Controller for Half-Bridge Resonant Converters# Technical Documentation: FAN7621 PWM Controller IC
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN7621 is a high-performance pulse-width modulation (PWM) controller IC designed primarily for switch-mode power supply (SMPS) applications. Its architecture supports both fixed-frequency voltage-mode control and quasi-resonant (QR) operation , making it versatile for various power conversion topologies.
Primary applications include:
- Flyback converters for AC/DC power supplies (30W-150W range)
- Forward converters in industrial power systems
- LED driver circuits requiring precise current regulation
- Adapter/charger designs for consumer electronics
- Auxiliary power supplies in larger systems (e.g., server PSUs, telecom equipment)
### 1.2 Industry Applications
Consumer Electronics:
- LCD/LED TV power supplies
- Laptop adapters and desktop PC standby supplies
- Printer/scanner power modules
- Gaming console power systems
Industrial/Commercial:
- Industrial control system power modules
- Telecom rectifiers and DC/DC converters
- Medical equipment power supplies (with appropriate safety certifications)
- Lighting ballasts and LED drivers for commercial lighting
Renewable Energy:
- Micro-inverter auxiliary power supplies
- Solar charge controller circuits
### 1.3 Practical Advantages and Limitations
Advantages:
- High efficiency (typically >85% at full load) through QR operation reducing switching losses
- Wide input voltage range (8.5V to 25V VCC) supporting universal input designs
- Built-in protection features including over-voltage protection (OVP), over-load protection (OLP), and over-temperature protection (OTP)
- Low standby power consumption (<100mW in no-load conditions)
- Frequency jittering reduces EMI emissions, simplifying filter design
- Soft-start function prevents inrush current during startup
Limitations:
- Maximum switching frequency limited to 130kHz, restricting ultra-compact designs
- Limited to voltage-mode control , potentially less responsive to load transients than current-mode alternatives
- External MOSFET required , increasing component count versus integrated solutions
- Minimum load requirements for stable operation in some configurations
- Thermal management critical in high-power applications due to package constraints
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Unstable Operation at Light Loads
- Problem: Oscillation or audible noise during light-load conditions
- Solution: Implement proper feedback compensation network and ensure adequate minimum load
Pitfall 2: Excessive EMI Emissions
- Problem: Failing conducted/radiated EMI tests
- Solution: Utilize built-in frequency jittering, implement proper snubber circuits, and follow layout guidelines
Pitfall 3: Thermal Runaway in High Ambient Temperatures
- Problem: Device overheating leading to premature failure
- Solution: Provide adequate PCB copper area for heat dissipation, consider external heatsinking if needed
Pitfall 4: Startup Failures with High-Capacitance Loads
- Problem: Insufficient startup current causing repeated restart cycles
- Solution: Adjust soft-start timing components and verify startup circuit design
### 2.2 Compatibility Issues with Other Components
MOSFET Selection:
- Ensure MOSFET gate charge (Qg) compatible with FAN7621's gate drive capability (typically 0.5A source/0.8A sink)
- Verify MOSFET VDS rating exceeds maximum reflected voltage with sufficient margin
- Consider MOSFET package thermal characteristics for target power level
Feedback Network Components:
- Optocoupler
