Abstract:
In accordance with one aspect of the present application, a lamp inverter circuit includes a switching portion that converts a bus voltage signal into an alternating current signal. An input portion receives the bus voltage signal, and a resonant load portion drives a lamp. A preheating portion heats the lamp prior to ignition, and thereafter renders itself inactive following ignition of the lamp. In accordance with another aspect of the present application, a method of starting a lamp includes receiving a bus voltage signal, converting the bus voltage signal into an alternating current signal, preheating the lamp to an ignition temperature, igniting the lamp and inactivating the preheating after the lamp has been ignited. In accordance with another aspect of the application, provided is a method of igniting an auxiliary lamp, including detecting a conductive state of a main lamp in a lamp ballast circuit, the detecting being by a switch that controls preheating of the main lamp. The integrity of the main lamp is detected, and current flow is switched from the main lamp to an auxiliary lamp in the event of a main lamp failure.

Description:
BACKGROUND 
   The present application relates to ballasts, or power supply circuits for gas discharge lamps. It finds particular application for use with current fed instant and/or rapid start electronic ballasts or power supply circuits and will be described with particular reference thereto. It is to be appreciated, however, that the present application is also applicable to other controllers, and is not limited to the aforementioned use. 
   Presently there are two prevailing starting methods for starting gas discharge lamps. One is instant start, and the other is rapid start. In both the instant start and rapid start methods, cathodes of the lamp are pre-heated before lamp ignition. With an instant start electronic ballast, the cathodes are preheated by a glow discharge current. This is the current that goes through the lamp before the lamp ignites. Typically, the voltage potential of the glow discharge current is high, and can range between approximately 400 and 500 V rms  or more. In the preheating phase, when the lamp is not conducting, such high potential differences can cause bombardment of the cathodes, resulting in some of the physical material of the cathode sputtering off each time the lamp is lit. Thus, lamps that utilize the instant start method of ignition tend not to have as long lives as lamps that utilize the rapid start method. Typically, a lamp that uses instant start will last about 80% as long as the same type of lamp using rapid start. 
   With a rapid start electronic ballast, the cathodes are pre-heated with a separate voltage on the cathodes, while maintaining low voltage across the lamp. Therefore, the glow discharge current is low, being less than about 10 ma in comparison with instant start circuits. In the rapid start mode, the time that high voltage potentials across the lamp are applied without the lamp conducting is significantly reduced during start-up, and the bombardment of the cathodes does not occur to the same extent as with the instant start method, significantly extending lamp life. 
   There is a drawback, however, to using the rapid start mode. Presently, once a preheating current is applied, it is generally not removed from the cathodes, even after the lamp ignites. Resultantly, while the lamp is lit, the low voltage heating current is continuously applied on the cathodes. Thus, lamps that utilize the rapid start method to start are consuming more power than lamps that use the instant start method. With a single lamp, it is likely that up to about 1.5 extra Watts of power will be consumed, and with a three lamp ballast, it is likely that between 4.5 and 6 extra Watts of power will be consumed. This extra power is consumed (simply producing heat) without producing any added light output, that is, without producing extra lumens. Thus, the trade-off from instant start to rapid start is greater lamp life for added power consumption. 
   BRIEF DESCRIPTION 
   In accordance with one aspect of the present application, a lamp inverter circuit includes a switching portion that converts a bus voltage signal into an alternating current signal. An input portion receives the bus voltage signal, and a resonant load portion drives a lamp. A preheating portion heats the lamp prior to ignition, and thereafter renders itself inactive following ignition of the lamp. 
   In accordance with another aspect of the present application, a method of starting a lamp includes receiving a bus voltage signal, converting the bus voltage signal into an alternating current signal, preheating the lamp to an ignition temperature, igniting the lamp and inactivating the preheating after the lamp has been ignited. 
