INTERNATIONAL PATENT APPLICATION
HIGH FREQUENCY ELECTRONIC BALLAST
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority under USC 119(e) of provisional patent application Serial No. 60/316,240 filed on August 31, 2001, entitled "High Efficiency Method for Driving Gas Discharge Lamps," which is hereby incorporated by reference in its entirety.
BACKGROUND
FIELD OF THE INVENTION
The present invention relates generally to electronic ballasts for gas discharge lamps. More specifically, this invention relates to the production of a high efficiency electronic ballast by unifying power and lamp control at a high, resonant frequency of alternating current applied directly to fluorescent lamps.
DESCRIPTION OF BACKGROUND ART
Fluorescent light operates by creating a discharge or arc across an ionized gas within a glass tube. In traditional fluorescent lighting, the gas tube is filled with mercury vapor which, when ionized, can collide with electrons of a current flow across the electrodes of a lamp, and emit photons. These photons strike fluorescent material on the inner wall of the glass tube and produce visible light.
Fluorescent lamps require a ballast to operate. The ballast conditions the electric power to produce the input characteristics needed for the lamp. When arcing, the lamp exhibits a negative resistance characteristic, and therefore needs some control to avoid a cascading discharge. Both manufacturers and the American National Standards Institute specify lamp characteristics, which include current,
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voltage, and starting conditions. Historically, 50-60 Hz ballasts relied on a heavy core of magnetic material; today, most modem ballasts are electronic.
Electronic ballasts can include a starting circuit and may or may not require heating of the lamp electrodes for starting or igniting the lamp. Prior to ignition, a lamp acts as an open circuit; when an arc is created the lamp starts, the entire ballast starting voltage is applied to the lamp. After ignition, the current through the lamp increases until the lamp voltage reaches equilibrium based on the ballast circuit. Ballasts can also have additional circuitry designed to filter electromagnetic interference (EMI), correct power factor errors for alternating current power sources, filter noise, etc.
Electronic ballasts typically use a rectifier and an oscillating circuit to create a pulsed flow of electricity to the lamp. Common electronic lighting ballasts convert 60 Hz line or input current into a direct current, and then back to a square wave alternating current to operate lamps near frequencies of 20-40 kHz. Some lighting ballasts further convert the square wave to more of a sine wave, typically through an LC resonant lamp network to smooth out the pulses to create sinusoidal waveforms for the lamp. See, for example, U.S. Pat. No. 3,681,654 to Quinn, or U.S. Pat. No. 5,615, 093 to Nalbant.
The square wave approach is common for a number of reasons. Many discrete or saturated switches are better suited to the production of a square wave than a sinusoidal wave. In lower frequency applications, a square wave provides more consistent lighting; a normal sinusoid at low frequency risks de-ionization of the gas as the voltage cycles below the discharge level. A square wave provides a number of other features, such as constant instantaneous lamp power, and favorable crest factors. With a square wave, current density in the lamp is generally stable, promoting long lamp life; similarly, there is little temperature fluctuation, which avoids flicker and discharge, damaging the lamp.
It is known that higher frequencies can produce more efficient lighting. In general, if de-ionization is minimized or avoided, then less energy is needed because there is no re-ionization of the gas; that is, a higher frequency avoids the cycle of
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decay and recovery of ionization within the lamp. Further, the anode fall voltage can be lower when the frequency is higher than the oscillation frequency of the plasma.
However, higher frequency ballasts suffer some problems. First, electronic ballasts can create harmonic disturbance, due in part to the use of pulses or square wave signals. Harmonics are signals in which the frequency is a whole number multiple of the system's fundamental frequency; the third harmonic is most damaging. The total harmonic distortion (or "THD") is one measure of ballast performance. Harmonics create unexpected or nonlinear loading of circuit elements; the harmonic signals cause voltage drops at points of impedance, at the frequency of the harmonic current. At high frequency, the circuitry required to convert a square wave into a sinusoidal wave may limit the available frequency of operation; high frequency voltage drops can change the voltage values of the fundamental wave. A ballast with a high THD may also create electromagnetic interference with nearby electrical equipment, necessitating additional circuitry to filter harmonics; however, such circuits can introduce additional problems such as high inrush current. Second, as discussed in U.S. Pat. No. 5,173,643 to Takehara, it is generally believed that operating frequencies above 50KHz may introduce stray capacitance into lamp circuitry. Finally, the semiconductor switches of many oscillating circuits in electronic ballasts have faced inefficiency or losses, including thermal dissipation, at high frequency driving. Thus, ballast technology has heretofore been limited, thereby also limiting the opportunity for improved energy efficiency.
