Application information

STCH02

14/24

DocID028766 Rev 1

Please note that the maximum IZCD sunk/sourced current has to not exceed ± 3 mA

(AMR) in all the Vin range conditions (88 - 265 Vac). No capacitor is allowed between

the ZCD pin and the auxiliary winding of the transformer.

The switching frequency is top-limited below 260 kHz, as the converter's operating

frequency tends to increase excessively at the light-load and high input voltage.

A starter block is also used to start-up the system when the signal on the ZCD pin is not high

enough to trigger the MOSFET

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

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

locked to transformer demagnetization, hence setting up the QR operation.

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

high enough to allow the ZCD triggering.

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

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

ringing cycles will be skipped (“valley-skipping mode”, Figure 7) and the switching frequency

Figure 7. Drain ringing cycle skipping as the load is progressively reduced

When the system operates in the 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.