Daewoo DTR-14D3VG, Service Manual

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(7) OCP (overcurrent protection) function
Overcurrent protection is performed pulse by pulse by directly
detecting collector current of the power transistor.
Configuration of the OCP circuit is shown at the right.
Detecting voltage is set to -1V below a reference point of
GND(ground). In addition, since the detecting voltage is set by
a comparator, very stable characteristics against temperature
is achieved and drift of the detecting voltage against
temperature change is almost 0V.
(8) Latch circuit
It is a circuit which sustains an output from the oscillator low and stops operation of the power supply when
overvoltage protection (OVP) circuit and thermal shutdown(TSD) circuit are in operation. As the sustaining current of
the latch circuit is 500µA maximum when V
IN
terminal voltage is 4V, the power supply circuit sustains the off state as
long as current of 500µA minmum flows to V
IN
terminal from a start-up resistor. In order to prevent a malfunction to be
caused by a noise and so on, delay time is provided by C1 incorporated in the IC and, therefore, the latch circuit
operates when the OVP or TSD circuit is in operation, or an external signal input is provided for about 10µsec or
longer. In addition, even after the latch circuit start operating, the constant voltage regulator (Reg) circuit is in
operation and the circuit current is at high level. As a result, V
IN
terminal voltage rapidiy decreases. When V
IN
terminal
voltage become lower than the shutdown voltage, V
IN
(
OFF
), (4.9V typical), it starts increasing as the circuit current is
below 500µA. When it reaches the ON-state voltage, V
IN
(
ON
) (8V typical), Vin terminal voltage starts decreasing
because the circuit current increases again.
(9) Thermal shutdown circuit
It is a circuit to trigger the latch circuit when the frame temperature of the IC exceeds 150˚C (typical). Although the
temperature is actually sensed at the control chip, it works against overheating of the power transistor as the power
transistor and the control IC are mounted on the same lead frame.
(10) Overvoltage protection circuit
It is a circuit to trigger the latch circuit when Vin terminal voltage exceeds 10V (typical). Although it basically functions
as protection of V
IN
terminal against overvoltage, since V
IN
terminal is usually supplied from the drive winding of the
transformer and the voltage is proportional to the output voltage, it also functions against the overvoltage of secondary
output which cause when the control circuit opens or in some other events.