Finding the Performance Parameters of a Push–Pull Converter The average (or dc) output voltage of the push–pull circuit in Figure 13.6 is Vo = 24 V at a resistive load of R = 0.8 Ω. The on-state voltage drops of transistors and diodes are Vt = 1.2 V and Vd = 0.7 V, respectively. The turns ratio of

Question

Finding the Performance Parameters of a Push–Pull Converter

The average (or dc) output voltage of the push–pull circuit in Figure 13.6 is [latex]V_o = 24 V[/latex] at a resistive load of [latex]R = 0.8 Ω[/latex]. The on-state voltage drops of transistors and diodes are [latex]V_t = 1.2 V[/latex] and [latex]V_d = 0.7 V[/latex], respectively. The turns ratio of the transformer is [latex]a = N_s/N_p = 0.25[/latex]. Determine (a) the average input current [latex]I_s[/latex], (b) the efficiency η, (c) the average transistor current [latex]I_A[/latex], (d) the peak transistor current [latex]I_p[/latex], (e) the rms transistor current [latex]I_R[/latex], and (f) the open-circuit transistor voltage [latex]V_{oc}[/latex]. Neglect the losses in the transformer, and the ripple current of the load and input supply is negligible. Assume duty cycle [latex]k = 0.5[/latex].

Accepted Answer

[latex]a = N_s/N_p = 0.25[/latex] and [latex]I_o = V_o/R = 24/0.8 = 30 A[/latex].

a. The output power [latex]P_o = V_o I_o = 24 × 30 = 720 W[/latex]. The secondary voltage [latex]V_2 = V_o + V_d = 24 + 0.7 = 24.7 V[/latex]. The primary voltage [latex]V_1 = V_2/a = 24.7/0.25 = 98.8 V[/latex]. The input voltage [latex]V_s = V_1 + V_t = 98.8 + 1.2 = 100[/latex] and the input power is

[latex]P_i = V_s I_s = 1.2I_A + 1.2I_A + V_d I_o + P_o[/latex]

Substituting [latex]I_A = I_s/2[/latex] gives

[latex]I_s(100 - 1.2) = 0.7 × 30 + 720[/latex]

[latex]I_s=\frac{741}{98.8}=7.5 A[/latex]

b. [latex]P_i = V_s I_s = 100 × 7.5 = 750 W[/latex]. The efficiency [latex]η= 720/750 = 96.0\%[/latex].

c. [latex]I_A = I_s/2 = 7.5/2 = 3.75 A[/latex].

d. [latex]I_p = I_s = 7.5 A[/latex].

e. [latex]I_R=\sqrt{k}I_p=\sqrt{0.5} imes 7.5=5.30 A [/latex], for 50% duty cycle.

f. [latex]V_{oc} = 2V_s = 2 × 100 = 200 V[/latex].

Question 13.3: Finding the Performance Parameters of a Push–Pull Converter ...

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