Abstract:
A metal halide lamp, having a cold fill pressure of at least 50 torr, incorporates a starter circuit with an active device to protect the diode included therein. The active portion of the starter circuit, either removes a voltage multiplying circuit portion, from the starter circuit or shorts out the diode by shunting action of the switch. The voltage multiplier circuit portion is connected across the main electrodes, wherein the voltage multiplier circuit portion increases the starting voltage applied to the starter electrode.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to high pressure arc discharge lamps and is particularly applicable to lamps containing a metal halide fill and employing a tungsten electrode wherein the fill pressure has been increased to such a high value that a starting circuit is required within the lamp. 
     High pressure metal halide discharge lamps (MHL) are generally comprised of a f used silica or quartz arc tube containing an ionizable fill and having a pair of main thermionic electrodes at the ends. In most applications the electrodes include a relatively high percentage of tungsten. T he electrodes are supported by inleads which include a thin molybdenum ribbon portion extending hermetically through a pinch or press seal in the end of the lamp. Generally, a starter electrode is disposed in the arc tube adjacent one of the main electrodes to facilitate starting. In most lamps, a discharge can be ignited between the starter and the adjacent main electrode at a much lower voltage than between the two main electrodes, and ignition of the arc between the main electrodes is thereby facilitated. 
     Maintenance of initial lumens in MHL lamps is a problem due to the highly complex chemical reactions occurring in the atmosphere within the arc discharge chamber. More specifically, at the operating temperature of 5,500° K at the center of the arc, to approximately 1,100° K at the wall of the arc tube, which defines a boundary of the plasma, many and various reactions occur. One negative reaction is the transport of metallic and inorganic compounds of tungsten (the main electrode constituent) from the electrode to the walls of the discharge tube during operation of the lamp. The tungsten, in its various compound forms so transported, creates an opaque barrier on the inner wall of the arc tube, thus preventing discharge radiation from being effectively transmitted. In short, significant losses to the level of lumens can occur. This loss of light level from within the discharge is perceived externally as a reduction of light output of the lamp, and thereby reduction in the maintenance of initial lumens. It is believed that the transport of tungsten and tungsten compounds to the walls of the discharge tube occurs through sputtering, evaporation and other chemical mechanisms. 
     To reduce the transport of tungsten and tungsten compounds to the walls of the discharge tube, a type of MHL lamp has been designed where the cold fill pressure of the inert gas in the arc discharge chamber is increased to a value of at least 50 torr, with the preferred fill pressure being 70 torr and possibly greater than 110 torr. While this design has successfully reduced the transfer of metallic and inorganic compounds, the increased fill pressure, requires a higher than normal starting voltage. Therefore, a voltage multiplier circuit is incorporated into the lamp to assist in the lamp ignition procedure. 
     The voltage multiplier circuit uses a semiconductor device, such as a diode. Unfortunately, the high temperature creates an environment where metal from the connections is able to diffuse into the semiconductor diode, gradually increasing the diode reverse leakage current. The increase of diode reverse leakage current is destructive and limits the diode usable life. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an active device and method to protect the starting circuit diode, in metal halide lamps from being destroyed. 
     A metal halide lamp incorporating this invention comprises an arc tube containing an ionizable medium and having electrodes, preferably tungsten, sealed into opposed ends of the arc tube. An outer envelope encloses the arc tube and includes one end accommodating inleads sealed therethrough. A base is attached to the outer envelope and includes input terminals, the input terminals being connected to the inleads which in turn are connected to the tungsten electrodes. The ionizable medium includes mercury, metal halides, and an inert gas selected from the group consisting of argon, krypton, xenon and mixtures thereof. The inert gas will be at a cold pressure of at least about 50 torr. The lamp incorporates a starting circuit, having a voltage multiplying circuit portion connected to the starter electrode, to provide an increased starting voltage to the starter electrode. Also included as part of the starter circuit is a disabling circuit portion, which either removes the voltage multiplying portion out of the circuit, or shorts the voltage multiplying circuit to the starter electrodes, after the lamp has been started. The disclosed active protection device is inexpensive to manufacture, requires little added labor, and is easily retrofitted to existing manufacturing processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of a prior art metal halide lamp with a starting circuit suitable for adaptation to the present invention; 
     FIG. 2A is a schematic diagram of the standard starting circuit for the lamp of FIG. 1; 
     FIG. 2B is a schematic diagram of a voltage multiplying starting circuit for use in a lamp such as shown in FIG. 1; 
     FIG. 3 is a schematic representation of a preferred embodiment of the starting circuit according to the present invention; 
     FIG. 4 is a schematic representation of an alternate embodiment of the starting circuit incorporating the invention; 
     FIG. 5 is a schematic representation of diode component testing performed to evaluate the effectiveness of the invention; and 
