Unbalanced-Flux Concept for High Power Density Magnetics and Review of the MHz LLC Transformers

High-power-density magnetics are often designed through the MHz-frequency approach, but this method is constrained by excessive core losses, which restrict operation to very low magnetic flux density B, typically lower than 70 mT. Since magnetic power scales with the square of B, this fundamentally limits achievable power density. In practice, the average power density of state-of-the-art MHz magnetic designs remains around 70 W/cm³, far below the requirements of emerging applications. To overcome these limitations, we propose the unbalanced-flux concept, a new magnetic architecture that replaces the traditional balanced-flux design paradigm. This approach reduces core loss by approximately 70%, enabling operation at substantially higher B. As a result, the concept achieves a threefold increase in power density compared to MHz-based designs. The approach is experimentally validated in a 1-kW, 380 V/48 V LLC transformer prototype. With natural air cooling, the transformer delivers 750 W at a record 224 W/cm³. With a heatsink, it achieves 190 W/cm³ at 1 kW, with a peak efficiency of 94.6%. These results demonstrate that unbalanced-flux magnetics provide a scalable pathway beyond the MHz, low-B limitations toward the next generation of ultra-high-power-density magnetic components.