Daewoo DTH-14Q1FS, Service Manual

Page: 21 / 78

19
Rs(R803,R804)
Ln
L1
9
V
IN
2
GND
C806
D804
AC
IN
Fig .1 Start-up Circuit
1
Fig .2 V
IN
Terminal Voltage Circuit Current V
IN
IN
20mA
200µA
4.9V 7.5V 8V Vin
4. EXPLAINATION OF CIRCUIT OPERATION
1) V
IN
terminal, start-up circuit
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
C806 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.
2) Osciliator, 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 CI 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°C). On the
other hand, C1 starts charging up through R1 from almost 0V and the voltage across C1 increases in
accordance with the inclination determined by the product of C1 and R1. When the voltage across C1 reaches
approx. 0.75V (Tc=25°C). 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 0V. When the power transistor turns off, C2 starts discharging through R2 and the voltage
across C2 decreases in accordance with the inclination derermined 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.