The function of filter capacitors in EMC
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The function of filter capacitors in EMC

Posted Date: 2024-01-15

The function of filter capacitors in EMC

Capacitors play an important role in electromagnetic compatibility (EMC). They can be used to control and manage electromagnetic interference (EMI) and improve the anti-interference ability of electronic equipment. Here are some of the main applications of capacitors in EMC:

Filters: Capacitors are often used as key components of filters. In electronic equipment, by placing capacitors on signal lines or power lines, high-frequency noise and electromagnetic interference can be effectively filtered, ensuring that the power supply and signal lines of the equipment are not interfered by external electromagnetic waves.

Power decoupling: In electronic circuits, capacitors are used as power decouplers to ensure that electronic components receive stable power when working. This helps prevent noise on the power lines from spreading into critical electronic components.

Suppress radio frequency interference: Radio frequency (RF) interference is a type of high-frequency interference that often affects wireless communication equipment and other high-frequency electronic equipment. Capacitors can be used to absorb and suppress these RF signals, preventing them from entering or leaving the device.

Anti-static discharge: In some environments, electrostatic discharge can be harmful to equipment. Capacitors can be used to absorb and release electrostatic energy, thereby reducing the impact of static electricity on equipment.

Differential mode noise filtering: In analog circuits, capacitors are often used to filter differential mode signals to help reduce the impact of noise on the signal.

Common-mode suppression: Capacitors are also used in common-mode suppression circuits to prevent common-mode signals (i.e., interference signals that act on two circuit wires at the same time) from affecting the equipment.

In EMC design, the selection and layout of capacitors are very critical. Proper capacitor selection can significantly improve the electromagnetic compatibility of the device, prevent mutual interference between different parts, and ensure stable operation of the device in the electromagnetic environment.

Capacitor self-resonance problem

The capacitor we use for filtering is not an ideal capacitor. In the system, it actually appears as a series connection of an ideal capacitor, an inductor and a resistor. as the picture shows.

When the multi-layer capacitor (Muti-Layer Capacitor) is assembled on the PCB board, it will produce a parasitic inductance of nearly 5nH, plus a lead resistance of about 30m ohms, and its frequency characteristics are shown as the curve shown in the figure. The filter capacitor will not be an ideal low-pass filter. The actual insertion loss characteristics appear as a band-pass filter circuit centered on the self-resonance point.

When two capacitors are connected in series, anti-resonance problems will occur due to the existence of ESL (equivalent series inductance) and ESR (equivalent series resistance).The figure below shows the equivalent principle of parallel connection of capacitors.

The figure below gives their true amplitude-frequency characteristics.

In a wide frequency band from nearly 15MHz to 175MHz, the impedance of the parallel capacitor is larger than the impedance of a single large capacitor. Since the two capacitors resonate, an impedance peak is generated at 150MHz, and the other parts of the system are Only a small portion of the energy generated in this frequency range can be bypassed to the ground plane.

When designing ordinary circuits, engineers usually focus on parameters such as capacitance value, withstand voltage value, package size, operating temperature range, and temperature drift. However, in high-speed circuits or power systems and some clock circuits that have high requirements on capacitance, capacitance is no longer just a capacitance. It is a circuit composed of equivalent capacitance, equivalent resistance and equivalent inductance. A simple structure as the picture shows.

Equivalent circuit of capacitor in high-speed circuit

In the figure, C is the required capacitance, ESR is the equivalent series resistance, ESL is the equivalent series inductance, and CP is the equivalent parallel capacitance.

Since this is a circuit, it is no longer as simple as an independent capacitor. The performance of this equivalent circuit is affected by many factors. When selecting this type of capacitor, you should not only pay attention to the parameters mentioned above, but also the equivalent parameters at a specific frequency. Take Murata's 1μF capacitor as an example. At the resonant frequency point, the corresponding equivalent capacitance is 602.625nF, the equivalent resistance is 11.5356mΩ, and the equivalent inductance is 471.621pH. Ideal capacitors and actual capacitors exhibit different properties. As shown in the figure is the impedance curve of the ideal capacitor and the actual capacitor.

