Abstract:
A method and apparatus for protecting a discharge lamp lighting device from damage due to mis-wiring of a source of electrical power to the discharge lamp lighting device. The protection apparatus includes a detector, a comparer and an inhibitor. The detector samples at least one monitor point associated with the discharge lamp lighting device to obtain at least one detection voltage. The comparer compares the at least one detection voltage with a reference voltage. The inhibitor inhibits an operation of the discharge lamp lighting device when the comparer determines that a mis-wiring of the source of electrical power to the discharge lamp lighting device exists.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a discharge lamp lighting device for lighting a discharge lamp, and a lighting system that includes the discharge lamp lighting device.  
         [0003]     2. Background and Related Information  
         [0004]     In recent years, electronic ballasts employing inverter technology have become popular for use as a discharge lamp lighting device for lighting a discharge lamp. Conventionally, built-in type ballasts (also referred to as OEM type ballasts) have been the primary form of commercial discharge lamp lighting device that have been produced. OEM type ballasts are defined as a discharge lamp lighting device that is delivered to a lighting fixture factory that incorporates the discharge lamp lighting device (ballast) into a lighting fixture produced in the factory, which then ships the finished product for sale.  
         [0005]     In recent years, the demand for so-called “indoor type ballasts” (also referred to as or retrofit type ballasts) has increased. A retrofit type ballast comprises a discharge lamp lighting device that is delivered to a job site for connection to at least one light fixture that has previously been installed on the job site. The retrofit type ballast is typically placed near the light fixture, or may be wired into the fixture itself.  
         [0006]     Retrofit type ballasts generally include an input terminal unit and an output terminal unit. The input terminal unit comprises, for example, a terminal block to which electrical leads are connected, or just electrical wires, that connect the ballast to a commercial power source that supplies AC electrical power. The output terminal unit comprises, for example, a terminal block to which electrical wires are connected, or just electrical wires, that connect the ballast to a lighting fixture (i.e., discharge lamp).  
         [0007]     It is more likely that a wiring error will occur with respect to the installation of a retrofit ballast by an electrician or do-it-yourself installer, as compared to the installation of an OEM ballast by a fixture manufacturer, especially when the ballast is to be installed in a place having poor visibility for the installer, such as, but not limited to, for example, on a ceiling. For example, the installer may mistake the output terminals for the input terminals, and connect the commercial power supply to the output terminal and thereafter, turn ON the commercial power supply (hereafter, this situation will be referred to as an input-output misconnection), damaging the ballast. The installer may also unintentionally connect one end (or both ends) of the output terminal to a fixture that is electrically grounded to earth, while the commercial power supply is connected to the input terminals and the high-pressure discharge lamp is connected to the output terminals (hereafter, this situation will be referred to as a ground misconnection), which again, may result in damage to the ballast when the commercial power supply is applied to the incorrectly wired ballast.  
         [0008]     The operation of a discharge lamp lighting device when an input-output misconnection occurs with a ballast comprising a buck chopper and polarity reversing combination topology will be described with reference to  FIG. 1B  of the drawings, which illustrates a portion of a discharge lamp lighting device of the present invention. It is noted that while the following discussion is provided with respect to a discharge lamp lighting device that employs a buck chopper circuit, the analysis is very similar for a full bridge circuit that omits the buck chopper circuit.  
         [0009]     When an installer mistakenly connects a commercial power supply  110  to an external output unit  112  and turns ON the external power supply, an AC power supply voltage is applied from the commercial power supply  110  through the external output unit  112  to connection point B associated with switching elements Q 3  and Q 4 , and connection point C associated with switching elements Q 5  and Q 6 . When this occurs, the AC power supply voltage is rectified by a diode bridge formed by diodes D 3 , D 4 , D 5  and D 6 , which are parasitic diodes of the switching elements Q 3 , Q 4 , Q 5  and Q 6 , respectively. The rectified voltage is applied to capacitor C 1  in a DC power supply  102  via inductor L 2  and diode D 7  (which is a parasitic diode of switching element Q 2 ) of a buck chopper circuit  104 , which charges the capacitor C 1 .  
         [0010]     When capacitor C 1  is charged, the voltage on capacitor C 1  (i.e., a voltage at point A in  FIG. 1B ) is supplied to control an auxiliary power supply unit  109 , which provides electrical power to a DC power supply controller  107  and an inverter controller  108 . Upon being supplied with the power supply voltage (electrical power) for operation, the inverter controller  108  starts a switching operation for lighting a high-pressure discharge lamp  113 . In other words, the switching elements Q 3 ,Q 4 ,Q 5  and Q 6  are switched ON and/or OFF, as shown in  FIG. 2-1 , to alternate the DC voltage output from the buck chopper circuit  104  and to generate a high pulse voltage in conjunction with the igniter circuit in the polarity reversing circuit  105 . The high pulse voltage is applied through the external output unit  112  to the high-pressure discharge lamp  113 .  
         [0011]     Since the AC power supply voltage from the commercial power supply  110  is being applied to the ballast through the external output unit  112  to connection point B of switching elements Q 3  and Q 4  and connection point C of switching elements Q 5  and Q 6 , when switching element Q 4  is switched ON by the inverter controller  108 , a current path is formed from connection point C to connection point B through switching element Q 4  and diode D 6 . A shunt current flows from connection point C to connection point B through the commercial power supply  110 , which is connected to the external output unit  112 . Thus, one or more of the switching element Q 4 , diode D 4 , and switching element Q 6  (with its parasitic diode D 6 ), may be damaged or destroyed.  
         [0012]     Similarly, when switching element Q 6  is switched ON by inverter controller  108 , a current path is formed from connection point B to connection point C through switching element Q 6  and diode D 4 . A shunt current flows between connection points B and C through the commercial power supply  110 , which is connected to the external output unit  112 . Thus, one or more of the switching element Q 6 , diode D 6 , and switching element Q 4  with its parasitic diode D 4 , may be damaged or destroyed.  
         [0013]     Thus, a problem arises. Specifically, when the installer mistakenly connects the commercial power supply  110  to the external output unit  112 , the discharge lamp lighting device  101  may fail. It is noted that such a problem is not limited to the above described example. Whenever a power supply voltage is applied to the external output unit  112 , the above-described problem may occur with respect to a discharge lamp lighting device having a configuration in which: (a) the auxiliary power supply unit  109  generates a power supply from the commercial power supply  110  for the operation of other circuit blocks; (b) the switching operation starts for an inverter unit  103  to supply an AC voltage to an external output unit  112  as soon as the inverter controller  108  is energized by the auxiliary power supply unit  109 ; and (c) the impedance looking into the output terminals of the external output unit becomes extremely small because of the switching action of the inverter unit.  
         [0014]     The following explains the operation of a discharge lamp lighting device  101  when the ground misconnection occurs in a ballast having the buck chopper and polarity reversing combination topology in which the protector of the present invention (to be discussed below) is not included. It is noted that the following analysis would be similar for a full bridge topology that omits the buck chopper circuit.  
