PCB News - Precautions for PCB board design of switching power supply

With the improvement of the performance of power semiconductor devices and the innovation of switching conversion technology, power electronics technology has been widely used in various power supply equipment. At present, the products of switching power supply tend to be small, high-speed and high-density. This trend has caused electromagnetic compatibility problems to become more and more serious. The high-frequency switching process of voltage and current produces a large amount of EMI (electromagnetic interference). If this part of the interference is not restricted, it will seriously affect the normal operation of the surrounding electrical equipment. Therefore, the PCB design of the switching power supply is a vital aspect to solve the electromagnetic compatibility problem of the switching power supply. The reason why PCB board is regarded as an indispensable and important component in the design of switching power supply is that it is responsible for the dual connection of the electrical and mechanical components of the switching power supply, and is the key to reducing the EMI design of electronic equipment.

1 Electromagnetic interference in PCB design

1.1 Electromagnetic coupling interference

In circuit design, electromagnetic coupling interference mainly affects other circuits through conduction coupling and common mode impedance coupling. From the perspective of EMC design, switching power supply circuits are different from ordinary digital circuits, and have relatively obvious interference sources and sensitive lines. Generally speaking, the interference sources of switching power supplies are mainly concentrated on components and wires with large voltage and current rate of change, such as power FETs, fast recovery diodes, high-frequency transformers, and wires connected to them. Sensitive lines mainly refer to control circuits and lines directly connected to interference measurement equipment, because these interference couplings may directly affect the normal operation of the circuit and the level of interference transmitted to the outside. The common-mode impedance coupling means that when the currents of two circuits pass through a common impedance, the voltage formed by the current of one circuit on the common impedance will affect the other circuit.

1.2 Crosstalk interference

The crosstalk interference between strips, wires, and cables in the printed circuit board (PCB) is one of the most difficult problems to overcome in the circuit of the printed circuit board. The crosstalk mentioned here is a crosstalk in a broader sense, no matter the source is useful signal or noise, crosstalk is expressed by the mutual capacitance and mutual inductance of wires. For example, a strip line on the PCB carries control and logic levels, and a second strip line close to it carries a low-level signal. When the parallel wiring length exceeds 10 cm, crosstalk interference is expected; When a long cable carries several sets of serial or parallel high-speed data and remote control lines, crosstalk interference also becomes a major problem. The crosstalk between adjacent wires and cables is caused by the electric field passing through mutual capacitance and the magnetic field passing through mutual inductance.

When considering the problem of crosstalk in the PCB strips, the main problem is to determine which of the electric field (mutual capacitance) and magnetic field (mutual inductance) coupling is more important. The determination of the coupling model mainly depends on the line impedance, frequency and other factors. Generally speaking, capacitive coupling is dominant at high frequencies, but if one or both of the source or receiver uses shielded cables and is grounded at both ends of the shield, magnetic field coupling will be dominant. In addition, low circuit impedance is generally lower at low frequencies, and inductive coupling is the main factor.

1.3 Electromagnetic radiation interference

Radiation interference is the interference introduced due to the radiation of electromagnetic waves in space. PCB electromagnetic radiation is divided into two types: differential mode radiation and common mode radiation. In most cases, the conduction interference generated by the switching power supply is dominated by common mode interference, and the radiation effect of common mode interference is far greater than differential mode interference. Therefore, reducing common mode interference is particularly important in the EMC design of switching power supplies.

2 PCB interference suppression steps

2.1 PCB design information

When designing a PCB, you need to understand the design information of the circuit board, which includes the following:

(1) Number of devices, device size, and device packaging;

(2) Requirements for overall layout, device layout location, presence or absence of high-power devices, and special requirements for heat dissipation of chip devices;

(3) The speed of the digital chip, whether the PCB is divided into low-speed, medium-speed and high-speed areas, and which are the interface input and output areas;

(4) The type and speed of the signal line and the transmission direction, the impedance control requirement of the signal line, the direction of the bus speed and the driving situation, the key signal and the protection measures;

(5) Power supply type, ground type, noise tolerance requirements for power supply and ground, setting and division of power supply and ground plane;

(6) The type and speed of the clock line, the source and destination of the clock line, the clock delay requirement, and the longest wiring requirement.

2.2 PCB layering

First, determine the number of wiring layers and power supply layers required to implement the function within an acceptable cost range. The number of layers of the circuit board is determined by factors such as detailed functional requirements, immunity, separation of signal categories, device density, and bus wiring. At present, circuit boards have gradually developed from single-layer, double-layer, and four-layer boards to more layers. The design of multilayer printed boards is the main measure to achieve electromagnetic compatibility standards. The requirements are:

(1) The distribution of separate power layer and ground layer can well suppress inherent common mode interference and reduce point source impedance;

(2) The power plane and the ground plane are as close as possible to each other, and the ground plane is generally above the power plane;

(3) It is best to lay out digital circuits and analog circuits in different layers;

(4) The wiring layer is preferably adjacent to the entire metal plane;

(5) Clock circuits and high-frequency circuits are the main sources of interference and should be dealt with separately.

2.3 PCB layout

The key to the EMC design of the printed circuit board is layout and wiring, which is directly related to the performance of the circuit board. The current EDA automation of circuit board layout is very low, requiring a lot of manual layout. Before layout, the PCB size that satisfies the function at the lowest possible cost must be determined. If the PCB size is too large and the device distribution is scattered during the layout, the transmission line may be very long, which will increase the impedance, reduce the anti-noise ability, and increase the cost. If the devices are placed in a centralized manner, the heat dissipation is not good, and the adjacent traces are prone to coupling crosstalk. Therefore, the layout must be carried out according to the circuit function unit, and factors such as electromagnetic compatibility, heat dissipation and interface must be taken into consideration at the same time. Some principles should be followed in the overall layout:

(1) Arrange each functional circuit unit according to the flow of the circuit signal to keep the signal flow in the same direction;

(2) Take the core component of each functional circuit unit as the center, and other components are laid out around it;

(3) Shorten the wiring between high-frequency PCB components as much as possible and try to reduce their distribution parameters;

(4) The components that are susceptible to interference should not be too close to each other, and the input and output components should be far away;

(5) Prevent mutual coupling between power lines, high-frequency signal lines and general wiring.