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302 - PFC Basics

Power Factor Correction fundamentals using boost converter topology.

Overview

PFC circuits shape the input current to be sinusoidal and in phase with the voltage, achieving near-unity power factor and reducing harmonic distortion.

Why PFC?

Without PFC, rectifiers draw peaky current that: - Causes harmonic pollution on the grid - Reduces effective power delivery - Fails regulatory standards (IEC 61000-3-2)

Boost PFC Topology

Circuit Configuration

        L           D
AC+ ──┬─⊐⊏──┬──────┤►├─┬── DC+
      │     │           │
     ⌇      │    SW     C    Load
Cin  ⌇      └────┤      │
      │          │      │
AC- ──┴──────────┴──────┴── DC-

Operating Principle

  1. Switch ON: Inductor charges, current rises
  2. Switch OFF: Inductor discharges through diode to output
  3. Current Shaping: PWM duty cycle modulated to make current follow voltage

Control Methods

Average Current Mode Control

  • Current loop tracks reference derived from voltage
  • Excellent current waveform
  • Complex implementation

Critical Conduction Mode (CRM)

  • Variable frequency operation
  • Zero-current switching
  • Simpler implementation
  • Higher peak currents

Key Equations

Duty Cycle (CCM): $\(D = 1 - \frac{V_{in}(t)}{V_{out}}\)$

Inductor Current Ripple: $\(\Delta I_L = \frac{V_{in} \cdot D}{L \cdot f_{sw}}\)$

Power Factor: $\(PF = \frac{P}{S} = \frac{V_{rms} \cdot I_{rms} \cdot \cos\phi}{V_{rms} \cdot I_{rms}}\)$

Design Parameters

Parameter Typical Value Notes
Output Voltage 385-400V DC Above peak AC
Switching Freq 65-130 kHz EMI tradeoffs
Inductor 200-500µH Ripple control
Output Cap 200-400µF Holdup time

Simulation Exercises

  1. Compare PF with and without correction
  2. Observe inductor current shaping
  3. Vary load and check regulation
  4. Analyze THD of input current