Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
1998-12-28
2001-02-27
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S194000, C315S276000, C315S224000, C315S244000, C315SDIG007
Reexamination Certificate
active
06194840
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
This invention relates to electronic ballasts for operating discharge lamps such as fluorescent lamps, and in particular to such ballasts having a minimum number of active components.
Most magnetically coupled self-oscillating inverters are manufactured in large quantities for sale in a highly competitive market. Half-bridge inverters generally have a lower cost because of a reduced parts count. Such inverters may be classified into two groups: those using a current transformer having a saturable core, generally together with power BJT's (bipolar junction transistors); and those using a current transformer having a linear core, generally together with MOSFETs (metal oxide semiconductor field effect transistors). As those of ordinary skill will recognize, in this context a linear core is one in which operation is over a region having a curved B-H characteristic, rather than a sharp B-H characteristic; that is, at all times the flux level is such that a significant increase in magnetizing current will be accompanied by a significant increase in flux level.
U.S. Pat. No. 5,349,270 discloses a MOSFET half-bridge ballast which has auxiliary windings coupled to a series resonance inductor which is connected between the load and the inverter output at the node between the two switching transistors. The load, a single 20 w fluorescent lamp, is in series with a 220 nf capacitor, and 10 and 15 nf capacitors are in a starting circuit in parallel with the lamp. These capacitors form a resonant circuit with the inductor. As a result, the signals provided by the auxiliary windings are proportional to the inverter output current. Each of the auxiliary windings is connected to a low-pass phase shifting circuit whose output provides a control signal to the gate electrode of the respective MOSFET. It will be clear to those of ordinary skill in the lamp ballast art that, if this ballast were applied to a multiple lamp fixture, removal of one lamp while the ballast was operating would cause a great change in the output voltage applied to the remaining lamp(s). The simple low-pass filter structure is not able to accommodate the proper frequency changes when the lamp number is changed.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to provide a low cost electronic lamp ballast whose output voltage remains substantially constant if the lamp, or all lamps, are removed.
Another object of the invention is to provide a low cost electronic lamp ballast which can operate one, two, three or four lamps with substantially the same output voltage.
A further object of the invention is to provide an electronic lamp ballast having an all passive component regulating circuit containing only passive components, yet permitting broad frequency variation.
According to the invention, a half-bridge converter for operating a plurality of discharge lamps includes two switching transistors having an output node between them, and a linear core transformer having a load winding through which load current passes, the load winding and the load being in series with one end of the series connection connected to the output node. The transformer includes at least one control winding, and the converter has an open loop regulation circuit, including a passive bandpass filter connected between the control winding and the control terminal of one of the switching transistors, for varying the oscillating frequency to maintain the output voltage constant.
A first embodiment of the invention offers the advantage that the output voltage remains constant even if the lamp, or one or more of multiple lamps, are extinguished or removed. The transformer has two control windings, each connected to a respective bandpass filter for controlling the gate of a respective MOSFET. To permit operation with anywhere from one to four lamps operating from the same ballast, the bandpass filter should have a substantially constant phase shift over a wide range of frequency, such as a ratio of 1:2. The desired value of phase shift is determined by other parameters of the converter circuit, especially those relating to the resonance of the load circuit and the circuitry which guarantees adequate ignition voltage for each discharge lamp. A variation of phase shift over a range of only 10°, such as within 5° of a median value, may be required, while the desired median value may be as low as 25° or as high as 65°. If the range of the number of lamps which will be lit is small, such as only 3 or 4 out of 4, then the required frequency range may be as low as approximately 5:6. Other combinations of lamps, such as 2 or 3 out of 3, or different circuitry arrangements, may require relatively constant phase shift over a broader frequency range of 4:5, and even 1 to 4 out of 4 may only require a range of 3:5.
The requirement for constancy of phase shift may also vary, and may in some circumstances permit the phase shift to vary over a range of approximately 20° in value. At the same time, the filter gain variation may interact with the phase shift variation requirement, and in commercially practicable converters the constancy of output voltage will usually be specified more narrowly if frequency variation is not great, such as 4:5, where only 8% total voltge range is permitted; while for a broader variation in numbers of lamps requiring frequency variation of 3:5 the acceptable range of output voltage may be 9% or 10%.
According to one preferred embodiment of the invention, which can operate between one and four lamps, the desired phase shift is between approximately 45° and approximately 55°, preferably about 50° over frequencies from 42 to 70 kHz. These filters are preferably pure AC bandpass filters utilizing only linear passive elements.
A filter providing good performance in this embodiment, for up to four discharge lamps, has at least one &pgr; section including an inductive element which is part of a low-Q parallel resonant circuit in the series or forward path of the filter, and at least one capacitor in the legs to signal ground.
In another embodiment of the invention, the converter is a low cost single stage electronic ballast based on the power feedback principle, having a power feedback path from the high frequency lamp circuit to the input DC voltage to the inverter, and having bandpass filters connected between control windings and gates of respective MOSFETs. The control windings are part of a transformer whose primary winding functions as a resonant inductor for the ballast load circuit. The bandpass filters are pure RC filters having a transfer characteristic with approximately constant attenuation and a phase shift constant within approximately 15°, over a frequency range greater than 2:3.
REFERENCES:
patent: 4920302 (1990-04-01), Konopka
patent: 5329209 (1994-07-01), Nepote
patent: 5349270 (1994-09-01), Roll et al.
patent: 5371668 (1994-12-01), Gurwicz et al.
patent: 5404082 (1995-04-01), Hernandez et al.
patent: 5410221 (1995-04-01), Mattas et al.
patent: 5422546 (1995-06-01), Nilssen
patent: 5424614 (1995-06-01), Maheshwari
patent: 5436529 (1995-07-01), Bobel
patent: 5438243 (1995-08-01), Kong
Franzblau Bernard
Philips Electronics North America Corporation
Philogene Haissa
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