Circuit Descriptions and Abbreviation List
EN 120 EM5A P/M9.
load. If the power supply is now loaded even more, the output
voltage will decay. The maximum primary current set by
R3108//3118 determines the maximum load.
8V6 Stabilisation and Feedback
In general with a fly-back supply with multiple output, 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.
Disadvantage of this approach is the power loss in the linear
voltage regulator.
For the EM5, a power economic solution is achieved, by
implementing a kind of secondary down-converter. The
advantage, compared to 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.
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 synchronised
with Q1.
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.
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. Automatically U1 will drop.
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 would fail, the secondary voltage will
increase. This would have disastrous consequences since
many ICs (e.g. 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 stabilisation task of the optocoupler,
however, with a considerable spread: from -20 to -25 V is a 25
% increase, thus U
OUT
will increase from 5.2 V to max. 6.5 V.
Tuner Supply
The Standby supply produces two voltages for the Tuner: +33V
(V
TUN
) and +5VT.
CL 26432041_080.eps
170402
C1
Q1
FEEDBACK
7103/04
C4
U2
(+8V6)
MOSFET
DRIVER
D
G
S
C2
Q2
D
G
S
C6
D1
D2
I2
I1
L1
U1
I3
C5
7133
R5
C7
C8
R1
R7
R6
R8
R9
R4
R3
FEEDBACK
DRIVER
U3
(+5V)
FEEDBACK
315V
+
+
+
-
-
-
CL 26432041_079.eps
170402
t0
Vg-Q1
Vg-Q2
V-D1
V-D2
I1
I2
I3
t1 t2 t3
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