WO1994027419A1

WO1994027419A1 - System and method for distributing power to gas discharge lamps

Info

Publication number: WO1994027419A1
Application number: PCT/US1994/005144
Authority: WO
Inventors: Fazle S. Quazi
Original assignee: Etta Industries, Inc.
Priority date: 1993-05-13
Filing date: 1994-05-13
Publication date: 1994-11-24
Also published as: CA2162889A1; AU6908894A

Abstract

A system and method for reducing installation costs of gas discharge lighting provides a central AC-DC power converter (302) which is connected into an AC lighting circuit and thereafter transmits DC power through the power distribution wires (306) to individual lighting locations (305). DC-powered resonant inverter driving circuits (304) are provided and connected to the DC power where lighting is desired. The system is particularly useful in retrofitting existing buildings or individual light fixtures which use incandescent lamps with more energy-efficient fluorescent, metal halide, and high pressure sodium types of gas discharge lamps.

Description

SYSTEM AND METHOD FOR DISTRIBUTING POWER TO GAS DISCHARGE LAMPS

Field of the Invention The present invention relates to driving circuits for gas discharge lamps and methods for constructing .and installing these circuits. Background of the Invention

Gas discharge lamps, such as fluorescent lamps, high pressure sodium lamps, and metal halide l.amps, require a driving circuit, or ballast, to operate properly. In recent years, electronic ballasts have gained popularity over magnetic ballasts because of their higher electrical power conversion efficiency. The efficiency improvement in using an electronic ballast rather than a magnetic ballast can exceed 30% depending on the lamp and ballast combination used. An electronic ballast based on resonant inverter circuitry has been described in U.S. Patent No. 4,933,605, assigned to the assignee of the present application. Electronic ballasts have typically been more expensive than magnetic ballasts. To reduce overall costs, a single ballast is often wired to multiple bulbs and particularly to multiple fixtures, as shown in the prior art example of Figure 1. A master lamp fixture 102 and a slave lamp fixture 104 share a ballast 106 located in master lamp fixture 102. .Lamp 107 in slave lamp fixture 104 is driven by signals carried on connection wires 108 which transmit high frequency driving signals from ballast 106 to lamp 107. However, such "master ballast" arrangements have not been used to drive a large number of slave fixtures. The inventor has studied this type of circuit and has determined that the sharing of ballasts between multiple fixtures is inherently limited by the following factors:

1. A single ballast can supply power only to a limited number of lamps; 2. The connection wires 108 supplying current from the ballast to individual lamps in these prior art systems carry high-frequency alternating current. Long wire runs will add significant inductance and capacitance between the lamp and ballast, affecting their operation and efficiency; and

3. High-frequency connection wires 108 must be shielded to reduce propagation of electromagnetic interference, so that installation of extensive networks of high frequency lines is made more difficult and expensive.

Often, incandescent or less-efficient discharge lamp units throughout a building are replaced to obtain energy savings. Typically, as shown in Figure 2, lighting fixtures in buildings are powered by AC current through wires 203 routed through single or multiple distribution boxes 202. Distribution boxes 202 connect wires 203 to fixtures 205. There are two known processes for replacement of fixtures 205 with more energy-efficient lighting. In the first process, the existing fixtures 205 are removed and new gas-discharge fixtures, preferably fluorescent fixtures, are directly substituted, each fixture including a ballast. Because of the problems associated with high-frequency power lines, it has not been considered feasible to retrofit .an existing building with centralized ballasts.

In a second method, a gas discharge adapter, preferably a fluorescent adapter, may be installed in place of each incandescent bulb. A variety of these adapters are commercially available. Generally, these devices screw into a standard incandescent bulb socket of a light fixture or table lamp to form electrical connections with the AC power. The adapters consist of a small fluorescent ballast operating from the AC socket, and a low-wattage fluorescent lamp, typically 9 to 15 watts, which provides greater light output efficiency than the replaced incandescent bulb. The applications of fluorescent adapters have been limited by the required size of the ballast circuit housing. In some cases these adapter are too large to fit into a given decorative fixture. In other cases, such as in chandeliers, the protruding ballast housing at the bulb base would significantly detract from the appearance of the fixture. Another problem with these adapters is the generation of electromagnetic interference. Adequate suppression of interference generated by these devices usually requires additional components which further increases the size of the devices. Therefore, the inventor believes that there is a need for a system which provides efficient electronic ballast circuits for a large number of gas discharge lamps at a reduced capital cost. It is further believed that there is a need for a more reasonably-priced method of retrofitting existing buildings and fixtures with highly-efficient ballast .and lamp units. Summary of the Invention Therefore, it is a general object of the present invention to provide an efficient electronic ballast circuit which will drive a large number of gas discharge lamps.

