AN3027 datasheet(33/41 Pages) STMICROELECTRONICS

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AN3027

L6563H: high-voltage startup transition-mode PFC

Doc ID 16134 Rev 4

33/41

Figure 21.

High-voltage startup behavior during latch-off protection

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

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 VccOFF threshold, the IC stops switching, reduces consumption and drops

more until the Vccrestart 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.

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