Abstract:
A ballast circuit includes a control loop to reduce a filament heating voltage below after the lamp filaments have been pre-heated. A filament cutback circuit includes a filament cut-back inductive component magnetically coupled to the resonant inductive component in the inverter to receive a filament cutback control voltage associated with an AC voltage for powering the lamps. During the pre-heating period, the filament cutback control voltage is not high enough to charge a chargeable component to a switch threshold level. However, during lamp ignition, the filament cutback control voltage is increased and charges the chargeable component to the switch threshold level. This causes a switch device to operate in a conductive switch state and the filament cutback circuit suppresses the pre-heat voltage.

Description:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     None 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     Proper heating of lamp filaments helps extend the life of gas discharge lamps. Prior to the ignition of the lamp filaments, a filament heating voltage may warm the lamp filaments and may be necessary to ignite certain types of high impedance gas-discharge lamps, such as T5 fluorescent lamps. Filament heating voltages also maintain lamp filament temperatures if the lamp current gets too low when a lamp is being dimmed. 
     Ballast circuits are used to power and operate gas-discharge lamps. The ballast circuit may include an inverter with at least one switch device utilized to convert a DC voltage into a pulsed voltage and a resonant circuit that converts the pulsed voltage into the required AC voltage for powering the gas-discharge lamp. A filament heating circuit is coupled to the inverter to transmit the filament heating voltage to the lamp filaments. These filament heating circuits may include filament resonant tanks tuned to receive the filament heating voltage during the pre-heat period and/or when the lamp is being dimmed. 
     While the filament heating circuits may be designed to provide the appropriate filament heating voltage to the lamp filaments, the electrical components utilized in these filament resonant tanks often have unacceptably large component tolerances. Because a filament resonant tank has such a high Q, this may result in excessively high filament heating voltages. 
     High impedance gas-discharge lamps, such as T5 lamps, are particularly sensitive to filament heating voltages during lamp operation. Filament heating voltages can cause unbalanced filament currents and over-current problems. Maintaining these filament voltages as low as possible when the lamp is being dimmed reduces these problems. 
     Prior art circuits do exist for cutting off a filament heating voltage during steady state operation. Unfortunately, these circuits are designed to eliminate the filament heating voltage after the pre-heat period. Thus, these circuits cannot be used during the pre-heat period or when a lamp is being dimmed because they are designed to prevent the filament heating voltage from heating the lamp filaments. 
     What is needed then is a ballast circuit that is capable of reducing the filament heating voltage below a desired maximum level during the pre-heat period and when the lamp is being dimmed. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention is directed to a ballast circuit that reduces a filament heating voltage to the lamp filaments of a gas discharge lamp when the filament heating voltage is at or above a desired maximum voltage level. The ballast circuit is capable of reducing the filament heating voltage during the pre-heat period and/or when the lamp is being dimmed. 
     The ballast circuit includes an inverter that converts a DC voltage into an AC voltage that powers a gas-discharge lamp. The inverter has inverter switch devices that convert the DC voltage into a pulsed inverter voltage and an inverter resonant circuit that converts the pulsed inverter signal into the AC voltage that powers the gas-discharge lamp. An inverter controller controls the switching frequency of the inverter switch devices and thus the magnitude of the AC voltage. A filament heating component is coupled to the inverter to receive a filament heating voltage. A voltage level of the filament heating voltage may also be adjusted by controlling the switch frequency of the inverter switch devices. The filament heating component may be a filament heating winding in a filament heating transformer that couples the filament heating voltage to the lamp filaments. 
     The inverter controller includes a feedback terminal that receives an overvoltage control signal and adjusts the switching frequency of the inverter switch devices in accordance with the signal level of the overvoltage control signal. A control loop is operably associated with the filament heating component and the feedback terminal. The control loop includes a voltage regulator that is responsive to the voltage level of the filament heating voltage. When the voltage regulator senses that the voltage level of the filament heating voltage is at or greater than the desired maximum voltage level, the voltage regulator may cause the control loop to generate the overvoltage control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic of one embodiment of a ballast circuit having a control loop in accordance with the invention. 
