Ways to suppress noise

　　The three elements that form electromagnetic interference are the interference source, the propagation path and the victim equipment. Therefore, the suppression of electromagnetic interference should also be started from these three aspects. First of all, the interference source should be suppressed and the cause of the interference should be directly eliminated; the second is to eliminate the coupling and radiation between the interference source and the victim equipment, and cut off the propagation path of electromagnetic interference; the third is to improve the immunity of the victim equipment and reduce its noise Sensitivity. The third point is not the scope of this article.

　　The use of power factor correction (PFC) technology and soft switching power conversion technology can greatly reduce the noise amplitude.

　　1. Measures on the circuit

　　The main reason for the electromagnetic interference of the adapter is the rapid change of voltage and current, so it is necessary to reduce the rate of change of voltage and current in the circuit (du/dt, di/dt) as much as possible. Adopting absorption circuit is also a good way to suppress electromagnetic interference. The basic principle of the absorption circuit is to provide a bypass for the switch when the switch is off, absorb the energy accumulated in the parasitic distribution parameter, thereby suppressing the occurrence of interference. Commonly used absorption circuits include RC, RCD, LC passive absorption networks and active absorption networks.

　　Filtering is a good method to suppress conducted interference. For example, connecting a filter to the input of the adapter can suppress the interference generated by the adapter and fed back to the power grid, and it can also suppress the noise from the power grid from harming the adapter itself. In the filter circuit, a lot of special filter components are also used, such as through-core capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filter characteristics of the circuit. Appropriate design or selection of filters, and correct installation of filters are an important part of anti-interference technology.

　　2. Structural measures: shielding

　　Shielding is one of the important means to solve the problem of electromagnetic compatibility, and the purpose is to cut off the propagation path of electromagnetic waves. Most of the electromagnetic compatibility problems can be solved by electromagnetic shielding. The biggest advantage of using electromagnetic shielding to solve electromagnetic interference problems is that it will not affect the normal operation of the circuit.

　　Shielding is divided into electrical shielding, magnetic shielding and electromagnetic shielding.

　　For the adapter, it is mainly necessary to do a good job of shielding the case, shielding the high-frequency transformer, shielding the switch tube and rectifier diode, and shielding the control and drive circuits, and improve the shielding effectiveness through various methods.

　　Common mistakes in power supply PCB design

　　When you use EDA to complete the PCB layout of the power adapter, and check that the network and DRC have not reported errors, is a PCB completed? The answer is of course no.

　　Many beginners also include some experienced engineers. Due to time constraints, impatience, or overconfidence, they neglect the later inspections. As a result, some very basic bugs appeared. The following lists some of the most basic and most error-prone elements for later inspection.

　　Power supply PCB layout

　　1. Component packaging

　　(1) Pad pitch. If it is a new device, draw the component package yourself to ensure that the spacing is appropriate. The pad spacing directly affects the soldering of the components.

　　(2) Via size (if any). For plug-in devices, the size of the vias should be left with enough margin.

　　(3) Contour silk screen. The outline silk screen of the device should be larger than the actual size to ensure that the device can be installed smoothly.

　　2. DC power adapter layout

　　(1) IC should not be close to the edge of the board.

　　(2) The components of the same module circuit should be placed close to each other. For example, the decoupling capacitor should be close to the power adapter pin of the IC, and the components that make up the same functional circuit should be placed in one area with a clear hierarchy to ensure the realization of the function.

　　(3) Arrange the position of the socket according to the actual installation. Sockets are all lead to other modules. According to the actual structure, in order to facilitate installation, the principle of proximity is generally adopted to arrange the position of the socket, and it is generally close to the edge of the board.

　　(4) Pay attention to the direction of the socket. The sockets are oriented, and if the direction is reversed, the wire must be re-customized. For flat sockets, the direction of the socket should be toward the outside of the board.

　　(5) The source of interference should be far away from sensitive circuits. High-speed signals, high-speed clocks, or high-current switching signals are all interference sources and should be kept away from sensitive circuits, such as reset circuits and analog circuits.

　　3. Wiring

　　(1) The size of the line width. The line width should be selected in combination with the process and the current carrying capacity, and the minimum line width cannot be less than the minimum line width of the PCB manufacturer. At the same time, the current carrying capacity is guaranteed.

　　(2) Differential signal line. For differential cables such as USB and Ethernet, please note that the cables should be of equal length, parallel, and in the same plane, and the spacing is determined by the impedance.

　　(3) Pay attention to the return path of the high-speed line. High-speed lines are prone to electromagnetic radiation. If the area formed by the routing path and the return path is too large, a single-turn coil will radiate electromagnetic interference. Therefore, when wiring, pay attention to the return path next to it. The multi-layer board is provided with a power layer and a ground plane to effectively solve this problem.

　　(4) Pay attention to the analog signal line. The analog signal line should be separated from the digital signal, and the wiring should avoid passing by the interference source as much as possible, and the wiring should be as short as possible.

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Pay attention to the logo before using the power adapter