Power Supply Protection; +135 Over Current Protection; Foldback - Sony WEGA KV27FV15 Training Manual

Power Supply Protection

Overview
There are three different circuits that can activate the protection latch.
One is the OCP (Over Current Protection) for the +135 volt line. The other
two are related to the voltage from T605/3 Foldback Winding. When one
of these circuits activates the latch, RY600 is turned OFF. This discon-
nects the AC from the circuit that develops Converter B+ and can be
noticed during troubleshooting when two clicks are heard in quick succes-
sion.
Latch
Placing a HIGH on Q653/B activates the latch. This turns ON Q653 that
turns ON Q652. Once Q652 turns ON, its conduction keeps Q653 ON
until power is disconnected. This places a LOW at Q648/B, turning it
OFF. This causes RY600 Power Relay to open and the set turns OFF.

+135 Over Current Protection

Over current is detected by monitoring the voltage across R652. When
this voltage, which rises as more current is drawn, gets to a level that
causes Q651 to turn ON, the latch will be activated.
Vcc Rectified Low Voltage Protect (LVP)
The signal from T605/3 is sent through R601 and rectified by D603. This
voltage is filtered by C618. It is important that this voltage be at a certain
level since it is used to control regulation. Therefore a circuit is con-
structed in which Q623 is normally ON. This transistor is ON because
D607 should always have sufficient bias to turn ON. This will place Q623/
C at close to ground level. This keeps D604 and D608 from conducting
and activating the latch circuit. If this voltage were to fall below a certain
level, D607 would turn OFF, causing Q623 to turn OFF. When Q623 is
OFF, the +12 volt line would cause D604 and D608 to turn ON, activating
the latch.
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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.