidirectional TVS diodes can be used to achieve a relatively low residual voltage.
In circuit design, the reverse breakdown voltage and current capacity of the TVS diode should be considered. In a DC loop, min(UBR) ≥ (1.3~1.6)Umax, where UBR is the reverse breakdown voltage of the DC TVS, and Umax is the peak voltage in the DC loop.
TVS diodes are mainly used for lightning protection of DC power supplies, signal lines, and antenna feed lines.
The failure mode of a TVS diode is primarily a short circuit. However, if the passing current is too large, the TVS diode may break and open. TVS diodes have a relatively long service life.
4. Voltage Switching Type Transient Suppression Diode (TSS)
The voltage switching type transient suppression diode (TSS, Thyristor Surge Suppressor) is similar to the TVS diode. It is also a voltage-limiting protection device made using a semiconductor process, but its working principle is similar to that of a gas discharge tube, different from a varistor and a TVS diode. When the overvoltage across the TSS diode exceeds its breakdown voltage, the TSS diode clamps the overvoltage to a level closer to 0V than the breakdown voltage. Afterward, the TSS continues this short circuit until the current flowing through the TSS drops below the critical value, at which point the TSS returns to the open state.
TSS is a voltage-switching type transient suppression diode, also known as a surge suppression transistor or a solid discharge tube. Brands such as LangTuo offer TSS devices. The TSS diode is a protection device made using a semiconductor process and is mainly used for lightning protection of signal circuits. It is not suitable for use on power ports. TSS devices typically have a flow capacity of up to 150A (8/20μs).
Both the TSS diode and the TVS diode are voltage-limiting protection devices made using a semiconductor process. However, the TSS diode is a voltage-switching type, while the TVS diode is a voltage-clamping type. The TSS diode has similar response times and junction capacitance to the TVS diode and is easy to fabricate into a surface-mount device, making it suitable for use on a single board. The TSS diode is suitable for signal circuit protection with high signal levels.
The TSS diode has the same response time and junction capacitance as the TVS diode. It is easy to use as a surface-mount device and is very suitable for use on a single board. When the TSS diode is activated, the overvoltage is pulled from near the breakdown voltage to a level close to 0V. At this point, the junction voltage drop of the diode is small, making it suitable for high-level signal lines, such as analog subscriber lines and ADSL. The TSS diode offers a higher flow rate than the TVS diode and provides better protection for high-level signal lines.
One issue to be aware of when using TSS diodes is that after the TSS diode breaks down under the action of overvoltage, it returns to the open state once the current flowing through it drops below the critical value. Therefore, when used in a signal line, the normal current of the signal line should be less than the critical recovery current of the TSS diode. The critical recovery current value varies depending on the type and design of the TSS diode. When using it, it is important to consult the exact value specified in the device manual.
In circuit design, the breakdown voltage (min(UBR)) and current capacity of the TSS diode should be considered. In a signal loop, min(UBR) ≥ (1.2 ~ 1.5)Umax, where Umax is the peak voltage of the signal loop.
TSS diodes are primarily used for lightning protection of signal lines.
The failure mode of a TSS diode is mainly a short circuit. However, if the passing current is too large, the TSS diode may break and open. The service life of a TSS diode is relatively long.
5. Positive Temperature Coefficient Thermistor (PTC)
The PTC is a current-limiting protection device that has an operating temperature value TS. When the temperature in the body is below TS, the resistance remains substantially constant, known as cold resistance. When the temperature of the positive temperature coefficient resistor body is above TS, the resistance increases rapidly, and the maximum resistance can be about 10^4 times higher than the cold resistance. Because its resistance increases rapidly with temperature, it is generally used in series as an overcurrent protection for transient large currents. PTC has applications on both signal and power lines.
The PTC reaction speed is slow, generally above the millisecond level, so its nonlinear resistance characteristics cannot play a role in lightning overcurrent. Its current limit can only be estimated based on its normal resistance (cold resistance). The thermistor's role is more often reflected in long-term overcurrent protection, such as power line contact, and is commonly used in the protection of subscriber lines.
Currently, PTC mainly comes in two forms: polymer PTC and ceramic PTC. Among these, ceramic PTC has better overvoltage withstand capability compared to polymer PTC, but polymer PTC has a faster response speed than ceramic PTC. Generally, ceramic PTC cannot achieve low resistance, and low-resistance PTC uses polymer materials.
