PWM AND SECONDARY COMBO IC# Technical Documentation: FAN7585SN Power Factor Correction (PFC) Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component Type : Critical Conduction Mode (CRM) Power Factor Correction Controller IC
Primary Function : Provides active power factor correction for AC-DC power supplies, improving efficiency and reducing harmonic distortion to meet regulatory standards like IEC 61000-3-2.
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## 1. Application Scenarios (Approx. 45% of Content)
### Typical Use Cases
The FAN7585SN is specifically designed as a controller for boost-type Power Factor Correction (PFC) pre-regulators. Its primary use case is to shape the input current drawn from the AC mains to be sinusoidal and in phase with the input voltage, thereby achieving a near-unity power factor (typically >0.95).
* Standalone PFC Front-End: It serves as the control core for a boost converter circuit placed between the bridge rectifier and the main DC-DC conversion stage (e.g., a flyback or LLC converter). It regulates a high-voltage DC bus (typically 380-400V) from a universal AC input (85-265VAC).
* Critical Conduction Mode (CRM) Operation: The IC operates the boost inductor in boundary conduction mode (transition between continuous and discontinuous modes). This minimizes switching losses at high line voltages and reduces reverse recovery issues in the boost diode, making it highly efficient for medium-power applications.
### Industry Applications
1. Switched-Mode Power Supplies (SMPS): Ubiquitous in power supplies requiring compliance with harmonic current limits.
* PC & Server Power Supplies: For Silver, Gold, or Platinum-rated PSUs.
* Industrial Power Systems: For machinery, motor drives, and automation equipment.
* Telecom & Networking Equipment: Routers, switches, and base station power modules.
2. Consumer Electronics:
* Flat-Panel TV & Monitor Power Boards: Especially in models above 32 inches.
* LED Lighting Drivers: For high-power commercial and industrial LED luminaires.
* Audio/Video Equipment: High-end amplifiers and receivers.
3. Appliances: Air conditioners, refrigerators, and other white goods with high-power motor drives or compressors.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: CRM operation reduces switching losses, leading to excellent efficiency across a wide input range.
* Simplified Design: Internal start-up timer and zero-current detection (ZCD) simplify control loop design.
* Robust Protection: Integrates comprehensive protection features (Over-Voltage, Under-Voltage, Over-Current, Open-Loop) enhancing system reliability.
* Reduced EMI: The variable frequency operation in CRM helps spread switching noise, easing EMI filter design.
* Low Component Count: Compared to fixed-frequency controllers, it can sometimes require fewer external components.
Limitations:
* Power Range Constraint: Best suited for medium power applications (typically up to 300-400W). At very low or very high power, efficiency or magnetic size may become suboptimal.
* Variable Frequency: The switching frequency varies with line voltage and load, which can complicate the design of certain magnetic components and requires careful consideration of frequency range for EMI.
* Higher Inductor Current Ripple: Peak currents are higher than in Continuous Conduction Mode (CCM), requiring a boost inductor with a higher peak current rating.
* Audible Noise Risk: At light loads, the switching frequency can drop into the audible range (<20kHz), potentially causing inductor or ceramic capacitor whine.
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## 2. Design Considerations (Approx. 35% of Content)
### Common Design Pitfalls and Solutions
