Working principle of capacitor filter

All components in electrical engineering can be classified into three basic types: resistance, inductance and capacitance. The resistance value of resistance is independent of the frequency of alternating current. The resistance value of inductance is called inductance XL2 π FL, which is in direct proportion to the frequency of alternating current. The higher the frequency is, the greater the inductance is. The capacitance component is opposite to the inductance component, and its capacitance xc1 / 2 π FC is in inverse proportion to the frequency of alternating current Therefore, in electrical engineering, LC element is often used to separate the reactance of different frequency AC power According to different function filters are generally divided into three categories: low-pass high pass band-pass. They are widely used in electrical circuits and electronic circuits. The simplest and most typical example is that the capacitance connected behind the rectifier circuit in our common DC stabilized voltage power supply is set to reduce the AC pulse. It is a low-pass filter. The output of the rectifier circuit they learned above Only in some special occasions, the general DC circuit needs an ideal straight-line like DC voltage. This is to smooth the pulsating voltage to achieve this measure, which is filtering. The filter is generally composed of inductance, capacitance, resistance and other components Capacitance filtering is simply said that the filtering is realized by using the small approximate short circuit of the capacitance's equivalent capacitive reactance to a specific frequency, which is independent of resonance.

The capacitive reactance xc1 / ω C1 / 2 π FC is 0.1uF ceramic capacitor for filtering high frequency - its equivalent capacitive reactance for 1MHz signal is only 1.6 ohm, and for 50Hz signal is approximately 32 kiloohm, so the equivalent capacitive reactance for filtering high frequency can only be filtered, and the equivalent capacitive reactance for filtering power frequency 2000uf capacitor can be equivalent to the equivalent capacitive reactance for 0.1uF for 1MHz signal. In the positive half cycle D conduction time, two currents are: one is the current IL to supply power to the load; the other is the IC to charge the capacitor. If the voltage drop of D is ignored, the voltage on the capacitor is equal to U2. When U2 reaches the maximum peak value, it begins to decline At this time, the voltage Uc of capacitor C will also gradually decrease due to discharge. When u2uc is used, the diode will turn on again. However, the output voltage of half wave rectifier filter still has the sawtooth component. Now, the principle of bridge rectifier filter circuit is the same as above

According to the above analysis, it can be seen that the capacitor filter has the following characteristics:

1. The fluctuating component of the load voltage is much reduced

2. The average value of load voltage is increased. When RL is fixed, the larger the filter capacitance is, the larger the UL is.

The design can estimate ul1-1.1u2 half wave ul1.2u2 full wave bridge rectifier filter capacitance determination:

When rlc3-5t half wave rectifier is rlc3-5t / 2 full wave bridge rectifier, t is the cycle of alternating current. The frequency of 220 alternating current is 50Hz, i.e. T1 / 50. It is very important for you to choose filter capacitor correctly in switching power supply. How to choose filter capacitor correctly, especially the selection of output filter capacitor, is a very concerned problem for every engineering and technical personnel. The pulse voltage frequency of the common electrolytic capacitor used in 50Hz power frequency circuit is only 100Hz, and the charging and discharging time is millisecond.

In order to obtain a smaller pulsation coefficient, the capacitance required is as high as hundreds of thousands of μ F. therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is to improve the capacitance, loss tangent and leakage current of the main capacitor, which are the main parameters to identify its advantages and disadvantages. While the sawtooth wave voltage frequency of the output filter electrolytic capacitor in the switching power supply is as high as tens of kHz or even tens of MHz. At this time, the capacitance is not the main index to measure the quality of the high-frequency aluminum electrolytic capacitor. The "impedance frequency" characteristic requires a lower equivalent impedance within the operating frequency of the switching power supply, and it also has a high-frequency peak signal generated when the semiconductor device works Good filtering effect. The common low-frequency electrolytic capacitors begin to show that the inductance can not meet the requirements of switching power supply at about 10kHz. The high-frequency aluminum electrolytic capacitor for switching power supply has four terminals, the two ends of which are respectively led out as the positive and negative electrodes of the capacitor.

The current flows from one positive end of the four terminal capacitor to the inside of the capacitor and then from the other positive end to the load. The current returned from the load also flows from one negative end of the capacitor to the negative end of the power supply. Because the four terminal capacitor has good high frequency characteristics, it provides a very favorable means to reduce the voltage ripple and suppress the switching peak noise. The high-frequency aluminum electrolytic capacitor also has the form of multi-core, that is, the aluminum foil is divided into short sections, which are connected in parallel with multi lead out pieces to reduce the impedance component in capacitance and reactance. And the low resistivity material is used as the lead-out terminal to improve the capacity of the capacitor to withstand large current. Inductor filter circuit inductor directionality no direction basic function filter, oscillation, delay, notch and other image terms "through DC resistance AC" It can form high pass or low pass filter with resistor or capacitor, phase-shifting circuit, resonance circuit and other transformers to carry out AC coupling, voltage transformation, current transformation and impedance transformation. According to the inductive reactance XL2 π FL, the larger the inductance L is, the higher the frequency f is, the larger the inductive reactance is. The voltage at both ends of the inductor is directly proportional to the inductance L and the current change speed △ I / △ T, which can also be expressed by the following formula: uldi / dt as long as the inductance L is large enough, even if the rectifier output voltage is low to 0, there is still a positive current in the inductor and the load maintains a certain positive voltage. The inductive coil is also an energy storage element. It stores electric energy in the form of magnetism. The size of electric energy stored can be expressed as WL1 / 2 in the following formula