         [0015]     When the installer mistakenly connects one end of the commercial power supply  110  to one end (or both ends) of the external output unit  112 , directly or indirectly through earth ground, and switches ON the external power supply while the commercial power supply  110  is connected to the external voltage receiving unit  111 , an AC power supply voltage is applied from the commercial power supply  110  to connection point B of switching elements Q 3  and Q 4  and/or to connection point C of switching elements Q 5  and Q 6 . The AC power supply voltage is rectified by bridge DB 1 , and applied to capacitor C 1  (via inductor L 1  and diode D 1 ) to charge capacitor C 1  to a peak value of the commercial power supply voltage.  
         [0016]     When capacitor C 1  is charged, the voltage on capacitor C 1  (e.g., the voltage at connection point A in  FIG. 1B ) energizes auxiliary power supply unit  109 , which in turn supplies electrical power to the DC power supply controller  107  and the inverter controller  108  for the operation of the DC power supply circuit  102  and the inverter unit  103 , respectively. Upon being supplied with electricals power, inverter controller  108  starts the switching operation to light the high-pressure discharge lamp  113 , as was described above. In other words, the switching elements Q 3 , Q 4 , Q 5  and Q 6  are switched ON and/or OFF, as shown in  FIG. 2-1 , to alternate the DC voltage output from the buck chopper circuit and to generate a high pulse voltage in conjunction with the igniter circuit in the polarity reversing circuit  105 . The high pulse voltage is applied through the external output unit  112  to the high-pressure discharge lamp  113 .  
         [0017]     Since one end of the commercial power supply  110  is connected to connection point B of switching elements Q 3  and Q 4  and/or connection point C of switching elements Q 5  and Q 6 , when switching element Q 4  is switched ON by the inverter controller  108 , a current path is formed from connection point B to switching element Q 4  to bridge DB 1  to commercial power supply  110  and back to connection point B. As a result, a very low impedance path is formed, and a shunt current flows through switching element Q 4 . Thus, switching element Q 4  may be damaged or destroyed.  
         [0018]     Similarly, when switching element Q 6  is switched ON by the inverter controller  108 , a low impedance current path is formed from connection point C through switching element Q 6 . Shunt current flows from connection point C through commercial power supply  110  and back to connection point C, potentially damaging or destroying switching element Q 6 .  
         [0019]     Therefore, when the installer mistakenly connects one end of the commercial power supply  110  to one or both ends of the external output unit  112  directly (or indirectly) through earth ground while the commercial power supply  110  is connected to the external voltage receiving unit  111 , the discharge lamp lighting device  101  may be damaged.  
         [0020]     It is noted that such a problem is not limited to the above described example. A similar problem may occur with respect to a discharge lamp lighting device  101  that has a configuration in which: (a) the auxiliary power supply unit  109  generates a power supply from a commercial power supply for the operation of other circuit blocks; (b) the switching operation starts for the inverter unit  103  to supply an AC voltage to an external output unit  112  as soon as the inverter controller  108  is energized by the auxiliary power supply  109 ; and (c) the impedance looking between one end of the input terminal and one or both ends of the output terminal  112  becomes extremely small because of the switching operation of the inverter unit  103 .  
         [0021]     The present invention addresses the above-described problems. According to a feature of the present invention, the occurrence of a failure due to an input-output misconnection and/or ground misconnection can be avoided or minimized. In the present invention, the auxiliary power supply unit  109  is deliberately energized at an initial start-up, so that the inverter controller  108  is energized. A protector is provided that functions to determine electrical connection characteristics of the discharge lamp lighting device and determine whether the polarity reversing circuit of the discharge lamp lighting device can be safely operated. If the protector determines that the electrical connection characteristics represent a mis-wiring situation, the protector inhibits the operation of the switching elements Q 3  to Q 6  of the discharge lamp lighting device.  
         [0022]     In order to achieve the above-describe objective, a discharge lamp lighting device of the present invention includes an external voltage receiving unit, a DC power supply unit, an inverter unit, an external output unit, a controller and a auxiliary power supply unit. The external voltage receiving unit receives an input voltage from the external power supply. The DC power supply unit generates a regulated DC voltage from the power supply voltage received at the external voltage receiving unit. The inverter unit converts the DC voltage that is generated by the DC power supply unit to a periodic AC voltage to light a high-pressure discharge lamp. The external output unit supplies the AC voltage generated by the inverter unit to the external discharge lamp. The inverter controller controls the operation of the inverter unit. The auxiliary power supply unit that is connected to the output of the DC power supply unit generates the power supply voltage for the operation of the inverter controller.  
         [0023]     According to this configuration, if the commercial power supply voltage is applied to the external output unit, the auxiliary power supply unit generates a power supply voltage for the operation of the inverter controller, while the protector functions to protect the discharge lamp lighting device from failure.  
         [0024]     Even if one end of the commercial AC power supply is connected to one or both ends of the external output unit, directly or indirectly, through earth ground while the commercial AC power supply voltage is applied to the external voltage receiving unit, the protector will function to protect the discharge lamp lighting device from failure.  
         [0025]     The protector comprises a detector, a comparer and an inhibitor. The comparer compares a voltage between at least one point of an internal circuit of the discharge lamp lighting device (or an equivalent value of the voltage), sampled by the detector, with a reference voltage (or an equivalent value of the reference voltage). The inhibitor restricts any switching operation of the polarity reversing circuit based upon the result of the comparison during a period from when the AC power supply voltage is applied to the discharge lamp lighting device (ballast) to when the switching operation starts for the inverter unit to output a voltage to the external output unit.  
         [0026]     According to the above, a regulated DC voltage is generated by the DC power supply unit from the power supply voltage received from the external voltage receiving unit. The regulated DC power supply voltage is converted to a periodic AC voltage by the inverter unit. The AC voltage is supplied to the external output unit to energize the discharge lamp. If a power supply voltage is mistakenly applied to the external output unit, or one end of the AC power supply is mistakenly connected to one or both ends of the external output unit, directly or indirectly, through earth ground while it is still connected to the external voltage receiving unit, a voltage between two points of the internal circuit is detected and compared. In response to the comparison, the operation of the inverter unit is selectively prevented. As a result, the formation of a shunt current loop through the power supply and the internal switching elements is prevented, thereby preventing damage to the discharge lamp lighting device (ballast).  
         [0027]     According to an object of the present invention, an apparatus is disclosed that protects a discharge lamp lighting device from damage resulting due to mis-wiring of a source of electrical power to the discharge lamp lighting device. The protector comprises a detector that samples at least one monitor point associated with the discharge lamp lighting device to obtain at least one detection voltage, a comparer that compares the at least one detection voltage with a reference voltage, and an inhibitor that inhibits an operation of the discharge lamp lighting device when a result of the comparison indicates that a mis-wiring of the power supply to the discharge lamp lighting device exists.  
         [0028]     According to a feature of the invention, the at least one detection voltage is obtained by sampling a voltage at a junction of a switching element associated with a polarity reversing circuit of the discharge lamp lighting device. The inhibitor determines that the mis-wiring exists when the at least one detection voltage is greater than the reference voltage that is less than the square root of 2 (e.g., approximately 1.414) times a commercial power supply.  
