AN3011 datasheet(25/37 Pages) STMICROELECTRONICS

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AN3011

Efficiency

Doc ID 16043 Rev 1

25/37

Figure 25.

Wattmeter connection scheme for high input current

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

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

30 minutes (warm-up period) immediately prior to conducting efficiency measurements.

After this warm-up period, the AC input power is monitored for a period of 5 minutes, to

assess the stability of the UUT. If the power level does not drift by more than 5% from the

maximum value observed, the UUT can be considered stable and the measurements can

be recorded at the end of the 5 minute period.

If the AC input power is not stable over a 5 minute period, the average power or

accumulated energy is measured over time for both AC input and DC output.

Some wattmeter models allow the integration of the measured input power in a time range,

and then measures the energy absorbed by the UUT during the integration time. Dividing

this by the integration time itself gives the average input power.

3.3

Overload protection

VIPer27 is protected against overload or output short-circuit. If the load power demand

increases the output voltage decreases and the feedback loop reacts by increasing the

voltage on the feedback pin. The PWM current set point is increased, leading to higher

power delivered to the output until this power equals the load power demands. If the load

power demand exceeds the converter power capability (fixed by the RLIM value) the voltage

on the feedback pin continuously rises, but the power delivered does not rise further. When

the feedback pin voltage exceeds VFB_lin (3.3 V typ), VIPer27 logic assumes it is a warning

for an overload event. Before shutting-down the system, the device waits for a period, fixed

by the capacitor present on the feedback pin. In fact, if the voltage on the feedback pin

exceeds VFB_lin, the internal pull-up is disconnected and the pin starts sourcing a 3 µA

current that charges the capacitor connected to it. As the voltage on the feedback pin

reaches the VFB_olp threshold (4.8 V typ.), VIPer27 stops switching and is not allowed to

switch again until the VDD voltage goes below VDD_RESTART (4.5 V typ.) and rises again up

to VDD_ON (14 V typ.).

The following waveforms show the behavior of the converter when the output is shorted.

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