AN4077 datasheet(18/33 Pages) STMICROELECTRONICS

18 / 33 page

Test results and significant waveforms

AN4077

18/33

Doc ID 022997 Rev 1

Another function integrated in the L6564H is the brownout protection, which is basically a

not-latched shutdown function that must be activated when a mains undervoltage condition

is detected. This abnormal condition may cause overheating of the primary power section

due to an excess of RMS current. Brownout can also cause the PFC pre-regulator to work in

open loop and this may be dangerous to the PFC stage itself and the downstream converter,

should the input voltage return abruptly to its rated value. Another problem is the spurious

restarts that may occur during converter power-down and that cause the output voltage of

the converter not to decay to zero monotonically. For these reasons it is usually preferable to

shut down the unit in case of brownout. Brownout thresholds are internally fixed at 0.88 V

(typ.) for enabling operation and 0.8 V (typ.) for disabling the L6564H. Sensing of the input

mains condition is done by an internal comparator connected to pin VFF (PIN #5) which

delivers a voltage signal proportional to the input mains.

Figure 22.

L6563 100 W TM PFC

demonstration board: input current

at 100 Vac-50 Hz CFF=0.47 µF,

RFF=390 k

Ω

Figure 23.

L6564H 100 W TM PFC

demonstration board: input current

at 100 Vac-50 Hz CFF=1 µF, RFF=1 M

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