Heteromagnetic Swinging Inductor (HMSI) And its Application for Power Factor Correction Converters
Variable inductors with current dependent inductances are found in various power electronics applications. Swinging inductors with discretely varying gap length have step-wise L-I curves. A sloped air gap (SAG) inductor is a swinging inductor that has a smooth L-I curve. But, its maximum inductance is limited by the thinnest gap length and by the thickest gap edge. The fringing field can cause significant increase in winding loss at high frequency. This paper introduces a swinging inductor (Fig.1) consisting of a core with simple, constant-length gap(s) that is (are) filled by materials whose magnetic properties are tailored for desired L-I curves. We term this configuration as the swinging inductor, hetero-magnetic swinging inductor (HMSI). The HMSI concept builds on our materials processing expertise in formulating filler materials with wide ranging magnetic properties. Together with the selection of gap length(s) and the core magnetic material, a HMSI offers greater flexibility for wider L-I swinging characteristics with low core loss and minimal fringe effect. In this study, we applied the HMSI concept in a critical conduction mode (CRM) PFC boost converter (shown in Fig. 2) aimed at reducing the range of switching frequency range. CRM PFC converters have been widely used for ac-dc regulation to obtain good power quality due to their simple controller structure and no diode reverse recovery. However, large switching frequency variations of the converters increase the complexity of EMI filter designs. In a conventional CRM PFC boost converter, the switching frequency variation comes from the changing of the input line voltage with a constant value boost inductor throughout the whole line. By implementing a variable inductor whose inductance follows a designed function to attenuate the influence of the changing input line voltage, the switching frequency variation range can be reduced. As a practice of combining different magentic materials in one core to make a flexible swinging inductor and thereby reduce fringing effect, a hetero-magnetic swinging inductor (HMSI) targets the application of reducing switching frequency variation range for critical conduction mode (CRM) power factor correction (PFC) converters in an effort to simplify its EMI filter design and improve the whole converters power density. The paper first introduces the concept of HMSI, then it provides the design of a HMSI for a given L-I curve. For application, a specific L-I function needed to limit the switching frequency range of a 150 W CRM PFC boost converter was calculated and a HMSI is designed and fabricated to meet this L-I function requirement. Finally, testing results of a 150 W CRM PFC boost converter with HMSI and a linear inductor are compared.