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The mean output voltage of a single-phase fully controlled bridge with firing angle \( \alpha \) is

  • A.\( (2V_m/\pi)(1 + \cos\alpha) \)
  • B.\( (2V_m/\pi)\cos\alpha \)
  • C.\( (2V_m/\pi)\sin\alpha \)
  • D.\( (V_m/\pi)\cos\alpha \)

Answer: B

Correct answer: \( (2V_m/\pi)\cos\alpha \)

Integrating the supply waveform over the conduction interval of a fully controlled bridge gives

\[ V_{dc} = \frac{2V_m}{\pi}\cos\alpha \]

The cosine is what makes the bridge so useful: at \( \alpha = 0 \) the output is the full \( 2V_m/\pi \), at 90 degrees it is zero, and beyond 90 degrees it is negative, so the same bridge inverts and returns power to the supply. The form with \( (1 + \cos\alpha) \) belongs to a half-controlled bridge, which cannot go negative.

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