Operation descriptions

VIPER28

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At the same time an internal current generator starts to charge the feedback capacitor

During the first start up phase of the converter, after the soft-start up time (typical value is

threshold that switches off the converter itself.

To avoid this event, the appropriate feedback network has to be selected according to the

output load. More the network feedback fixes the compensation loop stability. The Figure 24

on page 23 and Figure 25 on page 23 show the two different feedback networks.

using the formula:

Equation 5

compensate the feedback loop but also as element to delay the OLP shut down owing to the

time needed to charge the capacitor (see Equation 5).

the output capacitor could not be at its nominal value and the controller interpreters this

situation as an overload condition. In this case, the OLP delay helps to avoid an incorrect

device shut down during the start up face.

Owing to the above considerations, the OLP delay time must be long enough to by-pass the

initial output voltage transient and check the overload condition only when the output voltage

is in steady state. The output transient time depends from the value of the output capacitor

and from the load.

ensure enough OLP delay, an alternative compensation network can be used and it is

showed in Figure 25 on page 23.

is usually used to compensate the high frequency zero due to the ESR (Equivalent Series

Resistor) of the output capacitance of the fly-back converter.

The mathematical expressions of these poles and zero frequency, considering the scheme

in Figure 25 are reported by the equations below:

Equation 6

T

OLP

delay

–

C

FB

V

FBolp

V

FBlin

–

3

μA

----------------------------------------

×

=

1

FB

1

FB

ZFB

R

C

2

1

f

⋅

⋅

π

⋅

=