Concept:In a center-tapped rectifier, each half of the secondary winding is utilized only \( 50\% \) of the time, resulting in a lower Transformer Utilization Factor (\( \text{TUF} = 0.693 \) vs \( 0.812 \) for a bridge).
Formula:$$\text{VA Rating} = \frac{P_{\text{dc}}}{\text{TUF}} \implies \text{VA}_{\text{CT}} = \frac{P_{\text{dc}}}{0.693} \approx 1.44 P_{\text{dc}} \quad \text{vs} \quad \text{VA}_{\text{Bridge}} = \frac{P_{\text{dc}}}{0.812} \approx 1.23 P_{\text{dc}}$$
Solution:- Each half of the center-tapped secondary winding carries current only during its respective conducting half-cycle.
- This discontinuous current increases the RMS heating current relative to the delivered DC power.
- As a result, a center-tapped transformer requires a larger core and higher VA rating (\( \approx 1.44 P_{\text{dc}} \)) than a bridge rectifier transformer (\( \approx 1.23 P_{\text{dc}} \)).
Why other options are incorrect:- Option A: The higher rating requirement is due to winding duty cycle and TUF, not parasitic capacitance.
- Option B: The two diodes conduct on alternate half-cycles, not simultaneously.
- Option D: Operating frequency is determined by the AC source, not the transformer construction.
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