Philips EM3E, Руководство по обслуживанию, Страница: 46 / 120

Circuit Descriptions and Abbreviation List GB 98 EM3E 9.
Implementation
Figure 9-6
At start-up of the main supply, C2515 can be assumed as
being a short-circuit. U
AB
will be 15 V (R3513, D6510) and
U
GS
of the FET will be +5.4 V. The FET will be driven into
saturation (same as closing switch 'S' ). The drain-current will
increase linear in time. With other words: resistors R3513
and R3518 will start the oscillator.
The voltage across the co-coupled coil (4, 5) is also positive
and will keep the FET into conductivity.
The drive concept of the MOSFET TS7504 has changed
(compared to the EM2E chassis). In EM2E, TS7502 was a
high-voltage semiconductor, in EM3E it is changed to a low-
voltage semiconductor. The added opto-coupler 7505 is
used to bridge the different voltage levels.
Via this opto-coupler, the DC-current through R3504 is
influenced. The changed current through R3504, changes
the V
BE
of TS7502, which will influence the drive of MOSFET
TS7504 (= switch ‘S’ in Fig. 5).
The sudden current interruption in the primary coil, will
induce a counter-e.m.f. that wants to maintain the current via
the 'freewheel' diode D6508. This current is linear decreasing
in time and as it is also flowing through R3414//R3415,
TS7502 will be blocked after a certain time period. The gate
of the FET will be again made positive, is driven into
conductivity and the cycle starts again.
For safety reasons, transistor TS7530 is added as a back-up
solution for TS7502. If B-E of TS7502 is shorted, TS7530
takes over its function.
Stabilisation of V
BAT
The output voltage V
BAT
is determined by: V
BAT
= V
IN
* T
ON
/
(T
ON
+ T
OFF
) = V
IN
* duty-cycle.
To stabilise the output voltage, a feedback loop is
implemented, which will reduce T
ON
when V
BAT
increases
and vice versa.
Via a voltage divider, excisting of (1 %) resistors R3507,
R3510 and R3527, a voltage of 2.5 V (when V
BAT
= 141 V) is
fed to the input of precision shunt regulator 7506. This
regulator will conduct, a current will flow through the diode
part of the opto-coupler 7507. The base of TS7502 will now
be set at a certain positive voltage. As this transistor switches
the FET TS7504 on and off, this circuit can determine the
duty-cycle.
E.g. when the load increases, V
BAT
will decrease. As a
consequence, the input voltage of regulator 7506 will
decrease, resulting in a lower current. Via opto-coupler 7505
and transistor TS7502, T
ON
of the FET is changed (will
increase). The output voltage V
BAT
will rise.
If the load continues to increase, the regulator will block at a
certain moment. T
ON
will is now at maximum value. This is
the point where V
BAT
will go below 141 V and, at further
increasing load, is switched ‘off’ (the voltage across the co-
coupled coil (4, 5) will decrease, due to the increasing load.
Therefore the voltage on the gate of TS7504 comes below
the threshold voltage. The supply switches ‘off’ and an
audible hiccuping can be heard).
On the other hand when the load decreases, V
BAT
will rise. As
a consequence, the input voltage of TS7506 will also rise,
resulting in a higher current. This changes the base voltage
of TS7502, and through that the T
ON
(will decrease) of the
FET. The output voltage V
BAT
will be reduced.
If, for instance, V
IN
will decrease (e.g. U
MAINS
is 180 V i.s.o.
240 V), the slope of the drain-current will be flattened,
through which the FET will be longer into conductance,
keeping V
OUT
constant.
If, for any reason, the stabilisation circuit might fail, the output
voltage V
BAT
can never exceed 200 V (via D6514). D6514 will
form a short-circuit, V
BAT
will drop and the set will switch off
(this will result in an audible hiccuping of the supply).
Switch to ‘Standby’ (via RC)
When the set is switched to 'Standby' mode via the Remote
Control, the Main supply is switched ‘off’ by the circuit around
TS7529 (see diagram A1).
During 'on'-state, the Main supply is fed with line pulses via
the ‘SUP-ENABLE’ line. They are rectified and smoothed via
D6517, D6516 and C2530 and fed to TS7529. Because they
are less than -13 V, this transistor is blocked. When these
pulses are stopped, TS7529 will be saturated and TS7502
will switch ‘off’. This will switch ‘off’ the Main supply.
Set to ‘On’ (via ‘SUP-ENABLE’)
Via the ‘STANDBY’ command from the OTC, the MOSFETS
7141 and 7131 are switched ‘on’. When the +5V and +8V are
sensed by the OTC, a command is given to the HOP to start
the drive (via I
2
C).
When this is sensed via the ‘SUP-ENABLE’ line (at the base
of line transistor TS7421), the main supply is switched ‘on’
via TS7529.
Audio Supply
The pulses on the secondary winding of L5506 are rectified
by D6535 (+16 V) and D6536 (-16V), and smoothed by
C2542 and C2543.
D
A
S
G
3518
3512
5V6
6515
3514//15
3522
3508
3511
7530
7504
7502
6505
15V
6501
6510
15V
6535
7507
5506
COLD HOT
6536
6530
2503
3513 1503
2.5A
2512
+16V
-16V
2513
141V
5
Vbat
141V
Vbat
375V
7506
3506
3520
33k
3504
3507
2515
V - MAINS
CL 16532044_030.eps
160501
B
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