VIPER25LN datasheet(24/40 Pages) STMICROELECTRONICS

24 / 40 page

Operation description

VIPER25

24/40

Doc ID 15585 Rev 4

7.9

Starter

If the amplitude of the voltage on ZCD pin at the end of one oscillator cycle is smaller than

the VZCDAth arming threshold, in which case MOSFET's turn-ON could not be triggered, the

system would stop.

This is what normally happens during converter’s power-up or under overload/short circuit

conditions.

During the converter’s startup phase, the voltage on ZCD pin is not high enough to arm the

triggering circuit. Thus, the converter operates at a fixed frequency, FSTARTER (see Table 8

on page 8

). As the voltage developed across the auxiliary winding becomes high enough to

arm the ZCD circuit, MOSFET's turn-ON is locked to transformer demagnetization, hence

setting up quasi-resonant operation.

As protection, in case the ZCD voltage is permanently above the threshold VZCDAth, the

switching frequency is reduced to the minimum value, FOSCmin, reported on Table 8 on

page 8

7.10

Current limit set point and feed-forward option

The VIPER25 is a current mode converter and the drain current is limited cycle by cycle

according to the FB pin voltage value that is related with the feedback loop response and

the load. When the drain current, sensed by the integrated Sense-FET, reaches the current

limitation, after the internal propagation delay, the MOSFET is switched OFF. The current

limitation cannot exceed a certain value, IDlim, that can be adjusted acting on the current

sunk from the ZCD pin during MOSFET’s ON-time.

Usually a resistor, RLIM, connected from ZCD pin to ground is used to fix this sunk current

and then the peak drain current set-point: the lower the resistor is, the lower IDlim will be.

For a QR fly-back converter the power capability strongly depends on the input voltage. In

wide-range applications at maximum line the power capability can be more than twice the

value at minimum line, as shown by the upper curve in the diagram of Figure 31 on page 25.

To reduce this dependence, the current limit IDlim has to be reduced according to the

increment of the input voltage, implementing the so called line feed-forward. It’s realized with

a resistor, RFF, connected between the ZCD pin and the auxiliary winding, see the Figure 32

on page 26

. Since the voltage across the auxiliary winding during MOSFET’s on-time is

proportional to the input voltage through the auxiliary-to-primary turns ratio NAUX /NP, a

current proportional to the input voltage is sunk from the ZCD pin, thus lowering the over

current set point.

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