1W CMOS Power Amplifier# Technical Documentation: FAN7021M PWM Controller
Manufacturer: FAIRCHILD (ON Semiconductor)
Component: FAN7021M
Type: High-Voltage Quasi-Resonant PWM Controller IC
Primary Function: Provides control for offline flyback converters in switch-mode power supplies (SMPS).
---
## 1. Application Scenarios
### Typical Use Cases
The FAN7021M is specifically engineered for offline AC/DC power conversion in low-to-medium power applications. Its core function is to drive a power MOSFET in a quasi-resonant (QR) flyback topology. Key use cases include:
* Standby and Auxiliary Power Supplies: Providing low-power, always-on rails (e.g., +5VSB, +12V) in desktop PCs, servers, and appliances.
* Adapter/Charger Circuits: For consumer electronics such as laptops, monitors, routers, and set-top boxes, typically in the 30W to 90W range.
* LED Driver Power Stages: Acting as the primary-side controller for constant-voltage/constant-current LED drivers in lighting applications.
* Household Appliance Control Boards: Powering the logic and display circuits in white goods like refrigerators, air conditioners, and washing machines.
### Industry Applications
* Consumer Electronics: The dominant application area due to demand for high-efficiency, compact, and low-cost adapters.
* Information Technology: Used extensively in PC power supplies (particularly for the standby circuit) and external power bricks for peripherals.
* Industrial Electronics: For powering sensors, controllers, and human-machine interfaces (HMIs) where reliable offline power is required.
* Lighting Industry: As the core of efficient LED driver modules for commercial and residential lighting fixtures.
### Practical Advantages and Limitations
Advantages:
* High Efficiency: Quasi-resonant operation minimizes switching losses by turning on the MOSFET at the valley of the drain voltage (valley switching), leading to higher efficiency, especially at light loads.
* Low Standby Power: Incorporates a proprietary "Green Mode" function that reduces switching frequency under light/no-load conditions, easily meeting stringent energy efficiency standards (<75mW typical).
* Integrated Protections: Includes comprehensive built-in safeguards:
* Overload Protection (OLP)
* Over-Voltage Protection (OVP)
* Over-Temperature Protection (OTP) via external NTC
* Leading-Edge Blanking (LEB) on current sense to prevent false triggering from noise.
* Reduced EMI: Valley switching and frequency jittering technology help lower conducted and radiated electromagnetic interference, simplifying EMI filter design.
* Minimal External Components: Integrates a high-voltage startup circuit and a precision reference, reducing bill-of-materials (BOM) count and board space.
Limitations:
* Topology Constraint: Designed specifically for flyback converters. It is not suitable for forward, half-bridge, or LLC resonant topologies.
* Power Range: Optimized for low-to-medium power. For outputs significantly above 100W, other topologies or controllers may offer better performance.
* Frequency Variation: The quasi-resonant operation leads to variable switching frequency across line and load conditions, which can complicate the design of the output filter and EMI filter compared to fixed-frequency controllers.
* Parameter Tuning: Optimal performance requires careful tuning of external components (e.g., feedback loop, current sense resistor) for stability and protection thresholds.
---
## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Audible Noise at Light Load.
* Cause: "Green Mode" operation can drive the switching frequency into the audible range
