Patent Application
20150208492
The aspects of the present disclosure relate generally to the field of electric lighting, and in particular to ballast circuits used to drive gas-discharge lamps.
A gas-discharge lamp belongs to a family of electric lighting or light generating devices that generate light by passing an electric current through a gas or vapor within the lamp. Atoms in the vapor absorb energy from the electric current and release the absorbed energy as light. One of the more widely used types of gas-discharge lamps is the fluorescent lamp which is commonly used in office buildings and homes. Fluorescent lamps contain mercury vapor whose atoms emit light in the non-visible low wavelength ultraviolet region. The ultraviolet radiation is absorbed by a phosphor disposed on the interior of the lamp tube causing the phosphor to fluoresce, thereby producing visible light.
Fluorescent lamps exhibit a phenomenon known as negative resistance, which is a condition where increased current flow decreases the electrical resistance of the lamp. If a simple voltage source is used to drive a fluorescent lamp, this negative resistance characteristic leads to an unstable condition in which the lamp current rapidly increases to a level that will destroy the lamp. Thus, a fluorescent lamp needs to be driven from a power source that can control the lamp current. While it is possible to use direct current (DC) to drive a fluorescent lamp, in practice, alternating current (AC) is typically used because it affords better control of the lamp current. The current controlling circuits used to drive fluorescent lamps are generally referred to as ballast circuits or “ballasts”. In practice, the term ballast is commonly used to refer to the entire fluorescent lamp drive circuit, and not just the current limiting portion.
Current flow through a fluorescent lamp is generally achieved by placing cathodes at either end of the lamp tube to inject electrons into a vapor within the lamp. These cathodes are structured as filaments that are coated with an emissive material used to enhance electron injection. The emission mix typically comprises a mixture of barium, strontium, and calcium oxides. A small electric current is passed through the filaments to heat them to a temperature that overcomes the binding potential of the emissive material allowing thermionic emission of electrons to take place. When an electric potential is applied across the lamp, electrons are liberated from the emissive material coating on each filament causing a current to flow. While a lamp is in operation, and especially when a lamp is ignited, the emission mix is slowly sputtered off the filaments by bombardment with electrons and mercury ions. The rate of depletion of the emission mix varies from filament to filament. Thus as a lamp nears its end of life, the emission mix on one filament will deplete more quickly and exhibit lowered electron emissions while the other filament will continue to support normal electron emissions. This can lead to a slight rectification of the alternating current flowing through the lamp. Continued operation of a lamp after the emission mix is depleted can lead to overheating resulting in cracking of the glass allowing hazardous mercury vapor to escape. It is therefore desirable to detect when a lamp is nearing its end of life (EOL) and turn it off before overheating can occur.
Many lamp ballasts in use today are based on a voltage fed topology where the final inverter stage is driven by a voltage source. These voltage-fed topologies are capable of providing an inherent EOL protection function, which can prevent a lamp near EOL from overheating, without extinguishing other non-rectifying lamps that may be powered by the same ballast. The inherent EOL protection topology may be advantageously employed in parallel-connected systems to shut off EOL lamps while continuing to provide power to non-rectifying (non-EOL) lamps, thereby aiding visual identification of a lamp or lamps that need to be replaced. However when energy saving lamps or other types of secondary lamps are used in a ballast designed for regular lamps, the inherent EOL protection does not provide reliable results. This is because the voltage regulator of the voltage fed ballast is typically designed to keep the high frequency bus voltage, which is used to drive the lamps, at a constant level regardless of the type of lamp installed in the ballast. Therefore, these ballasts cannot accommodate certain types of fluorescent lamps, such as newer energy saving lamps, which have lamp voltages that are different than the design voltage of the ballast. Installing newer energy saving lamps, such as a 21-watt or 14-watt lamps, into ballasts of this type can lead to hazardous conditions as the lamps near EOL.
Accordingly, it would be desirable to provide lamp ballasts that address at least some of the problems identified above.
