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The authors report the use of a coreless printed circuit board transformer for power conversion with very high power density and efficiency. A coreless PCB transformer with an outermost radius of ~1 cm and 19 turns for both the primary and secondary windings can transfer 19 W at an efficiency of 90%, resulting in a record power density of 24 W/cm2. The power density and energy efficiency of a coreless PCB transformer are higher than those of core-based microtransformers. Coreless transformers are simpler in structure, easier to implement in silicon wafer and cheaper than core-based planar transformers.
Tang et al. (2000) studied this question.