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GB 96
9.
Main Supply is switched 'off' (via TS7529). A relay (1550) is
used to switch the Degaussing circuit. It is switched 'on' after
set start-up and switched 'off' by the m P after 12 s.
The Main Supply, an SMPS that is based on the 'down-
converter' principle, generates the 141 V (V
16 V for the audio part. Difference with former MG-sets is that
V
is not mains isolated ('hot') and is alignment free.
BAT
9.3
Power Supply (diagram A1 & A2)
The power supply has a number of main functions:
•
Mains harmonic filter.
•
Degaussing picture tube.
•
Standby power supply.
•
Main supply.
9.3.1
Mains Harmonic Filter (diagram A1)
3501
1590
3551
1501
2
3
2507
3521
V
mains
4M7
5503 or
1
4
5504
Figure 9-2
The mains harmonic filter has 2 functions: to prevent that
high frequency signals (harmonics) are transferred into the
mains and to protect the set from lightning damage.
C2507 prevents that the high frequency signals, generated
by the set, are conveyed into the mains (it forms a short-
circuit).
In case of a lightning surge between the 2 phases (differential
mode) the energy is immediately bled away through the VDR
(R3509) to the other phase.
In case of a lightning surge on both phases of the mains in
relation to the aerial earth, the filter acts as a high resistance
(U
=L * dI/dt), as a result of which the voltage across coil
EMK
L5503/04 increases. A spark gap (1590) prevents that the
voltage increases too much, which would lead to a damaged
coil. When ignited, the current will be discharged via this
spark gap.
Resistor R3500 is used for limiting the inrush-current.
9.3.2
Degaussing (diagram A1)
The 5V2 is present, as soon the set is switched 'on' via the
mains switch. As the 'DEGAUSSING' signal from the
processor (OTC) is 'low', transistor 7528 will conduct and
relay 1550 is activated. Initially a considerable current will
flow, via PTC 3516, through the degaussing coil. The PTC
will heat up, resistance will rise and the current will decay
rapidly. The OTC makes the 'DEGAUSSING' signal 'high'
after 12 s, which will switch the relay 'off'.
9.3.3
Standby power supply (diagram A2)
Principle
This power supply is of a SOPS type (Self-Oscillating Power
Supply) and is regulated by the controlled switching of an
oscillator. It uses the so-called 'Flyback' principle:
EM3E
Circuit Descriptions and Abbreviation List
) and the +/-
BAT
Mains harmonic
diversity
3
9503
9504
1
3500
2537
5511 or 5516
-
+
9504
2516
2
2
3
1
4
5502
CL 16532044_020.eps
140501
S
375V
S
375V
•
After closing switch 'S', the current ID will increase linear
in time. The magnetic energy in the primary coil is directly
proportional with the self-inductance of the coil and
current ID (thus with the time the switch is closed). The
voltage polarity at the secondary winding is negative
(due to different winding direction), meaning that diode D
+375V
will block. Capacitor C will discharge via RL, U
decrease.
•
Opening switch 'S' will generate a counter-e.m.f. in the
-
primary winding, trying to maintain current ID. Through
this the polarity of the secondary voltage will inverse. The
magnetic energy, stored in the coil, will now be
transformed to the secondary side. Diode D will now
conduct, capacitor C will be charged and
increase.
Implementation
3120
5102
6103
-13V
10R
3107
3106
1K
1K
2102
+
10µ
2111
3125
3126
15R
10K
7100
To apply this on the EM3E (diagram A2): replace switch 'S'
by FET TS7102, coil L by L5101/L5100, diode D by D6111
and C by C2104.
–
Time interval t0 - t1: After switching 'on' the TV-set, the
gate of MOSFET TS7102 will be high (max. 15 V due to
zener diode D6105). This will drive the FET into
saturation (U
transposed across the primary winding of L5101 (3, 5),
resulting in a linear increasing current through this coil.
The voltage across the co-coupled coil (1, 2) is also
positive and will keep the FET into conductivity via
C2101, R3103/3105/3102 and R3117 for some time.
The self-induction of the coil and the magnitude of the
supply voltage (+375 V) determine the slope of the
primary current. The maximum current is determined by
the time the FET stays into conductance (t0 - t1). This
time is directly determined by the voltage across R3108/
Id
D
+
-
+
C
+
-
D
-
+
Isec
+
C
-
+
Figure 9-3
HOT
COLD
U
A
5101
2
2101
6111
2n2
1
I
SEC
8
3103
1K
2114
9
GND-STB
10n
3110
U
3105
MAINS
+375V
1K
5
2R2
3102
3
ON
OFF
U
A
1K
3101
I
PRIM
U
10M
STARTUP
D
3117
7102
47R
U
D
D
V-START
G
U
MAIN
S
6105
3104
15V
47R
6105
I
PRIM
15V
7101
3108
I
//3118
SEC
6108
6106
0
t
t
1
GND-STB
15V
3127
3124
5K6
68R
3114
220R
7104
Figure 9-4
= 0 V). The DC-voltage U
DS
U out
RL
U out
RL
96532156_020.eps
210100
will
OUT
will
OUT
+5V2
U
2104
RL
OUT
2m2
t
N . Usec
3113
t
22R
t
t
t
2
6122
3V9
2109 +
2149
CL 16532044_019.eps
160501
will be
MAINS
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