Power factor correction techniques for AC to DC systems

Power factor correction techniques for AC to DC systems using passive filter

Power Factor Correction in AC–DC Systems using Passive Filters

When AC power is converted to DC using rectifiers (e.g., diode or thyristor rectifiers), the input current drawn is non-sinusoidal due to the switching nature of the devices. This leads to:

• Low Power Factor (PF)
• High Total Harmonic Distortion (THD)
• Poor utilization of AC supply

To overcome these issues, Passive Filters (consisting of inductors, capacitors, and sometimes resistors) are connected at the AC input to improve the input current waveform and thus correct the power factor.Principle of Passive PFC

• Passive filters are designed to filter out harmonics generated by the rectifier.
• By doing so, the input current waveform becomes more sinusoidal and aligned with the input voltage.
• As a result, the power factor is improved (closer to unity).

Common Passive Filter Techniques

1. Series Inductor (Input Choke Filter)

• A large inductor is placed in series with the AC input.
• It smooths the current waveform by limiting the di/dt.
• Reduces harmonics and improves displacement power factor.
• Drawback: bulky and heavy at low frequencies (50/60 Hz).

1. Shunt Capacitor Filter

• A capacitor is placed across the input to supply reactive power locally.
• Improves the displacement power factor.
• Drawback: not effective for harmonic reduction.

1. LC Filters (Low-Pass Filters)

• A combination of inductors and capacitors designed to filter specific harmonics (5th, 7th, etc.).
• Forces the input current to be closer to sinusoidal.
• Drawback: requires careful tuning, bulky for low-frequency systems.

1. L–C–L Filter

• More advanced configuration using multiple inductors and capacitors.
• Provides better harmonic suppression and smoother input current.

Advantages of Passive PFC

• Simple and reliable (no control circuit needed).
• Low cost compared to active PFC.
• Improves input power factor to acceptable levels.
• Reduces lower-order harmonics.

Limitations of Passive PFC

• Bulky components (large inductors and capacitors).
• Limited performance at variable loads.
• Not effective for high power factor correction (only partial correction).
• May cause resonance with the supply system.

Applications

• Low-cost rectifier systems.
• Small industrial power supplies.
• Low/medium power equipment where size is not critical.
• Systems with relatively constant loads.