Converter; Initial Start Up; Soft Start; Regulation - Sony WEGA KV27FV15 Training Manual

Converter

Overview
The function of the converter is to switch the Converter B+ voltage through
a transformer's primary winding. This switching will produce a signal that
will be coupled to the transformers secondary windings where they are
rectified and used to supply various voltages to the set. The frequency of
this switching is controlled to maintain a consistent value on the +135 volt
line. Changing the frequency of the switching can be used for regulation
due to the efficiency characteristics of the transformer. A second trans-
former with a cross-inductance winding is used to create a "dual tank
oscillator". The voltage across the cross-inductance winding or control
winding is used to change the value of the inductance in the tank circuit
and consequently change the frequency of switching.

Initial Start Up

When RY600 Power Relay (not shown) is closed, 300 VDC is developed
and applied to R607. Converter B+ is then applied to C613 and C615.
C613 ensures that the bottom transistor IC601-1 turns ON initially. When
IC601-1 turns ON, there is a current path through C615, T605/1 and 2,
T603/1 and 2, through IC601-1/ C-E to ground. The voltage applied to
IC601-1/B by T603/4 sustains this current path. This voltage at T603/4 is
developed by the expanding magnetic field being produced by the current
flowing through T603/1 and 2. When the magnetic field in T603 PRT
stops expanding, IC601-1 turns OFF. When this occurs, a voltage is
induced by the collapsing magnetic field at T603/3 that turns IC601-2 ON.
This provides a discharge path for C615. When C609 is fully charged,
IC601-2 will turn OFF. This cycle will continue to repeat itself. As the
voltage across T604/7 and 8 Control Winding changes, the inductance of
the other windings will change. This will determine the frequency of switch-
ing.
15

Soft Start

The voltage across T604/7 and 8 Control Winding determines the fre-
quency of operation. The control winding is a cross inductance winding.
At initial startup the converter has to switch above the optimum frequency
of T605 PIT. A soft start circuit is used to ensure that this occurs. The
soft start circuit will ensure that a voltage of approximately 7 volts will be
applied across T603/7 and 8 Control Winding. After the converter begins
to run, the voltage from the Vcc Rect. line is applied to T603/8 through
D612. The voltage applied to T603/7 will then come from the regulator
circuit instead of soft start.

Regulation

The frequency of the power supply is dependent on the load that it sees.
When the picture brightness increases, the load increases, lowering the
voltage across T603/7 and 8. When this occurs, the frequency of the
oscillator decreases, allowing T605 to supply more current to the second-
ary windings. This keeps the 135V line from lowering in voltage.
The following table shows what occurs with different loads on the supply.
Note that the oscillator frequency changed, but the 135V line remained
within a few tenths of a volt of its value. You should also be aware that the
frequency of operation is also dependent on the input AC line voltage.
Location
V across pins 7 and
8/T603
Freq. at IC601/E2
V at CN641/1
White Raster No Input
2.18 VDC 2.38 VDC
62 kHz 66 kHz
135.3 VDC 135.6 VDC