Electronic circuits are easily damaged by overvoltage, overcurrent, surges, and other conditions. With technological advancements, electronic circuit products are becoming increasingly diverse and complex, making circuit protection particularly crucial. Circuit protection components have evolved from simple glass tube fuses to a wider variety of types with superior protective performance.
What is the significance of circuit protection?
Overvoltage and overcurrent protection are becoming increasingly important in various electronic products. So what exactly is the significance of circuit protection? Let's discuss that today:
(1) As the integration of circuit boards is getting higher and higher, the price of the boards is also rising, so we need to strengthen protection.
(2) Semiconductor devices, ICs have a trend of decreasing operating voltage, while the purpose of circuit protection is to reduce energy loss, reduce heat generation, and extend service life.
(3) Vehicle-mounted equipment: Due to the harsher environmental conditions compared to general electronic products, the driving conditions of a car are highly variable, and a large instantaneous peak voltage is generated when the car starts. Therefore, overvoltage protection components are generally used in the power adapters of products that support these electronic devices.
(4) Communication equipment. Communication sites have certain requirements for lightning surge protection. It is important to use overvoltage protection and overcurrent protection components in these devices, as they are the key to ensuring the personal safety of users and normal communication.
(5) Most of the faults in electronic products are caused by overvoltage or circuit phenomena in the electronic equipment circuit. As our requirements for the quality of electronic equipment become higher and higher, the protection of electronic circuits has become more and more important.
Since circuit protection is so important, what are some commonly used circuit protection components? Today, we'll introduce a few:
Lightning protection devices
1. Ceramic gas discharge tube:
Among surge protection devices, ceramic gas discharge tubes are the most widely used. The reason they are considered the most widely used surge protection device is that they provide excellent protection against surges in both DC power supplies and various signals. Their most significant characteristics are high current carrying capacity, low inter-electrode capacitance, high insulation resistance, and a wide range of selectable breakdown voltages.
2. Semiconductor discharge tube:
A semiconductor discharge tube is an overvoltage protection device made using the thyristor principle. It relies on the breakdown current of the PN junction to trigger the device to conduct and discharge, allowing large surge currents or pulse currents to flow. The range of its breakdown voltage defines the overvoltage protection range. Solid-state discharge tubes can be directly connected across the protected circuit. They feature precise conduction, fast response (nanosecond-level response time), strong surge absorption capability, bidirectional symmetry, and high reliability.
3. Glass discharge tube:
Glass discharge tubes (high-efficiency discharge tubes, surge protectors) are surge protection devices newly introduced at the end of the 20th century. They combine the advantages of ceramic gas discharge tubes and semiconductor overvoltage protectors: high insulation resistance (≥10^8Ω), small inter-electrode capacitance (≤0.8pF), large discharge current (up to 3kA), bidirectional symmetry, fast response speed (no hysteresis phenomenon of impulse breakdown), stable and reliable performance, and low voltage after conduction. In addition, they also have advantages such as high DC breakdown voltage (up to 5000V), small size, and long life. Their disadvantage is that the DC breakdown voltage dispersion is relatively large (±20%).
Overvoltage devices
4. Varistor:
Varistors are also one of the most widely used voltage limiting devices. Utilizing the non-linear characteristics of varistors, when an overvoltage occurs between their terminals, the varistor clamps the voltage to a relatively fixed value, thus protecting downstream circuits. The response time of a varistor is in the nanosecond range, faster than an air discharge tube but slightly slower than a TVS diode. Generally, its response speed is sufficient for overvoltage protection in electronic circuits. The junction capacitance of a varistor is typically in the range of several hundred to several thousand pF. In many cases, it is not suitable for direct application in the protection of high-frequency signal lines. When used in AC circuit protection, its large junction capacitance increases leakage current, which needs to be carefully considered when designing the protection circuit. The current carrying capacity of a varistor is relatively large, but smaller than that of a gas discharge tube.
5. The function of surface mount varistors:
Surface mount varistors are mainly used to protect components and circuits from ESD generated in power supply, control and signal lines.
6. Transient voltage suppressor diode:
Transient voltage suppressor (TVS) diodes are widely used for the protection of semiconductors and sensitive devices, typically as secondary protection. They are generally used as secondary protection after ceramic gas discharge tubes, although some users directly use them for primary protection. Their characteristics include fast response speed (ps), small size, high pulse power, and low clamping voltage. Their 10/1000μs pulse power ranges from 400W to 30KW, and peak pulse current ranges from 0.52A to 544A; breakdown voltages are available in a series of values from 6.8V to 550V, facilitating use in circuits with various voltage requirements.
Overcurrent devices
7. Resettable fuse:
A PPTC (Polymer Resettable Tissue Circuit) fuse is an overcurrent electronic protection component. It is made by adding conductive particles to a high-molecular-weight organic polymer under high pressure, high temperature, and vulcanization conditions, and then processing it through a special process. A PPTC fuse is a positive temperature coefficient polymer thermistor used for overcurrent protection and can replace a current fuse. When the circuit is working normally, its resistance is very small (voltage drop is very small). When an overcurrent occurs and causes the circuit temperature to rise, the resistance increases sharply by several orders of magnitude, reducing the current in the circuit to below a safe value, thus protecting the downstream circuitry. After the overcurrent disappears, it automatically returns to a low resistance value.
Static components
8. ESD (Electrostatic Discharge) Diode:
ESD diodes are overvoltage and electrostatic discharge protection devices designed for I/O port protection in high-speed data transmission applications. They are used to prevent sensitive circuits in electronic devices from being affected by ESD (electrostatic discharge). They offer very low capacitance, excellent transmission line pulse (TLP) testing, and IEC 6100-4-2 testing capabilities, especially with high sampling counts up to 1000, thus improving the protection of sensitive electronic components.
9. The function of an inductor:
The relationship between electromagnetism is something most people are familiar with. The role of inductance is that when a circuit is initially unstable, if current flows through the inductor, an induced current will be generated in the opposite direction to the current (Faraday's law of electromagnetic induction). After the circuit has been running for a while, everything stabilizes, the current remains relatively stable, and electromagnetic induction will no longer generate current. At this point, the circuit is stable and there will be no sudden changes, thus ensuring the safety of the circuit. It's like a waterwheel; at first, it rotates slowly due to resistance, but later it gradually becomes smoother.
10. The function of magnetic beads:
Ferrite beads have high resistivity and permeability, and are equivalent to a resistor and an inductor in series, but both the resistance and inductance values vary with frequency. They offer better high-frequency filtering characteristics than ordinary inductors, exhibiting resistivity at high frequencies. Therefore, they can maintain high impedance over a fairly wide frequency range, thereby improving the frequency modulation filtering effect. They have been used in Ethernet chips.
