VIPER35 datasheet(24/44 Pages) STMICROELECTRONICS

24 / 44 page

Operation description

VIPer35

24/44

DocID026980 Rev 4

At medium/ light load, depending on the converter input voltage as well, the device enters

valley-skipping mode. An internal oscillator, synchronized to MOSFET turn-on, defines the

maximum operating frequency of the converter, FOSClim.

The VIPER35 is available as type 'L' or type 'H', depending on FOSClim value, see Table 7.

During the normal operation the converter works with a frequency below FOSClim, so the 'L'

type is suitable for applications where the priority is on the EMI filter minimization. The 'H'

type is suitable when an extended QR operation range or the transformer size reduction are

priorities.

As the load is reduced, and the switching frequency tends to exceed the oscillator’s one,

MOSFET turn-on doesn’t occur on the first valley but on the second one, the third one and

so on. In this way a “frequency clamp” effect is achieved, piecewise linear portion is showed

in Figure 32.

When the load is extremely light or disconnected, the converter enters burst mode

operation. By decreasing the load, the frequency is reduced even few hundred hertz, so to

comply with energy saving regulations or recommendations. As the peak current is low, no

audible noise occurs.

The above mentioned operation is based on ZCD pin. This pin is the input of the integrated

ZCD circuit which allows the power section turn-on at the end of the transformer

demagnetization. The input signal for the ZCD is obtained as a partition of the auxiliary

voltage used to supply the device, see Figure 33.

When the triggering circuit senses a negative-going edge below VZCDTth threshold

(seeTable 7), after an internal delay that helps to achieve minimum drain-source voltage

switch-on (“valley switching”), the power MOSFET turns on. However, to enable power

MOSFET turn-on, the triggering circuit has to be previously armed by a positive-going edge

exceeding VZCDAth threshold (see Table 7) on the same ZCD pin.

After the MOSFET turn-off, the blanking time, tBLANK, is generated to avoid an erroneous

arming and triggering due to the noise, generated by the leakage inductance resonance of

the transformer which rings and couples with ZCD pin.

Figure 32. Switching frequency vs power

VIPER35 数据表(PDF) 24 Page - STMicroelectronics