UCC2817, UCC2818, UCC3817, UCC3818

BiCMOS POWER FACTOR PREREGULATOR

SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009

9

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APPLICATION INFORMATION

and the square of RMS current:

P

COND +

R

DS(on)

K

I2

RMS

Calculating these losses and plotting against frequency gives a curve that enables the designer to determine

either which manufacturer’s device has the best performance at the desired switching frequency, or which

switching frequency has the least total loss for a particular power switch. For this design example an IRFP450

can be found in the Unitrode Power Supply Design Seminar SEM1200, Topic 6, Design Review: 140 W, [Multiple

Output High Density DC/DC Converter].

softstart

The softstart circuitry is used to prevent overshoot of the output voltage during start up. This is accomplished

slowly. Please use the following equation to select a capacitor for the softstart pin.

C

SS +

10

mA

t

DELAY

7.5 V

In an open-loop test circuit, shorting the softstart pin to ground does not ensure 0% duty cycle. This is due to

the current amplifiers input offset voltage, which could force the current amplifier output high or low depending

on the polarity of the offset voltage. However, in the typical application there is sufficient amount of inrush and

bias current to overcome the current amplifier’s offset voltage.

multiplier

The output of the multiplier of the UCC3817 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

MOUT +

I

IAC

V

VAOUT *

1

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.