Practical Tips | Working Principle, Characteristics, Selection Guide and PCB Layout Design of Decoupling Capacitors

1. Digital circuit - there will be a large current at the moment when the level flips, and a self-inductance voltage will be generated on the power supply line.

2. Why can capacitors be decoupled?

1) Power supply noise is generally a high-frequency AC component, and capacitors have the function of communicating and blocking DC, so capacitors can remove high-frequency noise components and achieve decoupling.

2) By reducing the output impedance of the power supply system, the impact of load changes on the power supply output voltage of the subsequent stage can be reduced, and the capacitor can achieve the requirement of reducing the output impedance. (Because the smaller the output resistance, the output voltage drop (that is, voltage fluctuation) caused by the sudden increase in load instantaneous current can be reduced).

(Another decoupling method to lower output impedance is to shorten the distance between the power and ground planes).

3. Characteristics of ideal capacitors and actual capacitors

Although we all know that capacitors have the function of communicating and blocking. But in actual use, we also need to understand the characteristics of the actual capacitor, so that we can choose the most appropriate capacitor based on the actual needs of the decoupling circuit.

1) Ideal capacitance VS actual capacitance:

An ideal capacitor: it does not produce any energy loss and is capacitive at any frequency.

Actual capacitance: In fact, because the material used to make the capacitor has resistance and the insulating medium of the capacitor has losses, the capacitor becomes imperfect for various reasons. The actual capacitance is equal to the equivalent series inductance ESL, the equivalent series resistance ESR, and the ideal capacitance in series, so its characteristics are related to frequency.

2) Model of actual capacitance:

The actual capacitor will have some energy loss, and behave externally like a resistor and capacitor connected in series ( equivalent series resistance ESR ). On the other hand, due to physical structural factors such as leads and windings, there is also an inductance component ( equivalent series inductance ESL ) inside the capacitor. There is some leakage or bulk resistance ( bulk resistance Rbulk ) in the capacitor in parallel with the ideal capacitance, ESL and ESR. The image below shows a real realistic capacitor model and impedance.

Since the dielectric material in the capacitor is very insulating, the value of Rbulk is usually very large (~100 GOhms), so it can be ignored when calculating the impedance of the capacitor. Therefore, we need to focus on the ESL and ESR values ​​when selecting capacitors.

3) The impact of ESR and ESL on capacitor filtering:

ESR: - will cause sudden changes in voltage!

ESR usually ranges from 100mΩ ~ 1000mΩ. If your chip power supply has a very short peak current of 100mA, and this current is almost provided by the decoupling capacitor, if your capacitor ESR has 1Ω, imagine 100mA When the current flows through this resistor and reaches the other end, there is already a voltage drop of 100mV.

ESL - will affect the filter's operating frequency and high-frequency filtering effect.

The graph below shows how ESL affects the impedance of a theoretical 10 nF capacitor with an ESR of 0.01 ohms. Various curves show impedance curves for different ESL values ​​(1 nH, 10 nH, and 100 nH).

From the above figure, we see that the impedance is capacitive before the self-resonant frequency (that is, the lowest point) (the impedance decreases as the frequency increases, showing capacitance) regardless of the ESL value; then above the self-resonant frequency After the resonant frequency, it becomes inductive (because at this time, the impedance increases with the increase of frequency and appears inductive).

The best filtering effect of the capacitor is at this self-resonant frequency, so for the RE radiation problem of EMC, we generally use the frequency of the radiation exceeding the standard point as the self-resonant frequency point, and then select the capacitor based on this curve.

This reduction of ESL has dual meanings:

1. Lowering ESL can increase the self-resonant frequency , that is, the frequency of the lowest point in the above figure shifts to the right. This allows the capacitor to maintain capacitance over a wider range.

Because: the LC self-resonant frequency Fs of the capacitor is calculated by the following formula:

2. Reduce ESL and reduce the impedance in the high-frequency area. Because after the frequency exceeds the self-resonant frequency fs, the capacitor presents inductive reactance, which is related to ESL. At this time, reducing ESL can reduce the impedance of the capacitor.

So the conclusion is: in order to improve the decoupling filtering effect of the capacitor, you must choose low ESR and low ESL capacitors!

(Generally, the higher the capacitance value, the greater the ESL. This is why large capacitors are generally used to filter low frequencies and small capacitors are used to filter high frequencies (because small capacitors have small ESL and high self-resonant frequency))

4. Selection of decoupling capacitors - capacitance and self-resonant frequency

1) Capacity value

For filtering at the chip power supply pin level, the selection can generally be made in terms of capacitance:

You can refer to the following three methods:

1.1) Manually calculate the overshoot voltage according to the chip specification sheet

1.2) Experience value (according to the 100 times principle: such as 10pF, 1nF, 0.1uF)

1.3) Component specification sheet (YYDS)

2) Self-resonant frequency

For EMC RE radiation exceeding the standard, or for some chips that are particularly sensitive to power supply ripple, the capacitor must be selected based on the noise frequency point and the self-resonant frequency of the capacitor.

• If you want to filter out a single point of noise , you can choose a capacitor whose noise frequency point is slightly lower than the self-resonant frequency. At this time, you can achieve the best filtering effect.

What should I do if I can’t find a suitable package?

Then you can choose capacitors with multiple capacitances and connect them in parallel. Using multiple identical capacitors in parallel will increase the total equivalent capacitance and reduce the PDN impedance, but will not change the resonant frequency. ( Multiple capacitors in parallel can reduce ESL )

• If the noise frequency point is in a range , then you need to choose capacitors with multiple capacitance values.
  

5. Placement requirements for decoupling capacitors

1) Proximity principle: The decoupling capacitor should be as close as possible to the power pin of the chip. Reduce the parasitic inductance of the wiring between the decoupling capacitor and the chip, and the decoupling effect will be better.

2) The principle of smaller and closer:
capacitors with small capacitance are closest to the chip, and then move away from the chip in sequence according to the principle of increasing capacitance (far away is relative, provided that the principle of proximity is followed). The small capacitor is responsible for high-frequency response and should be closer to the chip to shorten the response time. And small capacitors can filter out high-frequency noise. If they are too far away from the chip, the wiring between the capacitor and the chip will pick up the noise again, weakening the denoising effect.

3) The power line first passes through the decoupling capacitor and then connected to the chip pin

4) When multiple capacitors are connected in parallel, it is best not to place them in parallel (there is mutual inductance). You can change it to:

Source: Electronic Engineer Notes

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