Guanella Transformer/Balun Implemented with Planar Transformer Technology

Abstract

The planar transformer is a relatively new implementation of the conventional magnetically-coupled transformer which is widely used in switched-mode power converter designs with switching frequencies below 1 MHz. Planar transformers and the associated soft ferrite cores have evolved tremendously over the past two decades and provide outstanding performance. The planar transformer design is intrinsically compatible with printed circuit media. However, there are manifold additional merits to this approach, in particular heat transfer and power handling. As improvements are made in active devices the switching frequency of switched-mode converters has continually increased into the RF regime. The planar transformer is a conventional transformer relying exclusively on magnetic coupling for its operation; inter-winding capacitance is deleterious to its performance. Inter-winding capacitance exists in any conventional transformer, but it particularly pronounced in a planar transformer due to the stacked planar windings. Here we adapt the transmission-line or Guanella transformer techniques to the planar core to obtain improved high frequency performance. A full-wave electromagnetic simulation of a 1:4 impedance transforming Guanella balun is performed using the commercial finite element software HFSS. Essentially, the Guanella transformer/ balun absorbs the inter-winding capacitance into the design where it is complemented by the leakage inductance to provide flat high frequency response. This approach should benefit high-frequency, switched-mode power converter design as well as traditional RF applications, especially those with low impedance levels, such as matching to active devices, which are very difficult to achieve with traditional Guanella designs.