Soft Start/Regulation/Foldback; Soft Start; Regulation; Foldback - Sony WEGA KV27FV15 Training Manual

Soft Start/Regulation/Foldback

Overview
The three circuits discussed here all have an affect on the voltages across
T603/7 and 8. This winding of the PRT is a cross inductance winding or
control winding that controls the frequency at which the converter oper-
ates.

Soft Start

At power ON, C602, which is discharged, has a 0V potential at its + termi-
nal. This biases Q645 ON, via R647. With this transistor ON, it allows the
standby unregulated from Q646/E to be applied across T603/7 and 8.
This voltage is passed through Q646 Backup since the same line that
turns RY600 Power Relay ON turns it ON. The greater the voltage across
this winding, the higher the converter frequency and the lower the effi-
ciency of T605 PIT. This is discussed in the Converter section. There-
fore, the start up secondary voltages will be reduced considerably. As
C602 charges, the voltage difference between T603/7 and 8 diminishes.
As this voltage lowers, the secondary voltages will rise. When the Vcc
Rectifier voltage is larger than the Standby unregulated voltage, D612 will
begin to conduct and the Vcc Rectifier voltage will control T603/8. Also at
this time C602 will be fully charged and IC643 Control IC will take control
over T603/7. The difference in voltage between T603/7 and 8 will regu-
late the converters operating frequency.
Soft Start Reset
It is important that the soft start capacitor C602 be fully discharged when
the set is turned OFF. Turning Q648 OFF when the unit is turned OFF
does this. This removes the ground path from the relay, opening it up. It
also allows current from the Standby +5V to flow through the relay coil,
through R621, to Q647/B. This turns the Q647 ON and discharges C602
through R627.
Q646 Backup will also be turned OFF when the Power On line becomes
HIGH.
19

Regulation

The power supply is regulated by the control winding of PRT T603/7 and
8, in the following manner:
An increase in voltage across the control winding will reduce the induc-
tance of the windings in T603, consequently increasing the converter fre-
quency. When the converter frequency increases, it moves further away
from the optimum operating frequency of T605 PIT, reducing the voltage
at the secondary outputs. The opposite occurs when the voltage across
pins 7 and 8 decreases. A correction voltage is produced by IC643/4,
which varies inversely proportional to the 135V line. Pin 7 of the trans-
former control winding is connected to the correction voltage. The other
leg of the control winding (pin 8) is connected to the output from D603 Vcc
Rectifier. The difference in the voltage across this winding determines
the frequency of the converter which regulates the +135 Volt line as de-
scribed in the previous sections.

Foldback

The foldback circuit is a type of protection circuit that uses a winding on
T605 PIT to monitor the current drawn by its secondary. This voltage is
rectified and called Vcc Rect. Q624 is set up with the Vcc Rectifier volt-
age input to the emitter. Its base is connected to the Rectified +12 volt
line from the secondary side of the supply through D605 and R609. D605
is a zener diode whose breakover voltage is 13 volts.
This circuit will protect against two failures. The first is a rise in the Vcc
Rect. This voltage is developed by the foldback winding of T605. This
winding is used as a current mirror. This means that the more current
drawn on the secondary of T605 PIT, the higher the voltage at T605/3.
When too much current is drawn from the secondary, Q624 would turn
ON because of the increase in voltage across R610. If this occurs, the
latch and soft start circuits will be activated. Soft start is activated to lower
the output supply voltage by increasing the switching frequency. The
latch's purpose is to shut the supply down.
Secondly, if there were a loss of the +12 volt line then D605 would
breakover. This would cause sufficient current flow through R610 and
Q624 would turn ON. If this occurs, the latch and soft start circuits will be
activated.