1. Field of the Invention
This invention relates to the field of integrated circuits, and in particular to the design of an interleaved transformer device with improved coupling characteristics.
2. Description of Related Art
A transformer comprises two coils of wire that are electromagnetically coupled. An AC signal applied to one of the coils of wire, nominally termed the primary coil, induces a corresponding AC signal on the other coil, nominally termed the secondary coil. The efficiency of the coupling between the coils determines the efficiency of the energy transfer from one coil to the other, as well as the overall noise sensitivity of the transformer.
Transformers are generally used to provide electrical isolation between signals, often to allow for a change of voltage reference planes. Transformers are also often used to couple a signal to a resonant L-C circuit, one of the coils of the transformer providing the inductance, or a portion of the inductance. Radio-frequency (RF) transmitters typically include an RF transformer that couples an output stage to a resonant antenna stage.
Discrete transformers include two coils of wire that are wrapped around a core, which may be an air core, or a ferrite core, for improved coupling efficiency. The coils may be stacked, one atop the other on a common core, or may be wrapped one after the other on the common core, or may be formed by interleaving the wires and then wrapping the combination of the wires about the common core.
Stacked and interleaved transformers may be similarly constructed on multi-layer integrated circuits. Stacked coils can be implemented by forming conductive spirals of conductors on each of two layers, one atop the other. A transformer with interleaved coils is formed by laying the conductors of each coil adjacent to each other on the same conductive layer in a spiral pattern. A stacked core exhibits a high capacitance, because the two co-located spirals, one atop the other, effectively form two plates of a capacitor. This capacitive coupling between the coils introduces phase-shift and amplitude errors during the coupling process. In a transformer with interleaved coils, because the conductors are adjacent each other, rather than atop each other as in the stacked embodiment, the capacitive coupling is substantially less, thereby providing less phase-shift and less amplitude error.
FIG. 1 illustrates an example layout of a coplanar interleaved transformer 100. Terminals 111 and 112 are the terminals of a first coil 110, and terminals 121 and 122 are the terminals of a second coil 120. Different cross-hatching is used for the conductors of each coil, for ease of illustration and understanding. Both coils are substantially on the same integrated circuit layer, preferably the heavy metallic layer, for minimal resistance losses. Interconnection segments 102 are located on a second circuit layer, to allow for insulated cross-overs of routing, as required. Connections between layers are illustrated by the circular areas 105, for ease of distinction in the illustrations. The coplanar interleaved transformer of FIG. 1 provides less capacitive coupling between coils than a stacked configuration, but consumes more area than a stacked configuration, and provides less efficient coupling than the stacked configuration.
U.S. Pat. No. 5,543,773, xe2x80x9cTRANSFORMERS AND COUPLED INDUCTORS WITH OPTIMUM INTERLEAVING OF WINDINGSxe2x80x9d, issued Aug. 6, 1996 to Peter D. Evans and William J. B. Heffernan, teaches the interleaving of parallel segments of one of the coils, to achieve different turn-ratios, and is incorporated by reference herein. The referenced patent includes a coplanar transformer having a continuous primary coil spiral wound about a ferrite core, with interleaved segments of a secondary coil that are connected in parallel to effect a non-unity turns ratio. To improve the coupling between the primary and secondary coils, additional turns are conventionally used. For example, in the referenced patent, a 12:2 turns-ratio of actual turns is used to effect a 6:1 electrical turns-ratio; a 12:6 actual turns-ratio is used to effect a 2:1 turns-ratio, and so on. These additional actual turns increase the inductive coupling between the coils, while still maintaining the desired electrical turns-ratio that determines the ratio between input and output currents (n1*i1=n2*i2). Increasing the number of actual turns, however, increases the resistance, as well as the inductance, of each coil. In a coplanar transformer, increasing the number of actual turns substantially increases the area consumed by the transformer.
It is an object of this invention to provide a transformer structure that provides the relatively low capacitance of a conventional coplanar interleaved transformer, but with improved coupling efficiency and effectiveness. It is a further object of the invention to provide a coplanar transformer structure that provides improved coupling efficiency and effectiveness, without substantially increasing the inductance or resistance of the coils.
These objects and others are achieved by splitting each coil into two or more parallel segments, and interleaving the multiple segments of each coil with each other to form an interleaved coil that has a greater coupling efficiency than a non-segmented interleaved transformer. Because the multiple segments of each coil are connected in parallel, the resistance of the coil is decreased, or, the width of the conductor used to form the coil can be decreased while maintaining the same resistance. By reducing the width of the conductors used in a coplanar transformer, the additional parallel segments can be placed in substantially the same area as the conventional transformer, thereby maintaining the same inductance. To provide for maximum efficiency, each segment of each coil is embodied so as to have substantially equal length as each other segment of the coil.