Switching Power Supply; Primary Rectifier; Oscillator; Regulation - Sony KP-43T70 Training Manual

Switching Power Supply -

Primary Rectifier

When the set is turned ON System Control IC001/62 places a high on
the base of Q652 Relay Drive. Q652 turns ON and provides the ground
return path for Power Relay RY601. With the relay closed, AC is ap-
plied to a voltage doubler circuit comprised of bridge rectifier, D602 and
two capacitors, C607 and C608. This produces 300V at R608 with
respect to Hot Ground.
IC601
The switching transistors in previous power supply circuits have been
replaced by an IC in this chassis. Basically this IC is two transistors
fabricated on the same piece of silicon. This gives us the advantage of
having the gain and other electrical characteristics matched. In addition
there is a zener diode and a regular diode across the base emitter
junction for protection purposes.

Oscillator

IC601-1, C615, C618 and the winding between T604/4 and 5 form one
section of the oscillator for the switching regulator. IC601-2, C616, C617
and the winding on T604 between pins 2 and 3 form the other leg. T604
is the Power Regulating Transformer (PRT). The arrangement of the
circuit can be considered a "Dual Tank Oscillator". The operating fre-
quency is determined by the two LC circuits: C618, and the T604 wind-
ing between pins 4 and 5; C617, and theT604 winding between pins 2
and 3. IC601-1 and IC601-2 share in producing the oscillator signal.
IC601-2 is ON during the positive half, and IC601-1 is ON during the
negative half. The oscillator frequency is 97kHz when the TV produces
a white raster, and at 103kHz with a black raster.
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Start Up
The current path for initial start-up of the oscillator is through IC601-2,
through the winding of pins 2 and 1 of T604, the winding of pins 6 and 5
of T605 and then through C621. When current flows through this path a
magnetic field is created in the windings of T604. This field continues to
grow until C621 is fully charged. After C621 is fully charged the mag-
netic fields begin to collapse. This induces a voltage at T604/3 that that
turns OFF IC601-2 through C616 and R613. While this is occurring a
voltage is induced at T604/4 that turns IC601-1 ON. When this occurs it
provides a discharge path for C621. Once C621 is discharged it allows
the whole cycle to repeat itself.
As the circuit oscillates it produces a 300Vp-p waveform at Power Input
Transformer T605/6. This waveform is induced into the secondary
windings of the T605, producing all of the secondary voltages.

Regulation

The power supply is regulated by the control winding of PRT T604/7 and
8, in the following manner:
An increase in voltage across the control winding will reduce the induc-
tance of T604 therefore increasing the oscillator frequency. When the
oscillator frequency increases it moves further away from the resonant
frequency of T605, 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 IC651/4 which varies inversely
proportional to the 135V line. Pin 8 of the transformer control winding is
connected to the correction voltage The other leg of the control winding
(pin 7) is connected to the +18V line by D660. The voltage differential
across the control winding causes a dc current to flow through the
winding.