The most commonly used EMI/EMC precautions for electronic R&D engineers are shielding, filtering, grounding, and wiring. However, with the integration of electronic systems, cost, quality, and functionality are considered, and the speed of product launch is required. Engineers must conduct EMI/EMC predictive analysis and design at the initial stage of design, avoiding problems in the later stages of development, adopting passive control methods to save repairs, and receiving twice the result with half the effort. This article describes the issues that should be considered when controlling EMI/EMC at the beginning of product design. 1 PCB board design 1.1 PCB layer number and function distribution When designing a circuit board, the first thing to consider is the number of layers of the PCB and the distribution of signals, power, and ground. The number of layers is determined by functional specifications, noise suppression, signal type, trace distribution, impedance matching, active component density, number of networks, and so on. Pressing RF radiation at the PCB level is better than working on a plastic case in a case or metal. Table 1 shows the general distribution of the number of layers of the board and the signal, power, and ground. These allocation methods are not static and can be modified as required by the functional requirements and the number of required winding layers (RounTIng Layers). The important point to be grasped is that each winding layer must be adjacent to a complete plane. PCBs are generally even layers, and two-layer boards are often used for frequencies below 10 kHz. If more than 3 complete planes are provided, ie one power supply and two grounds, the best EMI effect is obtained by routing the highest speed clock to the adjacent ground plane and not adjacent to the power plane. This is the basic concept of suppressing EMI suppression on the PCB. 1.2 Power and Grounding The most important consideration in high-speed PCB design is to supply power to each part of the circuit to minimize noise. It is like developing a non-interfering power supply. A good grounding impedance should be zero, so a good reference voltage can be provided to all circuits without EMI. In fact, in a real power network, because there is a non-zero value transfer delay current, there should be some limited impedance, such as resistance, inductance, or capacitance, which are dispersed throughout the board. Another problem in high-speed PCB boards is the alternating electromagnetic field generated by the AC signal, which circulates in a closed loop surrounded by wires that can make the circuit's crosstalk and radiation more severe. The performance of the distributed power supply is determined by the different potentials of the circuits on the board. The purpose of the design is to minimize the impedance of the power supply network. There are usually two ways to solve this problem, using the power bus and the power plane. The average designer tends to use the power bus because it has a reasonable cost, so it is preferred, but the power bus shares the entire layout layer with the signal line, and there is a lot of power supplied to all devices. Usually the bus is long and narrow, so its impedance is relatively large, which is why the current is limited to the path defined by the bus. The EMI generated by the device is related to the devices on the power bus. As for the power plane because it is full of the entire layout layer, and the impedance of the power plane is a small part of the power bus. On the power plane, the noise current is scattered because the current path is not limited. The path impedance along the line is also lower, so the power plane is quieter than the power bus. Another function of the power plane is that there is a return path for all signal supply in the system that can be used to limit many high speed noise problems. At low speeds, current flows to the path of the lowest resistance. At high speeds, the inductance of the return current path is much lower than the effective resistance. The high-speed return current is the path of the lowest inductance, but this path is not the lowest. The path of the resistor. This lowest inductance return path is placed directly below a signal conductor and has a minimum total loop between the return current paths. The power plane provides a return path for all signals in the system, and the current can be returned via VCC or ground. 1.3 Terminal The shorter the route, the shorter the transmission delay. If the length of the wire exceeds 1/6 of the length of the rising edge of the electron, the signal delay is greater than the effective part of the transmission time, so the signal route must be regarded as a transmission line, an inappropriate The terminal transmission line is prone to reflections and thus to the signal. Too short a terminal line will produce a negative reflection to slow down the conversion time and slow the data flow; while a too long terminal line will produce a positive reflection which can be interpreted as a multi-tasking signal due to this frequency Underneath, with high impedance and transmission rate, it can be effectively coupled with adjacent path lines. Signals at the line load can be combined to form a ring to reduce the speed of the system, which can also cause erroneous timing and even disrupt system functionality. Therefore, the terminal resistance of the terminal line should be reduced, or limited to no reflection, so that the resistance value matches the characteristic impedance of the transmission line to effectively suppress the reflection. Incorporating a load-bearing resistor between parallel terminals reduces the load impedance, but it has the disadvantage that it has a higher current at the positive voltage output. This current can be passed through the power supply and the grounding resistor. The use of the two resistors is a so-called Thevenin effect; although this method is very good, because the resistance is between the power supply and the ground, a large power supply current is required. Another technique is to incorporate resistors and capacitors that allow AC shorts and DCs to open. This circuit can be referenced and considered an AC termination. The design of the load termination technology can limit the first reflection. Another option is to increase Zs to be equal to Zo and connect Zs to the power supply. When Zs is added, the power supply will generate a new impedance Zo. We can also consider using the terminator on both the power supply and the load side to make half of the received signal and reduce the huge reflection. In digital circuits, this technique is only used for lines connected to receiving components. Handheld devices place special emphasis on EMI design on printed circuit boards, and in most cases the surface mount components of the board produce emissions that are larger than the copper foil lines on the board. The same current flowing through the copper foil wire must flow through the IC as well, ensuring that the area between the copper foil wire and its nearest reference datum is less than the current loop area from the chip pin to the board and back to the device's power and ground pins. , the chip can emit more energy than the copper foil wire. In addition, if the two copper foil wires are the same length and carry the same signal, the radiation of the copper foil wire physically higher than the nearest solid reference plane will be compared. Big. Simply put, the higher the distance from the datum, the higher the radiation. Circuit Breaker Rocker Switches Circuit Breaker Rocker Switches Gray,Circuit Breaker Rocker Switches Black,Circuit Breaker Rocker Switches Black White,Circuit Breaker Rocker Switches Blue Yang Guang Auli Electronic Appliances Co., Ltd. , https://www.ygpowerstrips.com