Electromagnetic compatibility refers to the ability of a device or system to function properly in its electromagnetic environment without causing unacceptable electromagnetic interference to anything in that environment. The purpose of electromagnetic compatibility design is to ensure that electronic devices can suppress various external interferences, allowing them to operate normally in specific electromagnetic environments, while also reducing electromagnetic interference from the electronic devices themselves to other electronic devices. As the sensitivity of electronic devices increases, their ability to receive weak signals strengthens, and the frequency bands of electronic products widen while their sizes shrink, the requirements for electronic devices’ anti-interference capabilities become more stringent. Some electronic devices generate electromagnetic waves during operation, which can easily cause electromagnetic interference to surrounding electronic devices, leading to malfunctions or affecting signal transmission. Furthermore, excessive electromagnetic interference can result in electromagnetic pollution, endangering human health and damaging the ecological environment. This article analyzes several key technologies for electromagnetic compatibility in the design of PCBs (Printed Circuit Boards, also known as printed wiring boards).
1 Power Supply Design
The power supply of electronic devices is widely connected to other functional units. On one hand, useless signals generated in the power supply can easily couple into various functional units; on the other hand, useless signals from one unit may couple into other units through the common impedance of the power supply. Therefore, the following measures should be taken in power supply design.
(1) Based on the current size of the printed circuit board, try to increase the width of the power supply lines to reduce loop resistance, aligning the direction of the power and ground lines with the direction of data transmission; at the same time, in multilayer PCBs, use power and ground layers to minimize the length of power lines to the power or ground layers. This helps enhance noise immunity;
(2) Where possible, provide power separately for each functional unit, and keep all circuits using a common power supply as close to each other as possible to ensure mutual compatibility;
(3) Use power filters on AC and DC lines to prevent external interference from entering the device through the power supply, and to isolate switch transients and other signals generated internally from entering the primary power supply, effectively isolating the input and output lines of the power supply and the input and output lines of the filter;
(4) Implement effective electromagnetic shielding for the power supply, isolating high voltage power from sensitive circuits as much as possible, especially with switch-mode power supplies which can cause high-frequency radiation and conducted disturbances. Use electrostatic shielded power transformers to suppress common mode interference on power lines, with multiple shielded isolation transformers offering better performance;
(5) All circuit functional state power supplies should maintain low output impedance, even in the RF range, with output capacitors also exhibiting low impedance, while ensuring that regulators have a sufficiently fast response time to suppress high-frequency ripple and transient loading effects;
(6) Rectifier diodes should operate at the appropriate current density, providing sufficient RF bypass for voltage regulators;
(7) Power transformers should be symmetrically balanced rather than power-balanced transformers, and the core material used should be chosen based on its lower limit of saturation magnetic induction (Bm). Under no circumstances should the core be driven into saturation; the transformer core structure should preferably be of type D or C, with type E being second.
2 Ground Line Design
Ground noise refers to the potential differences that occur between various parts of the ground line within the system or the ground noise caused by grounding impedance. Due to potential differences in the grounding system, appropriate grounding methods must be selected based on the characteristics of the PCB during the product grounding design process. Grounding is an important method for controlling interference in electronic product design. If grounding and shielding can be correctly combined, most interference issues can be resolved. The grounding structure in electronic products generally includes system ground, chassis ground, digital ground, and analog ground. The following points should be noted in ground line design:
(1) Ground lines should be made as thick as possible. If the ground lines are too thin, the ground potential will fluctuate with current changes, causing the timing signal levels of electronic products to be unstable and reducing noise immunity. Therefore, during design, ground lines should be thick enough to handle three times the allowable current of the printed circuit board. If possible, the width of the ground line should exceed 3mm.
(2) Correctly select grounding methods. Single-point grounding is intended to prevent currents from two different reference levels in subsystems from causing common impedance coupling by following the same return path. This grounding method is more suitable for low-frequency PCBs, as it can reduce the effects of distributed transmission impedance. However, in high-frequency PCBs, the inductance of the return path becomes the main part of the line impedance at high frequencies; thus, for high-frequency PCBs, multi-point grounding is typically employed to minimize ground impedance. The length of the grounding leads is crucial, as longer leads mean greater inductance, increasing ground impedance and causing potential differences. A mixed grounding structure combines single-point and multi-point grounding. This structure is commonly used in PCBs with mixed high and low frequencies, presenting single-point grounding at low frequencies and multi-point grounding at high frequencies.
(3) Separate digital and analog grounds. If there are both high-speed logic circuits and linear circuits on the circuit board, they should be kept as separate as possible, and their ground lines should not mix but should connect separately to the power supply ground. The ground line for low-frequency circuits should preferably use single-point parallel grounding; if actual wiring is difficult, partial series grounding can be used before parallel grounding. High-frequency circuits should adopt multi-point series grounding, with ground lines kept short and thick, and a grid-like large area ground foil should be used around high-frequency components. The grounding area for linear circuits should be maximized.
