Philips EM5E AA Service Manual page 106

EN 106 9.
Amplification is done via a 'class D' integrated power amplifier
IC, the TDA7490.
The microprocessor, called OTC (OSD, Teletext and Control)
takes care of the analogue TXT input- and output processing.
The OTC, ROM, and RAM are supplied with 3.3 V, which is
derived from the +5V2.
The NVM (Non Volatile Memory) is used to store the settings,
one Flash-RAM contains the set software, the other Flash-
RAM (7012) is for EPG, and the DRAM is used for storing the
Teletext pages.
There is a separate Standby Supply, in order to reduce the
Standby power consumption. During Standby, the 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 P after
12 s.
The Main Supply, an SMPS based on the 'down-converter'
principle, generates the 141 V (V
audio part.
Note: Voltage V is not mains isolated ('hot'), but is alignment
BAT
free.
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 Mains harmonic filter circuitry
The mains harmonic filter has two 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 (common mode), the filter acts as a
high resistance (U = L * dI/dt), as a result of which the
EMK
voltage across coil 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)
As soon as the set is switched 'on' via the mains switch, the
5V2 is present. As the 'DEGAUSSING' signal from the
EM5E Circuit Descriptions and Abbreviation List
) and the +/- 16 V for the
BAT
Mains harmonic
diversity
3
9503
9504
1
3500
2537
5511 or 5516
- +
9504
2
2 3
1 4
5502
CL 16532044_020.eps
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 'off' the relay.
9.3.3 Standby power supply (Diagram A2)
Principle
This power supply is delivering the standby voltage, but also
the main voltages for the small signal part. It is a SOPS type
(Self-Oscillating Power Supply) and is regulated by the
controlled switching of an oscillator. It uses the so-called
'Flyback' principle:
375V
375V
After closing switch 'S', the current I
The magnetic energy in the primary coil is directly proportional
with the self-inductance of the coil and current I
time the switch is closed).
The voltage polarity at the secondary winding is negative (due
to different winding direction), meaning that diode D will block.
Capacitor C will discharge via R
Opening switch 'S' will generate a counter-e.m.f. in the primary
+375V
winding, trying to maintain current I
of the secondary voltage will inverse. The magnetic energy,
2516
stored in the coil, will now be transformed to the secondary
-
side. Diode D will now conduct, capacitor C will be charged and
U will increase.
OUT
140501
Implementation
3120 5102
-13V
10R
3107
1K
2102
10
2111
Figure 9-4 Standby supply circuitry
To apply this on the EM5E (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
S
Id
D
+
-
+
-
S
D
-
+
Isec
-
+
Figure 9-3 Flyback principle
will increase linear in time.
D
, U
L OUT
. Through this the polarity
D
HOT COLD
6103
U
5100/5101
A
2
2101
2n2
1
8
3103
1K
2114
10
GND-STB 10n
3110
U
3105
MAINS
+375V
1K
5
2R2
3102 3
U
A
1K
3101
I
PRIM
3106
U
10M
STARTUP D
3117
1K 7102
47R
U
V-START D D
G
U
MAIN
S
6105
3104
15V
47R
6105 I
PRIM
15V
+
7101
3108
I
//3118
SEC
6108
3125 3126 6106
GND-STB
15R 10K 15V
3127
5K6
7100
7103/7104
U out
+
RL
C
U out
+
RL
C
96532156_020.eps
210100
(thus with the
D
will decrease.
6111
+5V2
I
SEC
2104 U
RL
OUT
2m2
ON OFF
t
N . Usec
3113
t 22R
t
t
0 t t 1 t 2
3124 6122
68R 3V9
2109 +
3114
2149
220R
CL 26532041_062.eps
110402