AN4248 datasheet(8/46 Pages) STMICROELECTRONICS

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Mains feed-forward setting

AN4248

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DocID024214 Rev 1

Mains feed-forward setting

The VFF pin is used to feed the controller the input voltage information and accordingly the

internal current sense threshold is decided. The purpose of feed-forward input is to maintain

the constant input power of converter throughout the wide mains variation and fixes the

output overload current threshold constant throughout the wide mains variations.

The linear dynamics of the pin ranges from 0 to 3 V. A voltage higher than 3 V makes the IC

stop switching. If VFF function is not desired, tie the pin to GND (pin 3) directly if a latch-

mode output OVP is not required (see pin 11, ZCD) or through a 10 k min resistor if a latch-

mode OVP is required. Bypass the pin with a capacitor to GND (pin 3) to reduce noise pick-

up. The following figure shows the internal circuit of the controller for feed-forward function

and the variation of current sense threshold (Vcsx) versus mains voltage variations (VFF

signal).

Figure 3. L6566B mains feed-forward section

Figure 4. Power capability change vs. input voltage in QR flyback converters

The optimum value of k, kopt, which minimizes the power capability variations over the input

voltage range, is the one that provides equal power capability at the extremes of the range.

The approximation of this parameter is:

AN4248 Datasheet(PDF) 8 Page - STMicroelectronics