UCC2817A, UCC2818A

UCC3817A, UCC3818A

SLUS577B − SEPTEMBER, 2003 − REVISED FEBRUARY 2006

11

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APPLICATION INFORMATION

Multiplier

The output of the multiplier of the UCC3817A is a signal representing the desired input line current. It is an input

to the current amplifier, which programs the current loop to control the input current to give high power factor

operation. As such, the proper functioning of the multiplier is key to the success of the design. The inputs to the

as:

I

V

K

V

VFF

2

where K is a constant typically equal to 1

V

.

The

multiplier. Additionally, curves in Figures 10, 11, and 12 provide typical multiplier characteristics over its entire

operating range.

out of its linear range. A smaller current level is functional, but noise can become an issue, especially at low

of voltage rating constraints of standard 1/4-W resistor, use a combination of lower value resistors connected

in series to give the required resistance and distribute the high voltage amongst the resistors. For this design

example two 383-k

Ω resistors were used in series.

The current into the IAC pin is mirrored internally to the VFF pin where it is filtered to produce a voltage feed

forward signal proportional to line voltage. The VFF voltage is used to keep the power stage gain constant; and

to provid input power limiting. Please refer to Texas Instruments application note SLUA196 for detailed

explanation on how the VFF pin provides power limiting. The following equation can be used to size the VFF

resistance connected between the IAC pin and the rectified line voltage.

R

1.4 V

V

IN(min)

0.9

2

R

IAC

[ 30 kW

(9)

(10)