What is Power factor correction?

A circuit in equipment powered from an AC supply that shapes the input current into a sine wave of the same form and phase as the voltage, bringing the power factor close to 1 (a PFC circuit). The common approach places a boost converter after the rectifier and controls its switch so that the input current is proportional to the rectified voltage; the output is a DC voltage of about 380 to 400 V, higher than the peak input voltage. It reduces the harmonic currents of a rectifier with plain capacitor smoothing, and a downstream isolated DC-DC converter makes the required voltages from this DC.

Read the beginner explainer: AC vs DC Explained: Impedance, Phase and Power Factor

Japanese name: 力率改善回路

Power factor correction appears in 6 practice questions across 2 topics, including Power semiconductors, Power Electronics Circuits.

Field: Power Electronics Circuits

Try it: questions about Power factor correction

What is the purpose of a power factor correction (PFC) circuit used in power supplies?

ATo double the output voltage
BTo increase supply noise
CTo shape the input current into a sine wave matching the voltage, bringing the power factor close to 1

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Answer: C. To shape the input current into a sine wave matching the voltage, bringing the power factor close to 1

A simple capacitor-input rectifier draws current only in pulses near the voltage peaks, dropping the power factor to about 0.5–0.7 and injecting harmonics into the grid. A PFC circuit—typically a boost converter—shapes the input current into a sine wave matching the voltage, raising the power factor above 0.95. Harmonic regulations in Japan and Europe apply to equipment above certain power levels, so PFC is built into PC power supplies, LED lighting and more.

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.

Power factor correction in the explanations

A PFC circuit—typically a boost converter—shapes the input current into a sine wave matching the voltage, raising the power factor above 0.95.
What is the purpose of a power factor correction (PFC) circ…
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.
In a boost-type power factor correction circuit (PFC circui…
This brings the on-resistance of 600 V-class silicon MOSFETs to a fraction of the classical silicon limit, and such devices are widely used in power supplies and PFC circuits.
Which is a characteristic of a superjunction (SJ) MOSFET?
As freewheeling diodes in inverters and PFC circuits, they cut not only their own losses but also the turn-on losses of the complementary switch.
What is the main advantage of using SiC Schottky barrier di…
Applications include driving LEDs from a single battery cell, boosting solar-panel voltage, raising motor-drive voltage in hybrid cars, and PFC circuits.
Which DC-DC converter outputs a DC voltage higher than its …