AN2950 datasheet(20/37 Pages) STMICROELECTRONICS

20 / 37 page

Testing the board

AN2950

20/37

Doc ID 15494 Rev 1

Depending on the equipment supplied, there are several criteria to measure the standby or

light load performance of a converter. One of these is the measurement of the output power

when the input power is equal to one watt. Table 17 reports the output power needed to

obtain 1 W of input power in different line conditions. Figure 25 shows the results of this

measurement.

Figure 25.

Pout vs VIN at Pout = 1 W

2.6

Overload protection

The VIPer28 is a current mode converter. This means that the regulation of the output

voltage is made by increasing or decreasing the primary peak current on a cycle-by-cycle

basis as a consequence of the increase or decrease of the output power demand.

The peak current is internally sensed and converted into a voltage that is compared with the

FB pin voltage. The device is shut down as soon as the two voltages become equal.

When the FB pin voltage reaches VFBlin (3.5 V typical), the drain peak current reaches its

maximum value, IDlim (which is 0.8 A typical or a lower value according to the value of the

resistor if connected between the LIM and GND pins).

If the load power demand exceeds the converter’s power capability, the FB pin voltage

exceeds VFBlin and the device waits for a certain time, fixed by the value of the capacitor C7,

before shutting down the system: the internal pull-up is disconnected and the pin starts

sourcing a 3 µA current that charges the capacitor.

Table 17.

Output power when the input power is 1 W

VIN_AC

PIN (W)

POUT (W)

Eff. (%)

PIN-POUT (W)

0.81663

81.663

0.18337

115

0.83166

83.166

0.16834

150

0.81162

81.162

0.18838

180

0.78657

78.657

0.21343

230

0.77655

77.655

0.22345

265

0.77154

77.154

0.22846

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