Iron powder pot cores keep the flux inside

Iron powder pot cores keep the flux insidePot cores offer several interesting design options, stemming from their unique shape. Due to their encapsulated form that envelops the winding almost completely (see figure 1), only a negligible portion of magnetic flux leaves the device. This also applies to the additional gap in the middle leg that achieves smaller AL-values and adjusts the saturation curve (see figure 2). Therefore, pot cores are an optimal solution for EMI-sensitive environments. The inner volume of the pot core can be used completely for the coil allowing for an extremely compact design, especially when profile wire is used. Another advantage is the large smooth surface that enables easy attachment of a heat sink which is difficult to say the least on wound toroids. Similar to planar E-cores, by mounting a heat sink onto the flat surface it allow easy heat dissipation from winding through the core and the core itself. High insulation strength is achieved by additional tape layers or by keeping a defined space between the coil and the core which is later filled with potting compound.

Pot cores Figure 1: Coil in an iron powder pot core before potting

While the mere coil winding (on a bobbin or using a winding mandrel for bigger coils even with profile or flat wire) is relatively simple, the assembly of the winding into the core can be problematic due to relaxation of the copper coil as it expands after winding. This is critical as the coil needs to fit precisely into the core. Since the iron core may have sharp edges, fitting the coil into the core can damage the insulation of the wires and the additional tape insulation.

Pot CoresFigure 2: Lines and density of magnetic flux in the cross section of a coil in a gapped iron powder pot core

Another challenge when designing and manufacturing pot core inductors is the right choice and usage of the potting compound. On the one hand, it must be fluid enough to seep between the turns and the core during potting to avoid any air inclusions. This would affect a proper insulation between each winding and between the coil and the core. On the other hand, the volume change during curing must be minimized. Afterwards during cooling, the difference between the thermal expansion coefficients of the potting material and the core must be small enough to avoid thermal-mechanical stress as this can potentially damage the core. In addition, the thermal expansion must be considered over the complete operating temperature range of the inductor in its application.

You can avoid many of the pitfalls by using our experience to help you design pot cores and inductive components.

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