In the EMC design process, capacitors are the most widely used devices, mainly used to form various low-pass filters or as decoupling capacitors and bypass capacitors. Through practical data: In EMC design, proper selection and use of capacitors can not only solve many EMC problems, but also fully reflect the advantages of better results and convenience in use. If the capacitor is improperly selected or used, it may not achieve the intended purpose, and may even worsen the EMC level of the product.
Capacitors are basic filter components and are used as bypass components in low-pass filters. Utilizing its characteristic that its impedance decreases with increasing frequency, it can play a role in bypassing high-frequency interference. However, in actual use, we must pay attention to the non-ideality of the capacitor.
From a theoretical point of view, the larger the ideal capacitance, the smaller the capacitive reactance and the better the filtering effect. However, capacitors all have equivalent series inductance ESL, and large-capacity capacitors generally have large equivalent series inductance, and the equivalent series inductance is in series with the capacitor itself, so series self-resonance occurs. The greater the equivalent series inductance, The lower the self-resonant frequency, the worse the decoupling effect on high-frequency noise, or even no decoupling effect at all. The larger the physical size of the component, the lower the self-resonant frequency of a capacitor with the same capacitance value.
The circuit model and frequency impedance characteristics of the actual capacitor are shown in the following figure. It is a series network composed of equivalent inductance ESL, capacitance and equivalent resistance ESR. The inductance component is determined by the lead and capacitor structure, and the resistance is inherent to the dielectric material. The inductance component is the main indicator that affects the frequency characteristics of the capacitor. Therefore, when analyzing the bypass effect of the actual capacitor, the LC series network is used to be equivalent.
Capacitor equivalent circuit and frequency impedance characteristics
As shown in the figure, at the resonant frequency f0, L and C will resonate in series, and the impedance of the entire loop is the lowest at this time. At frequencies above the self-resonance point, the impedance of the capacitor increases with the increase in inductance. At this time, the capacitor will no longer play the role of bypass and decoupling. Therefore, bypassing and decoupling are affected by the lead inductance and capacitance of the capacitor, the wiring length between components, and the through-hole pad.
2. The influence of capacitance on filter characteristics
The actual capacitor is shown in the figure above. When f0=1/(2π(LC)1/2), series resonance will occur. At this time, the impedance of the capacitor is the smallest and the bypass effect is the best. After the resonance point is exceeded, the impedance characteristic of the capacitor presents the impedance characteristic of the inductance. As the frequency increases, the bypass effect begins to deteriorate. At this time, the capacitor used as a bypass device begins to lose its bypass function.
The impedance of an ideal capacitor decreases as the frequency increases, while the impedance of the actual capacitor exhibits capacitive characteristics when the frequency is lower, that is, the impedance decreases with the increase of frequency, and resonance occurs at a certain point. The impedance is equal to the equivalent series resistance ESR. Above the resonance point, due to the effect of ESL, the impedance of the capacitor increases with the increase of frequency, so the bypass effect of high-frequency noise is weakened or even disappeared.
The resonant frequency of the capacitor is jointly determined by ESL and C. The larger the capacitance or inductance, the lower the resonant frequency, that is, the worse the high-frequency filtering effect of the capacitor. In addition to ESL being related to the type of capacitor, the lead length of the capacitor is also a very important parameter. The longer the lead, the greater the inductance and the lower the resonant frequency of the capacitor.
According to the principle of LC circuit series resonance, the resonance point is not only related to the inductance, but also to the capacitance value. The larger the capacitance, the lower the resonance point. Many design engineers believe that the larger the capacitance of the capacitor, the better the filtering effect. This is a misunderstanding. The larger the capacitor, the better the bypass effect on low-frequency interference, but because the capacitor resonates at a lower frequency, the impedance starts to increase with the increase in frequency, so the bypass effect on high-frequency noise becomes worse.
Therefore, when choosing a capacitor, it does not depend on the size of the capacitance, but the self-resonant frequency of the capacitor, which matches the logic circuit and the operating frequency used. Below the self-resonant frequency, the capacitor is capacitive, and above the self-resonant frequency, the capacitor becomes inductive. When the capacitor is inductive, it has actually lost its due role. The self-resonant frequency of the two types of ceramic capacitors is shown in the table below. One is with 6.4mm pins, and the other is surface mount 0805 package.
Although from the point of view of filtering high-frequency noise, capacitor resonance is undesirable, but the resonance of the capacitor is not always harmful. When the frequency of the noise to be filtered is determined, the capacity of the capacitor can be adjusted to make the resonance point just fall on the disturbance frequency.