Impedance curves of ideal capacitors and real capacitors

In engineering practice, many engineers see many capacitors in reference board designs or boards designed by other engineers, and feel that if their products follow their designs, they may not cause problems. In fact, this is not the case, because the product applications may be different and the structures may be different, which may result in different PCB laminations and different flow planes in the product design, and these will cause inconsistencies in the power supply system.

In power supply system design, there are usually many types of capacitors. For example, there are 100μF, 47μF, 22μF, 10μF, 1μF, 0.1μF and other types of capacitors in a power supply system. Can so many types of capacitors be unified into one? What type of capacitor? As shown in the figure, the impedance curve of the capacitor is taken as an example for explanation.

Impedance curve of the circuit with the same capacitance value added

Add circuit impedance curves with different capacitance values

From the comparison of the two figures above, we can see that if the same type of capacitors are used, although the impedance is lower, the decoupling frequency range will hardly change; if different types of capacitors are used, the decoupling frequency range can be increased.

In a power supply system, more capacitors are not always better. In some systems, if there are too many capacitors, new noise points will appear.

Effect of ESR on amplitude-frequency characteristics of parallel capacitors

The peak value of impedance is inversely proportional to the ESR value of the capacitor. With the improvement of single board design level and device performance

The peak value of the parallel capacitor impedance will increase as the ESR decreases. The shape and position of the parallel resonance peak depend on the PCB board design and capacitor selection.

There are several principles you should understand:

1. As ESR decreases, the impedance of the resonance point will decrease, but the impedance of the anti-resonance point will increase:

2. When n identical capacitors are used in parallel, the minimum anode resistance is ESRIn:

3. When multiple capacitors are connected in parallel, the impedance does not necessarily occur at the resonance point of the capacitor;

4. For a given number of capacitors, a better choice is that the capacitance values ​​are evenly spread over a large range, and the ESR of each capacitance value is moderate; a worse choice is that there are only a small number of capacitance values, and the ESR of the capacitances are all very small.

Effect of ESL on amplitude-frequency characteristics of parallel capacitors

The capacitor package and structure are different, and the ESL is also different. The ESL of several typical package capacitors is shown in the table.

The ESL of the capacitor, together with the capacitance value, determines the frequency range of the capacitor's resonance point and the anti-resonance point of the parallel capacitor. In actual design, capacitors with small ESL should be used as much as possible.

Capacitor selection

For RF designs, ceramic capacitors, polyester fiber capacitors, and polystyrene film capacitors are all good choices. For EMI filters, the requirements for the dielectric material of the capacitor are not high. Common loose media such as X7R, Y5V and Z5U are good choices: usually the absolute capacitance value, temperature coefficient of the capacitor, voltage change coefficient, etc. are not important. Different types of capacitors with different capacitance values ​​have different filtering ranges. The following is a typical insertion loss comparison effect:

As can be seen from the above figure, the 001uF capacitor has better high frequency performance than the 0.1uF capacitor, both of which are chip ceramic capacitors in the 0805 package.

Filter characteristics: It is recommended that single boards with a board operating frequency higher than 50MHz (such as most transmission and MUSA boards) use 0.01uF filter capacitors instead of the 0.1uF filter capacitors we currently use in large quantities.

power supply output capacitor, input capacitor

We usually refer to the capacitance of the input and output circuits of the power module as filter capacitance. A simple understanding is that it is a capacitor that ensures the stability of input and output power supplies. In the power module, the principle of placing filter capacitors is "large first and then small". As shown in Figure 2.48.1, the filter capacitors are placed first larger and then smaller in the direction of the arrow.

When designing the power supply, pay attention to ensuring that the traces and copper sheets are wide enough and the number of vias is sufficient to ensure that the current flow capacity meets the requirements. Width and number of vias are evaluated in conjunction with current flow.

Power input capacitor

The power input capacitance and the switching loop form a current loop. The change amplitude of this current loop is large, the amplitude of Iout. Frequency is the switching frequency. Produced during the switching process of the DCDC chip, the current changes generated by this current loop include faster di/dt.