     FIG. 6 is a graphical representation of examples evaluating the effect of the invention on extending diode life. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a metal halide lamp  10  is depicted to which the present invention is basically suited. In one embodiment lamp  10  may be rated at 400 watts, however, the present invention may also be used with other rated lamps, for example in the range of 175-1500 watts. 
     Lamp  10  comprises an outer glass envelope  12  containing a quartz or fused silica arc tube  14  having flat pressed or pinched ends  16 ,  18 . Main electrodes  20 ,  22  are mounted in opposite ends of arc tube  14 , each including a shank portion  24 ,  25  which extends to a molybdenum foil  26 ,  27  to which current conductor  28 ,  29  are respectively connected. A lamp which corresponds to the lamp described above is disclosed in U.S. Ser. No. 09/290,008 entitled, Improved Metal Halide Lamp, Ramaiah et. al., filed 04/09/1999, and hereby incorporated by reference. 
     Main electrodes  20 ,  22  are connected by conductors  30 ,  32  to outer envelope inleads  34 ,  36  sealed through stem  38  of the outer envelope  12 . Starter electrode  39  is provided at the upper end of arc tube  14  close to main electrode  20 , and consists of an inwardly projecting end of a fine wire. The outer envelope inleads  34 ,  36  are connected to the contact surfaces of screw base  40  attached to the neck end of the envelope, that is to the threaded shell  42  and to the insulated center contact  44 . The threaded shell is designed to be connected to a signal source  45 . A starting circuit  46  is generally shown as block component. Also depicted is heat shield  47 . 
     A starting circuit, such as represented by block  46  of FIG. 1, is depicted as standard starting circuit  48  in FIG.  2 A. This circuit consists of bi-metal starter switch  49  and resistor  50 . The lamp is shown connected to ballast  52 , which includes a ballast capacitor  53  and ballast resistor arrangement  54 . Switch  49  is normally open, and when voltage is first applied to a cold lamp at inleads  34 ,  36  the voltage is applied across the lamp starter electrode  39  and main electrode  20 , facilitating starter-to-main electrode breakdown. Main-to-main electrode breakdown across electrodes  20 ,  22  follows shortly after starter-to-main breakdown. Once the lamp heats up, switch  49  closes, bringing electrodes  20 ,  39  to the same potential which prevents damage to pinch end  16  due to electrolysis. 
     As is now known, if the cold gas fill, primarily the inert gas, is increased sufficiently, there is a minimization of blackening on the quartz wall of the discharge tube. It is believed that this occurs because of a reduction in the mean free path of the sputtered tungsten atoms during the initial start up. Arc tube  14  is provided with an ionizable radiation-generating fill. One suitable filling comprises mercury, sodium iodide, scandium iodide, and at least 50 torr of an inert gas such as argon. It was found, however, that it is not possible to arbitrarily increase the cold fill of a metal halide lamp and still obtain a reliable start using standard metal halide ballasts and the standard starter circuit. 
     Turning to FIG. 2B, voltage multiplier starting circuit  55  is used to increase lamp startability in lamps having increased cold fill gas levels. The voltage multiplier circuit  55  consists of two components in addition to those found in the standard starting circuit: diode  56  and resistor  58 . When terminal  34  is positive with respect to terminal  36 , diode  56  is forward biased. Current flows through diode  56  and resistor  58 , allowing ballast capacitor  53  to charge. On the opposite half-cycle, when terminal  36  is positive with respect to terminal  34 , diode  56  is reverse biased. No current flows through diode  56  and resistor  58 . However, the voltage on capacitor  53  is added to the ballast output, thus multiplying the voltage available at lamp terminals  34 ,  36  and aiding in lamp starting. 
     In a lamp having increased cold fill pressure, acceptable starting is achieved by employing voltage multiplying circuit  55  in conjunction with a typically available ballast. However, this circuit allows current to flow through diode  56  during lamp operation. This leads to diode deterioration over time, manifesting itself through increased diode leakage current, which reduces the ability of the circuit to charge ballast capacitor  53 . This, in turn, decreases the voltage multiplication generated at the lamp terminals, and thus reduces startability of the lamp. 
     To address this problem a schematic of a first embodiment for a starting circuit  60  is shown in FIG.  3 . In this embodiment, starting circuit  60  is similar to starting circuit  49  of FIG. 2B but also includes a normally closed disabling bimetal switch  62  which removes diode  56  completely from the circuit once a predetermined temperature is reached. Opening of disabling switch  62  removes the voltage multiplier circuit components ( 56 ,  58 ) from starter circuit  60 , thereby removing any bias on diode  56 , and preventing diode damage due to current flow during lamp operation. 
     As a further aspect of the invention, heat shield  47 , as shown in FIG. 1, may be interposed between at least diode  56  and arc lamp  14 . As illustrated in FIG. 1, starting circuit  46  (depicted in this embodiment in accordance with the description of starting circuit  60 ) is within envelope  12  of lamp  10 . To reduce the heat received by diode  56 , heat shield  47 , formed of mica or other appropriate material, is interposed to limit the amount of heat radiated to diode  56 . It is to be noted that when the present embodiment includes shield  47 , the temperature at which disabling switch  62  is intended to open should be lower than in an embodiment where heat shield  47  is not used. The temperature at which the disabling switch is to open is between 100° C. and 250° C., depending on where it is located within the lamp. Use of disabling switch  62  has been found to extend diode life since it is possible to select values so no current flows through diode  56  at temperatures above the rated temperature (e.g. for a specific diode this temperature is 175° C.). Therefore, metal diffusion into the semiconductor diode, and thus diode damage, is greatly reduced. 
