LED LCD TV POWER SUPPLY CIRCUIT PLDC-P015A - USING ICS - FA5591- L6599 –LNK362
PHILIPS PLDC-P015A
USING ICS - FA5591- L6599 –LNK362
LED LCD TV POWER SUPPLY CIRCUIT
CHASSIS
PHILIPS L11M1.1L
PHILIPS Q552.2L
MODELS
PHILIPS 40PFL5606D
PHILIPS 40PFL3606D
PHILIPS 32PFL3606D
SMPS
D.C. to DC convertor
and DC to AC converter belong to the category of switched mode power supply
(SMPS). The various types of
voltage regulator used in linear power supplies (LPS), fall in the category of
dissipative regulator, as they have a voltage control element usually
transistor or zener diode which dissipates power equal to the voltage
difference between an unregulated input voltage and a fixed supply voltage
multiplied by the current flowing through it. The switching
regulator acts as a continuously variable power converter and hence its
efficiency is negligibly affected by the voltage difference. hence the
switching regulator is also known as “non-dissipative regulator” in a SMPS, The input DC supply is chopped at a higher frequency around
15 to 50KHz using an active device like the BJT power MOSFET or SCR and the
convertor transformer
LED DRIVE WORKING
There are large arrays of LEDs located behind
the LCD panel in a typical LCD TV LED. In this array are a large number of
parallel channels of LEDs connected in series depending on the size of the TV
and the type of backlighting, for example edge backlighting (less LEDs but more
in series) or direct backlighting (more LEDs in parallel) . The LED voltage
(VLED) is provided by the White LED Backlight Driver Board to each LED channel
and is regulated to a level needed by the highest voltage required to maximize
the light output of each LED string . Depending upon the power supply
requirements determined by the number of LEDs in the string or grouping of
parallel LED strings, the up-stream power source for the LED backlight driver
board may be a DC/DC step-up boost converter, a DC/DC step-down converter or
more commonly an AC/DC converter . In the case where supply voltage is lower
than the required VLED, a step-up boost converter will be used . As an example,
a LED boost converter LED backlighting system will be described in detail in
this paper for a direct backlighting application, however the theory of
operation will also apply to both the step-down converter and AC/DC converter
situation .
High brightness LEDs used in LCD backlighting
require high LED current which also equates to higher LED forward voltage . For
example, if a user wants to set the LED current to 80mA maximum, a minimum of 3
.65V forward voltage must be provided to each LED in the string . If the power
supply can only provide 3 .6V to each LED, then the maximum LED current is
limited to 74mA .
Boost Converter Driving
In a system where supply voltage (VIN) is
lower than VLED, a step-up boost converter is used to provide power to the LED
strings ., the output voltage of the boost converter drives all the LED strings
. The feedback signal CSFBO connected to CSFB on the boost converter provides
the lowest VCS level from all the LED strings and also controls how the VLED
voltage is regulated .
When the CSFB voltage is lower than the
dropout voltage that is necessary for the LEDs to operate correctly, the stepup
converter will boost the VLED level . However, when the CSFB voltage is higher
than the dropout voltage, the step-up converter will stop boosting VLED .
During this time, the LED current is provided by the boost output capacitor
(C5) . This boost is set as a forced PWM system, so the pass switch (T1) will
turn on with a minimum on-time (unless current limit or OVP is reached) to
provide output current to the LEDs as well . At some point when the required
LED current is higher than the current provided by the boost capacitor and the
minimum on-time of T1, the VLED will start to drop and CSFB will go below the
dropout voltage . At that time, the step-up converter will start boosting the
VLED voltage level .
In some systems it is desirable to maintain
the operating LED voltage when the LED current sinks are OFF . When the LEDs
are OFF, the voltage across the LED string decreases . When the LED string
turns off, the current sink voltage will rise . Without a sample and hold
technique, the LED voltage will regulate down in order to drive the current
sink voltage to the regulation point even though the LED string is OFF . Since
there is no power consumed when the LED string is off, regulating the current
sink voltage during the OFF time of the LED string is unnecessary . A
potentially unwanted effect of regulating the LED voltage during the OFF time
is that additional time is necessary to establish the proper LED voltage when
the current sink is turned back on as the LED voltage slews to the required
voltage level . During this time, the current in the LED string will not be regulated and will tend to be less
than the final desired LED current level . For the AAT2404, when the external
current sinks are ON, the CSFB is regulated to the internal reference (dropout)
voltage . When the external current sinks are OFF or CSFB voltage is greater
than an internal set voltage (2 .5V), the LED voltage is determined by the
voltage level left on the on the compensation capacitor (C3 in Figure 5) which
has been disconnected from the feedback loop . During this OFF time, since the
inductor current is proportional to the compensation capacitor’s voltage, VLED
will not decrease and will be either held or increase slightly until required
to turn on the LEDs .
POWER SUPPLY CIRCUIT
