GREEN MODE PWM CONTROL IC# Technical Documentation: FAN7601GX Green-Mode PWM Controller
Manufacturer : FAIRCHILD (Now part of ON Semiconductor)
Component Type : Current-Mode PWM Controller for Offline SMPS
Primary Function : Provides control circuitry for flyback and forward converters in low-power switch-mode power supplies (SMPS), featuring green-mode operation for reduced standby power consumption.
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## 1. Application Scenarios
### Typical Use Cases
The FAN7601GX is specifically engineered for offline power supplies requiring high efficiency and low standby power. Its primary use cases include:
* AC/DC Adapters and Chargers : For consumer electronics such as laptops, monitors, routers, and set-top boxes. The green-mode function is critical for meeting international energy efficiency standards (e.g., ENERGY STAR, EU CoC) by minimizing no-load power consumption.
* Auxiliary/Standby Power Supplies : Within larger systems like desktop PCs, servers, and industrial equipment, where it provides the always-on +5VSB rail.
* Low-Power Open-Frame SMPS : Used in applications requiring outputs from 5W to approximately 30W, such as for small appliances, IoT hubs, and peripheral devices.
### Industry Applications
* Consumer Electronics : The dominant application area, driven by the global demand for energy-efficient wall adapters and internal power boards.
* Industrial Controls : For powering control logic, sensors, and communication modules where reliability and efficiency are paramount.
* Telecommunications : In powering customer premises equipment (CPE) like modems and routers.
* Lighting : For the driver circuits of some LED lighting systems requiring isolated, constant-voltage supplies.
### Practical Advantages and Limitations
Advantages:
* Green-Mode Operation : Automatically reduces switching frequency under light-load conditions, dramatically cutting switching losses and achieving standby power often below 100mW.
* Built-in Protections : Integrates several key protection features:
* Overload Protection (OLP) : Cycle-by-cycle peak current limiting.
* Over-Voltage Protection (OVP) : Monitors the auxiliary winding voltage.
* Internal Thermal Shutdown (TSD) : Protects the IC from excessive junction temperature.
* High Startup and Low Operating Current : Features a high-voltage startup cell (~500V) that allows quick startup from the high-voltage DC bus, and a low operating quiescent current, enhancing overall efficiency.
* Minimal External Components : The integrated design reduces BOM count, board space, and assembly cost.
Limitations:
* Power Range : Optimized for low-to-mid power levels. For outputs significantly above 30W, alternative controllers or topologies may be more suitable.
* Fixed Maximum Duty Cycle : The internal maximum duty clamp (typically ~72%) limits the minimum input voltage range for a given output, which must be considered during design.
* Frequency Jitter (Trade-off) : The frequency jittering feature, which reduces EMI, can introduce slight output voltage ripple at certain load conditions that may need filtering in noise-sensitive analog circuits.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Insufficient VDD Capacitance :
* Pitfall : Under heavy load transients or during startup, the VDD voltage can dip below the UVLO (Under-Voltage Lock-Out) threshold, causing the IC to reset or hiccup.
* Solution : Calculate and select the VDD capacitor (`Cvdd`) based on the IC's startup current, operating current, and the time required for the auxiliary winding to take over. A value between 10µF and 22µF is typical, with low ESR.
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