FAIRCHILD AN-6741 Flyback Power Supply Controller using SG6741
Summary
The SG6741 is a highly integrated PWM controller engineered for developing efficient and compact flyback power supplies. This technical manual provides comprehensive application guidance for electrical engineers, detailing the design strategy, advanced startup circuitry, and critical PCB layout considerations. Readers will learn how to leverage its proprietary "green mode" and built-in protections—such as cycle-by-cycle current limiting and frequency hopping—to achieve ultra-low standby power consumption while maintaining stable performance across wide AC input ranges.
Page 1 Text Content
www.fairchildsemi.com
AN-6741 Flyback Power Supply Control with the SG6741 Summary Description This application note describes a design strategy for a high- The highly integrated SG6741 series of PWM controllers efficiency, compact flyback converter. Design provides several features to enhance the performance of considerations, mathematical equations, and guidelines for a flyback converters. printed circuit board layout are presented.
To minimize standby power consumption, a proprietary green mode provides off-time modulation to linearly
Features decrease the switching frequency at light-load conditions.
To avoid acoustic-noise problems, the minimum PWM
High-voltage startup
frequency is set above 22KHz. This green mode enables the
Low operating current: 4m A
power supply to meet international power conservation
Linearly decreasing PWM frequency to 22KHz
requirements. With the internal high-voltage startup
Frequency hopping to reduce EMI emission
circuitry, the power loss due to bleeding resistors is also
Peak-current-mode control
eliminated. To further reduce power consumption, the
Cycle-by-cycle current limiting
SG6741 is manufactured using Bi CMOS process, which
Leading-edge blanking
allows an operating current of only 4m A.
Synchronized slope compensation Gate output maximum voltage clamp: 18V The SG6741 integrates an internal frequency hopping VDD over-voltage protection (OVP) function to reduce the EMI emission of a power supply with VDD under-voltage lockout (UVLO) minimal line filtering; while the built-in synchronized slope Internal open-loop protection compensation maintains a stable peak-current-mode control. Constant power limit (full AC input range) The proprietary internal line compensation ensures constant
output power limit over a wide AC input voltages, from 90VAC to 264VAC. The SG6741 provides many protection functions. In addition to cycle-by-cycle current limiting, the internal open-loop protection circuit ensures safety should an open- loop or output short-circuit failure occur. The PWM output is disabled until the voltage drops below the UVLO lower limit, then the controller starts again. As long as VDD exceeds about 26V, the internal OVP circuit is triggered.
© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.2.1 • 10/20/08
Page Summary Contents For FAIRCHILD AN-6741 Flyback Power Supply Controller using SG6741
Manual Details
| Brand | Fairchild |
|---|---|
| Pages | 8 |
| File Size | 301.52 KB |
| Published | May 19, 2026 |
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Frequently Asked Questions
How does the SG6741 minimize standby power consumption?
It utilizes a proprietary green mode that employs off-time modulation, linearly decreasing the PWM frequency down to 22KHz at light loads.
What happens if an open loop or short circuit occurs?
The internal open-loop protection (OLP) keeps the PWM output disabled until the fault is corrected, thus limiting input power.
What are the essential PCB layout considerations for this controller?
Creepage distances must be at least 5mm. Additionally, keep the area enclosed by the transformer auxiliary winding and capacitors small.
What minimum operating current can the SG6741 support?
The device is capable of supporting a low operating current as low as 4mA.