STCMB1 datasheet(26/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

26/59

DocID028958 Rev 4

The PFC OCP is integrated on the ISEN_PFC pin too: it is equipped with an OCP

comparator, stopping the MOSFET conduction time in case the current reaches the

threshold VISEN_PFC (-0.5 V typ.), realizing a pulse-by-pulse current limiting.

Calculation of the sensing resistor can be done according to the following procedure:

Calculate the maximum peak current in the PFC inductor/MOSFET/diode:

Equation 13

Calculate the maximum sense resistor value:

Equation 14

Select the commercial value Rs, next to Rsx.

Calculate the ILrms current:

Equation 15

Calculate the sensing resistor(s) power dissipation:

Equation 16

7.11

PFC: zero-current detector

The PFC embedded in the STCMB1 uses the ZCD pin as the boost inductor's

demagnetization sensing input. A negative-going edge triggers the PFC MOSFET's turn-on.

The pin is provided with two clamps to protect the pin from voltage above its rating, so it can

be connected to an auxiliary winding of the boost inductor through a resistor. A series

limiting resistor has to be provided to limit the current injected in the pin voltage clamps. The

demagnetization can also be sensed via an RC series connected to the MOSFET.

For debug purposes it is possible disable the PFC stage by pulling this pin over 2.4 V at the

start-up, thus allowing the half-bridge section to work standalone.

7.12

Starter

Because any transition mode PFC is basically a self-oscillating converter, using a self-

generated signal for the MOSFET to turn-on, a starter signal is necessary to start the

operation at power-on or restart the operation after an idle state, when the ZCD signal is not

present.

The STCMB1 device is provided with an internal starter having a 10

s period and working

at wake-up, at the burst mode restart, after OVP or inrush current protection intervention.

max

min

max

Lth





max

min

PFC

ISEN







Lvy

Lrms





max

PRs ILrms



STCMB1 Datasheet(PDF) 26 Page - STMicroelectronics

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