What is a Decoupling Capacitor?

Catalog

I Introduction

Decoupling capacitors are used to filter out the interference of the output signal. They're often used in the amplifier circuit where AC is not needed to eliminate self-excitation and make the amplifier work stably.

In a circuit with one conductor, when the power is shared, if a device needs to provide an output, the voltage of the conductor will be simultaneously pulled down, generating noise coupled into the shared circuit. In a noisy environment, these electromagnetic waves will induce a voltage signal in the conductor, affecting the components in the loop. And in the digital circuit, the device is prone to generate the wrong signal due to the interference at the critical position, thereby causing malfunction.

Decoupling capacitors can reduce the above situations. They are generally placed at the power supply near the components to reduce the influence of the wiring impedance on the filtering effect. Most decoupling capacitors are ceramic capacitors, the value of which is determined by the fastest rising and falling speed of the voltage signal.

A typical application of using decoupling capacitors next to an IC

II Function of Decoupling Capacitors

1. Remove High Frequencies

The decoupling capacitor is mainly used to remove the interference of high frequencies such as the RF signal, which enters into the device through electromagnetic radiation.

In fact, the capacitor near the chip also has the function of energy storage. Assume that the main power supply is a reservoir, and every household in our building needs a water supply. The water does not come directly from the reservoir as it’s too far away, and we need to wait for a long time until the water comes. The actual water comes from the water tower on top of the building, which acts as a buffer.

From the micro view, when the high-frequency device is working, its current is discontinuous, the frequency is very high, and there is a certain distance between the device VCC and the main power supply. Even if the distance is not long, at high frequency, the impedance

and the inductance of the line will also be very large, thus the device can not be supplied with current in time. The decoupling capacitor can make up for this deficiency, which is one of the reasons why there are many small capacitors placed at the VCC pins of high-frequency devices on the circuit boards.

2. Provide DC Power Supply to Active Devices

When the active devices are turned on and off, high-frequency noise will be generated and transmitted along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and lead the noise to the ground.

III Bypass Capacitor VS. Decoupling Capacitor

1. Bypass capacitors

In the circuit, if you want to remove the signal above a certain frequency or all the AC components, you can use a filter capacitor. Traditionally, capacitors only with the filtering effect are usually called bypass capacitors.

For example, a capacitor connected in parallel with the emitter resistors of a transistor or the cathode resistor in a vacuum tube is a bypass capacitor, because an AC signal enters the ground through this capacitor. And as in a power supply circuit, except for the smoothing filter or anti-crosslinking capacitor of thousands of microfarad, the high-frequency capacitor of micrometers is usually used to bypass the high frequencies. The application circuit of the bypass capacitor is shown in the figure below.

Operation of a Bypass Capacitor

2. Decoupling Capacitors Vs. Bypass Capacitors

In electronic circuits, decoupling capacitors and bypass capacitors can both play the role of anti-interference, but they are placed in different positions.

For the same circuit, the bypass capacitor filters the high-frequency noise carried by the previous stage circuit in the input signal. While the decoupling capacitor filters the interference of the output signal and prevents the interference signal from returning to the power supply. This is the essential difference between them.

IV Decoupling Capacitor Calculation 

The original purpose of decoupling: to maintain the voltage limit within the specified allowable error regardless of the regulations and requirements on current fluctuations.

① Calculation Method 1

The capacitance C of the decoupling capacitor required by an IC can be calculated with the formula:

⊿U is the allowable decrease of the actual power bus voltage (V);

 I is the maximum required current(A);

⊿t is the duration time of this required capacitance.

② Calculation Method 2

The decoupling capacitor value is recommended to be greater than 1/m times the equivalent open circuit capacitance.

Here, m is the maximum percentage of the allowable power bus voltage change on the IC power supply pin, which will generally be given in the IC datasheet.

The equivalent open circuit capacitance:

In the formula:

P-total wattage dissipated by IC;

U-IC's maximum DC power supply voltage;

f-The clock frequency of the IC.

Once the equivalent open circuit capacitance is determined, multiply it by 1/m to find the total decoupling capacitor value required by the IC. Then the result is divided by the total number of power pins connected to the same power bus, and finally, the capacitance value near all power pins connected to each power bus is obtained.

V Selection and Layout of PCB Decoupling Capacitors 

Decoupling capacitors are not the more the better, and we should also pay attention to the filtering effect. When we design the printed circuit boards, a 10μF-100μF electrolytic capacitor is connected across the power input, and a 0.01μF ceramic capacitor is configured between the power and ground of each integrated chip. On the one hand, it provides and absorbs the instantaneous charge and discharge energy of the opening and closing of the integrated circuit; on the other hand, it bypasses the high-frequency noise of the device.

1. Classification of PCB Decoupling Capacitors

The decoupling capacitor can play a role of energy storage when compensating for the integrated chip or when there is a drop in the operating voltage of the circuit board. It can be divided into three types: overall, local and inter-board type.

① Overall Decoupling Capacitors

The overall decoupling capacitor works in the low-frequency (<1MHz) range. It provides one current source for the entire circuit board, to compensate for the noise current ΔI generated by the circuit board during operation and ensure the stability of the working power supply voltage. Its capacitance is 50 to 100 times the sum of all load capacitances on the PCB.

