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MICROPOWER CONTROL
General
The power supply is equipped with a micropower system
in order to reduce the standby power consumption to about
<200 mW (generally about 5 to 7W). To make this possible,
the power supply and degaussing circuitry must be totally
switched off in standby mode.
The only active part in standby mode is the micropower
circuitry which gets its supply voltage from a capacitive
coupled rectifier. This provides the supply voltage for re-
quired circuits, like a "primary" infrared receiver and a gate
circuit. When the TV set is switched on with the mains
switch or with a remote control command, the gate circuit
drives a triac switch to conduct, thereby supplying the
mains voltage to the power supply. At the same time, the
micropower circuit controls the main microcontroller via
optocouplers.
The micropower circuitry is not mains isolated.
Start up via the mains switch
The mains voltage is fed directly to the micropower circuit
which is located on the local control panel.
The circuitry gets its supply voltage from the capacitive
coupled rectifier consisting of capacitors Cfc24/26, resis-
tors Rfc23/24 and diodes Dfc6...Dfc9. The supply voltage is
regulated to +5.1 V by zener diode DZfc1 and fed onwards
to the infrared receiver Hfc2, NAND gate ICfc1 and LED
Dfc16, that indicates the mains voltage is connected.
The TV set can be switched on by firmly pressing the mains
switch. An additional contact in the mains switch causes a
low level pulse on pins 12/13 of the NAND gate. Due to the
effect of the NAND gate, pin 11 as well pins 1/2 are high
and pin 3 is low. Moreover, pins 5/6 go low via diode dfc13
and pin 4 is high.
Transistor tfc1 conducts and grounds the cathode of the
internal LED of the optocoupler ICfc2. The optocoupler then
conducts, feeding the mains voltage via resistor Rfc27 to
the gate of triac Dfc11. The triac's gate gets sufficient start-
ing voltage via Rfc27, after which resistor Rfc26 is con-
nected in parallel with Rfc27 to keep the triac in a conduct-
ing state. Thus the mains voltage is available for the power
supply.
Furthermore, when the TV set is started up with the mains
switch, a low level on pin 3 causes transistor tfc6 to con-
duct. Also the optocoupler ICfc4 conducts grounding the
cathode of diode dfc18. This indicates to the microcontroller
that the TV set is being powered on via the mains switch.
Because the additional contact causes only a momentary
low on pins 12/13, capacitor Cfc31 discharges and pins 12/
13 go high. Because of this, both transistor tfc6 and
optocoupler ICfc4 stop conducting.
The power supply then generates the supply voltages, the
microcontroller is reset, and its pins 48 (Pict_on) and 49
(Rec_on) go low. This causes optocoupler ICfc3 to conduct
holding pins 5/6 low in order to keep triac Dfc11 conduct-
ing.
If the mains switch is not pressed firmly, the additional
contact may not close for a long enough time and the TV
set may remain in standby mode.
Start up / switch off by the remote control
The TV set must, of course, be in standby mode. This means
the optocoupler Icfc3 does not conduct, voltage level on
pins 5/6 is high and capacitor Cfc28 is charged via rfc29.
When the remote control command is given, pulses on pin
3 of the "primary" IR receiver discharge capacitors Cfc28.
Thus pins 5/6 of the NAND circuit go low via diode Dfc12,
and the mains triac starts to conduct. The command must
be sufficiently long that the power supply has time to gen-
erate supply voltages, the microcontroller has time to be
reset and to receive the start command from the "second-
ary" IR receiver. After that, pins 48 (P_on) and 49 (Rec_on)
of the microcontroller go low and optocoupler ICfc3 con-
ducts and holds pins 5/6 of the NAND gate low.
When the TV set is switched off by the remote control, pins
48 (P_on) and 49 (Rec_on) of the microcontroller go high.
Therefore all Vp and Vr voltages will be switched off (hori-
zontal and vertical stages are switched off), the optocoupler
ICfc3 will not conduct causing pins 5/6 of the NAND gate
to go high. Optocoupler ICfc2 does not conduct and resis-
tor Rfc27 will be disconnected from the gate of Dfc11.
Resistor Rfc26 alone is not able to feed enough holding
voltage to the mains triac, and so the power supply will be
disconnected from the mains.
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