Highly Integrated Green- Mode PWM Controller # Technical Documentation: FAN6754MLMY Quasi-Resonant PWM Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN6754MLMY
Description : Monolithic Quasi-Resonant (QR) Current-Mode PWM Controller for High-Efficiency Switched-Mode Power Supplies (SMPS).
---
## 1. Application Scenarios
### Typical Use Cases
The FAN6754MLMY is specifically engineered as a high-performance, low-power quasi-resonant PWM controller. Its primary use cases include:
* Offline AC/DC Power Supplies : It serves as the core control IC in flyback converter topologies, converting high-voltage AC mains input (e.g., 85-265VAC) to isolated low-voltage DC output.
* Standby and Auxiliary Power Rails : Its low startup current (<10 µA) and low operating current make it ideal for providing the always-on +5VSB or +3.3VSB rails in desktop PC power supplies (ATX/SFX) and server PSUs.
* Adapter and Charger Designs : Commonly found in external power adapters for laptops, monitors, printers, and consumer electronics, as well as in battery chargers, due to its high efficiency across a wide load range.
* LED Driver Power Supplies : Used in constant-voltage (CV) or constant-current (CC) LED driver modules for lighting applications, benefiting from its precise control and protection features.
### Industry Applications
* Consumer Electronics : Power supplies for TVs, set-top boxes, gaming consoles, and audio equipment.
* Information Technology : ATX/SFX PC power supplies, server PSU auxiliary rails, and various peripheral adapters.
* Industrial Electronics : Low-to-medium power industrial control systems, sensor modules, and embedded computing.
* Lighting : LED drivers for commercial and residential lighting fixtures.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: Quasi-Resonant (QR) operation minimizes switching losses by turning on the power MOSFET at the valley of the drain voltage, significantly improving efficiency, especially at light loads.
* Low Standby Power: Meets stringent energy efficiency standards (e.g., ENERGY STAR, CoC, DoE Level VI) thanks to Burst Mode operation at light loads and ultra-low startup/operating currents.
* Robust Protection Suite: Integrates multiple protections including Overload Protection (OLP), Over-Voltage Protection (OVP), Over-Temperature Protection (OTP) via external NTC, and an open-loop protection.
* Frequency Jittering: Built-in frequency modulation reduces peak EMI emissions, simplifying filter design and helping to meet EMI regulatory standards (FCC, CISPR).
* Minimal External Components: Integrates critical functions like a high-voltage startup cell and soft-start, reducing BOM count and board space.
Limitations:
* Topology Constraint: Primarily optimized for flyback converters. It is not suitable for forward, half-bridge, or LLC resonant topologies without significant external circuitry.
* Power Range: Best suited for low-to-medium power applications (typically up to 100W). For higher power levels, other controllers or topologies may be more efficient.
* QR Operation Dependency: The efficiency benefit of valley switching is dependent on transformer design and line/load conditions. At very light loads or high line voltage, it may transition to fixed-frequency operation, reducing the QR advantage.
* External Sense Resistor Required: Current sensing for cycle-by-cycle current limiting requires an external low-ohm resistor, which contributes to a small power loss.
---
## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Transformer Design Causing Unstable
