Abstract:
The current invention provides a power supply that includes an igniter that generates an ignition voltage for igniting a DC lamp; an auxiliary power stage that outputs an auxiliary voltage for sustaining sufficient current in the DC lamp after the DC lamp is ignited; a voltage conversion stage coupled to the auxiliary power stage and generating a voltage at a level that is higher than the auxiliary voltage; and a switch that couples the auxiliary voltage to the DC lamp and the voltage conversion stage for a predefined period of time.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to power supplies and more particularly to power supplies that ignite and power high-intensity arc lamps. 
     BACKGROUND 
     High-intensity arc lamps emit light with extremely high brightness for use in projection display systems, for example, conference room projectors, home theatre projectors, etc. Such lamps are powered by a direct current (DC) voltage ranging from 12 V to 25 V and a DC current ranging from 20 A to 50 A. Operating the lamp requires a high voltage ignition pulse of up to 35 kV, depending on the temperature and gas pressure within the arc tube of the lamp. An arc sustaining circuit supplies a sufficient current that sustains the arc for turning on the lamp. As a result, a special power supply, known as a ballast, is utilized for these lamps. 
       FIG. 1  shows a block diagram of a known high-intensity arc lamp ballast that powers a lamp  107  by an alternating current (AC) power source  101 . The lamp ballast is composed of an EMI filter  102 , a bridge rectifier  103 , a power factor correction (PFC) circuit  104 , a DC/DC voltage converter  106 , an auxiliary power supply  108 , an arc sustaining circuit  109 , and an igniter  110 . The PFC circuit  104  converts an AC input voltage  101  to a DC voltage, i.e., V B  of 380 V˜400 V, and shapes the input current to reduce its harmonic contents and improve system efficiency. The full-bridge converter  106  converts DC voltage V B  to a voltage required by lamp  107 . The auxiliary power supply  108  generates suitable voltages for igniter  110  and arc sustaining of lamp  107 . 
     More detailed description of the ballast circuit of prior art for high-wattage arc lamps can be made by referring to  FIG. 2 . The PFC stage is not shown in the figure and well known by those skilled in the art. Both full-bridge DC/DC converter  209  and auxiliary power supply  108  (e.g. a flyback converter) receives PFC output voltage V B  as the input. The full-bridge DC/DC converter  209  is composed of switches Q 3 -Q 6 , DC voltage blocking capacitor Cb, transformer T 4 , diodes D 1  and D 2 , and inductor Lig. Because lamp  107  has aging effect, i.e., the lamp impedance increases with time, full-bridge DC/DC converter  209  powers lamp  107  preferably with a constant-power control during normal operation to avoid excessive lamp power when a constant-current control is used. Flyback converter  108  converts V B  to V C1  to provide an input for igniter  110  and an arc sustaining current through switch Q 2  and current-limiting resistor R 1  right after the lamp ignition. 
     When switch Q 2  is turned on, the voltage at the cathode of diodes D 1  and D 2  becomes voltage V C1 . The voltage at the anode of diodes D 1  and D 2  is the voltage across the secondary winding of transformer T 4 , which is equal to V B ·(N s /N p ), where N p  and N s  are the turn numbers of the primary and secondary windings of transformer T 4 , respectively. Voltage V C1  is typically in the range of 100 V˜200 V and ensures adequate arc sustaining current after lamp  107  is ignited. Assuming a V B  of 400 V and an N s /N p  ratio of 3/28, the voltage at the anode of diodes D 1  and D 2  would be 43 V. This voltage ensures that diodes D 1  and D 2  do not conduct when switch Q 2  is turned on since both diodes are reverse biased. 
     Igniter  110  of  FIG. 2  has two stages. The first stage includes a resistor Rig 1 , energy storage capacitor Cig 1 , silicon diode for alternating current (SIDAC)  226 , and transformer T 1 . SIDAC  226  conducts current in either direction but only after its breakdown voltage has been reached. Before lamp  107  is ignited, switch Q 2  is turned on, and voltage V C1  provides a charging current which flows through switch Q 2 , resistor R 1 , and resistor Rig 1  to charge capacitor Cig 1 . When the increased voltage across capacitor Cig 1  turns on SIDAC  226 , a voltage pulse is generated across the secondary winding of transformer T 1 , which charges storage capacitor Cig 2 . Capacitor Cig 1  discharges quickly