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AN3011

Efficiency

Doc ID 16043 Rev 1

23/37

Figure 22.

Efficiency vs AC input voltage when the input power is 1 W

3.2

Test equipment and measurement of efficiency and input

power

The converter input power was measured using a wattmeter. The wattmeter

contemporaneously measures converted input current (using its internal ammeter) and

voltage (using its internal voltmeter). The wattmeter is a digital instrument, therefore, it

samples the current and voltage and converts them into 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.

Figure 23

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 was connected to the output of the power converter (UUT) sinking the

load current. The electronic load also measures the load current. A voltmeter was used in

order to measure the output voltage of the power converter.

Once the input power and the output power can be measured, the efficiency in different

operating conditions can be calculated by properly setting the AC source output voltage and

the current sourced by the electronic load.

3.2.1

Measuring input power notes

With reference to Figure 23, the UUT input current causes a voltage drop across the

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

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

If the switch of Figure 23 is in position 1 (see also the simplified scheme of Figure 24) this

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

input which, 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 low-

load condition). In the case of high UUT input current the voltage drop can be relevant

(compared to the UUT real input voltage) and therefore, if this is the case, the switch in

Figure 23

can be changed to position 2 (see simplified scheme of Figure 25) where the UUT

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