Service manual

CP-850FX Service Manual
Europe R&D
73
5.10.5.2 Bottom-Skip Operation (Switching from Quasi-Resonant Operation)
The basic bottom-skip operation is that the load of the secondary side is detected by the drain
current value (actually OCP/BD terminal voltage), which switches to the Quasi-Resonant (at
heavy load) and the Bottom-Skip operation (at light load). The timing of distinguishing is made
by taking the OCP/BD terminal voltage in at start-down of the MOSFET gate voltage of the HIC.
Furthermore, the number of start-down (OCP/BD terminal voltage is under VOCPBD(TH1)) is
measured, which makes the MOSFET turn ON in accordance with the mode described above.
BottomSkip Quasi-Resonant
Quasi-Resonant
IDS
VDS
Detection
level
MOSFET Gate
（ PowerIC internal ）
Bottom detect signal
（ PowerIC internal ）
State signal
（ PowerIC internal ）
Hysteresis
OCP/BD
V OCPB D(LIM)
V OCPB D(TH2)
V OCPB D(TH１)
V OCPB D(BS1)
V OCPB D(BS2)
V OCP
図16- ボトムスキップ擬似共振動作タイミングチャート
1). Quasi-Resonant Operation ⇒ Bottom-Skip Operation
The Quasi-Resonant is operated under the mode that VOCP is higher than VOCPBD(BS2) at the
absolute rating. Where the load becomes lighter than that of the mode, the drain current falls.
As the result, the mode is switched to the Bottom-Skip operation when the VOCP becomes lower
than VOCPBD(BS2) at the absolute rating, and the standard voltage is automatically changed to
VOCPBD(BS1). Fig 16 shows the switching timing chart from the Quasi-Resonant to the
Bottom-Skip operation.
2). Bottom-Skip Operation ⇒ Quasi-Resonant Operation
The Bottom-Skip is operated under the mode that VOCP is lower
than VOCPBD(BS1) at the absolute rating. Where the load
becomes higher than that of the mode, the drain current rises. As
the result, the mode is switched to the Quasi-Resonant operation
when VOCP becomes higher than VOCPBD(BS1) at the absolute
0V
QR BottomSkip
Hysteresis
V OCPBD(LIM)
V OCPBD(BS2)
V OCPBD(BS1)
Fig.16. Bottom-Skip Quasi-Resonant Operation Timing Chart
Fig.17. Operation Mode switching
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