SSL4120T datasheet(21/47 Pages) NXP

21 / 47 page

SSL4120T

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Objective data sheet

Rev. 1 — 21 June 2012

21 of 47

NXP Semiconductors

SSL4120T

Resonant power supply control IC with PFC

on. In this way the non-overlap time is optimized automatically, minimizing switching

losses, even if the HB transition cannot be fully completed. Figure 10 illustrates the

operation of the adaptive non-overlap time function in Inductive mode.

The non-overlap time depends on the HB slope, but has upper and lower limits.

An integrated minimum non-overlap time, tno(min), prevents cross conduction occurring

under any circumstances.

The maximum non-overlap time is limited to the oscillator charge time. If the HB slope

lasts longer than the oscillator charge time (= 1⁄4 of HB switching period) the MOSFET is

forced to switch on. In this case the MOSFET is not soft switching. This limitation ensures

that, at very high switching frequencies, the MOSFET on-time is at least 1⁄4 of the HB

switching period.

7.8.4.2

Capacitive mode

The description above holds for normal operation with a switching frequency above the

resonance frequency. When an error condition occurs (e.g. output short, load pulse too

high) the switching frequency can be lower than the resonance frequency. The resonant

tank then has a capacitive impedance. In Capacitive mode, the HB slope does not start

after the MOSFET has switched off. Switching on the other MOSFET is not recommended

in this situation. The absence of soft switching increases dissipation in the MOSFETs. In

Capacitive mode, the body diode in the switched-off MOSFET may 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 SSL4120T will always wait until the slope

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

circumstances. Figure 11 illustrates the operation of the adaptive non-overlap time

function in Capacitive mode.

In Capacitive mode, half the resonance period may 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.

Fig 10. Adaptive non-overlap time function (normal inductive operation)

fast HB slope

VBoost

GATELS

GATEHS

slow HB slope

incomplete HB slope

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