ON Semiconductor NCP1351B, Application Note

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AND8344/D
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2
According to the US Department of Energy’s (DOE)
Energy Information Administration (EIA), by 2015
electronics products may account for 18% of total household
electricity demand – this will exceed lighting and appliances
as a percent of total residential electricity consumption. Part
of this is linked to the fact that TVs are ‘on’ more hours per
day. According to Nielsen Media Research (NMR), for the
September 2004 – September 2005 viewing season, the
average U.S. household was tuned into television an average
of 8 hours and 11 minutes per day. And this does not take into
account additional hours that a TV may be on due to the
usage of peripheral devices such as game consoles and
computers. As a result, effective November 30, 2008, a new
version of the ENERGYSTAR
®
standard for TVs will go into
effect that maintains the < 1 W standby power requirement
and adds active mode power consumption based on screen
size and format (normal or full High Definition). This
standard is technology neutral and applies to all TV displays
including LCD, Plasma, and rear projection.
Active mode has been included because flat panel
televisions being purchased by consumers can consume
more than twice the active mode power of the smaller CRT
televisions that they are replacing. Much of this increase in
power consumption is simply attributable to the increased
size of the products being sold now.
One of the key differentiating factors of a flat TV over
CRT TV is that the cabinet can be very thin. Unfortunately
since the amount of power can be high, the power density
(W/cm
3
) is much higher than CRT. Moreover since TVs are
used in the living room, audible noise can be a problem, so
the use of fans is limited. Finally due to the high density and
close proximity of audio amplifiers, power supply and
signal processing within the cabinet, excellent EMI
performance is necessary.
Region
Country
Program
Name
Requirements
for Televisions
Demoboard
Compliance
China CSC 3 W Yes
Korea Energy
Saving
3 W Yes
European
Union
EU
Eco−Label
1 W
9 W with a STB
Yes
European
Union
EU Code
of Conduct
3 W with a STB Yes
Europe GEEA 1 W Yes
US 1 Watt
Executive
Order/
ENERGY
STAR
1 W Yes
As a result high efficiency and a low EMI signature at
a reasonable cost are required. Classical topologies are not
ideal for meeting these needs:
•
Flyback: Transformer Usage is Far from Pptimal
•
Forward: the EMI Signature is Not Reduced to its
Minimum
Architecture Overview
First, the use of active power factor correction in the
front-end allows system optimization because the PFC
output voltage is well regulated. The implementation of the
active PFC front end is done using the NCP1606 controller.
The SMPS stage uses a Half Bridge Resonant LLC topology
since it improves efficiency, reduces EMI signature and
provides better magnetic utilization compared to
conventional topologies. The NCP1392 controller is used to
implement the Half Bridge Resonant LLC converter. For the
standby output circuit, a fly back topology driven by the
NCP1351 has been chosen. In summary, the architecture
selected for this reference design allows design optimization so
that the desired performance is achieved without significantly
increasing the component costs and circuit complexity.
Demoboard Specification
LCD TVs require various voltages to power different
parts of the TV. The most power (24 V at 6 A) is used for
backlighting. The 12 V rail is used for the audio amplifier
and it is also used to power the signal processing board.
These two rails are provided from the LLC power supply.
Most of the drivers and processors in the LCD TV have their
own DC/DC converters to convert voltage from the main
SMPS to the appropriate voltage. These DC/DC converters
and linear regulators are powered from the 5 V and 12 V
rails. In this application, there are two 5 V rails. One is used
for the standby power and the other 5 V rail is active only
when the main LLC is on.
The parameters required for this switched mode power
supply (SMPS) are as follows:
Requirement Min Max Unit
Input Voltage (ac) 85 265 V
Output Voltage 1 (dc) − 24 V
Output Current 1 0 6 A
Output Voltage 2 (dc) − 12 V
Output Current 2 0 3 A
Output Voltage 3 (dc) − 5 V
Output Current 3 0 2 A
Output Voltage STBY (dc) − 5 V
Output Current STBY 0 2 A
Total Output Power 0 200 W
Consumption for a 500 mW
Output Load in STBY Mode − 1 W
Consumption for a 100 mW
Output Load in STBY Mode − 400 mW
The NCP1392 contains the following features which are
ideal for this application:
Brown-Out (BO) Protection Input
This pin has two functions. First, the BO permanently
monitors the bulk voltage and ensures the SMPS works in
the proper V
bulk
range. The second function is activated if
this pin is pulling up to 2 V which stops all output switching.