AN1214 APPLICATION NOTE

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of switching frequencies are available to be used. This helps minimize the effects of the internal propagation

delay as well as the offset of the current sense comparator. In this way the current will track the voltage wave-

form better, in particular near the zero-crossings. This, however, must be weighed against the size increase of

the inductor because a lower switching frequency implies a larger inductance value.

When determining the lowest frequency, it must be noted that switching below 15kHz is not recommended since

this may interfere with the internal starter.

A special construction technique of the inductor can offer the optimum compromise: one that allows the use of

a low inductance value so as to minimize inductor size and, at the same time, have a lower switching frequency

near zero-crossings. The price to pay for that is an additional step in the inductor manufacturing flow.

It is the so-called “step-gap core” technique: the centre leg of one half of the ferrite core is ground so that the

air gap thickness has a step change, as shown in fig. 6.

At low inductor current the small thinner part of the air gap dictates a high inductance value (L1). As current

introduced so that, for a given switching frequency at the top of the rectified sinusoid - where current is high -

(that is, for a given L=L2), the switching frequency near the zero-crossings - where current is low - will not go

as high as with a linear inductor.

Figure 6. Step-gap ferrite grinding and its effect on inductance value

cation will be found empirically.

L

L1

L2

b

core half