PCB News - Analysis of Bypass Capacitor in High Speed PCB

Analysis of Bypass Capacitor in High Speed PCB

1 Introduction

As the volume of the system decreases and the operating frequency increases, the functions of the system become more complicated, which requires multiple different embedded function modules to work at the same time. Only when each module has good EMC and low EMI, can the realization of the whole system function be guaranteed. This requires the system itself not only to have a good shielding performance from external interference, but also to not produce serious EMI to the outside world when working with other systems at the same time. In addition, switching power supplies are more and more widely used in high-speed digital system design, and multiple power supplies are often needed in a system. Not only is the power system susceptible to interference, but the noise generated during power supply can cause serious EMC problems to the entire system. Therefore, in high-speed PCB design, how to better filter power noise is the key to ensuring good power integrity. This article analyzes the filter characteristics of capacitors, the impact of the parasitic inductance of capacitors on the filter performance, and the current loop phenomenon in the PCB, and then makes some conclusions on how to choose bypass capacitors. This article also emphatically analyzes the generation mechanism of power supply noise and ground bounce noise, and analyzes and compares the various placement methods of bypass capacitors in the PCB on the basis of them.

2 Insertion loss characteristics, frequency response characteristics and filter characteristics of capacitors

2.1 Insertion loss characteristics of ideal capacitors

The ability of the EMI power filter to suppress interference noise is usually measured by the insertion loss (Insertion Loss) characteristics. The insertion loss is defined as the ratio of the noise power P1 transmitted from the noise source to the load when there is no filter connected to the noise power P2 transmitted from the noise source to the load after the filter is connected, expressed in dB (decibel). Figure 1 shows the insertion loss characteristics of an ideal capacitor. It can be seen that the slope of the insertion loss curve corresponding to a 1μF capacitor is close to 20dB/10 times the frequency.

Observe one of the insertion loss characteristics. When the frequency increases, the insertion loss value of the capacitor increases, that is to say, the P1/P2 value increases. This means that after the system is filtered by the capacitor, the noise that can be transmitted to the load is reduced. The ability of the capacitor to filter high frequency noise is enhanced. From the analysis of the ideal capacitor formula, when the capacitor is constant, the higher the signal frequency, the lower the loop impedance, that is, the capacitor is easy to filter out high-frequency components. The conclusions drawn from the two aspects are the same.

Observe the curves corresponding to different capacitors. When the frequency is very low, the corresponding insertion loss values of various capacitors are approximately the same, but as the frequency increases, the insertion loss value of a small capacitor increases by a larger capacitance. If it is slower, the value of P1/P2 will increase more slowly, which means that large capacitors are easier to filter out low-frequency noise. Therefore, when designing high-speed circuit boards, we usually place a 1~10μF capacitor at the power input end of the circuit board to filter out low-frequency noise; place a 0.01~0.1 between the power supply and the ground of each device on the circuit board. The μF capacitor filters out high-frequency noise.

The impedance of the capacitor connected between the power supply and the ground can be calculated by the following formula: The purpose of capacitor filtering is to filter out the AC components superimposed in the power system. From the above formula, it can be seen that when the frequency is constant, the greater the capacitance value, The smaller the impedance in the loop, the easier it is for the AC signal to flow through the capacitor to the ground plane. In other words, it seems that the larger the capacitor value, the better the filtering effect. In fact, this is not the case, because the actual capacitor is not ideal. All the characteristics of capacitors. The actual capacitance has parasitic components, which are formed when the capacitor plates and leads are constructed, and these parasitic components can be equivalent to the resistance and inductance connected in series on the capacitor, usually called equivalent series resistance (ESR) and equivalent Series inductance (ESL). This capacitor is actually a series resonant circuit. In the actual circuit or PCB design, the presence of the parasitic inductance of the capacitor will have a great impact on the filtering performance of the capacitor, so a capacitor with a relatively small parasitic inductance should be selected in the system design.

2.2 High-frequency response characteristics of actual capacitors

From Section 2.1, we know that the actual capacitor is working because of the parasitic inductance, which makes the capacitor circuit a series resonant circuit. The resonance frequency is, where: L is the equivalent inductance; C is the actual capacitance. When the frequency is less than f0, it appears as a capacitance; when the frequency is greater than f0, it appears as an inductance. Therefore, the capacitor is more like a band-stop filter than a low-pass filter. The ESL and ESR of a capacitor are determined by the structure of the capacitor and the dielectric material used, and have nothing to do with the capacitance of the capacitor. The ability to suppress high frequencies will not be enhanced by replacing large-capacity capacitors of the same type. The impedance of a larger-capacity capacitor of the same type is smaller than the impedance of a small-capacity capacitor when the frequency is lower than f0, but when the frequency is greater than f0, ESL determines that there is no difference in impedance between the two. It can be seen that in order to improve the high-frequency filtering characteristics, a capacitor with a lower ESL must be used. The effective frequency range of any kind of capacitor is limited, and for a system, there are both low-frequency noise and high-frequency noise, so it is usually necessary to use different types of capacitors in parallel to achieve a wider effective frequency range.
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