The
switched MODE PSU sources work with semiconductor elements, which open and close
at high speeds as if they were switches that turn on and off at a
generally high frequency.
As
the switching signal is a square wave, in the case of a transistor, it
will be either cut or saturated, with no intermediate conduction
situations. Thus, the heat radiation will be very low, reducing the size of the heatsink, or even dispensing with its use in some cases.
This fact also allowed the use of smaller transistors and encouraged the use of mosfets in the function of power switchers.
The
operation at increasingly higher frequencies contributed to the
reduction of the size of the transformers and the use of capacitors of
increasingly lower values, thus reducing the size and weight of these
sources.
Consumption
has also dropped a lot, due to the high efficiency, since the switched
sources do not transform much energy into heat.
PRIMARY POWER SOURCE
It is the part involved with the input voltage of the network. The
primary source is formed by the rectification and filtering stages,
offering an output around 320 volts DC to supply the Power Switch
transistor, through the primary winding.
When
the source has a PFC circuit, a circuit that controls the power factor
of the source in order to save more energy, the primary voltage appears
with higher values.
DEPARTURE CIRCUIT
It
is a small circuit formed mainly by some high-value resistors, which
serves to power and start the oscillator at the moment when the source
is powered.
SECONDARY POWER SOURCE
These
are the voltages obtained by induction in the secondary of the
transformer, rectified and filtered by diodes and electrolytic
capacitors, which supply the television circuits. Many of them go through regulators before being used by TV circuits.
CONTROL CIRCUIT
It is a circuit that keeps the output voltages of the source within a reliable range. To
do this, it takes a sample of one of the voltages generated in the
secondary, and sends it to the primary through a photo coupler or
transformer, in order to use it as a control voltage to change the
frequency or width of the pulses. of the switching waves that are being
generated in the primary of the source.
BRIEF DESCRIPTION OF THE TOSHIBA 32HL86 AND 26HL86 LCD TV POWER SOURCE
Note: For better understanding, the reader must accompany the description with the device's source diagram in hand.
PRIMARY POWER SOURCE:
The
mains voltage passes through the F 801 fuse and other protection
components, reaching T 801 which has the function of the mains filter. This
filter prevents the oscillating frequencies of the source from reaching
the home's mains supply, which can interfere with other appliances in
the house.
The
mains voltage goes directly to the rectifier bridge DE01 belonging to
the STANDBY SOURCE, but depends on the SR81 relay to reach the bridge
D801 belonging to the MAIN SOURCE.
This
is because the voltages generated by the STANDBY SOURCE must be present
at all times, even when the device is in STBY, whereas the voltages
generated by the MAIN SOURCE will only appear when the device is turned
on, that is, when the device is removed from the mode STBY.
To remove the STBY device, the user must press the power key on the remote control or on the TV panel. At
that moment, pin 21 of the micro (QA01), sends a high level to the base
of Q852, which, when driving, feeds the base of Q 851 which, when
saturated, energizes the coil of relay SR81, closing the contact that
allows power the D801 rectifier bridge.
The voltage rectified by the bridge D801 is filtered by capacitor C 810 and proceeds to pin 1 of CI Q801. This
pin belongs to the internal transistor drain that will amplify the
switching pulses that are generated by an oscillator located inside the
IC.
Pin
8 of the CI is intended for its supply from D 813 and its starting,
provided by resistor R 827. Pin 9 of the CI is for controlling the over
voltage of the voltage generated and pin 10 for controlling input
voltage over-voltage.
The voltage rectified by bridge DE01 is filtered by capacitor C E10 and proceeds to pin 1 of transformer TE62. The
return of this winding (pin 3) is connected to pin 1 of CI QE01
(Internal Mosfet Drain) which, when amplifying the pulses generated in
the internal oscillator, transfers them to the primary winding (Pins 1, 2
and 3 of TE62).
Pin 4 of the CI is intended for its supply from DE06 and its starting, provided by resistors RE03 and RE04.
Pin 6 of CI QE01 belongs to the internal oscillator which is controlled by the photo coupler transistor QE26. This photo coupler is based on a voltage sample taken from the 18 volt source generated in the standby source.
MAIN SECONDARY PSU POWER SOURCE:
The
secondary of the main source generates two voltages: a voltage of 12.4
volts regulated by the CI Q860, and which serves to feed mainly the
audio circuits, and a voltage of 24 volts, which serves to supply the
power circuit of the Inverter and it is also reduced to 5 volts by the
regulating IC QR71 to supply integrated inverter and other device
circuits.
The
voltage of 5.1 volts is constant, as it is responsible for supplying
the microprocessor, but the voltage of 9 volts is switched by the micro
via the transistors QE53 and QE52, QE95 and the Mos QE96 switch.
The
transistors Q853, Q854, Q855 and Q856, form a circuit that warns the
computer about a possible lack of power, so that it saves the important
configurations of the device quickly, before the computer is totally
without power.
STANDBY SECONDARY POWER SOURCE:
The
Standby source generates 2 voltages: 40 volts which is later regulated
to 32 volts to supply the tuning circuit and a voltage of 18 volts which
is later regulated to 9 volts by the QE90 regulator and to 5.1 volts by
the QE40 regulator.