AN3028 datasheet(32/37 Pages) STMICROELECTRONICS

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Test equipment and measurement of efficiency and low-load performance

AN3028

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Doc ID 16135 Rev 1

Appendix A

Test equipment and measurement of

efficiency and low-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. 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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