Highly-Integrated Green-Mode PWM Controller # Technical Documentation: FAN6751MRMY Quasi-Resonant PWM Controller
Manufacturer : FAIRCHILD Semiconductor (On Semiconductor)
Component Type : Quasi-Resonant Current-Mode PWM Controller for Offline Power Supplies
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## 1. Application Scenarios
### Typical Use Cases
The FAN6751MRMY is a highly integrated pulse-width modulation (PWM) controller IC designed for high-efficiency, low-standby-power switch-mode power supplies (SMPS). Its primary use cases include:
* AC/DC Power Adapters and Chargers: For consumer electronics such as laptops, monitors, printers, and networking equipment. Its quasi-resonant (QR) operation enables high efficiency across a wide load range, meeting stringent energy standards like ENERGY STAR and CoC.
* Open-Frame Switching Power Supplies: Used in industrial control systems, appliance power boards, and auxiliary power modules. The built-in protection features enhance system reliability.
* LED Driver Power Supplies: Particularly in constant-voltage LED drivers where high power factor and low total harmonic distortion (THD) are beneficial, supported by the controller's inherent frequency modulation characteristics.
* Set-Top Boxes and TV Power Supplies: Applications requiring low audible noise (due to valley switching) and good dynamic response.
### Industry Applications
* Consumer Electronics: The dominant application area, driven by the need for compact, efficient, and reliable external power supplies (EPS).
* Industrial Electronics: For control logic power where robustness and wide input voltage range are critical.
* Telecommunications/Networking: Powering routers, modems, and other equipment requiring 24/7 operation with low standby loss.
* Lighting: Entry to mid-level LED lighting drivers where cost-effective quasi-resonant topology is preferred over more complex LLC designs.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: Quasi-resonant operation reduces switching losses by turning on the power MOSFET at the valley of the drain voltage (valley switching). This significantly improves efficiency, especially at light loads.
* Low Standby Power: Features like Burst Mode operation minimize switching frequency and associated losses under no-load or light-load conditions, easily achieving <75mW standby targets.
* Reduced EMI: The valley switching and frequency jittering function help spread the EMI spectrum, simplifying filter design and reducing conducted EMI.
* High Integration: Integrates critical functions like a high-voltage startup cell, soft-start, comprehensive protection (Overload Protection - OLP, Over-Voltage Protection - OVP, Over-Temperature Protection - OTP via external NTC), and a frequency clamp. This reduces external component count and board space.
* Robust Protection Suite: Enhances system reliability by protecting against fault conditions.
Limitations:
* Load Range Dependency: The benefits of valley switching diminish at very light or very heavy loads, potentially leading to higher switching losses at these extremes compared to some fixed-frequency or full-resonant topologies.
* Variable Frequency Operation: The switching frequency varies with line voltage and load. This can complicate the design of some noise-sensitive circuits or magnetics optimization compared to fixed-frequency controllers.
* Power Level: Typically optimized for low to mid-power applications (up to ~150W). For higher power levels, other topologies like LLC may offer better performance.
* Design Complexity: While integrated, designing a stable QR converter requires careful transformer design and loop compensation to manage the variable frequency characteristic.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Audible Noise in Burst Mode.
* Cause: The controller entering and exiting Burst Mode at a low, audible frequency.
* Solution: Adjust the feedback network (
