PFC + PWM Controller Combo# Technical Documentation: FAN4800A Power Factor Correction (PFC) Controller
Manufacturer : FSC (Fairchild Semiconductor, now part of ON Semiconductor)
## 1. Application Scenarios
### Typical Use Cases
The FAN4800A 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). Key use cases include:
* AC-DC Front-End Converters : Serving as the first stage in two-stage power supplies, where it provides a regulated high-voltage DC bus (typically 385-400V) from a universal AC input (85-265VAC).
* Single-Stage PFC Designs : In cost-sensitive applications, it can be configured in single-stage topologies that combine PFC and DC-DC conversion, though with some trade-offs in performance.
* Critical Conduction Mode (CrM) Operation : The controller is optimized for CrM (also known as Transition Mode or Boundary Conduction Mode), where the power switch turns on just as the inductor current reaches zero. This eliminates reverse recovery losses in the boost diode, improving efficiency.
### Industry Applications
* Computer Power Supplies : Desktop PC, server, and workstation power supplies requiring compliance with energy efficiency standards (e.g., 80 PLUS).
* Industrial Power Systems : Motor drives, automation equipment, and telecom rectifiers where stable, high-quality input power is necessary.
* Consumer Electronics : High-power adapters for gaming consoles, large LCD/LED TVs, and audio amplifiers.
* Lighting : High-intensity discharge (HID) lamp ballasts and high-power LED drivers.
### Practical Advantages and Limitations
Advantages:
* High Integration : Combines a PFC controller and a PWM controller (for the downstream DC-DC stage) in one 8-pin package, reducing component count and board space.
* Excellent PF and Low THD : Average-current-mode control inherently provides superior input current shaping compared to simpler methods like voltage-mode or peak-current-mode.
* Synchronized Operation : The internal PFC and PWM circuits are synchronized, with the PWM stage operating at exactly half the frequency of the PFC stage. This minimizes beat-frequency noise and simplifies EMI filtering.
* Protection Features : Includes over-voltage protection (OVP), under-voltage lockout (UVLO), and open-loop protection.
Limitations:
* Fixed Frequency Ratio : The 2:1 PFC-to-PWM frequency lock may not be optimal for all second-stage topologies or optimization goals.
* CrM Operation Constraints : While efficient, CrM operation leads to variable switching frequency, which complicates EMI filter design, especially at high line and light load conditions where frequency can rise significantly.
* Power Level : Best suited for medium power applications (approximately 100W to 500W). For very low or very high power, other controllers or modes (e.g., Continuous Conduction Mode) might be more appropriate.
## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Instability at Light Loads or High Line Voltage.
* Cause : In CrM, the switching frequency increases, potentially pushing the controller beyond its maximum specified frequency or causing control loop issues.
* Solution : Ensure the boost inductor is sized correctly to keep the maximum frequency within the IC's limit (typically ~300kHz for the PFC stage). Implement a frequency clamp circuit if necessary.
2. Pitfall: Excessive THD at Zero-Crossings of the AC Line.
* Cause : Dead
