Design of filter circuit

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After the alternating current is rectified by the diode, the direction becomes single, but the magnitude (current intensity) is still constantly changing. This kind of pulsating direct current cannot be used directly to power radio equipment. To turn the pulsating direct current into a direct current with a smooth waveform, it is necessary to do some "filling and leveling" work, which is filtering. In other words, the task of filtering is to reduce the fluctuation component in the output voltage of the rectifier as much as possible and transform it into a nearly constant direct current.

Capacitor filtering

A capacitor is a warehouse for storing electric energy. In a circuit, when voltage is applied to both ends of a capacitor, the capacitor is charged and the electric energy is stored in the capacitor; when the external voltage is lost (or reduced), the capacitor will release the stored electric energy. When charging, the voltage across the capacitor gradually increases until it approaches the charging voltage; when discharging, the voltage across the capacitor gradually decreases until it disappears completely. The larger the capacity of the capacitor
, the larger the load resistance value, and the longer the time required for charging and discharging. This characteristic of the voltage across the capacitor cannot change suddenly, which can be used to undertake the task of filtering.

Figure 5-9 is the simplest capacitor filter circuit. The capacitor is connected in parallel with the load resistor and connected behind the rectifier. The following is the working process of the capacitor filter with the half-wave rectification shown in Figure 5-9 (a). During the conduction period of the diode, e2 provides current to the load resistor Rfz while charging the capacitor C to the maximum value. After reaching the maximum value, e2 gradually decreases; while the voltage across the capacitor cannot change suddenly and still maintains a high voltage. At this time, D is subjected to reverse voltage and cannot be turned on, so Uc discharges through the load resistor Rfz . Since C and Rfz are large , the discharge speed is very slow. During the period when e2 decreases, the voltage on the capacitor C does not drop much. When the next cycle of e2 comes and rises to a value greater than Uc , the capacitor is charged again. Repeating this process, the two ends of the capacitor C (i.e. , the two ends of the load resistor Rfz ) maintain a relatively stable voltage, showing a relatively smooth waveform on the waveform diagram. Figure 5-10 ( a ) ( b ) shows the output waveforms before and after capacitor filtering during half-wave rectification and full-wave rectification respectively.

Obviously, the larger the capacitance, the better the filtering effect, the smoother the output waveform, and the higher the output voltage. However, after the capacitance reaches a certain value, increasing the capacitance has no significant effect on improving the filtering effect. Usually, the optimal capacitance should be selected according to the load current and the output current. The relationship between the capacitance and output current of the filter capacitor listed in Table 5-2 can be used for reference. The withstand voltage value of the capacitor is generally 1.5 times.

Table 5-3 lists the relationship between the voltages in the rectifier circuit with filter.

Output Current	About 2A	About 1A	About 0.5-1A	0.1-0.5A	100-50mA	50mA or less
Filter capacitor	4000u	2000u	1000u	500u	200u-500u	200u

For rectifier circuits using capacitor filtering, the output voltage and current vary greatly at any time, which is very unfavorable for variable loads (such as Class B push-pull circuits).

2. Inductor Filtering

　　Taking advantage of the fact that inductors have large impedance to AC but small impedance to DC, a filter can be made using a coil with an iron core. The output voltage of electromagnetic filtering is low, the phase output voltage fluctuation is small, and the change with load is also small, which is suitable for occasions with large load current.

3. Compound filter.

　　By placing the capacitor in the load parallel branch and the inductor or resistor in the series branch, a duplex filter can be formed to achieve better filtering effect. The shape of this circuit resembles the letter π, so it is also called a π-type filter.

　　Figure 5-12 shows an LC filter composed of an inductor and a capacitor . It has high filtering efficiency and almost no DC voltage loss. It is suitable for applications with large load currents and small ripples. However, this filter is relatively bulky and expensive due to its large inductor size and weight (which can be reduced at high frequencies), so it is generally not used much.

The RC filter composed of resistors and capacitors is shown in Figure 5-13 . This type of compound filter has a simple structure, can play the role of voltage reduction and current limiting, and has a high filtering efficiency. It is the last type of filter used. The above two types of compound filters have poor load capacity due to the connection of capacitors .