Anthony Catalano - EEWeb

TECHNICAL A System Perspective ARTICLEon
Specifying
Electronic Power Supplies:
Efficiency
A B
Bob Stowe
Power Supply Design Consultant
In the last installment of this series entitled “A
System Perspective on Specifying Electronic Power
Supplies,” we discussed the effects of source
characteristics upon power supply specification. In
this installment, we will learn about the importance of
efficiency for your system and how to specify it.
What is Power Supply Efficiency?
Figure 1 shows the typical power flow from a source,
through the power supply, and on to the load. Power
supplies are not ideal, so not all of the power supply
input power is transferred to the load as useful power.
A portion of the input power is instead dissipated to the
power supply environment as heat — as represented by
the red highlighting around the power supply.
Power supply efficiency, then, is a measure of how much
input power is transferred to the load as useful and
desirable power, rather than dissipated in the form of
heat in the power supply. Efficiency is expressed either
as a percentage figure, or as a decimal figure. When
expressed as a percentage, 100% is perfect efficiency
and typical efficiencies for power supplies range
SOURCE Power In POWER Power Out LOAD
SUPPLY
Figure 1
Waste Power
To Power Supply
Environment
anywhere from 35% to 96% depending on the type and
application of the supply. When expressed as a decimal,
1 is perfect efficiency and typical efficiencies range
anywhere from 0.35 to 0.96.
The following equation summarizes power flow:
Pin = Pout + Pwaste
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TECHNICAL ARTICLE