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SUMMARY OF THE INVENTION
The present invention is an electronic ballast that applies a high frequency sinusoidal wave directly to a gas discharge lamp. The basic idea consists of uniting a high frequency oscillator with elements traditionally located within a lamp network; the output of the oscillator is applied to a lamp, preferably using a center tapped primary winding of a transformer, with the secondary winding of such transformer is in series with the lamp electrode feeder. The lamp is maintained at its striking voltage, and energy consumption low. Feedback and tuning ensures that the lamp is operated at a very high, resonant fundamental frequency. In this way, power control and lamp control are integrated to achieve high levels of efficiency.
DESCRIPTION OF DRAWINGS
Fig. 1 shows a block diagram of typical electronic ballast.
Fig. 2 shows a block diagram of an embodiment of the present invention.
Fig. 3 shows a wiring diagram for an embodiment of the present invention adapted for receiving a direct current input.
Fig. 4 shows a wiring diagram for an embodiment of the present invention adapted for receiving an alternating current input.
REFERENCE NUMERALS
1 First Circuit
2 Second Circuit
5 Alternating Current Power Source 6 Direct Current Power Source
10 Rectifier
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20 Filter
23 LI Inductor
30 Oscillator
31 Ql Transistor 32 Q2 Transistor
40 Tl Transformer
41 P Primary Winding
41 A PI First Portion of Primary Winding
41B P2 Second Portion of Primary Winding 42 S Secondary Winding
43 SF Feedback Winding
45 N Center Tap of Transformer 40
47 Cl Resonating Capacitor
57 C2 Ballast Capacitor 60 FL1 Gas Discharge Lamp
DETAILED DESCRIPTION OF THE INVENTION
As noted above, the present invention is an electronic ballast that applies a high frequency sinusoidal wave directly to a gas discharge lamp. Figure 1 shows a basic block diagram of the typical approach to electronic ballasts; a rectifier converts an alternating current source into direct current, which is filtered and then passes to an oscillator to generate a square wave at a frequency from 20 to 40 KHz. A lamp network is required to condition the square wave for the lamp; this conditioning includes treatment of the wave as described above, such as filtering harmonic distortion and noise, and possibly modifying the square wave form to create more of a sinusoidal shape - if desired for the application. Figure 2 is a basic block diagram of the configuration of an embodiment of the present invention, adapted to receive an alternating current input. This design applies a fundamental, higher frequency, sinusoidal alternating current directly to the lamp using a unified approach. As
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shown, here also a rectifier converts the alternating current input into a direct current, which is then filtered to remove any alternating current ripple; in general, an L-C or Pi filter, or their equivalents would serve this function. This invention is not limited to alternating current sources of input power; the rectifier and filter may be omitted for applications involving a direct current input. A current limiting inductance receives the filtered direct current and applies it to a center tapped transformer. An oscillator, in conjunction with the center tapped transformer, converts the filtered direct current into a high frequency alternating current across the primary of current transformer. Feedback from the transformer is tuned by resonant capacitance, so that the oscillator operates at the fundamental frequency of the circuit.
Figure 3 shows in greater detail a general embodiment of the present invention for an alternating current input. The alternating current input 5 is converted to a direct current by rectifier 10, which may be one of any number of designs known in the art and capable of producing a direct current from an alternating current. A clean direct current that is free from any line or alternating current ripple is desired for embodiments with alternating current input in order to maintain the purity of the oscillator resonant frequency. Any ripple frequency energy could modulate the gas discharge lamp, and reduce efficiency. Accordingly, filter 20 is located after rectifier 10. Filter 20 is not shown in detail, as these are also well known in the art. In addition, those skilled in the art will recognize that with a direct current power supply or input, rectifier 10 and filter 20 may be omitted, as shown in Figure 4, or replaced with a single diode or other such components appropriate for that direct current input.
