Electronic ballast

Details Electronic ballast Electronic ballast

2001-12-05

2004-01-06

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Pulsating or a.c. supply

With power factor control device

C315S291000

Reexamination Certificate

active

06674248

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electronic ballasts for gas discharge lamps, such as fluorescent lamps.
BACKGROUND OF THE INVENTION
Electronic ballasts for fluorescent lamps typically can be analyzed as comprising a “front end” and a “back end”. The front end typically includes a rectifier for changing alternating current (AC) line voltage to a direct current (DC) bus voltage and a filter circuit for filtering the DC bus voltage. The filter circuit typically comprises an energy storage capacitor. Electronic ballasts also often use a boost circuit for boosting the magnitude of the DC bus voltage. Additionally, an electronic ballast is known that uses passive power factor correction means to reduce ballast input current total harmonic distortion. These means include line frequency filter circuits having a high impedance at line frequency and about the first 30 harmonics of the line frequency. The high impedance of line frequency filter circuits have a significant reducing effect on ballast input current total harmonic distortion. These filters are in contrast to EMI filters which have low impedance at line frequency and the related harmonics and therefore have no significant effect on ballast input current total harmonic distortion.
The ballast back end typically includes a switching inverter for converting the DC bus voltage to a high-frequency AC voltage, and a resonant tank circuit having a relatively high impedance for coupling the high-frequency AC voltage to the lamp electrodes. The ballast back end also typically includes a feedback circuit that monitors the lamp current and generates control signals to control the switching of the inverter so as to maintain a desired lamp current magnitude.
In order to maintain stable lamp operation, typical prior art electronic ballasts filter the DC bus voltage to minimize the amount of bus voltage ripple. This is usually accomplished by providing a bus capacitor having a relatively large capacitance and hence, a relatively large energy storage capacity. By providing a relatively large bus capacitor, the amount of decay from the rectified peak voltage is minimized from one half-cycle to the next half-cycle. Minimizing the amount of ripple on the DC bus also tends to minimize the current crest factor (CCF) of the lamp current. The CCF of the lamp current is defined as the ratio of the magnitude of the peak lamp current to the magnitude of the root-mean-square (RMS) value of the lamp current.
CCF
≡
I
pk
I
RMS
(
Equation
⁢
 
⁢
1
)
An important indicator of lamp current quality for a gas discharge lamp such as a fluorescent lamp is the current crest factor (CCF) of the lamp current. A low CCF is preferred because a high CCF can cause the deterioration of the lamp filaments which would subsequently reduce the life of the lamp. A CCF of 2.1 or less is recommended by Japanese Industrial Standard (JIS) JIS C 8117-1992, and a CCF of 1.7 or less is recommended by the International Electrotechnical Commission (IEC) Standard 921-1988-07.
However, using a relatively large bus capacitor to minimize ripple on the DC bus voltage comes with its disadvantages. The larger the bus capacitor, the more expensive it is, and the more area it consumes on a printed circuit board, or the like, and the more volume it uses within the ballast. Also, the bus capacitor is discharging whenever the bus voltage level is above the instantaneous absolute value of the AC line voltage, and hence the bus capacitor recharges only during a relatively short time within each line half-cycle, around the absolute value peak voltage of the AC line voltage. Thus, typical prior art ballasts draw a relatively large amount of current during the short time that the bus capacitor is charging, as shown in FIG.
1
. This results in a distorted ballast input current waveform giving rise to unwanted harmonics and undesirable levels of total harmonic distortion (THD).
In an AC power system, the voltage or current wave shapes may be expressed as a fundamental and a series of harmonics. These harmonics have some multiple frequency of the fundamental frequency of the line voltage or current. Specifically, the distortion in the AC wave shape has components which are integer multiples of the fundamental frequency. Of particular concern are the harmonics that are multiples of the 3
rd
harmonic. These harmonics add numerically in the neutral conductor of a three phase power system. Typically, total harmonic distortion is calculated using the first 30 harmonics of the fundamental frequency. Total harmonic distortion (THD) of the ballast input current is preferred to be below 33.3% to prevent overheating of the neutral wire in a three phase power system. Further, many users of lighting systems require ballasts to have a ballast input current total harmonic distortion of less than 20%.
One approach to lowering the ballast input current total harmonic distortion and improving the ballast power factor has been to employ well known active power factor correction (APFC) circuits. This approach has certain tradeoffs including added ballast complexity, more components, greater cost, potentially lower reliability and, possibly, increased power consumption. Moreover, the ballast with APFC typically uses a relatively large bus capacitor with its attendant disadvantages as noted above.
Another approach to lowering ballast input current total harmonic distortion has been to employ a valley fill circuit between a rectifier and an inverter. One disadvantage of typical prior art valley fill circuits is that they can have greater bus ripple, which results in even higher lamp current crest factor, which can in turn shorten lamp life.
Prior art approaches to providing electronic ballasts having improved power factor and THD are discussed in T.-F. Wu, Y.-J. Wu, C.-H. Chang and Z. R. Liu, “Ripple-Free, Single-Stage Electronic Ballasts with Dither-Booster Power Factor Corrector”, IEEE Industry Applications Society Annual Meeting, pp.2372-77, 1997; Y.-S. Youn, G. Chae, and G.-H. Cho, “A Unity Power Factor Electronic Ballast for Fluorescent Lamp having Improved Valley Fill and Valley Boost Converter”, IEEE PESC97 Record, pp. 53-59, 1997; and G. Chae, Y.-S. Youn, and G.-H. Cho, “High Power Factor Correction Circuit using Valley Charge-Pumping for Low Cost Electronic Ballasts”, IEEE 0-7803-4489-8/98, pp. 2003-8, 1998.
Prior art patents representative of attempts to provide electronic ballasts having improved power factor and total harmonic distortion include U.S. Pat. No. 5,387,847, “Passive Power Factor Ballast Circuit for the Gas Discharge Lamps”, issued Feb. 7, 1995 to Wood; U.S. Pat. No. 5,399,944, “Ballast Circuit for Driving Gas Discharge”, issued Mar. 21, 1995 to Konopka et al.; U.S. Pat. No. 5,517,086, “Modified Valley Fill High Power Factor Correction Ballast”, issued May 14, 1996 to El-Hamamsy et al.; and U.S. Pat. No. 5,994,847, “Electronic Ballast with Lamp Current Valley-fill Power Factor Correction”, issued Nov. 30, 1999.
Another reference is “Fluorescent Ballast Design Using Passive P.F.C. and Crest Factor Control” by Peter M. Wood, 1998. This reference shows a ballast of the type employing a line frequency filter having a substantial impedance at the line frequency and about the first 30 harmonics of the line frequency.
SUMMARY OF THE INVENTION
In accordance with a first feature of the invention, a novel electronic ballast for driving a gas discharge lamp includes a rectifying circuit to convert an AC line input voltage to a rectified voltage, a valley fill circuit including an energy storage device which is charged through a switched impedance, the energy in this device being used to fill the valleys between successive rectified voltage peaks to produce a valley filled voltage, and an inverter circuit having series connected controllably conductive devices to convert the valley filled voltage to a high frequency AC voltage. The energy storage device can be a capacitor or an inductor or any other energy storage component or combination of components. Charging the

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