Power Electronics Circuits
In a boost-type power factor correction circuit (PFC circuit) for a power supply that works from 100 to 240 V AC, which of the following correctly explains why the DC output voltage is often set to about 380 to 400 V?
- AThe higher the DC output voltage is set, the more the power factor rises above 1, so the input current can be reduced further
- BA boost converter can only output a voltage higher than its input, so the output must exceed the peak input voltage (about 339 V for 240 V RMS)
- CThe PFC circuit is a buck converter, so the DC output voltage is kept below the peak input voltage to protect the downstream isolated converter and smoothing capacitor from overvoltage
Show answer and explanation
Answer: B. A boost converter can only output a voltage higher than its input, so the output must exceed the peak input voltage (about 339 V for 240 V RMS)
A boost-type power factor correction circuit takes the rectified, pulsating voltage as its input and controls the switch so that the input current becomes a sine wave of the same shape as the voltage. A boost converter cannot output a voltage lower than its input, so the DC output voltage must always be higher than the peak input voltage. The peak of 240 V RMS is √2 × 240 ≈ 339 V, so the output is set to about 380 to 400 V with some margin. A downstream isolated DC-DC converter then turns this DC voltage into the voltages required.
- A. The power factor never exceeds 1.
- B. Correct. The output of a boost converter must be higher than the peak input.
- C. A typical power factor correction circuit is a boost converter and outputs a voltage higher than the peak input.