         [0029]     According to another feature of the invention, the at least one detection voltage is obtained by sampling an output voltage of a DC power supply of the discharge lamp lighting device, and the comparer determines that a mis-wiring of the power supply to the discharge lamp lighting device exists when the sampled output voltage does not exceed the reference voltage. Alternatively, the at least one detection voltage is obtained by sampling an output voltage of a buck chopper of the discharge lamp lighting device, and the comparer determines that a mis-wiring of the power supply to the discharge lamp lighting device exists when the sampled output voltage does not exceed the reference voltage. Still further, the at least one detection voltage may be obtained by sampling an output voltage of a rectifier of the discharge lamp lighting device, with the comparer determining that a mis-wiring of the power supply to the discharge lamp lighting device exists when the sampled output voltage does not exceed the reference voltage.  
         [0030]     According to another object of the invention, a method is disclosed for protecting a discharge lamp lighting device from damage due to mis-wiring of a source of electrical power to the discharge lamp lighting device. At least one monitor point associated with the discharge lamp lighting device is detected to obtain at least one detection voltage that is compared with a reference voltage. The operation of the discharge lamp lighting device, such as a switching operation of a polarity reversing circuit, is inhibited when a result of the comparison of the at least one detection voltage with the reference voltage determines that a mis-wiring of the power supply to the discharge lamp lighting device exists.  
         [0031]     According to a feature of the invention, an output voltage is detected at a junction of a pair of switching elements of the polarity reversing circuit of the discharge lamp lighting device, a switching operation of the pair of switching elements being inhibited when the comparing of the detected output voltage with the reference voltage indicates that the detected output voltage is greater than the reference voltage.  
         [0032]     According to another feature of the invention, the switching operation of the polarity reversing circuit of the discharge lamp lighting device is inhibited when the comparison of the at least one detection voltage with the reference voltage indicates that the at least one detection voltage exceeds the reference voltage.  
         [0033]     A still further feature of the invention is that the switching operation of the polarity reversing circuit of the discharge lamp lighting device is inhibited when the comparison of the at least one detection voltage with the reference voltage indicates that the reference voltage exceeds the at least one detection voltage.  
         [0034]     According to another object of the invention, an apparatus is disclosed for lighting a discharge lamp. The apparatus includes a DC power supply that generates a predetermined DC voltage in response to an AC power source from an external voltage receiver, a DC power supply controller that controls an operation of a switching element of the DC power supply, an inverter having a plurality of switching elements that changes the predetermined DC voltage to an AC voltage sufficient to light a discharge lamp, an inverter controller that controls an operation of the plurality of switching elements of the inverter, an external outputted that supplies the AC voltage from the inverter to the discharge lamp, an auxiliary power supply that generates an operating voltage to power the DC power supply controller and the inverter controller based upon the power source from an external voltage receiver, the auxiliary power supply being configured to generate the operating voltage to power the inverter controller even if the AC power source is supplied to the external outputter, and a protector that operates to inhibit the operation of the plurality of switching elements of the inverter in response to a comparison of a monitor voltage obtained from the discharge lamp lighting apparatus with a reference voltage.  
         [0035]     According to a feature of the invention, the monitor voltage represents a voltage at a junction of a pair of the plurality of switching elements of the inverter, and the protector inhibits the operation of the plurality of switching elements when the monitored voltage is determined to exceed the reference voltage, while enabling the operation of the plurality of switching elements when the monitored voltage is determined to be less than the reference voltage.  
         [0036]     According to another feature of the invention, the monitor voltage represents the predetermined DC voltage of the DC power supply, and the protector inhibits the operation of the plurality of switching elements when the predetermined DC voltage is determined to be less than the reference voltage, while enabling the operation of the plurality of switching elements when the DC power supply is determined to be greater than the reference voltage.  
         [0037]     According to a variation of the invention, the inverter includes a buck chopper, and the monitor voltage represents an output voltage of the buck chopper. The protector inhibits the operation of the plurality of switching elements when the output voltage of the buck chopper is determined to exceed the reference voltage, while enabling the operation of the plurality of switching elements when the output voltage of the buck chopper is determined to be less than the reference voltage. In this variation, the reference voltage is significantly less than a normal output voltage of the buck chopper.  
         [0038]     According to another variation, the inverter includes a buck chopper, with the monitor voltage representing an output voltage of the buck chopper. The protector functions to inhibit the operation of the plurality of switching elements when the output voltage of the buck chopper is determined to be less than the reference voltage, and enables the operation of the plurality of switching elements when the output voltage of the buck chopper is determined to be greater than the reference voltage. In this variation, the reference voltage approximates a normal output voltage of the buck chopper.  
         [0039]     In another variation, the DC power supply comprises a boost chopper, with the monitor voltage representing an output voltage of an AC-to-DC voltage converter. The protector inhibits the operation of the plurality of switching elements when the output voltage of the AC-to-DC voltage converter is determined to be less than the reference value, and enables the operation of the plurality of switching elements when the output voltage of the AC-to-DC voltage converter is determined to be greater than the reference value. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     The present invention is further described in the detailed description which follows, with reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:  
         [0041]      FIGS. 1A  to  1 C represent exemplary circuit topologies for discharge lamp lighting devices according to the present invention;  
         [0042]      FIG. 1A-1  illustrates one possible configuration of a RLC networks useable with the circuit topology of  FIGS. 1A and 1C ;  
         [0043]      FIG. 2-1  illustrates switching states of switching elements employed in the discharge lamp lighting devices of  FIGS. 1A and 1B ;  
         [0044]      FIG. 2-2  illustrate waveforms at several sampling points of the discharge lamp lighting devices during a no-load period and a normal operation period;  
         [0045]      FIG. 3  illustrates an example of a comparer of the present invention utilized with the present invention that operates to prevent damage to the circuitry of the discharge lamp lighting device during a mis-wiring situation;  
         [0046]      FIG. 4  illustrates another example of the comparer according to the present invention;  
         [0047]      FIG. 5  illustrates a variation of the comparer of the present invention; and  
         [0048]      FIG. 6  illustrates another variation of the comparer of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0049]      FIGS. 1A-1C  illustrate various embodiments of a discharge lamp lighting device of the present invention that lights a high-pressure discharge lamp, such as, but not limited to, for example, a mercury or metal-halide lamp. Each discharge lamp lighting device (also referred to as an electronic ballast)  101  comprises a DC power supply  102 , an inverter  103 , a DC power supply controller  107 , an inverter controller  108 , an auxiliary power supply  109 , and an external output  112 .  
         [0050]     The DC power supply  102  converts an AC power supply voltage, such as, for example, provided by a commercial power supply  110 , to a regulated DC voltage. The AC power supply is supplied to the DC power supply  102  via an external voltage receiver  111 , which comprises, for example, a terminal block or wires. The inverter  103  receives an output from the DC power supply  102  and produces a rectangular wave AC power output that is utilized to light a high-pressure discharge lamp  113 . The DC power supply controller  107  controls the operation of the DC power supply  102 , while the inverter controller  108  controls the operation of the inverter  103 . Auxiliary power supply  109  generates a supply voltage for operating the DC power supply controller  107  and the inverter controller  108 . External output  112 , comprising, for example, a terminal block or wires, supplies the rectangular wave AC power output from the inverter  103  to the externally connected high-pressure discharge lamp  113 . It is understood that reference to the high-pressure discharge lamp  113  includes a fixture and/or lamp fitting.  