   In accordance with another aspect of the application, provided is a method of igniting an auxiliary lamp, including detecting a conductive state of a main lamp in a lamp ballast circuit, the detecting being by a switch that controls preheating of the main lamp. The integrity of the main lamp is detected, and current flow is switched from the main lamp to an auxiliary lamp in the event of a main lamp failure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a lamp system in accordance with the present application; 
       FIG. 2  is a circuit diagram of a ballast inverter circuit included in the lamp system shown in  FIG. 1 , in accordance with one aspect of the present application; 
       FIG. 3A  shows the bus voltage over a time sequence for the rapid start electronic ballast according to the present application; 
       FIG. 3B  provides a function of the bus voltage versus starting time for a rapid start electronic ballast according to the present application; 
       FIG. 4  depicts the charge current of capacitor  30  of  FIG. 2  as a function of the bus voltage; 
       FIG. 5  shows an alternate architecture of a portion of the circuit of  FIG. 2 , with an additional winding tapped into the resonant inductor. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , lamp circuit A includes a high frequency inverter  10 , and a lamp assembly  12 . Inverter  10  is supplied with a bus voltage  11 , and can be either of a current fed or a voltage fed type of high frequency inverters. Both types of inverters utilize inductive circuitry, whether they be inductors or transformers. Tapped off of a portion of the inductive circuitry of inverter  10 , are additional inductive winding of winding arrangement  14  that supply power to a unidirectional timing switch circuit  16 . These windings supply power that will be used to pre-heat cathodes of lamp assembly  12 . Two windings are used for each lamp, as each lamp has two cathodes to pre-heat. For a three lamp configuration, only four to six windings from inductive winding arrangement  14  are tapped off of inverter  10 , since one cathode of the three lamps is in parallel and heated by a single winding. 
   Timing switch circuit  16  is selected to be unidirectional to avoid having to convert an AC control signal into a pulsating DC signal before being controlled by switch circuit  16 . This makes for a simpler, lower cost switch arrangement, and aids in allowing the provision of power to the cathodes by a single switch. 
   The single switch of timing switch circuit  16  is selected to have a zero voltage turn-on point and a zero current turn-off point. This allows the switch to turn on without excessively high voltages. Otherwise, larger, more expensive switches would be needed to do the same job. Also, utilizing zero voltage turn-on and zero current turn-off minimizes power dissipation when the switch is activated or deactivated. Inverter  10  and timing switch circuit  16  are both gated by a current limiting transformer arrangement  18  that regulates the start-up and currents supplied to lamp cathode assembly  12 . 
   With reference to  FIG. 2 , illustrated is a more detailed view of the lamp circuit A including, inverter starting circuit  10  in a current fed half bridge inverter implementation, in operation with a cathode cut-off circuit, comprised of inductive winding arrangement  14 , unidirectional timing switch circuit  16 , and current limiting transformer arrangement  18 . 
   In order to convert a DC bus signal into an AC signal, a first transistor  20  and a second transistor  22  alternate between periods of conductivity and periods of non-conductivity, out of phase with each other. That is, when the first transistor  20  is conductive, the second transistor  22  is non-conductive, and vice-versa. The transistors  20 ,  22  are part of a switching portion of the inverter circuit  10 . The action of alternating periods of conduction of the transistors provides an AC signal to the lamp assembly  12 . In the embodiment illustrated in  FIG. 2 , the transistors are bipolar junction transistors (BJTs), but it is to be understood the concepts of the present application may be incorporated in other switching networks, such as known in the art. For example, the following descriptions may be implemented with field effect transistors in both half-bridge current fed ballasts and push-pull type current fed electronic ballasts, among others. 
   In this embodiment, each transistor  20 ,  22  has a respective base, (B) emitter, (E) and collector (C). The voltage from base-to-emitter on either transistor defines the conduction state of that transistor. That is, the base-to-emitter voltage of transistor  20  defines the conductivity of transistor  20  and the base-to-emitter voltage of transistor  22  defines the conductivity of transistor  22 . In the illustrated embodiment neither of the transistors  20 ,  22  are conductive when current is initially supplied to the inverter starting circuit  10 . As will be expanded upon below, a start-up portion  24  of the inverter circuit prevents current from being supplied to the transistors  20 ,  22  before the bus voltage reaches a predetermined threshold voltage. The start-up portion includes Zener diode  26 , diode  28 , capacitor  30 , and diac  32 . 
