Soft Start; Limit - Sony KP-43T70 Training Manual

Switching Power Supply
Regulation (cont'd)
The frequency of the power supply is dependent on the load that it sees.
When the picture brightness increases the load increases which lowers
the voltage across T604/7 and 8. When this occurs the frequency of the
oscillator decreases which allows T605 to supply more current to the
secondary windings which keeps the 135V line from lowering in voltage.
Since the 135V line is the only one regulated there will be about a +/-
5% variation in the other supply voltages.
The following chart shows what occurs with different loads on the
supply. Note that the oscillator frequency changes but the 135V line
remains constant.
Location White Raster
V across pins 7+8/T604 2.58V
Freq. ,at IC601/E2 97Khz
V at CN653 135V

Soft Start

The soft start circuit prevents discharged capacitors on the secondary
lines from drawing excessive current during power "start up" and short-
ing the oscillator transistors. The soft start circuit brings the secondary
voltages up slowly.
At power ON, C666, which is discharged, has a 0V potential at its +
terminal. This biases Q654 ON, via R696. With this transistor ON, it
allows standby 12V from Q651/E to be applied across pins 7 and 8 of
T604. This increases the oscillator frequency and reduces efficiency.
Therefore, the start up secondary voltages will be reduced considerably.
The Standby 12V is switched through Q651 when Q652 Relay Drive
turns ON.
ix
As soon as a secondary voltage is produced, C666 starts charging. This
causes the voltage at the positive capacitor terminal to rise, causing
Q654 to decrease conduction, less dc current flows through the trans-
former control winding; the oscillator frequency decreases, and the
secondary voltage increases further.
C666 continues to charge and the secondary voltages continue to rise
until the capacitor charges to the point that its voltage potential is the
same as Q654/E, at this point the transistor stops conducting allowing
IC651 to control the regulation process.

Limit

Limit transistor has two functions:
•
•
No Input
2.73V
Voltage Limiter
103Khz
Zener D664 is a 24V zener that is connected between Q656/B and the
135V 18V line. During normal operation, the potential across the zener is
lower than the zener voltage. Therefore, the zener is OFF and cannot
supply a base voltage to Q656 therefore Q656 will be OFF. Should a
defect cause the 18V line to rise above 24V the zener diode breaks over
and applies bias to Q656/B, turning it ON. This turns Q654 ON, the
voltage across the transformer control winding increases (T604/8 drops
to 3.7V) and the oscillator frequency increases. The end result is a
decrease in transformer efficiency and a drop in the secondary voltages.
Soft Start Reset
Relay RY601 is powered by 12V produced by the Standby power supply
and the relay is turned ON by Q652.
Q652 is OFF when the unit is turned OFF. This removes the ground
path from the relay, opening it up. It also allows the 12V from the relay
to flow through the relay coil, through R693, to Q656/B. This turns the
transistor ON, and discharges C666.
It acts as a non latching voltage limiter.
It is a soft start reset. It discharges C666 when the unit is turned
OFF, preparing it for the next turn ON.