AN-6858 Datasheet, PDF (1/7 Pages) Fairchild Semiconductor – Control a Flyback Power Supply

www.fairchildsemi.com
AN-6858
Applying SG6858 to Control a Flyback Power
Supply
Summary
This application note describes a detailed design strategy for
a high-efficiency, compact flyback converter. Design
considerations, mathematical equations, and guidelines for
printed circuit board layout are presented.
Features
ƒ Green-mode PWM
ƒ Supports the “Blue Angel” standard
ƒ Low start-up current (10µA)
ƒ Low operating current (2.5mA)
ƒ Leading-edge blanking
ƒ Constant output power limit
ƒ Universal input
ƒ Built-in synchronized slope compensation
ƒ Current-mode operation
ƒ Cycle-by-cycle current limiting
ƒ Under voltage lockout (UVLO)
ƒ Programmable PWM frequency
ƒ V_DD over-voltage protection (auto restart)
ƒ Gate output voltage clamped at 17V
ƒ Low cost
ƒ Few external components required
ƒ Small SOT-26 and DIP-8 packages
Description
This highly integrated PWM controller provides several
special enhancements designed to meet the low standby-
power needs of low-power SMPS. To minimize standby
power consumption, the proprietary green-mode function
provides off-time modulation to linearly decrease the
switching frequency under light-load conditions. This green-
mode function enables the power supply to meet power
conservation requirements.
The BiCMOS fabrication process enables reducing the start-
up current to 10µA and the operating current to 2.5mA. To
further improve power conservation, a large start-up
resistance can be used. Built-in synchronized slope
compensation ensures the stability of peak current mode
control. Proprietary internal compensation provides a
constant output power limit over a universal line input range
(90V_AC to 264V_AC). Pulse-by-pulse current limiting ensures
safe operation even during short circuits.
To protect the external power MOSFET from being
damaged by excessive supply voltage, the SG6858’s output
driver is clamped at 17V. SG6858 controllers can be used to
improve the performance and reduce the production cost of
power supplies. The SG6858 replaces linear and RCC-mode
power adapters. It is available in 8-pin DIP and 6-pin SOT-
26 packages.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 10/9/08
www.fairchildsemi.com