Application information

HVLED805

16/29

Doc ID 18077 Rev 1

After the first few cycles initiated by the starter, as the voltage developed across the auxiliary

winding becomes large enough to arm the DMG circuit, MOSFET’s turn-on will start to be

locked to transformer demagnetization, hence setting up QR operation.

The starter is activated also when the IC is in CC regulation and the output voltage is not

high enough to allow the DMG triggering.

If the demagnetization completes – hence a negative-going edge appears on the DMG pin –

on again, with some delay to ensure minimum voltage at turn-on. If, instead, the negative-

cycles will be skipped (“valley-skipping mode”, Figure 13) and the switching frequency will

Note:

That when the system operates in valley skipping-mode, uneven switching cycles may be

observed under some line/load conditions, due to the fact that the OFF-time of the MOSFET

is allowed to change with discrete steps of one ringing cycle, while the OFF-time needed for

cycle-by-cycle energy balance may fall in between. Thus one or more longer switching

cycles will be compensated by one or more shorter cycles and vice versa. However, this

mechanism is absolutely normal and there is no appreciable effect on the performance of

the converter or on its output voltage.

5.4

Constant voltage operation

The IC is specifically designed to work in primary regulation and the output voltage is

sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier

diode.

Figure 14 shows the internal schematic of the constant voltage mode and the external

connections.

Figure 13.

Drain ringing cycle skipping as the load is progressively reduced

(limit condition)

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