In the synchronous BUCK method, the freewheeling path must pass through the GND pin of the chip, and the input capacitor must be connected between the GND and Vin of the chip. The path should be as short and thick as possible.

If the area of ​​this current loop is small enough, the external radiation of this current loop will be better.

Decoupling capacitors and bypass capacitors

1. Select the capacitor based on the self-resonance characteristics in the product information provided by the supplier to meet the needs of the designed clock rate and noise frequency.

2. Add as many capacitors as possible within the required frequency range. For example, the self-resonant frequency of a 22nF capacitor is approximately 11MHz, and the useful impedance (Z1 ohm) range is 6M~40MHz. You can add as many capacitors as possible within this frequency band to achieve the required decoupling level.

3. Place at least one decoupling capacitor as close as possible to each power pin of the IC to reduce parasitic impedance.

4. Place the bypass capacitor and IC on the same PCB plane as much as possible. One important thing to note: in both layouts, the Vcc net has only one point connected to the Vcc plane. In doing so, the noise inside and outside the IC must pass through this single via to the power plane. The additional impedance of the via helps prevent the noise from spreading to the rest of the system.

5. For multi-clock systems, the power plane can be divided as shown in Figure 3-14, and a capacitor with the correct value is used for each part. The power plane separated by the slit separates the noise in one part from the sensitive devices in other parts. separated while providing separation of mid-capacity values;

6. For systems where the clock frequency changes within a wide range, the selection of bypass capacitors is very difficult. A better solution is to place two capacitors with a capacitance close to 2:1 in parallel. This can provide a wider low impedance area and a wider bypass frequency, as you can see in the picture below. , the impedance peak is still generated, but it is less than 15 ohms, and the available frequency range (impedance less than 15 ohms) extends to the range of nearly 3.25MHz to 100MHz. This method of multiple decoupling capacitors is only needed in a single IC Use only when a wide bypass frequency range cannot be reached by a single capacitor. Furthermore, the capacitance values ​​must remain within the 2:1 range to avoid impedance peaks beyond the usable range.

The power pins of high-speed ICs require enough decoupling capacitors, preferably one for each pin. In actual design, if there is no space to place decoupling capacitors, they can be deleted as appropriate.

The capacitance of the decoupling capacitor of the IC power pin is usually relatively small, such as 0.1μF, 0.01μF, etc.; the corresponding package is also relatively small, such as 0402 package, 0603 package, etc. When placing decoupling capacitors, you should pay attention to the following points.

(1) Place it as close to the power pin as possible, otherwise the decoupling effect may not be achieved. Theoretically, the capacitor has a certain decoupling radius, so the proximity principle should be strictly implemented.

(2) The leads from the decoupling capacitor to the power supply pin should be as short as possible, and the leads should be thickened. Usually the line width is 8~15mil (1mil = 0.0254mm). The purpose of bolding is to reduce the lead inductance and ensure power supply performance.

(3) After the power and ground pins of the decoupling capacitor are led out from the pad, drill holes nearby and connect them to the power and ground planes. The lead should also be thickened, and the via hole should be as large as possible. If a hole with a diameter of 10mil can be used, an 8mil hole should not be used.

(4) Ensure that the decoupling loop is as small as possible. Common placement examples of decoupling capacitors are shown in Figure 2.48.2 to Figure 2.48.4. Figures 2.48.2 to 2.48.4 show the placement of IC decoupling capacitors in SOP packages. QFP and other packages are similar to this.

In common BGA packages, the decoupling capacitor is usually placed under the BGA, that is, on the back. Due to the high pin density of the BGA package, there are generally not many decoupling capacitors, but as many as possible should be placed, as shown in Figure 2.48.5.

Design of energy storage capacitor

The energy storage capacitor can ensure that the supply voltage does not drop when the load quickly reaches its heaviest. Energy storage capacitors can be divided into plate energy storage capacitors and device level energy storage capacitors:

A. Plate energy storage capacitor: ensures that when the load quickly reaches the heaviest level, the power supply voltage across the board will not drop. On high-frequency and high-speed single boards (and backplanes where conditions permit), it is recommended to evenly arrange a certain number of tantalum capacitors (luf, 10uf, 22uf, 33uf) with larger capacitances to ensure that the values ​​of the same voltage on the single board remain consistent. .

