AND8460 Datasheet PDF - ON Semiconductor

Part Number AND8460
Description Implementing a 12V / 240W Power Supply
Manufacturers ON Semiconductor 
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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
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 AllInOne 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.
90V – 265Vac
Frequency Clamped
Critical Conduction Mode
Power Factor Controller
Resonant Controller
with builtin
Half Bridge Driver
Resonant Technology
for Increased
Efficiency and Lower EMI
SR controller
Synchronous Rectification
for improved efficiency
12V / 20A
SR controller
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 turnon and turnoff
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 Allinone 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 Allinone PC SMPS that usually do not utilize an
additional standby power supply.
© Semiconductor Components Industries, LLC, 2010
August, 2010 Rev. 2
Publication Order Number:

AND8460 equivalent
flowing through inductor L7 and switch Q4 is higher than the
maximum current limit level, the CS pin current increases
above the OPC threshold (250 uA) and the driver is turned
off. The CS input is also used to detect coil demagnetization
for zero current detection. The zero current detection
prevents the MOSFET from turning on when current flows
through the coil. As long as there is no coil current, the
NCP1605 operates at a frequency determined by the internal
oscillator and external capacitor C38. Zero current detection
circuitry sensitivity is adjusted by resistor R70 and R81.
To protect the PFC from sudden drops in the line voltage,
the controller monitors the rectified line voltage via
brownout divider R15, R23, R31, R50, R71 and C39.
The driver output is connected to MOSFET Q4 via
resistors R25, R26 and diode D7 to regulate turnon speed.
Transistor Q7 is used to speed up the MOSFET turnoff time
and thus reduce turnoff losses.
Please refer to the application note AND8281/D for
detailed information on the PFC stage design and operation.
Figure 4. The LLC Stage Primary Side Connection
LLC Power Stage Primary Side
Primary Side Power Loop Connection
The PFC stage prepares a regulated voltage on bulk
capacitors C16 and C17 for the downstream LLC stage (refer
to Figure 3). The LLC stage power loop is closed through Q3
and Q5, transformer TR1 and resonant capacitors C7, C18
(Figure 4). The NCP1397 LLC controller features a 600 V
highside driver and is capable of driving the HB power
stage directly without the use of a driver transformer.
Resistors R54 and R55 are used to suppress ringing and
control EMI noise on the power MOSFET gates. Bootstrap
capacitor C53 provides the energy required for controlling
the high side MOSFET. When Q5 is turnedon, the HB pin
voltage drops and bootstrap capacitor C53 is charged
through resistor R96 and highvoltage diode D23. At turnon
and after any restart, the LLC controller turns on MOSFET
Q5 first to charge up the bootstrap capacitor.
The PFC and LLC controllers are powered from the
auxiliary winding W4 of transformer TR1. The PFC
controller charges up the VCC capacitors C3, C42 first when
the demoboard is plugged into the mains. Once the PFC
stage starts operation and the bulk voltage is within the
nominal operating range, the LLC stage is enabled. The
auxiliary winding also provides bias voltage for the X2
capacitor discharge circuitry via diode D1, resistor R1 and
capacitor C1. The X2 capacitor discharge circuitry is
described in the PFC Stage section (refer to page 3).
FB Loop and Skip Mode:
The minimum operating frequency of the LLC converter
is set by resistor R104 (refer to Figure 5). The maximum
operating frequency is set by resistor R102. The LLC stage


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