1. Field of the Invention
This invention relates to the field of electric transformers, and in particular to transformers that are used in ballast circuits of lamp assemblies.
2. Description of Related Art
Ballasts are commonly used in lamp assemblies to provide a preferred, or optimal, current and voltage to the lamp device. Most lamp ballasts employ a transformer to effect the required transformation of supply voltage to this preferred voltage. The reliability, or expected time-to-failure, of a transformer is inversely proportional to its operating temperature. Electric current flowing through the coils of the transformer generates heat, and this heat causes an increase in the operating temperature of the transformer, thereby reducing its reliability. The amount of heat generated can be reduced by using larger sized wires in each coil, but would result in a larger sized and more costly transformer. Alternatively, the operating temperature can be reduced by efficiently removing the generated heat from the transformer. A variety of techniques are currently available for increasing the efficiency of heat transfer from the coils of a transformer. Thermally conductive potting compounds have been used to conduct the heat from the transformer coils to an enclosure containing the transformer. These semi-fluid compounds, however, are somewhat difficult to handle, compared to solid devices and components, and often introduce reliability problems to other devices, for example, by flowing into the moving parts of switches, relays, connectors, and the like.
European Patent Application EP 0 254 132, filed Jul. 8, 1987, discloses fastening a metal shell about a transformer, wherein the shell preferably contacts the exterior layer of the coil windings, via a thin insulating layer. This shell is preferably connected to the ballast enclosure, to transfer the heat generated by the coils to the enclosure, both the shell and the enclosure being made of heat-conductive material. This exterior shell must be configured to allow air to circulate within the enclosure, else the thermal efficiency gained by providing the shell will be reduced by the reduction in radiant heat dissipation. Also, the wires that are connected to the coils must be routed through openings in the shell. To assure an efficient thermal contact between the shell and the coil, both the coil dimensions and the shell dimensions must be controlled. A loose fitting shell will be thermally inefficient, and a tight fitting shell may be difficult to fit onto the transformer.
It is an object of this invention to provide a method of manufacturing a transformer that provides for efficient heat-transfer from the coils of the transformer. It is a further object of this invention to provide a method of manufacturing a transformer that provides an efficient heat transfer without introducing additional manufacturing constraints. It is a further object of this invention to provide a transformer that includes an efficient heat transfer arrangement.
These objects and others are achieved by providing one or more heat-conducting devices coincident with the plastic bobbin that is typically used to form the coils of the transformer. The coil wire is subsequently wrapped around the bobbin and adjacent heat-conducting device assembly, using conventional coil winding techniques. The heat-conducting device is preferably configured to extend beyond the transformer so as to contact a heat-conducting surface, such as a ballast enclosure, when the transformer is appropriately mounted. Because the heat-conducting device is located adjacent the inner windings of the coil, which is typically the locale of the highest temperature build-up in a transformer, a highly efficient heat-transfer is achieved. Preferably, the heat-conducting device is a thick copper conductor having a thin layer of insulating tape separating it from the coil windings.