NCP1602 Datasheet, PDF (22/32 Pages) ON Semiconductor – Enhanced, High-Efficiency Power Factor Controller

High Current
No delay è CrM
NCP1602
Low Current
The next cycle is
delayed
Timer delay
Lower Current
Longer dead−time
Timer delay
Figure 72. Valley Switching Operation in CrM and DCM Modes
As illustrated in Figure 72, under high load conditions, the
boost stage is operating in CrM but as the load is reduced, the
controller enters controlled frequency discontinuous
operation.
Valley Synchronized Frequency Foldback (VSFF)
a/ Valley Synchronized (VS)
To further reduce the losses, the MOSFET turns on is
stretched until its drain−source voltage is at its valley. The
end of the dead time is synchronized with the drain−source
ringing.
DRV
DRV
Zero Current Detection
200−us
WATCHDOG
DEMAG
SENSING
END OF DEMAG
SENSING
DRV
Dead−Time (DT)
DT
DEAD TIME
GENERATOR
DRV
Ramp for DT Control
Vctrl
VCTRL
CS/ZCD
Vcsint
CSZCD
BUFFER
ZCD
CLK
ZCD TIMER
DRV
END OF DEAD TIME
SYNCHRONIZATION
Clock Generation
DRV
Figure 73. Valley Synchronized Turn−on Block Diagram
Valley Synchronized is the first half of the VSFF system.
Synchronizing the Turn−on with the drain voltage valley
maximizes the efficiency at both nominal and light load
conditions. In particular, the stand−by losses are reduced to
a minimum. The synchronization of Power MOSFET
Turn−on (rising edge of CLK signal) with drain voltage
valley is depicted on Figure 73. This method avoids system
stalls between valleys. Instead, the circuit acts so that the
PFC controller transitions from the n valley to (n+1) valley
or vice versa from the n valley to (n−1) cleanly as illustrated
by the simulation results of Figure 74. When the Line
voltage and inductor current are very low, or when the
amplitude of the drain voltage gets too low (in the case of
long dead times), the turn−on of the power MOSFET is no
longer synchronized with the drain valley but will start
exactly at the end of a programmed dead time looks to the
ZCD TIMER block.
If no demagnetization is sensed the power MOSFET will
be turned−on after a watchdog timing of 200−ms.
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