Green-Mode PWM Control IC# Technical Documentation: FAN7601B Green-Mode PWM Controller
Manufacturer : FAIRCHILD (now part of ON Semiconductor)
Component : FAN7601B
Description : Current-Mode PWM Controller with Integrated High-Voltage Startup and Green-Mode Operation
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The FAN7601B is primarily designed for offline switch-mode power supplies (SMPS) requiring high efficiency and low standby power. Its integrated architecture makes it suitable for compact, cost-effective designs.
* Primary-Side Regulation (PSR) Flyback Converters : The controller is optimized for flyback topologies, commonly used in low-to-medium power applications (up to approximately 75W). It provides stable output voltage regulation without a secondary-side feedback optocoupler by sensing the auxiliary winding voltage, simplifying the design and reducing component count.
* Battery Chargers : Used in adapters for laptops, power tools, and consumer electronics where constant voltage (CV) and constant current (CC) charging profiles are required. Its current-mode control provides inherent cycle-by-cycle current limiting.
* Auxiliary/Standby Power Supplies : Its Green-Mode function is critical for meeting global energy efficiency standards (e.g., ENERGY STAR, EU CoC Tier 2). During light or no-load conditions, the switching frequency is reduced, dramatically cutting switching losses and lowering standby power consumption to well below 100mW.
### 1.2 Industry Applications
* Consumer Electronics : AC-DC adapters for LCD monitors, TVs, set-top boxes, gaming consoles, and small appliances.
* Computer & Peripherals : Power supplies for desktop computers (standby rail), printers, and external hard drives.
* Industrial Systems : Low-power auxiliary rails for industrial control systems, where reliability and efficiency are paramount.
* LED Lighting : Drivers for LED bulbs and fixtures, leveraging its constant-current capability.
### 1.3 Practical Advantages and Limitations
Advantages:
* High Integration : Incorporates a high-voltage startup circuit (typically ~500V), eliminating the need for an external startup resistor and reducing power loss.
* Low Standby Power : Green-Mode operation and low startup current enable designs to achieve <30mW no-load consumption.
* Robust Protection Suite : Includes built-in Overload Protection (OLP), Over-Voltage Protection (OVP) via the feedback pin, and an internal Soft-Start to minimize inrush current stress.
* Cost-Effective : Reduces Bill of Materials (BOM) by integrating startup, omitting the optocoupler (in PSR designs), and requiring minimal external components.
* Fixed 67kHz Switching Frequency : Simplifies EMI filter design while offering a good balance between magnetic size and switching loss.
Limitations:
* Power Range : Best suited for low-to-medium power applications. For high-power flybacks (>100W), other topologies or controllers may be more appropriate.
* Regulation Precision : Primary-side regulation is inherently less precise than secondary-side feedback using an optocoupler, especially with wide input voltage ranges or highly dynamic loads. Typical load regulation is around ±5%.
* Minimum Load : Some PSR designs may require a small minimum load to maintain stable regulation at very light loads.
* Frequency Variation : In Green-Mode, the variable frequency can make EMI filtering more challenging across the entire load range.
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## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
* Pitfall 1: Unstable Feedback Loop in PSR Design.
* Cause : Improper selection of the feedback network (resistors, capacitor on FB pin) or transformer auxiliary
