Boundary Mode PFC Controller# Technical Documentation: FAN6961 PFC Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN6961 — Critical Conduction Mode (CrM) Power Factor Correction (PFC) Controller
Document Version : 1.0
Date : October 2023
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN6961 is a specialized integrated circuit designed as a power factor correction controller operating in Critical Conduction Mode (CrM) , also known as Transition Mode (TM). Its primary function is to shape the input current of AC-DC power supplies to follow the input voltage waveform, thereby achieving a high power factor (typically >0.95) and reducing harmonic distortion.
Primary Use Cases Include:
- Standalone PFC Pre-regulators : Positioned between the AC input rectifier and the main DC-DC converter stage in switched-mode power supplies (SMPS).
- LED Driver Power Supplies : Used in high-power LED lighting applications (e.g., street lights, stadium lighting) where efficient, high-PF drivers are mandated by regulations.
- Adapter/Charger Designs : For notebook PC adapters, server power supplies, and industrial battery chargers with output power typically ranging from 100W to 300W .
- Server & Telecom Power Systems : In 80 PLUS certified silver/gold/platinum efficiency power supplies for data centers and communication infrastructure.
### 1.2 Industry Applications
- Consumer Electronics : High-end audio/video equipment, gaming consoles, and large LCD/LED TV power supplies.
- Industrial Equipment : Motor drives, welding equipment, and automation control system power modules.
- IT & Communications : Power supplies for routers, switches, and base station amplifiers.
- Lighting Industry : High-bay industrial lighting, architectural lighting, and horticultural grow light drivers.
### 1.3 Practical Advantages and Limitations
Advantages:
- High Efficiency at Medium Power : CrM operation minimizes switching losses at lower currents, offering excellent efficiency in its target power range.
- Simplified Magnetics Design : The variable frequency operation reduces the size and cost of the boost inductor compared to fixed-frequency Continuous Conduction Mode (CCM) designs.
- Zero-Current Switching (ZCS) : The controller turns on the MOSFET at zero inductor current, eliminating turn-on losses and reducing EMI.
- Integrated Protections : Includes over-voltage protection (OVP), open-loop protection, and brown-out detection, enhancing system reliability.
- Low Start-up Current : (~25 µA typical) allows for simpler, lower-power start-up circuitry.
Limitations:
- Power Range Constraint : Practical for designs up to ~300W. At higher power levels, the high peak currents and variable frequency can lead to increased EMI filter size and MOSFET stress.
- Frequency Variation : The switching frequency varies with line voltage and load, complicating EMI filter design as it must attenuate over a wide frequency band.
- Audible Noise Risk : At light loads, the switching frequency can drop into the audible range (<20 kHz), potentially causing inductor/transformer whine.
- Input Current Distortion : Near zero-crossings of the AC line, CrM operation can cause slight distortion, which may not meet the strictest harmonic standards (e.g., IEC 61000-3-2 Class D) without careful design.
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## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
| Pitfall | Consequence | Solution |
| :--- | :--- | :--- |
| Inadequate Brown-Out Setting | PFC operates at low input voltage, causing overcurrent and potential failure. | Precisely set the brown-out threshold (via resistor divider on `BOP`/
