Abstract:
Prior to a load being activated, a first capacitive network and the load are operationally in parallel with each other, and the first capacitive network and a first inductor are in series with each other. A second inductor is magnetically coupled to the first inductor to boost a voltage supplied to the load. When the load is activated, a second capacitive network, the load, and the first inductor are operationally in series with each other. In a further embodiment, the first inductor and a second inductor are not capacitively coupled together, rather the second inductor generates lagging current at a first node to cancel leading current generated by the first capacitive network. Heating of the load is accomplished by the use of a cathode heater winding in operational connection with at least one of the cathodes.

Description:
BACKGROUND OF INVENTION 
     The present invention is directed to electronic ballasts, and more particularly to an inverter circuit topology which has improved operational efficiencies over existing electronic ballasts. 
     FIG. 1 illustrates a conventional parallel load, series resonant electronic ballast  10 . Electronic ballast  10  is supplied by an a.c. input source  12 . An input signal from input source  12  is rectified by full-bridge rectifier circuit  14  consisting of diodes  16 - 22 . The signal generated by full bridge rectifier circuit  14  is supplied, through an input filter  24 , to switching network  26 , consisting of switches  28  and  30 . Switches  28  and  30  are connected together at one end via node  37  , and may be controlled by a known controller  32 , such as a complementary switching system or other known design. Output from switching network  26  is supplied through inductor  34  to a lamp starting circuit  36 . Lamp starting circuit  36  includes d.c. blocking capacitor arrangement  38  with capacitors  40  and  42 , resonant capacitors  44  and  46 and a positive temperature co-efficient element  48  such as a thermister. D.C. blocking capacitors  40 ,  42  are connected to each other at node  43 . Lamp  50  is connected to ballast  10  via cathodes  52  and  54 . Capacitor  55  is used as an energy storage capacitor. D.C. blocking capacitor arrangement  38  and capacitor  55  are connected at a first end to circuit bus  56  and at a second end to reference bus  57 . Upon initiation of operation a signal from switching network  26  causes energization of the lamp starting circuit  36 , wherein cathodes  52  and  54  are heated prior to the igniting of lamp  50 . Additional circuit connections are well known in the art, and are not shown for purposes of clarity for the present description. 
     Ballast  10  may be considered a parallel load, series resonant circuit in that lamp  50  is placed in parallel with resonant capacitors  44  and  46  which are in series with resonant inductor  34 . Positive temperature coefficient element  48  is provided parallel to resonant capacitor  44  to preheat the cathodes. Ballast  10  is useful for operation in single lamp that has low lamp arc current. It provides sufficient voltage for starting of lamp  50 , and also works efficiently during the running of lamp  50  following the breakdown of gases in the discharge lamp. 
     A drawback to the described conventional parallel load, series resonant ballast and other similar ballasts is that high current stresses which exist on the resonant components and switching devices for high bus voltage implementations. High bus voltage, for example, in Europe is approximately 325 volts, and in the U.S. it is in the range of 390 volts for 277 RMS voltage input. 
     High currents are problematic since the resulting high lamp arc current not only goes through the switching devices but also goes through, for example, the resonant inductor  34 . Therefore, resonant inductor  34  sees a summation of current which includes the lamp arc current and the resonant capacitor current through capacitors  44  and  46 . The lamp arc current may vary, depending upon what type lamps are used. For example, for a 28-watt compact fluorescent lamp (CFL) T-4, the lamp arc current may be 210 milli-amps, while for a T-6 2D lamp, the lamp current may be 360 milli-amps or higher. This means the resonant inductor needs to be of a significant size to avoid becoming saturated and to ensure that the power dissipation is not excessive. It is also necessary to use switches such as Field Effect Transistors (FETs), Bipolar Junction Transistors (BJTs) or other known switching devices having high current ratings. 
     Another drawback of ballast  10  is that it&#39;s resonant circuit has a poor power factor, where the input tank current and voltage are significantly out of phase, especially for the lamp with high lamp&#39;s arc current. An issue is that the signal delivered by switching network  26  from node  37  has its current and voltage out of phase, wherein the current through inductor  34  is out-of-phase with the voltage from node  37  to  43 . This out-of-phase state means more current to the tank than necessary to drive the lamp. For example, if only 30 watts were necessary in a fully in-phase system, in an out-of-phase system it may be necessary to deliver 50 or 60 watts of apparent power from the output of switches  28  and  30 . The excess apparent power circulates between resonant circuit  36  and switch network  26  resulting in the dissipation of a large amount of power in the components. 
     In these high voltage implementations it is necessary to use components sized to handle the noted stresses and excess current. However, these larger than desired components are more expensive than smaller components, and take up more physical space. Since the electronics industry is increasingly striving to decrease the cost and size of the ballasts, the foregoing noted inefficiencies are impediments to the objectives of the industry. This is especially true for ballasts used to power lamps such as integral compact fluorescent lamps, high intensity discharge lamps and others. 
     Therefore, it is considered desirable to configure an inverter circuit topology which improves the power factor of the ballast&#39;s tank circuit, to reduce the current stress on the resonant components and switching devices, allowing the use of smaller sized components. It is also desirable to provide a circuit which improves the output regulation over lamp impedance variations due to thermal effects, to provide a flexibility in preheating of the circuit, and for an overall improved and more economical ballast. 
     SUMMARY OF INVENTION 
     A high frequency, high power factor inverter circuit is provided to generate current for a load. A first inductor is connected to receive an input voltage. A second inductor is connected at one end to the load and at a second end to a first node. The second inductor is further magnetically coupled to the first inductor in a configuration which increases or boosts the voltage to the lamp. A first capacitive network is connected in parallel across the load. A second capacitive network is connected in series with the load, wherein the second capacitive network has a capacitive value larger than the first capacitive network. Prior to the load being activated, the first capacitive network and the load are operationally in parallel with each other, and the first capacitive network and first inductor are in series with each other. When the load is activated, the second capacitive network, the load, and the first inductor are operationally in series with each other. In a further embodiment, the first inductor and second inductor are not coupled together, rather the second inductor generates lagging current at a first node which acts to cancel leading current generated by the first capacitive network at the first node. The summation current at the first node may be less than the current otherwise seen by the system. Heating of the load, when it is a gas discharge lamp having cathodes is accomplished by the use of a cathode heater winding in operational connection with at least one of the cathodes and magnetically coupled to the first inductor. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a conventional series resonant parallel load electronic ballast; 
     FIG. 2 depicts a first embodiment of an improved electronic ballast for use in higher lamp current implementations; and 
     FIG. 3 depicts a second embodiment of an electronic ballast for use in high frequency/high lamp current situations. 
    
