Protective Operation; Power Factor Control - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
Without secondary-side regulation, e.g. in
SAT standby operation, UB would increase to
approx. 180 V. In this mode of operation,
other voltages (e.g. the bias voltages for the U
5, U 12 branch) do not alter significantly. Sta-
bilization is also guaranteed for these voltages
in this mode of operation.
Due to the secondary-side stabilization with
UB as stabilization factor, UB without any load
would naturally remain constant. In this case,
the power supply would regulate down from
approx. 180 V (without secondary-side regula-
tion) to the previously set value of UB, typi-
cally 146 V. This would also result in all other
voltages being reduced. The bias voltages for
the stabilized voltages would be too low and
they would no longer be stabilized. The digital
electronics would search for errors.
In order to prevent this, the regulation acts via
U 7. This is accomplished via components D
672, R 693 and 666.
U 7 also drops when the line-output stage is
disconnected. When this has reached a value
of approx. 6V, the D 666 Schottky diode volt-
age has also fallen sufficiently for it becomes
conducting. A further drop in operating voltage
is prevented by the I 670 input. Stabilization of
secondary side voltages, e.g. for U 5 and U
12, still operate properly in this mode of
operation.
2.2.4

Protective operation

The IC contains a protective circuit to prevent
the control frequency from entering ranges
where it would otherwise damage the switch-
ing transistor during over- and underload. In
such cases the system is shut down.
Document Q 2500
The indicator for this is the voltage on pin 6 in
TDA4605. During normal operation this volt-
age is approx. 11 V. If the load increases, due
to a short circuit on the secondary side, for
example, the voltage on pin 6 drops. If it drops
below 7V, the logic circuit switches off. The
same occurs if the voltage on pin 6 exceeds
15 V due to complete discharge or a fault in
the control circuit. There is also a second pro-
tective circuit to safeguard against overload-
ing. If the current flowing through the working
winding in the transformer and therefore also
through the switching transistor is so great
that the voltage on pin 3 drops below 1 V, the
power supply also cuts out.
In addition, integrated thermal protection
switches the system off at chip temperatures
of over 150°.

2.2.5 Power Factor Control

All TV sets with power consumption of more
than 75 W, and delivered after 01.01.2001,
must be fitted with a Power Factor Control
(PFC) circuit.
The circuit power supplies used up to now
overlay a pulsed current on the sinusoidal
mains alternating voltage. The Power Factor
Control circuit ensures current extraction is
almost completely sinusoidal. This function is
implemented by changing the power supply's
input range
Power Factor Control
R613
D613
230V~
17
T639
C628
D617 L618 D623
C611 C620
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