Ascertainment of fringing-effect losses in ferrite inductors with an air gap by thermal compact modelling and thermographic measurements

Abstract

Recent years have been characterized by a rapid increase in the efficiency of energy conversion and power density, which are the key criteria in the design of high-frequency power converters and the best measures of their performance. The demand for reduced size and minimization of losses has led to a revaluation of the approach to building magnetic components, driving the operating frequency to ever higher levels in order to facilitate more compact and efficient designs. These could not have been achieved without the investigation of phenomena associated with electromagnetic fields and the utilization of thermal analysis tools. Thermal qualification is particularly crucial for magnetic components with an air gap, due to the fringing magnetic field around the air gap. The fringing flux induces excess eddy currents in the windings causing localized heating (hotspots) and reducing the overall efficiency of power conversion. This extra power loss is beginning to be a significant factor in an increasing number of designs. As shown herein, the power loss associated with the phenomenon can make up about 25% of the total losses in a magnetic component. This illustrates the potential hazards of designing magnetic components with an air gap, be they transformers or chokes, without taking the fringing effect into consideration.A novel approach is presented here to estimate the fringing-effect power loss. This method is based on compact thermal modelling and thermographic measurements. A DC-DC high-frequency forward converter and a centre-gapped ferrite core inductor were constructed so as to perform dynamic, time-dependent thermographic analysis on the latter. The obtained thermal characteristics, juxtaposed with the thermal compact model created for the inductor, allowed for the extraction of the fringing-effect loss from all power-loss sources present. The ascertainment of the fringing-effect loss is problematic, as the phenomenon is not limited only to the area above the air gap but has an effect on neighbouring copper wires, causing a distinctive pattern in temperature distribution in windings if measured along the gapped middle column of the ferrite core. The power-loss density in areas away from an air gap, where the fringing-effect component is not present or largely diminished, is nearly homogenous. This power-loss density, if referred to the entire volume of the coil, yields the power loss in the winding with no fringing-effect component. A simple deduction from the measured total power dissipated in the winding leads to the ascertainment of the fringing-effect loss.