         FIG. 2  is a frequency domain graph of the relevant signals and response curves associated with the ballast circuit shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a ballast circuit  10  in accordance with the present invention is shown. The ballast circuit  10  has an inverter  20  which in  FIG. 1  is arranged in a half-bridge inverter topology. However, it should be understood that the invention may be embodied in other inverter circuits and may be used with any type of gas discharge lamp  16 . The invention reduces a filament heating voltage  14  below a desired maximum voltage level. A half-bridge inverter topology is illustrated because this inverter topology is commonly used to power high-impedance gas-discharge lamps  16 . 
     Lamps  16  are particularly sensitive to unbalanced filament currents and over-current pin problems caused when the filament heating voltage  14  heats the lamp filaments  12  during lamp dimming. The present invention is useful in reducing these effects. In addition, the invention may be utilized with lamps that are not high impedance gas discharge lamps  16 . 
     Inverter  20  receives a DC voltage  26  at V_rail and converts the DC voltage  26  into an AC voltage  28  that powers the lamps  16 . Inverter  20  utilizes an inverter controller  30 , inverter switch devices  22 , and an inverter resonant circuit  24  that includes a resonant inductive component,  24 A, and a capacitive resonant component  24 B. Inverter resonant circuit  24  may be tuned to the appropriate frequency for powering the gas discharge lamps  16 . In this particular embodiment, the inverter resonant circuit  24  is coupled between the inverter switches  22  at terminal  25 . As is known in the art, inverter switches  22  are switched at a switching frequency to generate a pulsed voltage  25 A. Inverter resonant circuit  24  then filters the pulsed voltage  25 A to provide an AC voltage  28  at the appropriate frequency for powering the gas discharge lamps  16 . 
     Ballast circuit  10  may be operable to pre-heat the lamp filaments  12  prior to filament ignition and/or to dim the lamps  16  in accordance with a desired dimming level. In either case, the lamp filaments  12  are heated by a filament heating voltage  14 . However, the filament heating voltage  14  may reach excessively high levels when heating the filaments  12 . This may damage the lamps  16  and cause unbalanced filament currents and overcurrent pin problems when the lamps  16  are being dimmed. The invention reduces a voltage level of the filament heating voltage  14  below a desired maximum voltage level during the pre-heat period and/or during dimming to reduce these problems. 
     In this embodiment, the lamp filaments  12  are connected in series. A filament heating component  40  is coupled to the inverter resonant inductive component  24 A to receive a filament heating voltage  14  from the inverter  20 . The filament heating component  40  may be a primary transformer winding  50  in filament heating transformer  36 . Primary transformer winding  50  is connected to lamp filament  12 A and provides the filament heating voltage  14  to this lamp filament,  12 A. Primary transformer winding  50  may also be magnetically coupled to secondary windings  60  which receive filament heating voltage  14  to heat the lamp filaments  12 B,  12 C. 
     During a pre-heat period, the inverter controller  30  may operate in accordance with a pre-heat sequence to pre-heat the lamp filaments  12 . Because the filament heating voltage  14  in this embodiment is received from the inverter resonant circuit  24 , the filament heating voltage  14  is associated with the AC voltage  28 . Consequently, the switching frequency of the inverter switch devices  22  also determines the signal frequency of the filament heating voltage  14 . 
     Referring now to  FIGS. 1 and 2 , primary resonant winding  50  of filament heating transformer  36  may also be part of a filament resonant tank  52 . Inverter controller  30  operates the filament heating voltage  14  within a pre-heat frequency range  48  during the pre-heat period. Pre-heat frequency range  48  is normally much higher than the frequency of operation during steady state. The frequency response bandwidth  52 A of the filament resonant tank  52  passes the filament heating voltage  14  operating at the pre-heat frequency range  64 B. However, the filament heating voltage  14  is blocked at frequencies near the inverter resonant frequency  24 C of the inverter resonant circuit  24 . In this manner, the filament heating voltage  14  is coupled to the lamp filaments  12  during the pre-heat period but blocked during full-lamp operation. This helps balance the filament currents and reduce overcurrent pin problems during full lamp operation after the pre-heat period. 
