AN3028 Datasheet, PDF (32/37 Pages) M/A-COM Technology Solutions, Inc. – ESD Protection of Broadband GaAs CATV & FTTx Amplifiers

Test equipment and measurement of efficiency and low-load performance
Appendix A Test equipment and measurement of
efficiency and low-load performance
AN3028
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. 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 sec typ.).
Figure 34 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 34, 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 in Figure 34 is in position 1 (see also the simplified scheme of Figure 35), 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
low-load condition). In case of high UUT input current, the voltage drop can be relevant
(compared to the UUT real input voltage). If this is the case, the switch in Figure 34 can be
changed to position 2 (see simplified scheme of Figure 36) where the UUT input voltage is
measured directly at the UUT input terminal and the input current does not affect the
measured input voltage.
The voltage across the voltmeter causes a leakage current inside the voltmeter itself (which
is not an ideal instrument and doesn’t have infinite input resistance). If the switch of
Figure 34 is in position 2 (see simplified scheme of Figure 36), the voltmeter leakage current
is measured by the ammeter together with the UUT input current, causing a measurement
error. The error is negligible if the UUT input current is much higher than the voltmeter
leakage. If the UUT input current is low and not much higher than the voltmeter leakage
current, it is probably better to set the switch of Figure 34 to position 1.
If we are not sure which measurement scheme has the lesser effect on the result, we can try
with both and register the lower input power value.
As noted in IEC 62301, instantaneous measurements are appropriate when power readings
are stable. The UUT shall be operated at 100% of nameplate output current output for at
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