Abstract:
An igniter circuit ( 10 ) for an HID lamp ( 11 ) has a DC input (VBUS) for coupling to a source of DC voltage, and an output ( 23, 24 ) for coupling to the HID lamp. A resonant ignition circuit ( 12 ) operating at a controlled resonant frequency is coupled to the DC input for producing successive bursts of voltage having a frequency equal to the resonant frequency and having an amplitude that increases with time. The resonant ignition circuit ( 12 ) feeds the bursts of voltage across the output of the igniter until an HID lamp coupled thereto reaches breakdown.

Description:
FIELD OF THE INVENTION 
     This invention relates to igniters for high intensity discharge lamps. 
     BACKGROUND OF THE INVENTION 
     The function of a high-intensity discharge (HID) electronic ballast is to supply ignition to the lamp for starting and then operating the lamp, such as a metal halide lamp. A metal halide lamp is a gas discharge lamp in which metal halides are enclosed, for example, in a quartz envelope. 
     To initiate its operation, a metal halide lamp demands a high ignition voltage. Once the lamp is ignited, the voltages falls to low voltage of the order of 20 V and the lamp it is then maintained for a short time (typically between 1-2 minutes) in so-called “current mode” where the current is constant and the voltage rises until the lamp reaches nominal power, whereafter the ballast serves to stabilize the power. 
     Prior art igniter circuits are known where an uncontrolled oscillator frequency is swept from a frequency that is less than the resonant frequency such that when it reaches resonance the voltage reaches maximum value and the lamp strikes. However, during this operation the frequency continues to rise and the voltage therefore falls. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an igniter circuit for an HID lamp that employs a self-oscillating power supply for applying across the lamp a high ignition voltage that increases with time. 
     It is a further object to provide such an igniter circuit that is configured for coupling directly to an inverter having a half bridge topology for feeding low frequency current to the lamp after ignition. 
     These objects are realized in accordance with a first aspect of the invention by an igniter circuit for an HID lamp, the igniter circuit comprising: 
     a DC input for coupling to a source of DC voltage, 
     an output for coupling to the HID lamp, and 
     a resonant ignition circuit operating at a controlled resonant frequency coupled to said DC input for producing successive bursts of voltage having a frequency equal to the resonant frequency and having an amplitude that increases with time and for feeding said bursts of voltage across the output of the igniter until an HID lamp coupled thereto reaches breakdown. 
     According to a second aspect of the invention, there is provided a method for igniting a HID lamp, the method comprising: 
     using a resonant circuit connected across the lamp to generate successive bursts of voltage having a frequency equal to the resonant frequency and having an amplitude that increases with time; and 
     applying said bursts of voltage across the HID lamp until the lamp ignites, thereby loading the resonant circuit so that its Q factor falls sufficiently to stop the resonant circuit resonating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram showing the functionality of an igniter circuit for an HID lamp in accordance with an exemplary embodiment of the invention; 
         FIG. 2  is a schematic circuit diagram of the igniter circuit shown functionally in  FIG. 1 ; 
         FIG. 3  is a waveform showing graphically a series of ignition pulses fed to the with the igniter circuit shown in  FIG. 2 ; and 
         FIG. 4  is a waveform showing graphically a resonant frequency voltage whose amplitude increases with time and that is applied to the HID lamp prior to ignition. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a block diagram showing the functionality of an igniter circuit  10  according to the invention for an HID lamp  11 . The igniter circuit  10  is operated from a DC power source, VBUS, which is not itself part of the igniter circuit  10  and may be generated in manner well known to those skilled in the art. The DC power source, VBUS, is fed to a controlled self-oscillator  12  comprising a self oscillator  13  controlled by an ignition pulse control circuit  14 . The HID lamp  11  is coupled to the controlled self-oscillator  12  which constitutes an igniter circuit for igniting the lamp. The lamp is powered by a power supply  15 , which conveniently is coupled to the DC power source, VBUS, although it could be independent thereof. In order to disable operation of the igniter circuit after ignition of the HID lamp  11 , a power sensor  16  is responsively coupled to the DC power source, VBUS, so as to sense the current supplied thereby. Before the lamp  11  ignites the current sensed by the power sensor  16  is low; but once the lamp  11  ignites it draws power from the DC power source, VBUS. The power sensor  16  thus serves to indicate whether or not the HID lamp  11  has ignited. The ignition pulse control circuit  14  is responsively coupled to the power sensor  16  so as to cease operation when the power sensor  16  senses that the HID lamp  11  has ignited. For the sake of completeness, although not relevant to the igniter circuit per se, the power sensor  16  serves a second function in that the power supply  15  includes a power regulator (not shown) that is responsive to the power sensed by the power sensor  16  for stabilizing the nominal power to the lamp  11 . 