     Ballast circuit  10  may be a dimmable ballast and thus be operable to operate the lamps  16  at one or more dimming levels. Inverter controller  30  may receive a dimming control signal  54  indicating a desired dimming level for the lamps  16  and adjust the switching frequency of the inverter switch devices  22  in accordance with this desired dimming level. Typically, the switching frequency during lamp dimming is significantly higher than during full-lamp operation. At low dimming levels, the lamp current may be relatively low and thus may require that the lamp filaments  12  be heated to maintain the lamp filaments  12  at the appropriate temperature. The frequency response bandwidth  52 A of the filament resonant tank  52  may also be tuned to receive the filament heating voltage  14  at some or all of these dimming frequencies. 
     One of the problems with the resonant devices  24 ,  52  of the ballast circuit  10  is that the electrical component values have a high level of variability. Given the high Q of resonant devices  24 ,  52 , this may lead to excessively high filament heating voltages  14  during the pre-heat period and/or during lamp dimming. Accordingly, a control loop  38  is utilized to reduce the filament heating voltage  14  below a desired maximum voltage level. Other embodiments of the control loop  38  may be utilized to reduce the filament heating voltage  14  during other lamp conditions, as the invention may be utilized any time the filament heating voltage  14  needs to be maintained below a desired maximum voltage level. 
     In this embodiment, control loop  38  is connected to a feedback terminal  32  in inverter controller  30  and receives a feedback control signal  62  associated with a voltage level of the filament heating voltage  14  from the filament heating component  40 . Filament heating component  40  may be any component that receives the filament heating voltage  14  or a signal associated with the filament heating voltage  14 . In this case, feedback control signal  62  is the filament heating voltage  14  itself. A single lamp application of ballast circuit  10  may receive the filament heating voltage  14  on a winding magnetically coupled to the inverter inductive component  24 A. In other embodiments, filament control signal  62  may not be the filament heating voltage  14  itself and may be indirectly related to the voltage level of the filament heating voltage  14 . 
     In this embodiment, the feedback control signal  62  may be the same as the filament heating voltage  14  received on secondary winding  60  coupled to lamp filament  12 D. It should also be understood however that filament heating voltage  14  may be at a different voltage levels at each individual lamp filament,  12 A,  12 B,  12 C,  12 D. Thus, the voltage level at filament resonant winding  50  may be different than the voltage level at secondary winding  60  coupled to the lamp filament  12 D. Any of these voltage levels may be used to operate the control loop  38 . Also, the value of the desired maximum voltage level may be dependent upon where the voltage level of the filament heating voltage  24  is being measured. While these voltage levels may be different, all of them change in accordance with a change in the amount of power transmitted by the filament heating voltage  14 . Feedback control signal  62  is associated with the filament heating voltage  14  because its signal level also changes in accordance with changes in the amount of power transmitted by the filament heating voltage  14 . 
     As inverter controller  30  changes the switching frequency of the inverter switch devices  22 , a voltage level of the filament heating voltage  14  also changes. Response curve  64 A of filament resonant tank  52  may be shaped such that as the signal frequency  42  of the filament heating voltage  14  is moved away from a center frequency  52 C of the response curve  64 A, the voltage level of the filament heating voltage  14  is lowered. Center frequency  52 C is generally the resonant frequency of the filament resonant tank  52  and may be the associated with a pre-heat frequency for powering the lamps  16 . This response curve,  64 A, may be shaped so that the filament heating voltage  14  has desired voltage levels at different stages of the pre-heat period and/or at designated dimming levels. In addition, response curve  64 A is also shaped so that filament heating voltage  14  is received within the pre-heat and/or dimming frequency signal ranges  64 B of filament heating voltage  14  but is blocked at the frequency  63  of the filament heating voltage  14  during full-lamp operation. 
     Control loop  38  may have a high pass filter  64  with a response curve  54 A that has a corner frequency  66  at or near the edge of pre-heat and/or dimming frequency signal ranges  64 B. In this case, feedback control signal  62  is AC. As the signal frequency  42  of the filament heating voltage  14  is lowered as the voltage level of the feedback control signal  62  is also lowered. Once the feedback control signal  62  is outside the pre-heat and/or dimming frequency signal ranges  64 B, feedback control signal  62  is filtered out by high pass filter  64  and control loop  38  does not operate during full lamp operation. 