Inductor (LI) 23 receives the filtered direct current, and acts to limit current change.
As discussed below, inductor (LI) 23 plays a role in setting the voltage ultimately applied to lamp 60. The output of inductor (LI) 23 is applied to center tap (N) 45 of primary winding (P) 41 of transformer (Tl) 40; that is, center tap (N) 45 splits the primary winding (P) 41 of transformer (Tl) 40 into of primary winding 41 into a first portion (PI) 41 A and a second portion (P2) 4 IB, as shown in Figure 3.
In this embodiment, transistors (Ql) 31 and (Q2) 32 are joined collector to collector, with the junction occurring across primary winding (P) 41 of transformer (Tl) 40. That is, each end of primary winding (P) 41 connects to a collector of one of
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transistors (Ql) 31 and (Q2) 32. Secondary winding (S) 42, coupled to primary winding (P) 41, is positioned in series with gas discharge lamp (FLl) 60. It is desirable to introduce some capacitance in series with lamp (FLl) 60 and secondary winding (S) 42 in order to offer some ballast and provide direct current blocking for lamp (FLl) 60. This capacitance is represented by capacitor (C2) 57, but could also include alternate configurations of circuit design available to create a capacitance in the absence of a discrete, separate component, as is known in the art. By way of example and not limitation, such configurations may include alternatives such as placing two conductors near each other without touching. Thus, capacitor (C2) 57 is in series with secondary winding (S) 42 and lamp (FLl) 60, thereby making up second circuit 2 for this embodiment. This design may include other circuitry as desired for the particular application; for example, the invention may include one or more heaters, which are generally omitted for use with cold cathode fluorescent lamps. The no load voltage impressed on the secondary winding (S) 42 of transformer
(Tl) 40 is preferably approximately equal to the strike voltage of lamp (FLl) 60. The alternating current output voltage of oscillator 30 at transformer (Tl) 40 is a linear function of the voltage at inductor (LI) 23. Those skilled in the art will readily see how components of a variety of values could achieve this objective. A third or feedback winding (SF) 43 is provided by transformer (Tl) 40; conservatively, feedback winding (SF) 43 may be coupled either to primary winding (P) 41 or secondary winding (S) 42. Those skilled in the art will recognize that such feedback may be provided in a variety of ways to achieve a similar effect, possibly even arranged by a separate transformer. The bases of transistors (Ql) 31 and (Q2) 32 are joined across feedback winding (SF) 43, with a resonating capacitor (Cl) 47 in parallel with such third coil. Resonating capacitor (Cl) 47 may alternatively be located in parallel with primary winding (P) 41, secondary winding (S) 42, or a combination thereof.
For ease of description, this embodiment is shown in a configuration that supports single-phase applications. Those skilled in the art will recognize the invention's ready adaptability to multi -phase operation, with minor changes in
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individual components that are well known. For example, transformer (Tl) 40 is described above as comprising a primary winding (P) 41 with center tap (N) 45. For three-phase operation, transformer (Tl) 40 could be implemented with three primary windings, such as a WYE transformer, with a center tap common to the three primary windings. Accordingly, it is intended that elements such as windings, transistors, or other circuit components be construed as including all known modifications to enable multi-phase operation. That is, with respect to the above example, primary winding (P) 41 should be construed as a primary winding or windings appropriate for the number of phases of the application. Similarly, the above examples are shown with a single lamp (FLl) 60, which is intended to be construed as one or more gas discharge lamps, with such minor adaptations to second circuit 2 as are known in the art. Transistors (Ql) 31 and (Q2) 32 may be bipolar, FET, or other equivalents. In some embodiments, a power conversion stage may be included with a basic Royer circuit in order to regulate lamp power from line voltage changes. For the purposes of this specification and the appended claims, the terms "connected" or "joined" mean that there exists a conductive path, which may include elements that are not explicitly recited.
