SSL8516T datasheet(13/33 Pages) NXP

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SSL8516T

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Product data sheet

Rev. 1 — 5 May 2014

13 of 33

NXP Semiconductors

SSL8516T

Greenchip PFC and flyback controller

Valley switching is active in all operating modes.

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

to discontinuous mode operation with valley skipping. When the frequency limit is

reached, the quasi-resonant operation changes to DCM with valley skipping. The

frequency limit reduces the MOSFET switch-on losses and conducted EMI.

At medium and low power levels, the controller enters Frequency Reduction (FR) mode. A

Voltage Controlled Oscillator (VCO) controls the frequency. During frequency reduction

mode, the primary peak current is kept at an adjustable minimal level to maintain a high

efficiency. Valley switching is also active in this mode.

In frequency reduction mode, the PFC controller switches off when the flyback switching

frequency has dropped to below 53 kHz and VPFCTIMER has reached 3 V. The flyback

maximum frequency changes linearly with the control VFBCTRL (see Figure 8). Hysteresis

is added to ensure a stable PFC switch-on and switch-off. In no-load operation, the

switching frequency is reduced to (almost) zero.

7.3.2 Valley switching (HV pin)

A new cycle starts when the external MOSFET is switched on. VFBSENSE and VFBCTRL

determine the on-time. The MOSFET is then switched off and the secondary stroke starts

(see Figure 9). After the secondary stroke, the drain voltage shows an oscillation with a

frequency of approximately:

(3)

where Lp is the primary self-inductance of the flyback transformer and Cd is the

capacitance on the drain node.

When the secondary stroke ends and the internal oscillator voltage is high again, the

circuit waits for the lowest drain voltage before starting a new primary stroke.

Fig 8.

Flyback frequency control











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