🔌 Power Factor Correction (PFC)

A boost PFC converter shapes the input current to be sinusoidal and in phase with voltage. Power factor PF = cos(φ) → 1. See THD reduction and reactive power Q.

ElectronicsInteractive
Green: voltage · Amber: current without PFC · Teal: corrected current · P pause · R reset

How it Works

Without PFC, a simple bridge rectifier draws current in short pulses near the voltage peaks, creating high harmonic content and a poor power factor. The boost PFC converter uses fast switching and an average current control loop to force input current to track the sinusoidal voltage waveform, achieving near-unity power factor.

The canvas shows two AC cycles: the supply voltage (green), the distorted current without PFC (amber, with 3rd/5th/7th harmonics visible), and the shaped sinusoidal current with PFC active (teal). Toggle PFC on/off to compare. Adjust the phase angle and THD sliders to explore different load types.

PF = cos(φ) / √(1 + THD²) [true power factor]
Q = S × sin(φ) [reactive power]
S = P / PF [apparent power]
THD = √(Σ Iₙ²) / I₁ [harmonic distortion]

Frequently Asked Questions

What is power factor?

Power factor (PF) is the ratio of real power (W) to apparent power (VA): PF = P/S = cos(φ). A PF of 1 means all supplied power does useful work. A PF less than 1 indicates reactive or harmonic power that burdens the grid without doing useful work.

Why does power factor need correction?

Poor power factor causes increased current draw from the grid, higher I²R losses in wiring, overloaded transformers, and voltage distortion. Utilities may charge penalties for low power factor industrial loads. PFC reduces these issues and improves energy efficiency.

How does a boost PFC converter work?

A boost PFC converter uses a high-frequency switching MOSFET controlled by an average current mode loop. It modulates the duty cycle so that input current follows the sinusoidal shape of the input voltage, making the current appear resistive (unity power factor).

What is Total Harmonic Distortion (THD)?

THD is the ratio of the sum of harmonic currents to the fundamental current: THD = √(I₂²+I₃²+...)/I₁ × 100%. Without PFC, rectifier loads draw pulsed current with high THD (over 100%). PFC reduces THD to below 5%, which is required by standards like IEC 61000-3-2.

What is reactive power and how does PFC reduce it?

Reactive power Q = V·I·sin(φ) in VAR is the power that oscillates between source and load without doing work. PFC brings current into phase with voltage (φ → 0), so sin(φ) → 0 and Q → 0. The apparent power equals real power.

What is the difference between displacement PF and true PF?

Displacement PF = cos(φ₁) accounts for the phase shift between fundamental voltage and current. True PF also includes harmonic distortion: PF = DPF / √(1+THD²). Even with DPF=1, harmonics can reduce true PF significantly.

What standards govern power factor requirements?

IEC 61000-3-2 limits harmonic currents for equipment above 75W. EN 61000-3-2 is the European version. Energy Star and 80 PLUS require PF over 0.9 for computer power supplies. Many industrial standards require PF over 0.95.

What output voltage does a boost PFC produce?

A boost PFC converter always produces an output voltage higher than the peak input voltage. For 230V AC input (peak 325V), the PFC output bus is typically 400V DC. This voltage is then stepped down by a downstream converter to the required output voltage.

What is PSRR in the context of PFC?

Power Supply Rejection Ratio (PSRR) measures how well a downstream regulator rejects ripple on its supply rail. The PFC stage produces a 100/120 Hz ripple on the DC bus (at twice the line frequency) which the downstream converter must reject.

Can PFC be implemented passively?

Yes, passive PFC uses large inductors (chokes) in series with the AC input to reduce current harmonics and improve displacement power factor. However, passive PFC is bulky, heavy, and limited to PF around 0.7–0.9. Active boost PFC achieves PF over 0.99 with a compact circuit.

About this simulation

This simulation compares raw rectifier current against boost-PFC-corrected current against the same AC voltage waveform, live. Toggling PFC Active switches between a distorted current (built from fundamental plus 3rd, 5th, and 7th harmonics) and a clean sinusoid in phase with voltage, while the power-factor arc and stats panel show exactly how PF = cos(φ)/√(1+THD²) responds.

🔬 What it shows

Two overlapping AC cycles: voltage (green), distorted load current with harmonics (amber), and — when PFC is on — a corrected sinusoidal current (teal) nearly in phase with voltage, plus a rotating phasor arc visualizing the phase angle behind the power factor.

🎮 How to use

Adjust Phase Angle φ and THD (%) to shape the uncorrected current, set V_rms and Load Power P, then toggle PFC Active on/off to compare. Watch Power Factor PF, Real Power P, Apparent S, and Reactive Q update live. Pause with P, reset with R.

💡 Did you know?

Standards like IEC 61000-3-2 cap harmonic currents for equipment over 75W, which is why nearly every modern laptop charger and LED driver above that threshold contains an active boost-PFC stage even though the end user never sees it.

Frequently asked questions

Why does the amber current line look jagged instead of a smooth sine wave?

It's built from a fundamental sine plus 3rd, 5th, and 7th harmonics scaled by the THD slider — real rectifier loads draw current in narrow pulses near the voltage peak, which is mathematically equivalent to a fundamental heavily contaminated with these odd harmonics.

Why does toggling PFC Active instantly change the Power Factor stat to near 0.99?

This simulation models an idealized boost PFC converter that forces current to track the sinusoidal voltage almost perfectly, collapsing both phase shift and THD. Real converters approach but don't quite reach 0.99 due to switching non-idealities, but the model shows the theoretical target behavior.

Why does increasing Phase Angle φ shrink Real Power P's share of Apparent S?

Reactive power Q = S·sin(φ) grows with φ while real power P = S·cos(φ) shrinks, so apparent power S = P/PF must rise to deliver the same real power — you're supplying more current from the grid for the same useful work.

Why does raising THD hurt power factor even when Phase Angle φ is small?

True power factor combines both effects: PF = cos(φ)/√(1+THD²). Even with perfect phase alignment (φ=0), high harmonic distortion alone can drag true PF well below 1, which is why THD reduction matters independently of phase correction.

Why does a boost PFC converter need an output voltage higher than the input peak?

A boost topology can only step voltage up, never down, which is what lets it actively shape input current throughout the entire AC cycle including near the voltage zero-crossings. For 230V RMS input (325V peak), the PFC stage typically produces roughly 400V DC.