1. Field
The present invention generally relates to induction RF fluorescent light bulbs, and more specifically to reduction of electromagnetic interference from an induction RF fluorescent light bulb with a ferromagnetic core.
2. Description of Related Art
Discharge lamps create light by exciting an electrical discharge in a gas and using that discharge to create visible light in various ways. In the case of fluorescent lamps the gas is typically a mixture of argon, krypton and/or neon, plus a small amount of mercury. Other types of discharge lamps may use other gasses. The gas is contained in a partially evacuated envelope, typically transparent or translucent, typically called a bulb or arc tube depending upon the type of lamp.
In conventional discharge lamps electrically conductive electrodes mounted inside the bulb or arc tube along with the gas provide the electric field used to drive the discharge.
Use of electrodes can create certain problems. First, the discharge is typically designed to have a relatively high voltage in order to minimize losses at the electrodes. In the case of fluorescent lamps, this may lead to long, thin lamp structures, which function well for lighting office ceilings, but are not always a good fit for replacing conventional incandescent lamps. Fluorescent lamps designed to replace incandescent lamps, known as compact fluorescent lamps, or CFLs, are typically constructed by bending the long, thin tube, such as into multiple parallel tubes or into a spiral, which is now the most common form of CFLs. A plastic cover shaped like a conventional incandescent lamp is sometimes placed over the bent tubes to provide a more attractive shape, but these covers absorb light, making the lamp less efficient. Bent and spiral tube lamps also have wasted space between the tubes, making them larger than necessary. The use of a cover increases the size further.
The use of electrodes can create problems other than shape and size. Electrodes can wear out if the lamp is turned on and off many times, as is typical in a residential bathroom and many other applications. The life of the electrodes can also be reduced if the lamp is dimmed, because the electrodes are preferably operated in a specific temperature range and operation at different power levels can cause operation outside the preferred ranges, such as when operating at lower power, which can allow the electrodes to cool below the specified temperature range.
In addition, the long thin shape selected, because it is adapted to allow use of electrodes, tends to require time for mercury vapor to diffuse from one part of the tube to another, leading to the long warm-up times typically associated with many compact fluorescent lamps.
Finally, the electrodes are normally designed to be chemically compatible with the gas used in the lamp. While this is not usually a concern with typical fluorescent lamps, it can be a problem with other types of discharge lamps.
One way to avoid the problems caused by electrodes is to make a lamp that does not use electrodes, a so-called electrodeless lamp. In an electrodeless lamp, the discharge may be driven by, for example, 1) an electric field created by electrodes mounted outside the bulb or arc tube; 2) an electric field created by a very high frequency electromagnetic field, usually in combination with a resonant cavity, or 3) an electric field created by a high frequency magnetic field without the use of a resonant cavity. This latter lamp is called an induction-coupled electrodeless lamp, or just “induction lamp.”
In an induction lamp, a high frequency magnetic field is typically used to create the electric field in the lamp, eliminating the need for electrodes. This electric field then powers the discharge.
Since induction lamps do not require use of electrodes, they do not need to be built into long thin tubes. In fact, a ball-shaped bulb, such as the bulb used for conventional incandescent lamps, is a preferred shape for an induction lamp. In addition, since induction lamps do not use electrodes, they can be turned on and off frequently without substantial adverse impact on loss of life. The absence of electrodes also means that induction lamps can be dimmed without reducing lamp life. Finally, the ball-shaped lamp envelope allows rapid diffusion of mercury vapor from one part of the lamp to another. This means that the warm-up time of induction lamps is typically much faster than the warm-up time of most conventional compact fluorescent lamps.
Induction lamps fall into two general categories, those that use a “closed” magnetic core, usually in the shape of a torus, and those that use an “open” magnetic core, usually in the shape of a rod. Air core induction lamps fall into this latter category. Closed core lamps are usually operated at frequencies generally above 50 kHz, while open core lamps usually require operating frequencies of 1 MHz and above for efficient operation. The lower operating frequency of closed core induction lamps makes them attractive; however, the bulb design required to accommodate the closed core makes them generally unsuitable for replacing standard in incandescent lamps. Open core induction lamps, while requiring higher operating frequencies, allow the design of lamps that have the same shape and size as common household incandescent lamps. This disclosure is primarily is addressed to the open core category of induction lamps.
In spite of their obvious advantages, there are very few open core induction lamps on the market today. One reason for the lack of commercially successful products is the cost of the high frequency ballast. Conventional fluorescent lamps, including CFLs, can be operated at frequencies from 25 kHz to 100 kHz, a frequency range where low cost ballast technology was developed in the 1990s, and closed core induction lamps can be operated at frequencies from 50 kHz to 250 kHz, for which the ballasts are only slightly more expensive. However, open core induction lamps typically require operating frequencies of 1 MHz or higher. The United States Federal Communications Commission (FCC) has established a “lamp band” between 2.51 MHz and 3.0 MHz that has relaxed limits on the emission of radio frequency energy that may interfere with radio communication services. Cost effective open core induction lamps may preferably have an operating frequency of at least 2.51 MHz.
The lack of commercially successful open core induction lamps may be traced to the absence of a low cost ballast that can operate in the 2.51 MHz to 3.0 MHz band while meeting all the requirements of the FCC; that is small enough to fit into a lamp; that has a ballast housing that is the same size and shape as a conventional incandescent lamp; and that can be dimmed on conventional TRIAC dimmers found in homes in certain major markets, such as the U.S. The present disclosure addresses one or more of these issues. Therefore a need exists for improved induction lamps, especially in residential applications.