Patent Publication Number: US-2012025728-A1

Title: Hid lamp ignitor

Description:
The technical field of this disclosure is ignition circuits for lamps, particularly, ignitors for high intensity discharge (HID) lamps. 
     High Intensity Discharge (HID) lamps, such as mercury vapor, metal halide, high-pressure sodium and low-pressure sodium light sources, are used for a variety of lighting tasks. The HID lamps can be electrically driven by electromagnetic or electronic ballasts. The HID lamp resistance is large when the lamp is off, so a large voltage from an ignition circuit must be applied to the lamp to start the lamp. Unfortunately, conventional ignition circuits present a number of limitations. 
       FIG. 1  is a schematic diagram of an ignitor for an HID lamp. The ignitor  20  includes a timer circuit  30  and a high voltage (HV) pulse circuit  40 . The timer circuit  30  generates a series of narrow trigger signals  32 , which are provided to the HV pulse circuit  40 . The HV pulse circuit  40  generates a HV pulse at the lamp output  42  in response to the trigger signal  32 . 
     In operation, capacitor C 9  in the HV pulse circuit  40  charges to DC bus voltage through resistor R 5 . When the narrow trigger signal  32  turns on switch Z 4 , the capacitor C 9  and primary winding of transformer L 3  form an LC tank circuit. Current oscillates through the primary winding of transformer L 3  by travelling through the switch Z 4  in one direction and through the diode D 8  in the opposite direction. Transformer L 3  is a boost transformer, so the oscillating current generates the HV pulse at the lamp output  42  to ignite the lamp. After the trigger signal  32  turns off the switch Z 4 , the capacitor C 9  charges again to DC bus voltage through the resistor R 5  and the cycle can repeat. The charging time is longer than the time that the trigger pulse turns on the switch Z 4  to allow the capacitor C 9  to fully charge. 
     The component limitations in the ignitor  20  restrict the pulse repetition rate which can be achieved from the ignitor  20 . The capacitance of capacitor C 9  is selected to provide the desired high voltage for the HV pulse at the lamp output  42 . The resistance value of resistor R 5  determines how quickly the capacitor C 9  can be charged, so a small resistance is required when a high pulse repetition rate is desired. The pulse repetition rate, defined as the number of times capacitor C 9  can be discharged in one second and corresponds to the HV pulse rate. Unfortunately, a high repetition rate deposits more energy in the small resistor R 5  than it can dissipate, damaging the resistor R 5 . Therefore, resistor R 5  must be sized to limit the pulse repetition rate and avoid damage, even though a high repetition rate and HV pulse rate is more effective in igniting the lamp. Conventional ignition circuits are typically limited to a repetition rate of less than 1 kHz. 
     Conventional ignition circuits are also affected by variations at the lamp output for parameters such as lead length and output circuit components. Such variations can reduce the ignition pulse voltage, making it difficult or impossible to light the lamp. 
     It would be desirable to have a HID lamp ignitor that would overcome the above disadvantages. 
     One aspect of the present invention provides an ignitor for a lamp including a transformer having a primary winding inductively coupled to a secondary winding, the secondary winding being operably connected to a lamp output operable to receive the lamp; a first switch circuit operably connected between DC voltage and a junction point, the first switch circuit being responsive to a first switch signal; a second switch circuit operably connected between the junction point and common, the second switch circuit being responsive to a second switch signal; and an LC tank circuit having the primary winding operably connected in series with a capacitor, the LC tank circuit being operably connected between the junction point and the common. The first switch signal alternates with the second switch signal to close the first switch circuit and the second switch circuit. 
     Another aspect of the present invention provides an ignitor for a lamp including a transformer having a primary winding inductively coupled to a secondary winding, the secondary winding being operably connected to a lamp output operable to receive the lamp; an LC tank circuit having the primary winding operably connected in series with a capacitor, the LC tank circuit being operably connected between a junction point and common; and means for switching the junction point between DC voltage and the common at a predetermined frequency. 
