CCFL BACK LIGHT IC# Technical Documentation: FAN7548GX Power Factor Correction (PFC) Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component Type : Critical Conduction Mode (CRM) Power Factor Correction Controller IC
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## 1. Application Scenarios (Approx. 45% of Content)
### Typical Use Cases
The FAN7548GX is a specialized controller IC designed for building Critical Conduction Mode (CRM) Boost 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 high Power Factor (PF > 0.99 typical) and low Total Harmonic Distortion (THD).
Primary Use Cases Include:
* AC-DC Switch-Mode Power Supplies (SMPS): Serving as the front-end PFC stage in power supplies requiring compliance with international harmonic current emission standards (e.g., IEC 61000-3-2).
* Standalone PFC Boost Converters: Converting universal AC input (85V~265V) to a regulated high-voltage DC bus (typically 380V~400V) for downstream DC-DC conversion stages.
* Lighting Systems: High-power LED drivers, HID ballasts, and fluorescent electronic ballasts where efficient power usage and regulatory compliance are critical.
* Industrial Motor Drives & UPS Systems: Improving the quality of input power and reducing stress on upstream components.
### Industry Applications
* Consumer Electronics: High-end desktop computers, gaming consoles, large-screen LCD/LED TVs, and audio amplifiers.
* IT & Telecom: Server power supplies, telecom rectifiers, and network equipment power modules.
* Industrial Equipment: Power supplies for factory automation, test & measurement instruments, and welding equipment.
* Appliances: High-power air conditioners, refrigerators, and washing machines with variable-speed drives.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: CRM operation ensures that the boost inductor current returns to zero every switching cycle (Zero Current Switching - ZCS at turn-on), minimizing switching losses in the MOSFET, especially at high line voltages.
* Simplified Design: The controller inherently regulates the input current to be sinusoidal. It requires fewer external components for current sensing compared to average current mode control.
* Built-in Protections: Integrates essential safety features like Over-Voltage Protection (OVP), Open-Loop Protection (OLP), and Under-Voltage Lockout (UVLO).
* Low Start-up Current: Allows the use of high-value start-up resistors, reducing standby power consumption.
* Multiplier-Free Architecture: Simplifies the control loop by eliminating the traditional analog multiplier, enhancing reliability.
Limitations:
* Variable Frequency Operation: The switching frequency varies inversely with the input voltage and load. It is highest at low line and full load, which can complicate EMI filter design. Frequency can become very high at light loads.
* Higher Peak Currents: For a given output power, the peak inductor current in CRM is higher than in Continuous Conduction Mode (CCM). This requires a MOSFET and boost diode with higher peak current ratings.
* Audible Noise Risk: At light loads, the switching frequency can drop into the audible range (<20kHz), potentially causing inductor/transformer audible noise.
* Power Range: Typically best suited for low to medium power applications (up to ~300W-400W). For very high power, CCM controllers may be preferred for lower peak currents.
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## 2. Design Considerations (Approx. 35% of Content)
### Common Design Pitfalls and Solutions
1. Pitfall: Inductor Saturation at High Line, Light Load.
* Cause:
