Voltage Mode PWM Controller# Technical Document: FAN7556 PWM Controller IC
Manufacturer : FAIRCHILD (Now part of ON Semiconductor)
Component : FAN7556
Description : Monolithic IC for Current-Mode PWM Control
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## 1. Application Scenarios
### Typical Use Cases
The FAN7556 is a versatile pulse-width modulation (PWM) controller designed primarily for offline switch-mode power supplies (SMPS) . Its architecture supports both fixed-frequency current-mode control and free-running operation , making it suitable for various power conversion topologies.
* Primary Application : Flyback Converters in low to medium power ranges (typically up to 250W). Its integrated high-voltage startup circuit and current-sensing capability simplify design for AC/DC adapters, battery chargers, and auxiliary power supplies.
* Secondary Topologies : Can be configured for Forward Converters and Boost Converters , often used in power factor correction (PFC) pre-regulator stages, though it may require external circuitry for optimal PFC performance compared to dedicated controllers.
* Standby Power Supplies : Its low startup current (~30µA typical) and low operating current make it suitable for applications requiring high efficiency at light loads or in standby modes, such as in televisions, monitors, and home appliances.
### Industry Applications
* Consumer Electronics : Primary power supply for LCD/LED TVs, set-top boxes, DVD/Blu-ray players, and desktop computer standby power.
* Computer & Peripherals : Power supplies for printers, scanners, and external hard drives.
* Industrial Systems : Low-power auxiliary rails for motor drives, PLCs, and instrumentation.
* Lighting : Electronic ballasts for fluorescent lamps and drivers for LED lighting systems.
* Adapter/Charger Market : Universal input (85-265VAC) AC/DC adapters for laptops, monitors, and various portable devices.
### Practical Advantages and Limitations
Advantages:
* Integrated High-Voltage Startup : The HV pin allows direct connection to the rectified high-voltage DC bus, eliminating the need for a separate startup resistor network and reducing standby power consumption.
* Current-Mode Control : Provides inherent cycle-by-cycle current limiting, simplified feedback loop compensation, and faster response to line/load transients compared to voltage-mode control.
* Built-in Protections : Features include Undervoltage Lockout (UVLO) with hysteresis for reliable startup/shutdown, Overload Protection (OLP) via the current sense pin, and a Soft-Start function to reduce inrush current stresses.
* Adjustable Oscillator Frequency : External RC network allows frequency programming from approximately 1 kHz to 1 MHz, offering design flexibility for optimizing transformer size and efficiency.
Limitations:
* Maximum Duty Cycle Clamp : The internal maximum duty cycle is typically clamped (e.g., ~80%). This can limit its use in wide-input-range applications without careful transformer design.
* Peak Current Sensing : As a peak current-mode controller, it requires slope compensation for duty cycles >50% to prevent sub-harmonic oscillation, adding slight design complexity.
* No Integrated PFC : While usable in basic PFC stages, it lacks dedicated features like a multiplier for continuous conduction mode (CCM) PFC, making it less optimal than dedicated PFC controllers for high-performance requirements.
* Drive Capability : The totem-pole output is suitable for driving MOSFETs directly but may require a gate driver IC for very high-current or high-frequency MOSFETs.
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## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Sub-harmonic Oscillation at High Duty Cycles :
* Pitfall : Without slope compensation, operating above 50