as SIDAC  226  conducts current. The voltage across capacitor Cig 1  is charged up again when SIDAC  226  turns off as the current flowing through SIDAC  226  is lower than its holding current. This operation continues as long as switch Q 2  remains on. The second stage of igniter  110  includes spark-gap  219 , diode  227 , and transformer T 2 . Once the voltage across capacitor Cig 2  reaches the break-over voltage of spark-gap  219 , a voltage pulse is generated across the secondary winding Lig of transformer T 2  to strike lamp  107 . The benefit of using a two-stage igniter is that the input voltage at the primary side of ignition transformer T 2  is boosted by the first stage, thereby allowing the use of a lower turns ratio for the secondary-to-primary winding of transformer T 2 . A lower number of secondary turns decreases power loss at high current for lamp  107 . The turning on or off of switch Q 2  is controlled by a control circuit  229 . 
     After lamp  107  is ignited, switch Q 2  is kept on for a period of 100 μs-500 μs before it is turned off. During this period, energy-storage capacitor C 1  is discharged, and a current flows through switch Q 2 , resistor R 1 , and winding Lig to sustain the arc in lamp  107 . When the ignition period is over, igniter  110  stops generating voltage pulses as the maximum voltage across capacitor Cig 1  becomes comparable with the operating voltage of lamp  107 , which is well below the turn-on threshold of SIDAC  226 . Meanwhile, spark-gap  219  is turned off, leading to an open-circuit condition for the primary side of transformer T 2 . Thus, the secondary winding of transformer T 2  and its magnetic core form an inductor Lig. After switch Q 2  is turned off, full-bridge DC/DC converter  209  takes over and provides the required DC current through inductor Lig for operating lamp  107 . 
     As can be seen from  FIG. 2 , before lamp  107  is ignited, the voltage across diodes D 1  and D 2  is the sum of voltage V C1  and the reflected voltage V B ·(N s /N p ) across the secondary winding of transformer T 4 . As a result, diodes D 1  and D 2  should have a voltage rating higher than the sum of V B ·(N s /N p ) and V C1 . 
     Assuming the voltage rating of diodes D 1  and D 2  is V D , V C1  needs to be lower than V D −V B ·(N s /N p ) to ensure safe operation of these output diodes. Therefore, voltage V C1  for the igniter input is ultimately limited by the voltage rating of diodes D 1  and D 2 . This leads to the choice of either larger size and less reliable igniters or output diodes with high voltage ratings but an accompanying higher power loss of the diodes and subsequent significant loss of efficiency. 
     Therefore, there exists a need for a power supply having low power loss and high efficiency for igniting and powering a lamp with an arc sustaining circuit. 
     SUMMARY 
     Briefly, according to some embodiments of the present invention, a power supply for a DC lamp comprises an igniter, an arc sustaining circuit, an auxiliary power stage, a voltage conversion stage, and a full-bridge DC/DC converter. The igniter generates an ignition voltage for igniting the DC lamp. The auxiliary power stage outputs an auxiliary voltage for sustaining sufficient current in the DC lamp after the DC lamp is ignited. The voltage conversion stage coupled to the auxiliary power stage generates a voltage at a level that is higher than the auxiliary voltage and a switch couples the auxiliary voltage to the DC lamp and voltage conversion stage for a predefined period of time. 
     According to some of the more detailed features of the present invention, a control circuit controls the switch in response to detection of a drop of the auxiliary voltage after the DC lamp is ignited and the voltage conversion stage comprises a voltage multiplier. The auxiliary power stage can be a flyback power stage with at least one of a secondary winding or an auxiliary winding and a DC/DC converter that is coupled to the DC lamp after the predefined period, with the converter having output diodes with ratings commensurate with the auxiliary voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a block diagram of a conventional ballast for a high-intensity arc lamp. 
         FIG. 2  shows further details of the block diagram of  FIG. 1 . 
         FIG. 3  shows a block diagram of a power supply for igniting and sustaining the ignition arc according to an exemplary embodiment of the invention. 
         FIG. 4  shows one exemplary circuit diagram in the embodiment of  FIG. 3 . 
         FIG. 5  shows another exemplary circuit diagram in the embodiment of  FIG. 3 . 
