Service manual
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 VOCPBD(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 VDS
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 VOCPBD(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 VOCPBD(TH2)≒0.8V, the MOSFET remains OFF while
VOCPBD(TH1) ≒0.4V and more is imposed on. That is, the gap between VOCPBD(TH1) and
VOCPBD(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
３３
77
Cont
OCP
/BD
44
ROCP
C4
C5
R4
R5
C10
D3
D4
R9
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