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3DQDVRQLF
1.6 Power supply STR-S5707
1.6.1 Vin 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
Vin terminal (pin 9). At start up of a power supply, when a voltage at the Vin terminal reaches to 8V
(typical) by charging up C817 by the function of a start-up resistor, R804, a control circuit starts
operating by the function of the start-up circuit. After the control circuit starts its operation, power
source is obtained by smoothing voltage appearing at winding of pin 6,7 of T801.
1.6.2 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). The on-time is controlled by changing a current charged by
C1, which is as the result of that the detection winding of pin 5-7 of T801, which detects a change of
voltage in a secondary side, connected to the sensing terminal (pin 7). has the current in accordance
with an output signal from an output voltage detection circuit (an error amplifier) built in. As an AC
input voltage to the power supply gets the higher and a load current the smaller, the current flowing to
the SENS terminal gets the larger, and the on-time gets the shorter.
1.6.3 Function of INH terminal (pin 6), control of off-time
Signal to the INH terminal is used as inputs to COMP.1 and COMP.2 inside of the control IC. A
threshold voltage of COMP.1,VTH1 is set at 0.75V(Ta=25°) and an input signal to a drive circuit
becomes almost 0V (the power transistor is in OFF mode) when a voltage at the INH terminal reaches
the VTH1. A threshold voltage of COMP.2, VTH2, is set at 1.5V (Ta=25°). When the INH terminal
voltage reaches VTH2, an output from COMP.2 reverses (the power TR is in on mode).
* Quasi-resonant operation
By inputting the voltage of winding of pin 6-7 of T801 which is synchronized with the energy discharge
time of a secondary winding to the INH terminal through D810 and R808, quasi-resonant operation
can be achieved.
When the power transistor turns off and a voltage higher than VTH2 is applied to the INH terminal,C3
immediately discharges and then starts charging again. Even after the discharge of energy of a
secondary winding is completed, VINH does not immediately increases. when it gets lower than VTH1,
the transistor turns on.
1.6.4 Drive circuit
The STR-S5707 applies the proportional drive system in order to minimize turn-on and saturation loss,
and storage time.
1.6.5 OCP function
Over-current protection is performed pulse by pulse by directly detecting collector current of the power
transistor. detecting voltage is set to -1V below a reference point of GND (ground).
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