SSL4101T datasheet(9/29 Pages) NXP

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SSL4101T

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Product data sheet

Rev. 1 — 21 April 2011

9 of 29

NXP Semiconductors

SSL4101T

GreenChip III+ SMPS control IC

7.2.3 Frequency limitation

To optimize the transformer and minimize switching losses, the switching frequency is

limited to fsw(PFC)max. If the frequency for quasi-resonant operation is above the fsw(PFC)max

limit, the system switches over to discontinuous conduction mode. Also here, the PFC

MOSFET is only switched on at a minimum voltage across the switch (valley switching).

7.2.4 Mains voltage compensation (VINSENSE pin)

The mathematical equation for the transfer function of a power factor corrector contains

the square of the mains input voltage. In a typical application this results in a low

bandwidth for low mains input voltages, while at high mains input voltages the Mains

Harmonic Reduction (MHR) requirements may be hard to meet.

To compensate for the mains input voltage influence, the SSL4101T contains a correction

circuit. Via the VINSENSE pin the average input voltage is measured and the information

is fed to an internal compensation circuit. With this compensation it is possible to keep the

regulation loop bandwidth constant over the full mains input range, yielding a fast transient

response on load steps, while still complying with class-D MHR requirements.

In a typical application, the bandwidth of the regulation loop is set by a resistor and two

capacitors on the PFCCOMP pin.

7.2.5 Soft start-up (pin PFCSENSE)

To prevent audible transformer noise at start-up or during hiccup, the transformer peak

current, IDM, is increased slowly by the soft start function. This can be achieved by

inserting RSS1 and CSS1 between pin PFCSENSE and current sense resistor RSENSE1.

An internal current source charges the capacitor to VPFCSENSE =Istart(soft)PFC × RSS1. The

voltage is limited to Vstart(soft)PFC.

The start level and the time constant of the increasing primary current level can be

adjusted externally by changing the values of RSS1 and CSS1.

The charging current Istart(soft)PFC flows as long as the voltage on pin PFCSENSE is

below 0.5 V (typ). If the voltage on pin PFCSENSE exceeds 0.5 V, the soft start current

source starts limiting current Istart(soft)PFC. As soon as the PFC starts switching, the

Istart(soft)PFC current source is switched off; see Figure 5.

τsoftstart

SS1

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