Another general object of the present invention is to provide a novel system in which a high voltage DC front end provides DC current to a plurality of electronic inverters which drive gas discharge lamps. It is another general object of the present invention to provide a novel method for retrofitting existing buildings with highly-efficient baltested gas discharge lamps.

A further object of the present invention is to provide method of retrofitting existing buildings with gas discharge lamps in which existing AC wiring is used to transmit high voltage DC power from a newly installed central ballast front end to a plurality of electronic inverters providing driving signals for gas discharge lamps.

Another object of the present invention is to provide new and improved gas discharge lamp ballasts with reduced physical size. Yet Mother object of the present invention is to provide apparatus and methods for refitting existing lighting fixtures, such as chandeliers, with compact fluorescent light bulbs.

Further objects of the invention will be apparent to those skilled in the art upon review of the specification, drawings, and claims.

These objects are achieved in the present invention by providing a central AC-DC power converter which is connected into an AC lighting circuit and thereafter transmits DC power through the power distribution wires to individual lighting locations. DC-powered resonant inverter driving circuits are provided and connected to the DC power where lighting is desired. The system is particularly useful in retrofitting existing buildings or individual light fixtures which use incandescent lamps with more energy-efficient fluorescent lights. The invention described herein produces significant advantages, including: (1) a savings of at least 33% in ballast cost; (2) no change in the existing building wiring system, (3) a single front end can be used to power ballasts in multiple zones or rooms, (4) one front end can be used to operate single or multiple lamps, and (5) elimination of electromagnetic interference associated with long high frequency current carrying wires. Brief Description of the Drawings

Figure 1 is a diagram of a prior art master-slave fixture;

Figure 2 is a block schematic diagram of a conventional AC electrical lighting power distribution system;

Figure 3 is a block schematic diagram of the modified lighting power distribution system according to the present invention;

Figure 4 is a schematic diagram of the driver end used in each fixture in Figure 3;

Figure 5 is a schematic diagram of the front end provided in the distribution box as shown in Figure 3;

Figure 6 is a flowchart showing a method of retrofitting an existing fixture system with the system of the present invention; and

Figure 7 is a block schematic diagram showing the method according to the present invention as applied to a multibulb incandescent lighting fixture. Detailed Description of the Preferred Embodiments The present invention will be described herein in terms of preferred embodiments using fluorescent lamps. However, it will be understood that the invention is not limited to the specific embodiments disclosed herein. The embodiments disclosed may be modified by those skilled in the art within the scope of the invention, which is defined by the claims. The present invention relates to ballast circuits for gas discharge lamps, such as fluorescent lamps. A conventional electronic ballast, as disclosed in the inventor's U.S. Patent Nos. 4,933,605 and 4,864,482 which are incorporated herein by reference, converts conventional AC line voltage to high voltage DC, which is then switched at high frequencies by an inverter circuit to produce an excitation signal for the associated gas discharge lamp. In a first preferred embodiment of the present invention, as shown in Figure 3, a new type of ballast system is constructed by providing two physically separate types of components: a front end 302 which produces DC power, and a plurality of driver ends 304 each including a resonant inverter circuit for producing a lamp driving signal from the DC power of the front end 302. In the embodiment shown in Figure 3, this system is provided in a building electrical distribution system, where it may be installed as original equipment or as part of a retrofitting process. As shown in Figure 3, AC power from a circuit breaker panel 301 is connected by wires 203, typically including a white "neutral" wire and a black or red "hot" wire. Wires 203 run through a conduit 204 to a distribution box 202. In a conventional AC electrical distribution system, these wires would branch from the distribution box 202 to provide AC power to various lighting fixtures in the circuit. However, according to the present invention, wires 203 are connected as power inputs to a front end 302 located in distribution box 202.