       FIG. 2  is a schematic circuit diagram showing in detail a preferred embodiment of the igniter circuit  10  shown in  FIG. 1 . 
     Resonant Ignition Circuit 
     The oscillator  13  comprises a drive transformer T 1  having first, second and third windings  20 ,  21  and  22 , respectively, which are connected in the correct polarity for positive feedback (oscillation). A first end of the first winding  20  is coupled to the source terminal of a first MOSFET M 1  whose gate terminal is coupled via a resistor R 1  to a second end of the first winding  20 . The drain terminal of the first MOSFET M 1  is coupled to VBUS, typically having a voltage of 400 VDC. A pair of zener diodes D 1  and D 2  is coupled back to back across the first winding  20 , their anodes being commonly connected and their respective cathodes being connected to opposite ends of the first winding  20 . The zener diodes D 1 , D 2 , limit the gate voltage fed to the MOSFET M 1  and thereby ensure that when the resonant voltage increases, it does not damage the gate of the MOSFET M 1 . 
     In complementary trimmer, the first end of the second winding  21  is coupled via a resistor R 2  to the gate terminal of a second MOSFET M 2  whose source terminal is coupled to a second end of the second winding  21  and constitutes the ground rail, GND. The drain terminal of the second MOSFET M 2  is coupled to the source terminal of the first MOSFET M 1 . A pair of zener diodes D 3  and D 4  is coupled back to back across the first winding  20 , their anodes being commonly connected and their respective cathodes being connected to opposite ends of the second winding  21 . The zener diodes D 3  and D 4  limit the gate voltage fed to the MOSFET M 2  and thereby ensure that when the resonant voltage increases, it does not damage the gate of the MOSFET M 2 . The first end of the second winding  21  is coupled to an ‘ON’ control output of an ON-OFF splitter  25 , its second end being coupled to GND. An ‘OFF’ control output of the ON-OFF splitter  25  is connected to the gate of the MOSFET M 2 . An input of the ON-OFF splitter  25  is connected to an output of the ignition pulse control circuit  14 , as will be described in more detail below. The ON-OFF splitter  25  serves to convey an ignition pulse conveyed by the ignition pulse control circuit  14  to the winding  21  of the drive transformer T 1  to enable the oscillation process; and to convey a disable signal to the gate of the MOSFET M 2  to prevent oscillation after the lamp  11  has ignited. 
     A first end of the third winding  22  of the oscillator drive transformer T 1  is connected to a first capacitor C 1  connected in series with a first end of a resistor R 3 , whose second end is coupled to the common junction of a split winding of a transformer T 2 , comprising windings L 1 , L 2 . The coils L 1  and L 2  are wound such that a first end of the coil L 1  is connected to a second end  23  of the coil L 2 , whose first end is connected to a first end of the HID lamp  11 . A second end of the third winding  22  is connected to the common junction of the two MOSFETs M 1  and M 2 , i.e. to the source of M 1  and to the drain of M 2 . 
     The DC power source, VBUS, comprises pair of large series connected electrolytic capacitors C 2  and C 4  connected between VBUS and GND, their common junction  24  being connected to a second end of the HID lamp  11  and to the second end of the coil L 1  via a capacitor C 5 . The capacitors C 2  and C 4  serve as storage capacitors for storing DC voltage for powering the controlled self-oscillator  12  and the power supply  15 . The power supply  15  operates as a low frequency square wave current source controlled power shown as  26  in  FIG. 2  that is connected to the common junction of the coil L 1  and the capacitor C 5 . The low frequency square wave current source is produced in known manner by an inverter (not shown). Preferably, the inverter is a half-bridge topology of which the capacitors C 2  and C 4  are integral components. The junction of the capacitors C 2  and C 4  and the first end of the coil L 2  constitute output terminals of the igniter circuit  10  across which the HID lamp  11  is coupled. 
     Having described the topology of the resonant ignition circuit  12 , its operation will now be described. 
     The resonant ignition circuit is constituted by M 1  and M 2 , R 1 , R 2 , D 1 , D 2 , D 3 , D 4 , T 1 , C 1 , R 3 , L 1 , C 4  (short), C 5  (short) and its resonant frequency f 0  is determined by C 1 , L 1  in accordance with the equation: 
     
       
         