   The potential difference across capacitors  34  and  36  is equivalent to the bus voltage. In one embodiment, capacitors  34  and  36  are of equal value, so that the voltage across capacitor  34  is the same as the voltage across capacitor  36 . In parallel with capacitors  34  and  36  are resistors  38 ,  40 , and  42 . Resistors  38  and  40  form a voltage divider at node  44  and current is supplied to the start-up portion  24  through voltage divider  38 ,  40 . 
   When power is first applied to the inverter starting circuit  10 , Zener diode  26  and diode  28  prevent any significant current from passing through start-up portion  24 . As the bus voltage ramps up, after power is initially supplied to inverter starting circuit  10 , a portion of the circuit current charges capacitors  34  and  36 , other current charges resonant capacitor  46 , and the remaining current flows through resistors  38 ,  40 , and  42 . Initially, because half the bus voltage is divided by resistors  38  and  40 , a breakdown voltage of Zener diode  26  is not reached, and Zener diode  26  prevents current from passing through start-up portion  24 . Eventually, the bus voltage ramps to a level where the potential at node  44  is greater than the breakdown voltage of Zener diode  26  turning Zener diode  26  conductive, supplying increased current levels to start-up portion  24 , and more specifically, to capacitor  30 . In the illustrated embodiment, the breakdown voltage of Zener diode  26  is between about 60 to 80 V, and preferably 8V. 
   Once Zener diode  26  turns conductive (from left to right in  FIG. 2 ) capacitor  30  begins charging. At this point, current is being supplied to start-up portion  24 , but diac  32  prevents the base of transistor  20  from becoming conductive in the collector-emitter direction. As the bus voltage continues ramping up, capacitor  30  collects more charge, and eventually reaches a potential to overcome the breakover voltage of diac  32 . When the breakover voltage is reached, transistor  20  turns conductive, wherein inverter starting circuit  12  begins to oscillate, and cathodes of lamp assembly  12  are preheated. 
   After the breakover voltage of diac  32  is reached, capacitor  30  no longer has an opportunity to continuously collect charge. Current flows directly from node  44  to the collector of transistor  20 , since transistor  20  is conductive after diac  32  breaks down. Diode  28  provides a path to allow capacitor  30  to discharge, once per cycle. The inverter starting circuit  10  now operates as is typical, with no further activity from the start-up portion  24 . 
   With continuing attention to  FIG. 2 , switching transistors  20 ,  22  are driven by respective drive circuits  48 ,  50 . Drive circuit  48  incorporates diode  52 , resistor  54  combination supplied via coupling of windings  56 ,  68 . Drive circuit  50  incorporates diode  60 , resistor  62  combination, supplied via coupling of windings  64 ,  68 . Lamp assembly  12  is provided with power from inverter circuit  10  by a coupling between windings  68  and  70 , where winding  70  has a capacitor  72  across its length and are considered resonant load components. Windings  58  and  66  also serve as current limiting devices. 
   In the event of sudden load change, power Zener diodes  74  and  76  break down, clamping the voltage across the transistors (e.g., BJTS), to protect them from destruction. 
   With continuing attention to  FIG. 2 , breakover voltage of diac  32  is chosen to be proportional to an optimal ignition voltage of lamp assembly  12 . In the illustrated embodiment, the breakover voltage of diac  32  is chosen to be such that when the bus voltage (the voltage across capacitors  34  and  36 ) reaches a predetermined value, for example about 390 V, diac  32  reaches its breakover voltage. Stated differently, start-up portion  24  detects when the bus voltage reaches the preferred firing voltage by virtue of the chosen breakover voltage of Zener diode  26  and diac  32 . In the illustrated embodiment, the breakover voltage of the diac  32  is between 20 V and 40 V, and preferably about 32 V. 
   It is to be understood the above description that applies to first transistor  20  is also applicable to second transistor  22 . That is, in an alternate inverter starting circuit embodiment, the start-up portion  24  is connected to second transistor  22 , and it, instead of first transistor  20 , would initiate oscillations. 
   Also, the preferred firing voltage may be chosen to be less than typical operating voltages for lamps in instant start and rapid start applications, which, in some instances, are approximately 450 V and 500 V, respectively. The firing voltage is also chosen to be about 300 V or greater. 