In the front end 302 according to the present invention, 50 or 60 cycle AC power is converted into a high voltage DC power output of up to 600 volts, preferably by semiconductor rectifiers in a manner which will be described in detail with reference to Figure 5. Front end 302 is preferably made with a higher power output capability than the DC power available in a conventional fluorescent ballast, such as for example 2000 watts. The front end can be conveniently built in a housing with dimensions of 3 inches by 4 inches x 1.5 inches. If compactly designed, the front end 302 can easily be installed in a distribution box 202. The front end 302 is preferably a highly regulated power supply which active monitors and shapes the DC output. Preferably, the front end 302 may also correct power factors, shape power line harmonics, and protect against inrush currents.

The output voltage of front end 302 may vary depending on the requirements of the system. Preferably, for use with a 120 volt AC system, the DC output voltage will be at 250 volts DC, and preferably 350 VDC. If 277 VAC is used as an input, 500 VDC output may be produced. Finally, if 440 VAC is provided as the power input, 580 VDC output may be produced. It is desirable to keep the output voltage less than 600 VDC to stay within the power rating of standard building wiring. As shown in Figure 3, the DC output of front end 302 is connected by wires 306 to a plurality of fixtures 305, where the DC power supplies a plurality of driver ends 304. An electronic dimmer circuit 320 may optionally be connected to driver ends 304.

Wires 306 may preferably be identical to wires 203, that is, conventional insulated household or office power distribution wires. Electrical codes in the U.S., and the ratings of existing wiring, typically permit this wiring to carry up to either 600V AC or 600V DC in a standard conduit. Therefore, wires 306 may be wires designed for AC current, and in fact may be wires that were originally installed to carry AC current to lighting fixtures but have now been connected to the front end 302 to carry DC current.

Thus, lamp fixtures 305 are powered through single or multiple distribution boxes 202 which connect the single front end 302 to the plural driver ends 304 at the fixtures 305.

Each driver end 304 receives the DC output generated by front end 302. In the driver end, this DC power is converted into a high frequency (usually higher than 20 kHz) source which supplies power to one or more lamps, by any of a variety of known circuits. The preferred 2000-watt front end 302 according to the present invention can safely drive at least 20 driver ends 304 which may be designed to operate two 40 watt, four-foot T-12 lamps.

The circuits provided in the driver end 304 are the inverter switching circuits which produce an excitation signal for the gas discharge lamps 107. Since no power conditioning or conversion circuits are provided in driver ends 304, driver ends 304 can be made in a more compact and less expensive manner as compared to a conventional gas discharge ballast. Preferably, the driver end circuit is a resonant inverter circuit as disclosed in U.S. Patent Nos. 4,933,605 or 4,864,482. The circuitry of driver end 304 will be discussed in greater detail with reference to Figure 4. Driver ends 304 may take the form of conventional hard wired gas discharge ballasts having wire connections for power input, or driver ends 304 may be constructed with a bulb socket base 311 to provide power input terminals. The bulb socket base 311 threadedly connects with a standard light bulb socket 309. Driver ends 304 which are constructed with bulb socket bases 311 may also be provided with a mounting for a lamp 107, which may be a compact, low voltage fluorescent lamp, circular fluorescent lamp, or other appropriate design for use in retrofitting existing lighting fixtures. Driver ends 304 of this type can be appropriately used when increased efficiency is desired in the lighting circuit but replacement of existing incandescent lighting fixtures is not desired. Through the use of bulb-mount driver ends, incandescent lighting can be converted to fluorescent lighting using the system of the present invention while minimizing installation costs and avoiding replacement of the entire fixture.

While the bulb mount driver ends of this type are physically similar in appearance to known fluorescent incandescent replacement devices, they are different from these prior devices in that they operate from DC current received through the bulb base, and in that the driver end circuit 304 is smaller and less obtrusive since it includes no AC-DC converter circuitry.

