With the continuous advancement of society and the rapid development of the Internet of Things, the outdoor application scenarios of electronic products are growing at an unprecedented rate. As a result, electronic devices have become widely used across various fields, including public utilities, commercial applications, and civilian use. This widespread adoption has led to a diversification of product functions and an increase in the complexity of application environments. As more features are added to devices, their functional interfaces have also expanded, such as network interfaces (with POE functionality), analog video interfaces, audio interfaces, alarm interfaces, RS485 interfaces, and RS232 interfaces.
While the functionality of these devices continues to evolve, there is a growing demand for smaller form factors. This trend increases design complexity and exposes products to more potential threats, such as lightning strikes during the rainy season, which can cause batch damage, or static electricity issues during winter installation that may lead to device malfunctions. To address these challenges, this article focuses on the basic application of common protective devices in electronic products. By implementing protection circuits, we can enhance the product's resistance to electrostatic discharge (ESD) and surge interference, ultimately improving its overall stability.
In practical applications, overvoltage and overcurrent caused by lightning strikes can damage device ports. Therefore, appropriate protection circuits must be designed. Each port should be tailored based on the product family, network status, target market, and application environment. The protection circuit design varies depending on factors such as signal type and implementation cost.
1. **Gas Discharge Tube (GDT)**
A gas discharge tube is a switching-type protection device that works through gas discharge. When the voltage between its two terminals reaches a certain threshold, it breaks down from an insulating state to a conductive state, acting like a short circuit. The voltage maintained in the conductive state is typically between 20 and 50V, making it suitable for protecting downstream circuits. Key parameters 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 protection, with subsequent stages using components like varistors or TVS diodes. GDTs are known for their high insulation resistance, low interelectrode capacitance (typically below 5pF), and minimal leakage current, making them ideal for signal lines. However, they are not recommended for direct use in AC power supply circuits due to potential issues with freewheeling interruption after multiple discharges.
2. **Varistor**
A varistor is a voltage-limiting protection device that uses nonlinear characteristics to clamp overvoltage to a fixed level. It has a fast response time (in the nanosecond range) and is commonly used in DC and AC power supply circuits. Key parameters include varistor voltage, current capacity, junction capacitance, and response time.
The varistor voltage (min(U1mA)) should be chosen carefully to ensure sufficient safety margin. For example, in DC loops, min(U1mA) ≥ (1.8–2) × Udc, while in AC loops, min(U1mA) ≥ (2.2–2.5) × Uac. Varistors are typically used in DC power supplies, low-frequency signal lines, and antenna feeders. Their failure mode is usually a short circuit, and they degrade after multiple surges.
3. **TVS Diode (Transient Voltage Suppressor)**
TVS diodes are also voltage-limiting devices that respond extremely quickly (in the picosecond range). They are ideal for fine protection in high-speed signal lines. Key parameters include reverse breakdown voltage, maximum clamping voltage, instantaneous power, junction capacitance, and response time.
TVS diodes are known for their excellent nonlinear characteristics and ability to provide a lower residual voltage compared to varistors. However, their current capacity is lower than that of GDTs and varistors, so they are typically used in the final stage of protection. TVS diodes are easy to integrate and are commonly used on single boards.
4. **TSS Diode (Thyristor Surge Suppressor)**
TSS diodes are another type of voltage-limiting protection device, similar to TVS but with a switching mechanism. They operate like a gas discharge tube, clamping overvoltage to near-zero volts once triggered. TSS diodes are suitable for high-level signal lines and have a higher current capacity than TVS diodes.
However, care must be taken when selecting TSS diodes, as they require the current to drop below a critical value before returning to the open state. This makes them unsuitable for all signal line applications.
5. **PTC Thermistor (Positive Temperature Coefficient)**
PTC thermistors are current-limiting devices that increase resistance rapidly when temperature rises above a certain threshold. They are often used in series with circuits to protect against transient overcurrents. PTCs are available in polymer and ceramic types, each with different performance characteristics.
6. **Fuses, Fuses, and Air Switches**
Fuses, fuses, and air switches are essential for protecting circuits from overcurrent and short-circuit faults. They help prevent electrical fires and ensure equipment safety. In protection circuits involving GDTs, varistors, and TVS diodes, fuses are necessary to protect against damage to the protection system itself.
7. **Inductors, Resistors, and Wires**
Although not protection devices themselves, inductors, resistors, and wires play a crucial role in coordinating multiple protection components. Inductors can limit current in power circuits, while resistors are used in signal lines to avoid signal distortion. Wires can also serve as a means of coordination in high-current applications.
8. **Transformers, Optocouplers, and Relays**
These components provide isolation, helping to improve the overvoltage withstand capability of port circuits. Transformers, optocouplers, and relays can isolate the internal circuit from external surges, reducing the need for complex protection circuits. Proper selection and design are essential to ensure effective isolation and prevent breakdown under high-voltage conditions.
By integrating these protective devices and components into the design, engineers can create robust and reliable systems capable of withstanding a wide range of environmental and electrical challenges.
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