BELINEA 10 55 70 Service Manual Download Page 24

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Non-VA Version

3.6. Switching Power Supply

The switching power supply (SPS) used in this display is a

mode type. The power supply provides seven

outputs

and

Please

refer to schematic diagram for details of the circuit layout. The
input voltage is from

270VAC with an input

frequency of

as shown in Figure 3-l 3.

The current first passes through the

control circuit and

is regulated to DC by the bridge diodes

and filter

capacitor

During rectification a large current surge is

generated and as

has a very low impedance while being

charged the fuse, on/off switch and bridge diode are all liable
to be damaged. For this reason, a thermal resistor
is added before the bridge diode in order to limit the large
current surge generated during the charging of the capacitor.

During rectification, Cl2 is charged through R7, R12 and

When Cl2 is charged to

IC2 3842A starts to operate (for

details, of the functions of this IC, please refer to

relevant

data sheet) and outputs a pulse signal from Pin 6 to set the
transistor

in the ON state. At this time, transformer

which is connected in parallel, starts to store power. When the

current passing through the resistor R45, and the supplemen-
tary current from R39, R41 and R27 (Pin 4 to Pin 3) into Pin
3 of

reaches

IC2 is reset, causing the energy stored

by the transformer to reach the rated value. In order to prevent
the transformer from being saturated and causing damage to
the transistor, when transistor

is in the OFF state, the

energy stored in the transformer

is released into the secon-

dary coil and is regulated through the various output loops and
filters and converted to the required DC output. In addition to
this, at the appropriate time,

windings pin 1

pin2 supply

Pin 7 of

with a fixed power supply for normal operation.

Also, when windings

pin3 are in power saving active

state, power is supplied to Pin 7 of

for normal operation.

In any of the above cased, the output pulse is terminated and
the

is turned off, causing the voltage on the output of the

to rise rapidly. and the voltage across the winding of the

primary to reverse in polarity, thus tending to reset the flux
within the core. At this point, the diodes

D22 and

on the secondary supply winding become

biased

and begin to conduct, thus transferring energy from primary
to the secondary, and charging the secondary capacitors.

There is also secondary winding the primary side of the power
supply which, through diode D8 and

recharges the control

reservoir capacitor C12. This supply then keeps the IC

running. In the event of a secondary short circuit, the supply
fails to recharge, thus the voltage across Cl2 drops to a
threshold limit below which the IC cuts out and returns to its
low current load operation.

During normal operation, the supply rails charge until the
error amplifier realized by IC4 on the secondary begins to turn
on the opto-coupler,

At this point, the photo-transistor

of this opto-coupler on the primary side begins to conduct,
draining current

the primary control IC supply through

diode D6. This reverse biased the gate of the thyristor

ISOLATION

CIRCUIT

FMFBT

CONTROL

SWITCH

CIRCUIT

FUSE