SSL4101T

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© NXP B.V. 2011. All rights reserved.

Product data sheet

Rev. 1 — 21 April 2011

12 of 29

NXP Semiconductors

SSL4101T

GreenChip III+ SMPS control IC

At high output power the converter switches to quasi-resonant mode. The next converter

stroke is started after demagnetization of the transformer current. In quasi-resonant mode

switching losses are minimized as the converter only switches on when the voltage across

the external MOSFET is at its minimum (valley switching, see also Section 7.3.2).

To prevent high frequency operation at lower loads, the quasi-resonant operation changes

to discontinuous mode operation with valley skipping in which the switching frequency is

on when the voltage across the MOSFET is at its minimum.

At very low power and standby levels the frequency is controlled down by a Voltage

Controlled Oscillator (VCO). The minimum frequency can be reduced to zero. During

resistor and the maximum sense voltage.) As the primary peak current is low in frequency

frequencies in the audible range. Valley switching is also active in this mode.

In frequency reduction mode the PFC controller is switched off and the flyback maximum

frequency changes linearly with the control voltage on the FBCTRL pin (see Figure 7 ).

For stable on and off switching of the PFC, the FBCTRL pin has a 50 mV (typical)

hysteresis. At no load operation the switching frequency can be reduced to (almost) zero.

7.3.2 Valley switching (HV pin)

Refer to Figure 8. A new cycle starts when the external MOSFET is activated. After the

on-time (determined by the FBSENSE voltage and the FBCTRL voltage), the MOSFET is

switched off and the secondary stroke starts. After the secondary stroke, the drain voltage

shows an oscillation with a frequency of approximately:

(1)

Fig 7.

Frequency control of flyback part

1.5 V

discontinuous

with valley

switching

quasi resonant

frequency

reduction

PFC off

switching frequency

014aaa159

PFC on

1

2

π

×

L

p

C

d

×

()

×

()

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