AN3027 Datasheet, PDF (33/41 Pages) STMicroelectronics – How to design a transition-mode PFC pre-regulator

AN3027
L6563H: high-voltage startup transition-mode PFC
Figure 21. High-voltage startup behavior during latch-off protection
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As a result, shown in Figure 21, the voltage at pin Vcc oscillates between its turn-on and
turn-off thresholds until the HV bus is recycled and drops below the startup threshold of the
HV generator. The high-voltage startup circuitry is capable of guaranteeing safe behavior if
a short-circuit is present on the DC-DC output when Vcc of both controllers is generated by
the same auxiliary winding.
Figure 22. High-voltage startup, managing the DC-DC output short-circuit
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Figure 22 shows how the PFC manages Vcc cycling and the associated power transfer.
At short-circuit the auxiliary circuit is no longer able to sustain Vcc which starts dropping.
Reaching its Vcc_OFF threshold, the IC stops switching, reduces consumption and drops
more until the Vcc_restart threshold is tripped. The high-voltage startup generator restarts and
when Vcc crosses again its turn-on threshold, the IC starts switching. In this manner the
power is transferred from the mains to the PFC output only during a short time for each trip
cycle.
Doc ID 16134 Rev 4
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