Proper configuration of PCB decoupling Capacitor

Original article address: PCB decoupling capacitor reasonable configuration Author: ker11f

I. Category of decoupling capacitors in PCB

The decoupling capacitor can be used for energy storage when the operating voltage of the integrated chip or circuit board is compensated. It can be divided into three types: integral, partial, and inter-board.

The overall decoupling capacitor is also called a bypass capacitor. It operates in a low frequency (<1 MHz) range and provides a current source for the entire circuit board to compensate for the Delta I noise current generated when the circuit board is working, ensure the voltage stability of the working power supply. Its size is 50 ~ Of all load capacitors and ~ 100 times. It should be placed close to the PCB external power cord and ground line, printing line density is high. This will not only reduce the low-frequency Decoupling, but also provide space for the layout of key printed lines on the PCB.

Local decoupling capacitors have two functions. First, for functional consideration, the power supply voltage of the integrated chip is relatively stable through capacitor charging and discharging, and the function of the integrated chip is not affected due to the temporary voltage drop. Second, for EMC considerations: Provides the nearest high-frequency channel for the transient current of the integrated chip, so that the current does not pass through the power supply line with a large loop area, thus greatly reducing the outward radiation noise. At the same time, because each integration piece has its own high-frequency channel, there is no public impedance between them, and its impedance coupling is restrained. The local decoupling capacitor is installed between the power terminal and the ground terminal of each integration piece, and is as close as possible to the Integration piece.

The decoupling capacitor between the Board refers to the capacitance between the power supply plane and the ground plane. It is the main source of decoupling current at high frequencies. The Inter-board capacitor can be increased by adding the area between the power supply layer and the connected ground layer. In the PCB, some grounding surfaces can be laid to the power supply layer, and these grounding surfaces can be removed. The capacitance between boards can be increased when replaced by the power isolation area.

Ii. Size of decoupling capacitor in PCB

In the DC power supply circuit, load changes may cause power noise. For example, in a digital circuit, when a circuit is switched from one state to another, a large peak current is generated on the power cord to form a transient noise voltage. Configuring the decoupling capacitor can suppress noise caused by load changes. It is a common practice for the reliability design of printed circuit boards. A good high-frequency Decoupling capacitor can remove high-frequency components up to 1 GHz. The high-frequency characteristics of ceramic chip capacitor or multilayer ceramic capacitor are better. When designing a printed circuit board, a decoupling capacitor must be added between the power supply and ground of each integrated circuit. Decoupling capacitor has two functions: one is the energy storage capacitor of the integrated circuit, which provides and absorbs the charge and discharge energy when the integrated circuit is opened and closed; the other is to bypass the high-frequency noise of the device. The decoupling capacitor configuration principles are as follows:

1 power supply distribution filtering capacitor

The power input end is connected to a 10μf ~ 100 μF electrolytic capacitor, if the position of the printed circuit board allows, the above electrolytic capacitor will be better anti-interference effect. 1 μF, 10 μF capacitor, parallel resonance frequency above 20 MHz, the removal of high-frequency noise is better. It is often advantageous that the power supply enters the Printed Board and a high-frequency capacitance of 1 μF or 10 μF, Which is required even for battery-powered systems.

2 chip configuration decoupling Capacitor

Configure a 0.01 μF ceramic capacitor for each IC chip. A typical decoupling capacitor of 0.1/μF in a digital circuit has a 5 NH distributed inductance, and its parallel resonance frequency is around 7 MHz, that is to say, there is a good decoupling effect for noise below 10 MHz, and there is almost no effect on noise above 40 MHz. If the printed circuit board cannot be installed in a small space, it can be 4 ~ Configure 1 μF ~ for 10 chips ~ 10μf ta electrolytic capacitor, the High-Frequency Impedance of this device is particularly small, in kHz ~ The impedance within 20 MHz is less than 1 μF ~ 10 μF and the leakage current is very small (less than 0.5 μA ). The decoupling capacitor value is not strictly selected. It can be calculated based on C = 1/F, that is, 0.1 μF for 10 MHz. For a system consisting of a microcontroller, take 0.1 μF ~ Between 0.01 μF.

3. Add storage and capacitor when necessary

Each 10-piece integrated circuit requires a charge-discharge capacitor, or storage capacitor. The capacitance size can be 10 μF. The large capacitor is usually used as the electrolytic capacitor, but when the filtering frequency is high, it is best not to use the electrolytic capacitor. The electrolytic capacitor is rolled up by two layers of film. The structure of this winding is expressed as an inductor at a high frequency, it is best to use the TA capacitor or polycarbonate capacitor.

