Power Factor Correction Controller# Technical Documentation: FAN7527B Power Factor Correction (PFC) Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN7527B
Description : Critical Conduction Mode (CRM) PFC Controller IC
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## 1. Application Scenarios
### Typical Use Cases
The FAN7527B is specifically designed as a controller for building active Power Factor Correction (PFC) pre-regulators . Its primary function is to shape the input current drawn from the AC mains to closely follow the input voltage waveform, thereby achieving a near-unity power factor and minimizing harmonic distortion. Typical use cases include:
* Standalone PFC Front-Ends: Used as the first conversion stage in AC-DC power supplies, positioned between the AC input rectifier and the main DC-DC converter (e.g., a flyback or LLC resonant converter). It boosts the rectified voltage to a stable, higher DC bus voltage (typically 385-400V DC).
* CRM/BCM Operation: The controller operates in Critical Conduction Mode (CRM) , also known as Boundary Conduction Mode (BCM). In this mode, the boost inductor current returns to zero at the end of every switching cycle. This eliminates the reverse recovery losses of the boost diode, improves efficiency, and is ideal for low-to-medium power applications.
### Industry Applications
The FAN7527B is widely adopted in power supplies requiring compliance with international harmonic current emission standards such as IEC 61000-3-2 .
* Consumer Electronics: SMPS for LCD/LED TVs, monitors, audio amplifiers, and gaming consoles.
* Computer & IT Equipment: Desktop PC power supplies (particularly in the PFC stage of ATX PSUs), server PSUs, and external adapters for laptops.
* Industrial Power Systems: Low-to-medium power industrial motor drives, automation equipment, and lighting ballasts (e.g., for LED drivers).
* Appliances: Power supplies for white goods and other household appliances where efficiency and regulatory compliance are critical.
### Practical Advantages and Limitations
Advantages:
* High Power Factor: Enables designs to achieve PF > 0.99, ensuring compliance with stringent energy standards.
* Simplified Design: The CRM control methodology simplifies the control loop compared to Continuous Conduction Mode (CCM) PFC controllers, as it inherently avoids the right-half-plane zero.
* High Efficiency: Zero-current switching (ZCS) of the boost diode minimizes switching losses and eliminates diode reverse recovery issues, leading to higher efficiency, especially at lower power levels.
* Integrated Features: Includes essential protection features like over-voltage protection (OVP), open-loop protection, and a disable function, reducing external component count.
* Low Start-up Current: Consumes very low current during startup (~50µA typical), easing the design of the startup circuit.
Limitations:
* Power Range Constraint: CRM operation is best suited for applications typically below 300W-400W . At higher power levels, the high peak inductor currents increase conduction losses and EMI filter size, making CCM controllers more suitable.
* Variable Frequency Operation: The switching frequency varies with input voltage and load. It is highest at the peak of the AC line and minimum load, which can complicate EMI filter design.
* Higher RMS Currents: Compared to CCM, CRM has higher root-mean-square (RMS) currents in the inductor and MOSFET, which can impact component sizing and conduction losses.
* Audible Noise Risk: At very light loads, the switching frequency can drop into the audible range (<20kHz), potentially causing audible noise from magnetic components.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pit
