UCC1857

UCC2857

UCC3857

PRELIMINARY

DESCRIPTION

The UCC3857 provides all of the control functions necessary for an Iso-

lated Boost PFC Converter. These converters have the advantage of trans-

former isolation between primary and secondary, as well as an output bus

voltage that is lower than the input voltage. By providing both power factor

correction and down conversion in a single power processing stage, the

UCC3857 is ideal for applications which require high efficiency, integration,

and performance.

The UCC3857 brings together the control functions and drivers necessary

to generate overlapping drive signals for external IGBT switches, and pro-

vides a separate output to drive an external power MOSFET which pro-

vides zero current switching (ZCS) for both the IGBTs. Full programmability

is provided for the MOSFET driver delay time with an external RC network.

ZCS for the IGBT switches alleviates the undesirable turn off losses typi-

cally associated with these devices. This allows for higher switching fre-

quencies, smaller magnetic components and higher efficiency. The power

factor correction (PFC) portion of the UCC3857 employs the familiar aver-

age current control scheme used in previous Unitrode controllers. Internal

circuitry changes, however, have simplified the design of the PFC section

and improved performance.

(continued)

Isolated Boost PFC Preregulator Controller

8

15

4

VD

13

7

3

VIN

CAO

CA–

MOUT

PKLMT

14

MOSDRV

16

IGDRV1

18

IGDRV2

17

PGND

20

CT

6

AGND

19

RT

20

SS

5VREF

12

DELAY

11

VAO

10

VA–

2

CRMS

1IAC

REF

REF

Z

V

Z

C

R

S

Q2

Q1

T1

FEEDBACK

CKT

OPTO

BIAS

SUPPLY

QA

R

AC

V

OUT

+

–

RECTIFIED

AC INPUT

REF

UCC3857

C

F

TYPICAL APPLICATION CIRCUIT

FEATURES

• Single Power Stage

• Zero Current Switched IGBT

• Programmable ZCS Time

• Corrects PF to >0.99

• Fixed Frequency, Average Current

Control

• Improved RMS Feedforward

• Soft Start

• 9V to 18V Supply V Range

• 20-Pin DW, N, J, and L Packages

02/99

UDG-98065