3. Proper decoupling capacitor layout in PCB

(1) equivalent capacitance model

When using capacitors to suppress electromagnetic harassment and filtering, the most easily overlooked problem is the effect of capacitor leads on the filtering effect. The capacitance is inversely proportional to the frequency. Using this feature, the capacitor is connected in parallel between the signal line and the ground line to bypass high-frequency noise. However, in actual engineering, many people find that this method does not have the expected effect of noise filtering and is helpless in the face of stubborn electromagnetic noise. One reason for this is that the effect of capacitor leads on the bypass effect is ignored.

As shown in the Circuit Model 4-7 of the actual capacitor, it is a series network consisting of an equivalent inductance (ESL), a capacitor and an equivalent resistance (ESR.

The impedance of the ideal capacitor decreases with the increase of the frequency, and the impedance of the actual capacitor is the impedance of the network shown in Figure 6-7. When the frequency is low, the capacitance is displayed, that is to say, the impedance decreases with the increase of frequency, and resonance occurs at a certain point. The impedance of the capacitor at this point is equal to the equivalent series resistance (ESR. Due to the effect of ESL, the capacitance impedance increases with the increase of the frequency, which presents the Impedance Characteristics of the inductance. As the impedance of the capacitor increases above the resonant point, the bypass effect on high-frequency noise is weakened or even disappears. Therefore, when assigning the decoupling capacitor, you must pay attention to the influence of the distribution parameters of the capacitor on the filtering.

(2) functions of capacitor leads

The resonance frequency of the capacitor is jointly determined by ESL and C. The larger the capacitance value or Inductance Value, the lower the resonance frequency, that is, the lower the high-frequency filtering effect of the capacitor. Apart from the capacitor type, the length of the capacitor lead is a very important parameter. The longer the lead, the larger the inductance, the lower the resonance frequency of the capacitor. Therefore, in actual engineering, the correct installation method and incorrect installation method of the capacitor should be as short as possible, as shown in Figure 4-8.

According to the series resonance principle of LC circuit, the resonance point is not only related to the inductance, but also the capacitance value. The larger the capacitance, the lower the resonance point. Many people think that the larger the capacitor capacity, the better the filtering effect. This is a misunderstanding. The higher the capacitance, the better the bypass effect on low-frequency interference. However, because the capacitor is resonant at a lower frequency, the impedance increases with the increase of the frequency. Therefore, the bypass effect on high-frequency noise becomes worse, therefore, when filtering, select the appropriate capacitor. Table 4-2 shows the auto-resonance frequency of the capacitor with different capacities. The lead length of the capacitor is 1.6mm.

Although from the perspective of filtering out high-frequency noise, capacitor resonance is not always harmful. When the noise frequency to be filtered out is determined, you can adjust the capacity of the capacitor so that the resonance point falls on the harassment frequency.

(3) Effect of Temperature on Capacitance

The temperature also has a great impact on the characteristics of the capacitor. Because the medium parameters in the capacitor are affected by the temperature change, the capacitor value also changes with the temperature. Different media vary with the temperature, and the capacity of some capacitors decreases by more than 70% when the temperature increases. The commonly used filter capacitor is the porcelain medium capacitor. Porcelain capacitor has three types: Ultra-stable (such as cog or NPO), stable (such as X7R), and general (such as Y5V or z5u. The Temperature Characteristics of capacitors of different media are shown in 4-9.

From Figure 4-9, we can see that the capacity of the cog capacitor almost changes with the temperature, and the capacity of the X7R capacitor changes below the rated operating temperature range by 12%, the ysv capacitor capacity changes by more than 70% within the rated operating temperature range. These features must be noted, otherwise the performance of the filter may change at high or low temperatures, resulting in electromagnetic compatibility problems.

Although cog dielectric capacitor has little influence on temperature and stable characteristics, its dielectric constant is low, generally ranging from 10 ~ 100, so when the volume is small, the capacity is small. The dielectric constant of the XTR dielectric capacitor is much higher, which is 2000 ~ 4000, so a small volume can generate a large capacitor. The maximum dielectric constant of the ysv dielectric capacitor is 5000 ~ 25000. It is usually used where a small volume is required and a large volume is required.

Many people choose capacitor, one-sided pursuit of the small size of the capacitor, although the capacitor medium has a high medium constant, but the temperature stability is very poor, this will lead to equipment Temperature Characteristics deteriorated. This requires special attention when selecting capacitors, especially in military equipment.

