SSL8516T datasheet(14/33 Pages) NXP

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SSL8516T

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Product data sheet

Rev. 1 — 5 May 2014

14 of 33

NXP Semiconductors

SSL8516T

Greenchip PFC and flyback controller

Figure 9 shows the drain voltage, valley signal, secondary stroke signal and the internal

oscillator signal.

Valley switching allows high frequency operation because capacitive switching losses are

reduced (see Equation 4). High frequency operation makes small and cost-effective

magnetic components possible.

(4)

7.3.3 Current mode control (FBSENSE pin)

Current mode control is used for the flyback converter because of its good line regulation.

The FBSENSE pin senses the primary current across an external resistor and compares it

to an internal control voltage. The internal control voltage is proportional to VFBCTRL (see

Figure 10).

The FBSENSE pin outputs a current of 2.1

A. This current runs through the resistors

from the FBSENSE pin to the sense resistor RSENSE2 and creates an offset voltage.

Resistor RSS2 causes the offset voltage because RSENSE2 is relatively small. The minimum

flyback peak current is adjusted using this offset voltage. Adjusting the minimum peak

current level, changes the frequency reduction slope (see Figure 7).

(1) Start of a new cycle at lowest drain voltage.

(2) Start of a new cycle in a classical Pulse-Width Modulation (PWM) system without valley detection.

Fig 9.

Signals for valley switching

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