PFC + PWM Controller Combination# Technical Documentation: FAN4800C Power Factor Correction (PFC) Controller
Manufacturer : Fairchild Semiconductor (FSC)
## 1. Application Scenarios
### Typical Use Cases
The FAN4800C is a highly integrated, average-current-mode PFC controller designed for offline switch-mode power supplies (SMPS). Its primary function is to shape the input current waveform to match the input voltage waveform, thereby achieving a high power factor (typically >0.99) and low total harmonic distortion (THD). Typical use cases include:
* Active PFC Front-Ends : Serving as the control core for boost-type PFC pre-regulators in power supplies ranging from 100W to over 1000W.
* Single-Stage Conversion : In some designs, it can be configured to control both the PFC stage and a downstream PWM stage (like a forward or flyback converter) from a single IC, simplifying the overall architecture.
* Universal Input Voltage Supplies : Enabling power supplies to operate efficiently from wide AC input ranges (e.g., 85VAC to 265VAC) while maintaining compliance with international harmonic current standards such as IEC 61000-3-2.
### Industry Applications
* Computer & Server Power Supplies : Desktop PC, workstation, and server PSUs where high efficiency (80 Plus certification) and compliance with regulatory standards are mandatory.
* Industrial Power Systems : Power supplies for factory automation, motor drives, and telecommunications equipment requiring robust and reliable AC-DC conversion.
* Consumer Electronics : High-end audio/video equipment, gaming consoles, and large-format LED TV power supplies.
* Lighting : High-power LED drivers and ballasts for commercial/industrial lighting systems.
### Practical Advantages and Limitations
Advantages:
* High Integration : Combines a PFC controller and a PWM controller in one 8-pin package, reducing component count and board space.
* Average Current Mode Control : Provides inherently stable operation, excellent noise immunity, and low distortion of the input current.
* Critical Conduction Mode (CrM) Operation : The PFC stage operates at the boundary between continuous and discontinuous conduction mode, minimizing switching losses at high line and allowing the use of smaller magnetics.
* Internal Start-up Timer : Simplifies the design of the start-up sequence and protection circuitry.
* Low Start-up and Operating Current : Enhances light-load efficiency and reduces standby power consumption.
Limitations:
* Fixed Frequency for PWM Stage : The downstream PWM stage operates at a fixed frequency (half the oscillator frequency), which may not be optimal for all load conditions compared to variable-frequency controllers.
* Power Level Constraints : While versatile, its CrM PFC topology is most efficient in a medium power range (up to ~300W for single-phase). For very high power (>1kW), continuous conduction mode (CCM) controllers might offer better performance.
* Thermal Management : As an 8-pin DIP or SOIC package, its power dissipation capability is limited. Careful PCB layout for thermal relief is essential.
## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Poor Power Factor at Light Loads.
* Cause : The PFC stage may drop out of critical conduction mode.
* Solution : Ensure the current sensing network (`R_sense`) and multiplier/divider circuit components are accurately calculated. Verify operation across the full load range.
2. Pitfall: Excessive THD or Current Distortion at High Line Voltage.
* Cause : Inadequate compensation of the current loop or incorrect setting of the multiplier input.
* Solution : Properly design the compensation network for the current error amplifier (pins related