     Another aspect of the present invention provides an ignitor system for a lamp including a timer operable to generate a timing signal; a level shifter responsive to the timing signal to generate a first switch signal and a second switch signal; a transformer having a primary winding inductively coupled to a secondary winding, the secondary winding being operably connected to a lamp output operable to receive the lamp; a first switch circuit operably connected between DC voltage and a junction point, the first switch circuit having a first switch operably connected in parallel with a first diode, the first switch being responsive to the first switch signal, a first diode cathode of the first diode being operably connected to the DC voltage; a second switch circuit operably connected between the junction point and common, the second switch circuit having a second switch operably connected in parallel with a second diode, the second switch being responsive to the second switch signal, a second diode cathode of the second diode being operably connected to the junction point; and an LC tank circuit having the primary winding operably connected in series with a capacitor, the LC tank circuit being operably connected between the junction point and the common. The first switch signal alternates with the second switch signal to alternately close the first switch and the second switch. 
     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention, rather than limiting the scope of the invention being defined by the appended claims and equivalents thereof. 
    
    
     
         FIG. 1  is a schematic diagram of an ignitor system for an HID lamp; 
         FIG. 2  is a schematic diagram of an ignitor system for an HID lamp in accordance with the present invention; 
         FIGS. 3A-3D  are voltage traces for an ignitor for an HID lamp in accordance with the present invention; 
         FIGS. 4A &amp; 4B  are schematic diagrams of an ignitor with a voltage limiter for an HID lamp in accordance with the present invention; 
         FIG. 5  is a schematic diagram of an ignitor system with lamp feedback for an HID lamp in accordance with the present invention; 
         FIGS. 6A &amp; 6B  are voltage traces for an ignitor with lamp feedback for an HID lamp in accordance with the present invention; 
         FIGS. 7A &amp; 7B  are voltage traces for an ignitor with lamp feedback employing a pulse polarity mode and synchronization mode for an HID lamp in accordance with the present invention; 
         FIG. 8  is a schematic diagram of another embodiment of an ignitor for an HID lamp in accordance with the present invention. 
     
    
    
       FIG. 2  is a schematic diagram of an ignitor for an HID lamp in accordance with the present invention. The ignitor system  60  includes a timer  70 , a level shifter  80 , and an ignitor  90 . Two switches in the ignitor  90  alternate with each other to alternately charge and discharge a LC tank circuit and provide a high voltage (HV) ignition pulse to a lamp. 
     The ignitor  90  includes a transformer  92 , a first switch circuit  100 , a second switch circuit  120 , and an LC tank circuit  140 . The transformer  92  has a primary winding  94  inductively coupled to a secondary winding  96 . The secondary winding  96  is operably connected to a lamp output  98  operable to receive a lamp (not shown). 
     A pair of switch circuits switches a junction point to which the LC tank circuit is operably connected between DC voltage and common at a predetermined frequency. A first switch signal  112  from the level shifter  80  alternates with the second switch signal  132  from the level shifter  80  to alternately close a first switch  106  and a second switch  126 . The first switch circuit  100  is operably connected between DC voltage  102  and a junction point  104 . The first switch circuit  100  has the first switch  106  operably connected in parallel with a first diode  108 . The first switch  106  is responsive to the first switch signal  112  and a first diode cathode  110  of the first diode  108  is operably connected to the DC voltage  102 . The second switch circuit  120  is operably connected between the junction point  104  and common  124 . The second switch circuit  120  has the second switch  126  operably connected in parallel with a second diode  128 . The second switch  126  is responsive to the second switch signal  132  and a second diode cathode  130  of the second diode  128  is operably connected to the junction point  104 . 
     The LC tank circuit  140  includes the primary winding  94  of the transformer  92  operably connected in series with a capacitor  142 . The LC tank circuit  140  is operably connected between junction point  104  and common  124 . 
