Philips Semiconductors Linear Products

Product specification

UC3842

Current-mode PWM controller

August 31, 1994

1106

OFF-LINE FLYBACK REGULATOR

UC3842

4.7 1W

673-3

AC

INPUT

250V

56k

1W

16V

20k

150k

3.6k

100pF

COMP

10k

2

1

8

4

7

6

3

5

GND

CUR

SEN

OUT

27

20k

1k

0.85

UFN432

1N3613

ISOLATION

BOUNDARY

2.5k

820pF

1N3613

400V

4.7k

4W

T1

USD735

10V

DC

OUT

(5V 2–5A)

+

–

220

µF

10

µF

20V

0.01

µF

0.01

µF

0.0047

µF

0.01

µF

4700

µF

470pF

VFB

VREF

RT/CT

VCC

NOTES:

T1: Coilcraft E-4140-B

Primary – 97 turns

single AWG24

Secondary – 4 turns

4 parallel

AWG22

control – 9 turns

–3 parallel AWG28

SPECIFICATIONS

Input line voltage:

Input frequency:

50 or 60Hz

Switching frequency:

40kHz

±10%

Output power:

25W maximum

Output voltage:

5V

±5%

Output current:

2 to 5A

Line regulation:

0.01%/V

Load regulation:

Efficiency @ 25 W,

70%

65%

Output short-circuit current:

2.5A average

NOTE:

This circuit uses a low-cost feedback scheme in which the DC

voltage developed from the primary-side control winding is sensed

by the UC3842 error amplifier. Load regulation is therefore

dependent on the coupling between secondary and control

windings, and on transformer leakage inductance. For applications

requiring better load regulation, a UC1901 Isolated Feedback

Generator can be used to directly sense the output voltage.