         FIG. 6  shows still another exemplary circuit diagram in the embodiment of  FIG. 3 . 
         FIG. 7  shows yet another exemplary circuit diagram in the embodiment of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 3  shows a block diagram for an arc-lamp ballast that incorporates an exemplary embodiment of the invention. The lamp ballast is composed of an EMI filter  102 , a bridge rectifier  103 , a PFC circuit  104 , a DC/DC voltage converter  106 , an auxiliary power supply  108 , an arc sustaining circuit  109 , a voltage multiplier  302 , a lamp status and control circuit  229 , and an igniter  110 . Voltage V AUX1  is for providing an arc-sustaining current after the lamp ignition and also serves as one input of voltage multiplier  302 . Voltage V M  is for driving igniter  110 . These two voltages are generated from auxiliary power supply  108  independently. Switch  301  is used to connect/disconnect one of the auxiliary outputs, i.e., V AUX1 , to/from voltage multiplier  302  and arc sustaining circuit  109 . Auxiliary output voltage V AUX2  is connected to voltage multiplier  302 . Before the lamp ignition, switch  301  is turned on by lamp status detection and control circuit  229  to provide an input voltage for voltage multiplier  302  and a path for arc sustaining current  109  to flow right after the lamp ignition. As voltage V M  increases, an ignition pulse is generated at the output of igniter  110  to ignite lamp  107 . After lamp  107  is ignited and turned on for a few hundred microseconds, switch  301  is turned off so that no arc sustaining current continues to flow to lamp  107  and voltage V AUX1  is disconnected from multiplier  302 . Output voltage V M  of voltage multiplier  302  then decreases and no further ignition pulse is generated during normal operation of lamp  107 . The DC/DC voltage converter  106  takes over and continues to provide driving current for lamp  107  immediately after arc sustaining circuit  109  stops the current flow. 
       FIG. 4  shows one exemplary circuit implementing full-bridge DC/DC converter  209 , auxiliary power supply  108 , voltage multiplier  302 , arc sustaining circuit  109 , and igniter  110 . Full-bridge DC/DC voltage converter  209  and auxiliary power supply  108  are powered by DC voltage V B , which can be the output of a PFC stage (not shown). Auxiliary power supply  108  serves two functions. The first function is to generate igniter input voltage V M  at the output of voltage multiplier  302 . The other is to provide an arc sustaining voltage immediately after lamp  107  is ignited. In  FIG. 4 , the input of igniter  110  is generated across capacitor C 2 . Under this arrangement, igniter voltage V M  equals V C2  and voltage V AUX1 , generated across capacitor C 1 , equals V C1 . An arc sustaining current flows through switch  301 , diode D 5 , and current limiting resistor R 1 . Full-bridge DC/DC converter  209  converts voltage V B  (e.g. 380 V˜400 V DC) to a voltage required by lamp  107  during normal operation. 
     After DC voltage V B  is applied to the input of auxiliary power supply  108 , auxiliary power converter  108  starts operating and switch  301  is also turned on. When switch Q 1  is turned on, the secondary winding of flyback transformer T 3  induces a negative voltage V AUX2  at the anode of diode D 3  so that diode D 3  is turned off since it is reverse biased. At the same time, diode D 4  is forward biased and current i charge  flows through the secondary winding of flyback transformer T 3 , capacitor C 1 , switch  301 , capacitor C 2 , and resistor R 2 , charging capacitor C 2 . During conduction of switch Q 1 , magnetic energy is stored in flyback transformer T 3 . 
     When switch Q 1  is turned off, the secondary winding of flyback transformer T 3  induces a positive voltage at the anode of diode D 3  so that diode D 3  starts conducting and diode D 4  is turned off. As a result, the stored magnetic energy is released into capacitor C 1 , increasing the voltage across capacitor C 1 . This operation continues until voltage V C1  across capacitor C 1 , reaches a preset voltage. 
     During the conducting period of switch Q 1 , voltage V AUX2  at the anode of diode D 3 , referred to the secondary ground, is: 
                 V     AUX   ⁢           ⁢   2       =       -       N   sec       N   pri         ⁢     V   B         ,         
where N pri  and N sec  are the primary and secondary turns number of flyback transformer T 3 , respectively. As a result, voltage V C2  across capacitor C 2 , i.e., the igniter input voltage V M  is:
 
