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Guidelines for PCB board layout
PCB Blog
Guidelines for PCB board layout

Guidelines for PCB board layout

2022-04-26
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Author:pcb

Principles of PCB board layout Operation Skills,Filter Capacitors, Decoupling Capacitors, Bypass Capacitors,Reasons for Paralleling a Small Capacitor to a Large Capacitor. Summarize several common operating techniques: try to place decoupling capacitors and filter capacitors around the corresponding components. The arrangement of decoupling capacitors and filter capacitors is an important measure to improve the power quality of the circuit board and improve the anti-interference ability. In fact, the traces, pin connections and wiring of the printed circuit board may bring about a large inductance effect. Placing a 0.1uF decoupling capacitor can effectively filter out high frequency ripple. If a chip capacitor is used on the circuit board, the chip capacitor can be placed close to the power supply pin of the component. For some power conversion chips, or power input terminals, a 10uF or larger capacitor is also arranged to further improve the quality of the power supply. Make sure to mark the footer when making a component library. If you are not sure about the size of the component, print it out at 1:1 and compare it directly with the actual part. Importing a netlist generated from a schematic, with flying leads displayed on the PCB can greatly aid in layout and routing. Do not use the X, Y keys to flip the components during component layout, otherwise it will not be able to be soldered. One way to route the two-layer board is to run only the horizontal line on one side and the vertical line on the other side. Do not have unrelated vias near the pads. The design rules mainly set the line width Width and spacing Clearance.

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Filter capacitors, decoupling capacitors, bypass capacitors
Filter capacitors are used in power rectifier circuits to filter out AC components. Make the output DC smoother. Decoupling capacitors are used in amplifier circuits where AC is not required to eliminate self-excitation and make the amplifier work stably. The bypass capacitor is used when there is a resistance connection, and is connected to both ends of the resistance to allow the AC signal to pass through smoothly.

1. Understanding of the energy storage function of decoupling capacitors
1) The decoupling capacitor is mainly to remove the interference of high frequency such as RF signal, and the way of interference is through electromagnetic radiation. In fact, the capacitor near the chip also has the function of storing energy, which is second. You can think of the main power supply as Miyun Reservoir. Every household in our building needs water supply. At this time, the water does not come directly from the reservoir. It is too far away. When the water comes, we are already thirsty. The actual water comes from the water tower on the top of the building, and the water tower is actually a buffer. From a microscopic point of view, when a high-frequency device is working, its current is discontinuous and the frequency is very high, and the device VCC has a certain distance from the total power supply, even if the distance is not long, in the case of a high frequency, the impedance Z =i*wL R, the influence of the inductance of the line will also be very large, which will cause the device to not be supplied in time when the current is required. Decoupling capacitors can make up for this deficiency. This is one of the reasons why many boards place small capacitors at the VCC pins of high frequency devices (usually a decoupling capacitor is placed in parallel with the vcc pins so that the AC component is grounded from this capacitor.)

2) High-frequency switching noise generated by active devices during switching will propagate along the power line. The main function of decoupling capacitors is to provide
A localized DC power supply to active devices to reduce the propagation of switching noise on the board and to direct the noise to ground

2. The difference between bypass capacitors and decoupling capacitors
Decoupling: The removal of RF energy from high frequency devices into the power distribution network when the device is switched. Decoupling capacitors can also provide a localized DC voltage source for the device, which is especially useful in reducing inrush currents across the board.
Bypass: Divert unwanted common-mode RF energy from components or cables. This is mainly to eliminate unintentional energy entering the sensitive part by generating AC bypass, and can also provide baseband filtering function (bandwidth limited).
We can often see that a decoupling capacitor is connected between the power supply and the ground. It has three functions: one is to act as the energy storage capacitor of the integrated circuit; the other is to filter out the high-frequency noise generated by the device and cut off its The path that propagates through the power supply loop; the third is to prevent the noise carried by the power supply from interfering with the circuit. In electronic circuits, both decoupling capacitors and bypass capacitors play an anti-interference role. The location of the capacitor is different, and the name is different. For the same circuit, the bypass capacitor is used to filter the high-frequency noise in the input signal, and the high-frequency clutter carried by the front stage is filtered out, and the decoupling capacitor is also called decoupling. Capacitance is to filter out the interference of the output signal.

The reason for connecting a small capacitor in parallel with a large capacitor
Large capacitors are generally larger in size due to their large capacity, and are usually made by multi-layer winding, which results in a relatively large distributed inductance (also called equivalent series inductance, or ESL for short). As we all know, the impedance of inductors to high-frequency signals is very large, so the high-frequency performance of large capacitors is not good. On the other hand, some small-capacity capacitors are just the opposite. Due to their small capacity, the volume can be made very small (the lead is shortened, and the ESL is reduced, because a section of wire can also be regarded as an inductance), and plate capacitors are often used. Structure, such a small capacity capacitor has a small ESL so that it has good high frequency performance, but due to the small capacity, the impedance to low frequency signals is large. Therefore, if we want to allow low-frequency and high-frequency signals to pass well, we use a large capacitor and then add a small capacitor. The commonly used small capacitor is a 0.1uF ceramic capacitor. When the frequency is higher, a smaller capacitor can be connected in parallel, such as a few pF or several hundreds of pF. In digital circuits, a 0.1uF capacitor is generally connected in parallel with the power pin of each chip to ground (this capacitor is called a decoupling capacitor, and of course it can also be understood as a power filter capacitor, the closer to the chip, the better), because The signals in these places are mainly high frequency signals, which can be filtered with smaller capacitors on PCB board.