The capacitor used in electromagnetic compatibility design requires the frequency as high as possible, so that it can play an effective filtering role in a wide frequency range (10KHz ~ 1GHz). There are two ways to increase the resonant frequency: one is to shorten the length of the lead as much as possible; the other is to select capacitors with smaller inductance.
In the table, taking the 1uF capacitor as an example, the resonant point of the high-frequency capacitor of the plug-in (6.4mm lead) is 2.5MHz, and its impedance is the smallest at the resonant point. Its surface mount (0805 package) high-frequency capacitor has a resonance point of 5MHz and its impedance is the smallest at the resonance point.
According to the reference data in the above table, when the lead of this type of device is too long, its parasitic parameters at high frequencies will reduce its own resonant frequency. It is recommended to use mounted devices as much as possible when performing high-frequency filtering. A common practice is to select capacitors whose parameters differ by 100 times to be connected in parallel to ensure that the capacitance characteristics are always maintained in its wider frequency range.
However, in practical applications, due to the difference in the distance between the capacitor pins and traces and the digital chip when the capacitor is placed, different leads or trace inductances will be brought about, and the large capacity can play the role of energy storage and filtering. Therefore, for the decoupling design of digital chips, especially the digital power supply pins with rich high-order harmonics, large-capacity capacitors are usually used in parallel with 0.1μF capacitors and 0.1uF capacitors with the same capacitance value, which is better. Effect.
The self-resonant frequency of the surface mount capacitor is relatively high. In practical applications, the equivalent series inductance of its connecting line will also reduce its original advantage. Surface mount capacitors have a higher self-resonant frequency because the lead inductance of the radial and axial capacitors in the small package size is small. According to actual experience, the self-resonant frequency of surface mount capacitors of different package sizes varies within ±(2～5)MHz with the change of the lead inductance of the package.
The plug-in capacitor is nothing but the result of the surface mount device plus the pin leads. For a typical plug-in capacitor, its equivalent series inductance averages 2.5nH/2.54mm. The equivalent series inductance of surface mount capacitors is 1nH on average. Based on the above, the equivalent series inductance of the capacitor needs to be considered when using decoupling capacitors. Surface mount capacitors have better performance at high frequencies than plug-in capacitors because of their low equivalent series inductance.
Since the equivalent series inductance is the main factor that causes the capacitor to lose its application above the self-resonant frequency, in actual circuit applications, the connecting line inductance of the capacitor in the PCB, including vias, must be taken into consideration. In some circuits, if the operating frequency is very high and the frequency is much higher than the self-resonant frequency range of the capacitor in the circuit, then the capacitor cannot be used.
For example, a 0.1uF capacitor is not suitable for filtering a 100MHz clock signal, and a 0.001uF capacitor is a good choice without considering the inductance of leads and vias. This is because 100MHz and its harmonics have exceeded the resonant frequency of the 0.1uF capacitor.
In practical applications, generally choose ceramic capacitors, ultra-small polyester or polystyrene film capacitors are also possible, and their size is equivalent to ceramic capacitors. There is also a three-terminal capacitor because the capacitor lead inductance is very small, it can extend the frequency range of small ceramic capacitors from below 50MHz to above 200MHz, which is very useful for suppressing noise in higher frequency bands. To obtain a better filtering effect at a higher frequency or higher frequency, especially to protect the shield from being penetrated, a feedthrough capacitor must be used, which is a kind of three-terminal capacitor.
The following diagrams are the frequency impedance relationship diagrams of capacitors with different capacitances and capacitance values. The self-resonant frequency points can be seen from the diagrams, which can be used for reference.
Comparison of insertion loss characteristics of three-terminal capacitors and ordinary capacitors
The distributed capacitance between the power layer and the ground plane in the PCB is an ideal plate capacitance. The current flows in from one side and flows out from the other side, and the inductance is almost zero. In this case, the plate capacitor is still capacitive at high frequencies. Therefore, in the multi-layer PCB design, the plate capacitor formed between the power layer and the ground layer is of great significance for the high-frequency decoupling of high-frequency digital circuits. , The size of the plate capacitance formed between the power layer and the ground layer in the PCB increases with the decrease of the distance between the power layer and the ground layer, and increases with the increase of the area of the power layer and the ground layer. Therefore, there is a parallel relationship between the high frequency capacitance added in the digital circuit and the plate capacitance, which is equivalent to the parallel connection of capacitors. In this way, the anti-common resonance point of the parallel capacitor will appear in the circuit Or
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