LG PF-43A10 Service Manual page 27

4) DRIVE Circuit
• BASE DRIVE current (IB1) and Sink current (IB2) control
terminal for starting POWER TRANSISTOR.
• COMPARISON DRIVE mode for reduce the switching
Loss of POWER TRANSISTOR.
5) OCP Function (Protective Function Against Over
Current)
This function is operated in the pulse by pulse way by
detecting collector current of power TR Figure 2-g shows
OCP circuit's organization. The detected voltage is set to 1V
or the vasis of GND of intemal control IC and, just because
the detected voltage is set by comparator variation of
temperature approximately makes the drift of detected
voltage )V. This indicates stable feature compared with
temperature.
6) Latch circuit
This is stop to power circuit operating circuit by keeping the
output of oscillator to be low when the two circuits which
protects against over-voltage and over-heat operation.
Sustaining current of latch circuit becomes 500uA MAX
when VIN terminal voltage is 4V.So, If the current flows with
above 500uA in VIN terminal by starting voltage, power
circuit sustains stop condition. To prevent wrong operation
by noise, set delay time with C1 contained HIC. When
OVP,TSD circuit operates and external signal input continues
approximately above 10u sec, latch circuit operates. Even
though latch circuit is operating, static voltage power (Reg)
circuit operates and the circuit current becomes high. As a
result of this, VIN terminal voltage becomes low rapidly. If
VIN terminal voltage is below operating stop voltage
4.9V(TYP), circuit current becomes below 500uA and VIN
terminal voltage starts to ascend. So, if the voltage reaches
starting operation voltage 8V (TYP), circuit current is
increased and VIN terminal voltage rises and falls between
4.9V (TYP) and 8V (TYP). This prevents VIN terminal voltage
from rising abnormally. figure 2-h shows the waveform of
VIN terminal voltage when latch circuit operates.
7) Protect circuit against over-heat.
This is a circuit which makes latch circuit operate when the
frame temperature of HIC is above 150
termperature detectiong is controlled by control circuit
element. But this is a function against over-heat of power-
TR because the system is composed that power-TR and
control circuit element is on the same frame.
8) Protect circuit against over-voltage.
This circuit let latch circuit operate when the VIN terminal
voltage exceeds 10V. (Typ.) Basically, this is used as a
portect circuit against over-voltage of the control circuit VIN
terminal.
But commonly, VIN terminal is supplied with from subsidiry
wire wound of Trans. And for this voltage is proportion to
output voltage, when 2nd output voltage is over or in control
circuit is open, it operates. At this case, 2nd output voltage
when the protect circuit against over-voltage is gained as
following expressing.
Output voltage Vout when normal operation
Vout(0VP) ≒
VIN terminal voltage when normal operation
O C (TYP). Actual
x 100V (Typ)
- 27 -
Vin
9
TR
OSC DRIVE
2
TR
<Figure 2-e> Drive circuit composition
IB
1
IB
2
<Figure 2-f> Power TR(TR1) waveform of each part
OSC
<Figure 2-g> Protective circuit against over-current
VIN
8V
(TYP)
4.9V
(TYP)
<Figure 2-h> V
IN
when latch circuit is operating
IB
1
DRIVE D
1
5 3
R
D
R
S
4
SINK
C
1
3
IB
2
V
CE
I
C
V
BE
I
B
DRIVE
-1V
6 2 GND
OCP
Rocp
t
terminal waveform
NP
1
TR
1
(POWER
TR)
E
2