Abstract:
In accordance with one aspect of the present application, a continuous mode electronic ballast for operating an HID lamp includes an inverter circuit configured to generate a control signal. A resonant circuit is operationally coupled to the inverter circuit and to the lamp and configured to generate resonant voltage in response to receiving the control signal generated by the inverter circuit. A clamping circuit is operationally coupled to the resonant circuit to limit the voltage across the resonant circuit to protect components of the ballast. A multiplier circuit is operationally coupled to the resonant circuit to boost the voltage clamped by the clamping circuit to a value sufficient to permit starting of the lamp. The clamping circuit and the multiplier circuit cooperate to facilitate a continuous starting of the lamp.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present application is directed to high frequency resonant inverter circuits that operate a lamp. More particularly, the present application is directed to the resonant inverter circuit that operates continuously from an open circuit condition at the lamp&#39;s output terminals to a short circuit condition at the lamp&#39;s output terminals and will be described with particular reference thereto.  
         [0002]     Typically, high frequency inverters use a resonant mode to ignite the lamp. The resonant mode of operation requires the inverter to operate a resonant circuit near its resonant frequency to enable the output voltage to reach sufficient amplitude, usually 2 kV-3 kV, to ignite the lamp. At the fundamental switching frequency, resonant mode starting causes high currents to flow through the semiconductor devices and the ballasting components. The components of the resonant circuit have to be larger and more expensive than typically needed for steady state operation. In addition, higher currents, although achieving the required output voltage, cause the inverter to dissipate more power during initial start up than during steady state operation. To reduce power dissipation, the inverter must be turned “ON” and “OFF” to reduce power dissipation.  
         [0003]     To correct the above problems, a resonant mode at the frequencies higher than the fundamental frequency might be employed, which requires less current to flow through the inverter components. However, since a square wave is applied to the circuit that resonates at,the third harmonic or higher of the fundamental switching frequency, the desired zero switching cannot be achieved. The inverter circuit might also encounter a capacitive mode of operation that would cause damage to the intrinsic diodes of the power MOSFETs. The inverter still cannot be operated continuously without excessive power dissipation in the inverter and must be pulsed “ON” and “OFF” to reduce power dissipation.  
         [0004]     It is desirable to operate the inverter continuously without high power dissipation. The present application contemplates a new and improved method and apparatus that overcomes the above-referenced problems and others.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     In accordance with one aspect of the present application, a continuous mode electronic ballast for operating an HID lamp includes an inverter circuit configured to generate a control signal. A resonant circuit is operationally coupled to the inverter circuit and to the lamp and configured to generate resonant voltage in response to receiving the control signal generated by the inverter circuit. A clamping circuit is operationally coupled to the resonant circuit to limit the voltage across the resonant circuit to protect components of the ballast. A multiplier circuit is operationally coupled to the resonant circuit to boost the voltage clamped by the clamping circuit to a value sufficient to permit starting of the lamp. The clamping circuit and the multiplier circuit cooperate to facilitate a continuous starting of the lamp. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates a ballast circuit according to the concepts of the present application.  
         [0007]      FIG. 2  depicts in more detail a multiplier used in the ballast circuit.  
         [0008]      FIG. 3  depicts in more detail a pulsing circuit used in the ballast circuit.  
         [0009]     FIGS.  4 A-B depict a charge pump circuit that controls a power controller of the pulsing circuit.  
         [0010]      FIG. 5  shows a graph of the charge pump current vise time during the open circuit condition.  
         [0011]      FIG. 6  shows a graph of the charge pump current vise time during the time when the lamp is initially lit.  
         [0012]      FIG. 7  shows a graph of the charge pump current vise time during the steady state operation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     With reference to  FIG. 1 , a ballast circuit  10  includes an inverter circuit  12 , a resonant circuit  14 , a clamping circuit  16  and a pulsing circuit  18 . A DC voltage is supplied to the inverter  12  via a voltage conductor  20  running from a positive voltage terminal  22  and a common conductor  24  connected to a ground or common terminal  26 . A lamp  28  is powered via lamp connectors  30 ,  32 .  
         [0014]     The inverter  12  includes switches  34  and  36  such as MOSFETs, serially connected between conductors  20  and  24 , to excite the resonant circuit  14 . Typically, the resonant circuit  14  includes a resonant inductor  38  and a resonant capacitor  40  for setting the frequency of the resonant operation. A DC blocking capacitor  42  prevents excessive DC current flowing through lamp  28 . A snubber capacitor  44  allows the inverter  12  to operate with zero voltage switching where the MOSFETs  34  and  36  turn ON and OFF when their corresponding drain-source voltages are zero.  