         [0051]     The DC power supply  102  comprises a so-called boost chopper circuit, which boosts the inputted AC power supply voltage and generates a regulated DC voltage. In the embodiments of  FIGS. 1A  to  1 C, the DC power supply  102  comprises a diode bridge DB 1  that converts an inputted AC voltage to a DC voltage, an inductor L 1 , a diode D 1 , a switching element Q 1 , and a capacitor C 1 . However, it is understood that variations in the configuration of the DC power supply  102  may be made without departing form the spirit and/or scope of the present invention.  
         [0052]     In the embodiments illustrated in  FIGS. 1A and 1B , the inverter  103  comprises a buck chopper circuit  104  and a polarity reversing circuit  105 . The embodiment illustrated in  FIG. 1C  does not employ the buck chopper circuit  104 . The buck chopper circuit  104  bucks down the DC voltage from the DC power supply  102  and adjusts the power supplied to the high-pressure discharge lamp  113  in accordance with a first control signal supplied by the inverter controller  108 . In the disclosed embodiments, the buck chopper circuit  104  comprises a switching element Q 2 , a diode D 2 , an inductor L 2 , a capacitor C 5 , and a diode D 7  that acts as a parasitic diode with respect to switching element Q 2 . It is understood that variations in the configuration of the buck chopper circuit  104  may be made without departing from the scope and/or spirit of the present invention.  
         [0053]     The polarity reversing circuit  105  generates rectangular wave AC power by alternating the DC voltage (provided by the buck chopper circuit  104  in  FIGS. 1A and 1B , or directly from the DC power supply  102  in  FIG. 1C ) according to a second control signal provided by the inverter controller  108 . The polarity reversing circuit  105  comprises a full bridge circuit and an igniter circuit. The full bridge circuit is formed by switching elements Q 3  and Q 4  that are connected in series, and switching elements Q 5  and Q 6  that are connected in series. The igniter circuit, which generates a high voltage pulse of a few thousand volts to activate (ignite) the high-pressure discharge lamp  113 , comprises a pulse transformer Ti, a capacitor C 8 , a switching element Q 7  (such as, but not limited to, for example, a voltage responsive element such as a SAIDAC), and a resistor R 10 . Again, it is understood that the disclosed construction of the polarity reversing circuit is presented merely for purposes of explaining the present invention, and thus, modifications and variations may be made thereto without departing from the scope and/or spirit of the invention.  
         [0054]     Switching elements Q 2  to Q 6  comprise, for example, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). However, it is understood that other types of switching elements may be employed without departing from the spirit and/or scope of the present invention. Parasitic diodes D 7  and D 3  to D 6  of respective switching elements Q 2  to Q 6  are connected in reverse directions. A voltage at connection (monitor) point A (corresponding to the voltage output from the DC power supply circuit  102 ) is supplied to the auxiliary power supply  109 . As noted above, the auxiliary power supply  109  generates and supplies a supply voltage to the DC power supply controller  107  and the inverter controller  108 .  
         [0055]     Connection (monitor) point B of switching elements Q 3  and Q 4 , and connection (monitor) point C of switching elements Q 5  and Q 6  are connected to the external high-pressure discharge lamp  113  through the pulse transformer T 1  and the external output unit  112  (see  FIG. 1B ).  
         [0056]     The above discussion has been presented with respect to a discharge lamp lighting device having a buck chopper circuit  104  and a polarity reversing circuit  105  with a pulse ignition, as depicted, for example, in  FIG. 1B . Other topologies are also possible, such as, but not limited to, for example, a discharge lamp lighting device in which the buck chopper circuit is eliminated, leaving only the polarity reversing circuit. The topology of a polarity reversing circuit may include, for example, a full bridge circuit and an igniter circuit.  FIG. 1C  depicts an example of a discharge lamp lighting device that comprises a full bridge configuration without the buck chopper circuit. In the embodiment of  FIG. 1C , switching elements Q 3  to Q 6  function as both the buck chopper circuit and the polarity reversing circuit.  
         [0057]     In the present discussion, the igniter circuit is generally referred to as pulse ignition. Another type of ignition, referred to as resonant ignition, is possible when the pulse transformer Ti, along with any other component(s) related to the pulse ignition, are replaced by two interconnected RLC/Semi networks  114  and  115 . Networks  114  and  115  form a generic circuit topology for either pulse ignition or resonant ignition.  FIG. 1A-1  shows one possible configuration of networks  114  and  115 . The specific configuration of the pulse ignition or resonant ignition is not critical to the operation of the present invention, the disclosed configurations being non-limiting examples presented to assist in the understanding of the present invention.  
         [0058]      FIG. 1A-1  illustrates an example of the networks  114  and  115  useable with the present invention. In the illustrated example, network  114  comprises capacitive elements C 100  and C 102 , an inductive element L 100  and a multi-tap inductive element L 102 , while network  115  comprises an inductive element L 104 .  
         [0059]     A first end of capacitive element C 100  is electrically connected to terminal point B, shown in  FIGS. 1A and 1C , and a first end of the multi-tap inductive element L 102 . A second end of the capacitive element C 100  is electrically connected to a first end~of the inductive element L 100  and a first end of inductive element L 104  of network  115 . A second end of the inductive element L 100  is electrically connected to terminal point  203 , shown in  FIGS. 1A and 1C . A second end of the multi-tap inductive element L 104  is electrically connected to terminal point  202 , shown in  FIGS. 1A and 1C , while the tap is electrically connected to a first end of capacitive element C 102 . A second end of the capacitive element C 102  is electrically connected to terminal point O, shown in  FIGS. 1A and 1C . A second end of inductive element L 104  is electrically connected to terminal point C, shown in  FIGS. 1A and 1C . It is understood that alternative networks may be used without departing from the scope and/or spirit of the invention.  
         [0060]     The following discussion describes an operation sequence of the discharge lamp lighting device  101 , with respect to a circuit topology having the buck chopper circuit  104  and the polarity reversing circuit  105 , as shown in  FIGS. 1A and 1B . An AC power supply voltage from a commercial power supply  110  connected to an external voltage receiving unit  111 , which is generally supplied by turning ON an external power supply switch (not shown), is converted to a DC voltage via a DC power supply circuit  102 . In the disclosed embodiment, the DC power supply circuit  102  comprises a diode bridge DB 1 , an inductor L 1 , a diode D 1  and a capacitor Cl. The DC voltage charged to capacitor C 1  is supplied to an auxiliary power supply unit  109 , which supplies a predetermined voltage (or voltages) to a DC power supply controller  107  and an inverter controller  108 .  
         [0061]     In the disclosed embodiment, the auxiliary power supply unit  109  comprises a DC-DC converter circuit that outputs a constant DC voltage, or voltages, from, but not limited to, for example, approximately several tens to several hundreds of volts. The construction of DC-DC converters are known to those skilled in the art, and thus, a detailed description thereof is omitted herein.  
