CCFL BACK LIGHT IC# Technical Documentation: FAN7548G Power Factor Correction (PFC) Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN7548G
Description : Critical Conduction Mode (CRM) Power Factor Correction Controller IC
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## 1. Application Scenarios
### Typical Use Cases
The FAN7548G is a specialized controller IC designed for building efficient, compact, and cost-effective Power Factor Correction (PFC) pre-regulator stages. Its primary function is to shape the input current drawn from an AC line to closely follow the sinusoidal waveform of the input voltage, thereby achieving a high Power Factor (PF > 0.99 typical) and low Total Harmonic Distortion (THD < 10% typical).
Primary Use Cases Include:
* AC-DC Switch Mode Power Supplies (SMPS): Serving as the front-end PFC stage in power supplies with output power ranging from approximately 100W to 300W. It converts the rectified but unfiltered AC input into a stable, high-voltage DC bus (typically 380-400V).
* LED Lighting Drivers: In medium-to-high-power LED drivers where compliance with harmonic current standards (like IEC 61000-3-2 Class C) is mandatory and high efficiency is critical.
* Adapter/Charger Systems: For laptop adapters, server power supplies, and industrial battery chargers that require universal input (85-265VAC) operation and must meet regulatory efficiency mandates (e.g., 80 PLUS).
### Industry Applications
* Consumer Electronics: High-end TVs, gaming consoles, audio amplifiers.
* IT & Telecom: Server PSUs, telecom rectifiers, PoE midspans.
* Industrial: Motor drives, automation control power supplies, test equipment.
* Lighting: Commercial and industrial LED luminaires, street lighting drivers.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: Critical Conduction Mode (Boundary Conduction Mode) operation minimizes switching losses, especially at high line voltages, leading to excellent efficiency across the load range.
* Simplified Design: The CRM operation inherently provides zero-current switching (ZCS) turn-on for the MOSFET, reducing stress and EMI. It does not require a current sense transformer; a simple sense resistor suffices.
* Integrated Functions: Includes essential protection features like Over-Voltage Protection (OVP), Open-Loop Protection (OLP), and Under-Voltage Lockout (UVLO), reducing external component count.
* Low Standby Power: The IC features a low startup current and can be designed for low quiescent power, aiding in meeting standby power regulations.
Limitations:
* Power Range: Optimized for low-to-medium power applications. For powers significantly above 300W, the variable, high-frequency operation at low line can lead to increased core losses in the inductor and higher EMI filter complexity. Continuous Conduction Mode (CCM) controllers are often preferred for >300W.
* Variable Frequency: The switching frequency varies with line voltage and load (highest at low line, full load). This complicates EMI filter design, as the filter must be effective across a wide frequency band.
* Input Current Distortion: Near the zero-crossings of the AC line, the CRM operation can cause slight distortion in the current waveform, potentially limiting the achievable THD compared to some CCM controllers under certain conditions.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Excessive THD at Light Loads.
* Cause: The controller's minimum off-time can cause the current to dip near the AC zero-crossings.
* Solution: Implement a small "pre-load