OPERATION OF THE BALLAST
The electronic ballast described above is designed to produce a more efficient conversion of input energy into light. By applying a sinusoidal alternating current to the lamp at high frequency, preferably between lOOKHz to 250KHz, the ballast prevents de-ionization and improves efficiency. A unified approach to the generation and application of a sinusoidal wave eliminates the two step creation of a discrete square wave that must then treated by an L-C or other circuit to render it more sinusoidal. Such a two step approach is vulnerable to harmonic distortion, electromagnetic interference, and noise. The generation of a pure sine wave, which suits gas discharge lamps, and is cleaner. Although the present invention is operable with lamps of a variety of sizes, the use of physically smaller lamps, e.g., Tl through T3 (those of a diameter of 1/8 to 3/8 inches), demonstrated better lighting
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performance. The use of tri- or quad- phosphor lamps will further increase light output bandwidth within the visible frequency spectrum.
With reference to Figure 3, a current-fed oscillator may be employed to convert a direct current into a sinusoidal alternating current for driving a lamp. A direct current is applied to inductor (LI) 23. The oscillation is formed when transistors (Ql) 31 and (Q2) 32 alternatively switch, conducting against the impedance of inductor (LI) 23, into center tap (N) 45, and across the respective portions of primary winding (P) 41 to form a sinusoidal alternating current. The voltage of the alternating current is a linear function of the voltage at inductor (LI) 23, determining the wave amplitude. A base signal for transistors (Ql) 31 and (Q2) 32 is provided by feedback winding (SF) 43, which is timed by parallel capacitor (Cl) 47. Selection of the values of the individual components of the unified electronic ballast should preferably produce a no load voltage for the alternating current equal to the strike voltage of lamp (FLl) 60. An induced sinusoidal alternating current is produced in secondary (S) 42 by its coupling with primary (P) 41. The current at lamp (FLl) 60 is ballasted by a small, high-voltage capacitor (C2) 57 positioned in series with lamp (FLl) 60. Capacitor (C2) 57 may also perform direct current blocking to resist lamp mercury migration.
In general, the operating frequency of an oscillator is determined by the resonant frequency of the tank circuit formed by the capacitive and inductive components, and the load that is coupled across the output, such as transformer (Tl) 40. In this case the oscillation occurs at the loaded resonant frequency of the network formed by capacitor (Cl) 47, the magnetizing inductance of primary winding (P) 41, and the reflected impedance of the output load at secondary winding (S) 42 (lamp, capacitor (C2) 57, and any stray capacitance). Capacitor (Cl) 47 may be placed across, (i.e., in parallel with) any winding or combination of windings of transformer (Tl) 40 to achieve the desired effect. Preferably this oscillator operates at a frequency between lOOKHz and 250KHz.
The sinusoidal shape of the alternating current is dependent upon the quality factor or "Q value" of the loaded circuit. The loaded Q value is preferably greater than 3 to ensure stable operation; a value between 6 and 12 may be typical. Another aspect
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of the relatively high Q factor is that a large amount of energy circulates within the circuit relative to the amount of power delivered to the gas discharge lamp. In a less efficient design at high frequency, this characteristic could cause stray capacitance, losses, noise, and interference, particularly if the lamp requires greater energy. In the preferred embodiment, a relatively high current circulates on side of primary winding (P) 41 of transformer (Tl) 40 at a relatively low voltage, and a lower current circulates on the side of secondary winding (S) 42 of transformer (Tl) 40, at a relatively higher voltage. Inasmuch as it is an object of the present invention to reduce power consumption for lighting, the topologies described are well suited to operation at lower power levels. For example, the present invention has shown the ability to provide 100 Watts of effective lighting for 15 Watts of power in a hot cathode lamp and 7.5 Watts of power in a cold cathode lamp.
While the preferred embodiment of the invention, and its operation have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
What is claimed is: