The Circuit Designer's Companion - diagramas.diagram...

102 The Circuit Designer’s Companion(above 1MHz or so) other types of ferrite core, or iron dust cores, are better. For the beststability and initial tolerance, a lower permeability material is preferable.As well as the intrinsic stability of the material, for these applications it is importantto consider mechanical stability. Any movement or distortion of the core, or movementof the winding relative to the core, will affect the magnetic path and hence theinductance. Also, any mechanical, magnetic or thermal shock to the core causes animmediate change in permeability followed by a long, slow relaxation towards theoriginal value. This is known as “disaccommodation”. These effects mean that the corecharacteristics have to be very carefully considered when a stable inductance is requiredin a high-shock or high-vibration environment. It is common for the winding, bobbinand core to be encapsulated in varnish to enhance mechanical stability. Hardencapsulating compound should not be used as the high shrinkage could mechanicallydamage the brittle core.Power circuitsEnergy storage chokes and power transformers, as used for example in switching powersupplies, have a quite different set of important parameters. In these, inductancestability is not required but high volumetric efficiency is. Energy stored in the choke isgiven by L·I 2 and so a material which shows a high saturation flux density, allowing ahigher magnetising current, is to be preferred. At higher operating frequencieshysteresis becomes the dominant loss mechanism, and limits the power handlingcapacity of the core. A small gap in the magnetic circuit, usually obtained by grindingaway a part of the core, allows higher saturation at the expense of lower effectivepermeability. These considerations point to the use of gapped manganese-zinc ferritesor iron dust cores in which the air gap is inherent in the material.SuppressionIn contrast to the previous applications in which low core losses were required for highQ or high power handling, suppression chokes work best if they have high losses. Asuppression circuit has to reflect or absorb high-frequency interference energy andprevent it from being propagated beyond the suppressor. The more energy is absorbedwithin the choke the better will be its circuit performance. Clearly, high-loss ferrites arethe best type for these applications; all ferrites when used well above their intendedfrequency range exhibit high losses, but materials specifically designed andcharacterised for this purpose are available. The ferrite bead (Figure 3.22) is an extremeexample, in which a straight piece of wire is transformed into a high-frequency chokemerely by stringing a bead onto it. The losses induced in the ferrite at high frequenciesgive the assembly a complex impedance (resistance + reactance) of several tens ofohms. The same principle is applied to monolithic ferrite chip components, in which theconductor is passed between layers of ferrite to create a surface mount part.|Z|RL(typical)=5MHz50MHz500MHzfFigure 3.22 The ferrite bead