         [0015]     Switches  34  and  36  cooperate to provide a square wave at a node  46  to excite the resonant circuit  14 . Gate or control lines  48  and  50 , running from the switches  34  and  36  respectively, each include a respective resistance  52 ,  54 . Diodes  56 ,  58  are connected in parallel to the respective resistances  52 ,  54 , making the turn-off time of the switches  34 ,  36  faster than the turn-on time. Achieving unequal turn-off and turn-on times provides a time when the switches  34 ,  36  are simultaneously in the non-conducting states to allow the voltage at the node  46  to transition from one voltage state, e.g. 450 Volts, to another voltage state, e.g. 0 Volts, by a use of residual energy stored in the inductor  38 .  
         [0016]     With continuing reference to  FIG. 1  and further reference to  FIG. 3 , gate drive circuitry, generally designated  60 ,  62 , further includes inductors  64 ,  66  which are secondary windings mutually coupled to inductor  68 . Gate drive circuitry  60 ,  62  is used to control the operation of respective switches  34  and  36 . More particularly, the gate drive circuitry  60 ,  62  maintains switch  34  “ON” for a first half of a cycle and switch  36  “ON” for a second half of the cycle. The square wave is generated at node  46  and is used to excite resonant circuit  14 . Bi-directional voltage clamps  70 ,  72  are connected in parallel to inductors  64 ,  66  respectively, each include a pair of back-to-back Zener diodes. Bi-directional voltage clamps  70 ,  72  act to clamp positive and negative excursions of gate-to-source voltage to respective limits determined by the voltage ratings of the back-to-back Zener diodes.  
         [0017]     With continuing reference to  FIG. 1 , the output voltage of the inverter  12  is clamped by series connected diodes  74  and  76  of clamping circuit  16  to limit high voltage generated to start lamp  28 . The clamping circuit  16  further includes capacitors  78 ,  80 , which are essentially connected in parallel to each other. Each clamping diode  74 ,  76  is connected across an associated capacitor  78 ,  80 . Prior to the lamp starting, the lamp&#39;s circuit is open, since an impedance of lamp  28  is seen as very high impedance. A high voltage across capacitor  42  is generated by a multiplier  82  that ignites the lamp. The resonant circuit  14  is composed of capacitors  40 ,  42 ,  78 ,  80  and inductor  38  and is driven near resonance. As the output voltage at node  84  increases, the diodes  74 ,  76  start to clamp, preventing the voltage across capacitors  78 ,  80  from changing sign and limiting the output voltage to the value that does not cause overheating of the inverter  12  components. When the diodes  74 ,  76  are clamping capacitors  78  and  80 , the resonant circuit becomes composed of the capacitor  40  and inductor  38 . Therefore, the resonance is achieved when the diodes  74 ,  76  are not conducting.  
         [0018]     With continuing reference to  FIG. 1  and further reference to  FIG. 2 , multiplier circuit  82  boosts the voltage limited by the clamping circuit  16 . The multiplier  82  is connected across capacitor  42  to terminals  84 ,  86  to achieve a starting voltage by multiplying inverter  12  output voltage at node  84 . At the beginning of the operation, inverter  12  supplies voltage to the terminals  84 ,  86 . Capacitors  90 ,  92 ,  94 ,  96 ,  98  cooperate with diodes  100 ,  102 ,  104 ,  106 ,  108 ,  110  to accumulate charge one half of a cycle, while during the other half of the cycle the negative charge is dumped into capacitor  42  through terminal  86 . Typically, when inverter  12  voltage is 500V peak to peak, the voltage across terminals  84 ,  86  rises to about −2 kVDC.  
         [0019]     The multiplier  82  is a low DC bias charge pump multiplier. During steady-state operation the multiplier  82  applies only a small dc bias (about 0.25 Volts) to the lamp which does not affect the lamp&#39;s operation or life.  
         [0020]     With continuing reference to  FIG. 1 , pulsing circuit  18  is used to turn inverter  12  “ON” and “OFF.” Typically, when lamp  28  is in an open circuit, the power dissipation of inverter  12  is about 12 to 15 W. Normally this would not cause a problem, except the cabling has to withstand a voltage of about 1.6 kVDC, setting a limitation on the use of standard cables which are typically rated at 600V RMS. The pulsing circuit  18  turns inverter  12  “ON” supplying a constant high voltage to lamp  28  for about 40-50 msec and “OFF” for the rest of the cycle. The resultant RMS is only 600V, permitting a use of conventional 600V wiring cables. In addition, such duty cycle reduces the power dissipation in the open circuit to about ⅔ W, because the inverter circuit is shut down for about 90% of the cycle.  
