Power Factor Correction (PFC) Controller# Technical Documentation: FAN7527DTF Power Factor Correction Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN7527DTF
Description : Critical Conduction Mode (CRM) Power Factor Correction (PFC) Controller IC
Package : DTF (SOP-8)
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## 1. Application Scenarios
### Typical Use Cases
The FAN7527DTF is specifically designed as a controller for building active Power Factor Correction (PFC) pre-regulators in switch-mode power supplies (SMPS). Its primary function is to shape the input current drawn from the AC mains to closely follow the sinusoidal waveform of the input voltage, thereby achieving a high power factor (typically >0.95) and reducing harmonic current distortion.
Primary Circuit Topology : It operates in Critical Conduction Mode (CRM) , also known as Transition Mode (TM) or Boundary Conduction Mode (BCM). In this mode, the controlled MOSFET turns on exactly when the inductor current reaches zero. This eliminates the reverse recovery losses of the boost diode, simplifies control, and is highly efficient for medium-power applications.
### Industry Applications
1. Consumer Electronics Power Supplies :
* AC-DAdapters & Chargers : For laptops, monitors, gaming consoles, and TVs requiring compliance with international energy efficiency and harmonic standards (e.g., IEC 61000-3-2, ENERGY STAR).
* LED Driver Power Supplies : In high-brightness LED lighting systems (e.g., street lights, commercial fixtures) where high PF is required for grid compatibility and to maximize power utilization from the mains.
2. Industrial & IT Equipment :
* Server & Telecom Power Supplies : Used in front-end PFC stages of 80+ Bronze/Silver certified SMPS units for servers, routers, and base stations.
* Industrial Motor Drives & UPS Systems : As the input conditioning stage to improve overall system efficiency and reduce input current harmonics.
### Practical Advantages and Limitations
Advantages :
* High Power Factor : Enables easy compliance with harmonic current regulations.
* Simplified Design : CRM operation requires no input voltage sensing or complex multiplier circuitry, reducing external component count.
* Zero Current Switching (ZCS) : Turn-on of the MOSFET at zero inductor current minimizes switching losses and EMI noise.
* Internal Start-up Timer : Reduces standby power consumption by disabling the controller during fault conditions or light loads.
* Integrated Protections : Features include Over-Voltage Protection (OVP), Open-Loop Protection (OLP), and Under-Voltage Lockout (UVLO), enhancing system reliability.
Limitations :
* Power Range Constraint : CRM operation leads to variable, high-frequency switching (frequency inversely proportional to load). This makes it most suitable for low to medium power applications (typically up to 300W). At higher powers, switching losses and EMI management become challenging.
* Inductor Size : The peak current in CRM is roughly double the average current, requiring a larger, more expensive boost inductor compared to Continuous Conduction Mode (CCM) designs for the same power level.
* Ripple Current : Higher input current ripple requires more substantial input filtering to meet conducted EMI standards.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Unstable Output Voltage or Poor PF at Light Loads.
* Cause : The controller's internal minimum off-time (`t_OFF(MIN)`) forces a frequency clamp at light load. If the control loop is not properly compensated, it can cause instability.
* Solution : Carefully design the voltage feedback loop (pin 1: `INV`) using Type-2 compensation
