Abstract:
A voltage control startup circuit for a lighting ballast includes first and second transistors for converting direct current from a voltage source into alternating current to operate a lamp. The circuit includes an input portion for receiving a bus voltage signal, a resonant load portion for receiving a lamp load. The ballast also includes a start-up portion that delays firing of the lamp based on the detected bus voltage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present application relates to ballasts, and 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 inverter circuits, and is not limited to the aforementioned use.  
         [0002]     In the late eighties, and early nineties, the lighting industry began to make a shift from passive power and harmonic correction circuits to active power correction and harmonic circuits in the form of active pre-regulators for use in conjunction with electronic lamp ballasts. An advantage of active power factor and harmonic correction via active pre-regulators is that bus voltage variation can be virtually eliminated even though there are still voltage variations on the input line. The visible effect of this change is less variation in lumen output, that is, lamps connected to active pre-regulator circuits exhibit steadier intensities than lamps connected to circuits without active pre-regulation.  
         [0003]     While the use of active pre-regulators has provided improved performance in certain areas, new problems have arisen when these pre-regulators are put into operation with rapid and/or instant-start ballasts or power supply circuits. Particularly, systems employing active pre-regulators require a significant amount of time to reach steady state operating conditions during start-up. This may result in undesirable operating conditions for the gas discharge lamps when the less than steady state operating voltages are passed through the converter section during this transient start-up condition.  
         [0004]     During normal operation, which is a steady state condition, the active pre-regulator will provide a pre-determined DC voltage output, whose value will be dependent on the circuit design and/or lamp being driven, but in many instances may be up to a 500 V DC output. During the transient start-up condition, the output will be substantially below the desired steady state voltage conditions. Therefore, when operating in rapid and instant start modes the voltage supply will not be at steady state, and may result in an undesirable effect of unacceptable “preheat” or glow periods at this lower voltage. Instant-start lamps are typically specified to be operated in a glow discharge mode for a very short time period, approximately for no more than 100 milliseconds. This is a requirement since longer “preheat” periods will act to shorten lamp life due to excessive electrode erosion during these glow discharge conditions. Additionally, when operating in low voltage (i.e. non-steady state conditions), undesirable visible phenomena such as lamp flickering may occur. Therefore, it is considered desirable to delay the start-up operation of an electronic ballast for instant-start type fluorescent lamps until a pre-determined DC bus voltage has been substantially reached.  
         [0005]     One particular attempt to address this issue is set forth in U.S. Pat. No. 5,177,408 to Marques which issued Jan. 5, 1993. This patent disclosed a time delay circuit of an electronic ballast for “instant-start” type fluorescent lamps of the type having an electronic converter powered by an active electronic pre-regulator. The inverter is described as an inductive-capacitive parallel-resonant push-pull circuit or other type of current-fed power-resonant circuit. The start-up circuit may be either a resistor and Zener diode or a resistor, capacitor, and diac network programmable uni-junction transistor circuit connected between the pre-regulator output and an oscillation-enabling input of the inverter. The active electronic pre-regulator is designed so that it takes a pre-determined start-up time to reach steady state operating conditions. This delay device is connected between the pre-regulator and the converter.  
         [0006]     Drawbacks to the above disclosed design exist. For example, to minimize design and development cost, to lower the number of different products (i.e. SKUs), to simplify inventory control, and to address global market needs, ballasts or power supply circuits having universal input capabilities have become a key selling point. In theory, a device is considered a universal input device if it is capable of operating cooperatively with the various standardized line voltages supplied in different parts of the world. For example, the standard line voltage in the United States is 120 V, in China it is 220 V, and in Europe, 230 V. A universal device would also preferably be able to operate with industrial line voltages which is currently 277 V in the United States.  
         [0007]     The aforementioned U.S. Pat. No. 5,177,408 is, however, dependent on the input line voltage to obtain its time delay. This means to obtain a pre-determined time delay, it would be necessary to take into consideration the line voltage with which the device will be operating. Such a device would not therefore be considered a universal input ballast or power supply. Particularly, if a unit were used with a 120 V input line, the time delay would be different than if that unit were receiving a 230 V input line. Thus, this approach does not take full advantage of active power factor correction control.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0008]     In accordance with one aspect of the present application, a lamp inverter circuit is provided. The lamp inverter circuit includes a switching portion that converts a DC signal to an AC signal. Further, the circuit includes an input portion for receiving a line voltage signal, a resonant load portion for receiving a lamp load, and a voltage controlled start-up portion that controls the ignition of the lamp based on a detected voltage.  