     FIG. 3A  provides a graphed time sequence of a rapid start electronic ballast incorporating inverter starting circuit  10  of the present application. As seen from this figure, the sequence includes three distinct transitions. From turn-on (0) to t 0  the bus voltage transitions from its starting voltage (e.g. 169 V) to a preferred pre-heat voltage (e.g. 390 V). The time duration to t 0 -t 1  is a pre-heat time (e.g. steady 390 V), and from t 1  to t 2 , the bus voltage ramps up to its steady state (e.g. 500 V). Turning attention to  FIG. 3B , depicted is a chart showing inverter starting time for a rapid start electronic ballast incorporating inverter starting circuit  10 . Viewing  FIGS. 3A and 3B  together emphasizes the starting time is controlled by the bus voltage of the circuit. For example if the bus voltage is less than 300 V, the inverter circuit will take approximately 10 seconds to start, however, when the bus voltage is 300 V or more, the start time is reduced to approximately 40 milliseconds.  FIG. 3B  illustrates the voltage dependency of the circuit, and emphasizes that operation to start the circuit is not a time dependent factor but is rather a voltage controlled concept. There is no pre-determined time following energization that the oscillations will begin. Rather, in the present design, following energization of the circuit, as long as the bus voltage is below a certain value (e.g. 300 V) there will, ideally, be no oscillations and only when the voltage is at or above the breakover voltage (e.g. 300 V) will the oscillations begin. Thus it is shown the starting of the circuit is controlled by the value of the bus voltage. 
   Turning now to  FIG. 4 , depicted is operation of charge capacitor  30  of  FIG. 2 , which illustrates its two distinct charging rates. Charge capacitor  30  will always have an amount of stored energy to be used for the breakover of diac  32 . As seen, when the bus voltage is over 300 V, capacitor  30  charges at a very quick rate, and when below 300 V bus voltage, capacitor  30  is being charged only due to leakage current. Particularly, when the bus voltage is less than 300 V, Zener diode  26  never turns conductive in its reverse direction, and allows only a leakage current  73   a  to charge capacitor  30 . After the bus voltage reaches 300 V, a significantly higher charging current  73   b  is available to capacitor  30 . 
   Another consideration in selecting the threshold voltage is the starting bus voltage. For a 120 V line input, the output bus voltage ramps up from about 169 V. For a 277 V line input, the output bus voltage ramps up from about 390 V. As stated earlier, the start time ( FIG. 3B ) is about 40 milliseconds at 390 V. After inverter circuit is oscillating, the bus voltage continues to ramp up to steady state operating voltage V. Thus, one exemplary firing voltage is 390 V, because it is greater than the 300 V required for mode transition, is less than common steady state operating voltages, and triggers the inverter circuit as soon as possible, before the bus voltage reaches steady state. Of course, greater or lesser firing voltages can be chosen, based on known line voltages and desired universality of the inverter. 
   Returning to  FIG. 2 , attention is now drawn to the cathode cutoff or preheat control circuit ( 14 ,  16 ,  18 ). In this design, winding  70  and  68 —in addition to being designed for resonant inductance—also provides isolation to the ballast. Secondary inductive winding  80  of winding arrangement  14  steps up the voltage to and provides isolation for switch circuit  16 . Resonant winding  70  includes a gap with a relatively low magnetizing inductor. That inductor acts as a resonant component, resonating with capacitors  72  and  46  before the lamp starts. Additionally, capacitors  75 ,  77 , and  78  reflected back to the primary side  70  after lamps of lamp assembly  12  are ignited. Winding  70  combined with capacitors  46 ,  72 ,  75 ,  77 , and  78  determine one or multiple operating frequencies of the assembly A. 
   Inductor winding  80  is also tapped off of resonant inductor winding  70 . Winding  80  supplies power to switching circuit primary winding  82 . Winding  82  in turn supplies power to cathode windings  84 ,  86 ,  88 , and  90 . Cathode windings  84 ,  86 ,  88 , and  90  pre-heat the cathodes of the lamps of lamp assembly  12 . It is to be understood that cathode windings  84 ,  86 ,  88 , and  90  act as the secondary of the primary winding  82 . The primary winding has a higher number of turns and thus, a lower current is needed in the primary winding  82 . Otherwise, costlier devices would be called for switching device  94  that can accommodate higher currents. Lower current devices are also desired to reduce power dissipation. Capacitor  92  limits the current that winding  80  supplies to primary winding  82 . If the value of capacitor  92  is chosen to be sufficiently low, it limits the maximum current supplied to winding  82 . Capacitor  92  serves a dual purpose; it acts as a DC blocking cap when transistor  94  is inactive. After a few cycles, this removes winding  82  from the circuit. That is, when transistor  94  goes inactive, no heating is being supplied to the cathodes. Diode  96  is connected back between capacitors  34  and  36  and protects transistor  94  from being supplied with excess voltage when transistor  94  goes inactive, during its transient state. 
