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voltage will decay. The maximum primary current set by
R3108//3118 determines the maximum load.
8V6 Stabilization and Feedback
In general, with a fly-back supply with multiple output, as used
in former chassis, one of the output voltages is controlled via
the primary feedback loop. The additional secondary output
voltages are determined via the turn-ratio of the transformer.
It is often seen that a linear voltage regulator is used for post
regulation of the non-primary regulated secondary voltages.
The disadvantage of this approach is the power loss in the
linear voltage regulator.
For this chassis, a power economic solution is achieved, by
implementing a kind of secondary down-converter. The
advantage, compared to a conventional down-converter, is that
no extra coil is required. It is using the inductance of the main
transformer. In this way one large current coil, a power diode
and one elcap are saved.
DRIVER
U1
-
I1
315V
+
+
C1
-
-
L1
+
Q1
D
MOSFET
G
DRIVER
S
FEEDBACK
7103/04
Figure 9-6 Circuit principle
A basic fly-back converter is used, with a MOSFET Q1,
transformer L1, and a primary feedback circuit. The output of
the primary controlled voltage is U1.
The additional secondary controlled supply consists of D2 and
Q2, with output voltage U2.
The main fly-back supply is working independently, where the
duty cycle is controlled via the primary feedback, and the
MOSFET Q1 is switching at a certain frequency. MOSFET Q2
is also switching at the same frequency, as it is synchronized
with Q1.
Circuit Descriptions and Abbreviation List
D2
I3
Q2
D
S
G
FEEDBACK
C5
R5
C7
R8
7133
R7
R3
R1
R4
R6
C6
C8
R9
D1
I2
C2
FEEDBACK
Vg-Q1
Vg-Q2
V-D1
V-D2
I1
I2
U2
(+8V6)
C4
I3
U3
(+5V)
•
•
CL 26432041_080.eps
170402
•
•
The two controllers, the primary feedback for U1, and the
secondary feedback for U2, all work independently.
The secondary voltage U2 is controlled by the 'on' time of Q2.
As soon as the load on U2 increases, the 'on' time of Q2 (the
period t1-t2) is automatically increased by the secondary
feedback. More energy will be taken by the output U2, and less
energy will be transferred to U1. Voltage U1 will drop
automatically. The primary feedback loop will change the
primary drive to enlarge the total amount of energy to be
transferred, from the primary side, and U1 will rise again.
Protection
If the optocoupler fails, the secondary voltage will increase.
This would have disastrous consequences since many ICs (for
example, OTC, Flash-RAM and DRAM) are fed with this 5.2 V.
In other words, very expensive repairs would be required.
We already know that the negative supply is directly dependent
upon the secondary 5.2 V, because of which the negative
supply will increase proportionally as the secondary voltage
increases.
If the negative supply in the mean time reaches -25 V, D6106
will start to zener and therefore TS7101 will start conducting.
D6106 will take over the stabilization task of the optocoupler,
however, with a considerable spread: from -20 to -25 V is a 25
% increase, thus U
Tuner Supply
The Standby supply produces two voltages for the Tuner: +33V
(V
TUN
EM5A NTSC
t0
t1 t2
CL 26432041_079.eps
Figure 9-7 Timing diagram
Time interval t0-t1: The primary MOSFET Q1 is switched
'on,' both diodes D1 and D2 are blocked.
Time interval t1-t2: MOSFET Q1 is switched 'off' and Q2
is switched 'on.' During this period, the energy is
transferred to output U2 of the supply. Diode D1 is blocked,
because U3 is lower than U1.
Time t2: Q2 is switched 'off.'
Time interval t2-t3: During this period, the rest of the
energy will be transferred to output U1.
will increase from 5.2 V to max. 6.5 V.
OUT
) and +5VT.
9.
EN 121
t3
170402
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