(4) effect of Voltage on capacitance in PCB

The electric capacity of the capacitor not only changes with the temperature, but also changes with the working voltage, which must be noted in the actual project. The voltage characteristics of capacitors of different media materials are shown in 6-10. It can be seen that the capacity of the X7R capacitor is reduced to 70% of the original value while that of the ysv capacitor is reduced to 30% of the original value, when selecting a capacitor, you must set aside margin on the voltage or capacitance. Otherwise, the filter will fail to achieve the expected effect under the rated operating voltage.

Considering the effect of temperature and voltage, the capacitance changes 4-11.

Therefore, when placing the filter capacitor, you must fully consider the filtering effect of the capacitor, instead of the more the better, the larger the better. Eliminate the misunderstanding of blind use of capacitors and the viewpoint of not using them.

Iv. Proper configuration of PCB decoupling capacitors

(1) Reasonably arrange the power filter/decoupling capacitor: Generally, only a few power filter/decoupling capacitors are drawn in the schematic diagram, but they are not pointed out where they should be connected. In fact, these capacitors are set for switching devices (door circuits) or other components that require filtering/decoupling. The layout of these capacitors should be as close as possible to these components, it does not work if you are too far away. When the power supply filtering/decoupling capacitor is properly arranged, the problem of the grounding point is not so obvious.

(2) storage devices such as ROM and Ram, which have weak noise and great changes in current upon shutdown, should be in the chip's power cord (VCC) and ground line (Gnd) directly connected to the decoupling capacitor.

(3) the lead of the decoupling capacitor cannot be too long. The shorter the lead, the better the decoupling effect. In particular, high-frequency bypass capacitors cannot contain leads.

(4) The decoupling is not the better, but the filtering effect. The number and size of capacitors are selected based on the time of the circuit board and the device.

(5) when the requirement is high, the decoupling capacitor does not need the porcelain chip capacitor or electrolytic capacitor, so that their capacity value is accurate and the distribution inductance is large. Use more accurate ta capacitor or polyester capacitor.

(6) When there are many chips and many decoupling capacitors, you can install a charge-discharge capacitor to accumulate the accumulation of charge-discharging circuit switching.

 Original article address: PCB decoupling capacitor reasonable configuration Author: ker11f

I. Category of decoupling capacitors in PCB

The decoupling capacitor can be used for energy storage when the operating voltage of the integrated chip or circuit board is compensated. It can be divided into three types: integral, partial, and inter-board.

The overall decoupling capacitor is also called a bypass capacitor. It operates in a low frequency (<1 MHz) range and provides a current source for the entire circuit board to compensate for the Delta I noise current generated when the circuit board is working, ensure the voltage stability of the working power supply. Its size is 50 ~ Of all load capacitors and ~ 100 times. It should be placed close to the PCB external power cord and ground line, printing line density is high. This will not only reduce the low-frequency Decoupling, but also provide space for the layout of key printed lines on the PCB.

Local decoupling capacitors have two functions. First, for functional consideration, the power supply voltage of the integrated chip is relatively stable through capacitor charging and discharging, and the function of the integrated chip is not affected due to the temporary voltage drop. Second, for EMC considerations: Provides the nearest high-frequency channel for the transient current of the integrated chip, so that the current does not pass through the power supply line with a large loop area, thus greatly reducing the outward radiation noise. At the same time, because each integration piece has its own high-frequency channel, there is no public impedance between them, and its impedance coupling is restrained. The local decoupling capacitor is installed between the power terminal and the ground terminal of each integration piece, and is as close as possible to the Integration piece.

The decoupling capacitor between the Board refers to the capacitance between the power supply plane and the ground plane. It is the main source of decoupling current at high frequencies. The Inter-board capacitor can be increased by adding the area between the power supply layer and the connected ground layer. In the PCB, some grounding surfaces can be laid to the power supply layer, and these grounding surfaces can be removed. The capacitance between boards can be increased when replaced by the power isolation area.

