STCMB1 datasheet(30/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

30/59

DocID028958 Rev 4

7.16

Dc brown-out function (HB enable and disable)

Figure 15. Dc brown-out block diagram

The dc brown-out is a function dedicated to manage the correct sequencing of the

converters at the power-on and shutdown. This function is necessary to ensure that the

resonant HB stage starts after the PFC has started up, and PFC output voltage is close to

the nominal level. The function also guarantees that at power-off the resonant HB stops the

operation at a precise input voltage level, thus allowing to properly manage the hold-up

requirements of the power supply. Actually, the PFC bulk capacitor acts as a reservoir

battery and powers the resonant stage when the ac mains disappears.

The function monitors the PFC output voltage on the FB pin and enables the resonant HB

stage operation as the pin voltage reaches 2.4 V (VFB_E typ. value). The resonant stage is

then stopped if the voltage on the FB pin drops to 1.75 V (VFB_D typ. value). The resonant

HB is re-enabled as the voltage on the FB pin rises again above the enable voltage

reference VFB_E.

Considering a PFC delivering a typical 390 V output voltage at nominal Vref, we can easily

calculate the resonant HB stage enable and disable voltages:

Equation 17

VPFC

390V

Vref

2.5V

VFB E

–

2.4V

VFB D

–

1.75V

VHB Start

–

VPFC

VFB E

–

Vref

------------------



374.4V

VHB Stop

–

VPFC

VFB D

–

Vref

------------------



273V

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