Capacitor-Input Filters

One of the assumptions implied at the beginning of this chapter, namely, that transformer and rectifier voltage drops are negligibly small, cannot usually be made when capacitor-input filters are used, because of the large peak currents drawn by the capacitor during the charging interval. Such charging currents drawn through finite resistances affect both the d-c output voltage and the ripple in a complicated manner, and simple analysis such as that given for inductor-input filters is no longer possible. Figure 86 is a plot of the ripple in the load of capacitor-input filters with various ratios of source to load resistance, and for three types of single-phase rectifiers. These curves are useful also when resistance is used in place of an inductor at the input of a filter, ω is 2π times the a-c supply frequency, C is the capacitance, R_L is the load resistance, and R_S the source resistance.

When L-C filter stages follow a capacitor-input filter, the ripple of the latter is reduced as in Fig. 85, except that the value of P_A must be taken from Fig. 86.

Fig. 86. Rms ripple voltage of capacitor-input filters

When an R-C filter stage follows any type of filter, the ripple is reduced in the ratio R/X_C represented by the R-C stage.