B. Device-level energy storage capacitor: ensure that when the load quickly reaches its heaviest, the power supply voltage around the device will not drop. For devices with high operating frequency, high speed and large power consumption, it is recommended to place 1-4 molybdenum capacitors with larger capacitance (luf, 10uf, 22uf, 33uf) around them to ensure the working voltage of the device when it changes rapidly. constant.

The design of energy storage capacitors should be distinguished from the design of decoupling capacitors. There are following design suggestions:

1. When there are multiple power supply voltages on a single board, only one capacitance capacitor is selected for one power supply voltage energy storage capacitor. Generally, Tantalum capacitors (tantalum capacitors) in surface mount packages are used. You can choose 10uf, 22uf, 33uf, etc.;

2. Chips with different power supply voltages form a community, and energy storage capacitors are evenly distributed within this community, as shown in the figure below:

The function of the energy storage capacitor is to ensure that the IC can provide electrical energy in the shortest time when using electricity. The capacitance of energy storage capacitors is generally relatively large, and the corresponding packaging is also relatively large. In the PCB, the energy storage capacitor can be farther away from the device, but not too far, as shown in Figure 2.48.6. The common energy storage capacitor fan hole method is shown in Figure 2.48.7.

The principles of capacitor fan holes and fan lines are as follows.

(1) The leads should be as short and thick as possible to reduce parasitic inductance.

(2) For energy storage capacitors or devices with relatively large overcurrent, you should drill as many holes as possible when drilling.

(3) Of course, the fan hole with the best electrical performance is the hole in the plate.Actual needs to be considered comprehensively

The use of capacitors in filter circuits

EMC filter usually refers to a low-pass filter composed of L and C. The difference between LC filters with different structures lies in the connection methods of capacitors and inductors. The effectiveness of an LC filter is not only related to its structure, but also to the impedance of the connecting network. For example, a single capacitor filter works well in high impedance circuits but poorly in low impedance circuits. Traditionally, the characteristics of the filter are described under the condition that the termination impedance at both ends of the filter is 50 ohms, but in practice the source impedance Zs and load impedance Zi are very complex, and it may be unknown at the frequency point to be suppressed . If one or both ends of the filter are connected to a reactive component, resonance may occur, causing the insertion loss at certain frequency points to become an insertion gain.

As shown in the figure, in a signal path, L and C form a low-pass filter circuit. Since the source impedance Zs and load impedance Zi at a certain frequency point are unknown, when using it, we must avoid combining parameters to make useful Frequency components are filtered out. In many cases, engineers often prefer to use capacitors with a capacitance of 102,104 without calculation, which may sometimes be counterproductive.

Generally, the resonance of a capacitor does not exist alone. Generally, the self-resonance of a capacitor is composed of the equivalent inductance of the capacitor and its own pin or the inductance formed by the wire connecting the capacitor. In actual work, we can know from the calculation formula:

F=1/(2*π*√LC)

When the LC in the series structure resonates, the impedance at both ends is the smallest, which is equivalent to a short circuit; when the LC in the parallel structure resonates, the impedance at both ends is the largest, which is equivalent to an open circuit. As shown in Figure 1, when L and C resonate, from the signal flow analysis (shown by the red arrow), it is a series resonance, which is equivalent to a short circuit in terms of the characteristics of the series resonance circuit. If the resonant frequency of LC happens to be the interference frequency we want to filter out, then the path formed by L and C is equivalent to a short circuit, which can effectively filter out noise.

