AN1686 Datasheet, PDF (1/7 Pages) STMicroelectronics – AN L5991-BASED CONVERTER WITH

AN1686
APPLICATION NOTE
AN L5991-BASED CONVERTER WITH
TEMPORARY EXTRA POWER CAPABILITY
by C. Adragna
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.
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
Symbol
Parameter
Value
Unit
V_in
Input Voltage Range
88 to 264
V_ACrms
f_L
Mains Frequency
50/60
Hz
P_outx
Maximum Continuous Output Power
45
W
P_outpk Peak Output Power (t ≤ 0.5 s) (*)
75
W
V_out
Regulated Output Voltage (@ P_out =0 ÷ P_outpk, V_in = 88÷264 V_AC)
18V ± 2%
V
∆V_out Output Voltage Ripple (@P_out = P_outx, V_in =88÷264 V_AC)
2
%
F_osc
Normal Operation Switching Frequency
70
kHz
F_SB
Light Load Switching Frequency
18
kHz
η
Target Efficiency (@P_out = P_outx, V_in =88÷264 V_AC)
80
%
Maximum Input Power (@P_out = 0.5 W, V_in =88÷264 V_AC)
≤2
W
Maximum Input Power (Open load, V_in =88÷264 V_AC)
≤1
W
Notes:
(*) If P_out is such that P_outx < P_out ≤ P_outpk the converter shall be shutdown (latch mode) within 1÷2 s; a load exceeding
P_outpk shall cause converter’s shutdown (latch mode) within 10÷100 ms.
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,
March 2003
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