AN2521 datasheet(16/33 Pages) STMICROELECTRONICS

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Functional check

AN2521

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voltage to the flyback converter as a result of the PFC turning on or off. As the figures show,

both transitions are clean and there is no output voltage, VCC dip or restarting attempt that

could affect proper power supply operation.

The input power consumption of the board has also been checked at light load conditions,

simulating an adapter powering a laptop PC during power-saving operation. The results are

shown in Table 4, 5 and 6 below, where the low load efficiency with standard inputs of 115 V

and 230 V is calculated.

As visible in Table 4, 5 and 6, the input power consumption is always very low and the

efficiency remains significantly high even at output power levels where the power supply

efficiency normally drops. This is achieved thanks to the burst mode adjustable threshold of

the L6668 SKIPADJ pin and the PFC management by the PFC_STOP pin, as previously

described.

3.3

Over current and short circuit protection

An important function of any power supply is its ability to survive instances of output

overload or short circuit, avoiding any consequent failure. Additionally, the power supply

must be compliant with safety rules which require that the components will not melt or burn-

out in fault conditions. It’s common to find circuits with good protection capability against

load shorts but which do not survive dead shorts such as those of an output electrolytic

capacitor or a secondary rectifier, or in cases of transformer saturation. Moreover, in cases

of a shorted rectifier the equivalent circuit changes and the energy are delivered even during

the ON time, as in forward mode. In this evaluation board the over-current is managed by

U5, a CC/CV controller. Inside the IC there is a reference and two Or-end operational

amplifiers, one dedicated to act as the error amplifier of the voltage loop and the other

dedicated to act as the error amplifier of the current loop. If the output current exceeds the

programmed value, the current loop error amplifier takes over and, via the optocoupler,

Table 4.

Light load efficiency (0.5 W)

Vinac [Vrms]

Pout [W]

Pin [W]

Efficiency

115

0.52

0.75 (1)

Compliant to US Executive order 13221 “1W _Standby“

68.67%

230

0.52

0.88 (1)

58.52%

Table 5.

Light load efficiency (1.2 W)

Vinac [Vrms]

Pout [W]

Pin [W]

Efficiency

115

1.2

1.55

77.86%

230

1.2

1.71

70.35%

Table 6.

Light load efficiency (2.4 W)

Vinac [Vrms]

Pout [W]

Pin [W]

Efficiency

115

2.41

2.93

82.14%

230

2.4

3.14

76.56%

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