     Referring again to  FIG. 1 , during the pre-heat period and/or lamp dimming, control loop  38  is operable to generate an overvoltage control signal  34  if the filament heating voltage  14  is above a desired maximum voltage level. Inverter controller  30  responds to reduce the overvoltage control signal  34  by adjusting the switching frequency of inverter switch device  22 . In this embodiment, this adjusts the signal frequency  42  of the filament heating voltage  14  and thus places the signal frequency  42  at a different position on the response curve  64 A of the filament resonant tank  52 . In turn, this lowers the voltage level of the filament heating voltage  14 . The inverter controller  30  may continue to adjust the switching frequency until the overvoltage control signal  34  has been eliminated. 
     Overvoltage control signal  34  may therefore be generated when the voltage level of the filament heating signal  14  is at or above a desired maximum voltage level. As mentioned above, the voltage level of the filament heating voltage  24  in this control loop  38  is the voltage across secondary winding  60  associated with heating lamp filament,  12 D. After feedback control signal  62  is filtered by high pass filter  64 , the feedback control signal  62  may be received by a converter  68 . Converter  68  converts feedback control signal  62  from AC into a pulsed DC control signal  70 . In this embodiment, the converter  68  is a half-wave rectifier  68 A coupled to a capacitor C 2 . Only one half-cycle of the feedback control signal  62  is transmitted through the half-wave rectifier  68 A. These half-cycles are then smoothed out by capacitor C 2  to form the pulsed DC control signal  70 . 
     Pulsed DC control signal  70  may provide a voltage across the voltage regulator  44  in the control loop  38 . So long as the voltage level of the pulsed DC control signal  70  is below an activation voltage level of the voltage regulator  44 , the voltage regulator  44  does not transmit and no overvoltage regulation signal  34  is generated. However, once the voltage level of the pulses of the pulsed DC control signal  70  are at or above the activation voltage level, an overvoltage control signal  34  is generated. Thus, the activation voltage level of the voltage regulator  44  should be selected based on the desired maximum voltage level of the filament heating voltage  14 . In this embodiment, the voltage regulator  44  is a reverse biased Zener diode and the breakdown voltage of the Zener diode corresponds with the desired maximum voltage level of the filament heating voltage  14 . 
     Inverter controller  30  may be any type of control circuit utilized to control the switching frequency of an inverter switch device. In this embodiment, inverter controller  30  is an IC control chip, specifically the UBA2014 driver chip. The circuit can take advantage of the characteristics of the chip to generate the overvoltage control signal  44  from the pulsed DC control signal  70 . To do this, a bootstrapped component R 1  is coupled to the feedback terminal  32  of the inverter controller  30 . Bootstrapped component R 1  converts the output of the voltage regulator  44  into a smooth DC signal. 
     A bootstrapped component R 1  is simply a component in which both the input and output of the component are driven substantially in unison. Of course, practical limitations prevent the inputs and outputs of a bootstrapped component R 1  to be driven in perfect unison. However, bootstrapping techniques are known for approximating this effect. In this example, the bootstrapped component R 1  is a resistor coupled to a second resistor R 2  which is also connected to ground. When the voltage regulator  44  is activated, current is fed to resistor R 2 . Because resistor R 1  is coupled to the chip, this raises the voltage on both sides of bootstrapped component R 1 . Overvoltage control signal  34  is thus generated as a smooth DC signal. As the filament heating voltage  14  is lowered by the inverter controller  30 , the overvoltage control signal  34  is also lowered in a smooth fashion until the overvoltage control signal  34  is eliminated and the filament heating voltage  14  is below the desired maximum voltage level. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful BALLAST CIRCUIT FOR A GAS DISCHARGE LAMP WITH A CONTROL LOOP TO REDUCE A FILAMENT HEATING VOLTAGE BELOW A MAXIMUM HEATING LEVEL it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.