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Electronics Engineering QUESTION #11709
Question 1
In a PFC (Power Factor Correction) boost pre-regulator, the input current waveform is shaped to follow the input voltage. At rated load with $\text{PF} = 0.99$, what is the approximate THD of the input current?
  • ~14%✔️
  • ~5%
  • ~1%
  • ~40%
Correct Answer Explanation
The relationship between PF, displacement factor (DF), and THD is: $\text{PF} = \text{DF} \times \dfrac{1}{\sqrt{1+(\text{THD})^2}}$. For near-unity DF and PF = 0.99: $0.99 \approx \dfrac{1}{\sqrt{1+\text{THD}^2}}$, giving $\text{THD} \approx \sqrt{(1/0.99)^2 - 1} \approx \sqrt{0.0203} \approx 14\%$. Active PFC targets THD < 5% for high-quality designs.