AN3286 datasheet(13/42 Pages) STMICROELECTRONICS

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AN3286

Testing the board

Doc ID 17998 Rev 1

13/42

While the frequency fold-back is active, uneven switching cycles may be observed, due to

the fact that the OFF time of the MOSFET is allowed to change with discrete steps

(MOSFET is switched on always in the valley) while the OFF time needed for cycle-by-cycle

energy balance may fall in between. One or more longer switching cycles is then

compensated by one or more shorter ones, and vice versa.

This phenomenon (shown in Figure 10) is completely normal and there is no appreciable

effect on the performance of the converter and its output voltage.

Figure 11 shows the changes of the switching frequency vs. the transformer input power

(almost proportional to the output power) for two different input voltages.

Figure 11.

Switching frequency vs. transformer input power

Figure 9.

Drain current and voltage at

230 VAC 0.7 A (valley skip.)

Figure 10.

Drain current and voltage at

115 VAC 0.6 A (uneven cycles)

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