PCB News - How to choose a combination of capacitors in PCB design

How to choose a combination of capacitors in PCB design

As we mentioned earlier, the change of transient current is equivalent to a step signal, which has a wide frequency spectrum. Therefore, to compensate for this current demand, it is necessary to provide a sufficiently low power supply impedance over a wide frequency range. However, the effective frequency range of different capacitors is different, which is related to the resonant frequency of the capacitor (strictly speaking, it should be the resonant frequency after installation). The effective frequency range (the frequency range where the capacitor can provide sufficiently low impedance) is a small section near the resonance point. frequency. Therefore, to provide a sufficiently low power supply impedance in a wide frequency range, a combination of many different capacitors is required.

You might say that with only one capacitance value, as long as the number of parallel capacitors is large enough, the same low impedance can be achieved. This is true, but in practical applications you can calculate that most of the time, the number of capacitors required is large. If you really want to do this, your circuit board may be densely packed with capacitors. Neither professional nor necessary.

When choosing a capacitor combination, there are many issues to consider, such as what package to choose, what material, what capacitance value, what is the interval between capacitance values, the frequency of the main clock and its harmonic frequencies, signal rise time, etc. Specially designed according to specific design.

Usually, tantalum capacitors or electrolytic capacitors are used for board-level low-frequency decoupling. The calculation method of capacitance has been mentioned before, but it should be reminded that it is best to use several or more capacitors in parallel to reduce the equivalent series inductance. The Q value of these two capacitors is very low, the frequency selectivity is not strong, very suitable for board level filtering.

The selection of high-frequency small capacitors is a bit troublesome and needs to be calculated by frequency band. The frequency range that needs to be decoupled can be divided into several sections, each section is calculated separately, and multiple capacitors of the same capacitance are used in parallel to meet the impedance requirements, and different capacitance values are selected for different frequency bands. However, in this method, the division of frequency bands must be continuously adjusted according to the calculation results.

Generally, 3 to 4 frequency bands are sufficient, so 3 to 4 capacitance levels are required. In fact, the more capacitance levels you choose, the flatter the impedance characteristics will be, but there is no need to use a lot of capacitance levels. The flatness of the impedance is of course good, but our ultimate goal is that the total impedance is less than the target impedance, as long as this requirement can be met. That's it.

The choice of the capacitance value in a certain level depends on the system clock frequency. As mentioned earlier, there is anti-resonance in the parallel connection of capacitors. Pay attention to the design and try not to let the harmonics of the clock frequency fall near the anti-resonance frequency. For example, if you choose 0.47, 0.22, 0.1 or other values on the fraction of a microfarad level, you need to calculate the following resonant frequency after installation.

One more thing to note is that the level of capacitance should not exceed 10 times. For example, you can choose a combination like 0.1, 0.01, 0.001. Because this can effectively control the amplitude of the anti-resonance point impedance, too large an interval will make the anti-resonance point impedance very large. Of course, this is not absolute. It is best to use software to look at it. The ultimate goal is that the anti-resonance point impedance can meet the requirements.

The selection of high-frequency small capacitors, in order to obtain the optimal combination, is a process of iterative search for the optimal solution. The best way is to roughly calculate the approximate combination first, then use the power integrity simulation software to simulate, and then make partial adjustments to meet the target impedance requirements. This is intuitive and convenient, and it is easier to control the anti-resonance point. And the capacitance of the power plane can also be added for joint design.

Figure 1 is an example of a capacitor combination. The capacitors used in this combination are: 2 680uF tantalum capacitors, 7 2.2uF ceramic capacitors (0805 package), 13 0.22uF ceramic capacitors (0603 package), 26 0.022uF ceramic capacitors (0402 package). In the figure, the upper flat curve is the impedance curve of the 680uF capacitor, and the other three capacitance curves are the three V-shaped curves in the figure, which are 2.2uF, 0.22uF, and 0.022uF once from left to right. The overall impedance curve is the thick envelope at the bottom of the figure.

This combination realizes keeping the power supply impedance below 33 milliohms in the 500kHz to 150MHz range. At the 500MHz frequency point, the impedance rises to 110 milliohms. It can be seen from the figure that the impedance at the anti-resonance point is controlled very low.

The medium of small capacitors is generally ceramic capacitors in conventional designs. The ESR of the NP0 dielectric capacitor is much lower, and it can be used for parts with stricter impedance control, but note that the Q value of this capacitor is very high, which may cause severe high-frequency ringing, so be careful when using it.

The choice of package, as long as the processing capacity allows, of course, the smaller the better, so that you can get a lower ESL, and you can also leave more wiring space. However, different packages have different resonance frequency points and different capacitance value ranges, which may affect the final number of capacitors. Therefore, the capacitor package size and capacitance value must be considered jointly. In short, the ultimate goal is to achieve the target impedance requirement with the least amount of capacitance, and reduce the pressure of installation and wiring.

The above is an introduction to how to choose a combination of capacitors in PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.