         [0009]     In accordance with another aspect of the present application, a method of firing a lamp is provided. An AC line voltage is supplied and converted into a DC bus voltage. A charging capacitor is charged by the bus voltage. A breakdown voltage of a diac is overcome, turning the diac conductive, supplying current to oscillation of the inverter circuit.  
         [0010]     In accordance with another aspect of the present application, a lamp ballast is provided. The lamp ballast includes a switching portion that includes first and second bipolar junction transistors. The ballast also includes a resonant load portion for receiving a lamp, a power factor correction circuit for delivering a bus voltage, and a voltage dependent start-up portion that controls firing of the lamp until the bus voltage ramps up to a pre-determined threshold. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0012]      FIG. 1  is a block diagram of a lamp system;  
         [0013]      FIG. 2  is a circuit diagram of ballast inverter circuit included in the lamp system shown in  FIG. 1  with a start up portion operably connected with a high side switch of the inverter circuit;  
         [0014]      FIG. 3  is a circuit diagram similar to the ballast of  FIG. 2 , however the implementation of the start-up portion is on a low side switch of the inverter circuit;  
         [0015]      FIG. 4   a  shows the bus voltage over a time sequence for the rapid start electronic ballast according to the present application;  
         [0016]      FIG. 4   b  provides a function of the bus voltage versus starting time for a rapid start electronic ballast according to the present application; and  
         [0017]      FIG. 5  depicts the charge current of capacitor  30  of  FIG. 2  as a function of the bus voltage. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     With reference to  FIG. 1 , lamp circuit A includes a lamp assembly  10  operably connected to a bus voltage sensing and self-oscillating inverter/starting circuit  12 . The lamp assembly  10  can be a gas discharge lamp or a plurality of gas discharge lamps, such as linear fluorescent or compact fluorescent lamps that operate at a particular frequency or range of frequencies. In one embodiment, the inverter starting circuit  12  is connected to power factor correction (PFC) circuit  14 , such as an active power factor correction circuit which regulates a line voltage, corrects harmonics and supplies a bus voltage to inverter starting circuit  12 . It is to be understood that PFC circuit  14  may provide passive power correction in an alternate embodiment. An AC voltage source  16  supplies an alternating current signal to the PFC circuit  14 . The voltage source  16  can deliver a wide range of signals. Currently in the United States, the standard wall socket delivers a 120 V RMS voltage. The standard line voltage in China is 220 V, and Europe is higher, at about 230 V. Other sources, such as ones used for more industrial applications can deliver voltages of 277 V or higher. In one embodiment, the resulting bus voltages produced by PFC  14  range from 169 V (with a 120 V input) to 390 V (with a 277 V input), or more. The PFC circuit  14  can accept an input line voltage in the above disclosed range, in addition to accommodating higher or lower input voltages. Active and/or passive power factor correction circuits of this type are well known in the art, and therefore a detailed description of their operation is not undertaken here.  
         [0019]     With reference to  FIG. 2 , illustrated is a detailed view of the inverter starting circuit  12  in a current fed half bridge inverter implementation. 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  12 . The action of alternating periods of conduction of the transistors provides an AC signal to the lamp assembly  10 . 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 inverter circuits, such as known in the art. For example, the following descriptions may be implemented with BJTs in both half-wave current fed ballasts and push-pull type current fed electronic ballasts, among others.  
         [0020]     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 by the PFC circuit  14  to the inverter starting circuit  12 . 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 from the PFC circuit  14  reaches a predetermined threshold voltage. The start-up portion includes Zener diode  26 , diode  28 , capacitor  30 , and diac  32 .  
         [0021]     The potential difference across capacitors  34  and  36  is equivalent to the bus voltage from the PFC circuit  14 . 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 .  
