SSL4120 datasheet(18/48 Pages) NXP

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SSL4120

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Product data sheet

Rev. 2 — 1 November 2012

18 of 48

NXP Semiconductors

SSL4120

Resonant power supply controller IC with PFC for LED lighting

The valley sensing block connected to the SNSAUXPFC pin detects the valleys. This

block measures the PFC transformer auxiliary winding voltage which is a reduced and

inverted copy of the MOSFET drain voltage. When a valley of the drain voltage (= top at

SNSAUXPFC voltage) is detected, the MOSFET is switched on.

If a top is not detected on the SNSAUXPFC pin (a valley at the drain) within tto(vrec) =4 s

after demagnetization is detected, the MOSFET is forced to switch on.

7.7.5 PFC frequency and off-time limiting

The switching frequency is limited to fmax(PFC) for transformer optimization and to minimize

switching losses. If the frequency for quasi-resonant operation > fmax(PFC), the system

switches to DCM. The PFC MOSFET is switched on when the drain-source voltage is at a

minimum (valley switching).

The minimum off-time is limited at toff(PFC)min to ensure correct control of the PFC

MOSFET under all circumstances.

Fig 8.

Demagnetization and valley detection

demagnetized

VRect/N

VRect

Vboost

(Vboost - VRect)/N

Vdemag(SNSAUXPFC)

magnetized

Demagnetization

lTr(PFC)

Aux(PFC)

Dr(PFC)

GATEPFC

Valley

(= top for detection)

off

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