AN2950 datasheet(32/37 Pages) STMICROELECTRONICS

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Test equipment and measurement of efficiency and low load performance

AN2950

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Doc ID 15494 Rev 1

Appendix A

Test equipment and measurement of

efficiency and low load performance

The converter’s input power has been measured using a wattmeter. The wattmeter

simultaneously measures the converter’s 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 into digital forms. The digital samples are then

multiplied, yielding 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 51

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

source, as well as the wattmeter’s internal block diagram.

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

one 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 of the load current vs. output

voltage.

The ratio between the output power, calculated as previously said, and the input power,

measured by the wattmeter, is the converter’s efficiency. It has been measured in different

input/output conditions acting on the AC source and on the electronic load.

A.1

Notes on input power measurement

This section shows two possible connections between the wattmeter and the unit under test

(UUT) for power measurements, each one represented in Figure 51 by the connection of the

switch either in position 1 or in position 2.

Figure 51.

Connection of the UUT to the wattmeter for power measurements

If the switch shown in Figure 51 is in position 1 (see also the simplified scheme in

Figure 52

), the ammeter's internal shunt resistance (which is higher than zero) has to be

taken into account. This resistance produces a voltage drop (then an input measured

voltage) higher than the input voltage at the UUT's input. This voltage drop is generally

negligible if the UUT's input current is low (for example, when measuring the input power of

the UUT at low load conditions), but at heavy load conditions, when the UUT input current

increases, the error introduced in the measurement with this setting can be relevant.

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