Datasheet

AN4164 Test equipment and measurement of efficiency and light load performance
Doc ID 023660 Rev 1 33/38
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 form. The 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 second typ.).
Figure 52 shows how the wattmeter is connected to the UUT (unit under test) and to the AC
source and the wattmeter internal block diagram.
An electronic load has been connected to the output of the power converter (UUT), allowing
to set and measure the converter's load current, 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 52 , the UUT input current causes a voltage drop across the
ammeter's internal shunt resistance (the ammeter is not ideal as it has an internal
resistance higher than zero) and across the cables connecting the wattmeter to the UUT.
If the switch of
Figure 52 is in position 1 (see also the simplified scheme of Figure 53 ), 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 measuring the input power of UUT in light
load condition).
Figure 52. Connection of the UUT to the wattmeter for power measurements
AM13105v1
A
V
DISPLAY
X
AVG
WATT METER
1
2
AC
SOURCE
U.U.T
(Unit Under test)
INPUT
Multiplier
Voltmeter
Ammeter
OUTPUT
+
Switch
AM1
3105v1
A
V
DISPLAY
X
AVG
WATT METER
1
2
AC
SOURCE
U.U.T
(Unit Under test)
INPUT
Multiplier
Voltmeter
Ammeter
OUTPUT
+
Switch