Summary of filter capacitors, decoupling capacitors, and bypass capacitors
Time：2021.08.26
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The filter capacitor is used in the power rectifier circuit to filter out AC components. Make the output DC smoother.
The decoupling capacitor is used in the place where AC is not needed in the amplifying circuit to eliminate self-excitation and make the amplifier work stably.
The bypass capacitor is used when a resistor is connected, and it is connected to both ends of the resistor to allow the AC signal to pass smoothly.
1. Understanding of the energy storage function of decoupling capacitors
1) The decoupling capacitor is mainly to remove the interference of high frequency such as RF signal, and the way of interference is through electromagnetic radiation. In fact, the capacitor near the chip also has the function of storing energy, which is second. You can think of the total power source as the Miyun Reservoir. Every household in our building needs water supply. At this time, the water does not come directly from the reservoir. The distance is too far. When the water comes, we are already thirsty. The actual water comes from the water tower on the top of the building, which is actually a buffer. From a microscopic point of view, when the high frequency device is working, its current is discontinuous and the frequency is very high. However, there is a distance between the device VCC and the total power supply. Even if the distance is not long, the impedance Z =i*wL+R, the inductance of the line will also be very large, which will cause the device to be unable to be supplied in time when it needs current. The decoupling capacitor can make up for this deficiency. This is also one of the reasons why many circuit boards place small capacitors at the VCC pin of the high-frequency device (a decoupling capacitor is usually connected in parallel to the vcc pin, so that the AC component is grounded from this capacitor).
2) The high-frequency switching noise generated by the active device during switching will propagate 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 to guide the noise to the ground.
2. The difference between bypass capacitor and decoupling capacitor
Decoupling: Remove the RF energy that enters the power distribution network from high-frequency devices during device switching. The decoupling capacitor can also provide a localized DC voltage source for the device, which is particularly useful in reducing the surge current across the board.
Bypass: Transfer unwanted common-mode RF energy from components or cables. This is mainly to eliminate unintentional energy from entering the sensitive part by generating an AC bypass. In addition, it can also provide a baseband filtering function (limited bandwidth).
We can often see that a decoupling capacitor is connected between the power supply and the ground. It has three functions: one is to serve as the energy storage capacitor of the integrated circuit; the other is to filter out the high-frequency noise generated by the device and cut it off. The path for propagation through the power supply loop; the third is to prevent the noise carried by the power supply from interfering with the circuit.
In electronic circuits, decoupling capacitors and bypass capacitors both play an anti-interference role. The location of the capacitor is different, and the name is different. For the same circuit, the bypass capacitor takes the high-frequency noise in the input signal as the filtering object to filter out the high-frequency clutter carried by the previous stage, and the decoupling capacitor is also called decoupling. Capacitors take the interference of the output signal as the filtering object.
High-frequency bypass capacitors are generally relatively small. According to the resonance frequency, they are generally 0.1u, 0.01u, etc., while decoupling capacitors are generally larger, 10u or greater, depending on the distribution parameters in the circuit and the magnitude of the drive current change.
The typical decoupling capacitor value in digital circuits is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz. That is to say, it has a better decoupling effect for noise below 10MHz, and has little effect on noise above 40MHz. Capacitors of 1μF and 10μF, and the parallel resonance frequency is above 20MHz, the effect of removing high-frequency noise is better. Every 10 pieces of integrated circuits need to add a charge and discharge capacitor, or an energy storage capacitor, about 10μF can be selected. It is best not to use electrolytic capacitors. Electrolytic capacitors are rolled up with two layers of film. This rolled up structure behaves as an inductance at high frequencies. Use tantalum capacitors or polycarbonate capacitors. The selection of the decoupling capacitor is not strict, you can press C=\'1\'/F, that is, 0.1μF for 10MHz and 0.01μF for 100MHz.
Precautions for capacitor selection and use:
1. Generally, paper or polyester capacitors can be used in low-frequency coupling or bypass and low electrical characteristics requirements; in high-frequency and high-voltage circuits, mica capacitors or ceramic capacitors should be used; in power supply filtering and decoupling circuits, Electrolytic capacitors can be used.
2. In the oscillation circuit, delay circuit, and tone circuit, the capacitor capacity should be as consistent with the calculated value as possible. In various filters and networks (frequency selective networks), the capacitor capacity is required to be accurate; in decoupling circuits and low-frequency coupling circuits, the requirements for the same two-level accuracy are not too strict.
3. The rated voltage of the capacitor should be higher than the actual working voltage, and there must be enough room. Generally, a capacitor whose withstand voltage is more than twice the actual working voltage is selected.
4. It is preferred to choose capacitors with high insulation resistance and low loss, but also pay attention to the use environment.