         [0022]     When power is first applied to the inverter starting circuit  12 , 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  12 , a portion of the circuit current charges capacitors  34  and  36 , other current charges snubber capacitor  46 , and the remaining current flows through resistors  38 ,  40 , and  42 . Initially, because 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 .  
         [0023]     Eventually, the bus voltage from PFC  14  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 64.5 and 71.5 V, and preferably 68 V.  
         [0024]     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 after approximately 0.7 seconds, lamp assembly  10  is ignited.  
         [0025]     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 capacitor  30 , 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  12  now operates as is typical, with no further activity from the start-up portion  24 .  
         [0026]     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 winding  58 . Drive circuit  50  incorporates diode  60 , resistor  62  combination, supplied via coupling of windings  66 . Lamp assembly  10  is provided with power from inverter starting circuit  12  by a coupling between windings  68  and  70 , where winding  70  has a capacitor  72  across its primary winding and are considered resonant load components.  
         [0027]     In the event of an over voltage occurring during lamp start-up or sudden load removal, power Zener diodes  74  and  76  will clamp the voltage to protect the BJTs from over voltage damage.  
         [0028]     With continuing attention to  FIG. 2 , breakover voltage of diac  32  is chosen to be an optimal bus voltage for starting the inverter circuit and ignition voltage of lamp assembly  10 . 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 pre-determined 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 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.  
         [0029]     It is to be understood the above description that applies to first transistor  20  is also applicable to second transistor  22 . That is, as shown in  FIG. 3  in an alternate inverter starting circuit  12 ′ embodiment, the start-up portion  24  is connected to second transistor  22 , and it, instead of first transistor  20 , would initiate oscillations. Components having similar operation and use as components in  FIG. 2  are similarly numbered as in  FIG. 2 .  
         [0030]     The firing voltage is chosen to be about 300 V or greater for rapid start ballasts.  
         [0031]      FIG. 4   a  provides a graphed time sequence of a rapid start electronic ballast incorporating inverter starting circuit  12  of the present application. As seen from this figure, the sequence includes three distinct transitions. Fo a 120 V input line, 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. 4   b,  depicted is a chart showing inverter starting time for a rapid start electronic ballast incorporating inverter starting circuit  12 . Viewing  FIGS. 4   a  and  4   b  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 lamp 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. 4   b  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.  
         [0032]     Turning now to  FIG. 5 , 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  80  to charge capacitor  30 . After the bus voltage reaches 300 V, a significantly higher charging current  82  is available to capacitor  30 .  
         [0033]      0 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. 4   b ) is about 40 milliseconds at 390 V. After lamp assembly  10  is ignited, 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 fires the lamp as soon as possible, before the bus voltage reaches steady state. Of course, greater or lesser firing voltages can be chosen, for example in some applications the bus voltage may experience an overshoot during start-up, based on known line voltages and desired universality of the inverter.  
         [0034]     Thus, from the foregoing, it is shown ( FIGS. 2 and 3 ) are two implementations of a new starting circuit in conjunction with a current fed, half-bridge inverter circuit. The main bus voltage is 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 pre-determined level, the Zener diode breaks down, allowing the charging capacitor to 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. Possible applications of the present invention include General Electric&#39;s 4 ft. and 8 ft. T12 and T8 electronic lamp ballasts.  
         [0035]     Exemplary component values for the circuits of  FIGS. 2 and 3  are as follows:  
                                                                                                                                                                                                                                             Part Description   Part Number   Nominal Value                                        Lamp Assembly   10   40   Watts           Line Voltage   16   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           Zener Diode   26   68   V                Diode   28   UF 4007                Capacitor   46   1.2   nf           Charging Capacitor   30   0.1   μf                Diac   32   HT-32           Zener Diode   74   P6KE440A           Zener Diode   76   P6KE440A                Inductive Winding   56   5   mh           Inductive Winding   64   5   mh                Base Diode   52   1N5817           Base Diode   60   1N5817                Base Resistor   54   75   Ω           Base Resistor   62   75   Ω           Inductive Winding   70   0.85   Henries           Inductive Winding   68   1.27   Henries           Capacitor   72   12   nf                      
 
         [0036]     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.