Patent Publication Number: US-7592753-B2

Title: Inductively-powered gas discharge lamp circuit

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to gas discharge lamps, and more particularly to circuits for starting and powering gas discharge lamps. 
   Gas discharge lamps are used in a wide variety of applications. A conventional gas discharge lamp includes a pair of electrodes spaced apart from one another within a lamp sleeve. Gas discharge lamps are typically filled with an inert gas. In many applications, a metal vapor is added to the gas to enhance or otherwise affect light output. During operation, electricity is caused to flow between the electrodes through the gas. This causes the gas to discharge light. The wavelength (e.g. color) of the light can be varied by using different gases and different additives within the gas. In some applications, for example, conventional fluorescent lamps, the gas emits ultraviolet light that is converted to visible light by a fluorescent coating on the interior of the lamp sleeve. 
   Although the principles of operation of a conventional gas discharge lamp are relatively straightforward, conventional gas discharge lamps typically require a special starting process. For example, the conventional process for starting a conventional gas discharge lamp is to pre-heat the electrode to produce an abundance of electron around the electrodes (the “pre-heat” stage) and then to apply a spike of electrical current to the electrodes with sufficient magnitude for the electricity to arc across the electrodes through the gas (the “strike” stage). Once an arc has been established through the gas, the power is reduced as significantly less power is required to maintain operation of the lamp. 
   In many applications, the electrodes are pre-heated by connecting the electrodes in series and passing current through the electrodes as though they were filaments in an incandescent lamp. As current flows through the electrodes, the inherent resistance of the electrodes results in the excitation of electrons. Once the electrodes are sufficiently pre-heated, the direct electrical connection between the electrodes is opened, thereby leaving a path through the gas as the only route for electricity to follow between the electrodes. At roughly the same time, the power applied to the electrodes is increased to provide sufficient potential difference for electrons to strike an arc across the electrodes. 
   Starter circuits come in a wide variety of constructions and operate in accordance with a wide variety of methods. In one application, the power supply circuit includes a pair of transformers configured to apply pre-heating current across the two electrodes only when power is supplied over a specific range. By varying the frequency of the power, the pre-heating operation can be selectively controlled. Although functional, this power supply circuit requires the use of two additional transformers, which dramatically increase the cost and size of the power supply circuit. Further, this circuit includes a direct electrical connection between the power supply and the lamp. Direct electrical connections have a number of drawbacks. For example, direct electrical connections require the user to make electrical connections (and often mechanical connections) when installing or removing the lamp. Further, direct electrical connections provide a relatively high risk of electrical problems bridging between the power supply and the lamp. 
   In some applications, the gas discharge lamp is provided with power through an inductive coupling. This eliminates the need for direct electrical connection, for example, wire connections and also provides a degree of isolation between the power supply and the gas discharge lamp. Although an inductive coupling provides a variety of benefits over direct electrical connections, the use of an inductive coupling complicates the starting process. One method for controlling operation of the starter circuit in an inductive system is to provide a magnetically controlled reed switch that can be used to provide a selective direct electrical connection between the electrodes. Although reliable, this starter configuration requires close proximity between the electromagnet and the reed switch. It also requires a specific orientation between to the two components. Collectively, these requirements can place meaningful limitations on the design and configuration of the power supply circuit and the overall lamp circuit. 
   SUMMARY OF THE INVENTION 
   The present invention provides an inductive power supply circuit for a gas discharge lamp that is selectively operable in pre-heat and operating modes through variations in the frequency of power applied to the secondary circuit. In one embodiment, the power supply circuit generally includes a primary circuit with a frequency controller for varying the frequency of the power applied to the primary coil and a secondary circuit with a secondary coil for inductively receiving power from the primary coil, a gas discharge lamp and a pre-heat capacitor. The pre-heat capacitor is selected to pre-heat the lamp when the primary coil is operating within the pre-heat frequency range and to allow normal lamp operation when the primary coil is operating within the operating frequency range. In one embodiment, the pre-heat capacitor is connected in series between the lamp electrodes. 