     The timer  70  generates a timing signal  72 , which is provided to the level shifter  80 . The level shifter  80  generates the first switch signal  112  and the second switch signal  132  in response to a timing signal  72 . The first switch signal  112  alternates with the second switch signal  132  to alternately close the first switch  106  and the second switch  126 . The level shifter  80  can include an IR2104S half bridge driver integrated circuit. In one embodiment, the timing signal  72  is a square having a 50 percent duty cycle, i.e., the timing signal  72  is high for one half of a cycle and low for the other half of the cycle. The timer  70  can include a 555alt timer integrated circuit. 
     In operation, the timer  70  provides the square wave timing signal  72  to the level shifter  80 , which alternately provides the first switch signal  112  and the second switch signal  132  to the first switch  106  and the second switch  126 , respectively. When the first switch  106  is initially closed with the second switch  126  open, the DC voltage  102  is provided to the junction point  104  and across the LC tank circuit  140 . The current through the LC tank circuit  140  charges the capacitor  142  and oscillates to induce an ignition pulse in the lamp through the transformer  92 . Current passes alternately in one direction from the DC voltage  102  through the first switch  106  and in the opposite direction through the first diode  108 . The second switch  126  is open and the second diode  128  blocks current flow from the junction point  104  to common  124  through the second switch circuit  120 . The capacitor  142  charges to the voltage of the DC voltage  102 . 
     The timer  70  changes the state of the timing signal  72 , which causes the level shifter  80  to reverse the states of the first switch signal  112  and the second switch signal  132 . This closes the first switch  106  and opens the second switch  126 , so the junction point  104  is switched to common  124 . The current through the LC tank circuit  140  discharges the voltage across the capacitor  142  and oscillates to induce an ignition pulse in the lamp through the transformer  92 . Current passes alternately in one direction from the junction point  104  through the second switch  126  and in the opposite direction through the second diode  128 . The first switch  106  is open and the first diode  108  blocks current flow from the DC voltage  102  junction point  104  to the junction point  104  through the second switch circuit  120 . The capacitor  142  discharges to zero. 
     In another embodiment, the ignitor system  60  optionally includes an open circuit voltage (OCV) feedback circuit that monitors OCV at the lamp output  98 . 
       FIGS. 3A-3D  are voltage traces for an ignitor for an HID lamp in accordance with the present invention. The voltage traces are measured for the ignitor system of  FIG. 2 . In one example, the ignition pulse has a voltage of about 2.3 kV with a frequency of about 2 kHz as shown in  FIGS. 3C &amp; 3D . In another example, the frequency can be as high as about 4 kHz or the like as shown in  FIGS. 3A &amp; 3B . The maximum frequency achievable can be limited by tank current decay time. The first switch circuit  100  is turned on after the second switch circuit  120  current decays to zero, and vice versa, so in theory the maximum frequency equals 1/(2T decay ). In one embodiment, a small inductor can be placed in series with the LC tank circuit  140  to limit current change rate, such as between junction point  104  and the primary winding  94  of the transformer  92 . Power dissipation in the small inductor increases with frequency, so the small inductor can limit maximum frequency in some embodiments. 
       FIG. 3A  is a voltage trace across the lamp at the lamp output. The ignition pulse is superimposed on the more slowly cycling open circuit voltage provided by the lamp ballast.  FIG. 3B  is a voltage trace across the capacitor in the LC tank circuit. The voltage oscillates initially on each state change as the first and second switches alternately open to generate the ignition pulse at the lamp. After the initial oscillation peak, the voltage across the capacitor alternately decays to DC voltage or zero.  FIGS. 3C &amp; 3D  are details on an expanded time scale of the voltage trace across the lamp at the lamp output. 
       FIGS. 4A &amp; 4B , in which like elements share like reference numbers with  FIG. 2 , are schematic diagrams of an ignitor with a voltage limiter for an HID lamp in accordance with the present invention. The output compensation accounts for variations in lead length, output circuit components, and the like, by limiting the voltage across the primary of the transformer to a predetermined voltage. This assures that the ignition pulse is sufficient regardless of variations. 