Generally, overall decoupling capacitors should be placed close to the power extension cord and ground wires of the PCB, where the printed line density is very high. This provides space for the placement of critical printed lines on the PCB without reducing low-frequency decoupling.

② Local Decoupling Capacitors

The local decoupling capacitor has two functions:

The local decoupling capacitor is installed between the power supply terminal and the ground terminal of each integrated chip, and as close to the integrated chip as possible.

③ Inter-board Decoupling capacitors 

The Inter-board Decoupling capacitor refers to the capacitor between the power plane and the ground plane. It is the main source of decoupling current at high frequencies. The capacitance between the planes can be increased by increasing the area between the power layer and the ground layer.

In the PCB, some ground planes can be distributed to the power plane. Remove these ground planes and replace them with power isolation areas can increase the capacitance between the planes.

2. PCB Decoupling Capacitor Value

In the DC power supply circuit, changes in the load can generate power supply noise. For example, in digital circuits, when the circuit is transformed from one state to another, a large peak current will be generated on the power lines, forming a transient noise voltage.

Configuring decoupling capacitors can suppress noise caused by load changes. It is a common practice in the reliability design of PCB. A good high-frequency decoupling capacitor can remove high-frequency components up to 1GHz. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics.

When we design a printed circuit board, a decoupling capacitor must be added between the power supply and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, which provides and absorbs the instantaneous charge and discharge energy of the integrated circuit when it is opened and closed; On the other hand, it bypasses the high-frequency noise of the device.

Capacitor Placed in Parallel for Decoupling

The configuration principles of decoupling capacitors are as follows:

① Filter Capacitors Assigned to Power Supply

A 10μF ~ 100μF electrolytic capacitor is connected across the power input. If the position of the PCB allows, the anti-interference effect of this electrolytic capacitor will be better.

1μF and 10μF capacitor with parallel resonance frequency above 20MHz has a better effect of removing high-frequency noise. Applying this capacitor in the area where a power source enters the printed board is often advantageous, even a battery-powered system needs this kind of capacitor.

② Decoupling Capacitors Allocated for Chips

Configure a 0.01μF ceramic capacitor for each integrated circuit chip. A typical decoupling capacitor in a digital circuit is of 0.1/μF. The decoupling capacitor has a distributed inductance of 5nH, and its parallel resonance frequency is around 7MHz, which means that it has a good decoupling effect for noise below 10MHz, and has little effect for noise over 40MHz.

If the printed circuit board space is too small, we can apply a 1μF ~ 10μF tantalum electrolytic capacitor for every 4- 10 chips. The high-frequency impedance of this capacitor is particularly small, which is less than 1μF-10μF in the range of 500kHz-20MHz, and the leakage current is very small (below 0.5μA).

The selection of the decoupling capacitor value is not strict, it can be calculated with the formula:

For the system formed by the microcontroller, the capacitance can be between 0.1μF- 0.01μF.

③ Charge and Discharge Capacitors 

Every 10 or so integrated circuits need to add a charge and discharge capacitor with a capacitance of 10μF. The large capacitors usually used are electrolytic capacitors. However, when the filtering frequency is relatively high, the electrolytic capacitors are rolled up with two layers of films, and this rolled structure behaves as an inductor at high frequencies. In this situation, a tantalum capacitor or polycarbonate capacitor is used.

3. Factors in PCB Decoupling Capacitor Layout

① Influence of Capacitor Leads

When we use capacitors to suppress electromagnetic disturbance and filtering, the most easily overlooked problem is the influence of capacitor leads on the filtering effect.

The capacitive reactance of the capacitor is inversely proportional to the frequency. Based on this feature, the capacitor is connected in parallel between the signal line and the ground line to bypass the high-frequency noise. However, in the actual project, many people find that this method does not have the expected effect of filtering noise, and is helpless in the face of stubborn electromagnetic noise. One reason for this is that the influence of the capacitor leads on the bypass effect is ignored.

The actual capacitor is a series network composed of equivalent series inductance (ESL), capacitance, and equivalent series resistance (ESR).

The impedance of an ideal capacitor decreases with the increase of frequency, while the impedance characteristics of an actual capacitor are shown in the figure.

Impedance Characteristics of an Actual Capacitor

When the frequency is low, it shows the capacitance characteristic, that is, the impedance decreases as the frequency increases. At a certain point, resonance occurs, and the impedance of the capacitor is equal to the ESR. Above the resonance point, due to the effect of ESL, the impedance of the capacitor increases with the increase of frequency, which makes the capacitor exhibit the impedance characteristics of the inductor. As the impedance of the capacitor increases, the bypass effect on high-frequency noise is reduced or even disappeared.

Therefore, when we arrange the decoupling capacitor, we must pay attention to the influence of the distribution parameter of the capacitance on the filtering.

② Function of Capacitor Leads

The resonant frequency of the capacitor is determined by both ESL and C. The larger the capacitance or the inductance, the lower the resonant frequency, which means the high-frequency filtering effect of the capacitor is worse.

