Daewoo DTF-2950-100D, Руководство по обслуживанию, Страница: 73 / 92

CP-850FX Service Manual
Europe R&D 72
5.10.5 Quasi-Resonant and Bottom-Skip Operation
5.10.5.1
Quasi-Resonant Operation
The Quasi-Resonant operation is to match the timing of
the MOSFET Turn-ON to the bottom point of the voltage
resonant waveform after a transformer releases the
energy (i.e., 1/2 cycle of the resonant-frequency).
As shown in Fig.15, the voltage resonant condenser C4
is connected between the drain and source, and the
delay circuit, C10, D3, D4, and R9 are connected
between the auxiliary winding D and OCP/BD terminal
(Pin No.7). Where the MOSFET is turned OFF, the
Quasi-Resonant signal is made of the fly-back voltage
generated in the auxiliary winding, which operates BD
comparator, and it provides the Quasi-Resonant
operation. Due to the operation of the delay circuit, even
if the energy of the transformer is released to complete,
the Quasi-Resonant signal imposed on Pin No. 7 terminal
does not fall immediately. This is why the C10 is
discharged by R4, and after a certain period, the voltage
falls to the threshold voltage V
OCPBD
(TH1)
≒
0.4V and
below. Consequently, the delay- time needs to be set by
adjusting C10 monitoring the operating waveform in
order to turn ON the MOSFET at that time when the V
DS
of the MOSFET reaches the lowest point.
In addition to the Quasi-Resonant operation, in order to control the increase of the oscillating
frequency at light to medium load, the Bottom-Skip operation widening OFF time is built-in in
accordance with the load volumes. The switching timing between the Quasi-Resonant and
Bottom-Skip operation is described in the item No. 8.5.2.
Where the Quasi-Resonant signal voltage imposed on OCP/BD terminal is below V
OCPBD(TH2)
≒
0.8V, the internal oscillator starts PWM operation with the fixed oscillating frequency (
≒
22kHz).
The PWM operation is also provided at power supply start-up or low auxiliary winding voltage
such as
winding-short, which lowers oscillating frequency, and the stress of the MOSFET is fairly reduced.
After the Quasi-Resonant signal is over V
OCPBD(TH2)
≒ 0.8V, the MOSFET remains OFF while
V
OCPBD
(TH1)
≒ 0.4V and more is imposed on. That is, the gap between V
OCPBD(TH1)
and
V
OCPBD(TH2)
prevents the HIC from operating improperly.
While, in the setting up R9 and R4, the Quasi-Resonant signal imposed on the OCP/BD terminal
needs to be 5V or below since the OCP/BD terminal voltage is maximum 5V. At the normal
condition, it should be 1.5V approximately.
Fig.15. Quasi-Resonant and Delay Circuit
P
D D
S/GND
V
CC
11
３ ３
77
Cont
OCP
/BD
44
ROCP
C4
C5
R4
R5
C10
D3
D4
R9
Downloaded From TV-Manual.com Manuals