    
     DETAILED DESCRIPTION 
     In addition to a first inductor  34 , also provided is a second inductor  62  and an external cathode beater winding  64 . First inductor  34  and second inductor  62  being connected at a first node  76 . Each of inductors  34 ,  62  and heater winding  64  are shown to be magnetically coupled. Inductors  34  and  62  are coupled in a phase relationship such as to act as an auto-transformer providing a voltage step-up of the input signal This step up or boost function is useful in permitting the ballast to be used with a variety of lamps. For example, where a CFL lamp is known as an easy starting lamp since it can be started at relatively lower voltages, an HID lamp, or other high-pressure discharge lamp is difficult to start, requiring higher starting voltages. Using the step-up transformer configuration formed by inductors  34  and  62  allows for the increase of voltage necessary for sing high voltage lamps. Cathode heater winding  64 , coupled to inductors  34  and  62 , provides a manner of supplying voltage in order to heat cathode  54 . 
     The configuration of circuit  58  of FIG. 2 provides a new topology wherein prior to operation of lamp  50 , during the heating stage, the circuit functions in a manner different from that during its running-time operation stage. Prior to the breakdown of the lamp, i.e. during the heating stage, a resonant circuit is formed by inductor  34 , and the combination of a first capacitive network of resonant capacitors  44  and  46 . However, in this embodiment, unlike that of FIG. 11 a second capacitive network or combination  40  and  42  does not function only as a d.c. blocking capacitor configuration. Rather, following the breakdown of the lamp, during the operation of lamp  50 , they become part of the resonant circuit  60 , as their values are lowered to affect the resonant circuit. Although the combination of capacitors  40  and  42  are at a lower value than the same numbered capacitors in FIG. 1, they are nevertheless much larger than capacitors  44  and  46 . 
     In addition to inductor  34 , also provided is a second inductor  62  and an external cathode heater winding  64 . Each of inductors  34 ,  62  and heater winding  64  are shown to be magnetically coupled. Inductors  34  and  62  are coupled in a phase relationship such as to act as an auto-transformer providing a voltage step-up of the input signal. This step-up or boost function is useful in permitting the ballast to be used with a variety of lamps. For example, where a CFL lamp is known as an easy starting lamp since it can be started at relatively lower voltages, an HID lamp, or other high-pressure discharge lamp is difficult to start, requiring higher starting voltages. Using the step-up transformer configuration formed by inductors  34  and  62  allows for the increase of voltage necessary for starting high voltage lamps. Cathode heater winding  64 , coupled to inductors  34  and  62 , provides a manner of supplying voltage in order to heat cathode  54 . 
     The configuration of circuit  58  of FIG. 2 provides a new topology wherein prior to operation of lamp  50 , during the heating stage, the circuit functions in a manner different from that during its running-time operation stage. Prior to the breakdown of the lamp, i.e. during the heating stage, a resonant circuit is formed by inductor  34 , and the combination of resonant capacitors  44  and  46 . However, in this embodiment, unlike that of FIG. 1 , the capacitor combination  40  and  42  does not function only as a d.c. blocking capacitor configuration. Rather, following the breakdown of the lamp, during the operation of lamp  50 , they become part of the resonant circuit  60 , as their values are lowered to affect the resonant circuit. Although the combination of capacitors  40  and  42  are at a lower value than the same numbered capacitors in FIG. 1 , they are nevertheless much larger than capacitors  44  and  46 . 
     Prior to breakdown and starting of lamp  50 , ballast  58  is a parallel load, series resonant circuit, somewhat similar to that of FIG.  1  . However, when the lamp is in the running or operational state, the functioning of the components changes and capacitors  40  and  42  function as part of the resonant circuit. 
     Once the lamp ignites, operation of ballast  58  changes, and it begins loading up, due to the size selected for capacitors  44  and  46 . Now the circuit resonance is dominated by the resonance between capacitors  40  and  42  and inductors  34  and  62 . The combination of capacitors  40 and  42  allows for its equivalent circuit to be put in parallel whereby the combination of capacitors  40 ,  42 , lamp  50  and inductors  34 ,  62  are in series. Therefore, the resonant circuit is now converting to a series load, series resonant circuit . This is distinct from operation during the heating pre-lamp operation time, where the circuit is more of a parallel load, series resonant. At that time lamp  50  is in parallel with capacitors  44  and  46 as no current is flowing. However, once the lamp ignites, circuit operation is altered. This is true because capacitors  44  and  46  are small enough that their operation as parallel capacitors to load  50  is diminished whereby the larger capacitor combination  40  and  42  is configured to act as if it is in series with lamp  50  and inductor  34 . 
