Daewoo DTQ-14Q1FS, Руководство по обслуживанию

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A Start-up circuit is to start and stop a operation of a control IC by detecting a voltage appearing at a V
IN
terminal(pin-
9).At start up of a power supply, when a voltage at the V
IN
terminal reaches to 8V by charging up C809 by the function
of a start-up resistor, Rs, a control circuit starts operating by the function of the start-up circuit. As shown in Fig. 2, since
a circuit current is suppressed 200
maximum (at V
IN
= 7.5V) until the control circuit starts its operation.
After the control circuit starts its operation, power source is obtained by smoothing voltage appearing at L1 winding.
Once the control circuit starts operating, as its voltage doesn’t reach the fixed voltage at once, V
IN
terminal voltage
starts dropping. However, as a shut-down voltage is set low(at 4.9V), while V
IN
terminal voltage reaches a shutdown
voltage, L1 winding voltage reaches the fixed voltage earlier so that the control circuit can continue on operating.
4. EXPLANATION OF CIRCUIT OPERATION
1) V
IN
terminal, start-up circuit
2) Oscillator, F/B terminal voltage (Pin #7)
A oscillator generates pulse signals which turns a power transistor on and off by making use of charge and discharge of
C1 and C2 incorporated in the Hybrid IC.
Constant voltage control of a switch-mode power supply is performed by changing both ON-time and OFF-time except
when the load is light (ex. remote control stand-by mode of TVs).
Fig. 4 shows how the oscillator works when the Hybrid IC independently operates (with no F/B nor INH signals). When
the power thransistor is on, C2 Is charged to the set voltage (approx 2.3V at Ta = 25 ). On the other hand, C1 starts
charging up through R1 from almost OV and the voltage across C1 reaches approx. 0.75V (Tc = 25 ). the output from
the oscillator is reversed and the power transistor turns off. At the same time C1 is quickly discharged by the function of
a internal circuit of the oscillator and the voltage across it decreases to almost OV. When the power transistor turns off,
C2 starts discharging through R2 and the voltage across C2 decreases with the inclination determined by the product of
C2 decreases to about 1V. the output from the oscillator is reversed again and the transistor consequently turns on. The
power transistor continues turning on and off by repeating the above-mentioned operations.