What are the common circuit protection components in PCBA processing?

What is the meaning of circuit protection?

In all kinds of electronic products, the setting of overvoltage protection and overcurrent protection is becoming more and more important, then the meaning of circuit protection is what in the end, today to talk to you:

(1) As the integration of the circuit board is getting higher and higher, the price of the board is also rising, so we need to strengthen protection.

(2) semiconductor devices, IC working voltage has a lower and lower trend, and the purpose of circuit protection is to reduce energy loss, reduce heating phenomenon, prolong the service life.

(3) The on-board equipment, due to the use of the environment conditions are more severe than the general electronic products, the driving condition of the car changes, the car starts to produce a large instantaneous peak voltage, etc. Therefore, in the power adapters that accompany these electronic devices, overvoltage protection elements are generally used.

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

(5) Most of the failures of electronic products are caused by the overvoltage or circuit phenomenon in the electronic equipment circuit. As we have higher and higher requirements for the quality of electronic equipment, electronic circuit protection has become more and more important.

So circuit protection is so important, what are the commonly used PCB circuit protection components? I'm going to introduce some of them today.

First, lightning protection device

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 that whether it is the lightning protection of DC power supply or the lightning protection of various signals, the ceramic gas discharge tube can play a very good protective role. Its biggest characteristics are large through flow, small capacitance between stages, high insulation resistance, breakdown voltage optional range.

2. Semiconductor discharge tube:

Semiconductor discharge tube is a kind of overvoltage protection device, is made of thyristor principle, relying on the breakdown current of PN junction to trigger the device conduction discharge, can flow through a large surge current or pulse current. The range of the breakdown voltage constitutes the range of overvoltage protection.

Solid discharge tube can be directly bonded at both ends of the protected circuit. It has the characteristics of precise conduction, fast response (ns response time), strong surge absorption capacity, bidirectional symmetry and high reliability.

3. Glass discharge tube:

Glass discharge tube (strong discharge tube, detonator) is a new lightning protection device launched at the end of the 20th century, it has the advantages of ceramic gas discharge tube and semiconductor overvoltage protector: High insulation resistance (≥10^8Ω), small electrode capacitance (≤0.8pF), large discharge current (up to 3 kA), two-way symmetry, fast reaction (no impact breakdown hysteretic phenomenon), stable and reliable performance, low voltage after conduction, in addition to high DC breakdown voltage (up to 5000V), small volume, long life and other advantages. Its disadvantage is that the DC breakdown voltage dispersion is larger (±20%).

Two, overvoltage device

1. Varistor:

Varistor is also one of the most used voltage limiting devices. Using the nonlinear characteristics of the varistor, when the overvoltage appears between the poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, so as to realize the protection of the later stage circuit.

The response time of varistor is ns, faster than that of air discharge tube and slightly slower than TVS tube. Generally, the response speed of varistor can meet the requirements for overvoltage protection used in electronic circuits. The junction capacitance of varistor is generally in the order of several hundred to several thousand pF range. In many cases, it is not suitable to be directly applied in the protection of high frequency signal lines. When applied in the protection of AC circuits, its junction capacitance is larger than that of AC circuits, which will increase the leakage current. Varistors have larger through-flow capacity than gas discharge tubes.

2. Patch varistor:

Patch varistors are mainly used to protect components and circuits from ESD generated in power supply, control and signal lines.

3. Transient suppression diode:

Transient suppressor TVS diodes are widely used in semiconductor and sensitive device protection, usually for secondary protection. It is basically used for the secondary protection after the ceramic gas discharge tube, and some users directly use it for the primary protection of the product.

It is characterized by fast reaction speed (ps class), small volume, large pulse power and low clamping voltage. The pulse power of 10/1000μs wave ranges from 400W to 30KW, and the peak pulse current ranges from 0.52A to 544A. The breakdown voltage has a series of values from 6.8V to 550V, to facilitate the use of a variety of different voltage circuits.

Three, the current device

Self-restoring fuse:

Self-restoring fuse PPTC is a kind of overcurrent electronic protection element, using high molecular organic polymer in high pressure, high temperature, vulcanization reaction conditions, mixed with conductive particle material, after a special process. Self-restoring fuse (PPTC: polymer self-restoring fuse) is a positive temperature coefficient polymer thermistor, used for overcurrent protection, can replace the current fuse.

When the circuit works normally, its resistance value is very small (voltage drop is very small). When the circuit overcurrent makes its temperature rise, the resistance value increases sharply by several orders of magnitude, so that the current in the circuit is reduced to the safety value below, so that the following circuit is protected. After the overcurrent disappears, the resistance value is automatically restored to the low value.

Four, electrostatic components

1, ESD electrostatic discharge diode:

ESD electrostatic discharge diode is a kind of overvoltage, anti-static protection element, is designed for high-speed data transmission application I/O port protection device. ESD Electrostatic diodes are used to protect sensitive circuits in electronic devices from ESD(electrostatic discharge).

Provides very low capacitance, excellent transmission line pulse (TLP) testing, and IEC6100-4-2 testing capability, especially after multisampling up to 1000, thus improving protection of sensitive electronic components.

2. Inductance:

I believe everyone is clear about the electromagnetic relationship. The function of inductance is that at the beginning of the circuit, when everything is unstable, if there is a current passing through the inductance, an induction current in the direction opposite to the current will be generated (Faraday's law of electromagnetic induction). After the circuit has been running for a period of time, everything will be stable and the current will not change. Electromagnetic induction will not produce current, at this time is stable, there will be no sudden changes, so as to ensure the safety of the circuit, just like the water wheel, at the beginning due to the resistance of the rotation is relatively slow, and then slowly become peaceful.

3. Magnetic beads:

Magnetic beads have very high resistivity and permeability, which is equivalent to resistance and inductance in series, but the resistance value and inductance value vary with frequency. It has better high-frequency filtering characteristics than ordinary inductors, showing resistance at high frequency, so it can maintain a relatively high impedance in a wide frequency range, so as to improve the frequency modulation filtering effect, which has been used in Ethernet chips.

Electronic circuits are easy to be damaged when overvoltage, overcurrent, surge and other conditions occur. With the development of technology, PCB circuit products are increasingly diversified and complicated, and circuit protection becomes particularly important. Circuit protection components from simple glass tube fuse, become more types, more superior protection performance.