         [0062]     The DC power supply controller  107  and the inverter controller  108 , which are energized with the supply voltage from the auxiliary power supply unit  109 , generate control signals that are supplied to the DC power supply circuit  102  and the inverter unit  103 . The inverter unit  103  begins the switching operation for lighting the high-pressure discharge lamp  113 . Specifically, when the high-pressure discharge lamp  113  is not lit, buck chopper circuit  104 , which receives the DC voltage generated by the DC power supply circuit  102 , receives a signal from the inverter controller  108  to output a maximum voltage that is allowed by an application. Polarity reversing circuit  105 , which receives the DC voltage output from the buck chopper circuit  104 , alternates the input DC voltage and begins the operation of the igniter circuit to activate (illuminate) the external high-pressure discharge lamp  113 .  
         [0063]      FIG. 2-1  illustrates the switching operation of the polarity reversing circuit  105 . By having switching elements Q 4  and Q 5  OFF when switching elements Q 3  and Q 6  are ON and vice versa, the voltage between connection point B and connection point C of the polarity reversing circuit  105  (see, for example,  FIG. 1B ) becomes a rectangular wave voltage Vb-c (see  FIG. 2 - 2 ( a )). Voltage Vb-c is formed because of alternating the DC voltage output from the buck chopper circuit  104 . Upon receiving the rectangular wave voltage Vb-c, which depends on a time constant formed by resistor R 10  and capacitor C 8 , capacitor C 8  is gradually charged to a voltage VC 8 , as shown in  FIG. 2 - 2 ( b ).  
         [0064]     Switching element Q 7  is turned ON when the voltage on capacitor C 8  reaches a break-over voltage Vbo of the switching element Q 7 . Generally, the break-over voltage Vbo of the switching element Q 7  is designed to be less than a maximum output voltage of the buck chopper circuit  104  when the discharge lamp  113  is not lit, and larger than the output voltage when the high-pressure discharge lamp  113  is lit. When switching element Q 7  is turned ON, the electrical charge accumulated in capacitor C 8  is discharged via capacitor C 8 , switching element Q 7 , and a primary winding N 1  of pulse transformer T 1 . The pulse voltage generated in the pulse transformer T 1  is boosted up (increased), and a high pulse voltage (equal to, for example, several thousand volts) is generated in secondary winding. N 2  of the pulse transformer T 1 . The high pulse voltage is superimposed on the rectangular wave voltage Vb-c to generate voltage VIa (see  FIG. 2 - 2 ( c )) between the two ends of the high-pressure discharge lamp  113 .  
         [0065]     By applying the high pulse voltage between the two ends of the high-pressure discharge lamp  113 , the high-pressure discharge lamp  113  is ignited (activated). The impedance of the high-pressure discharge lamp  113 , after dropping rapidly, increases gradually as it approaches a steady state. The inverter controller  108  determines the switching frequency and the duty cycle of the buck chopper circuit  104 , and generates a necessary control signal to operate switching element Q 2 , based on the impedance of the high-pressure discharge lamp  113 . The DC voltage output from the buck chopper circuit  104  becomes nearly the same value as an absolute value of the voltage VIa between the two ends of the high-pressure discharge lamp  113 . Polarity reversing circuit  105  continues the switching operation shown in  FIG. 2-1  even after the high-pressure discharge lamp  113  has been activated.  
         [0066]     It is noted that the operating principle for a full bridge discharge lamp lighting circuit that omits the buck chopper circuit and employs a resonant ignition or a pulse ignition is similar.  
         [0067]     The following description is provided with respect to several embodiments of the present invention with reference to the drawings. Similar elements are assigned the same numerical characters throughout the various embodiments, and thus repetitive descriptions will be omitted. The embodiments presented herein are non-limiting, the embodiments being presented for the purpose of explaining the present invention. Thus, the invention is not to be limited to that shown herein. Variations and modifications to that disclosed herein are expressly envisioned without departing from the spirit and/or scope of the invention.  
         [0068]     A first embodiment of a protector employed with a discharge lamp lighting device of the present invention is illustrated with reference to  FIGS. 1A  to  1 C and  3 .  
         [0069]     Discharge lamp lighting device  101  includes a voltage detector, such as, but not limited to, for example, a processor IC 101  that detects a voltage VB at connection point B and a voltage VC at connection point C (see  FIG. 3 ). In the disclosed embodiment, scaling resistors R 1  to R 5  are used to linearly scale down the voltage VB at connection point B to be equal to a conversion value Vb. Similarly, scaling resistors R 6  to R 10  are used to linearly scale down the voltage VC at connection point C to a conversion value Vc. The scaled voltages Vb and Vc are applied to A/D converter input terminals  3  and  4 , respectively, of the processor IC 101 . As shown in  FIG. 3 , a first optional smoothing capacitor (not labeled) may be provided between the junction of scaling resistors R 4  and R 5  to smooth voltage Vb. Similarly, a second optional smoothing capacitor (not labeled) may be provided at the junction of scaling resistors R 9  and R 10  to smooth voltage Vc. However, it is noted that such smoothing capacitors are generally not required with today&#39;s processors and thus, they may be omitted.  
         [0070]     The voltage at connection point B, or the voltage at connection point C will be equal to, in the disclosed embodiment, approximately, 465V, which is approximately the same voltage as the output voltage of the DC power supply circuit  102 . A voltage division ratio of the scaling resistors R 1  to R 5  and scaling resistors R 6  to R 10  are set so that the conversion value Vb or Vc to be applied to A/D converter terminals  3  and  4  of the processor IC 101  is less than a maximum value allowed for the processor, which is typically 5V in most applications. The conversion value Vb and/or conversion Vc is read by the processor IC 101  as 10-bit data. When the output voltage at connection point B or C is at approximately 465V, that is, when the conversion value Vb or Vc is approximately 5V, a maximum data value of D 1024  is read by the processor IC 101 . Considering tolerances and for ease of calculation, the conversion value of Vb or Vc is selected to be substantially equal to 5V when the output voltage at connection points B and C, respectively, are each substantially equal to 500V.  
         [0071]     In the disclosed embodiment, a reference voltage VREF 1  is set to be substantially equal to 50 volts. This voltage is set at a level lower than the peak voltage of approximately 108 volts AC, which reflects an estimated 10 percent deviation from a nominal 120 volts AC voltage provided from the commercial power supply  110 . In the disclosed embodiment, conversion value VREF 1  of reference voltage VREF 1  is stored in the processor IC 101  as 10-bit data. The conversion value VREF 1  is set at D 102 , which is calculated based on a ratio of VREF 1  (approximately equal to 50 volts) to a maximum output voltage VB or VC (approximately equal to 500 volts), when the maximum output voltage VB or VC at connection point B or point C, respectively, is set at D 1024 .  
         [0072]     When the commercial power supply voltage  110  is initially applied to the discharge lamp lighting device  101  having the above-described configuration, a voltage equal to approximately 1.414 times the input voltage is smoothed and applied to capacitor C 1  of the DC power supply circuit  102 . The voltage at capacitor C 1  (connection A) is additionally supplied to the auxiliary power supply unit  109  to activate the processor IC 101 .  
