AN-4150 Datasheet, PDF (1/16 Pages) Fairchild Semiconductor – Design Guidelines for Flyback Converters

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Application Note AN-4150
Design Guidelines for Flyback Converters Using
FSQ-series Fairchild Power Switch (FPS™)
1. Introduction
Compared to conventional hard-switched converters with
fixed switching frequencies, the quasi-resonant converter
(QRC) topology is a very attractive alternative for power
supply designers. The increasing popularity of the QRC
approach is based on its ability to reduce electromagnetic
interference (EMI) while increasing power conversion
efficiency.
The FSQ-series FPS™ (Fairchild Power Switch) is an
integrated Pulse Width Modulation (PWM) controller and
Sense FET specifically designed for quasi-resonant off-line
Switch Mode Power Supplies (SMPS) with minimal external
components. Figure 1 shows the internal block diagram of
the FSQ-series. Compared with discrete MOSFET and PWM
controller solution, it can reduce total cost, component count,
size and weight, while simultaneously increasing efficiency,
productivity, and system reliability. The FSQ-series employs
an advanced control technique that allows converter to
operate with narrow frequency variation, while keeping the
quasi-resonant operation. When the converter operates in
discontinuous conduction mode (DCM), the controller finds
the valley of the drain voltage and turns on the MOSFET at
the minimum drain voltage. Meanwhile, the converter can
operate with fixed frequency when operating in continuous
conduction mode (CCM), which allows converter design as
simple as conventional PWM converters.
This application note presents practical design consider-
ations of a flyback converter employing the FSQ-series
FPS™. It covers designing the transformer, output filter, and
sync network; selecting the components; and closing the
feedback loop.
Sync
4
V_str
V_cc
Drain
5
2 678
FB
3
+
0.7V/0.2V
-
+
0.35/0.55V
V_CC
V_ref
V_Burst
-
I_delay
I_FB
3R
R
Soft-
Start
OSC
PWM
SQ
RQ
+
V_ref
V_CC good
-
8V/12V
Gate
driver
LEB
200ns
LPF
RC=80ns
6V
V_SD
Sync
6V
V_ovp
TSD
2.5μs time
delay
SQ
RQ
AOCP
V_cc good
Figure 1. Block Diagram of FSQ-Series
V_OCP
(1.1V)
1
GND
© 2006 Fairchild Semiconductor Corporation
FSQ-Series Rev. 1.0.0 10/23/06
www.fairchildsemi.com