1. Field of the Invention
The invention relates to an electronic ballast for fluorescent lamp, and more particularly, to an electronic ballast suitable for using a conventional thyristor lighting controller for an incandescent lamp to control the lighting of a fluorescent lamp.
2. Description of Related Art
In most of the lighting-control electronic ballasts for fluorescent lamps used today, a pulse-width modulator IC (e.g. modulator in D2220 lighting-control electronic ballast, Osram, Germany) or a frequency modulator IC (e.g. modulator in MD140 lighting-control electronic ballast, MEDERY, U.S.A.) is used as a control circuit to control the output power of a ballast for the purpose of flux or luminance control in a fluorescent lamp. These solutions are complicated and expensive. In addition, there is a high-power-factor solution implemented by two transistors with a capacitor to isolate DC components of total high frequency output current from a lamp and an additional diode to rectify the total high output current feed back to one DC output terminals of a bridge rectifier through the capacitor, such as M128 lighting-control electronic ballast (made by MITOR in the U.S.A.). However, this solution is low in efficiency and hard to prevent from flashing.
As well known, there are three types of electronic ballast: low-power-factor electronic ballast, current-pursuing type electronic ballast, and constant-voltage type electronic ballast. In a combination of low-power-factor electronic ballast and conventional thyristor lighting controller for incandescent lamp, interrupted power supply for the fluorescent lamp, and thus flashing, will occur, since both the input current waveform of the ballast and the output current waveform of the thyristor controller are discontinuous. In the current-pursuing type electronic ballast, peak ratio of its input current is high ( greater than 1.7) and the life-time of a fluorescent lamp will be greatly reduced when it is combined with a thyristor lighting controller. Also, the constant-voltage type electronic ballast can not be combined with a thyristor lighting controller to control the lighting due to its constant voltage supplied to the fluorescent lamp.
The invention is directed to overcoming the problems set forth above. Therefore, the object of the invention is to provide a simple structure, low cost and high efficiency electronic ballast capable of combining with a conventional thyristor lighting controller for an incandescent lamp to control the lighting of a fluorescent lamp.
Thus, in accordance with the invention, an electronic ballast suitable for the lighting control of a fluorescent lamp comprises a bridge rectifier for rectifying the input commercial power supply voltage and a high frequency oscillator to provide the loading fluorescent lamp a high frequency supply voltage.
The electronic ballast further comprises a linear active switching mode filter circuit for filtering the output voltage of the rectifier by a linear active switching mode and feeding a supply voltage to the high frequency oscillator.
The above-mentioned electronic ballast for fluorescent lamp according to the invention has the advantage of simple structure, low cost and high efficiency in the case of stand-alone applications. Furthermore, it can be combined with a conventional thyristor lighting controller for incandescent lamp to form a lighting-control electronic ballast for fluorescent lamp having the advantages of easy prevention of flashing, high input power factor and low lamp current peak ratio.