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AN4345

Test equipment and measurement of efficiency and light load performance

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Appendix A

Test equipment and measurement of

efficiency and light load performance

The converter input power has been measured using a wattmeter. The wattmeter measures

simultaneously the converter input current (using its internal ammeter) and voltage (using its

internal voltmeter). The wattmeter is a digital instrument so it samples the current and

voltage and converts them to digital forms. Digital samples are then multiplied giving the

instantaneous measured power. The sampling frequency is in the range of 20 kHz (or higher

depending on the instrument used). The display provides the average measured power,

averaging the instantaneous measured power in a short period of time (1 sec typ.).

Figure 37 shows how the wattmeter is connected to the UUT (unit under test) and to the AC

source and the wattmeter internal block diagram.

Figure 37. Connections of the UUT to the wattmeter for power measurements

An electronic load has been connected to the output of the power converter (UUT), allowing

the converter load current to be set and measured, while the output voltage has been

measured by a voltmeter. The output power is the product between load current and output

voltage. The ratio between the output power, calculated as previously stated, and the input

power, measured by the wattmeter, is the converter's efficiency, which has been measured

in different input/output conditions.

A.1

Measuring input power

With reference to Figure 37, the UUT input current causes a voltage drop across the

ammeter's internal shunt resistance (the ammeter is not ideal so it has an internal resistance

higher than zero) and across the cables connecting the wattmeter to the UUT.

If the switch of Figure 37 is in position 1 (see also the simplified scheme of Figure 38), this

voltage drop causes an input measured voltage higher than the input voltage at the UUT

input that, of course, affects the measured power. The voltage drop is generally negligible if

the UUT input current is low (for example when we are measuring the input power of UUT in

light load conditions).

AM13843V1

A

V

DISPLAY

X

AVG

WATT METER

1

2

AC

SOURCE

U.U.T

(Unit Under test)

INPUT

Multiplier

Voltmeter

Ammeter

OUTPUT

+

Switch