Analysis of PCB decoupling capacitors

- Dec 07, 2019-

Analysis of PCB decoupling capacitors

abstract:  

The global electronics manufacturing industry is entering a period of innovation-intensive and fast-growing enterprises. As the basic engineering and core components of electronic components, the PCB circuit board industry has also accelerated the pace of upgrading and iteration. In order to better capture the international leading PCB manufacturing technology and fully display the domestic circuit board industry manufacturing level, from August 30 to September 1, the 2016 Shenzhen International Circuit Board Procurement Exhibition (CS Show 2016) will be held at the Shenzhen Convention and Exhibition Center Officially kicked off. Today we will first analyze a difficult problem of the PCB-why should decoupling capacitors be placed nearby?

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I believe that for engineers who are hardware engineers, when they start to work in the company after graduation, when designing a PCB, the old engineer will tell him that the PCB traces should not go at right angles, the traces must be short, and the capacitors must be placed nearby.

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But at the beginning, we may not understand why this is done. It is not enough for us to rely on their few experiences. Of course, if you do nt pay attention to these details, if you commit it again in the future, you may be scolded by them. "How many times have I said that the capacitor must be placed nearby, and the effect cannot be achieved when it is far away." Often, experience tells us that those old engineers are only a part of the talents who really master the mystery. We will not use it at first Sad, look at the information and you will be able to grasp it quickly.

Until I was scolded several times, we went back to find relevant information. Why should I design the PCB capacitors nearby? After reading the information, I can understand some, but the information on the Internet is very sparse, and rarely can I find a comprehensive explanation. . The following contents are about the decoupling radius of the capacitor. I believe that after reading it, you can answer x and avoid similar problems.

An important problem of capacitor decoupling is the decoupling radius of the capacitor. Most of the materials will mention that the capacitor should be placed as close to the chip as possible, and most of the materials are from the perspective of reducing the loop inductance to discuss this placement distance. It is true that reducing inductance is an important reason, but there is another important reason that most materials have not mentioned, and that is the problem of capacitor decoupling radius. If the capacitor is placed too far away from the chip and exceeds its decoupling radius, the capacitor will lose its decoupling effect.

The best way to understand the decoupling radius is to examine the phase relationship between the noise source and the capacitor compensation current. When the chip's demand for current changes, a voltage disturbance will be generated in a small local area of the power plane. To compensate for this current (or voltage), the capacitor must first sense this voltage disturbance. It takes a certain time for the signal to propagate through the medium, so there is a time delay between the occurrence of a local voltage disturbance and the perception of this disturbance by the capacitor. Similarly, it takes a delay for the compensation current of the capacitor to reach the disturbance zone. Therefore, the phase inconsistency between the noise source and the capacitor compensation current must be caused.

A specific capacitor is best for noise compensation with the same self-resonant frequency. We use this frequency to measure this phase relationship.

When the distance from the disturbance zone to the capacitor is reached, the phase of the compensation current is exactly 180 degrees from the phase of the noise source, that is, completely inverted. At this time, the compensation current no longer works, the decoupling effect fails, and the compensation energy cannot be delivered in time. In order to effectively transfer the compensation energy, the phase difference between the noise source and the compensation current should be as small as possible, preferably in the same phase. The closer the distance is, the smaller the phase difference is, and the more the compensation energy is transferred. If the distance is 0, the compensation energy is transferred to the disturbance zone 100%. This requires the noise source to be as close as possible to the capacitor, and much smaller than it. In practical applications, this distance is best controlled between (λ / 40 -λ / 50), which is an empirical data.

For example: 0.001uF ceramic capacitor, if the total parasitic inductance is 1.6nH after mounting on the circuit board, then the resonance frequency after mounting is 125.8MHz and the resonance period is 7.95ps. Assuming the signal travels on the board at 166 ps / inch, the wavelength is 47.9 inches. The decoupling radius of the capacitor is 47.9 / 50 = 0.958 inches, which is approximately equal to 2.4 cm.

The capacitor in this example can only compensate for power supply noise within a range of 2.4 cm around it, that is, its decoupling radius is 2.4 cm. Different capacitors have different resonance frequencies and different decoupling radii. For large capacitors, because the resonance frequency is very low and the corresponding wavelength is very long, the decoupling radius is large. This is why we don't pay much attention to the location of large capacitors on the circuit board. For small capacitors, because the decoupling radius is small, it should be as close as possible to the chip that needs to be decoupled. This is what is repeatedly emphasized in most materials. Small capacitors should be placed as close to the chip as possible.

I believe that after reading this article today, colleagues in the industry will have a new understanding of the radius of the decoupling capacitance of the PCB. If you want to know more about PCB expertise, please stay tuned for the CS Show 2016 Shenzhen International Circuit Board Procurement Exhibition which will be held in Shenzhen Convention and Exhibition Center on August 30th. A complete and successful PCB circuit board procurement exhibition, in addition to the participation of professional visitors, can not do without the enthusiasm of PCB exhibitors. As an industry exhibition focusing on PCB / FPC procurement in China, CS Show 2016 has attracted a large number of well-known domestic and foreign exhibitors to participate in it. It is a stage to establish industry awareness and show strength. Various innovative, environmentally friendly, intelligent circuit board equipment, original equipment The materials will be perfectly presented at the opening of the exhibition.

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