A family of LLC converters with magnetic control

Although frequency control is an effective and common control strategy for resonant converters, it still has some drawbacks: 1) wide operating switching frequency range is required; 2) degraded circuit electro-magnetic interference (EMI) performance; 3) in some applications, like interleaving and multiple output operations, not enough control freedoms to achieve independent control since frequency will affect operation of each phase or output channel. On the other hand, magnetic control or variable inductor control (VIC) can regulate the resonant converter without adjusting the switching frequency and duty cycle, which can effectively address the above-mentioned issues. Therefore, in this article, all possible LLC converters with different VIC control strategies are introduced. The suitable applications for each VIC control strategy are summarized. Key design guidelines for variable inductor are presented. Experimental results for variable resonant inductance, variable magnetizing inductance, and a novel variable resonant and magnetizing inductance based LLC converters are presented. Other applications of VIC LLC converters are also discussed.