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  ON Semiconductor Electronic Components Datasheet  

AND8460D Datasheet

Implementing a 12V / 240W Power Supply

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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
http://onsemi.com
APPLICATION NOTE
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 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.
EMI
Filter
90V – 265Vac
NCP1605
PFC
Controller
Frequency Clamped
Critical Conduction Mode
Power Factor Controller
NCP1397B
Resonant Controller
with builtin
Half Bridge Driver
Resonant Technology
for Increased
Efficiency and Lower EMI
Bias
circuitry
NCP4303B
SR controller
Synchronous Rectification
for improved efficiency
12V / 20A
NCP4303B
SR controller
TL431
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
1
Publication Order Number:
AND8460/D


  ON Semiconductor Electronic Components Datasheet  

AND8460D Datasheet

Implementing a 12V / 240W Power Supply

No Preview Available !

AND8460/D
Based on the above considerations, the following is the
required specifications of the SMPS reference design:
Table 1. DEMOBOARD SPECIFICATION
Requirement
Min Max Unit
Input voltage (ac)
90 265
V
Output voltage (dc)
12 V
Output current
0 20 A
Total output power
0 240 W
Consumption for a 500 mW
output load in STBY mode
1.7 W
Consumption for a 100 mW
output load in STBY mode
1.2 W
No load consumption SR
operating
870 mW
No load consumption SR
1W
turned off, no bypass Shottky
used
Load regulation
20 mV
The NCP4303A/B provides the following beneficial
features for SR implementation in an LLC power stage:
Precise Zero Current Detection with Adjustable
Threshold
The NCP4303 SR controller provides a default Zero
Current Detection (ZCD) threshold of 0 mV. A 100 mA
current source on the CS input allows the customer to
decrease this basic ZCD threshold by using a resistor in
series with the CS input. The turnoff current threshold can
therefore be precisely adjusted down to 0 A to maximize the
SR MOSFET conduction time. The result is optimized
system efficiency.
Typically 40 ns Turnoff Delay from Current Sense
Input to Driver Output
Once the CS input detects that the secondary current has
reached zero, it is necessary to turnoff the SR MOSFET as
fast as possible. The extremely low 40 ns propagation delay
of the NCP4303 assures that the SR MOSFET will be
turnedoff quickly, avoiding reverse current flow back into
the transformer winding from the secondary filtering
capacitor.
Automatic Parasitic Inductance Compensation Input
The high secondary RMS current in the LLC stage has a
high di(t)/dt product that can induce a high error voltage on
the parasitic inductances of the SR MOSFET package
(TO220 for instance). Parasitic error voltages shift the drain
to source voltage and affect the accuracy of the ZCD system.
As a result, the SR MOSFET is turnedoff prematurely and
efficiency is decreased. NCP4303 offers a method to
compensate for this effect via a special input that offsets the
ZCD comparator threshold with a compensation voltage.
Thanks to this feature, the ZCD comparator can perform
precise detection independent of the secondary current
di(t)/dt product. This technique allows the use of standard
leaded SR MOSFETs, which can reduce assembly process
costs (SMT MOSFETs usually require a more expensive
PCB and soldering).
Current Sense Pin Capability of 200 V
The high voltage capability of the CS pin allows for direct
connection to the SR MOSFET drain. This avoids the use of
a high impedance series resistor which would delay the CS
signal.
Disable Input to Enter Standby or Low Consumption
Mode
The trigger/disable input integrates two functions: 1st it
can be used to turnoff the SR MOSFET in Continuous
Current Mode applications (like CCM flyback).
2nd it can be used to switch the controller into standby
mode. The SR standby mode decreases SMPS power
consumption when the output is not loaded. Parallel
Schottky diodes can be used for conduction in this mode
rather than the SR MOSFETs.
Adjustable Minimum On and Off Times Independent of
VCC Level
Due to the various impedances in the application
(parasitic inductances and capacitances) spurious ringing
can occur after the SR MOSFET is turned on or off. To
overcome controller false switching due to this parasitic
ringing, the NCP4303 utilizes adjustable minimum on and
off times. The driver state cannot be changed during these
minimum periods. The duration of the minimum on time and
minimum off time can be adjusted independently of each
other and independent of the IC Vcc level.
5 A / 2.5 A Peak Current Sink / Source Drive Capability
The SR MOSFETs for high current applications usually
feature high input capacitance. The strong sink driver
capability of the NCP4303 decreases the turnoff time and
thus allows for optimized conduction time of the SR
MOSFET.
Operating Voltage Range Up to 30 V
The NCP4303 VCC input can be connected directly to the
application output voltage without any additional
preregulation. This feature simplifies driver
implementation and reduces application cost.
Gate Driver Clamp of Either 12 V (NCP4303A) or 6 V
(NCP4303B)
Some of today’s SR MOSFETs provide low channel
resistance for lower gate voltages (< 6 V). Thus it is
beneficial to clamp the driver voltage at a lower level and
reduce driving losses. This technique helps to maintain high
efficiency, especially under medium and light load
conditions. On the other hand, some MOSFETs still require
higher gate voltage. NCP4303A provides 12 V gate driver
clamp for these cases. Please refer to the datasheet for more
http://onsemi.com
2


Part Number AND8460D
Description Implementing a 12V / 240W Power Supply
Maker ON Semiconductor
Total Page 30 Pages
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