The capacitor is composed of two conductive parallel plates, a passive device that stores energy in the form of an electric field, and releases the stored energy out of the circuit when needed. Capacitors are divided into bypass capacitors, decoupling capacitors, filter capacitors, etc., which are often used in power circuits.
When the function of the capacitor in the circuit is to provide low impedance to the AC signal, it is called a bypass capacitor. When the function of the capacitor in the circuit is to increase the AC coupling between the power supply and the ground and reduce the influence of the AC signal on the power supply, it is called a decoupling capacitor. When a capacitor is used in a filter circuit, it is called a filter capacitor. For direct current, the capacitor can also be used as a circuit to store energy, using charge and discharge to function as a battery.
The bypass capacitor is used when there is a resistance connection. It is connected to both ends of the resistance to make the AC signal pass smoothly. It uses the high-frequency noise in the input signal as the filtering object to filter out the high-frequency clutter carried by the previous stage.
The decoupling capacitor is used in the place where AC is not needed in the amplifying circuit to eliminate self-excitation and make the circuit work stably. A decoupling capacitor is connected between the power supply and the ground, and its main function is the energy storage capacitor of the integrated circuit, which filters out the high-frequency noise generated by the device, cuts off its propagation path through the power supply loop, and prevents the noise carried by the power supply from composing the circuit. interference.
The filter capacitor is used in the power rectifier circuit to filter out the AC component and make the DC output smoother. Large capacitors are made by multi-layer winding, so their capacity and volume are large, and the inductance has a large impedance to high-frequency signals, so their high-frequency performance is not good. However, small capacitors have large impedance to low-frequency signals because of their small capacity and volume. For experienced people, three types of capacitors, large, medium and small, will be used for different filtering. Large capacitors in parallel are used to filter waves with slow frequencies, and small capacitors in parallel are used to filter waves with fast frequencies.
The capacitor is connected between the power supply and the ground for the purpose of energy storage and filtering. If there is no capacitor connected in the application, when the power consumption increases, it will cause the power supply voltage to drop, start to produce noise, ringing and other phenomena. If there is a capacitor, it will release the stored electrical energy to stabilize the circuit. The current in the circuit is often pulsating, such as synchronizing the frequency in a digital circuit, which will cause the power supply to pulsate and generate noise. In order to improve its stability, if there is a capacitor, these noises can be bypassed to the ground.
The choice of input and output capacitors is mainly based on the three aspects of the capacitor's rated withstand voltage value, capacity value, and service life. Choose from the rated voltage value of the capacitor, such as the selection of the capacitor voltage value in AC-DC rectification and filtering. The theoretical DC voltage value after rectification is calculated by the formula. Generally, the voltage resistance value is 1.1-1.3 times the theoretical calculation. The voltage value of the capacitor. In DC-DC applications, the withstand voltage of the external input capacitor is generally 1.3-1.4 times the withstand voltage.
From the selection of the capacitance of the input capacitor, according to the selection of the filter capacitor C, according to the formula, the larger the filter capacitor C, the better. In theory, increasing the capacity of the filter capacitor C in the circuit can make the output voltage waveform smoother and less undulating, but at the moment the circuit is turned on, the inrush current factor generated in the circuit cannot be ignored.
The choice of external output capacitor generally considers the output ripple noise and capacitive load of the power supply. Many people think that the larger the external output filter capacitor, the better the effect of reducing the interference of ripple noise and the more stable the system power supply. In fact, it is wrong. The choice of output filter capacitor is not only considering the ripple noise factor, but also the starting ability of the power supply and the ability to withstand the input inrush current.
