20
As the circuit in Fig. 3 shows, the ON-time is controlled by changing a current charged by C1, which is as the
result of that the detection winding (L1),which detects a change of voltage in a secondary side, connected to the
sensing terminal (Pin No. 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.
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 V
TH1
is set at 0.75V (Ta=25°C) 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 V
TH1
. As long as the INH terminal
voltage does not get lower than V
TH1
. the power transistor sustains OFF mode. On the other hand, a threshold
voltage of COMP.2 V
TH2
, is set at 1.5V (Ta=25°C).When the INH terminnal voltage reaches V
TH2
, an output from
COMP.2 reverses and, as a result, C2 starts firing and a voltage across C2 drops to almost 0V in a moment. As
the result of this immediate discharge of C2, the OFF-time of the oscillator which has been determined by the
product of C2 and R2 (
⁄
55
¥
sec) can be quicker up to approx. 2
¥
sec. As long as the INH terminal voltage
does not get lower than V
TH2
, AVoltage across C2 stays at almost 0V and a output from the oscillator keeps the
power transistor being on. The relation between the INH terminal voltage and the function of the oscillator
described above is shown in Fig. 6 and Fig. 7
R6
R7
PIN +2Di
Reg
Drive
Cfrcu1
R7
R8
R9
R1
R2
C1
C2
+
–
+
–
OSD
L1
COMP 1
COMP 2
VTH1
VTH2
INH
Tre
Fig 3 Oscillator Circuit Configulation
C2 ACROSS
Voltage
C1 ACROSS
Voltage
Osc.
Output
PTR
Fig 4 Operation Waveforms of Oscillator without F/B nor
INH Signal
1V
2.3V
0V
0V
ON
OFF
0.75V
Fig 5 Operation Waveforms of Oscillator with F/B Signal
C2 ACROSS
Voltage
C1 ACROSS
Voltage
Osc.
Output
PTR
1V
2.3V
0V
0V
0.75V
OFF
Ter. Voltage
Osc.
Output
PTR
Fig .6 Operation Waveforms of INH Terminal(V
TH1
)
ON
INH
V
TH1
C2 ACROSS
Voltage
Osc.
Output
PTR
Fig .7 Operation Waveforms of INH Terminal(V
TH2
)
2.3V
ON
OFF
Ter. Voltage.
INH
V
TH2
0V
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