As described herein, the exemplary embodiments overcome one or more of the above or other disadvantages known in the art.
One aspect of the present disclosure relates to an electric lighting device. In one embodiment, the electric lighting device includes a voltage-fed inverter configured to receive a DC voltage and produce an AC lamp voltage. A lamp load is coupled to the AC lamp voltage and this lamp load includes a gas discharge lamp and a sensing capacitor coupled in series with the gas discharge lamp. A failing gas discharge lamp places a DC bias voltage on the sensing capacitor. A voltage regulator is included that is configured to receive the AC lamp voltage, generate a reference voltage and adjust the inverter frequency to regulate the AC lamp voltage at a generally constant level corresponding to the reference voltage. An EOL protection circuit is included that is configured to receive the DC bias voltage and adjust the reference voltage such that the AC lamp voltage is lowered when a magnitude of the DC bias voltage exceeds a predetermined threshold voltage.
Another aspect of the exemplary embodiments relates to a ballast circuit or assembly for driving a gas discharge lamp. In one embodiment, the ballast circuit includes a voltage-fed resonant inverter configured to receive a DC input voltage and produce a high frequency AC voltage. The ballast circuit also has a voltage regulator coupled to the inverter that is configured to receive the high frequency AC voltage. The voltage regulator generates a reference voltage and adjusts the inverter frequency so that the high frequency AC voltage is maintained at a generally constant voltage corresponding to the generated reference voltage. A sensing capacitor is coupled in series with the gas discharge lamp. When the gas discharge lamp nears its end-of-life (EOL), a DC bias voltage is placed on the sensing capacitor. An EOL protection circuit is also included that receives the DC bias voltage and adjusts the reference voltage generated by the voltage regulator. The EOL protection circuit adjusts the reference voltage such that the high frequency AC voltage is lowered when the gas discharge lamp nears its end-of-life.
A further aspect of the present disclosure relates to a method for driving one or more gas-discharge lamps. In one embodiment, the method uses a resonant inverter to convert a DC voltage into a regulated AC lamp voltage. The AC lamp voltage is coupled to one or more gas-discharge lamps such that an AC lamp current flows through each of the gas-discharge lamps. The AC lamp current is monitored for current imbalances created by a failing lamp, and a bias voltage imparted on a sensing circuit by the current imbalances is detected. It is then determined if the magnitude of the DC bias voltage exceeds a predetermined threshold magnitude. The AC lamp voltage is reduced when the magnitude of the DC bias voltage exceeds the predetermined threshold magnitude.
These and other aspects and advantages of the exemplary embodiments will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings illustrate presently preferred embodiments of the present disclosure, and together with the general description given above and the detailed description given below, serve to explain the principles of the present disclosure. As shown throughout the drawings, like reference numerals designate like or corresponding parts.
Aspects of the present disclosure are directed to electronic lighting and more particularly to ballasts with end-of-life protection for use in connection with fluorescent lamps and will be described with particular reference thereto, although the exemplary ballasts described herein can also be used in other lighting applications and configurations, and are not limited to the aforementioned application. For example, various disclosed advances can be employed in single-lamp ballasts, series-coupled multiple-lamp ballasts, and the like.
