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Summary of the Core Role and Necessity of Power Factor Correction (PFC) in Power Supply
Author.
LCS
Source:
Post time:
2026-10-14
The core function of PFC (Power Factor Correction) power factor correction circuit is to improve the current waveform drawn by the power supply from the grid, so that it follows the sinusoidal waveform of the voltage as much as possible and is in phase with the voltage, thereby improving the power factor PF and suppressing harmonic currents.
1、 What problems will not be caused by adding PFC?
1. Heavy burden on the power grid side and waste of resources:
Without a PFC power supply, the input current exhibits a spike pulse shape, resulting in a low power factor PF (approximately 0.5-0.6).
Current and voltage are out of phase, causing waveform distortion and generating a large amount of high-order harmonic currents.
These harmonic currents cannot do work, but they will occupy the capacity of the power grid (manifested as the apparent power S being much greater than the active power P), causing overload and heating of equipment (cables, transformers, circuit breakers) on the grid side, reducing their available capacity, and resulting in increased investment costs.
The grid side must provide a current capacity far exceeding the actual useful power required, which reduces the efficiency of grid utilization.
2. Power quality pollution and interference with other equipment:
Distorted harmonic currents will flow back into the power grid, causing voltage waveform distortion and deteriorating power quality.
Other devices on the same power grid may experience interference, which can lead to lighting flicker, abnormal measurement of precision instruments, and misoperation of industrial control equipment.
3. Regulations mandate that the product cannot be marketed:
This is the main reason why high-power power supplies must be equipped with PFC. International and national mandatory harmonic standards (such as EN61000-3-2, GB/T14549) impose strict limits on the current of each harmonic for AC-DC power supplies with input power greater than 75W.
Power supplies without PFC are almost impossible to pass these certifications, so they cannot be legally sold and used.
4. Engineering design difficulties:
The DC bus voltage without PFC power supply will fluctuate with the input AC voltage and have a wide range. This results in the need for the downstream DC-DC converter to handle an extremely wide range of input voltages, increasing the design difficulty of transformers and MOS transistors.
At the moment of power on, charging a large capacity electrolytic capacitor will generate a huge surge current, which can easily damage the components.
2、 The two main types of PFC
1. Passive PFC (Passive PFC):
Principle: Connect a large volume power frequency inductor in series at the AC input terminal, and use the inductor to block sudden changes in current to smooth the current waveform.
Features: Simple structure and relatively low cost. However, the PF improvement is limited (about 0.7-0.85), the harmonic suppression ability is poor, the inductance volume is large and heavy, and the effect is greatly reduced when inputting low voltage, making it impossible to stably output DC bus voltage. At present, it has gradually been phased out in the high-power field.
2. Active PFC (Active PFC/APFC):
Principle: Essentially, it is a Boost boost converter composed of PFC-MOS, PFC inductor, PFC diode, and control chip. The chip controls the input current waveform in real-time to follow the voltage sine waveform.
Features:
High PF value (up to 0.95-0.99), excellent harmonic suppression effect, can easily meet regulatory requirements.
It can stably boost the rectified fluctuating voltage to a fixed high-voltage DC bus (usually 380-400V), even if the input voltage is wide (90-264Vac), and provide stable working conditions for the subsequent DC-DC, simplifying the design of the subsequent stage.
Due to working at high frequencies, PFC inductors are relatively compact in size.
It is currently the mainstream solution for high-power AC-DC power supplies.
Disadvantages: complex circuit, high BOM cost, additional losses introduced by one stage transformation (but modern designs can control them at a very low level, such as 96-98%), suboptimal light load efficiency (often solved by PFC bypass or intermittent mode), complex loop design, and reliability challenges for high-voltage electrolytic capacitors.
3、 Clarify important misconceptions
PF ≠ Efficiency (η): This is the most crucial difference. PF is an indicator that describes the quality of the current waveform on the grid side, while efficiency is an indicator of the energy conversion loss of the power source itself. PFC cannot directly improve the efficiency of the power supply itself, and theoretically, an additional stage of PFC conversion will also introduce additional losses.
PFC does not directly save electricity costs for users: electricity costs are calculated based on active power. The value of PFC lies in optimizing the operation of the power grid, reducing the burden on the distribution system, minimizing power grid losses, and improving power quality. The beneficiaries are mainly power supply and distribution system managers, rather than individual device users. However, in scenarios such as computer rooms and factories where multiple devices are connected in parallel, PFC can significantly reduce the total input current, thereby lowering the investment cost and operational losses of the distribution system.
4、 Application selection of PFC
Active PFC must be used: products with input power greater than 75W and meeting harmonic standards (such as server power supplies, energy storage inverters, high-power fast charging, industrial power supplies, etc.); Applications that require global wide voltage input and stable DC bus; The scenario of deploying multiple devices in a centralized manner to reduce the total input current and distribution pressure.
PFC can be omitted: low-power adapters with input power less than 75W (such as mobile phone chargers) are exempt from regulations; Internal equipment that is not sold externally, does not require certification, and has low power.
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