     Referring to  FIG. 4A , the ignitor  90  further includes a voltage limiter, which in this embodiment is a transient voltage suppressor (TVS)  160  operably connected in parallel with the primary winding  94  of the transformer  92 . The TVS  160  conducts when the voltage across the primary winding  94  exceeds a predetermined voltage, so that the voltage at the lamp output  98  remains constant at the desired value. The predetermined voltage can be selected as desired for a particular application as desired by selecting a particular TVS. The TVS  160  dissipates energy from current through the TVS  160  as heat. 
     Referring to  FIG. 4B , the ignitor  90  further includes a voltage limiter, which in this embodiment is a full wave bridge  170 . The full wave bridge  170  is operably connected across the primary winding  94  of the transformer  92  through tertiary winding  172 , and operably connected to the DC voltage  102 . The full wave bridge  170  conducts when the voltage across the primary winding  94  exceeds a predetermined voltage, so that the voltage at the lamp output  98  remains constant at the desired value. The predetermined voltage can be selected as desired for a particular application by selecting the turns ratio between the primary winding  94  and the tertiary winding  172 . The full wave bridge  170  returns energy from current through the full wave bridge  170  to the DC bus, so the energy is recovered. 
     In selecting the components for the ignitor with a voltage limiter, the longest lead wire desired at the lamp output or an equivalent capacitance can be specified. The capacitance of the capacitor in the LC tank circuit selected so the ignition pulse height at the lamp output is greater than or equal to the minimum desired pulse height. The lead wire at the lamp output or an equivalent capacitance can then be switched to the shortest lead wire desired. The predetermined voltage at which the voltage limiter conducts can be selected as the voltage which limits the ignition pulse height at the lamp output to the maximum desired pulse height. For the TVS, the voltage conduction value can be specified in selecting the TVS device. For the full wave bridge, the voltage conduction value can be selected by specifying the turns ratio between the primary and tertiary windings in the transformer in the LC tank circuit. 
       FIG. 5 , in which like elements share like reference numbers with  FIG. 2 , is a schematic diagram of an ignitor system with lamp feedback for an HID lamp in accordance with the present invention. Lamp feedback allows the ignition pulses to be coordinated with the open circuit voltage to the lamp. The lamp ballast can operate in a frequency switching mode, a pulse polarity mode, and/or in a synchronization mode. Referring to  FIG. 5 , lamp ballast  200  provides power to a lamp  202  at lamp output  98 . The lamp ballast  200  includes an ignitor system  60 , which includes a timer  70 , a level shifter  80 , and an ignitor  90 ; a lamp power supply  210 ; and a lamp feedback circuit  220 . The lamp power supply  210  provides AC power  212  to the lamp  202  during and after ignition. In one embodiment, the AC power  212  is a square wave. In another embodiment, the AC power  212  is a sine wave. The lamp feedback circuit  220  is responsive to an open circuit voltage (OCV) signal  222  from the lamp output  98  to generate a lamp operation state signal  226  provided to the lamp power supply  210  and/or a timing signal  224  provided to the level shifter  80 . 
     In operation in a frequency switching mode, the lamp feedback circuit  220  monitors the OCV signal  222  to determine whether the lamp  202  is in startup or steady state operation, and sets the frequency of the AC power  212  to one frequency when the lamp  202  is in startup operation and another frequency when the lamp  202  is in steady state operation. In one embodiment, the frequency is lower when the lamp is in startup operation and higher when the lamp is in steady state operation. 
     During startup operation, the OCV signal  222  indicates the lamp  202  is off, i.e., the OCV is high, and the lamp feedback circuit  220  generates a lamp operation state signal  226  indicating the lamp  202  is off. The lamp power supply  210  is responsive to the lamp operation state signal  226  and sets the AC power  212  to a lower frequency. When the lamp is in steady state operation, the OCV signal  222  indicates the lamp  202  is on, i.e., the OCV is low, and the lamp feedback circuit  220  generates a lamp operation state signal  226  indicating the lamp  202  is on. The lamp power supply  210  is responsive to the lamp operation state signal  226  and sets the AC power  212  to a higher frequency. In one embodiment employing the frequency switching mode, the lamp ballast  200  is a low frequency square wave electronic ballast operating in accordance with the appropriate ANSI Standard. 