   Winding arrangement  14  of  FIG. 1  also includes an inductor winding  98 , which supplies current through diode  100  and charges capacitor  102 . Capacitor  102  is subject to the RC time constant defined by capacitor  106  and resistor  104 . The time constant is selected to remove heating from the cathodes a safe time after lamp assembly  12  ignites, for example, it may be several seconds to 10 seconds or more after the inverter circuit is ignited. Capacitor  102  is connected to the gate of FET  108  via resistor  104 . When the RC time constant defined by capacitor  106  and resistor  104  is fulfilled, charge developed on capacitor  102  causes FET  108  to become conductive. The gate of FET  94  is then brought down to a lower voltage level, causing FET  94  to become inactive. As stated previously, when FET  94  turns inactive, the voltage on the cathodes switch of circuit  16  is removed, thereby removing heating to the cathodes. 
   Transistors  94  and  108  are depicted as MOSFETs, but it is to be understood that a similar circuit architecture could be accomplished using bipolar junction transistors or other switching devices. 
   Turning to  FIG. 5 , depicted is an alternate cathode cut-off circuit  14 ′,  16 ′,  18 ′ embodiment with a portion of a half bridge voltage fed rapid start electronic inverter  10 ′. The inverter  10 ′ uses FET switches  20 ′,  22 ′. This design incorporates an additional winding  110  tapped to resonant inductor  112 . Power to the cathodes  114 ,  116  is derived from winding  110  on the resonant inductor  112  via capacitor  92  and primary winding  82  elements with similar functions to those in  FIG. 2  are numbered similar to those in FIG.  2 . 
   Thus, from the foregoing, it is shown ( FIGS. 2 and 5 ) are two implementations of a new starting circuit in conjunction with current or voltage fed, half-bridge inverter circuits, which also implements a cathode cut-off circuit, which employs a unidirectional switching current design. The main bus voltage may be sensed by a three resistor divider circuit. A portion of the bus voltage is applied to a Zener diode and a charging capacitor. When the voltage reaches a predetermined level, the Zener diode breaks down, allowing the charging capacitor to 10 charge. A diac then breaks down, causing the self-oscillating inverter to be triggered. A diode prevents the charging capacitor from charging, allowing it to discharge every half-cycle, when a first transistor is on. The component values are selected such that the Zener breakdown voltage is at least double the diac breakdown voltage, or higher. The unidirectional switch circuit controls the energy delivered to the lamp cathodes, and current limiting capacitor prevents current from rising to dangerous levels. This protects the arrangement from possible miswirings, such as if one or more cathodes were shorted. Single unidirectional transistor switch  94  is turned on with zero voltage prior to oscillations of inverter transistors  20  ( 20 ) and  22  ( 22 ′), and turns off with zero current when the parasitic antiparallel diode of the FET—or a diode in parallel with the BJT—is conducting. Transistor  108  controls the removal of cathode heating after the lamp has started and stabilized. Possible applications of the present invention include General Electric&#39;s 4 ft. and 8 ft. T12 and T8 electronic lamp ballasts. 
   Additional embodiments of the described designs may be found in starting auxiliary power circuits when a main lamp is down. For example, with reference to  FIG. 1 , as shown by the dotted lines, the preheating circuit ( 14 ,  16 ,  18 ) may be used to provide auxiliary power when the main lamp (e.g., of assembly  12 ) has failed or is otherwise not in use. In this situation, an auxiliary lamp  120  may be connected to the preheat circuit ( 14 ,  16 ,  18 ). The auxiliary lamp may be a low-power fluorescent lamp, incandescent lamp or other lighting element. Therefore, in place of the additional windings  14 ,  18  being used to pre-heat the cathodes of lamp assembly  12 , they are connected to the auxiliary lamp  120 . This connection may be accomplished by the coupling of windings in a manner as discussed in connection with  FIGS. 2 and 5  (e.g., winding  82 ), to provide power and control for the auxiliary lamp  120  or use a independent inverter circuit to power and control the auxiliary lamp. This design provides a very low cost control to the auxiliary lighting in part due to the use of a single switch (e.g.,  94 ) control, in place of a two-switch system. The auxiliary lighting may be applicable  11  in a variety of situations such as back-up lighting and emergency lighting situations. Thus, instead of heating a cathode, the preheating circuit ( 14 ,  16 ,  18 ) may be used to deliver power to an auxiliary lamp  120 . 
   Some exemplary component values for the circuits of  FIGS. 2 and 5  are as follows: 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
           
