Start-Up; Normal And Control Operation - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
2.2.2

Start-up

The power supply section is started up by an
additional starting circuit. The required voltage
is rectified using a diode path in the bridge
rectifier. It is fed via resistor R 621 to pin 6 in
TDA4605 where capacitor C 622 is then
slowly charged. During this charging phase,
capacitor C 637 is charged to 6.6 V on pin 2
via an internal IC path. A reference voltage of
1V that is required during the start-up phase
and later during normal operation is also gen-
erated in the IC.
If the voltage at pin 6 reaches 12 V the IC op-
erates and makes the switching transistor
conducting via pin 5. A current flows through
both the transformer working winding and the
switching transformer drain source path. Dur-
ing this period, magnetic energy is being
stored in the transformer. During this conduc-
tive phase, a drain current simulator C 637,
integrated in the IC, is charged on pin 2. If the
internal reference value of 1 V is reached the
IC blocks the switching transistor. The mag-
netic field in the transformer breaks down and
this induces voltages in the windings.
The start-up procedure recommences and the
system swings/oscillates to normal operation.
This is arrived at when a voltage of 400 mV
has built up at pin 1.

2.2.3 Normal and control operation

A static state is set in normal operation at
constant load. The operating voltage for the IC
is drawn from the transformer winding pin 5-6
and rectified with D622. The voltage at pin 6 in
the IC is then 11 V. The control input at pin 1
is 400 mV and the duty cycle for the zero pas-
sage detector at pin 8 in the IC is set.
The switching transistor is controlled with a
fixed frequency of between 20 and 40 kHz,
which corresponds to the instantaneous load.
Document Q 2500
If the load changes, the duty cycle at pin 8 in
the IC also changes. The negative edge indi-
cates to the IC when the energy stored in the
transformer has been dissipated. If the load
increases, this occurs more quickly and the IC
reduces the control frequency. If the load de-
creases, the control frequency increases. This
means that load variations between approx.
40 and 260 W and mains voltage fluctuations
between 180 and 270 V can be compensated
for.
In order to achieve a higher UB voltage stabil-
ity, regulation on the secondary side is now
used which influences the primary circuit's I
669 opto-coupler. The I 669 opto-coupler is
controlled via I 670. The control mechanism is
influenced via several paths:
With d.c. voltage via R 663
-
Alternating, by coupling to the R 680 di-
-
ode.
When not under load, via D 672
-
The operating voltage is set with the P662
potentiometer. A small resistance here indi-
cates a high value of UB. Correspondingly, a
high value of R means a low value for UB.
If UB increases, e.g. due to a smaller load on
the line output stage, UB exerts a stronger
influence via R 663 at the input of I 670. This
results in the I 670 cathode outputting a
smaller voltage. The photodiode in I 669 re-
ceives more current through R 665. The path
between pin 3/4 of I 669 has a lower resis-
tance allowing for a higher voltage on pin 1 of
I 611. Component I 611 then regulates output
voltage until a value of 400 mV on pin 1 is
reached again. If the value of UB becomes
lower, then the regulation process is exactly
the inverse of that described here.
Via D 680, R 669 and C 669, ripple voltage
components are coupled into the regulating
circuit. In particular, this branch prevents a low
50 hz ripple voltage arising on UB. With 60 hz
signal sources (NTSC, PAL 60 Hz or VGA
operation), this would result in humming inter-
ference in the picture.
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