First, the function of the filter capacitor in EMC
Capacitors are the most widely used device in THE EMC design of PCBs. According to the different functions, capacitors can be divided into three types:
Decouple: Breaks the coupling between ports in a system or circuit to ensure proper operation.
Bypass: Provides a low-impedance path to ground where transient energy is generated. It is one of the necessary conditions for good decoupling.
Bulk: The energy storage capacitor can guarantee that the voltage will not fall when the load quickly changes to the heaviest.
Second, the problem of capacitance self-resonance
The capacitor we use to filter is not an ideal capacitor, and actually behaves in the system as an ideal capacitor in series with inductor and resistance, as shown in the following figure:
The multilayer capacitor (Muti-LayerCapacitor) produces a parasitic inductance of nearly 5nH when assembled on the PCB board, coupled with a lead resistance of about 30mω, and its frequency characteristics are shown as shown in the curve. The filter capacitor will not be ideal for a low-pass filter, and the actual insertion loss characteristics will be a band-pass filter circuit centered on the self-resonant point.
When two capacitors are connected in series, a desonant resonance problem occurs due to the presence of ESL (Equivalent Series Inductor) and ESR (Equivalent Series Resistor). The figure below shows the equivalent principle of capacitor paralleling:
The graph below shows their true amplitude-frequency characteristics:
In a wide frequency band of nearly 15MHz to 175MHz, the impedance of the parallel capacitor is larger than the impedance of a single large capacitor, and because the two capacitors produce resonances, an impedance peak is generated at 150MHz, and only a small part of the energy generated by the rest of the system in this frequency range can be bypassed to the ground plane.
Third, the effect of ESR on the amplitude and frequency characteristics of parallel capacitors
The peak of the impedance is inversely proportional to the value of the capacitor's ESR, as the single board design level and device performance improve, the peak of the impedance of the shunt capacitor will increase with the decrease of the ESR, and the shape and position of the shunt resonance peak depends on the design of the PCB board and the choice of capacitor. There are a few principles you should know:
As the ESR decreases, the impedance of the resonant point decreases, but the impedance of the anti-resonant point increases;
When n identical capacitors are used in parallel, the minimum cationic port can be small dry ESRIn;
When multiple capacitors are connected in parallel, the impedance does not necessarily occur at the resonant point of the capacitor;
For a given number of capacitors, the better option is that the capacitance values are evenly spread over a larger range, and the ESR of each capacitance value is moderate: the worse choice is that there is only a small amount of capacitance values, and the ESR of the capacitors is very small.
Fourth, the influence of ESL on the amplitude and frequency characteristics of parallel capacitors
The ESL is different for different capacitor packages and structures, and the ESL for 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 resonant point of the capacitor and the reverse resonant point of the parallel capacitor. In the actual design, capacitors with small ESL should be used as much as possible.
Fifth, the choice of capacitors
Ceramic capacitors, polyester fiber capacitors, and polystyrene film capacitors are all good choices for RF designs.
For EMI filters, the dielectric material requirements for capacitors are not high, and common loose dielectrics such as X7R, Y5V and Z5U are good choices: usually the absolute capacitance value, the temperature coefficient of the capacitor, the voltage change coefficient, etc. are not important. Different types of capacitor filter ranges with different capacitance values are different, and the following is a typical insertion loss comparison effect:
As can be seen from the figure above, the ceramic capacitor of the same 0805 package, the capacitor of 001UF has better high-frequency filtering than the capacitor of 0.1UF; it is recommended that the single board with a working frequency of more than 50MHZ (such as most of the single boards of transmission and MUSA) uses a filter capacitor of 0.01UF, instead of the 0.1UF filter capacitor that we currently use a lot.
Sixth, the design recommendations for decoupling capacitors and bypass capacitors
1. Select the capacitor based on the self-resonant characteristics on the product information provided by the supplier to meet the needs of the designed clock rate and noise frequency.
2. Add as much capacitance as possible in the required frequency range. For example, a 22nF capacitor has a self-resonant frequency of nearly 11MHz and a useful impedance (Z1 ohm) range of 6M to 40MHz, and you can add as much capacitance as possible in this band to reach the level where decoupling is required.
3. Place at least one decoupling capacitor as close as possible to each power pin of the IC to reduce the parasitic impedance.
4. The bypass capacitor and the IC are placed on the same PCB plane as much as possible. There is one thing to note: in both layouts, the Vcc network has only one point connected to the Vcc plane. This allows noise inside and outside the IC to go through this unique via to the power plane, and the additional impedance of the vias helps avoid noise diffusion 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 capacitance value is used for each part, and the power plane separated by a slit separates the noise of one part from the sensitive devices of other parts, and at the same time provides the separation of the middle capacity value.
6. For systems where the clock frequency changes over a wide range, the selection of bypass capacitors is very difficult. A better solution is to place two capacitors with a capacitance value close to 2:1 in parallel.
This provides a wide low impedance region and a wider bypass frequency, as you can see in the figure below, the impedance peak is still generated, but less than 15 ohms, while the available frequency range (impedance less than 15 ohms) is extended to a range of nearly 3.25MHz to 100MHz, this method of multi-decoupling capacitors is only used when a single IC requires a wide bypass frequency range and a single capacitor cannot reach this band. Also, the capacitance value must be kept in the 2:1 range to avoid peak impedance exceeding the available range.
Seven, the design of energy storage capacitors
The energy storage capacitor guarantees that the supply voltage does not fall when the load quickly changes to its heaviest. Energy storage capacitors can be divided into two types: board-pole energy storage capacitors and device-level energy storage capacitors:
First, the board pole energy storage capacitor: to ensure that the load quickly changes to the heaviest, the supply voltage around the single board will not fall. In high-frequency, high-speed veneers (and backplanes where conditions permit), it is recommended to evenly arrange a certain number of larger capacitive tantalum capacitors (luf, 10uf, 22uf, 33uf) to ensure that the value of the same voltage of the single board is consistent.
Second, the device-level energy storage capacitor: to ensure that the load quickly changes to the heaviest, the supply voltage around the device will not fall. For devices with high operating frequency, high rate, and large power consumption, it is recommended to discharge 1-4 molybdenum capacitors (luf, 10uf, 22uf, 33uf) around it to ensure that the operating voltage remains unchanged during fast device changes.
The design of the energy storage capacitor should be distinguished from the design of the decoupling capacitor, and the following design recommendations are available:
1, when the single board has a variety of supply voltages, for a kind of power supply voltage energy storage capacitor is still only selected a capacitor of capacitance, generally choose the surface mount package of Totalum capacitor (tantalum capacitor), you can choose 10uf, 22uf, 33uf and so on as needed.
2. The chips with different supply voltages form a community, and the energy storage capacitors are evenly distributed in this community, as shown in the following figure: