Versatile PWM Controller# Technical Documentation: FAN7554 PWM Controller IC
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN7554
Description : Low-Power Current-Mode PWM Controller for Offline Switching Power Supplies
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## 1. Application Scenarios
### Typical Use Cases
The FAN7554 is a specialized pulse-width modulation (PWM) controller IC designed primarily for offline flyback converters in low-to-medium power applications. Its integrated high-voltage startup circuit and low operating current make it particularly suitable for standby power supplies and auxiliary power units where efficiency at light loads is critical. Common implementations include:
* Fixed-Frequency Flyback Converters : The IC provides stable PWM control for discontinuous conduction mode (DCM) flyback topologies, commonly used in AC-DC adapters and chargers.
* Quasi-Resonant (QR) Flyback Converters : When combined with appropriate external timing and sensing circuitry, the controller can be configured for QR operation, reducing switching losses and electromagnetic interference (EMI) by enabling valley switching.
* Battery Charger Circuits : Its precise current-mode control and protection features are well-suited for constant-current/constant-voltage (CC/CV) charging stages in consumer electronics chargers.
### Industry Applications
* Consumer Electronics : Primary application domain. Used in AC-DC adapters for laptops, monitors, TVs, set-top boxes, and game consoles.
* Industrial Power Supplies : Provides control for low-power auxiliary rails (e.g., 5VSB, 12V) within larger industrial systems, motor drives, and automation equipment.
* LED Lighting Drivers : Employed in offline LED driver circuits requiring isolated, constant-current output.
* Office Equipment : Found in power supplies for printers, scanners, and networking devices like routers and modems.
### Practical Advantages and Limitations
Advantages:
* Integrated High-Voltage Startup : Eliminates the need for an external startup resistor, reducing power loss and simplifying the bill of materials (BOM).
* Low Operating Current : Typically below 3 mA, which enhances light-load and no-load efficiency, crucial for meeting modern energy efficiency standards (e.g., ENERGY STAR, EU CoC).
* Built-in Protections : Includes Overload Protection (OLP) , Over-Voltage Protection (OVP) via the feedback loop, and an internal Soft-Start function to limit inrush current.
* Current-Mode Control : Provides inherent cycle-by-cycle current limiting, simplified feedback loop compensation, and faster transient response compared to voltage-mode control.
* Frequency Modulation for EMI Reduction : The switching frequency decreases under light-load conditions, spreading the EMI spectrum and easing filter design.
Limitations:
* Limited Maximum Power : Best suited for applications below approximately 100W due to its current-handling and drive capabilities. For higher power, a secondary driver stage or a different controller is recommended.
* Fixed Internal Logic : Lacks programmability of parameters like switching frequency or protection thresholds, which are set by external passive components. This offers less flexibility than modern digital controllers.
* Sensitivity to Noise : As a current-mode controller, it is susceptible to noise on the current sense pin, which can cause subharmonic oscillation if slope compensation is not properly designed for duty cycles >50%.
* Heat Dissipation in Startup Circuit : The internal high-voltage startup transistor can generate heat during prolonged startup or abnormal conditions; PCB thermal design must account for this.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Subharmonic Oscillation:
* Pitfall : At duty cycles exceeding 50%, current-mode controllers can become unstable, manifest
