This invention relates to electronic ballasts for gas discharge lamps and, in particular, to removing heat from electrical components through a thermally insulating, plastic enclosure.
A fluorescent lamp is a non-linear load, i.e. the current through the lamp is not proportional to the voltage across the lamp and the current will increase rapidly unless there is a ballast in series with the lamp to limit current. An electronic ballast typically includes a converter for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the DC to high frequency AC. Converting from AC to DC is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. Some ballasts include a "boost" circuit to improve power factor or to increase the voltage on the filter capacitor from approximately 140 volts to 300 volts or higher (from a 120 volt AC input). The inverter changes the DC to high frequency AC at 140-300 volts for powering one or more fluorescent lamps.
A boost circuit and an inverter each includes at least one "magnetic" (transformer or inductor). Because electronic ballasts operate at high frequency (e.g. 30 khz.), the magnetics in an electronic ballast are relatively small and this fact has been exploited to make ballasts physically smaller than in the past because the magnetics are usually the largest components in a ballast. While the magnetics can be made in almost any shape, it can be shown that the most efficient and least expensive magnetics are essentially cubic. Thus, the height of the magnetics above the printed circuit board sets a lower limit on the thickness of the ballast. The various heights of all the components makes the surface of a populated circuit board relatively uneven.
The efficiency of a ballast is the power consumed by a lamp divided by the total power supplied to the ballast. No ballast can deliver all the applied power to a lamp (one hundred percent efficiency). Some power is always dissipated in a ballast and the more compact designs of modern electronic ballasts cause difficulty in coping with the heat being generated in a small space. Components, particularly the magnetics, can overheat, thereby reducing the life and reliability of the components. A plastic enclosure for a ballast simplifies the problem of electrically insulating the ballast but complicates the problem of removing heat from the ballast.
A heat sink, i.e. a large thermal mass for absorbing heat from a source, within a ballast is not compatible with the idea of a compact ballast. Further, thermally coupling components to the heat sink is difficult clue to the various heights of the components on the circuit board. Exposing a heat sink, or some components, to outside the ballast is not feasible because of the need to electrically insulate the circuitry of the ballast.
In view of the foregoing, it is therefore an object of the invention to provide a heat spreader for conducting heat away from sources within the ballast through a plastic enclosure to a sink outside the ballast.
Another object of the invention is to provide a heat spreader that can accommodate heat sources of different heights.
A further object of the invention is to provide a heat spreader that does not reduce the electrical insulating benefits of a plastic enclosure.
Another object of the invention is to provide a heat spreader that does not increase the size of an electronic ballast.