           
             
               f 
               0 
             
             = 
             
               1 
               
                 2 
                 ⁢ 
                 π 
                 ⁢ 
                 
                   
                     L 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       1 
                       · 
                       C 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
               
             
           
         
       
     
     C 4  and C 5  have very low impedance at the resonant frequency and so practically behave as short circuits. The Q factor is determined by the values of R 1 , R 2 , R 3 . The resistors R 1  and R 2  together with the input capacitances of the gates of the two MOSFETs M 1  and M 2  create a phase shift which causes a reduction in the resonant voltage fed to the lamp. 
     The Q factor determines the maximum peak voltage that may be fed to the HID lamp  11  before breakdown, which may be several kilovolts, whereafter the voltage fed to the lamp falls to a low voltage, typically in the order of 20V and is maintained at constant current until it reaches the nominal power of the lamp. 
     A train of ignition pulses shown graphically in  FIG. 3  at the resonant frequency f 0  is fed to the junction between the source of M 1  and the drain of M 2  through the resonant circuit constituted by C 1  and L 1 , so that the resonant circuit resonates with increasing amplitude for the duration of each ignition pulse as shown graphically in  FIG. 4  due to the positive feedback produced by the windings of the drive transformer, T 1 . At the end of each ignition pulse, the amplitude of the resonant lamp voltage decreases until it reaches substantially zero until the arrival of the next ignition pulse, when the cycle is repeated. As noted, C 4  has low impedance at the resonant frequency and acts as a short circuit. 
     When the lamp  11  starts to conduct, the lamp acts as a low impedance, and the current through the lamp fed by the low frequency current source  26  (corresponding to the power supply  15  shown in  FIG. 1 ) flows through L 1  and L 2  which together operate as a choke, which filters some of the high frequency ripple. C 5  acts as a first filter for removing the high frequency ripple superimposed on the low frequency current. C 2  and C 4  whose mid-point voltage is equal to half VBUS form part of a half bridge inverter that serves to supply low frequency current to the lamp  11  after ignition; and are thus integral components of the power supply shown as  15  in  FIG. 1  and of the low frequency current source shown as  26  in  FIG. 2 . 
     Before lamp breakdown, the transformer T 2  serves as the lamp igniter; and after breakdown when the lamp starts to conduct in the current mode, it serves as a choke for removing the high frequency ripple. 
     The object is to generate a high voltage waveform with increasing amplitude that is applied to the lamp as shown graphically in  FIG. 4 . When the lamp voltage reaches a certain voltage (1 kV-4 kV depending on lamp temperature), the lamp ignites. When this happens, the lamp impedance falls to a low value and loads the resonant circuit so that its Q factor falls significantly and it stops resonating. The self-oscillation circuit stops the oscillator coil T 1  from oscillating. 
     Ignition Pulse Control Circuit 
     As noted above, the oscillator  13  stops oscillating when the HID lamp  11  ignites owing to the fact that the low lamp impedance after ignition loads the resonant circuit causing a marked reduction in its Q factor. However, rather than rely on this alone, it is considered preferable to disable the ignition circuit once the lamp has ignited, this being achieved by the igniter pulse control circuit  14 . The igniter pulse control circuit  14  comprises a comparator  27  having a positive input to which a reference voltage signal PREF is fed and having a negative input coupled to the power sensor  16  so as to receive a voltage signal PIN that is proportional to the power across the HID lamp  11 . Ignition pulses shown graphically in  FIG. 3  having a duty cycle determined by T ON  and T OFF  are fed to one input of a 2-input AND-gate  28  while the logic signal at the output of the comparator is fed to the second input of the AND-gate  28 . Before the lamp starts conducting, PIN is low and the comparator output is logic HIGH; the AND-gate  28  therefore conveys the ignition pulses to the ON-OFF splitter  25 . When the lamp ignites, PIN is larger than PREF and the output of the comparator  27  goes to LOW, whereupon the AND-gate  28  stops feeding the ignition pulses to the ON-OFF splitter  25 . 
     The oscillator  13  is self-controlled to operate at the resonant frequency as determined by C 1  and L 1  such that although differences in the values of C 1  and L 1 , as may occur in mass production owing to component tolerances will give rise to different resonant frequencies, the oscillator  13  will always operate at resonant frequency. 
     Moreover, the resonant frequency at which the oscillator  13  resonates is also a function of the parasitic capacitance of the wires connecting the HID lamp  11  to the resonant ignition circuit  12 , being a function of their length. Therefore, the oscillator  13  resonates at resonant frequency regardless of the length of the wires connecting the HID lamp  11  to the resonant ignition circuit  12 .