         [0073]     As a condition for outputting driving control signals for controlling the switching of switching elements Q 3  to Q 6  of the polarity reversing circuit  105 , the inverter controller  108  is configured to compare the conversion value Vb (and/or Vc) with the conversion value V REF1 , based on a program executing in the processor IC 101 , so as to satisfy both of the following: 
 
Vb&lt;V REF1 , and 
 
Vc&lt;V REF1 . 
 
         [0074]     When the AC power supply voltage is supplied to the external voltage receiving unit  111  of the discharge lamp lighting device  101 , and the polarity reversing circuit  105  is not switching, the output voltage VB at connection point B (or the output voltage VC at connection point C) becomes equal to approximately 0V. Thus, a normal switching operation may be performed so as to satisfy Vb&lt;V REF1  and Vc&lt;V REF1  at all time within a full line frequency cycle.  
         [0075]     On the other hand, if the power supply voltage  110  is inadvertently connected to the external output unit  112  of the discharge lamp lighting device  101 , a half wave rectified voltage with a peak value of approximately 1.414 times the AC power supply voltage appears at connection point B and/or at connection point C of the polarity reversing circuit  105 . Because the conversion value Vb will be greater than the conversion value VREF 1  and/or Vc will be greater than V REF1  at some point within a full line frequency cycle, processor IC 101  maintains the polarity reversing circuit  105  in a standby state. Therefore, the switching operation of the polarity reversing circuit  105  does not start, which prevents damage to the switching elements Q 3  to Q 6  of the polarity reversing circuit  105 .  
         [0076]     If the commercial power supply  110  is connected to the external voltage receiving unit  111  of the discharge lamp lighting device  101  and at least one end of the external output unit  112  is connected (directly or indirectly) to earth ground while the polarity reversing circuit  105  is not switching, the output voltage VB at connection point B and/or the output voltage C at connect point C is a half wave rectified voltage with a peak value of approximately 1.1414 times the power supply voltage. In other words, Vb will be greater than V REF1  and/or Vc will be greater than V REF1  at some point within a full line frequency cycle, resulting in the processor IC 101  maintaining the polarity reversing circuit  105  in a standby state. Therefore, the switching operation of the polarity reversing circuit  105  does not start, which prevents damage to the switching elements Q 3  to Q 6  of the polarity reversing circuit  105 .  
         [0077]     It is noted that the above analysis is equally applicable to an inverter unit  103  that does not include the buck chopper circuit  104 , but only includes the full bridge only topology.  
         [0078]     A second embodiment of the present invention will now be described. The discharge lamp lighting device  101  of the second embodiment of the present invention is discussed with reference to FIGS.  1 A- 1 C and  4 .  
         [0079]     Discharge lamp lighting device  101  includes a boost chopper circuit that provides a regulated voltage of approximately 465 volts as an output voltage VC 1  for a commercial power supply voltage input of approximately 120 volts to 277 volts. A conversion method for the output voltage VC 1  is configured as shown in  FIG. 4 . Specifically, scaling resistors R 11  to R 15  are used to linearly scale down the output voltage VC 1  to a conversion value V C1 , which is smoothed by the inclusion of a smoothing capacitor C 9  connected between electrical ground and the junction of scaling resistors R 14  and R 15 . The smoothed conversion value V C1  is applied to A/D converter input terminal  1  of processor IC 101 , which includes an AND conversion function. It is noted that since modern processors are sufficiently fast, the smoothing capacitor C 9  is not necessary for most applications, and may be omitted without adversely affecting the operation of the present invention.  
         [0080]     In the second embodiment, when the output voltage VC 1  of the DC power supply circuit is substantially equal to 465 volts, the voltage division ratio is set so that conversion value V C1  to be applied to the processor IC 101  is less than the maximum value allowed for the microprocessor, which is typically 5 volts in most applications. The conversion value V C1  is read by the processor IC 101  as 10-bit data. When the output voltage VC 1  is substantially equal to 465 volts, that is, when the conversion value V C1  is substantially equal to 5 volts, the maximum value of D 1024  is read by the processor IC 101 . Considering tolerances and for ease of calculation, the conversion value V C1  is selected to be substantially equal to 5 volts when the output voltage V C1  is substantially equal to 500 volts.  
         [0081]     A reference voltage VREF 4 , which represents a nominal output voltage VC 1  (equal to approximately 465 volts) at the output of the DC power supply circuit  102 , is set to approximately 440 volts. This voltage is set at a level that is lower than a 2 to 3 percent deviation of the output voltage VC 1  (465*0.97=451 volts) but higher than a peak value of a maximum voltage of 305 volts for a commercial power supply (305*1.414=431). Conversion value V REF4  of the reference voltage VREF 4  is stored in the processor IC 101  as 10-bit data. The digital form of conversion value V REF4  is set at D 900 , which is calculated based on the ratio of VREF 4  (equal to approximately 440 volts) to a maximum output voltage VC 1  (equal to approximately 500 volts).  
         [0082]     In the disclosed embodiments, processor IC 101  comprises a part of the inverter controller  108 . However, the processor IC 101  and the scaling resistors that comprise the protector may be separate from the inverter controller  108  (that is, not incorporated into the inverter controller  108 ) without departing from the spirit and/or scope of the invention. Inverter controller  108  outputs signals for operating the switching element Q 2  of the buck chopper circuit  104  and the switching elements Q 3  to Q 6  of the polarity reversing circuit  105  in a buck chopper and polarity-reversing combination topology. In a polarity reversing circuit with a full bridge topology that does not include the buck chopper circuit (such as shown in  FIG. 1C ), the inverter controller  108  outputs signals for the switching operation of the full bridge circuit only.  
         [0083]     When the commercial power supply voltage is initially applied to the discharge lamp lighting device  101  having the above-described configuration, a voltage that is approximately equal to 1.414 times the input voltage is smoothed and applied to capacitor C 1  of the DC power supply circuit  102 . Auxiliary power-supply unit  109  outputs a power supply voltage to activate the processor IC 101 , based on the voltage across capacitor C 1 .  
         [0084]     Driving control signals for the buck chopper circuit,  104  and the polarity reversing circuit  105  are selectively output by the inverter controller  108  in accordance with instructions executed by the processor. IC 101  as a result of the comparison of conversion value V C1  and conversion value V REF4 , and a determination that the conversion value V C1  is greater than the conversion value V REF4 . When this condition is satisfied, it means the power supply voltage is supplied to the external voltage receiving unit  111  of the discharge lamp lighting device  101 , the DC power supply controller  107  is activated after receiving the power supply voltage for the control operation, which is output from the auxiliary power supply unit  109 , and the DC power supply circuit  102  executes a boost chopper circuit operation by which capacitor C 1  at the output of the DC power supply unit is charged to approximately 465 volts. In other words, because the conversion value V C1  is greater than the conversion value V REF4 , a normal switching operation occurs.  
