Flyback & Forward PWM Controllers# Technical Documentation: FAN7602 Green-Mode PWM Controller
## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN7602 is a current-mode PWM controller IC specifically designed for offline flyback converters in low-to-medium power applications. Its primary function is to regulate output voltage by controlling the switching of a power MOSFET.
Primary applications include:
- AC/DC Power Adapters : For consumer electronics (laptops, monitors, routers)
- Auxiliary Power Supplies (SMPS) : In industrial equipment, appliances, and computing systems
- Battery Chargers : For lead-acid and lithium-ion battery systems
- LED Driver Circuits : In constant-voltage LED power supplies
### 1.2 Industry Applications
Consumer Electronics : Widely used in power adapters under 75W due to integrated protections and green-mode operation for standby compliance.
Industrial Systems : Employed in control panel power supplies where reliability and compact design are critical.
Telecommunications : Powers networking equipment requiring stable auxiliary rails (12V, 5V, 3.3V).
Lighting Industry : Drives LED arrays in commercial lighting fixtures requiring constant voltage.
### 1.3 Practical Advantages and Limitations
Advantages:
- Green-Mode Operation : Reduces switching frequency under light loads, improving efficiency and meeting energy standards (ENERGY STAR, EU CoC).
- Integrated Protections : Includes over-voltage protection (OVP), over-load protection (OLP), and latch-off for fault conditions.
- Low Startup Current : Typically 25µA, enabling smaller startup circuit components.
- Frequency Jittering : Reduces EMI emissions, simplifying filter design.
- Wide VCC Range : Operates from 10V to 30V, accommodating various design topologies.
Limitations:
- Power Range : Optimized for <75W applications; higher powers require external components.
- Fixed Maximum Frequency : 67kHz typical limits high-density designs compared to newer >100kHz controllers.
- Minimum Load Requirement : May need pre-load in some configurations to maintain regulation at no-load.
- Thermal Considerations : Requires adequate PCB cooling in compact designs due to DIP-8/SOP-8 package constraints.
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Transformer Saturation
*Cause*: Improper current sensing or transformer design.
*Solution*: Use a current-sense resistor (Rcs) calculated per datasheet. Ensure transformer core has adequate margin (Bmax < 3000 Gauss for ferrite).
Pitfall 2: VCC Undervoltage Lockout (UVLO) Cycling
*Cause*: Insufficient VCC capacitor or high startup resistor value.
*Solution*: Size VCC capacitor (typically 22-47µF) to maintain voltage above UVLO threshold (typically 9V) until auxiliary winding takes over.
Pitfall 3: EMI Compliance Issues
*Cause*: Poor layout or missing snubber circuits.
*Solution*: Implement RC snubber across transformer primary. Use frequency jittering feature effectively by proper COMP pin compensation.
Pitfall 4: Overload Protection False Triggering
*Cause*: Noise on current sense pin or slow feedback response.
*Solution*: Place RC filter (1kΩ, 1nF) close to CS pin. Ensure optocoupler has adequate bandwidth (>5kHz).
### 2.2 Compatibility Issues with Other Components
MOSFET Selection : Compatible with most N-channel MOSFETs with Vds rating >600V for 85-265VAC inputs. Gate charge should be <30nC for direct driving.
Optocouplers : Requires standard