             
             
             
             
             
           
         
             
                 
             
             
                 
               Nominal 
                 
             
             
               Part Description 
               Value 
               Nominal Value 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Lamp Assembly 
               10 
                 
               40 
               Watts 
             
             
               Line Voltage 
               11 
                 
               120-277 
               Volts 
             
           
        
         
             
               First Transistor 
               20 
               BJT SPB 11NM60 
             
             
               Second Transistor 
               22 
               BJT SPB 11NM60 
             
           
        
         
             
               Bus Capacitor 
               34 
                 
               33 
               μf 
             
             
               Bus Capacitor 
               36 
                 
               33 
               μf 
             
             
               Bus Resistor 
               38 
                 
               400 
               kΩ 
             
             
               Bus Resistor 
               40 
                 
               620 
               kΩ 
             
             
               Bus Resistor 
               42 
                 
               1 
               MΩ 
             
             
               Diode 
               28 
                 
               UF 
               4007 
             
             
               Capacitor 
               46 
                 
               1.2 
               nf 
             
             
               Charging Capacitor 
               30 
                 
               0.1 
               uf 
             
           
        
         
             
               Diac 
               32 
               HT-32 
             
             
               Zener Diode 
               76 
               P6KE440A 
             
             
               Base Diode 
               52 
               1N5817 
             
             
               Base Diode 
               60 
               1N5817 
             
           
        
         
             
               Base Resistor 
               54 
                 
               75 
               Ω 
             
             
               Base Resistor 
               62 
                 
               75 
               Ω 
             
             
               Inductive Winding 
               58 
                 
               5m 
               Henries 
             
             
               Inductive Winding 
               66 
                 
               5m 
               Henries 
             
             
               Inductive Winding 
               70 
                 
               0.85 
               mH 
             
             
               Inductive Winding 
               68 
                 
               1.27 
               mH 
             
             
               Capacitor 
               72 
                 
               0.01 
               uf 
             
           
        
         
             
               Zener Diode 
               74 
               P6KE440A 
             
           
        
         
             
               Capacitor 
               75 
                 
               0.0056 
               uf 
             
             
               Zener Diode 
               26 
                 
               68 
               V 
             
             
               Capacitor 
               77 
                 
               0.0056 
               uf 
             
             
               Capacitor 
               78 
                 
               0.0056 
               uf 
             
             
               Winding 
               80 
                 
               0.47 
               mh 
             
             
               Primary Cathode Winding 
               82 
                 
               1 
               mh 
             
             
               Secondary Cathode Winding 
               84 
                 
               2 
               uh 
             
             
               Secondary Cathode Winding 
               86 
                 
               2 
               uh 
             
             
               Secondary Cathode Winding 
               88 
                 
               2 
               uh 
             
             
               Secondary Cathode Winding 
               90 
                 
               2 
               uh 
             
             
               Capacitor 
               92 
                 
               4.7 
               nf 
             
             
                 
             
           
        
       
     
   
   It is to be understood that the foregoing components and values may be altered depending on the specific implementation, and values not listed may be selected in accordance with such implementations. 
   The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.