   In one embodiment, the pre-heat capacitor, pre-heat frequency and operating frequency are selected so that the impedance of the electrical path through the lamp is greater than the impedance of the electrical path through the electrodes at the pre-heat frequency, and so that the impedance of the electrical path through the lamp is lesser than the impedance of the electrical path through the electrodes at the operating frequency. 
   In one embodiment, the secondary circuit further includes an operating capacitor disposed in series between the secondary coil and the lamp. The capacitance of the operating capacitor may be selected to substantially balance the inductance of the secondary coil. In this embodiment, the pre-heat capacitor may have a capacitance that is approximately equal to the capacitance of the operating capacitor. 
   In one embodiment, the primary circuit is adaptive to permit the primary to operate at resonance at the pre-heat frequency and at the operating frequency. In one embodiment, the primary circuit includes a tank circuit with variable capacitance and a controller capable of selectively varying the capacitance of the tank circuit. The primary circuit may include alternative circuitry for varying the resonant frequency of the tank circuit, such as a variable inductor. 
   In one embodiment, the variable resonance tank circuit includes a plurality of capacitors that may be made selectively operational by actuation of one or more switches. The switch(es) may be actuatable between a first position in which the effective capacitance of the tank circuit is set to provide resonance of the primary at approximately the pre-heat frequency and a second position in which the effective capacitance of the tank circuit is set to provide resonance of the primary at approximately the operating frequency. 
   In one embodiment, the tank circuit may include a tank operating capacitor that is connected between the primary coil and ground and a tank pre-heat capacitor that is connected between the primary and ground along a switched line in parallel to the pre-heat capacitor. In operation, the switch may be actuated to selectively enable or disable the pre-heat capacitor, thereby switching the resonant frequency of the primary between the pre-heat frequency and the operating frequency. 
   In another aspect, the present invention provides a method for starting and operating a gas discharge lamp. In one embodiment of this aspect, the method may include the steps of pre-heating the lamp by applying power to the secondary circuit at a pre-heat frequency at which the impedance of the electrical path through the lamp is greater than the impedance of the electrical path through the pre-heat capacitor for a period of time sufficient to pre-heat the lamp, and operating the lamp by applying power to the secondary circuit at an operating frequency at which the impedance of the electrical path through the lamp is lesser than the impedance of the electrical path through the pre-heat capacitor. 
   In one embodiment, the pre-heat frequency corresponds approximately to the resonant frequency of the secondary circuit taking into consideration the combined capacitance of the pre-heat capacitor and the operating capacitor, and the operating frequency corresponds approximately to the resonant frequency of the secondary circuit taking into consideration only the capacitance of the operating capacitor. 
   In one embodiment, the method further includes the step of varying the resonance frequency of the primary to match the pre-heat frequency during the pre-heating step and to match the operating frequency during the operating step. In one embodiment, this step is further defined as varying the effective capacitance of the tank circuit between the pre-heating step and the operating step. In another embodiment, this step is further defined as varying the effective inductance of the tank circuit between the pre-heating step and the operating step. 
   The present invention provides a simple and effective circuit and method for pre-heating, starting and powering a gas discharge lamp. The present invention utilizes a minimum number of components to achieve complex functionality. This reduces the overall cost and size of the circuitry. The present invention also provides the potential for improved reliability because it includes a small number of components, the components are passive in nature and there is less complexity in the manner of operation. In typical applications, the system automatically starts (or strikes) the lamp when the primary circuit switches from the pre-heat frequency to the operating frequency. The initial switch causes sufficient voltage to build across the electrodes to permit electricity to arc across the electrodes through the gas. Once the lamp has been started, the impedance through the lamp drops even farther creating a greater difference between the impedance of the electrical path through the lamp and the electrical path through the pre-heat capacitor. This further reduces the amount of current that will flow through the pre-heat capacitor during normal operation. In applications in which the resonant frequency of the primary circuit is selectively adjustable, the primary circuit can be adapted to provide efficient resonant operation during both pre-heat and operation. Further, the components of the secondary circuit can be readily incorporated into a lamp base, thereby facilitating practical implementation. 