Ii. Size of decoupling capacitor in PCB

In the DC power supply circuit, load changes may cause power noise. For example, in a digital circuit, when a circuit is switched from one state to another, a large peak current is generated on the power cord to form a transient noise voltage. Configuring the decoupling capacitor can suppress noise caused by load changes. It is a common practice for the reliability design of printed circuit boards. A good high-frequency Decoupling capacitor can remove high-frequency components up to 1 GHz. The high-frequency characteristics of ceramic chip capacitor or multilayer ceramic capacitor are better. When designing a printed circuit board, a decoupling capacitor must be added between the power supply and ground of each integrated circuit. Decoupling capacitor has two functions: one is the energy storage capacitor of the integrated circuit, which provides and absorbs the charge and discharge energy when the integrated circuit is opened and closed; the other is to bypass the high-frequency noise of the device. The decoupling capacitor configuration principles are as follows:

1 power supply distribution filtering capacitor

The power input end is connected to a 10μf ~ 100 μF electrolytic capacitor, if the position of the printed circuit board allows, the above electrolytic capacitor will be better anti-interference effect. 1 μF, 10 μF capacitor, parallel resonance frequency above 20 MHz, the removal of high-frequency noise is better. It is often advantageous that the power supply enters the Printed Board and a high-frequency capacitance of 1 μF or 10 μF, Which is required even for battery-powered systems.

2 chip configuration decoupling Capacitor

Configure a 0.01 μF ceramic capacitor for each IC chip. A typical decoupling capacitor of 0.1/μF in a digital circuit has a 5 NH distributed inductance, and its parallel resonance frequency is around 7 MHz, that is to say, there is a good decoupling effect for noise below 10 MHz, and there is almost no effect on noise above 40 MHz. If the printed circuit board cannot be installed in a small space, it can be 4 ~ Configure 1 μF ~ for 10 chips ~ 10μf ta electrolytic capacitor, the High-Frequency Impedance of this device is particularly small, in kHz ~ The impedance within 20 MHz is less than 1 μF ~ 10 μF and the leakage current is very small (less than 0.5 μA ). The decoupling capacitor value is not strictly selected. It can be calculated based on C = 1/F, that is, 0.1 μF for 10 MHz. For a system consisting of a microcontroller, take 0.1 μF ~ Between 0.01 μF.

3. Add storage and capacitor when necessary

Each 10-piece integrated circuit requires a charge-discharge capacitor, or storage capacitor. The capacitance size can be 10 μF. The large capacitor is usually used as the electrolytic capacitor, but when the filtering frequency is high, it is best not to use the electrolytic capacitor. The electrolytic capacitor is rolled up by two layers of film. The structure of this winding is expressed as an inductor at a high frequency, it is best to use the TA capacitor or polycarbonate capacitor.

3. Proper decoupling capacitor layout in PCB

(1) equivalent capacitance model

When using capacitors to suppress electromagnetic harassment and filtering, the most easily overlooked problem is the effect of capacitor leads on the filtering effect. The capacitance is inversely proportional to the frequency. Using this feature, the capacitor is connected in parallel between the signal line and the ground line to bypass high-frequency noise. However, in actual engineering, many people find that this method does not have the expected effect of noise filtering and is helpless in the face of stubborn electromagnetic noise. One reason for this is that the effect of capacitor leads on the bypass effect is ignored.

As shown in the Circuit Model 4-7 of the actual capacitor, it is a series network consisting of an equivalent inductance (ESL), a capacitor and an equivalent resistance (ESR.

The impedance of the ideal capacitor decreases with the increase of the frequency, and the impedance of the actual capacitor is the impedance of the network shown in Figure 6-7. When the frequency is low, the capacitance is displayed, that is to say, the impedance decreases with the increase of frequency, and resonance occurs at a certain point. The impedance of the capacitor at this point is equal to the equivalent series resistance (ESR. Due to the effect of ESL, the capacitance impedance increases with the increase of the frequency, which presents the Impedance Characteristics of the inductance. As the impedance of the capacitor increases above the resonant point, the bypass effect on high-frequency noise is weakened or even disappears. Therefore, when assigning the decoupling capacitor, you must pay attention to the influence of the distribution parameters of the capacitor on the filtering.

(2) functions of capacitor leads

The resonance frequency of the capacitor is jointly determined by ESL and C. The larger the capacitance value or Inductance Value, the lower the resonance frequency, that is, the lower the high-frequency filtering effect of the capacitor. Apart from the capacitor type, the length of the capacitor lead is a very important parameter. The longer the lead, the larger the inductance, the lower the resonance frequency of the capacitor. Therefore, in actual engineering, the correct installation method and incorrect installation method of the capacitor should be as short as possible, as shown in Figure 4-8.