     In an alternate embodiment, starting circuit  70  of FIG. 4 is depicted. In this embodiment, starting circuit  70  is similar to starting circuit  49  of FIG. 2B, but bimetal starter switch  49  and resistor  50  are connected in an alternative arrangement. The voltage multiplying circuit portion ( 56 ,  58 ) remains the same as in starter circuit  55  of FIG.  2 B. Particularly, the diode/resistor arrangement ( 56 ,  58 ) is connected across the inputs  34  and  36 . However, now connected between these series components, is starter switch  49  being used not only for starter electrode  39 , but also used for disabling of the voltage multiplying circuit portion ( 56 ,  58 ), in order to protect diode  56 . More specifically, switch  49  is now connected in parallel with diode  56 . Resistor  50  is connected at a node between diode  56 /switch  49  and resistor  58  at one end, and to starter electrode  39  at another. This alternate configuration effectively shorts diode  56  once a predetermined temperature is reached. This feature extends the diode life by minimizing current flow through the diode at temperatures above the rated temperature of diode  56 , and hence, greatly reduces metal diffusion into the semiconductor diode. It is understood that some small currents can still flow through diode  56  even when shorted. It is to be appreciated that temperature shield  47  (as depicted in FIG. 1) may also be employed in this embodiment. 
     Component testing shown schematically in FIG. 5 depicts a testing procedure undertaken to illustrate the usefulness of removing diode  56  from a biased state. The test consisted of two sets of diodes and resistor pairs, installed into high temperature fixtures and placed in an oven. The oven was set to 240° C. to simulate diode temperature during lamp operation. One set of resistor diode pairs was biased with 200 VAC to simulate voltage potential during operation, and one set was left unbiased to simulate operation in a lamp incorporating the invention. 
     The components were left in the oven at 240° C. for 700 hours. The oven was then turned off, and once the components cooled, the reverse leakage current of the diodes was checked on a curve tracer. The data was fit to a three parameter Weibull distribution (the data is not normally distributed), and the plot comparing reverse leakage current in microamps for the two groups is displayed in FIG.  8 . There is a statistically significant difference in damage, as evidenced by reverse leakage current, between the two groups—the unbiased diodes displayed considerably less damage than the biased diodes. This test demonstrates that eliminating the current to the diodes during operation can significantly decrease damage. 
     Furthermore, in two separate tests, a total of  21  lamps were built using the embodiment displayed in FIG.  3 . These lamps contained higher fill pressure arc tubes  14 , filled to about 90 torr argon fill pressure. Test 1, consisting of 8 lamps, has successfully burned for 2,000 hours. Test 2, consisting of 13 lamps, has successfully burned for 1,000 hours. Voltage multiplier circuit effectiveness is evaluated using the metric of peak voltage multiplier (PVM) as shown below:        PVM   =         V   peak          (   enhanced   )         V   peak                              
     In accordance with the defined relationship, the higher the PVM, the greater the starting voltage generated across the lamp. The PVM value generated will depend on the characteristics of the ballast used, and on the condition of the voltage multiplier circuit in the lamp under test. 
     Test results for 1,000 and 2,000 hours are shown below in Table 1. Test A is a test containing lamps having a voltage multiplier circuit, but does not include the diode protection concepts of the present invention. Tests 1 and 2 used voltage multiplier circuits which incorporate the diode protection concepts of the present invention. 
     The ballast used in the above testing generated a PVM of 1.74 for a new lamp. It is noted that a lamp with no voltage multiplier circuit, generates a PVM of 1.0. The average PVM for Test A lamps has deteriorated to 1.22 by 1000 hours, and further deteriorated by 2000 hours. Deterioration of this magnitude is unacceptable so early in lamp life. 
     Lamps in Tests 1 and 2 showed no PVM deterioration at 1000 hours, and at 2000 hours the lamps of Test 1 still showed very little deterioration in the generated PVM, demonstrating the effectiveness of the invention. The results of Test 2, for 2000 hours were not available at the time of this writing. 
     
       
         
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Average PVM 
               
             
          
           
               
                   
                 Burn Time 
                 Test 1 
                 Test 2 
                 Test A 
               
               
                   
                 (hours) 
                 n = 8  
                 n = 13 
                 n = 21 
               
               
                   
                   
               
               
                   
                 1,000 
                 1.74 
                 1.74 
                 1.22 
               
               
                   
                 2,000 
                 1.67 
                   
                 1.17 
               
               
                   
                   
               
             
          
         
       
     
     Thus, it is apparent that there has been provided, in accordance with the invention, an active method to protect the diode in metal halide lamps that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appending claims.