   These and other objects, advantages, and features of the invention will be readily understood and appreciated by reference to the detailed description of the current embodiment and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a gas discharge lamp system in accordance with an embodiment of the present invention. 
       FIG. 2  is a circuit diagram of the secondary circuit and the tank circuit. 
       FIG. 3  is a flow chart showing the general steps of a method for starting and operating a gas discharge lamp. 
       FIG. 4  is a circuit diagram of an alternative tank circuit. 
       FIG. 5  is a flow chart showing the general steps of a method for starting and operating a gas discharge lamp. 
       FIG. 6  is a circuit diagram of a second alternative tank circuit. 
   

   DESCRIPTION OF THE CURRENT EMBODIMENT 
   A gas discharge lamp system  10  in accordance with one embodiment of the present invention is shown in  FIG. 1 . The gas discharge lamp system  10  generally includes a primary circuit  12  and a secondary circuit  14  powering a gas discharge lamp  16 . The primary circuit  12  includes a controller  20  for selectively varying the frequency of the power inductively transmitted by the primary circuit  12 . The secondary circuit  14  includes a secondary coil  22  for inductively receiving power from the primary coil  18  and a gas discharge lamp  16 . The secondary coil  22  further includes an operating capacitor  30  connected between the secondary coil  22  and the lamp  16  and a pre-heat capacitor  32  connected in series between the lamp electrodes  24  and  26 . In operation, the controller  20  pre-heats the lamp  16  by applying power to the secondary circuit  14  at a pre-heat frequency selected so that the impedance of the electrical path through the pre-heat capacitor  32  is less than the impedance of the electrical path through the gas in the gas discharge lamp  16 . After pre-heating, the controller  20  applies power to the secondary circuit  14  at an operating frequency selected so that the impedance of the electrical path through the pre-heat capacitor  32  is greater than the impedance of the electrical path through the gas in the gas discharge lamp  16  This causes the pre-heat capacitor  32  to become “detuned,” which, in turn, results in the flow of electricity along the electrical path through the gas in the gas discharge lamp  16 . 
   As noted above, a schematic diagram of one embodiment of the present invention is shown in  FIG. 1 . In the illustrated embodiment, the primary circuit  12  includes a primary coil  18  and a frequency controller  20  for applying power to the primary coil  18  at a desired frequency. The frequency controller  20  of the illustrated embodiment generally includes a microcontroller  40 , an oscillator  42 , a driver  44  and an inverter  46 . The oscillator  42  and driver  44  may be discrete components or they may be incorporated into the microcontroller  40 , for example, as modules within the microcontroller  40 . In this embodiment, these components collectively drive a tank circuit  48 . More specifically, the inverter  46  provides AC (alternating current) power to the tank circuit  48  from a source of DC (direct current) power  50 . The tank circuit  48  includes the primary coil  18  and may also include a capacitor  52  selected to balance the impedance of the primary coil  18  at anticipated operating parameters. The tank circuit  48  may be either a series resonant tank circuit or a parallel resonant tank circuit. In this embodiment, the driver  44  provides the signals necessary to operate the switches within the inverter  46 . The driver  44 , in turn, operates at a frequency set by the oscillator  42 . The oscillator  42  is, in turn, controlled by the microcontroller  40 . The microcontroller  40  could be a microcontroller, such as a PIC18LF1320, or a more general purpose microprocessor. The illustrated primary circuit  12  is merely exemplary, and essentially any primary circuit capable of providing inductive power at varying frequencies may be incorporated into the present invention. The present invention may be incorporated into the inductive primary shown in U.S. Pat. No. 6,825,620 to Kuennen et al, which is entitled “Inductively Coupled Ballast Circuit” and was issued on Nov. 30, 2004. U.S. Pat. No. 6,825,620 is incorporated herein by reference. 
