Chapter 2

Q. 2.13

Calculate the ripple factor for the half-wave rectifier of Example 2.10   (a) without a filter and   (b) with a shunt capacitor filter as in Fig. 2-15(a).



Verified Solution

(a)  For the circuit of Example 2.10,
F_r = \frac{Δv_L}{V_{L0}}  = \frac{V_{Lm}}{V_{Lm/π}} =  π ≈ 3.14

(b)  The capacitor in Fig. 2-15(a) stores energy while the diode allows current to flow, and delivers energy to the load when current flow is blocked. The actual load voltage v_L that results with the filter inserted is sketched in Fig. 2-15(b), for which we assume that v_S = V_{Sm} \sin ωt and D is an ideal diode. For 0 < t ≤ t_1,  D is forward-biased and capacitor C charges to the value V_{Sm}. For t_1 < t ≤ t_2,  v_S is less than v_L, reverse-biasing D and causing it to act as an open circuit. During this interval the capacitor is discharging through the load R_L, giving
v_L = V_{Sm} e^{-(t – t_1)/R_LC} \quad   (t_1 < t ≤ t_2)         (2.9)

Over the interval t_2 < t ≤ t_2 + δ,  v_S forward-biases diode D and again charges the capacitor to V_{Sm}. Then v_S falls below the value of v_L and another discharge cycle identical to the first occurs.
Obviously, if the time constant R_{L}C is large enough compared to T to result in a decay like that indicated in Fig. 2-15(b), a major reduction in Δv_L and a major increase in V_{L0} will have been achieved, relative to the unfiltered rectifier. The introduction of two quite reasonable approximations leads to simple formulas for Δ{v_L} and V_{L0}, and hence for F_r, that are sufficiently accurate for design and analysis work:
1. If Δ{v_L} is to be small, then δ → 0 in Fig. 2-15(b) and t_2  –  t_1 ≈ T.
2. If Δ{v_L} is small enough, then (2.9) can be represented over the interval t_1 < t ≤ t_2 by a straight line with a slope of magnitude V_{Sm}/R_LC.
The dashed line labeled ‘‘Approximate v_L’’ in Fig. 2-15(b) implements these two approximations. From right triangle abc,
\frac{Δ{v_L}}{T} = \frac{V_{Sm}}{R_LC} \quad   \text{or}  \quad  Δ{v_L} = \frac{V_{Sm}}{fR_LC}

where f is the frequency of v_S. Since, under this approximation,
V_{L0} = V_{Sm}  –  \frac{1}{2} Δ{v_L}

and  R_LC/T = fR_LC is presumed large,
F_r = \frac{Δv_L}{V_{L0}}  = \frac{2}{2fR_L C  –  1} ≈ \frac{1}{fR_L C}           (2.10)