The driver end 304 may also be provided with a dimming circuit to control the brightness of tamp 107, as disclosed in the inventor's copending U.S. Patent Applications, Serial No. 07/410,480 filed September 21, 1989, titled "Electronic Dimming Methods for Solid State Electronic Ballasts", and Serial No. 07/789,268 filed November 8, 1991, both incorporated herein by reference. In general, as disclosed in more detail in these parent applications, a pulse width modulating circuit may be provided as part of electronic dimmer circuit 320 (shown in Figure 3), with the width of the pulses produced by the pulse width modulating circuit varying with the desired brightness of the connected lamp 107. These pulses are then integrated and supplied to the driver end 304, and particular to terminal 409 of driver end 304 as shown in Figure 4, which is connected to the noninverting input of inverter 76. The variable width pulses generated in electronic dimmer circuit 320, upon integration, produce a variable DC dimming control signal which causes variation in the voltage level at pin 2 of inverter 76. This variation in voltage varies the duty cycle of the output pulses at pins 11 and 14, thus varying the brightness of the lamp 107. A separate dimming circuit 320 may be provided for each driver end 304, or a single dimming circuit may be used with multiple driver end inverter circuits as disclosed in the above-identified copending applications. Optionally, the electronic dimming circuit 320 may be provided with an ambient light sensor input. In this embodiment, a photocell detects the amount of ambient light through a light collection device, and the desired brighmess signal of the dimming circuit is automatically varied inversely with the ambient light level so that less artificial light is provided at times when there is a large amount of ambient light. Further, driver ends 304 may be provided with switches 308, which may be any conventional lighting switch. Preferably, these switches may be of the low-voltage control type which operate using relays or other means to switch high voltages without high voltages being present in the components contacted by the operator of the switch. Figure 4 is a schematic diagram of a preferred embodiment of the driver end 304 of the present invention, although it will be understood that the invention is not limited to the particular circuit shown.

As shown in Figure 4, driver end 304 is provided with terminals 402 and 404 for receiving DC power from a centralized source, such as front end 302 (shown in Figure 3). Driver end 304 comprises an inverter pulse width modulator 76 which provides a pulse signal on two separate outputs, pins 11 and 14. Inverter pulse width modulator 76 is preferably a conventional integrated circuit such as the SG 2525 manufactured by Motorola, U.S.A.

Inverter pulse width modulator 76 has an internal oscillator controlled flip flop circuit and dual output NOR gates to provide dual pulse outputs at pins 11 and 14. The internal NOR gate outputs of the inverter pulse width modulator 76 are connected by internal output transistor pairs to pins 11 and 14. The pulse outputs on pins 11 and 14 occur sequentially as the internal flip flop circuit changes state.

A low voltage power supply is applied to the inverter pin 15. This power supply may take any desired form, but in the embodiment shown comprises a transformer 406 with its primary winding connected in series with the positive DC current input from terminal 402. The secondary winding is connected between terminal 404 and through a diode 408 to inverter power input pin 15. Current transferred from the primary winding to the secondary winding of transformer 406 passes through a regulating diode 74 to charge a capacitor 75 and provide low-voltage DC power for operating inverter pulse width modulator 76.

The combination of capacitor 98 and resistor 100 determines the frequency of the oscillator 78 for the inverter pulse width modulator, while a resistor divider formed by a resistor 410 and a variable resistor 412 determines the .amount of DC voltage applied to the noninverted terminal (pin 2) of the internal error amplifier. This causes the error amplifier to set the magnitude of the duty cycle of the output pulses at pins 11 and 14. These output pulses drive the inverter switches of a resonant inverter 413 which converts the DC signal on line 414 to high frequency AC to drive bulb 110, which in the present example is a fluorescent bulb but may be any gas discharge lamp load. The resonant inverter 413 includes switches 112 and 114 which are connected to pins 14 and 11, respectively, of the inverter pulse width modulator 76. These switches are suitable solid state or mechanical switches which are driven on and off in response to the pulses at the outputs of the inverter pulse width modulator. Since these pulses occur at different times, the switch 112 will be on when the switch 114 is off and vice verse. During the time that the switch 112 is on or conductive, energy flows from the line 414 through the switch 112 and a resonant inductor 116 to charge a capacitor 118. Then when the switch 112 is off and the switch 114 turns on, stored energy from the capacitor 118 flows back through the resonant inductor and the switch 114. Preferably, the pulse repetition frequency which operates the switches is identical with the resonance frequency of the LC network formed by the resonωt inductor 116 and capacitor 118, so that inverter 413 operates effectively as a resonant inverter.

Referring now to Figure 5, the basic AC-DC conversion circuit, or front end, of the present invention is indicated generally at 302. Front end 302 includes a full wave bridge rectifier 12 having input terminals 501 and 503 connected to an AC power line. The rectifier includes outputs 18 .and 20 with a negative temperature coefficient thermistor 22 connected in a series with the output 18. A storage capacitor 24 is connected to the outputs 18 and 20 of the bridge rectifier 12 via an inductor 28 and a rectifier diode 36. Full wave bridge rectifier 12 and storage capacitor 24, like the other components of front end 302, are selected to have the desired current and power output capacity. In the preferred embodiment described previously with reference to Figure 3, the power output capacity of front end 302 is 2000 watts. Therefore, full wave bridge rectifier 12 and storage capacitor 24 are selected to provide this power handling capacity.