For example, in this signal path, the useful frequency is 5MHz, and the L value in the circuit is 1uH. If we want to filter out the 10MHz interference signal on the signal path, we must avoid the resonance points of the added filter capacitors C and L falling on Around 5MHz, thereby filtering out useful signals. If we select a 1000pF capacitor based on empirical values ​​and calculate the resonance point through the above resonance formula, the resonance point is calculated to be 5.03MHz. At this time, the LC is equivalent to a short circuit, and the useful frequency passes through the LC directly to the ground. This does not achieve the effect we need, but makes the circuit work. unusual. We should select the appropriate capacitance value based on the interference frequency that needs to be filtered. Substitute the resonant frequency formula into the calculation. The value of C is 253.3pF. We can just take the closest value. It should also be noted that if pin components are used, the pins should be as short as possible. If possible, it is best to use SMD components with the smallest ESL. It can be seen that the correct selection of filter structure and component parameters is crucial. In actual circuit applications, empirical values ​​are important, but in some cases, empirical values ​​are not worth promoting. Especially when dealing with harmonic components of useful frequencies, the correct method must be used to estimate the value before taking the value. .

As shown in the figure, to filter out noise interference signals on the wire harness, low-cost capacitors are preferred. Sometimes, some interference noise will be directed to other paths, resulting in an antenna effect and enhanced radiation. When choosing a capacitor, you must clearly understand that the capacitor itself only transfers energy, and the energy is not consumed. Only when the capacitor is connected to a low-impedance network can the filtering effect be achieved. In practical work, the reverse transfer characteristics of capacitance are often ignored by engineers. Everyone mistakenly believes that the ground is always pure and that as long as it is grounded, the problem can be solved. Therefore, grounding has become a panacea for rectification engineers.As shown below

Assume that there is 10dBm electromagnetic noise on the signal line that needs to be filtered. Usually, the first thing you will think of is using a capacitor for filtering. At this time, the ground where the capacitor needs to be transferred must be paid attention to. Whether the ground is clean, whether it has low resistance, and whether it exists. Ground bomb effect, whether it will cause loop effect, etc. Assuming that the noise energy on the ground is even higher than the energy of the filter object, adding capacitance will reversely transfer the noise on the ground to the signal line, and the signal line becomes the most ideal radiation medium.

Common mode capacitor

"Common mode capacitance" usually refers to the common mode capacitance in differential signals, which is an important parameter in circuits, especially in differential amplifiers and communication systems.

In a differential signal, there are two signals: differential mode signal and common mode signal.

Differential Mode Signal: This is the difference part of the two input signals, that is, the algebraic difference of the two signals.

Common Mode Signal: This is the average or common part of the two input signals.

Common mode capacitance refers to the capacitance of the common mode part of the signal to ground. This capacitance can be detrimental to some circuits, especially in differential amplifiers. In an ideal world, a differential amplifier only amplifies differential-mode signals and does not respond to common-mode signals. However, there are always some imperfections in real circuits, and one effect is common-mode capacitance.

Common mode capacitors can cause problems such as:

Common Mode Noise: If there is common mode noise in the input signal, the common mode capacitor may cause the noise to be amplified, thus affecting the performance of the circuit.

Common Mode Rejection Ratio (CMRR): This is an important indicator to measure the differential amplifier's ability to suppress common-mode signals. The presence of common-mode capacitance may affect CMRR, reducing the differential amplifier's ability to suppress common-mode signals.

As shown in Figure 1, 3 is a differential mode capacitor, 2 is a common mode inductor, and 4 is a common mode capacitor.

Generally, filters do not use differential mode coils alone. Because the windings on both sides of the common mode inductor are inconsistent, the inductances must not be the same, so they can play a certain role as differential mode inductors. If the differential mode interference is serious, a differential mode coil must be added.

Differential mode capacitor

It can be seen that the capacitive characteristics are high impedance at low frequency and low impedance at high frequency. The filter uses the low impedance of the capacitor to short-circuit the differential mode interference at high frequencies. (As shown in the figure below: ) When the frequency is 50Hz, the capacitor impedance approaches infinity, which is equivalent to a short circuit and does not have any attenuation effect. When the frequency is 500kHz, the capacitor impedance is very small. According to the above formula, we can see the differential mode The current attenuation of the load approaches 0. For example, when the frequency is 500kHz, the capacitive reactance of the load of 50 ohms is 0.05 ohms.

At this time, the capacitor shares 99.9% of the differential mode interference current, while the load only shares 0.1% of the differential mode interference current. That is to say, at 500kHz, the capacitor reduces the differential mode interference by 30dB.

Review Editor: Huang Fei


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