Electronic circuits are easily damaged in situations such as overvoltage, overcurrent, surges, etc. With the development of technology, electronic circuit products have become increasingly diversified and complex, making circuit protection especially important. Circuit protection components have evolved from simple glass tube fuses to a wider variety with superior protective performance.
What is the Significance of Circuit Protection?
In various electronic products, setting up overvoltage and overcurrent protection has become increasingly important. So what is the significance of circuit protection? Today, let’s discuss it:
- As circuit boards have become more integrated and their prices have risen, we need to enhance protection.
- Semiconductor devices and ICs are trending toward lower operating voltages. The purpose of circuit protection is to reduce energy loss, minimize heating, and extend service life.
- In-car equipment, due to harsher environmental conditions compared to general electronic products and the variable driving conditions, generates significant instantaneous peak voltage when the car starts. Therefore, overvoltage protection components are generally used in the power adapters of these electronic devices.
- Communication equipment: communication sites require certain protection against lightning surges. Overvoltage and overcurrent protection components are crucial in these devices to ensure user safety and normal communication.
- Most failures in electronic products are caused by overvoltage or overcurrent phenomena in electronic circuits. As our quality requirements for electronic devices increase, circuit protection has become even more crucial.
Given the importance of circuit protection, what are the commonly used circuit protection components? Today, let me introduce a few.
01
Lightning Protection Components
① Ceramic gas discharge tube
The most widely used lightning protection device is the ceramic gas discharge tube. The reason why the ceramic gas discharge tube is considered the most widely used lightning protection device is that it can provide excellent protection for both DC power and various signals.
Its main characteristics are large surge current capacity, small interstage capacitance, high insulation resistance, and a wide range of selectable breakdown voltages.
② Semiconductor discharge tube
The semiconductor discharge tube is an overvoltage protection device made using the principle of the thyristor. It relies on the breakdown current of the PN junction to trigger conduction and discharge, allowing it to handle large surge currents or pulse currents. The range of its breakdown voltage constitutes the range of overvoltage protection.
The solid discharge tube can be directly connected across the protected circuit. It has features such as precise conduction, fast response (response time in nanoseconds), strong surge absorption capacity, bidirectional symmetry, and high reliability.
③ Glass discharge tube
Glass discharge tubes (high-efficiency discharge tubes, lightning protection tubes) are newly introduced lightning protection devices from the late 20th century. They combine the advantages of ceramic gas discharge tubes and semiconductor overvoltage protectors: high insulation resistance (≥10^8Ω), low inter-electrode capacitance (≤0.8pF), large discharge current (up to 3 kA), bidirectional symmetry, fast response (no delay in impact breakdown), stable and reliable performance, and low voltage after conduction.
Additionally, they have the benefits of high DC breakdown voltage (up to 5000V), small size, and long lifespan. However, their drawback is the wide variability in DC breakdown voltage (±20%).
02
Overvoltage Components
① Pressure-sensitive resistor
A varistor is also one of the most commonly used voltage-limiting devices. By utilizing the non-linear characteristics of varistors, when overvoltage occurs across the two terminals of the varistor, it can clamp the voltage to a relatively fixed value, thereby protecting the subsequent circuit.
The response time of the varistor is in the nanosecond range, faster than gas discharge tubes but slightly slower than TVS diodes. In general, it can meet the requirements for overvoltage protection in electronic circuits. The junction capacitance of varistors is typically in the range of hundreds to thousands of picofarads. In many cases, it is not suitable for direct application in the protection of high-frequency signal lines. When applied in the protection of AC circuits, its relatively large junction capacitance can increase leakage current, thus requiring careful consideration in the design of protective circuits. Varistors have a higher current capacity than gas discharge tubes but smaller than TVS diodes.
② The role of surface mount pressure sensitive resistors
Surface mount varistors are mainly used to protect components and circuits, preventing Electrostatic Discharge (ESD) in power supplies, controls, and signal lines.
③ Transient Voltage Suppression Diode
Transient voltage suppressor (TVS) diodes are widely used for protection in semiconductor and sensor devices, typically employed as secondary protection. They are commonly used as secondary protection after ceramic gas discharge tubes, although some users directly apply them for primary protection of their products.
Their features include fast response time (in the picosecond range), small size, high pulse power handling capability, and low clamping voltage. Their 10/1000μs surge pulse power ranges from 400W to 30KW, with peak pulse current ranging from 0.52A to 544A. Breakdown voltages are available in series from 6.8V to 550V, facilitating their use in circuits with various voltage requirements.
03
Overcurrent Protection Components
① Self-recovery fuse
The self-recovery fuse PPTC is a type of overcurrent electronic protection component, which uses high polymer organic polymers under conditions of high voltage, high temperature, and sulfurization reaction. After adding conductive particle materials and undergoing special processing techniques. The self-recovery fuse (PPTC: Polymer Positive Temperature Coefficient Resettable Fuse) is a positive temperature coefficient polymer thermistor used for overcurrent protection, which can replace current fuses.
When the circuit is operating normally, its resistance value is very low (small voltage drop). When an overcurrent occurs in the circuit causing its temperature to rise, the resistance value increases sharply by several orders of magnitude, reducing the current in the circuit to below a safe value, thereby protecting the subsequent circuits. After the overcurrent disappears, it automatically recovers to a low resistance value.
04
Electrostatic Component
① ESD static discharge diode
The ESD static discharge diode is an overvoltage and anti-static protection component designed for protecting I/O ports in high-speed data transmission applications. ESD static diodes 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 IEC6100-4-2 testing capabilities, particularly after multiple samples up to 1000, thereby enhancing the protection of sensitive electronic components.
② The function of inductance
The relationship between electromagnetism is well known. The role of an inductor in a circuit is that at the initial stage when everything is still unstable, if there is current passing through the inductor, it will generate an induced current opposite to the direction of the current (Faraday’s Law of Electromagnetic Induction). After the circuit has been running for a while everything has stabilized, and there is no change in the current, electromagnetic induction will no longer produce current. At this point, it stabilizes and no sudden changes occur, ensuring the safety of the circuit, similar to a water wheel that initially rotates slowly due to resistance and then gradually stabilizes.
③ The function of magnetic beads
Ferrite beads have high resistivity and magnetic permeability, and they are equivalent to a resistor and inductor in series, but both the resistance value and inductance value vary with frequency. They have better high-frequency filtering characteristics than ordinary inductors, appearing resistive at high frequencies. Therefore, they can maintain high impedance over a wide frequency range, improving frequency modulation filtering effects. They have been used in Ethernet chips.
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