The front end 302 is designed to effectively limit inrush current under all circuit operating conditions, even during a short power interruption. This is accomplished in part by a boost switching regulator 26 which includes an inductor 28 connected in series with the thermistor 22. This inductor serves the dual purpose of providing the primary winding for a transformer 30 having a secondary winding 32, and also of storing energy during the time that a switch 34 is turned on. Switch 34 is formed by any suitable semiconductor or mechanical switch which can be selectively rendered conductive by a control signal to complete a circuit. When the switch 34 is turned off, energy stored in the inductor 28 is transferred to the storage capacitor 24 through rectifier diode 36 connected in series therebetween. The output voltage from the boost switching regulator 26 is greater than the input voltage thereto. The boost switching regulator 26 also includes a pulse width modulator controller 38 to drive the switch 34. This pulse width modulator controller is a commercially available integrated circuit, such as an SG 2843 manufactured by Motorola, U.S.A.

The pulse width modulator controller 38 includes an internal oscillator which provides pulses to an internal latching pulse width modulator. The input signal on pin 4 sets the frequency of the oscillator and determines the pulse output frequency which is provided from the latching pulse width modulator through an internal comparator to output pin 6. A current sense input for the latching pulse width modulator is provided by pin 3.

A low voltage supply is applied to pin 7 in order to provide power to the integrated circuit. Pin 7 is also connected internally to an undervoltage lockout circuit and a 5 volt reference circuit. This 5 volt reference circuit is connected to pin 8 and also provides a 2.5 divider to a non-inverting input of an internal error amplifier. Another, inverted input of the error amplifier is connected to pin 2, while an output compensation terminal for the error amplifier is provided by pin 1. The reference voltage is also provided by means of an internal undervoltage lockout circuit to the internal latching pulse width modulator.

The start-up current for the pulse width modulator controller 38 is provided by a resistor 56 connected to the output side of thermistor 22. The pulse width modulator controller 38 requires only a 1 milliampere start-up current, and this is provided to pin 7 by the resistor 56. The start-up current must be low so that power dissipation in the resistor 56 is insignificant.

With start-up power being provided to the pulse with modulator controller 38, the output frequency of its internal oscillator is set by the series combination of a timing resistor 58 and a timing capacitor 60. This in turn determines the pulse output frequency on pin 6 which drives the switch 34. The magnitude of the output voltage across the storage capacitor 24 is set by sensing the voltage ratio between the resistors 62 and 64 connected in series across the capacitor 24 at the output side of the rectifier diode 36. An impedance 66 is provided to maintain loop stability of the latching pulse width modulator and is connected between the compensation and the non-inverted terminals of the error amplifier (pins 1 and 2). The signal from a current sense resistor 68, which is connected in series with the switch 34, is provided to pin 3 of the pulse width modulator controller for short circuit protection and for limiting current flow through switch 34.

A start-up current through resistor 56 which builds a 1 ma level in a series capacitor 70, causes a slow start-up of the pulse width modulator controller 38. Short duration pulsating signals from the output (pin 6) then start to turn the switch 34 on and off. Inductor 28 stores energy during the on period of switch 34 and transfers this energy to the storage capacitor 24 through rectifier diode 36 during the off period of the switch. A few turns of secondary winding 32 now can provide energy to capacitor 70 through a rectifier diode 72. This energy flowing into the capacitor 70 supplies the additional power necessary to make the pulse width modulator controller fully functional. Next, the output voltage sense signal from the resistors 62 and 64 is applied to the feedback terminal, that is, the inverting terminal (pin 2) of the error amplifier to automatically provide the regulator for maintaining a constant output voltage from the boost switching regulator 26 irrespective of input voltage variations. The front end 302 is provided with output terminals 502 and 504 which can be connected respectively to the power input terminals 402 and 404 of a plurality of driver ends 302 (shown in Figure 3).

