GREEN MODE PWM CONTROL IC# Technical Documentation: FAN7601 Green-Mode PWM Controller
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN7601 is a current-mode PWM controller IC specifically designed for offline switch-mode power supplies (SMPS). Its primary use cases include:
* AC/DC Power Converters : Used in flyback converter topologies for low-to-medium power applications (typically up to 30W)
* Standby Power Supplies : Provides efficient power conversion in standby circuits of larger systems
* Adapter/Charger Circuits : Commonly implemented in wall adapters for consumer electronics (laptops, monitors, routers)
* Auxiliary Power Supplies : Serves as the control IC for auxiliary power rails in industrial equipment and appliances
### 1.2 Industry Applications
* Consumer Electronics : Power supplies for LCD monitors, set-top boxes, audio equipment, and small appliances
* Computer Peripherals : External hard drive enclosures, printer power supplies, and networking equipment
* Industrial Controls : Low-power auxiliary supplies for PLCs, sensors, and control panels
* Lighting Systems : LED driver power supplies and ballast control circuits
* Office Equipment : Fax machines, scanners, and small copiers
### 1.3 Practical Advantages and Limitations
Advantages:
* Green-Mode Operation : Implements frequency reduction and burst mode operation at light loads, significantly reducing standby power consumption (typically <100mW at no load)
* Built-in Protections : Includes over-voltage protection (OVP), over-load protection (OLP), and over-temperature protection (OTP) with auto-restart functionality
* Low Startup Current : Typically 30μA, enabling smaller startup circuit components
* Integrated High-Voltage Startup : Built-in 700V startup switch reduces external component count
* Fixed 67kHz Switching Frequency : Eliminates audible noise in normal operation while maintaining good efficiency
Limitations:
* Fixed Frequency : Cannot be synchronized to external clocks, limiting use in noise-sensitive applications requiring frequency synchronization
* Maximum Power Constraint : Optimized for applications below 30W; higher power designs require additional circuitry or different controllers
* Minimum Load Requirements : May exhibit instability at extremely light loads (<1% of rated power) without proper compensation
* Temperature Constraints : Maximum junction temperature of 150°C may require thermal management in high-ambient environments
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Transformer Saturation
* Problem : Improper transformer design causing saturation during startup or load transients
* Solution : Ensure proper core selection with adequate margin, implement soft-start circuitry, and verify maximum flux density calculations
Pitfall 2: EMI Compliance Issues
* Problem : Radiated and conducted emissions exceeding regulatory limits
* Solution : Implement proper input filtering, use snubber circuits on transformer primary, maintain tight component placement, and consider adding common-mode chokes
Pitfall 3: Light Load Instability
* Problem : Oscillation or audible noise during green-mode operation
* Solution : Optimize feedback compensation network, ensure proper optocoupler selection with adequate bandwidth, and verify minimum load requirements
Pitfall 4: Startup Failures
* Problem : Insufficient startup current or voltage causing failed initialization
* Solution : Verify VCC capacitor sizing (typically 10-22μF), ensure proper startup resistor calculation, and check for excessive leakage in startup circuit
### 2.2 Compatibility Issues with Other Components
Optocoupler Selection:
* Requires optocouplers with CTR (Current Transfer Ratio) between 80-160% for stable operation
* Recommended: PC817, LTV817, or equivalent with bandwidth