(4) Form closed loop ground lines. When designing the ground system for a printed circuit board composed solely of digital circuits, creating a closed loop for the ground line can significantly improve noise immunity. This is because there are many integrated circuit components on the printed circuit board, especially when dealing with high-power components, where the thickness of the ground line can cause large potential differences, reducing noise immunity. By forming a loop with the ground line, the potential difference can be minimized, enhancing the noise immunity of electronic devices.
(5) Use optical isolators to cut off ground loop interference. Optical connections typically involve optocouplers and fiber optic connections. The parasitic capacitance of optocouplers is generally 2pF, which provides good isolation for high frequencies. Fiber optic connections have almost no parasitic capacitance but are expensive and inconvenient to install and maintain.
3 Bypass and Decoupling Design
Bypassing refers to transferring unwanted common mode RF energy from components or cables, while the primary function of bypass capacitors is to create an AC component that eliminates unwanted energy entering the susceptible area. Decoupling, on the other hand, refers to removing RF energy from high-frequency devices entering the power distribution network during device switching. The main role of decoupling capacitors is to provide a local DC power supply to components, reducing the propagation of switching noise on the board and directing noise to ground.
3.1 Capacitor Selection
Selecting bypass and decoupling capacitors can be calculated based on the logic series and the clock speed used to determine the self-resonant frequency of the required capacitors, and the capacitance value can be chosen based on frequency and capacitive reactance in the circuit. Packaging should preferably choose SMT capacitors with lower lead inductance rather than through-hole capacitors. Additionally, product design often incorporates parallel decoupling capacitors to provide a wider operating frequency range and reduce ground imbalance. In parallel capacitor systems, when the operating frequency exceeds the self-resonant frequency, large capacitors exhibit inductive impedance that increases with frequency, while small capacitors exhibit capacitive impedance that decreases with frequency. At this time, the overall impedance of the capacitor circuit is lower than that of a single capacitor.
3.2 Bypass Capacitor Configuration
Bypass capacitors are generally used as high-frequency bypass devices to reduce transient power requirements for the power module. Typically, aluminum electrolytic capacitors and tantalum capacitors are suitable for use as bypass capacitors, with capacitance values depending on the transient current requirements on the PCB, generally in the range of 10~470µF. If the PCB has many integrated circuits, high-speed switching circuits, and power supplies with long leads, larger capacitance capacitors should be chosen.
3.3 Decoupling Capacitor Configuration
(1) A 10~100µF electrolytic capacitor should be connected across the power input. If possible, a capacitor over 100µF is preferable;
(2) In principle, each integrated circuit chip should be arranged with a 0.01µF ceramic capacitor. If space on the printed circuit board is insufficient, a 1~10µF tantalum capacitor can be arranged for every 4~8 chips;
(3) For devices with weak noise immunity and significant power changes when turned off, such as RAM and ROM storage devices, decoupling capacitors should be directly connected between the chip’s power and ground lines;
(4) Capacitor leads should not be too long, especially high-frequency bypass capacitors should not have leads;
(5) Due to components such as contactors, relays, and buttons on the printed circuit board, which generate significant spark discharge during operation, an RC circuit must be used to absorb discharge current. Generally, R should be 1~2K, and C should be 2.2~47µF;
(6) CMOS has high input impedance and is easily influenced; therefore, the unused terminal should be grounded or connected to the positive power supply during use.
4 Mixed Signal Circuit Board Design
Understanding the path and method of current return to ground is key to optimizing mixed signal circuit board design. It is not enough to consider where the signal current flows; the specific path of the current must not be overlooked. If it is necessary to split the ground plane, and wiring must be done through the gaps between the splits, single-point connections can be made between the split grounds to form a connection bridge, followed by wiring through that bridge. This way, a direct current return path can be provided beneath each signal line, minimizing the loop area formed. The mixed signal PCB design process should pay attention to the following points:
(1) Divide the PCB into independent analog and digital sections, achieving separation of analog and digital power, with A/D converters placed across sections;
(2) Do not split the ground. A unified ground should be laid beneath the analog and digital sections of the circuit board;
(3) In all layers of the circuit board, digital signals should only be routed in the digital section, and analog signals should only be routed in the analog section;
(4) Routing should not cross gaps between split power planes; signal lines crossing gaps between split power should be routed close to large area ground layers;
(5) Analyze the actual path and method of return ground current flow;
(6) Adopt correct layout and routing rules.
In conclusion, as electronic products become more complex, high-speed, and densely packed, the design requirements for PCBs are increasingly stringent, particularly regarding electromagnetic compatibility. The key to solving electromagnetic compatibility issues lies in the rational design of power supplies, grounds, bypassing, decoupling, and mixed signal circuits.
Source:PCB Electronic Circuit Technology
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