During normal operation, when power is first received at the rectifier input terminals 501 and 503, the thermistor 22 offers sufficiently high resistance to limit the inrush current to the storage capacitor 24. However, when power if first received, it is important to insure that the pulse width modulator controller 38 starts operation only after the storage capacitor 24 becomes charged to nearly the input voltage peak. Otherwise, switch 34 might be driven on while current is still flowing to the filter capacitor through the inductor 28. Depending on the amount of current flow, the inductor 28 may be fully or partially saturated, and in this condition, if the switch 34 is turned on, the inductor may offer only partial or no inductance.

As a result, switch 34 will experience a virtual short circuit and possible resultant damage.

The required delay in the operation of the switch 34 until the capacitor 24 has charge is provided by the resistor 56, the capacitor 70 and the internal undervoltage lockout unit of pulse width modulator controller 38. The undervoltage lockout unit will prevent operation of the pulse width modulator controller 38 until the input voltage exceeds the undervoltage lockout v.alue. The time that the voltage across the capacitor 70 exceeds the undervoltage lockout value depends upon the values of resistor 56 and capacitor 70. These can be set to insure that the specified input voltage level is not reached until storage capacitor 24 is given time to charge. As soon as the pulse width modulator controller 38 starts working, switch 34 will start to turn on and off. Inductor 28 will periodically store and release energy and secondary winding 32 will start to supply additional energy to capacitor 70 through diode 72. The capacitor 70 now supplies the necessary operating power to the pulse width modulator controller. When the front end 302 experiences a short term power interruption, power may be reestablished while the resistance of the thermistor 22 is still low. In this situation, the amount of inrush current drawn by the storage capacitor 24 depends on the amount of charge retained by this capacitor at the end of the power interruption. If the retained charge is low, a large inrush current will occur.

To control the inrush current to a minimal, nondamaging level, the charge on the storage capacitor 24 is maintained during a short term power interruption by removing the lamp 107 (shown in Figure 4) as soon as the power interruption occurs. If this is done, the storage capacitor will retain its full potential with the only power drain therefrom being caused by the resistors 62 and 64. However, the resistor divider formed by these resistors has a very high impedance, and the energy drawn thereby is extremely small.

Referring again to Figure 4, lamp 107 is effectively disconnected from the storage capacitor 24 by the removal of power from the inverter pulse width modulator 76. This removes the output signals from pins 11 and 14, and the switches 112 and 114 will remain open. During a power interruption, the secondary winding of transformer 406 cannot supply energy to the capacitor 75, and the storage capacity of capacitor 75 is selected to be very small so that the inverter pulse width modulator 76 will almost instantly shut down. Rapid shutdown can be insured by having the internal undervoltage lockout operate at a lockout voltage close to the voltage storage capacity of the capacitor 75. As soon as the voltage across the capacitor 75 drops below this lockout voltage, the undervoltage lockout shuts down the inverter pulse width modulator 76. Thus, the front end and driver end are designed in conjunction with each other to produce desired operating characteristics.

Figure 6 is a flowchart showing a method for retrofitting an existing lighting installation with a system according to the present invention. As noted above, a significant feature of the present invention is the compatibility of the system with existing building wiring. In particular, the front end 302 of the present invention can be installed between an existing AC lighting circuit .and an AC power source so that the wiring of the AC lighting circuit carries DC current. Then, driver ends 304 operating on DC current can be installed in lighting fixtures connected to the now-converted DC lighting circuit. As shown in Figure 6, this conversion process begins in block 602 with the provision of an AC-DC converter, such as front end 302. In the next step, shown in block 604, the front end circuit is connected to the AC input source in place of the existing lighting circuit, which is disconnected. In the preferred embodiment, as described above, the front end is installed in a junction box provided for the distribution of AC power through the lighting circuit.

In block 606, the wiring of the lighting circuit, which formerly carried AC power, is connected to the DC output of the front end circuit, thus converting the existing conventional lighting circuit to a high voltage DC circuit.

In block 608, the existing conventional AC fixtures connected to the lighting circuit are replaced by DC powered circuits, such as the driver ends disclosed above. The new DC driver ends could be installed as part of an entirely new DC-powered lighting fixture, or the driver ends could be installed as a retrofit of an existing incandescent or fluorescent fixture. In particular, existing incandescent fixtures may be provided with screw-in DC-powered driver ends designed to connect to a standard light bulb socket.

Finally, when wiring has been completed, the retrofitted lighting circuit is operated with DC power.

