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AN1686

APPLICATION NOTE

March 2003

Introduction

Purpose of this note is to provide a few brief design guidelines on the switch-mode power supply whose elec-

trical specification is summarized in the following table.

Table 1. Design Electrical Specification

The special requirement for this converter is the ability to cope with a peak power demand from the load that

exceeds the maximum continuous power by over 66% for a limited time (

≤ 0.5 s), still maintaining output voltage

regulation, and to automatically shut down in case this overload lasts more than a specified time. Furthermore,

Symbol

Parameter

Value

Unit

Input Voltage Range

88 to 264

Mains Frequency

50/60

Hz

Maximum Continuous Output Power

45

W

Peak Output Power (t

≤ 0.5 s) (*)

75

W

18V

± 2%

V

%

Normal Operation Switching Frequency

70

kHz

Light Load Switching Frequency

18

kHz

η

80

%

≤ 2W

≤ 1W

Notes:

by C. Adragna

AN L5991-BASED CONVERTER WITH

TEMPORARY EXTRA POWER CAPABILITY

In some applications the SMPS, normally supposed to deliver a certain amount of power, from time to

time undergoes load peaks that can be even two or more times as much. Such peaks are often too long

to be properly handled by oversizing the output capacitors, but short if compared to the thermal time

constants of the power components. Typical examples of such loads are motors and audio systems.

In this case, designing the SMPS for the peak power demand from the load would lead to a poorly used

and more expensive system. It is more cost-effective to design for the maximum continuous power and

allow the peak power to pass. However, if for any reason the load demands such peak power level for

a long time, some of the power components, not sized for withstanding this, will definitely fail unless the

system is stopped somehow.

In this application note a design example of such a system, based on the ST's advanced PWM control-

ler L5991, is carried out.