PCB Technical - What are the commonly used electronic components for power board PCB processing?

Power board PCB processing is a common category in PCB processing. Some electronic components are unique to power board PCB processing, and some components are common to power board PCBs. Understanding and familiarizing with the common components in the PCB processing of the power supply board and what their uses are, it is helpful to understand the processing of the power supply board PCB.

Introduction of commonly used electronic components in power board PCB processing

AC input socket
This is the first hurdle for AC power input from the outside. In order to block the interference from the power line and avoid the exchange noise generated by the power supply from spreading out through the power line to interfere with other electrical devices, the AC input terminal will install first to second order EMI (Electromagnetic Interference) Filter, whose function is a low-pass filter, bypasses or guides the high-frequency noise contained in the alternating current to the ground line, and only allows the wave of about 60Hz to pass.
Power board PCB AC input socket

In the above photo, the center is an integrated EMI filter power socket, and the filter circuit is completely wrapped in an iron shell, which can more effectively prevent noise from leaking; the one on the right is made of small circuit boards to make EMI filter circuits, which are usually used when there is not enough The power supply with integrated EMI filter is deeply installed, and there will be some noise leakage without the iron shell; and only Cx and Cy capacitors are added to the socket on the left (will be introduced later), and the power supply of this type of design is used. The EMI filter circuit usually needs to be built on the main circuit board. If the EMI circuit area on the main circuit board is empty, it means that the components in this area have been omitted.

At present, the internal space of the power supply with 12 cm fan cannot fit the integrated EMI filter, so most of them adopt the method on the left and right sides of the photo.

X capacitor (Cx, also known as cross-line filter capacitor)
This is used to bridge the capacitor between the live wire (L) and the neutral wire (N) in the composition of the EMI filter circuit. The purpose is to eliminate the low normal state noise from the power line.
Power board PCBA processing-X capacitor (Cx, also known as jumper line filter capacitor)

The appearance is square as shown in the photo, with X or X2 on the top.

Y capacitance (Cy, also known as line bypass capacitor)
The Y capacitor is connected across the floating ground (FG) and the live wire (L)/neutral wire (N) to eliminate high normal state and common state noise.
Power board PCB processing-Y capacitor (Cy, also known as line bypass capacitor)

The appearance of the Y capacitor is like a pie in the photo

The FG point in the computer power supply is connected to the metal casing, the ground wire (E) and the output terminal 0V/GND. Therefore, when the ground wire is not connected, the potential difference of half of the input power will be divided by two Cy capacitors in series. (Vin/2), after the human body touches it, it may cause electric induction.

Common choke (cross-connected inductance)
Power board PCB processing-common mode choke (cross-connected inductance)

The common mode choke coil is connected in series with the live wire (L) and the neutral wire (N) in the filter circuit to eliminate the low-pass common mode and radio frequency noise of the power line. The input circuit of some power supplies is designed to be wound on the magnetic core, and it can also be regarded as a simple common mode choke.

Its appearance has a toroidal shape and a transformer-like square shape, and some exposed coils can be seen.

The so-called common mode noise represents the noise between the L/N line and the ground line E, while the normal noise is the noise between the L and N lines. The function of the EMI filter is mainly to eliminate and block these two types of noise. After the EMI filter circuit is an instantaneous protection circuit and a rectifier circuit, the common components are as follows.

fuse
The fuse is when the current value flowing through it exceeds the rated limit, it will fuse to protect the connection to the back-end circuit. Generally, the fuse used in the power supply is a fast-acting type, and the better one is an explosion-proof type. The biggest difference between a fuse and a general fuse is that the outer tube is a beige ceramic tube, which is filled with fireproof material to avoid sparks when it is fused.
Power board PCB components-fuse

The way it is installed on the circuit board is the fixed type above the picture (the two ends are directly covered with the wire holder and soldered to the circuit board) and the detachable type in the center of the picture (fixed with a metal clip). The square component below is a thermal fuse. This type of fuse is fixed on the high-power cement resistor or the heat sink of the power component. It is mainly used for over-temperature protection to avoid component damage or fire caused by overheating. This type of fuse is also combined with current fuse The version of, double protection for current and temperature.

Negative temperature coefficient resistance (NTC)
Because the high-voltage side electrolytic capacitor in the power supply is in a non-electric state when the power is turned on, excessive current surge and line voltage drop will be generated at the moment of charging, which may cause the bridge rectifier and other components to exceed their rated current and burn out. When NTC is used, it is connected in series on the L or N line, and its internal resistance value can limit the current value at the moment of charging. The definition of negative temperature coefficient is that its resistance will decrease as its temperature rises, so as the current flows through the body, After the temperature gradually rises, its resistance will decrease accordingly to avoid unnecessary power consumption.
Power Board PCB Components-Negative Temperature Coefficient Resistance (NTC)

Its appearance is mostly black and dark green disc-shaped elements

However, the disadvantage is that when the power supply is started in a warm state, its protection effect will be discounted, and even if the impedance can be reduced with temperature, it will still consume a little power. Therefore, most high-efficiency power supplies currently use more advanced instantaneous protection circuits.

Metal Oxide Varistor (MOV)
The varistor is connected across the live wire and the ground wire at the back of the fuse. Its operating principle is that when the voltage difference between its two ends is lower than its rated voltage value, the body presents high impedance; when the voltage difference exceeds its rated value, the body resistance will drop rapidly. There is an approximate short-circuit state between LN. The front-end fuse will be blown by the increased current due to the short-circuit to protect the back-end circuit. Sometimes when the body bears too much power, it also self-destructs to warn the user that the device has appeared. problem.
Power Board PCB Components-Metal Oxide Varistor (MOV)

It is usually used at the AC input end of a power supply. When the input AC has an overvoltage, the fuse can be blown in time to avoid damage to the internal components. The color and appearance are very similar to Cy capacitors, but they can be different from the words and model numbers on the components.

Bridge rectifier
The internal bridge rectifier composed of four diodes alternately connected, its function is to perform full-wave rectification of the input AC for use by the back-end switching circuit.
Power board PCB components-bridge rectifier

Its appearance and size will vary with the rated voltage and current of the component. Some power supplies will fix it on the heat sink to help it dissipate heat for stable long-term operation. After rectification, it enters the switching circuit on the primary side of the power stage. The components here determine the maximum output capacity of each channel of the power supply, which is a very important part of the power supply.