AN2950 datasheet(33/37 Pages) STMICROELECTRONICS

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AN2950

Test equipment and measurement of efficiency and low load performance

Doc ID 15494 Rev 1

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Figure 52.

Suggested connection for low power measurements (switch in position 1)

In this case, it is advisable to connect the switch shown in Figure 51 to position 2 (see

simplified scheme in Figure 53): the UUT's input voltage is measured directly to the UUT's

input terminal and the input current does not affect the measured input voltage.

Figure 53.

Suggested connection for high power measurements

(switch in position 2)

With this setting, the measurement error is introduced by the shunt resistance of the

voltmeter, which is not infinite and then causes a leakage current inside the voltmeter itself.

This current is measured by the ammeter together with the UUT's input current, but the error

is negligible at heavy loads, when the UUT's input current is much higher than the

voltmeter's leakage current.

On the other hand, at low load conditions, when the UUT's input current decreases and

approaches the voltmeter's leakage current, the measurement error introduced with this

setting becomes significant.

To conclude, we could say that the setting shown in Figure 52 should be used for low loads

and stand-by measurements, the setting shown in Figure 53 for heavy loads and efficiency

measurements. In you are not sure which measurement scheme has the least effect on the

results, you can try with both and register the input power's lower value.

As noted in IEC 62301, instantaneous measurements are appropriate when power readings

are stable. The UUT should be operated at 100% of the nameplate output current output for

at least 30 minutes (warm-up period) immediately prior to conducting efficiency

measurements. After this warm-up period, the AC input power should be monitored for a

period of 5 minutes to assess the stability of the UUT.

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