According to the series resonance principle of LC circuit, the resonance point is not only related to the inductance, but also the capacitance value. The larger the capacitance, the lower the resonance point. Many people think that the larger the capacitor capacity, the better the filtering effect. This is a misunderstanding. The higher the capacitance, the better the bypass effect on low-frequency interference. However, because the capacitor is resonant at a lower frequency, the impedance increases with the increase of the frequency. Therefore, the bypass effect on high-frequency noise becomes worse, therefore, when filtering, select the appropriate capacitor. Table 4-2 shows the auto-resonance frequency of the capacitor with different capacities. The lead length of the capacitor is 1.6mm.

Although from the perspective of filtering out high-frequency noise, capacitor resonance is not always harmful. When the noise frequency to be filtered out is determined, you can adjust the capacity of the capacitor so that the resonance point falls on the harassment frequency.

(3) Effect of Temperature on Capacitance

The temperature also has a great impact on the characteristics of the capacitor. Because the medium parameters in the capacitor are affected by the temperature change, the capacitor value also changes with the temperature. Different media vary with the temperature, and the capacity of some capacitors decreases by more than 70% when the temperature increases. The commonly used filter capacitor is the porcelain medium capacitor. Porcelain capacitor has three types: Ultra-stable (such as cog or NPO), stable (such as X7R), and general (such as Y5V or z5u. The Temperature Characteristics of capacitors of different media are shown in 4-9.

From Figure 4-9, we can see that the capacity of the cog capacitor almost changes with the temperature, and the capacity of the X7R capacitor changes below the rated operating temperature range by 12%, the ysv capacitor capacity changes by more than 70% within the rated operating temperature range. These features must be noted, otherwise the performance of the filter may change at high or low temperatures, resulting in electromagnetic compatibility problems.

Although cog dielectric capacitor has little influence on temperature and stable characteristics, its dielectric constant is low, generally ranging from 10 ~ 100, so when the volume is small, the capacity is small. The dielectric constant of the XTR dielectric capacitor is much higher, which is 2000 ~ 4000, so a small volume can generate a large capacitor. The maximum dielectric constant of the ysv dielectric capacitor is 5000 ~ 25000. It is usually used where a small volume is required and a large volume is required.

Many people choose capacitor, one-sided pursuit of the small size of the capacitor, although the capacitor medium has a high medium constant, but the temperature stability is very poor, this will lead to equipment Temperature Characteristics deteriorated. This requires special attention when selecting capacitors, especially in military equipment.

(4) effect of Voltage on capacitance in PCB

The electric capacity of the capacitor not only changes with the temperature, but also changes with the working voltage, which must be noted in the actual project. The voltage characteristics of capacitors of different media materials are shown in 6-10. It can be seen that the capacity of the X7R capacitor is reduced to 70% of the original value while that of the ysv capacitor is reduced to 30% of the original value, when selecting a capacitor, you must set aside margin on the voltage or capacitance. Otherwise, the filter will fail to achieve the expected effect under the rated operating voltage.

Considering the effect of temperature and voltage, the capacitance changes 4-11.

Therefore, when placing the filter capacitor, you must fully consider the filtering effect of the capacitor, instead of the more the better, the larger the better. Eliminate the misunderstanding of blind use of capacitors and the viewpoint of not using them.

Iv. Proper configuration of PCB decoupling capacitors

(1) Reasonably arrange the power filter/decoupling capacitor: Generally, only a few power filter/decoupling capacitors are drawn in the schematic diagram, but they are not pointed out where they should be connected. In fact, these capacitors are set for switching devices (door circuits) or other components that require filtering/decoupling. The layout of these capacitors should be as close as possible to these components, it does not work if you are too far away. When the power supply filtering/decoupling capacitor is properly arranged, the problem of the grounding point is not so obvious.

(2) storage devices such as ROM and Ram, which have weak noise and great changes in current upon shutdown, should be in the chip's power cord (VCC) and ground line (Gnd) directly connected to the decoupling capacitor.

(3) the lead of the decoupling capacitor cannot be too long. The shorter the lead, the better the decoupling effect. In particular, high-frequency bypass capacitors cannot contain leads.

(4) The decoupling is not the better, but the filtering effect. The number and size of capacitors are selected based on the time of the circuit board and the device.

(5) when the requirement is high, the decoupling capacitor does not need the porcelain chip capacitor or electrolytic capacitor, so that their capacity value is accurate and the distribution inductance is large. Use more accurate ta capacitor or polyester capacitor.

(6) When there are many chips and many decoupling capacitors, you can install a charge-discharge capacitor to accumulate the accumulation of charge-discharging circuit switching.