Electronic circuits are easily damaged when overvoltage, overcurrent, surge, etc. occur. With the development of technology, electronic circuit products are becoming more diversified and more complicated, and circuit protection has become particularly important. Circuit protection components have also changed from simple glass tube fuses to more types and superior protection performance.

1. What is the meaning of circuit protection?

In all kinds of electronic products, setting over-voltage protection and over-current protection is becoming more and more important. So what is the meaning of circuit protection? Let’s talk with you today:

(1). As the integration level of circuit boards is getting higher and higher today, the price of the boards is also rising with the tide, so we must strengthen the protection.

(2) The working voltage of semiconductor devices and ICs has a tendency to become lower and lower, while the purpose of circuit protection is to reduce energy consumption, reduce heat generation, and extend service life.

(3) For vehicle-mounted equipment, due to the use of environmental conditions that are more severe than those of general electronic products, the driving conditions of the car are constantly changing, and a large instantaneous peak voltage is generated when the car is started. Therefore, in the power adapters of these electronic equipment supporting products, overvoltage protection components are generally used.

(4) Communication equipment and communication places have certain requirements for lightning and surge protection. The use of overvoltage protection and overcurrent protection components in these equipment becomes important, and they are the key to ensuring the user‘s personal safety and normal communication.

(5). Most of the faults in electronic products are caused by overvoltage or circuit phenomena in electronic equipment circuits. As our requirements for the quality of electronic equipment become higher and higher, electronic circuit protection has become more incompatible. Ignore.

So circuit protection is so important, what are the commonly used circuit protection components? Today I will introduce a few of them:

2. Lightning protection devices
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 the most widely used lightning protection device is because whether it is lightning protection for DC power supplies or lightning protection for various signals, ceramic gas discharge The tube can play a very good protective role. Its biggest feature is the large flow rate, small inter-stage capacitance, high insulation resistance, and a large range of selectable breakdown voltages.

2. Semiconductor discharge tube

The semiconductor discharge tube is a kind of overvoltage protection device, which is made using the principle of thyristor. It relies on the breakdown current of the PN junction to trigger the on and discharge of the device, which can flow a large surge current or pulse current. The range of its breakdown voltage constitutes the range of overvoltage protection. The solid-state discharge tube can be directly connected across both ends of the protected circuit when in use. It has the characteristics of precise conduction, fast response (response time ns level), strong surge absorption capacity, two-way symmetry, and high reliability.

3. Glass discharge tube

The glass discharge tube (powerful discharge tube, lightning protection tube) is a new lightning protection device introduced at the end of the 20th century. It has the advantages of a ceramic gas discharge tube and a semiconductor overvoltage protector: high insulation resistance (≥10^8Ω), extreme Small inter-capacitance (≤0.8pF), large discharge current (up to 3kA), two-way symmetry, fast response speed (there is no hysteresis of impact breakdown), stable and reliable performance, low voltage after turn-on, in addition It has the advantages of high DC breakdown voltage (up to 5000V), small size and long life. The disadvantage is that the DC breakdown voltage has a large dispersion (±20%).

Three, overvoltage devices

1. Varistor

Varistor is also one of the most used voltage-limiting devices. Using the non-linear characteristics of the varistor, when an overvoltage appears between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit. The response time of the varistor is ns, which is faster than the air discharge tube and slightly slower than the TVS tube. Under normal circumstances, the response speed of the overvoltage protection for electronic circuits can meet the requirements. The junction capacitance of varistor is generally in the range of several hundred to several thousand pF. In many cases, it is not suitable to be directly applied to the protection of high-frequency signal lines. When applied to the protection of AC circuits, because its large junction capacitance will increase leakage Current needs to be fully considered when designing the protection circuit. The current capacity of the varistor is larger, but smaller than that of the gas discharge tube.

2. The role of chip varistor

SMD varistors are mainly used to protect components and circuits to prevent ESD from power supply, control and signal lines.

3. Transient suppression diode

Transient suppressor TVS diodes are widely used in the protection of semiconductors and sensitive devices, and are usually used for secondary protection. Basically, it is used for the secondary protection after the ceramic gas discharge tube, and some users directly use it for the primary protection of the product. Its characteristics are fast response speed (ps level), small size, large pulse power, and low clamping voltage. The 10/1000μs wave pulse power ranges from 400W to 30KW, and the pulse peak current ranges from 0.52A to 544A; the breakdown voltage has a series of values ??from 6.8V to 550V, which is convenient for circuits with different voltages.

Four, overcurrent devices

Self-recovery fuse

Self-recovery fuse PPTC is a kind of overcurrent electronic protection component, which is made of high-molecular organic polymer under the conditions of high pressure, high temperature and vulcanization reaction, mixed with conductive particle materials, and processed by a special process. Self-recovery fuse (PPTC: polymer self-recovery fuse) is a positive temperature coefficient polymer thermistor used for overcurrent protection and can replace current fuse. When the circuit is working normally, its resistance value is very small (the voltage drop is small). When the circuit has over-current to increase its temperature, the resistance value increases sharply by several orders of magnitude, so that the current in the circuit is reduced to below a safe value. So that the following circuit is protected, and the low resistance value is automatically restored after the overcurrent disappears.

Five, electrostatic components

1. ESD electrostatic discharge diode

ESD electrostatic discharge diode is a kind of overvoltage and anti-static protection component, which is designed for the protection of I/O ports in high-speed data transmission applications. ESD electrostatic diodes are used to prevent sensitive circuits in electronic equipment from being affected by ESD (electrostatic discharge). It can provide very low capacitance, excellent transmission line pulse (TLP) test, and IEC6100-4-2 test capability, especially after the multi-sample number is as high as 1000, thereby improving the protection of sensitive electronic components.

2. The role of inductance

I believe everyone is aware of the relationship between electromagnetics. The role of inductance is that at the beginning of the circuit, when everything is still unstable, if there is a current in 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 period of time, everything is stable, the current has not changed, and electromagnetic induction will not produce current. At this time, it is stable and there will be no sudden changes, thus ensuring the circuit The safety is like a waterwheel. At first, it rotates slowly due to resistance, and then gradually calms down.

3. The role of magnetic beads

Magnetic beads have high resistivity and permeability, which is equivalent to a series connection of resistance and inductance, but the resistance value and inductance value change with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and shows resistance at high frequencies, so it can maintain a higher impedance in a relatively wide frequency range, thereby improving the FM filtering effect. It is used on Ethernet chips. pass.