 V   M   =V   C2   =V   AUX1   −V   AUX2   =V   C1   +V   B ( N   sec   /N   pri )  (1)
 
where V C1  is the voltage across capacitor C 1 , V C2  is the voltage across capacitor C 2 , and V B  is the bus voltage provided by PFC circuit  104 .
 
     As can be seen from the above equation 1, igniter input voltage V M  is always higher than arc sustaining voltage V C1 . In one exemplary embodiment, arc sustaining voltage V C1  is in the range of 100 V-200 V. This level provides adequate arc sustaining current after lamp  107  is ignited. However, the voltage at the anode of diodes D 1  and D 2  is much lower, e.g., 43 V for V B =400 V and N s /N p =3/28. This results in diodes D 1  and D 2  being reverse biased while switch  301  remains turned on. 
     The exemplary embodiment of igniter  110  of the current invention includes two stages. In the first stage, capacitor Cig 1  is charged by voltage V C2  through resistor Rig 1 . When the voltage across capacitor Cig 1  reaches the turn-on threshold of SIDAC  226 , SIDAC  226  starts conducting and generates a voltage pulse across the secondary winding of transformer T 1  to charge storage capacitor Cig 2  in the second stage. Once the voltage across capacitor Cig 2  reaches the break-over voltage of spark-gap  219 , spark-gap  219  turns on and a voltage pulse is generated across the secondary winding of transformer T 2  to strike lamp  107  with an ignition voltage pulse. 
     Once ignited, lamp  107  exhibits low impedance, and a discharging current of capacitor C 1  flows to lamp  107  through switch  301 , diode D 5 , and resistor R 1 . This leads to a sudden drop of voltage V C1 . The lamp status detection and control circuit  229  detects the drop and after a predefined delay turns off switch  301 . The delay enables the discharging current of storage capacitor C 1  to flow through lamp  107  and sustain the arc in lamp  107 . Resistor R 1  limits the discharging current to prevent damage to lamp  107 . Diode D 5  prevents capacitor C 2  from being charged by the voltage at the cathode of diodes D 1  and D 2 , thereby avoiding undesired operation of igniter  110  after lamp  107  is turned on. 
     In the embodiment of the invention as shown in  FIG. 4 , the arc sustaining voltage is V C1  and the igniter input voltage is V C2 , where V C2  is higher than V C1  according to equation 1. The maximum rating voltage for diodes D 1  and D 2  is:
 
 V   D   =V   B ( N   s   /N   p )+ V   C1 .  (2)
 
