Daewoo DTH-14Q1FS, Service Manual

Page: 23 / 78

21
9
VIN
6
INH
2
GND
C808
R807 L1
D805
D803
S1
+
Fig 8 Operation Circuit
Fig. 9 shows waveforms of V
CE
, Ic and V
INH
of the transistor, voltage across C808 in the oscillator as well as an
output from the oscillator when operating in quasi-resonant mode.
when the power transistor turns off and a voltage higher than V
TH2
is applied to the INH terminal, C808
immediately discharges and then starts charging again. Even after the discharge of energy of a secendary
winding is completed, V
INH
does not immediately increases. When it gets lower than V
TH2
after the time,tr, which
is determined by the production of internal impedance of the IC and C
INH
, has past, the transistor turns on.
4) Quasi-resonant operation
By inputting a voltage signal which is synchronized with the energy discharge time of a secondary winding of a
transformer to the. INH terminal, quasi-resonant operation can be achieved. As shown in Fig. 8 , the voltage of
L1 winding which is synchronized. with the energy discharge time of a secondary winding. S1. shall be input to
the INH terminal through D805 and R807. Since V
TH2
is set at 1.5V typical, a voltage at the INH terminal. V
INH
,
shall be set at 2V.
0V
0V
Fig 9 Waveforms of V
CE
and V
INH
at Quasi-resonant Operation
V
CE
INH Terminal
Voltage
tr
V
THI