VIPER35 datasheet(25/44 Pages) STMICROELECTRONICS

25 / 44 page

DocID026980 Rev 4

25/44

VIPer35

Operation description

Figure 33. Zero-current detection circuit

8.7

Frequency foldback function and valley-skipping mode

The switching frequency, in quasi-resonant mode, is not fixed and it depends on both the

load and the converter input voltage. The switching frequency increases when the load

decreases, or when the mains voltage increases, and vice versa. To avoid that, the

VIPER35 taps the maximum switching frequency of the application thanks to its control

logic.

The frequency limit is given by an internal oscillator switching at 136 kHz for the VIPER35L

or at 225 kHz for the VIPER35H, (see parameter FOSClim in Table 7). This oscillator is

synchronized with the power MOSFET turn-on. When the power MOSFET is off, if the first

negative-going edge voltage of the ZCD pin, resulting from transformer demagnetization,

appears after at least one oscillator cycle has been completed, the MOSFET turns on and

the oscillator is synchronized again.

Otherwise, if the first negative-going edge voltage appears before completing one oscillator

cycle, the signal is ignored. Due to the ringing of the drain voltage, the ZCD pin experiences

another positive-going edge voltage that arms the circuit and a negative-going edge voltage.

Again, if this appears before the oscillator cycle is completed, it is ignored, otherwise the

MOSFET turns on and the oscillator is synchronized. In this manner, one or more drain

ringing cycles are skipped (Figure 34 shows the so called “valley-skipping mode”) and the

switching frequency doesn’t exceed FOSClim limit.

Figure 34. Drain ringing cycle skipping as the load progressively reduces

VIPER35 Datasheet(PDF) 25 Page - STMicroelectronics