SSL4120 datasheet(23/48 Pages) NXP

23 / 48 page

SSL4120

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Product data sheet

Rev. 2 — 1 November 2012

23 of 48

NXP Semiconductors

SSL4120

Resonant power supply controller IC with PFC for LED lighting

MOSFETs. In capacitive mode, the body diode in the switched off MOSFET can start

conducting. Switching on the other MOSFET at this instant can result in the immediate

destruction of the MOSFETs.

The advanced adaptive non-overlap time of the SSL4120 always waits until the slope at

the half-bridge node starts. It guarantees safe switching of the MOSFETs in all

circumstances. Figure 11 shows the adaptive non-overlap time function operation in

capacitive mode.

In capacitive mode, half the resonance period can elapse before the resonant current

changes back to the correct polarity and starts charging the half-bridge node. The

oscillator is slowed down until the half-bridge slope starts to allow this relatively long

waiting time. See Section 7.8.5 for more details on the oscillator.

The MOSFET is forced to switch on when the half-bridge slope fails to start and the

oscillator voltage reaches Vu(CFMIN).

The switching frequency is increased to eliminate the problems associated with capacitive

mode operation (see Section 7.8.11).

7.8.5 HBC slope controlled oscillator (CFMIN and RFMAX pins)

The slope-controlled oscillator determines the half-bridge switching frequency. The

oscillator generates a triangular waveform between Vu(CFMIN) and Vl(CFMIN) at the external

capacitor Cfmin.

Figure 12 shows how the frequency is determined.

Fig 11. Adaptive non-overlap time function (capacitive operation)

VSNSFB

ISNSFB

Vpu(SNSFB)

Ifmin(SNSFB)

VRFMAX

Ifmax(SNSFB)

VSSHBC = 8.4 V

Iolp(SNSFB)

Volp(SNSFB)

Vfmin(SNSFB)

Vfmax(SNSFB)

Vclamp(SNSFB)

VSNSFB

Iclamp(SNSFB)

014aaa862

Vfmax(ss)(RFMAX)

Vfmax(fb)(RFMAX)

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