This general method can also be advantageously applied to the power distribution system within a single lighting fixture, as shown in Figure 7. Figure 7 shows a typical lighting fixture 702, which may be for example a decorative chandelier. Fixture 702 is mounted to hang from an electrical box 706 mounted in the ceiling. AC wires 703 are provided in electrical box 706 to power light fixture 702. Light fixture 702 may have a plurality of bulb sockets 704, each connected by two wires 708 to the AC wires 703 in a conventional installation of this type of fixture. However, as shown in Figure 7, the present invention can be advantageously applied to such a fixture in the following manner. A front end 302 according to the present invention is installed in electrical box 706 and is connected to receive power from AC wires 703. Front end 302 has a power output 707, which is a DC high voltage power output. According to this application of the present invention, DC output 707 is connected to fixture wires 708 to provide DC power to bulb sockets 704. A driver end 304 is provided with a threaded coupling which mates with bulb sockets 704 to connect the power available in bulb sockets 704 to driver end 304. Driver end 304 provides an excitation signal for a compact fluorescent bulb 712.

Thus, lighting fixture 702 can be retrofitted to operate using efficient fluorescent bulbs by installing a front end 302 between AC wires 703 and fixture wires 708, and by providing special compact driver ends which can be installed in bulb sockets 704 and which drive small, low voltage fluorescent bulb 712. For this application, it will be desirable to provide these compact driver ends with various threaded bases to correspond to the sockets of the fixture. For many chandeliers, a more compact base will be needed (e.g. a candelabra base).

While the front end 302 .and driver end 304 in this embodiment of the invention are generally similar to the preferred embodiments described previously, there are some differences in accordance with the special requirements of this particular application. For example, front end 302 will generally have a lower power output capacity than the front end 302 used in a building electrical distribution system. For example, for a 10 bulb chandelier which is to be operated with nine watt compact fluorescent bulbs, a front end power output of 100 watts would be sufficient. Also, to reduce the risk of electrical shock due to the presence of large voltages in wires 708, the DC output of front end 302 in this application is preferably much less than 350 volts, perhaps even less than 120 volts.

Because the driver ends 304 in this application can be constructed in a very compact manner, a large, visually intnisive housing associated with conventional screw-in fluorescent replacement bulbs is eliminated. Preferably, bulbs 712 are formed in an appropriate decorative manner. For use in a chandelier, these bulbs may be formed in a shape approximating the shapes of conventional incandescent lamps used in chandeliers, such as a "flame tip" shape.

The present invention provides particular cost advantages as compared to the systems and methods of the prior ait. Through study of the prior art systems, the inventor has determined that the front end of the electronic ballast accounts for 20 % to 60 % of the overall cost of each electronic ballast. For example, an advanced front end which consists of a highly regulated power reservoir, as described by U.S. Patent No. 4,864,482, is the most expensive section of the ballast. This is shown in Figure 3.

In the prior art, the DC power supply in the ballast typically provided a relatively low power output keyed to the design of the fixture. For example, the rated power output of the ballast might be 80 watts in a popular type of ballast designed to drive two four-foot, 40-watt fluorescent tubes. The power handling capability of a front end primarily depends on semiconductor power ratings and filter or storage capacitor capacity ratings. According to the present invention, the front end, as shown in Figure 3, is constructed so that it can deliver a high power output, such as 2000 watts. Presently, a 2000-watt front end can be constructed for about $14, only $4 more than the cost of an 80- watt front end. A complete 80-watt fluorescent lamp ballast, as disclosed in U.S. Patent Nos. 4,933,605 and 4,864,482, costs approximately $17. The cost of a driver end is approximately $9. Because 20 driver ends can be used with a single front end, the cost of the front end attributable to each fixture is less than $1 and the overall ballast cost drops from $17 to less than $10 per fixture.

In addition to the size and cost advantages resulting from the system according to the present invention, it has been found that separating the DC power supply and the resonant inverter circuitry -as disclosed in the present invention reduces electrical losses in the driver end. Also, because the heat produced by the DC power supply section is not produced in the vicinity of the driver end, the circuits of the driver end operate at lower temperatures. Longer operating component life is obtained as a result. An additional advantage of the system according to the present invention is that the isolation of the lighting circuit from the AC line produced by front end 302 protects the lighting circuit from lightning, voltage spikes, etc. Further, the filtering functions performed by the large front end 302 prevent electromagnetic interference from interfering with lighting circuit operation, and reduce overall electromagnetic interference generated by the system.

