Engineers engaged in power supply design know that printed circuit board design is not good, even if the circuit schematic design is correct, it will have many adverse effects on the reliability of electronic equipment. Therefore, when designing a printed circuit board, it is very important to pay attention to the correct method. This article will share some practical EMC tips in power circuit design. In the power supply market, printed circuit boards are still the main assembly method for various electronic devices and systems. Engineers engaged in power supply design also know that printed circuit board design is not good, even if the circuit schematic design is correct, it will have many adverse effects on the reliability of electronic equipment. For example, two thin parallel lines are close together. A delay in the signal waveform is formed, and reflection noise is generated at the end of the transmission line. Therefore, it is very important to pay attention to the correct method when designing a printed circuit board.
This article will share some practical EMC techniques in power circuit design. Ground Design In electronic equipment, grounding is an important method of controlling interference. If the grounding and shielding are properly combined, most of the interference problems can be solved. The ground wire structure in the electronic device is roughly systematic, chassis ground (shielded ground), digital ground (logically), and analog ground. Pay attention to the following points in the ground line design:
(1) Correct selection of single-point grounding and multi-point grounding In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the influence of the inductance between the wiring and the device is small, and the circulating current formed by the grounding circuit has a great influence on the interference, so Use a little grounding. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible. Grounding should be used at multiple points. When the operating frequency is 1~10MHz, if grounding is used, the grounding length is used. Should not exceed 1/20 of the wavelength, otherwise multi-point grounding should be used;
(2) Separating the digital circuit from the analog circuit On the circuit board, there are both high-speed logic circuits and linear circuits. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they are connected to the ground of the power supply. Try to increase the grounding area of ​​the linear circuit as much as possible;
(3) Try to thicken the grounding wire. If the grounding wire is very thin, the grounding potential changes with the change of the current, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass the three allowable currents on the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm;
(4) When the grounding wire is formed into a closed-loop circuit. When the grounding system of the printed circuit board consisting of only digital circuits is designed, the grounding wire can be made into a closed-loop circuit to significantly improve the anti-noise capability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components with high power consumption, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the ground junction, causing the noise resistance to decrease. If the grounding structure is looped, the potential difference will be reduced, and the noise resistance of the electronic device will be improved. The electromagnetic compatibility design electromagnetic compatibility means that the electronic device can still work in a coordinated and effective manner in various electromagnetic environments. ability.
The purpose of the electromagnetic compatibility design is to enable the electronic device to suppress various external interferences, enable the electronic device to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of the electronic device itself to other electronic devices.
(1) Choosing a reasonable wire width Because the transient interference generated by the transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of a printed conductor is proportional to its length and inversely proportional to its width, so that short and precise conductors are advantageous for suppressing interference. Signal lines for clock leads, row drivers, or bus drivers often carry large transient currents, and the printed conductors should be as short as possible. For the discrete component circuit, when the width of the printed conductor is about 1.5mm, the requirement can be fully satisfied; for the integrated circuit, the width of the printed conductor can be selected between 0.2 and 1.0mm;
(2) Adopting the correct wiring strategy to use equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is better to use a well-shaped mesh wiring structure, which is the side of the printed board. The lateral wiring is routed on the other side and then connected at the intersection holes with metallized holes. In order to suppress the crosstalk between the printed circuit board wires, the long-distance equal routing should be avoided when designing the wiring;
(3) The decoupling capacitor is placed in the DC power supply circuit, and the load change will cause power supply noise. For example, in a digital circuit, when a circuit transitions from one state to another, a large spike current is generated on the power line to form a transient noise voltage. The configuration of decoupling capacitors can suppress the noise generated by load changes. It is a common practice for the reliability design of printed circuit boards. The configuration principle is as follows: the power input terminal is connected to a 10~100uF electrolytic capacitor, if the printed circuit The position of the board allows the use of electrolytic capacitors above 100uF for better anti-interference effects. A 0.01 uF ceramic capacitor is provided for each integrated circuit chip. If you encounter a small printed circuit board space and can not fit, you can configure a 1 ~ 10uF tantalum electrolytic capacitor every 4 ~ 10 chips, the high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz ~ 20MHz, Moreover, the leakage current is small (0.5 uA or less). For devices with weak noise capability, large current changes during shutdown, and memory devices such as ROM and RAM, decoupling capacitors should be directly connected between the power supply line (Vcc) and ground (GND) of the chip. The leads of the decoupling capacitors must not be too long, especially if the high frequency bypass capacitors are not leaded. Printed circuit board size and device layout Printed circuit board size should be moderate. When the size is too large, the printed lines are long and the impedance is increased. Not only the anti-noise ability is reduced, but also the cost is high. If it is too small, the heat dissipation is not good and the heat is not easy. Near line interference. In terms of device layout, as with other logic circuits, the related devices should be placed as close as possible to achieve better noise immunity. Clock generators, crystal oscillators, and CPU clock inputs are prone to noise. They should be close to each other. Devices that are prone to noise, small current circuits, and high-current circuits should be kept away from logic circuits as much as possible. If possible, another circuit board should be used. This is very important.
The heat dissipation design is preferred from the perspective of facilitating heat dissipation. The printed plate is preferably installed upright. The distance between the plates and the plate is generally not less than 2 cm, and the arrangement of the devices on the printed plate should follow certain rules:
(1) For devices using free convection air cooling, it is preferable to arrange the integrated circuits (or other devices) in a vertically long manner; for devices using forced air cooling, it is preferable to divide the integrated circuits (or other devices) horizontally. Long way row
(2) Devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation. Devices with low heat generation or poor heat resistance (such as small signal transistors, small scale integrated circuits, electrolytic capacitors, etc.) should be placed. At the uppermost flow (at the inlet) of the cooling airflow, devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow;
(3) In the horizontal direction, the high-power devices are placed as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, the high-power devices are placed as close as possible to the top of the printed board to reduce the operation of these devices to other devices. The effect of temperature;
(4) The temperature-sensitive device is preferably placed in the lowest temperature region (such as the bottom of the device). Do not place it directly above the heat-generating device. Multiple devices are preferably staggered on a horizontal plane; 5) The heat dissipation of the printed circuit board in the equipment mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area.
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