AN-6203 Datasheet, PDF (1/8 Pages) Fairchild Semiconductor – Applying SG6203 to Control a Synchronous Rectifier

www.fairchildsemi.com
AN-6203
Applying SG6203 to Control a Synchronous Rectifier of a
Flyback Power Supply
Abstract
This application note describes a detailed design strategy for
a high-efficiency compact flyback converter. Design
considerations and mathematical equations are presented. A
12V, 120W evaluation board is built to evaluate the
performance improvement by the SG6203.
VCC
IN+
Output
IN-
GND
RT
VZ
DET
Figure 1. SG6203 Pin Configuration
Description
Generally, the conduction loss of the output diode is the
dominant loss component in low output voltage power
supplies. This loss can be reduced by using a low on
resistance MOSFET. Improved efficiency can be obtained
and a smaller heat sink can be used.
The SG6203 is designed to control and drive the
synchronous rectifier for the flyback converter. The
synchronous signal of the primary switch is obtained by a
single diode connected between the transformer secondary
winding and the SG6203. Using the SG6203, no additional
transformer winding is required and the circuit complexity
can be minimized.
In heavy load conditions, the flyback converter is usually
designed to operate in continuous conduction mode (CCM)
to have higher efficiency. To prevent the so called “shoot
through” or “cross conduction” problem in synchronous
rectifier applications, one external resistor and an internal
automatic tracking circuit is used to program a suitable dead
time for the primary switch and the synchronous rectifier.
On the other hand, the converter enters discontinuous
conduction mode (DCM) in light load conditions. To
improve light load efficiency, the synchronous rectifier is
turned off when the stored energy in the transformer is fully
released to the output. This feature can prevent reverse
energy flow and green-mode PWM IC in the primary side
can be used. To detect the secondary current flow and
determine the driving signal for the synchronous rectifier, a
current shunt or RC network for the output capacitor can be
used. An RC network is recommended for better efficiency.
Detailed operation and application information are presented
in the following section. A 12V, 120W evaluation board
using the SG6203 is built and the performance is measured
on this unit to show its improvement on the efficiency.
© 2006 System General Corporation, a subsidiary of Fairchild Semiconductor Corporation
(IAO37.0011.B1) Rev. 1.1.1 • 1/17/08
www.f