© Semiconductor Components Industries, LLC, 2000

October, 2000 – Rev. 1

1

Publication Order Number:

AND8031/D

AND8031/D

Isolated Precision

Regulation of a Single

1.8 Volt Output from a

Universal Line Input

Prepared by: Jason Hansen

ON Semiconductor

INTRODUCTION

The following Application Note describes an off–line

switching power supply utilizing a precision programmable

reference to regulate a 1.8 volt output. The center of the app

note is the MC33363B, a monolithic SMPS controller with

a 700 volt power switch, and the NCP100, a sub–one volt

precision programmable reference. The system design and

analysis will be described in detail.

The design requirements are for a universal off–line

converter with a 1.8 volt, 1.0 ampere single output with less

than 50 millivolts ripple and operates at 100 kHz. Most of the

components selected are surface mount. This design is

separated into the generic circuit of the off–line converter

and the feedback network.

Generic Off–Line Conversion Circuit

To reduce the total number of components and thus

minimize circuit board area, an SMPS controller with an

integrated power switch is selected. With the low output

power requirement, the MC33363B is selected as the control

and power switch IC. The MC33363B contains an externally

programmable frequency and current limit, internal startup

circuit and can handle up to 8 watts of output power.

The basic off–line conversion circuit is illustrated in

Figure 1. The line input is filtered through the EMC circuit

then rectified and filtered. The rectified voltage is converted

to a lower voltage via the transformer and the MC33363B.

The secondary of the transformer is rectified to a DC voltage

and filtered. The output voltage controls the duty cycle of the

switcher via the isolated feedback network. The calculated

values are a starting point and do not replace bench testing.

Figure 1. MC33363B Basic Flyback Circuit Schematic

1

3

4

5

6

7

8

16

13

12

11

10

9

MC33363B

Universal

Input

1N4005

R2

4.7

R1

4.7

C1

1 n

D1

D2

D3

D4

U1

4.7

µ

400 V

R4

3.9 k

C6

10

µ

D6

MURS120

D5

MURS160

C5

47 p

R6

2.7 k

R3 30 k

C3 390 p

C4 1

µ

U3

SFH615–4

C10

1

µ

U4

NCP100

R10

7.5 k

R9

12 k

C9

0.1

µ

R12

120

R11

10

Q1

BC858ALT1

D7

MBRD835L

C7

820

µ

+

–

1.8 V

C8

820

µ

C12

1

µ

U2 CTX22–14966

http://onsemi.com

APPLICATION NOTE