     Circuit  72  is similar to previously described circuit  60  including a parallel load portion and a series circuit portion formed by the first capacitive network of resonant capacitors  44  and  46 . However, in this embodiment, a second inductor  74  is not magnetically coupled to a first inductor  75 . This is different from FIG. 2 where second inductor  62  is coupled magnetically to first inductor  34  to form a type of voltage boost auto-transformer. 
     Turning to FIG. 3, ballast  70  is a further embodiment of the present invention. In circuit  70 , capacitors  40  and  42  function as d.c.-blocking components and are not used as part of the resonant circuit, as used in the configuration of FIG.  2 . 
     Circuit  72  is similar to previously described circuit  60 , including a parallel load portion and a series circuit portion formed by resonant capacitors  44  and  46 . However, in this embodiment, an inductor  74  is not magnetically coupled to inductor  75 . This is different from FIG.  2 where inductor  62  is coupled magnetically to inductor  34  to form a type of voltage boost auto-transformer. 
     Once lamp  50  ignites, it is placed in series with inductor  74 . This results in a lagging current at node  76 . The current through the path including resonant capacitors  44  and  46  on the other hand, results in a leading current at node  76 . Summation of the leading and lagging currents, result in at least a partial cancellation of these currents thereby providing for an improved unified signal and an improved power factor. This allows for the use of smaller sized magnetics or inductors  74  and  75 . For example, inductor  74  may only need to be sized to handle the lamp current. Further, inductor  75  may be smaller than inductor  34  used in the circuit of FIG.  1  . Particularly, while inductor  34  of FIG. 1 must be sized to handle both the lamp current and any capacitive current, inductor  75  may be sized smaller due to the cancellation of current occurring at node  76 . Due to the cancellation of current at node  76 , the possibility exists for inductor  75  to see current even lower than lamp current. 
     Inductor  75  and external cathode heater winding  64  are magnetically coupled. This provides the source for energization of the cathode for a preheat operation to assist in lamp starting. 
     In FIG. 1 the preheating of the cathodes is accomplished by use of the current going through capacitors  44  and  46 , and therefore both sides of lamp  50  are heated by the same source. However, due to the implementation of the embodiment shown in FIGS. 2 and 3, it is not possible to access cathode  54  in the same manner. Therefore, winding  64  is magnetically coupled to at least one of the inductors in order to supply voltage to cathode  54 . It is to be appreciated that either or both of the cathodes may be coupled in this manner. 
     The heating of cathodes  52  and  54  are shown in the manner described when the present invention is implemented using fluorescent lamps. However, for other lamps, such as HID lamps, heater winding  64  would not be needed since only a single electrode post is implemented in the HID lamps. Component values for the circuits of FIGS. 3 described in the foregoing, would include: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Diode Bridge 14 
                 1N4005 
               
               
                   
                 Filter Inductor 24 
                 680 uh 
               
               
                   
                 Switches 28, 30 
                 IRFR320&amp;LQD4P40 
               
               
                   
                 Inductor 34 
                 1.85 mh 
               
               
                   
                 Capacitors 40 
                 0.22 uf 
               
               
                   
                 Capacitors 42 
                 0.22 uf 
               
               
                   
                 Capacitor 44 
                 10 nf 
               
               
                   
                 Capacitor 46 
                 0.068 nf 
               
               
                   
                 Lamp 50 
                 F38W2D 
               
               
                   
                 Inductor 74 
                 680 uh 
               
               
                   
                 Inductor 75 
                 1.85 mh 
               
               
                   
                   
               
             
          
         
       
     
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.