       FIGS. 6A &amp; 6B  are voltage traces for an ignitor with lamp feedback employing a frequency switching mode for an HID lamp in accordance with the present invention.  FIG. 6A  is an example of AC power voltage at a lower frequency when the lamp is in startup operation and  FIG. 6B  is an example of AC power voltage at a higher frequency when the lamp is in steady state operation. The frequency switching mode can be used to determine the number of ignition pulses that occur in each cycle of the AC power. The amplitude of the voltage is not to scale for clarity of illustration. 
     Referring to  FIG. 6A , the AC power voltage  250  for the lamp in startup operation includes a square wave component  251  at a lower frequency with two ignition pulses  252  superimposed every half cycle. Referring to  FIG. 6B , the AC power voltage  254  for the lamp in steady state operation includes a square wave component  255  at a higher frequency with one ignition pulse  252  superimposed every half cycle. The lower frequency is one half the higher frequency in this example. In one embodiment, the lower frequency is about 70 Hz, the higher frequency is about 140 Hz, the ignition pulse frequency is 280 Hz, the ignition pulse width is greater than 1.0 microseconds, and the ignition pulse voltage is about 2.7 kV. Those skilled in the art will appreciate that the parameters for the AC power and ignition pulses can be selected as desired for a particular application. 
     Referring to  FIG. 5 , in operation in a pulse polarity mode, the lamp feedback circuit  220  monitors the OCV signal  222  to determine whether the polarity of the AC power  212  is positive or negative, and sets the polarity of the ignition pulse to match the polarity of the AC power  212 . Having the AC power and the ignition pulse of like polarity provides the greatest voltage to ignite the lamp since the momentary AC power voltage adds to the ignition pulse voltage, increasing the chance of ignition. 
     During startup operation, the lamp feedback circuit  220  monitors the OCV signal  222 . When the polarity of the instantaneous OCV is positive, the timing signal  224  directs the level shifter  80  to close the first switch  106  and open the second switch  126 , so the initial swing in the ignition pulse is positive. This adds the power pulse voltage to the instantaneous OCV. For example, when the instantaneous OCV is +300 Volts and the initial swing in the ignitor pulse is +2700 Volts, the resulting ignition voltage to the lamp  202  is +3000 Volts. When the polarity of the instantaneous OCV is negative, the timing signal  224  directs the level shifter  80  to close the second switch  126  and open the first switch  106 , so the initial swing in the ignition pulse is negative. This subtracts the power pulse voltage from the instantaneous OCV. For example, when the instantaneous OCV is −300 Volts and the initial swing in the ignition pulse is −2700 Volts, the resulting ignition voltage to the lamp  202  is −3000 Volts. Those skilled in the art will appreciate that the pulse polarity mode can be used for ignitors with lamp power supplies providing AC power as a square wave or a sine wave. The effect in the superposition of like polarity instantaneous OCV with ignition pulse can be seen in  FIG. 3C : the amplitude of consecutive ignition pulses on a single polarity OCV half cycle switches between consecutive ignition pulses, with the greater magnitude when the OCV and ignition pulse are of the polarity. 
     Referring to  FIG. 5 , the pulse polarity mode can optionally include operation in a synchronization mode in which the ignition pulse is synchronized with the AC power cycle to occur at a given time, or the synchronization mode can be used independently of the pulse polarity mode. During startup operation, the lamp feedback circuit  220  monitors the OCV signal  222  for zero crossing. After the lamp feedback circuit  220  detects a zero crossing, the timing signal  224  directs the level shifter  80  to change the state of the first switch  106  and the second switch  126  after a predetermined time. Thus, the ignition pulse occurs both with the desired like polarity as the OCV and at the desired time in the AC power cycle. When the AC power  212  is a square wave, the desired time can be selected so the ignition pulse occurs in a stable portion of the square wave. When the AC power  212  is a sine wave, the desired time can be selected so the ignition pulse occurs at the peak of the like polarity instantaneous OCV to maximize ignition pulse voltage to the lamp  202 . 