Referring to
In the exemplary embodiment shown in
In the embodiment shown in
The switching devices Q1 and Q2 are alternately activated to provide a square wave output with amplitude of approximately one-half the DC input voltage 150 at the inverter output node 158. This square wave inverter output at the inverter output node 158 excites the resonant tank 156 to produce the high frequency AC voltage A at node A1. The high frequency AC voltage A is used to drive one or more lamps 1-n. As shown in
Switching control signals for operating the pair of switching devices Q1, Q2, are provided by a pair of gate drive circuits 162, 164 respectively. Gate or control lines 166 and 168 respectively include resistors R1 and R2 to provide control signals to the control terminals of Q1 and Q2, respectively. The first gate drive circuit 162 is coupled between the inverter output node 158 and a first circuit node 170, and the second drive circuit 164 coupled between the ground rail 154 and the gate control line 168. The first and second gate drive circuits 162, 164 include a first and second driving inductors L1-2 and L1-3 respectively, which are mutually magnetically coupled to the resonant inductor L1-1 of the resonant tank 156 to induce voltage in the first and second driving inductors L1-2, L1-3 that is proportional to the instantaneous rate of change of current in the resonant tank 156 for self-oscillatory operation of the inverter 100. First driving inductor L1-2 is magnetically coupled in reverse polarity from second driving inductor L1-3 to resonate inductor L1-1 to provide alternate switching of Q1 and Q2 to form the square wave at inverter output node 158. In addition, the first and second gate drive circuits 162, 164 include secondary inductors L2-2 and L2-3, respectively, where each secondary inductor L2-2, L2-3 is serially connected through a respective capacitor C1 and C2, to the respective first and second driving inductors L1-2, L1-3 and to their respective gate control lines 166, 168. The secondary inductors, L2-2 and L2-3, are magnetically coupled to a tertiary winding L2-1 located in the exemplary voltage regulator 200 illustrated in
In operation, the first and second gate drive circuits 162, 164 maintain the switching device Q1 in an “ON” state and switching device Q2 in an “OFF” state for a first half of a cycle. The switching device Q2 is in an “ON” state and the switching device Q2 is in an “OFF” state for a second half of the cycle to generate the generally square wave at the inverter output node 158 for excitation of the resonant tank circuit 156. In one embodiment, the gate-to-source voltage of each of the switching devices Q1 and Q2 is limited by bi-directional voltage clamps formed by respective pairs of diodes Z1 and Z2, and Z3 and Z4, shown in this example as back-to-back zener diodes. As is shown in
To start the resonant inverter 100, series connected resistors R4 and R5 across the DC input voltage 150 cooperate with a resistor R6 (coupled between the inverter output node 158 and ground rail 154) to initiate regenerative operation of the gate drive circuits 162, 164. As shown in
In steady state operation of the inverter 100, the square wave voltage at the inverter output node 158 has amplitude of approximately one-half of the DC input voltage 150, and the initial bias voltage across C1 drops. A first series resonant circuit formed by inductance L2-2 and capacitor C1 and a second series resonant network formed by L2-3 and capacitor C2 are equivalently inductive with an operating frequency above the resonant frequency of the first and second series resonant networks. In steady state oscillatory operation, this results in a phase shift of the first gate drive circuit 162 to allow the current flowing through the inductor L1-1 to lag the fundamental frequency of the voltage produced at the inverter output node 158, thus facilitating steady-state soft-switching of the inverter 100. In one embodiment, the output voltage of the inverter 100 at inverter output node 158 is clamped by the serially connected clamping diodes D1 and D2 to limit high voltage seen by the capacitors C111 and C112. As the output voltage of the inverter 100 at node 158 increases, the clamping diodes D1, D2 start to clamp, preventing the voltage across the capacitors C111 and C112 from changing polarity and limiting the output voltage at node 158 to a value that prevents thermal damage to components of the inverter 100.
As one of the lamps 1-n, nears its end-of-life, generally referred to herein as the end-of-life state, the emission mix at a filament of the failing lamp starts to become depleted. When this happens, electron emission from the depleted filament is less than electron emission from the non-depleted filament creating an imbalance between the forward and reverse current flowing through the operable lamps 1-n. This imbalance results in a rectification of the lamp current. Rectification of the lamp current imparts a bias voltage 20 on the sensing circuit 110 that can be used as an EOL signal at node B.
