Converter; Initial Start Up; Soft Start; Regulation - Sony KV-32FV16 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 transformer's secondary windings where the signal
is 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 regula-
tion due to the efficiency characteristics of the transformer. A second
transformer 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 therefore change the frequency of switching to maintain the 135 volt
line.

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 by keep-
ing Ic601-2 OFF. When IC601-1 turns ON, there is a current path through
C615, T605/1 and 2, and 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 be-
ing 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 T603/7 and 8 Control Winding changes,
the inductance of the other windings will change. This will determine the
frequency of switching.
43

Soft Start

The voltage across T603/7 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 operate above the optimum power
transfer 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 approxi-
mately 7 volts will be applied across T603/7 and 8 Control Winding during
initial turn ON. 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. This lowers
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 White Raster
V across pins 7 & 2.41 VDC
8/T603
Freq. at IC601/E2 74 kHz
V at CN641/1 135.5 VDC
No Input
2.62 VDC
78 kHz
135.7 VDC