         [0021]     With continuing reference to  FIG. 1  and further reference to  FIG. 3 , a charge pump circuit  120  operates a control circuit  122  of pulsing circuit  18 . In one embodiment, the control circuit  122  is a UC3861 circuit manufactured by Texas Instruments, although it is to be understood that any other appropriate control circuit may also be used. The control circuit  122  is connected to terminals  26  and  86 , and to a terminal  124  of charge pump circuit  120 . The charge pump circuit  120  derives power from clamping circuit  16  through a terminal  126 . Initially, when lamp  28  is not lit, inverter  12  drives multiplier circuit  16  to a negative voltage, in this embodiment to nearly −2 kV, charging an electrolytic capacitor  128  of pump charge circuit  120 . A depletion mode switch  130  is in the conducting mode. As the negative voltage rises, voltage at a gate of switch  130  decreases negatively until switch  130  shuts off, allowing a capacitor  132  to charge through a series connected resistance  134 . The resistance  134  is connected to a 5V reference voltage of control circuit  122  through a line  136 . When capacitor  132  charges to about 2V, it enables a fault pin  138  of control circuit  122  shutting down control circuit  122  and inverter  12 . More specifically, output drivers of control circuit  122  connected to lines  140 ,  142  become disabled, turning off the primary winding  68  that supplies voltage to mutually coupled inductors  64 ,  66  of inverter  12 . The electrolytic capacitor  128  ceases to charge through the inverter  12 . The negative voltage gradually decreases reaching the value of the Under Voltage Lockout (UVLO) of control circuit  122 . At this time, control circuit  122  is reset and enters into a low quiescent current state. The low quiescent current of 15 μA allows the electrolytic capacitor  128  to charge through a line  144  connected to terminal  124 . The capacitor  128  charges through series connected resistances  146 ,  148 . When the voltage rises to about 16.5V, e.g. UVLO threshold voltage of the UC386881, the control circuit  122  enables the output drivers which turn “ON” inverter  12 . The inverter  12  starts driving multiplier  82 , negatively charging capacitor  128 . The process repeats until lamp  28  ignites.  
         [0022]     With continuing reference to  FIGS. 1 and 3  and further reference to FIGS.  4 A-B, charge pump circuit  120  derives power from a component of inverter  12  resonant capacitance. FIGS.  4 A-B illustrate an operational flow occurring in charge pump circuit  120  when it is powered by a power source  152 . More particularly, when inverter  12  is in the “ON” state, capacitor  80  is periodically charged and discharged through capacitor  128 . With continuing reference to  FIG. 4A , during the first half of the cycle, capacitor  80  accumulates the charge as the current through capacitor  80  flows counterclockwise. With continuing reference to  FIG. 4B , during the second half of the cycle, the accumulated charge is dumped into capacitor  128 . More specifically, during the second half of the cycle, the current changes direction to clockwise. A diode  160 , connected in series with capacitor  80  and capacitor  128 , is conducting, allowing capacitor  128  to charge through capacitor  80 . The voltage is regulated by a Zener diode  162  which is connected across capacitor  128 . Typically, the voltage is limited to 14V.  
         [0023]     With reference to  FIGS. 5-7 , charge pump circuit  120  is shown to be independent of the lamp&#39;s state. When lamp  28  is in an open circuit, its resistance is about 1MΩ, and the current flowing into charge pump  120  is about 77 mA as illustrated in  FIG. 5 . When lamp  28  first lights, its resistance is about 5Ω, and the current flowing into charge pump circuit  120  is about 51 mA as illustrated in  FIG. 6 . When lamp  28  is in a steady state, its resistance is about 51K, and the current flowing into charge pump circuit  120  is about 68 mA as illustrated in  FIG. 7 . As shown in FIGUES 5-7, the current flowing into charge pump circuit  120  and control circuit  122  does not substantially change when the lamp changes its state from the open circuit to steady state. This design acts to prevent high heat dissipation on Zener diode  162 .  
         [0024]     While it is to be understood the described circuit may be implemented using a variety of components with different components values, provided below is a listing for one particular embodiment when the components have the following values:  
                                             Component                Name/Number   Component Values                       Switch 34   20NMD50           Switch 36   20NMD50           Inductor 38   90 μH           Capacitor 40   22 nF, 630 V           Capacitor 42   33 nF, 2 kV           Capacitor 44   680 pF, 500 V           Resistor 52   100 Ω           Resistor 54   100 Ω           Diode 56   1N4148           Diode 58   1N4148           Inductor 64   1 mH           Inductor 66   1 mH           Diode Clamp 70   1N4739, 9.1 V           Diode Clamp 72   1N4739, 9.1 V           Diode 74   8ETH06S           Diode 76   8ETH06S           Capacitor 78    1 nF, 500 V           Capacitor 80    1 nF, 500 V           Capacitors 90, 92, 94, 98, 100   150 pF, 2 kV           Diodes 100, 102, 104, 106, 108, 110   1 kV           Capacitor 128   100 μF, 25 V           Switch 130   2N4391           Capacitor 132   47 nF           Resistor 134   1 MΩ           Resistors 146, 148   220 KΩ           Diode 160   1N4148           Zener Diode 162   14 V                      
 
         [0025]     The exemplary embodiment has been described with reference to the illustrated embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.