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© Semiconductor Components Industries, LLC, 2010
August, 2010 − Rev. 2
1 Publication Order Number:
AND8460/D
AND8460/D
Implementing a 12 V /
240 W Power Supply with
the NCP4303B, NCP1605
and NCP1397B
Prepared by: Roman Stuler, Jaromir Uherek and Ivan Seifert
ON Semiconductor
Overview
The following document describes a 12 V / 20 A output
switch mode power supply (SMPS) intended for use as an
ATX power supply main converter or as an All−In−One PC
power supply. The reference design circuit consists of a
double sided 135 x 200 mm printed circuit board with a
height of only 35 mm. An overview of the entire SMPS
architecture is provided in Figure 1. Careful consideration
was given to optimizing performance while minimizing the
total solution cost.
EMI
Filter
90V – 265Vac
Frequency Clamped
Critical Conduction Mode
Power Factor Controller
Resonant Technology
for Increased
Efficiency and Lower EMI
TL431
12V / 20A
NCP4303B
SR controller
NCP4303B
SR controller
NCP1397B
Resonant Controller
with built−in
Half Bridge Driver
Bias
circuitry
NCP1605
PFC
Controller
Synchronous Rectification
for improved efficiency
Figure 1. Demoboard Block Diagram
Architecture Overview
The circuit utilizes the NCP1605 for an active power
factor correction front end. This stage provides a well
regulated PFC output voltage that allows optimization of the
downstream converter. The NCP1605 controller operates
using a Frequency Clamped Critical conduction Mode
control technique. The SMPS stage uses a Half Bridge
Resonant LLC topology since it improves efficiency,
reduces EMI signature and provides better transformer
utilization compared to conventional topologies. The
NCP1397B controller is used to control the Half Bridge
Resonant LLC converter. To maximize efficiency of the
LLC power stage, Synchronous Rectification (SR) has been
implemented on the secondary side. The NCP4303B SR
controller is used to achieve accurate turn−on and turn−off
of the SR MOSFETs.
In summary, the architecture selected for this reference
design allows system optimization so that the maximum
efficiency is achieved without significantly increasing the
component cost and circuit complexity.
Demoboard Specification
Most of today’s computing applications like ATX PC,
game consoles and All−in−one PC use 12 V as the main
power rail. This voltage is then further decreased to 5 V and
3.3 V by DC/DC step down converters. Because nearly all
power passes through the 12 V output, it is critical that the
efficiency of the main power stage be optimized. Most
designs today utilize an LLC topology for the power stage
to provide high efficiency at a reasonable cost. The LLC
power stage provides inherently high efficiency results
thanks to zero voltage switching (ZVS) on the primary side
and zero current switching (ZCS) on the secondary side.
Efficiency however decreases for higher output currents as
the secondary RMS current reaches a high level. The
solution for these losses on the secondary side is to use
synchronous rectification instead of conventional rectifiers
(Schottky diode). Consideration was also give to optimizing
light and no load efficiency, which is particularly important
in All−in−one PC SMPS that usually do not utilize an
additional standby power supply.
APPLICATION NOTE
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