PFC Basic Principles

What is Power Factor?

The power factor is defined as the ratio of the actual power (P) drawn in an AC circuit to the apparent power (S):

• $P$ : Actual power (Watt)
• $S$ : Apparent power (VA)
• $\phi$: Voltage-current phase difference

When PF < 1, the network carries unnecessary reactive power and system efficiency decreases.

PFC Topologies and Selection Criteria

PFC circuits are primarily divided into passive and active topologies. Each approach has different applications and advantages.

Passive PFC:
Passive topologies are typically implemented with a combination of a coil and capacitor at the AC input. They are simple and low-cost; however, the power factor generally remains around 0.7–0.8, and harmonic filtering is limited.

Active PFC:
Active PFC circuits use AC-DC converters and control circuits. The boost converter is a typical example. Its advantages include a high power factor (>0.95), harmonic reduction, and a more stable output voltage. Its disadvantage is the complexity of the design and increased cost. Today, most high-power devices use active PFC.

Basic Calculations and Design Criteria

Inductor and capacitor selection is critical in PFC design. Let's perform a sample calculation using the boost topology.

Input parameters:

• AC input: 230V RMS
• Output DC: 400V
• Maximum power: 600W
• Switch frequency: 100 kHz

Inductance Calculation:$$L = \frac{V_{in} \cdot (V_{out}-V_{in})}{\Delta I_L \cdot f_s \cdot V_{out}}$$

Capacitor Calculation:$$C = \frac{I_{out}}{f \cdot \Delta V_{out}}$$

The output ripple voltage is controlled by ΔV_out. This ensures that the output voltage remains stable and is minimally affected by load changes.

4. PCB Design and Implementation Tips

The performance of a PFC circuit is directly related not only to the topology and calculations, but also to the PCB layout.

• High-current paths should be short and thick, thereby reducing voltage drop and heating.
• Thermal management is critical: Appropriate cooling paths must be designed for the boost diode and MOSFET.
• The EMI filter should be placed before the PFC circuit, and the input current paths should be kept as short as possible.
• The real-time feedback (voltage & current sensing) circuit on the PCB should have short signal traces, and parasitic capacitance and inductance should be minimized.

"For example, the PFC card we developeduses an active topology to deliver high PF (>0.95) and low THD (<10%) at power levels up to 3000W, and is optimized for industrial and laboratory applications."

Advantages and Disadvantages

PFC circuits offer many advantages in terms of energy efficiency and power quality, but they also have disadvantages such as design complexity and cost.