         [0085]     On the other hand, if the power supply voltage is inadvertently supplied to the external output unit  112  of the discharge lamp lighting device  101 , the DC power supply circuit  102  receives no voltage at its input terminals. As a result, capacitor C 1  of the DC power supply unit is charged to approximately 1.414 times the power supply voltage through the polarity reversing circuit  105  and the buck chopper circuit. In other words, V C1  will be less than V REF4 , so the processor IC 101  maintains the discharge lamp driving device in a standby state. Therefore, the switching operation for the buck chopper circuit  104  and the polarity reversing circuit  105  does not start, preventing damage to the switching elements of the polarity reversing circuit  105 .  
         [0086]     A third embodiment of the present invention will now be discussed with reference to  FIGS. 1A, 1B , and  5 .  
         [0087]     In the third embodiment, an output voltage VC 5  of capacitor C 5  associated with the output of buck chopper circuit  104  is sampled and provided to the inverter controller  108 , as shown in  FIG. 5 . Scaling resistors R 16  to R 20  are provided to linearly scale down the voltage VC 5  to a conversion value V C5 .  FIG. 5  depicts the DC voltage of the conversion value V C5  being smoothed by a smoothing capacitor C 10  that is connected between electrical ground and the junction of scaling resistors R 19  and R 20 , however, the inclusion of the smoothing capacitor C 10  may be omitted without affecting the operation of the present invention. The conversion value V C5  is applied to an A/D converter terminal of processor IC 101  (pin  2  of processor IC 101  in  FIG. 5 ), which has an A/D conversion function.  
         [0088]     When the output voltage VC 5  of the buck chopper unit  104  is approximately 465 volts, which is substantially equivalent to the output voltage of the DC power supply circuit  102 , the voltage division ratio is set so that conversion value V C5  to be applied to the processor IC 101  is less than a maximum value allowed for the processor IC 101 , which is approximately 5 volts in most applications. Conversion value V C5  is read by processor IC 101  as 10-bit data. When the output voltage VC 5  is set at approximately 465 volts, that is, when the conversion value V C5  is set at approximately 5 volts, the processor IC 101  reads the data as a maximum value of D 1024 . Considering tolerances and ease of calculation, the conversion value of V C5  is set to approximately 5 volts when the output voltage VC 5  is approximately 500 volts.  
         [0089]     In addition, a reference voltage VREF 5 , for output voltage VC 5  at the output of the buck chopper circuit  104 , is set at approximately 50 volts, which is significantly less than a normal output voltage of the buck chopper circuit  104 . This voltage is set to a level that is lower than a peak voltage of 108 volts, which reflects an estimated 10 percent deviation from the nominal voltage of 120 volts typically provided by the commercial power supply  110 . Conversion value V REF5  of the reference voltage VREF 5  is stored in the processor IC 101  as 10-bit data. Conversion value V REF5  is set in the processor IC 101  at D 102 , which is calculated based on a ratio of VREF 5  (equal to approximately 50 volts) to a maximum output voltage of the output voltage VC 5  (equal to approximately 500 volts), when the maximum output voltage of the output voltage VC 5  at the buck chopper circuit  104  is set at D 1024 .  
         [0090]     When the commercial power supply voltage is applied to discharge lamp lighting device  101  having the above-described configuration, a voltage approximately equal to 1.414 times the input voltage is smoothed and applied to the output terminal of capacitor C 1  of the DC power supply circuit  102 , as described above. Auxiliary power supply unit  109  outputs a power supply voltage for a control operation based on the voltage at capacitor C 1 , to activate processor IC 101 .  
         [0091]     Inverter controller  108  determines whether to output the driving control signals to the buck chopper circuit  104  and the polarity reversing circuit  105  based upon the comparison of the conversion values V C5  and V REF5  The driving signals are output when the following equation is satisfied: 
 
V C5 &lt;V REF5 . 
 
         [0092]     When the commercial power supply  110  is connected to the external voltage receiving unit  111  of the discharge lamp lighting device  101  and the buck chopper circuit  104  is not operating, the output voltage VC 5  of the buck chopper circuit  104  is approximately equal to 0 volts. Thus, a normal switching operation may take place, as the conversion value V C5  will be less than the conversion value V REF5 . On the other hand, if the external power supply  110  is accidentally connected to the external output unit  112  of the discharge lamp lighting device  101 , a DC voltage that is approximately equal to 1.414 times the power supply voltage will be provided across capacitor C 5 , even though the buck chopper circuit  104  is not operating. Thus, the conversion value V C5  will be greater than the conversion value V REF5 , and the processor IC 101  will maintain the discharge lamp lighting device  101  in the standby state. That is, the buck chopper circuit  104  and the polarity reversing circuit  105  will not start, preventing damage to the switching elements Q 3  to Q 6  of the polarity reversing circuit  105 .  
         [0093]     It is noted that this embodiment does not apply to the full bridge only topology shown in  FIG. 1C , as that topology omits the buck chopper circuit  104 .  
         [0094]     A fourth embodiment of the invention will now be described with reference to  FIGS. 1A, 1B , and  5 .  
         [0095]     In the fourth embodiment, discharge lamp lighting device  101  comprises a boost chopper circuit  102  that outputs a regulated voltage of approximately 465 volts, as output voltage VC 1  of the DC power supply circuit  102  for a commercial power supply input voltage of approximately 120 volts to approximately 277 volts. The buck chopper circuit  104  outputs a DC voltage that is approximately the same as the output voltage VC 1  when a high-pressure discharge lamp  113  is turned OFF, and outputs a voltage related to the impedance of the high-pressure discharge lamp  113  while the high-pressure discharge lamp  113  is turned ON (i.e., lit). A conversion method for output voltage VC 5  of the above-noted buck chopper is configured as shown in  FIG. 5 , and discussed above in the third embodiment. Hence, a discussion of the specific configuration is dispensed With in this embodiment.  
         [0096]     In the fourth embodiment, when the output voltage VC 5  of the buck chopper circuit is substantially equal to 465 volts, which is approximately the same as the output voltage of the DC power supply circuit  102 , the voltage division ratio is set so that conversion value V C5  applied to processor IC 101  is less than a maximum value typically allowed for the processor (i.e., 5 volts in most applications). Conversion value V C5  is read by processor IC 101  as 10-bit data. When the output voltage VC 5  is approximately 465 volts, that is, when conversion value V C5  is approximately 5 volts, a maximum data value of D 1024  is read by the processor IC 101 . Considering tolerances and ease of calculation, the conversion value of V C5  is set to be substantially equal to 5 volts when the output voltage VC 5  is substantially equal to 500 volts.  
         [0097]     In addition, a reference voltage VREF 6 , for the output voltage VC 5  at buck chopper circuit  104  (which is approximately equal to a normal buck chopper output voltage of 465 volts in the disclosed embodiment), is set to be equal to a slightly lower value, such as, for example, approximately 440 volts. This voltage is set to a level that is lower than a 2 to 3 percent deviation from the output voltage VC 5  (465*0.97 equals 451 volts) and higher than a peak value of a maximum voltage of 305 volts for a commercial power supply (305*1.414 equals 431 volts). Conversion value V REF6  of reference voltage VREF 6  is stored in the processor IC 101  as 10-bit data. The digital form of the conversion value V REF6  is set at D 900 , which is calculated based on the ratio of VREF 6  (approximately equal to 440 volts) to the maximum output voltage of output voltage VC 1  (approximately equal to 500 volts). As noted above, while the processor IC 101  comprises a part of the inverter controller  108  in the disclosed embodiment, it is understood that the processor could be separate from the inverter controller without departing from the scope and/or spirit of the invention.  
