The second key point of the effective use of coupling capacitance is to reduce the ESL of capacitance (i.e. equivalent series inductance). Although it is said to "reduce ESL", since it is impossible to change the ESL of a single product, it means that "even if the capacitance value is the same, the capacitance with small ESL should be used". By reducing ESL, high-frequency characteristics can be improved and high-frequency noise can be reduced more effectively.
Even if the capacitance value is the same, use a smaller capacitor
For multilayer ceramic capacitors (MLCC), sometimes several packages with the same capacitance but different sizes are prepared. ESL depends on the structure of the pin position. The capacitance with smaller size is basically smaller at the pin, and usually the ESL is smaller.
The right picture is an example of the frequency characteristic of capacitance with the same capacitance and size. As shown in the figure, the resonance frequency of the smaller 1005 size is higher, and the frequency range impedance of the later inductive region is lower. As described in "frequency characteristics of capacitor", the resonant frequency of capacitor is based on the following formula. It can be seen from the formula that as long as the capacitance value is the same, the lower the ESL, the higher the resonant frequency. In addition, the impedance characteristic of inductive region depends on ESL, which has also been introduced.
For noise countermeasures, when it is necessary to reduce the noise of higher frequency band, the capacitor with small size can be selected.
Use capacitors designed to reduce ESL
Among the multilayer ceramic capacitors, some models adopt the shape and structure designed to reduce ESL.
As shown in the figure, the electrode of ordinary capacitor is on the short side, while the electrode of LW reversal type is on the contrary, on the long side. Because l (length) and w (width) are opposite, it is called "LW reversal type". The type of ESL is reduced by increasing the width of the electrode.
Three terminal capacitor is a capacitor whose structure is optimized to improve the frequency characteristics of ordinary capacitor (two pins). The three terminal capacitor is to extend the other end of one pin (electrode) of the two pin capacitor outward as the through pin and the other pin as the GND pin. In the figure above, the input and output electrodes are equivalent to the straight pins protruding from both ends, and the left and right electrodes are of course conductive. There is a dielectric between the input and output electrode (through pin) and GND electrode, which plays the role of capacitance.
Insert the input and output electrodes in series into the power supply or signal line (connect one end of the input and output electrode to the input end and the other end to the output end), and GND electrode is grounded. In this way, since the ESL of the input and output electrodes is not included in the grounding terminal, the impedance of the grounding becomes very low. In addition, the ESL of the input and output electrodes is directly inserted into the noise path, which is conducive to reducing the noise (increasing the insertion loss).
ESL can be suppressed by configuring GND electrodes on the long side in pairs; Then the parallel mode can halve the ESL.
Based on this structure, the three terminal capacitor not only has a very low ESL, but also can maintain a low ESR. Compared with the double pin capacitor with the same capacitance and the same size, it can significantly improve the high-frequency characteristics.
Key points:
There are two key points in the effective use of decoupling capacitors: ① use multiple capacitors and ② reduce the ESL of capacitors.
By reducing the ESL of the capacitor, the high-frequency characteristics can be improved and the high-frequency noise can be reduced more effectively.
Although some capacitors have the same capacitance value, their ESL is smaller due to different size and structure.
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