INTRODUCTION

The minimization of the power drawn from the mains under light load conditions (Standby, Suspend or

some other idle mode) is an issue that is recently becoming of great interest, above all else because

new and more severe standards are coming into force.

This is already well-established in the area of computer monitors, where norms define precisely the vari-

ous idle modes and the relevant maximum consumption admitted, but more and more often power sup-

plies for other pieces of office equipment (i.e. printers, photocopiers, fax machines, AC-DC adaptors,

etc.), are required to accomplish with specifications concerning energy saving.

Anyway, minimizing the power wasted by a lightly loaded switch-mode converter is a demanding chal-

lenge for power supply designers and, to achieve the goal, an appropriate design strategy is required.

March 2000

®

AN1049

APPLICATION NOTE

MINIMIZE POWER LOSSES OF LIGHTLY LOADED FLYBACK

CONVERTERS WITH THE L5991 PWM CONTROLLER

by Claudio Adragna

The L5991 PWM controller is particularly suitable for SMPS of equipment that must comply with

standards concerning energy saving. The device, optimized for flyback topology, monitors the power

demanded by the load and changes the operating frequency of the converter accordingly: high fre-

quency at heavy load, low frequency at light load.

In this way, power losses dependent on frequency are reduced at light load maintaining, at the same

time, the advantages offered by a high switching frequency at heavy load.

The frequency reduction is very helpful but is not the only means needed to minimize power losses.

This note surveys the above mentioned functionality of the L5991 (called "Standby" function) as well

as the most significant points to consider in order to achieve the goal of a very efficient lightly loaded

flyback.

+

-

TIMING

2

3

+

-

14

T

Vref

CLK

2.5V

+

-

1.2V

13

BLANKING

PWM

FAULT

SOFT-START

R

SQ

21V

10V

VREF

OK

DIS

+

-

E/A

1V

R

2R

DIS

2.5V

7

6

5

11

10

9

4

8

15

1

17V

+

-

PWM

UVLO

12

SGND

COMP

SS

ISEN

DIS

DC

RCT

SYNC

DC-LIM

VREF

VFB

PGND

OUT

OVER

CURRENT

16

ST-BY

VREF

STANDBY

+

-

STANDBY

2.5/4.0V

Figure 1. L5991 Internal Block Diagram

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