Claims

I Claim:

1. A system for driving fluorescent, high pressure sodium, and metal halide type gas discharge lamps, comprising: a centrally located AC to DC conversion circuit having a power input adapted for connection to an AC current source and a DC power output; a plurality of driver end means connected to said DC power output of said central conversion circuit, each for receiving DC power from said conversion circuit and converting said DC power into an excitation signal suitable for driving at least one gas discharge lamp associated with said driver end means.

2. The system of claim 1 wherein said driver end means is a resonant inverter circuit for driving a fluorescent lamp.

3. The system of claim 1 wherein said centrally located conversion circuit is mounted in a distribution housing for an AC building circuit.

4. The system of claim 1 wherein said driver end means are mounted in DC-powered fluorescent lighting fixtures.

5. The system of claim 3 wherein sώd centrally located conversion circuit is connected to said driver end means by wires of an AC building power circuit.

6. The system of claim 1 wherein at least 10 driver end means are connected to receive power from a single said conversion circuit.

7. The system of claim 1 wherein the DC output voltage of said conversion circuit is at least 250 volts.

8. The system of claim 1 wherein said driver end means are provided with dimming means for electronically reducing the duty cycle of the power provided to said gas discharge lamp in response to a control signal indicating a desired brighmess level.

9. A system for driving gas discharge lamps, comprising: a centrally located AC to DC conversion circuit having a power input adapted for connection to an AC current source and a DC power output; a plurality of resonant inverter circuits, each connec sd to said DC power output of said central conversion circuit and each having a lamp

connectable to at least one fluorescent lamp, each said resonant inverter circuit comprising an integrated circuit pulse width modulator and switching means, said switching means connected to said DC power output and said lamp output, said switching means controlled by said pulse width modulator to convert said DC power into a high frequency AC excitation signal at said lamp output for driving said lamp.

10. A method for increasing the efficiency of an electrical lighting system, comprising the steps of: identifying a lighting circuit to be upgraded, said lighting circuit having an AC power source connected by distribution wires to a plurality of existing lighting components; providing an AC to DC converter circuit having an AC input and a DC output, and providing a plurality of gas discharge lighting drivers of a type powered by a DC power source external to said gas discharge lighting drivers; disconnecting said lighting circuit from said AC power source and connecting said converter circuit to said AC power source at the point of said disconnection; connecting said lighting circuit to said DC output of said converter circuit; and replacing said existing lighting components with said gas discharge lighting drivers to form a localized DC gas discharge lighting circuit.

11. The method of claim 10 wherein at least four gas discharge lighting fixtures are connected to receive power from a single converter circuit.

12. The method of claim 10 wherein said gas discharge lighting drivers comprise a resonant inverter circuit for driving a fluorescent lamp.

13. The method of claim 10 wherein s d centrally located conversion circuit is mounted in a distribution box for an AC building circuit.

14. The method of claim 10 wherein said gas discharge lighting drivers are mounted in existing lighting fixtures.

15. The method of claim 14 wherein said gas discharge lighting drivers are provided with bases which mate with standard lamp sockets in the existing fixtures to provide DC power connections of the gas discharge lighting drivers.

16. The method of claim 10 wherein the DC output voltage of said conversion circuit is at least 250 volts.

17. A method of increasing the efficiency of an electrical lighting fixture having a plurality of bulb sockets each connected by wires to an AC power source, comprising the steps of: providing a compact AC to DC converter circuit having an AC input and a DC output; disconnecting the wires of said lighting fixture from said AC power source and connecting said converter circuit to said AC power source at the point of said disconnection; connecting said wires to said DC output of said converter circuit; and replacing existing bulbs in said bulb sockets with compact gas discharge lighting driver circuits having bulb socket bases, said driver circuits operating to receive DC power from said conversion circuit and convert said DC power into an excitation signal suitable for driving a compact gas discharge lamp associated with said driver circuit.

18. The method of claim 17 wherein the lamp associated with said driver circuit has a shape simulating the shape of an incandescent light bulb.

19. The method of claim 17 wherein the driver circuits comprise a resonant inverter of a type controlled by an integrated circuit pulse width modulator.

20. The method of claim 19 wherein said driver circuit is mounted in a compact housing in the area of the bulb socket base.