   As noted above, the secondary circuit  14  includes a secondary coil  22  for inductively receiving power from the primary coil  18 , a gas discharge lamp  16 , an operating capacitor  30  and a pre-heat capacitor  32 . Referring now to  FIG. 2 , the gas discharge lamp  16  includes a pair of electrodes  24  and  26  that are spaced apart from one another within a lamp sleeve  60 . The lamp sleeve  60  contains the desired inert gas and may also include a metal vapor as desired. The lamp  16  is connected in series across the secondary coil  22 . In this embodiment, the first electrode  24  is connected to one lead of the secondary coil  22  and the second electrode  26  is connected to the opposite lead of the secondary coil  22 . In this embodiment, the operating capacitor  30  is connected in series between the secondary coil  22  and the first electrode  24  and the pre-heat capacitor  32  is connected in series between the first electrode  24  and the second electrode  26 . In  FIG. 2 , the tank circuit  48  is shown with primary coil  18  and capacitor  52 . Although not shown in  FIG. 2 , the tank circuit  48  is connected to the inverter  46  by connector  49 . 
   Operation of the system  10  is described with reference to  FIG. 3 . The method generally includes the steps of applying 100 power to the secondary circuit  14  at a pre-heat frequency. The pre-heat frequency is selected as a frequency in which the impedance of the electrical path through the lamp is greater than the electrical path through the pre-heat capacitor  32 . In one embodiment, the frequency controller  20  pre-heats the lamp  16  by applying power to the secondary circuit  14  at a pre-heat frequency approximately equal to the series resonant frequency of the operating capacitor  30  and the pre-heat capacitor  32 , referred to as ƒs. A formula for calculating ƒs in this embodiment is set forth below. At the pre-heat frequency, the pre-heat capacitor  32  is sufficiently tuned to provide a direct electrical connection between the electrodes  24  and  26 . This permits the flow of electricity directly across the electrodes  24  and  26  through the pre-heat capacitor  32 . This flow of current pre-heats the electrodes  24  and  26 . The system  10  continues to supply power at the pre-heat frequency until the electrodes  24  and  26  are sufficiently pre-heated  102 . The duration of the pre-heating phase of operation will vary from application to application, but will typically be a predetermined period of time and is likely to be in the range of 1-5 seconds for conventional gas discharge lamps. After pre-heating, the controller  20  applies  104  power to the secondary circuit  14  at an operating frequency selected as a frequency in which the impedance of the electrical path through the lamp is lesser than the electrical path through the pre-heat capacitor  32 . In this embodiment, the operating frequency is approximately equal to the resonant frequency of the operating capacitor  30 , referred to as ƒo. A formula for calculating ƒs in this embodiment is set forth below. This change in frequency causes the pre-heat capacitor  32  to become detuned, which, in effect, causes current to flow through the lamp  16 . Although the change in frequency will not typically cause the pre-heat capacitor to act as an open circuit, it will limit the flow of current through the pre-heat capacitor a sufficient amount to cause current to arc through the gas in the gas discharge lamp  16 . As a result, the switch to operating frequency causes the power generated in the secondary circuit  14  follows an electrical path from one electrode  24  to the other electrode  26  through the gas in the lamp sleeve  60 . Initially, this change in frequency will cause the lamp to start (or to strike) as the detuned pre-heat capacitor permits a sufficient voltage to build across the electrodes  24  and  26  to cause the current to arc through the gas. After the lamp has started, the lamp will continue to run properly at the operating frequency. In other words, a single change in the frequency applied to the secondary circuit  16  causes the lamp to move from the pre-heat phase through the starting (or striking) phase and into the operating phase. 