       FIGS. 7A &amp; 7B  are voltage traces for an ignitor with lamp feedback employing a pulse polarity mode and synchronization mode for an HID lamp in accordance with the present invention.  FIG. 7A  is an example of the AC power as a square wave, such as delivered by an electronic HID ballast, with a single like polarity ignition pulse occurring each OCV half cycle.  FIG. 7B  is an example of the AC power as a sine wave, such as delivered by an electro-magnetic ballast, with a single like polarity ignition pulse synchronized to the peak of each OCV half cycle. 
       FIG. 8  is a schematic diagram of another embodiment of an ignitor for an HID lamp in accordance with the present invention. Lamp feedback allows the ignition pulses to be coordinated with the open circuit voltage to the lamp. The lamp ballast can operate in a pulse polarity mode and/or in a synchronization mode. 
     Lamp ballast  300  provides power to a lamp  302  at lamp output  398 . The lamp ballast  300  includes an ignitor  390 , a lamp power supply  310 , and a lamp feedback circuit  320 . The lamp power supply  310  provides AC power  312  to the lamp  302  during and after ignition. In one embodiment, the AC power  312  is a square wave, such as provided by an electronic HID ballast. In another embodiment, the AC power  312  is a sine wave, such as delivered by an electro-magnetic ballast. The lamp feedback circuit  320  is responsive to an open circuit voltage (OCV) signal  322  to alternately generate a first switch timing signal  312  and a second switch timing signal  332  provided to the ignitor  390 . 
     The ignitor  390  includes a first switch  306 , second switch  326 , and a center tap transformer  392 . The center tap of the primary winding  394  of the center tap transformer  392  is operably connected to DC voltage  302  and the ends of the primary winding  394  are operably connected to common  324  through the first switch  306  and second switch  326 . The first switch  306  is responsive to the first switch timing signal  312  and the second switch  326  is responsive to the second switch timing signal  332 . The first switch  306  and second switch  326  can be field effect transistors (FETs), bipolar transistors, or insulated gate bipolar transistors (IGBTs). The lamp  302  is operably connected to the lamp power supply  310  through the secondary winding  396  of the center tap transformer  392 . 
     During startup operation, the lamp feedback circuit  320  monitors the OCV signal  322 . For operation in the pulse polarity mode, the first switch timing signal  312  directs the first switch  306  to close and the second switch timing signal  332  directs the second switch  326  to remain open when the lamp feedback circuit  320  determines that the instantaneous OCV is positive. The current through the primary winding  394  generates an ignition pulse of the same positive polarity as the instantaneous OCV. When the lamp feedback circuit  320  determines that the instantaneous OCV is negative, the second switch timing signal  332  directs the second switch  326  to close and the first switch timing signal  312  directs the first switch  306  to remain open. The current through the primary winding  394  generates an ignition pulse of the same negative polarity as the instantaneous OCV. 
     For operation in the synchronization mode during startup operation, the lamp feedback circuit  320  monitors the OCV signal  322  for zero crossing. The lamp feedback circuit  320  directs closure of one of the first switch  306  or the second switch  326  a predetermined time after the lamp feedback circuit  320  detects a zero crossing. The lamp feedback circuit  320  generates the first switch timing signal  312  which directs the first switch  306  to close to generate a positive ignition pulse. The lamp feedback circuit  320  generates the second switch timing signal  332  which directs the second switch  326  to close to generate a negative ignition pulse. The predetermined time can be selected so the ignition pulse is synchronized with the desired point in the AC power  312 . The synchronization mode can be used in conjunction with the pulse polarity mode to generate an ignition pulse of the same polarity as the AC power synchronized to the desired point on the AC power.  FIGS. 7A &amp; 7B  as discussed above provide exemplary voltage traces for operation in the pulse polarity mode and synchronization mode. 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the scope of the invention. For example, those skilled in the art will appreciate that switches other than transistors can be used as desired for a particular application. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.