AN3286 datasheet(12/42 Pages) STMICROELECTRONICS

12 / 42 page

Testing the board

AN3286

12/42

Doc ID 17998 Rev 1

In quasi-resonant operation the switching frequency depends on the input/output conditions,

increasing with increasing mains voltage and with decreasing output load. In fact, as the

input voltage increases, the ON time is reduced and so the switching frequency increases;

as the load decreases, the drain peak current (then also the needed ON time and the

demagnetization time) decreases, therefore, the switching frequency increases.

In order to avoid the frequency increasing too much at medium/light load (which would result

in an increase of all the frequency-related losses), the device is provided with a frequency

fold-back function with an internal limit of 136 kHz (typ.); as the converter switching

frequency approaches the limit, this function acts by inhibiting the MOSFET switch-on, on

the first valley, and allowing it on the second, or the third and so on, depending on the

working condition, as shown in Figure 9 and 10 below. This mode of operation is referred to

as “valley skipping mode”.

In Figure 9, one ringing cycle of the drain voltage is always skipped. In this figure, what

should be the switching frequency without the frequency fold-back function (fQR), the

MOSFET turn-on inhibit time (1/fFF), and the actual converter switching frequency

(fValley_Skipping), is highlighted.

Figure 7.

Drain current and voltage at max.

load 90 VAC

Figure 8.

Drain current and voltage at max.

load 265 VAC

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