L6566ATR datasheet(28/51 Pages) STMICROELECTRONICS

28 / 51 page

Application information

L6566A

28/51

Figure 16.

Typical power capability change vs input voltage in QR flyback

converters

It acts on the overcurrent setpoint Vcsx, so that it is a function of the converter’s input voltage

Vin (output of the PFC pre-regulator) sensed through a dedicated pin (15, VFF): the higher

the input voltage, the lower the setpoint. This is illustrated in the diagram on the left-hand

side of Figure 17 on page 29: it shows the relationship between the voltage on the pin VFF

and Vcsx (with the error amplifier saturated high in the attempt of keeping output voltage

regulation):

Equation 5

Note:

If the voltage on the pin exceeds 3 V switching ceases but the soft-start capacitor is not

discharged. The schematic in Figure 17 on page 29 shows also how the function is included

in the control loop.

With a proper selection of the external divider R1-R2, i.e. of the ratio k = R2 / (R1+R2), it is

possible to achieve the optimum compensation described by the lower curve in the diagram

of Figure 16.

The optimum value of k, kopt, which minimizes the power capability variation over the input

voltage range, is the one that provides equal power capability at the extremes of the range.

The exact calculation is complex, and non-idealities shift the real-world optimum value from

the theoretical one. It is therefore more practical to provide a first cut value, simple to be

calculated, and then to fine tune experimentally.

Assuming that the system operates exactly at the boundary between DCM and CCM, and

neglecting propagation delays, the following expression for kopt can be found:

Equation 6

Vin

Vinmin

11.5

22.5

3.5

0.5

1.5

2.5

system optimally

compensated

system not

compensated

k = 0

k = kopt

Vin

VFF

csx

−

()

max

min

max

min

opt

⋅

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