6. Fuses, Fuses, Air Switches
Fuses, fuses, and air switches are all protective devices that can disconnect short-circuit or overcurrent loads on the line in case of internal faults in the equipment, preventing electrical fires and ensuring the safety of the equipment.
Fuses are generally used for protection on the board, while fuses and air switches can be used for the protection of the entire system. The following is a brief introduction to the use of fuses.
For protection circuits composed of gas discharge tubes, varistors, and TVS diodes on the power supply circuit, a fuse must be included to protect the equipment from safety issues if the internal protection circuit fails. Figure 4-5 shows two examples of fuse applications. In the a circuit, the protection circuit shares a fuse with the main circuit. If the protection circuit shorts, the main circuit power supply will also be disconnected. In the b circuit, the main circuit and the protection circuit have separate fuses. If the protection circuit fails, the protection circuit is disconnected, and the main circuit can still operate normally. However, if the port is overvoltage again, the port may be damaged due to the loss of protection. Both circuits have their own advantages and disadvantages and can be selected based on the design requirements. It is not necessary to use a fuse for the protection of feeder-free signal lines and antenna feed lines.
Fuses have characteristics such as rated current and rated voltage. The rated voltage is divided into DC and AC.
The voltage rating marked on the fuse indicates that the fuse can safely and reliably interrupt its rated short-circuit current in a circuit with a voltage equal to or less than its rated voltage. The voltage rating series is included in N.E.C. regulations and is also a requirement of Underwriters Laboratories, as a protection against fire hazards. For most small-size fuses and micro fuses, manufacturers use standard voltage ratings of 32, 63, 125, 250, and 600V.
In summary, a fuse can be used at any voltage less than its rated voltage without compromising its fusing characteristics. For the fuse in the protection circuit, an explosion-proof type slow-fuse fuse should be used.
7. Inductance, Resistance, Wire
Inductors, resistors, and wires themselves are not protection devices, but they can play a role in protection circuits composed of multiple different protection devices.
Among the protective devices, the gas discharge tube is characterized by a large flow rate but a slow response time and a high impact breakdown voltage; the TVS diode has a small flow rate, the fastest response time, and the best voltage clamping characteristics; the varistor falls between the two. When a protection circuit requires a large overall flow rate and can achieve fine protection, the protection circuit often needs these types of protection devices to achieve ideal protection characteristics. However, these protective devices cannot be simply used in parallel. For example, a varistor with a large flow rate and a TVS diode with a small flow rate are directly connected in parallel. Under the action of an overcurrent, the TVS diode will be damaged first, and the varistor cannot utilize its advantage of a large flow rate. Therefore, when using several protective devices, it is often necessary to cooperate between different protective components such as inductors, resistors, and wires. The following describes each of these components:
Inductor: In the series DC power supply protection circuit, there should be no large voltage drop on the feeder. Therefore, the inter-pole circuit can be equipped with a hollow inductor, as shown below:
Figure 6 Using an inductor to achieve the coordination of two levels of protection devices
The role of electrical induction: When the protection circuit reaches the design flow rate, the overcurrent on the TVS should not reach the maximum flow rate of the TVS tube, so the inductor needs to provide sufficient current limiting capability for lightning overcurrent.
In the power circuit, the design of the inductor should pay attention to several problems:
1. The inductor coil should work normally without overheating when flowing through the fully equipped operating current of the device;
2. Try to use a hollow inductor. An inductor with a magnetic core will be magnetically saturated under the action of overcurrent. The inductance in the circuit can only be calculated by the inductance without the core.
3. The coil should be wound as much as possible, which can reduce the parasitic capacitance of the coil and at the same time enhance the resistance of the coil to transient over-voltage.
4. The insulating layer on the wire of the wound inductor coil should have sufficient thickness to ensure that the breakdown of the coil does not occur under the action of transient over-voltage.
In the protection circuit design of the power port, the inductance is usually 7~15uH.
Resistance: In the signal line, the components connected in series on the line should have as little suppression as possible on the high-frequency signal. Therefore, the resistor can be used for the inter-pole coordination, as shown in the following figure:
Figure 7 Using a resistor to achieve the coordination of two levels of protection devices
The function of the resistor should be substantially the same as that of the aforementioned inductor. The above figure is an example. The calculation method of the resistance is: measuring the impact breakdown voltage value U1 of the gas discharge tube, checking the TVS device manual to obtain the maximum through flow I1 of the TVS tube 8/20us inrush current and the highest clamp of the TVS tube. For the bit voltage U2, the minimum value of the resistor is: R≥(U1-U2)/I1.
