Saturable inductors

As for the MAX of the current limit, whenever we deem that inductor saturation is of real concern to us (as in high-voltage applications), we must look at the MAX of the current limit to decide upon the size of the inductor — that being the worst-case in terms of peak current under overloads, inductor energy storage, and its possible saturation. 

Saturable Inductors to Limit Rectifier Reverse Recovery Current... 

Figure 4-9 Typical use of saturable inductors in reducing reverse recovery losses.

This equation is useful for determining how close to saturation an inductor or transformer is operating, which could avoid a catastrophe. 

My suggestion is to open the pdf datasheet of any prospective switcher IC and carry out a text search (Ctrl + F) for the world foldback. If you find it, question the vendor about its full impact before you select the part for your application. Foldback is, in general, a good idea in terms of protecting the converter under abnormal conditions, but it should be used very judiciously so as not to impact normal behavior. For example, the Simple Switcher family has a hidden second-level current limit protection at which frequency foldback (or skipped pulses) occurs. But that trip level can only be encountered under very severe conditions—namely, a sudden overload with a completely incorrectly sized inductor that hard-saturates in the process. At other times it is not encountered and doesn t therefore interfere. It is considered transparent to all but the most novice engineers. And that is what I consider the right type of foldback. 

Blog Entry 2 What inductor are you using You could in fact be in current limit by using too low an inductor value, or saturating the inductor. I would measure your inductor current and make sure peaks are <5.75A. Maybe check the design on virtual bench too. After all you re trying to deliver >125W. 

Author s Comments That s obviously not true. The customer has had problems at 1A load too. So this explanation, even if true (and it is not) does not really fit the set of clues provided by the customer. It could be one reason, but is not the explanation. As indicated previously, the saturation rating of most inductors is not a cliff over which you suddenly fall off into a timeless abyss. Saturation usually just means the inductance has fallen by about 20 to 30% from its initial value. So in going further, say from 3A to 3.5A, you can expect the inductance to fall to about 50% of its initial value. But rarely any more For sure, it doesn t suddenly morph into a resistor. ... 

The inrush current of an EMI filter is usually examined to ensure that no parts are overstressed during power-up. If the inductor does not saturate, the inrush current is described by Faraday s law and can easily be modeled by mathematics or a simple SPICE model. It is also not too difficult to determine if a core is saturated during turn-on. A slightly more difficult calculation is to determine what the maximum current will be under a given turn-on condition. The hardware used for measurements used a transformer made of two stacked 55025 cores... 

To model the transformer, an ideal center-tapped transformer is combined with a nonlinear core model for the F material. As this circuit counts on the saturation of the core, a SPICE primitive inductor will not work. By adding this nonlinear core model across the input of the center-tapped transformer, the magnetizing inductance and saturation characteristics of the core are realized. 

Apparently this is not the case. Although, in general, better binders are better assembly inductors, there are compounds that significantly deviate from the best regression line (see Fig. 4a of [36], and Fig. 4a [20]), while a saturating behaviour of critical concentration Cr (inverse of apparent elongation constant Kelapp) with ligand concentration is observed (Fig. 2 of [36] and Fig. 5 of [35]). 

Ferrites having a square hysteresis loop (e.g. (Mg, Mn, Zn) Fe204), developed for the now outdated computer core memories, have found applications in the form of a toroidal saturable inductor for regulating the output currents in switched-mode power supplies. 

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