In the illustrated inverter 100 of
When one or more of the lamps 1-n of
When the reference voltage 24 at node C is lowered, the high frequency AC voltage at node A will also be decreased by the voltage regulator 200 as described above, which will reduce the current through, or extinguish the lamp 1-n, that is nearing end-of-life. This can prevent overheating of the lamp that is nearing end-of-life while maintaining sufficient power to the remaining lamps to keep them in a lighted state. Lowering of the reference voltage 24 can be accomplished in a variety of ways. In one embodiment, an EOL protection circuit 300, such as that shown in
In one embodiment of the exemplary EOL protection circuit 300, the switching devices or transistors Q301 and Q302 in each of the switching circuits S301 and S302 comprise bipolar junction transistors (BJTs). In alternate embodiments, any other suitable type of switching devices may be employed, other than including bipolar junction transistors, such as for example, metal oxide semiconductor field effect transistors (MOSFETs). The zener diode Z301 is used to set a threshold voltage for detection of an EOL condition. Once the filtered EOL signal at node 310 exceeds the breakdown voltage of the zener diode Z301, an EOL condition is present and current begins to flow through resistor R301, thereby applying a base-to-emitter voltage to the switching device Q301 resulting in current being conducted through the switching device Q301 which lowers the reference voltage 24 at node C of
Similarly, when the filtered EOL signal at node 310 is a positive voltage, diode D301 prevents current from flowing in the first switching circuit S301, while zener diode Z302 sets a threshold voltage in the second switching circuit S302. Once the voltage of the filtered EOL signal at node 310 exceeds the breakdown voltage of zener diode Z302, the second switching device Q302 begins to conduct, lowering the reference voltage 24 at node C of FIG. 3, which is coupled to output node 320 of
In the above embodiment, the EOL protection circuit 300 has its output node 320 coupled to the reference voltage at node C of the voltage regulator 200, and is used to lower the reference voltage 24 whenever an EOL condition is detected at node B. Alternatively, the EOL protection circuit 300 may be used to lower the voltage at other nodes of the exemplary voltage regulator 200 to achieve a similar effect of lowering the high frequency AC voltage A at node A1 when an EOL condition is detected at node B. For example, by coupling the output node 320 of the EOL protection circuit 300 to the cathode of zener diode Z230, or by coupling the output node 320 of the EOL protection circuit 300 to the bias voltage input node Vbias of the exemplary voltage regulator 200 shown in
In certain embodiments, it is advantageous to shut down the inverter 100 when an EOL condition is detected. When an inverter is shut down, its switching action is stopped and the output voltage, i.e. the high frequency AC voltage A at node A1, is reduced to about zero volts. An exemplary embodiment of a technique for shutting down the inverter 100 will be now be illustrated. In one embodiment, the inverter 100 can be shut down when an EOL condition is detected for example by coupling the output node 320 of the EOL protection circuit 300 in
In self-oscillating voltage-fed inverters, such as the inverter 100 illustrated in
In one embodiment, the bias voltage Vbias that the rectified lamp current imparts on the series connected sensing circuit 110 is detected 510. Depending on which filaments begin to deplete first, the bias voltage Vbias can have either a positive or a negative polarity. In either case, the positive or negative magnitude of the bias voltage Vbias can be compared 512 to a predetermined threshold voltage to determine if an EOL condition exists on any of the gas discharge lamps. In the exemplary embodiment of
If the comparison 512 indicates that that the absolute value |Vbias| of the bias voltage Vbias is not greater than or below 513 the predetermined threshold voltage, no EOL condition exists and the normal operating AC lamp voltage is applied 514. If the comparison 512 indicates that the absolute value |Vbias| is above or greater than 515 the predetermined threshold, it is determined that an EOL condition exists on the gas discharge lamp and the operating AC lamp voltage is lowered or reduced 516 to a level that will protect the failing lamps from damage. The level or magnitude of the lowered AC lamp voltage level is chosen so that if any of the lamps have been replaced with lower voltage energy efficient lamps, they will also be protected from damage.
Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Moreover, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201210368351.9 | Sep 2012 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2013/056094 | 8/22/2013 | WO | 00 |