Except for the types of capacitors, the length of the lead is also a very important parameter for ESL. The longer the lead, the greater the inductance and the lower the resonance frequency of the capacitor. Therefore, in actual operation, the lead wire of the capacitor should be as short as possible. The correct installation method and the incorrect installation method of the capacitor are shown in the picture below.

Installation Method of Filter Capacitors

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 people think that larger capacitance indicates a better filtering effect, which is a misunderstanding. Though the larger the capacitance is, the better the bypass effect of low-frequency interference is. However, because the capacitor resonates at a lower frequency, and the impedance starts to increase with the increase of frequency, the bypass effect of high-frequency noise becomes worse. The table shows the self-resonant frequency of ceramic capacitors with different capacities. The lead length of the capacitor is 1.6mm.

Although the resonance of the capacitor is undesirable from the perspective of filtering out high-frequency noise, it is not always harmful. When the frequency of the noise to be filtered is determined, the capacitance can be adjusted to make the resonance point just fall on the disturbance frequency.

③ Effects of Temperature

Temperature also has a great influence on the characteristics of the capacitor. Since the medium parameters in the capacitor are affected by temperature changes, the capacitance value of the capacitor also changes with temperature. Different media have different temperature-changing rules, and the capacitance of some capacitors will decrease by more than 70% when the temperature increases.

The commonly used filter capacitors are ceramic capacitors. There are three types of ceramic capacitors:ultra-stable, stable, and the universal type. The temperature characteristics of capacitors with different dielectrics are shown in the picture.

Temperature Characteristics of Different Dielectric Capacitors

It can be seen that the capacitance of the COG capacitor almost does not change with temperature. The capacitance of the X7R capacitor changes within 12% of the rated operating temperature range, and the capacity of the YSV capacitor changes by more than 70% within the rated operating temperature range. We should pay attention to these characteristics, otherwise, there will be changes in the performance of the filter at high or low temperatures, resulting in electromagnetic compatibility problems.

Although the COG dielectric capacitor is less affected by temperature and has stable characteristics, its dielectric constant is low, generally 10 to 100, so when the volume is small, the capacitance will be small. The dielectric constant of the XTR dielectric capacitor is much higher, from 2000 to 4000, thus a smaller volume can produce a larger capacitance. The dielectric constant of the YSV dielectric capacitor is the highest, which is 5000- 25000. It is usually used where smaller volume and larger capacitance are required.

When choosing capacitors, many people unilaterally pursue the small size of the capacitor. Although the dielectric constant of this capacitor is high, the temperature stability is very poor, which will lead to poor temperature characteristics of the device. This should be paid special attention to when we select capacitors, especially for military equipment.

④ Influence of PCB Voltage

The capacitance of the capacitor changes not only with the temperature but also with the operating voltage. This must be noticed in the actual project.

Figure 6-10 shows the voltage characteristics of capacitors with different dielectric materials. In the figure, under the rated voltage, the capacitance of the X7R capacitor is reduced to 70% of the original value and the capacity of the YSV capacitor is reduced to 30% of the original value. Therefore, when selecting a capacitor, we should leave some margin for the voltage and capacitance value, otherwise, the filter will not achieve the desired effect under the rated operating voltage.

When considering the influence of temperature and voltage together, the change of capacitance is shown in the figure.

Voltage Characteristics of Capacitors

Therefore, the filter effect of the capacitor must be fully considered when we place the filter capacitor.

Temperature / Voltage Characteristics of Capacitors

4. Reasonable Placement of PCB Decoupling Capacitors

① Generally, only a few power decoupling capacitors are drawn in the schematic, but it is not indicated where they should be connected. In fact, these capacitors are set for switching devices (gate circuits) or other components that require decoupling, so they should be placed as close to these components as possible. When the power supply decoupling capacitors are properly arranged, the grounding point problem becomes less obvious.

② For devices with weak antinoise capability and large current changes when the power is removed and storage devices such as ROM and RAM, decoupling capacitors should be directly connected between the power supply line (VCC) and ground (GND) of the chip.

③ The lead of the decoupling capacitor should not be too long. The shorter the lead, the better the decoupling effect. In particular, high-frequency bypass capacitors must not have leads.

④ The amount of decoupling is not the more the better, instead, we should pay attention to the effect of filtering, and select the number and size of capacitors according to the time of the circuit board and device.

⑤ Ceramic capacitors and electrolytic capacitors are not used when the decoupling requirement is higher because of their poor capacitance accuracy and large distributed inductance. Instead, tantalum capacitors or polyester capacitors should be applied.

⑥ In places with many chips and decoupling capacitors, a charge and discharge capacitor can be installed to process the charges generated during the operation of the circuit switch.

To Sum up

In this essay, we first learned about the 2 main functions of decoupling capacitors. Then, we discuss the similarities and differences between decoupling and bypass capacitors. Next, the calculation method of decoupling capacitance was introduced. And in the final part, we focus on the selection and layout of PCB decoupling capacitors. Hope you can have a basic understanding of the decoupling capacitor after reading this article!

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