Implementing the line-modulated fixed-off-time

AN3112

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Doc ID 16820 Rev 3

technique as line-modulated fixed-off-time (LM-FOT) [

5].

This modification, although simple, introduces profound changes in the timing relationships,

with a positive influence on the energetic relationships. From the control point of view,

characteristics. Consequently, all of the properties of the standard FOT control are

and R0, its effects are discussed only qualitatively and the quantitative aspects are provided

graphically for a specific case in [

5].

As a practical rule, it is convenient to first select a capacitor and then to calculate the resistor

fulfill the following inequalities:

Equation 6

maximum value, and VF the forward drop on D.

When working at high line/light load the on-time of the power switch becomes very short and

Cs is then used in parallel to Rs. This capacitor causes an almost instantaneous charge of C

the steep edge caused by Cs does not reach the clamp level, otherwise the internal clamp of

the L6564 undergoes uncontrolled current spikes (limited only by the dynamic resistance of

the 1N4148 and the ESR of Cs) that could overstress the IC. Cs must then be:

Equation 7

ZCDclamp

F

ZCDclamp

GD

ZCDclamp

ZCDx

F

ZCDclamp

GDx

V

V

V

V

R

Rs

R

V

I

V

V

V

−

−

⋅

<

<

+

−

−

F

ZCDclamp

GDx

ZCDclamp

V

V

V

V

C

Cs

−

−

<