         [0098]     When the commercial power supply  110  is applied to the discharge lamp lighting device  101  having the above-described configuration, a voltage that is approximately equal to 1.414 times the input voltage is smoothed and applied to the output terminal of capacitor C 1  of the DC power supply circuit  102 , as described above. Auxiliary power supply unit  109  outputs a power supply voltage for the control operation based on the voltage of capacitor C 1  to control the operation of the processor IC 101 .  
         [0099]     Inverter controller  108  operates to output a control signal that starts a switching operation exclusively for the buck chopper circuit  104 . Specifically, the buck chopper circuit  104  is switched to regulate the output voltage VC 5  of the buck chopper circuit  104  from the output voltage VC 1  (substantially equal to 465 volts) at the DC power supply circuit  102 .  
         [0100]     The conversion values V C5  and V REF6  are compared by a program executed by the processor. IC 101  to determine the operational state of the polarity reversing circuit  105 . When the conversion value V C5  is greater than the conversion value V REF6 , driving control signals are outputted to the polarity reversing circuit  105 .  
         [0101]     When the commercial power supply  110  is connected to the external voltage receiving unit  111  of the discharge lamp lighting device  101 , the auxiliary power supply unit  109  provides a voltage to the DC power supply controller  107 . The DC power supply circuit  102  then executes a boost chopper circuit operation, by which capacitor C 1  is charged to approximately 465 volts. Output voltage VC 5  of the buck chopper circuit  104  becomes equal to approximately the same level (i.e., 465 volts), such that the conversion value V C5  is greater than the conversion value V REF6 , and thus, the process proceeds to a normal switching operation to turn ON the discharge lamp  113 .  
         [0102]     On the other hand, if the commercial power supply  110  is accidentally connected to the external output unit  112  of the discharge lamp lighting device  101 , the DC power supply circuit  102  does not receive any voltage at its input terminals. As a result, capacitor C 5  of the buck chopper circuit  104  is charged to approximately 1.414 times the commercial power supply voltage through the polarity reversing circuit  105 . Thus, the conversion value V C5  will be less than the conversion value V REF6 , and the processor IC 101  will operate to maintain the discharge lamp lighting device  101  in the standby state. Therefore, the switching operation of the polarity reversing circuit  105  does not start, preventing damage to the switching elements Q 3  to Q 6  of the polarity reversing circuit  105 .  
         [0103]     It is noted that this embodiment does not apply for the full bridge only topology, such as shown in  FIG. 1C , because the buck chopper circuit  104  is omitted therein.  
         [0104]     A fifth embodiment of the invention will now be described. The fifth embodiment of the present invention will be described with reference to  FIGS. 1A-1C  and  6 .  
         [0105]     Discharge lamp lighting device  101  includes a boost chopper circuit that produces a regulated voltage of approximately 465V as an output voltage VC 1  by the DC power supply circuit  102  for a commercial power supply voltage of approximately 120 volts to approximately 277 volts. An AC-to-DC voltage rectifier, such as rectifier DB 1 , provides an output voltage VDB 1 , which is supplied to the inverter controller  108 , as shown in  FIG. 6 . Scaling resistors R 21  to R 25  are used to linearly scale down the output voltage VDB 1  to a conversion value V DB1 . The conversion value V DB1  is inputted to an A/D converter terminal of the processor IC 101  (i.e., pin  5  of processor IC 101  as shown in  FIG. 6 ) that includes an A/D conversion function. Further, a smoothing capacitor may optionally be provided between the junction of scaling resistors R 24  and. R 25  and electrical ground, although modern microprocessors are sufficiently fast, and thus, the smoothing capacitor C 11  is generally not necessary.  
         [0106]     When the output voltage VDB 1  of the rectifying circuit DB 1  is at an approximate maximum value of 431 volts (which corresponds to 1.414 times a maximum AC power supply voltage of 305 volts), a voltage division ratio is set so that the conversion value V DB1  applied to the processor IC 101  does not exceed a maximum permissible value allowed by the processor, which, in most applications, is typically 5 volts. Conversion value V DB1  is read by the processor IC 101  as 10-bit data. When the output voltage VDB 1  is approximately equal to 431 volts, that is, when the conversion value V DB1  is substantially equal to 5 volts, a maximum data value of D 1024  is read by the processor IC 101 . Considering tolerances and ease of calculation, the conversion value of V DB1  is selected to be substantially equal to 5 volts when the output voltage of VDB 1  is substantially equal to 500 volts.  
         [0107]     In addition, a reference voltage VREF 7  (associated with the output voltage VDB 1 ) is set to be equal to approximately 50 volts. This voltage is selected to be set at a level that is lower (smaller) than a peak voltage of 108 volts, which reflects an estimated 10 percent deviation from a nominal voltage of 120 volts for a commercial AC power supply. Conversion value V REF7  of the reference voltage VREF 7  is stored in processor IC 101  as 10-bit data. Conversion value V REF7  is set at D 102 , which is calculated based on a ratio of V REF7  (which is substantially equal to 50 volts) to a maximum output voltage of VREF 7  (which is substantially equal to 500 volts), when a maximum output voltage of the rectifying circuit DB 1  is set at D 1024 .  
         [0108]     When the commercial power supply  110  is applied to the discharge lamp lighting device  101  having the above-described configuration, a voltage that is approximately equal to 1.414 times the input voltage is smoothed and applied across capacitor C 1  of the DC power supply circuit  102 . Auxiliary power supply unit  109  outputs a power supply voltage for the control operation based on the voltage across the capacitor C 1 , to activate the processor IC 101 .  
         [0109]     Driving control signals from the inverter controller  108  are selectively output to drive the polarity reversing circuit  105  in response to a comparison of conversion values V DB1  and V REF7  by a program executed by the processor IC 101 .  
         [0110]     When the commercial power supply  110  is supplied to the external voltage receiving unit  111  of the discharge lamp lighting device  101 , the output voltage VDB 1  becomes equal to approximately 1.414 times the power supply voltage. As a result, the conversion value V DB1  is greater than the conversion value V REF7 . Therefore, a normal switching operation may commence. On the other hand, if the commercial power supply  110  is accidentally connected to the external output unit  112  of the discharge lamp lighting device  101 , output voltage VDB 1  will be equal to approximately 0 volts, as diode D 1  will prevent a voltage backflow. Thus, the conversion value V DB1  will be less than the conversion value V REF7 . As a result, the processor IC 101  will maintain the discharge lamp driving device  101  in the standby state. Therefore, the switching operation of the polarity reversing circuit  105  does not start, which prevents damage to the switching elements of the polarity reversing circuit  105 .  
         [0111]     It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.  
         [0112]     The present invention is not limited to the above-described embodiment, and various variations and modifications may be possible without departing from the scope of the present invention.