9
Circuit description new circuits
31
L7.3A
9
Circuit description new circuits
Power supply (diagram A1)
9.1
Introduction
9.1.1
General
The switched mode power supply (SMPS) is mains isolated.
The control IC7520 (MC44603AP) gives pulses for driving FET
7518 with duty cycle control at fixed frequency of nominal 40
kHz in normal operation. In stand-by, slow-start and overload
situation the SMPS runs at other frequencies than these 40
kHz.
Basic characteristics of SMPS :
–
Mains Isolated flyback Converter type
–
Input range : 90 - 276 Volts AC ( multi voltage ) or 150 - 276
Volts AC ( single voltage ).
–
Secondary Sensing by Opto Coupler
–
IC7520 is Featured with a Slow-Start circuitry
–
Over-and undervoltage Protection Circuit of the secondary
supply voltages. In case the load decreases under a
certain threshold level the SMPS will switch into stand-by-
mode (in stand-by SMPS is in the so called "reduced
frequency mode"; this is nominal about 20 kHz).
The (VBATT (+140 Volts for A/P and +130 Volts for LATAM
output gives a stabilized (VBATT for 25" and 29" tubes in
normal operation and approx. 140 Volts DC for A/P and 130
Volts for LATAM in stand-by mode. IC7541 (=TDA8139) having
a stable regulator for +5 Volts and +8 Volts supply and also a
built-in protection circuit for +5 Volts and +8 Volts supply is
present. If pin 4 is low in the standby mode, the IC disables the
supply output voltage of (8Volts, so IC7225 (TDA8374C) line
output stage will be shut "down". If no 8 Volts supply is present
transistor 7542 will switch on, this will cut off the +VBATT; it
also performs a fast CRT discharge. If the power is switched
off, pin 6 of IC7541 will be low and the protection circuit starts
to shut down the +5 Volts and +8 Volts.
9.1.2
Output voltages
Power output secondary output voltages:
•
14 Volts for the audio amplifier
•
13 Volts for the audio processing
•
5 Volts for the control part (is also available in standby/
reset)
•
8 Volts for the video processing and Nicam
•
140 Volts for A/P and (130 Volts for LATAM for the line
output stage and the tuning system
9.1.3
The switching periods of TS7518
The duty cycle of the power supply depends on T-on of FET
TS7518 and this is controlled by pin 3 of IC7520. This IC
detects the variations of the (VBATT via sensing-winding 8-9 at
the primary side of T5545. The switching period of TS7518 can
be divided into three main phases: Duty cycle T-on, T-off and
T-dead
•
During T-on, FET 7518 will switch on and conducts, so the
energy that is extracted from the mains is stored into the
primary winding 2-5 of transformer T5545 with a linear
increasing primary current. The slope depends on the
voltage across C2508. Via T-on regulation by pin 3 of
IC7520 the duty cycle of the SMPS and so the (VBATT is
controlled.
•
During T-on FET 7518 will switch off and does not conduct,
so all energy "inside" the transformer is supplied to the load
via secondary windings of T5545 and the secondary
diodes (D6550, D6560 and D6570,D6590). The current
through the secondary side of the transformer decreases
with a linear slope of T5545.
•
During T-dead FET 7518 does not conduct and no energy
is extracted from the mains of supplied side, this gives
space for T-on and T-off regulation.
9.2
Primary side
9.2.1
Mains input and degaussing
–
Mains voltage: this voltage is filtered by L5500 and 5502,
rectified by a diode bridge rectifier 6505 and smoothed by
C2508 to the DC input voltage for the SMPS at pin 5 of
T5545 (e.g. 300V DC for 220V AC mains).
–
Multi voltage: Only small adaptations of some component
values are needed to achieve this.
–
Degaussing : R3504 is a dual PTC (2PTC's in one
housing). When switching "on" the set, the PTC is cold and
low-ohmic. This would give very high degaussing current.
After degaussing the PTC will be heated up and become
high-ohmic; during normal operation the degaussing
current is very low.
9.2.2
Start up and take over
–
Start-up : Via the start-up circuitry 3510, 3530 and 3529 the
voltage coming from 220V AC mains is used to start-up
IC7520 via the supply pin 1. As long as pin 1 has not
reached 14.5 Volts, IC7520 does not start up and only
sinks 0.3mA. If Vpin 1 reaches the 14.5 Volts, IC7520 will
start (FET 7518 will conduct) and pin 1 sinks a typical
supply current of about 17 mA. This supply current cannot
be delivered by the start-up circuitry, so a take-over circuit
must be present. If no take-over will take place the voltage
on pin 1 will decrease and IC7520 will switch off; in that
case the restart procedure will start again.
–
Take over of IC7520: During start-up a voltage across
winding 8 - 9 is built up. At the moment the voltage across
winding 8 - 9 reaches approx. (14.5 Volts, D6540 start
conducting and takes over the supply voltage Vpin 1 of
IC7520 (take over current is approx. 17mA).
Note: This power supply is a SMPS (= Switched Mode Power
Supply) and not a SOPS (= Self Oscillating Power Supply).
9.3
Control circuitry; see page 37 for blockdiagram.
9.3.1
IC7520 control mechanisms
IC7520 controls the T-on time of FET 7518 in all three
operation modes:
•
"Secondary-output-sensing" controls the secondary output
voltages via the feedback voltage pin 14.
•
"1-prim current sensing" controls both the secondary
output voltages and the maximum 1-prim via the current
sense voltage pin 7.
•
"Demagnetization control" prevents the transformer T5545
from going into saturation via the so called "DEMAG"
function at pin 8; this will cause slow-start operation.
9.3.2
Secondary output voltage's feedback (pin 14 of IC7520)
Voltage feedback for T-ON control : Regulation of the SMPS is
via pin 14. Winding 2 -5 has the same polarity as winding 16 -
18, so variations of the +VBATT can be sensed and feed-back
to pin 14. The control voltage of winding 2 -5 during off period
of TS7518 the Opto-coupler 7581 conducted rectified by
D6540, smoothed by 2540 and coupled via 7581 and
