Capacitor Input Filter : Part2

The same concept can now be extended to the capacitor filter used in full wave rectifier circuit as shown in the Fig. 1.

Fig. 1(a)

Fig. 1(b)

       Immediately when power is turned on, the capacitor C gets charged through forward biased diode D₁ to E_sm, during first quarter cycle of the rectified output voltage. In the next quarter cycle from π/2 to π, the capacitor starts discharging through R_L. Once capacitor gets charged to E_sm, the diode D₁ becomes reverse biased and stops conducting. So during the period from π/2 to π, the capacitor C supplies the load current. It discharging to point B showing in the Fig. 2. At point B, lying in the quarter π to 3π/2 of the rectified output voltage, the input voltage exceeds capacitor voltage, making D₂ forward biased. This charges capacitor back to E_sm at point C.

Fig. 2  Capacitor filter with FWR

       The time required by capacitor C to charge to E_sm is quite small and only for this period, diode D₂ is conducting. Again at point C, diode D₂ stops conductor and capacitor supplies load and starts discharging upto point D in the next quarter cycle of the rectified output voltage as shown in the Fig. 2. At this point, the diode D₁ conducts to charge capacitor back to E_sm. The diode currents are shown shaded in the Fig. 3.

Fig. 3  Operation of capacitor filter for full wave rectifier

1.2.1 Expression for Ripple Factor

       The ripple voltage in the output of capacitor filter with full wave rectifier is practically assumed to be triangular as shown in the Fig. 4.

Fig. 5  Triangular approximation of ripple voltage

       The peak to peak ripple voltage V_r is given by,

       Where        f = Supply frequency in Hz

                         C = Capacitor filter in F

                         R_L = Load resistance in  Ω.

Note : The diodes are not conducting for the entire half cycle but only for a part of the half cycle, during which the capacitor is getting charged.

       When the capacitor is discharging through the load resistance R_L, both the diodes are non-conducting. The capacitor supplies the load current.

Note : As the time required by capacitor to charge is very small and it discharges very little due to large time constant, hence ripple in the output gets reduced considerably.

       Though the diodes conduct partly, the load current gets maintained sue to the capacitor. This filter is very popularly used in practice