   
     
       
         
           
             
               
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   Although the formulas provided for determining pre-heat frequency and operating frequency yield specific frequencies, the terms “pre-heat frequency” and “operating frequency” should each be understood in both the specification and claims to encompass a frequency range encompassing the computed “pre-heat frequency” and “operating frequency.” Generally speaking, the efficiency of the system may suffer as the actual frequency gets farther from the computed frequency. In typical applications, it is desirable for the actual pre-heat frequency and the actual operating frequency to be within a certain percentage of the computed frequencies. There is not a strict limitation, however, and greater variations are permitted provided that the circuit continues to function with acceptable efficiency. For many applications, the preheat frequency is approximately twice the operating frequency. The primary circuit  12  may continue to apply power to the secondary circuit  14  until  106  continued operation of gas discharge lamp  16  is no longer desired. 
   If desired, the primary circuit  12 ′ may be configured to have selectively adjustable resonance so that the primary circuit  12 ′ operates at resonance at both the pre-heat frequency and the operating frequency. In one embodiment incorporating this functionality, the primary circuit  12 ′ may include a variable capacitance tank circuit  48 ′ (See  FIG. 4 ) that permits the resonant frequency of the tank circuit  48 ′ to be selectively adjusted to match the pre-heat frequency and the operating frequency.  FIG. 4  shows a simple circuit for varying the capacitance of the tank circuit  48 ′. In the illustrated embodiment, the tank circuit  48 ′ includes a tank operating capacitor  52   a ′ connected between the primary coil  18 ′ and ground and a tank pre-heat capacitor  52   b ′ connected along a switched line between the primary coil  18 ′ and ground in parallel with the tank operating capacitor  52   a ′. The switched line includes a switch  53 ′ that is selectively operable to open the switched line, thereby effectively removing the tank pre-heat capacitor  52   b ′ from the tank circuit  48 ′. Operation of the switch  53 ′ may be controlled by the frequency controller  20 , for example, by microcontroller  40 , or by a separate controller. The switch  53 ′ may be essentially any type of electrical switch, such as a relay, FET, Triac or a custom AC switching devices. 
   Operation of this alternative is generally described with reference to  FIG. 5 . The primary circuit  12 ′ adjusts  200  the resonant frequency of the tank circuit  48 ′ to be approximately equal to the pre-heat frequency. The primary circuit  12 ′ then supplies power  202  to the secondary circuit at the pre-heat frequency. The primary circuit  12 ′ continues to supply power to the secondary circuit at the pre-heat frequency until the electrodes  24  and  26  have been sufficiently pre-heated  204 . Once the electrodes are sufficiently pre-heated, the primary circuit  12 ′ adjusts  206  the resonant frequency of the tank circuit  48 ′ to be approximately equal to the operating frequency. The primary circuit  12 ′ switches its frequency of operation to supply  208  power to the secondary circuit  14 ′ at the operating frequency. The primary circuit  12 ′ may continue to supply power until it is no longer desired  210 . The system  10  may also include fault logic that ceases operation when a fault condition occurs (e.g. the lamp is burnt out or has been removed, or a short circuit has occurred). 
   Variable capacitance may be implemented through the use of alternative parallel and series capacitance subcircuits. For example,  FIG. 6  shows an alternative tank circuit  12 ″ in which the tank pre-heat capacitor  52   b ″ is connected in series with the tank operating capacitor  52   a ″, but a switched line is included for shorting the circuit around the pre-heat capacitor  52   a ″ by operation of switch  53 ″ to effectively remove the pre-heat capacitor  52   b ″ from the circuit. 
   Although described in connection with a variable capacitance tank circuit  48 ′, the present invention extends to other methods for varying the resonant frequency of the tank circuit  48 ′ or the primary circuit  12 ′ between pre-heat and operating modes. For example, the primary circuit may include variable inductance. In this alternative (not shown), the tank circuit may include a variable inductor and a controller for selectively controlling the inductance of the variable inductor. As another example (not shown), the tank circuit may include a plurality of inductors that can be switched into and out of the circuit by a controller in much the same way as described above in connection with the variable capacitance tank circuit. 
   The above description is that of the current embodiment of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.