Principle of capacitance filter

The principle of capacitance filtering is shown in the figure below. The filtering capacitance has a large capacity, so the electrolytic capacitance is generally used. Pay attention to the positive and negative electrodes of the electrolytic capacitance when wiring. The capacitor filter circuit makes use of the charge and discharge function of the capacitor to make the output voltage smooth.

1、 Filtering principle ★ when U2 is positive half cycle and the value is greater than the voltage Uc at both ends of the capacitor, diode D1 and D3 are on, D2 and D4 are off, one current path flows through the load resistance RL, and the other is charged to capacitor C. When UC > U2, D1 and D3 tubes are reversed

When the capacitor is biased and cut off, it discharges through the load resistance RL, and UC slowly decreases according to the exponential law. ★ when the amplitude of U2 is negative half cycle and changes to just greater than UC, D2 and D4 become on state due to positive voltage, U2 charges C again, UC rises to the peak value of U2 and then starts to decline; when it drops to a certain value, D2 and D4 become off, C discharges RL, UC decreases according to the exponential law; when it is discharged to a certain value, D1 and D3 become on, repeat the above process. The influence of RL and C on charging and discharging the capacitor charging time constant is RDC, because the RD of diode is very small, the charging time constant is small and the charging speed is fast; RLC is the discharging time constant, because RL is large, the discharging time constant is far greater than the charging time constant, so the filtering effect is determined by the discharging time constant.

The larger the capacitance C, the greater the load resistance RL, the smoother the output voltage after filtering, and the larger the average value, as shown in the figure above on the right.

2、 The average value of the output voltage is derived from the analysis. When the load is open circuit, it is real-time; when RLC = (3 ~ 5) t / 2, it can achieve better filtering effect. Considering that the fluctuation range of grid voltage is ± 10%, the withstand voltage value of capacitor shall be greater than.

3、 The fluctuation coefficient calculated by the above formula is larger than the actual value.

4、 The conduction angle of the rectifier diode ★ before the filter capacitor is added, the conduction angle of the diode in the rectifier circuit θ is π. ★ after the filter capacitor is added, the diode will be on only when the capacitor is charged, so the on angle of each diode is less than π. The larger the RLC value, the better the filtering effect and the smaller the conduction angle θ. As the average output current increases after capacitor filtering, and the conduction angle of diode decreases instead, the rectifier diode will flow a large impulse current to charge the capacitor in a short period of time, as shown in the right figure, which is not good for the life of diode. Therefore, the rectifier diode with large capacity must be selected, and its maximum average rectifier current if is usually greater than the load electricity Two to three times the flow.

5、 Output characteristic and filter characteristic output characteristic of capacitor filter circuit: when filter capacitor C is selected, the relationship between output voltage average uo (AV) and output current average IO (AV) is called output characteristic. Filtering characteristics: the relationship between the pulse coefficient s and the average output current IO (AV) is called filtering characteristics. As shown in the figure, the curve shows that the larger the C is, the stronger the load capacity of the circuit is, and the better the filtering effect is; the larger the IO (AV) (i.e. the smaller the load resistance RL), the lower the uo (AV), and the greater the S value. Summary: the capacitor filter circuit is simple and the average value of output voltage is high. It is suitable for the situation of small load current and small change. [example 9.3.1] in the circuit as shown in the figure, the average output voltage uo (AV) is required to be 15V, the average load current IL (AV) to be 100mA, and uo (AV) to be 1.2u2. Test questions: (1) the size of the filter capacitance; (2) considering the fluctuation range of the grid voltage is ± 10%, the voltage withstand value of the filter capacitance. Solution: (1) according to uo (AV) ≈ 1.2u2, it can be seen that the value of C meets the condition of RLC = (3 ~ 5) t / 2. The capacity of the capacitor is (2) the effective value of the secondary side voltage of the transformer is the withstand voltage value of the capacitor. Actually, the capacitor with the capacity of 300 μ F and the withstand voltage of 25 V can be selected as the filter capacitance of the circuit.