Selection of decoupling capacitors

There are many comrades in the process of designing the circuit and making the PCB, there is still a lack of understanding of the choice of decoupling capacitor, or even a lot of blindness. Here we will talk about the problems involved.

It is a commonly used method in engineering to filter out the high-frequency noise from the power supply on the circuit board using the decoupling capacitor. A good high-frequency Decoupling capacitor circuit can remove high-frequency components up to 1 GHz. When designing a printed circuit board, a decoupling capacitor must be added between the power supply and ground of each integrated circuit. Generally, the high-frequency characteristics of porcelain dielectric capacitor and multilayer porcelain dielectric capacitor are better.

Decoupling capacitors are generally used as follows:

(1) high-frequency noise of the bypass device (low impedance path is provided for high-frequency noise between power supply and ground ). Generally, the higher the operating frequency, the larger the capacitance value, the smaller the impedance of the capacitor.

2) The energy storage capacitor used as an IC can provide and absorb the charge and discharge capabilities instantly when the IC is opened and closed.

In practical applications, the typical 0.1uf decoupling capacitor in digital circuits has a distribution inductance of 5 NH, the parallel resonance frequency is about 7 MHz (only the noise at this frequency has a better decoupling effect ). The capacitance of 1 UF and 10 UF is equal to the parallel resonance frequency above 20 MHz, which is better to remove high-frequency noise. It is often advantageous to place a high-frequency Decoupling capacitor of 1 UF or 10 UF at the place where the power supply enters the Printed Board. This capacitance is also required even for battery powered systems.

Each 10-piece IC requires a charge-discharge capacitor (storage capacitor). The size is generally 10 UF. It is best not to use electrolytic capacitors, because the electrolytic capacitors are rolled up by two-layer films. This structure is expressed as inductance at high frequencies, and it is best to use the TA capacitor or polycarbonate capacitor.

The decoupling capacitor value acquisition method is not strict. Generally, it can be calculated according to the formula C = 1/f. Select 0.1uf ~ for the micro-Control System ~ 0.01uf. At the same time, we should try to use the patch in the high-frequency circuit. By the way, in practical application, the larger the capacitance is, the more favorable the high-speed circuit is. On the contrary, the small capacitor can be applied to high-frequency circuits. The reason is the influence of parasitic parameters of the capacitor (such as equivalent series resistance, equivalent series inductance, leakage resistance, medium absorption capacitance, and medium absorption resistance) on the filtering effect. The resonance frequency of a capacitor is determined by its equivalent series inductance and capacity C. Both changes will affect the resonance frequency of the capacitor. The capacitance has the lowest impedance near the resonant point. Therefore, it is best to select a capacitor with the same resonance frequency as the actual working frequency during design. If the frequency range of operation is large, you can use some small capacitors with lower resonance frequencies and higher harmonic vibration frequencies.

In addition, the dielectric constant, insulation, temperature characteristics, and pressure resistance of the capacitor must be considered during the design process. The capacitor processing in High-Speed PCB can be summarized as follows:

A. Reduce the lead or pin length of the capacitor

B. Try to use a wide line

C. Prioritize and select the mounted capacitor whenever possible

D. The capacitor should be as close as possible to the power pin of the device and be directly connected to it

E. Do not share holes between Capacitors

F. The capacitor should be close to its pad as much as possible (the best fit on the pad)

To put it simply, reduce the inductance.

 

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