Dual-Output, Critical Conduction Mode PFC Controller# Technical Documentation: FAN7528 Critical Conduction Mode PFC Controller
Manufacturer : FSC (Fairchild Semiconductor, now part of ON Semiconductor)
Component Type : Critical Conduction Mode (CRM) Power Factor Correction (PFC) Controller IC
Primary Function : Provides control for a boost converter to achieve high power factor and low total harmonic distortion (THD) from AC line input.
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## 1. Application Scenarios (≈45% of Content)
### Typical Use Cases
The FAN7528 is specifically designed for offline AC-DC power supplies requiring active power factor correction to meet regulatory standards. Its primary use case is as the control core for a boost-type PFC pre-regulator stage.
* Standalone PFC Front-End: It drives an external MOSFET to shape the input current waveform to follow the input voltage waveform, typically boosting the rectified line voltage to a stable DC bus voltage (e.g., 385V) for a downstream DC-DC converter.
* Critical Conduction Mode Operation: The controller operates at the boundary between continuous and discontinuous conduction modes (CrM/TM). This ensures the inductor current reaches zero at the end of every switching cycle, allowing for zero-current switching (ZCS) of the boost diode. This minimizes diode reverse recovery losses, improves efficiency, and reduces EMI.
### Industry Applications
1. Switched-Mode Power Supplies (SMPS): Ubiquitous in power supplies for:
* IT & Consumer Electronics: Desktop PC power supplies (>80 PLUS certification), LCD TV/monitor power boards, gaming consoles, and audio amplifiers.
* Industrial Equipment: Power modules for motor drives, automation controllers, and test/measurement instruments where clean input current is mandated.
2. LED Lighting Drivers: Medium-to-high power LED drivers (e.g., street lights, high-bay industrial lighting) where high PF and efficiency are critical for performance and compliance.
3. Adapter/Charger Applications: Higher-power AC-DC adapters for laptops, all-in-one PCs, and small servers.
### Practical Advantages and Limitations
Advantages:
* High Power Factor: Enables designs to easily achieve PF >0.95, complying with standards like IEC 61000-3-2.
* Inherent ZCS: Eliminates boost diode turn-off losses, leading to higher system efficiency, especially at high line voltages.
* Simple Design: CrM operation simplifies control loop compensation compared to Continuous Conduction Mode (CCM) PFC controllers. The variable switching frequency is a natural characteristic.
* Integrated Features: Includes key protection features (over-voltage protection - OVP, open-loop protection, undervoltage lockout - UVLO) and a precision internal multiplier, reducing external component count.
* Low Start-up Current: Minimizes losses in the start-up circuitry.
Limitations:
* Variable & High Frequency: Switching frequency varies with line voltage and load, reaching its maximum at the peak of the AC line and at light loads. This complicates EMI filter design, as the filter must be effective over a wide frequency range.
* High Peak Currents: For a given output power, CrM operation results in higher peak inductor and switch currents compared to CCM. This necessitates components with higher current ratings, potentially increasing cost and size.
* Power Range: Best suited for low-to-medium power applications (typically up to 300W-400W). At very high power, high peak currents and frequency variation become significant drawbacks.
* Audible Noise Risk: The variable frequency can, under certain light-load conditions, enter the audible range (<20kHz), causing inductor or capacitor whine if mechanical design is inadequate.
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## 2. Design Considerations (≈35% of Content)
### Common Design
