I. Introduction
The converters with small profile and high efficiency are more and more required in industry applications. As the switching frequency goes up, high power density is satisfied. However, there is a limit in power density when the switching frequency reaches a high level [1]–[2]. The power density is mostly influenced by the magnetic components. The structure of inductors and transformers also has effect on the power density. The PCB windings are frequently used in high frequency power converters due to the benefit of high quality factors and low profile. Conventionally, the PCB transformers are integrated tightly to the other components horizontally [3]–[5]. The transformer occupies a large PCB area that impacts the power density. Therefore, it is important to reduce the transformer occupied area to minimize the converter volume. With the great need of the power density, the transformer core is embedded into the PCB to increase the integration density [6]. The power density of a 2 W converter with the switching frequency of 1 MHz reaches 73 W/in3. However, the transformer is cored so that the magnetic field is constrained. In VHF (30 MHz-300 MHz) range, the transformers are typically air-cored so that the transformer overall height is only the PCB thickness. Therefore, the VHF converters are suitable for the applications requiring low thickness. However, with the elimination of the magnetic core the magnetic field induced by the air-core transformer spreads widely, causing severe EMI and eddy current in the coppers. So the decoupling design of the transformer and other components is important to the converter efficiency and reliability.