In signal lines, there are several issues to be aware of when using resistors:
1. The power of the resistor should be large enough to avoid damage to the resistor under overcurrent;
2. Try to use a linear resistor as much as possible to minimize the effect of the resistor on normal signal transmission.
Wire: The full-scale working current of some AC/DC equipment is very large, exceeding 30A. In this case, the interference between the poles of the protection circuit and the inductor will be too large. To solve this problem, the protection circuit can be divided into two parts. In the two parts, the front-level protection and the rear-level protection are not designed on the same circuit board, and the feeders of the specified length can be used for cooperation between the two-stage circuits.
Figure 8 uses wire to achieve the coordination of two levels of anti-devices
In the protection circuit formed by the combination, the function of the specified length of the feeder is the same as that of the inductor, because the inductance of the 1 meter long wire is between 1 and 1.6 uH, and the feeder reaches a certain length, it plays a good role in cooperation. The wire diameter of the feeder can be flexibly selected according to the size of the full working current, which overcomes the shortcoming that the inductor cannot flow a large working current when the pole is matched.
8. Transformers, Optocouplers, Relays
Transformers, optocouplers, and relays are not themselves protective devices, but the design of port circuits can take advantage of the isolation characteristics of these devices to improve the port circuit’s ability to withstand overvoltage.
There are two ways to design a port lightning protection common mode protection:
1. Install a voltage limiting protector on the line to the ground. When the line introduces a lightning overvoltage, the voltage limiting protector becomes a short circuit state and discharges the overcurrent to the earth;
2. Design an isolation component on the line, and the circuits on both sides of the isolation component are not common. When the circuit introduces a lightning overvoltage, the transient overvoltage is applied to both sides of the isolation component. As long as the overvoltage acts on the isolation element, the isolation element itself is not broken down by insulation, and the high voltage signal line of the isolation element does not break down to other low voltage parts, the lightning overvoltage on the line cannot be converted into overcurrent into the device. The internal circuitry of the device is also protected. At this time, only differential mode protection needs to be designed on the line, and the protection circuit can be greatly simplified. For example, the protection of the Ethernet port can adopt this idea. The components that can achieve this isolation are: transformers, optocouplers, and relays.
The transformer here mainly refers to various signal transmission transformers for signal ports. The transformer generally has an initial/secondary insulation withstand voltage. The impulse withstand voltage of the transformer (for lightning strikes) can be converted according to the DC withstand voltage value or the AC withstand voltage value. The approximate estimation formula is: impact withstand voltage = 2 × DC withstand voltage = 3 × AC withstand voltage value.
Figure 9 is isolated with a transformer
The figure above shows a signal port protection circuit design that incorporates a transformer. When lightning strikes, the common-mode over-voltage induced on the cable outside the device acts between the primary and secondary of the transformer, as shown in Figure 9. As long as the primary/secondary insulation breakdown does not occur, the overvoltage on the cable outside the device will not be converted into an overcurrent into the device. At this time, the port only needs to be differential mode protection, and the isolation characteristics of the device such as a transformer are used to simplify the lightning protection circuit of the port.
The design of this method should be noted that the insulation withstand voltage of components such as transformers, optocouplers, and relays should be very high (for example, the withstand voltage is greater than 4kV), otherwise insulation breakdown will easily occur under the action of overvoltage. Can not play the role of improving the port withstand voltage. In addition, when using the isolation characteristics of the transformer, it is necessary to pay attention to the distributed capacitance between the primary/secondary of the transformer. In some cases, the common mode overvoltage on the external cable can be coupled from the primary to the secondary through the distributed capacitance, thereby entering the internal circuit, which destroys the isolation effect of the transformer. Therefore, the transformer with the primary interpole shield should be used as much as possible, and the outer lead of the transformer shield is grounded in the board, as shown in Figure 9. At this time, the effective insulation withstand voltage of the transformer becomes the insulation withstand voltage between the primary and the shield ground. Another problem to be aware of when using a common mode isolation design is that the primary circuit should be separated from the other circuits on the board and the ground traces on the board and have sufficient insulation distance. Generally, two printed traces with a 1 mm edge on the printed board can withstand a shock voltage of about 4 kV of 1.2/50us.
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