VIPER35 datasheet(28/44 Pages) STMICROELECTRONICS

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Operation description

VIPer35

28/44

DocID026980 Rev 4

For a quasi-resonant flyback 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, see Figure 36.

To reduce this dependence, the IDlim has to be reduced according to the increment of the

input voltage, this is the line feed-forward. It's given by a resistor, RFF, connected between

the ZCD pin and the auxiliary winding, see Figure 37. Since the voltage across the auxiliary

winding during MOSFET on-time is proportional to the input voltage through the auxiliary-to-

primary turn ratio NAUX /NP, a current proportional to the input voltage is sunk by the ZCD

pin, thus the overcurrent set-point lowers.

Figure 36. Typical power capability vs input voltage in quasi-resonant converter

In order to select the RFF resistance value (see Figure 37), when the proper overcurrent set-

points are known at minimum and at the maximum converter input voltage, in Figure 15 the

needed current to sink during MOSFET on-time is visible. With the following approximated

formula, the value of RFF resistor can be calculated:

Equation 3

where

•

Vin_Max and Vin_min are the maximum and minimum converter rectified input voltage

•

NAUX is the primary-to-auxiliary winding turn ratio

•

IZCD1, and IZCD2 are the currents needed to sink from the ZCD pin, in order to obtain

the selected overcurrent set-points, at maximum and minimum flyback input voltage,

see Figure 15.

Given RFF value, RLIM value can be calculated by the following formula:

in_Max

in_min

–

AUX

ZCD1

ZCD2

–

()

⋅

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