AN2941 datasheet(14/37 Pages) STMICROELECTRONICS

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Efficiency measurement

AN2941

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Doc ID 15447 Rev 4

requirements. The major computer manufacturers require efficiency higher than 70% from 1

W to 2 W input power and efficiency higher than 75% from 2 W to 3 W input power.

Figure 6

shows the demonstration board’s measured efficiency at light load, comparing it

with requirements coming from the reference ENERGY STAR® document.

Figure 6.

Light-load efficiency measurements

EPA rev. 5.0 computers power supply compliance

As indicated in Table 3, this converter is also compliant with the newer, most stringent “EPA

rev. 5.0 computers” limits for active-load efficiency.

This latest ENERGY STAR® document, whose effective date is July 2009, introduces a new

method of testing the energy performance of computers. It sets a limitation on typical energy

consumption (TEC) of computers which is a value for typical annual electricity use, by

measurements of average operational mode power levels scaled by an assumed typical

usage model (duty cycle). This new approach, like the previous one, does not establish

direct limits to the power supply minimum efficiency for each different state. In fact it

depends on the actual load applied by computer itself. At the moment this application note

has been written, not all the major computer manufacturers have produced specific

requirements for EPA rev. 5.0 compliant designs. Anyway, the good margin achieved against

the EPA rev. 4.0 limits proves that this board can be a viable solution even for upcoming EPA

5 computer designs.

50%

55%

60%

65%

70%

75%

80%

85%

90%

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Pin [W]

230V-50Hz

115V-60Hz

EPA4

Table 3.

Overall efficiency compared to “EPA rev. 5.0 computers”

Notes

230V-50Hz

115V-60Hz

EPA5

Vout

[V]

Iout

[mA]

Pout

[W]

Eff.

Vout

[V]

Iout

[mA]

Pout

[W]

Eff.

20% load eff.

19.12

785

15.02

18.17

82.6%

19.10

785

14.99

17.39

86.2%

82.0%

PASS

50% load eff.

19.13

1960

37.50

42.87

87.5%

19.11

1963

37.52

42.12

89.1%

85.0%

PASS

100% load eff.

19.15

3917

75.01

84.07

89.2%

19.14

3920

75.00

85.26

88.0%

82.0%

PASS

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