     For example, assuming an arc sustaining voltage V C1  of 100 V, a V B  of 400 V, an N p  of 28, and an N s  of 3, the reverse bias voltage across diodes D 1  and D 2  is 145 V. In comparison, the circuit of  FIG. 2  with a V C1  of 200V under similar conditions has a reverse bias voltage of 243 V across diodes D 1  and D 2 , almost 100 V higher. As a result, the current invention enables the use of output diodes with much lower voltage ratings than known in the art while providing much higher igniter input voltage. 
     In the exemplary embodiment of  FIG. 4 , diodes with lower than 200 V ratings, such as Schottky diodes with low forward voltage drop and fast recovery, can be used to implement the present invention. Therefore, the power loss associated with the output diodes is reduced significantly. 
     Moreover, in  FIG. 4 , by selecting an N pri  of 102 and an N sec  of 63, voltage V M (=V C2 ), at the input of voltage multiplier  302 , can be as high as 350 V. With higher voltage V M  supplied to igniter  110 , SIDAC  226  can have a higher breakdown voltage, leading to a higher primary voltage pulse for transformer T 1  when SIDAC  226  is turned on. A higher voltage pulse across the primary winding of transformer T 1  enables the use of lower secondary-to-primary turns ratios, leading to reduction of the sizes of transformers T 1  or T 2 . With a lower secondary-to-primary turns ratio, transformer T 2  can use a smaller turns number for its secondary winding Lig, resulting in a significant reduction of power loss of secondary winding Lig when the current through lamp  107  is high. 
     Finally, according to some embodiments of the current invention, energy storage capacitor Cig 2  can be charged to a higher voltage because of the higher primary voltage of transformer T 1 . This significantly reduces the probability of failure to fire spark-gap  219  resulting from tolerance of the break-over voltage and aging effect of SIDAC  226 . 
     While arc sustaining circuit  109  can be implemented by a flyback transformer, any suitable arrangement may be used, including providing igniter input voltage V M  via a variety of voltage multipliers. 
       FIG. 5  shows another exemplary implementation according to the invention. Capacitor C 2  is charged by voltage V C1  and the voltages across the secondary winding of flyback transformer T 3 , when switch  301  is turned on. In this embodiment, the igniter input voltage is:
 
 V   M   =V   C2   =V   C1   +V   B (N sec1   +N   sec2 )/ N   pri ,  (3)
 
where N sec1  and N sec1  are the turns number of the first and second secondary winding of flyback transformer T 3 , respectively. With an arc sustaining voltage V C1  of 100 V and V B  of 400 V, the reverse bias voltage across output diodes D 1  and D 2  is approximately 145 V, if N p =28 and N s =3. Meanwhile, voltage V C2  can be as high as 594 V by selecting N pri 102, N sec1 =63, and N sec2 =63. Adjusting N sec2  can lead to a desired input voltage for igniter  110 .
 
       FIG. 6  shows still another exemplary implementation according to the invention where the igniter input voltage V M  is:
 
 V   M   =V   C2   +V   C3 =2( V   C1   +V   B   ·N   sec   /N   pri ).  (4)
 
     Output diodes D 1  and D 2  still exhibit a voltage stress of approximately 145 V, whereas the input voltage for igniter  110  can be as high as 694 V if V C1 =100 V, V B =400 V, N pri =102, and N sec =63. This embodiment requires capacitors C 2 , C 3  and C 4  to have a voltage rating of at least the sum of V C1  and V B N sec /N pri . 
     An even higher voltage rating can be obtained with further extensions to voltage multiplier  302  in  FIG. 6 . 
       FIG. 7  shows yet another exemplary implementation according to the invention where the igniter input voltage V M  is:
 
 V   M   =V   C4 =2( V   C1   +V   B   N   sec   /N   pri ).  (5)
 
     The voltage stress for output diodes D 1  and D 2  is the same as that in  FIG. 6 . However, this embodiment requires capacitors C 3  and C 4  to have a higher voltage rating. Specifically, a voltage rating of at least 2V C1 +V B N sec /N pri  for capacitor C 3  and a voltage rating of 2(V C1 +V B N sec /N pri ) for capacitor C 4 , respectively. Persons of ordinary skill in the art will know how to achieve even higher voltage rating with further extension to voltage multiplier  302  in  FIG. 7  by following the true spirit of this invention. 
     The examples and embodiments described herein are non-limiting examples. The invention is described in detail with respect to exemplary embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the claims is intended to cover all such changes and modifications as fall within the true spirit of the invention.