Highly-Integrated Green-Mode PWM Controller# Technical Documentation: FAN6755 PWM Controller for Offline Quasi-Resonant Flyback Converters
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN6755 is a highly integrated pulse-width modulation (PWM) controller IC specifically designed for offline quasi-resonant (QR) flyback converters . Its primary function is to regulate output voltage in isolated power supplies while optimizing efficiency across varying load conditions.
Primary applications include:
- AC/DC Adapters and Chargers : Particularly for consumer electronics (laptops, monitors, printers) requiring 30W to 150W output power
- Auxiliary Power Supplies (AUX) : For industrial equipment, appliances, and telecommunications systems
- LED Driver Circuits : For constant-voltage or constant-current LED lighting applications
- Set-Top Boxes and TV Power Supplies : Where standby power consumption is critical
### 1.2 Industry Applications
- Consumer Electronics : Power adapters for portable devices, gaming consoles, and home entertainment systems
- Industrial Automation : Control system power modules, sensor network power supplies
- Telecommunications : Power for routers, modems, and network switches
- Lighting Industry : Commercial and residential LED driver modules
### 1.3 Practical Advantages and Limitations
Advantages:
- High Efficiency : Quasi-resonant operation reduces switching losses by enabling valley switching, achieving efficiencies up to 92% at full load
- Low Standby Power : Burst mode operation reduces consumption below 100mW at no-load conditions
- Integrated Protection : Comprehensive protection features including over-voltage protection (OVP), over-load protection (OLP), over-temperature protection (OTP), and brown-out protection
- Frequency Jittering : Reduces EMI emissions, simplifying filter design and compliance with EMC standards
- Wide VDD Range : Operates from 8V to 30V, accommodating various input voltage conditions
Limitations:
- Power Range Constraint : Optimized for medium power applications (typically <150W); not suitable for high-power (>250W) or very low-power (<5W) applications
- QR Mode Limitations : At very light loads, the controller transitions to burst mode, which may cause audible noise in certain transformer designs
- External Component Dependency : Performance heavily depends on proper selection of external components (transformer, optocoupler, feedback network)
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Transformer Design Inadequacies
- Problem : Improper transformer design causing excessive leakage inductance, leading to voltage spikes and reduced efficiency
- Solution : Implement proper interleaving techniques in transformer winding, maintain tight coupling between primary and secondary windings, and use appropriate snubber circuits
Pitfall 2: Feedback Loop Instability
- Problem : Oscillations in output voltage due to poorly compensated feedback network
- Solution : Carefully design Type II or Type III compensation network based on crossover frequency and phase margin requirements (typically 1/5 to 1/10 of switching frequency)
Pitfall 3: Thermal Management Issues
- Problem : Overheating of controller or power MOSFET during continuous operation
- Solution : Ensure adequate PCB copper area for heat dissipation, consider thermal vias under the IC, and verify MOSFET selection with proper derating
Pitfall 4: EMI Compliance Challenges
- Problem : Failure to meet conducted or radiated EMI standards
- Solution : Utilize the built-in frequency jittering feature, implement proper input filtering, maintain minimal loop areas in high-current paths, and use shielded transformers
### 2.2 Compatibility Issues with Other Components
Optocoupler Selection:
- The F
