Abstract:
For safety reasons, industrial lighting fixtures are required to have backup lighting systems so that if the primary lights should fail, there will still be enough light to ensure safe maneuvering. Typically these backup lighting systems have their own power or drive source. The present application contemplates a lighting ballast circuit that is able to power a primary high intensity discharge (HID) lamp and is also able to power an auxiliary lamp in the event of temporary or permanent failure of the HID lamp.

Description:
BACKGROUND OF THE INVENTION 
   The present application is directed to the lighting arts. Typically, high frequency ballast inverter circuits provide power for operating a lamp, and the present application is directed to one of these ballast circuits. More particularly, the present application is directed to a ballast circuit that has the capability to power both a primary and a secondary, backup lamp, and will be described with particular reference thereto. 
   Typically, in an industrial lighting setting, it is often desirable to have a backup light source available should primary lights fail. Sometimes, in certain settings, it is even required to have automatic backup lighting. Many industrial lighting settings use high intensity discharge (HID) lamps because of their long life, high lumen output, and relative reliability. Yet even the best of light sources will fail from time to time. HID lamps in particular will “drop out” from time to time, meaning that they temporarily stop producing light, but are not dead, or permanently spent. 
   After lamp drop out, or if power is interrupted to the HID lamp, the lamp cannot be restarted until the lamp cools down. This can take up to twenty minutes, and during that time, the area in which the lamp is fixed will be without light. Temporary lighting is desirable, and often required where failure of primary light sources could present dangerous conditions. Typically, these temporary, backup light sources were powered by their own dedicated drive circuits. It has been typical that these auxiliary drive sources tap the ballasting inductor to provide a 120 V signal to the auxiliary lamp. This means that added space is required in an often already space lacking environment to house the backup drive circuits, and their associated power sources. 
   When an HID lamp drops out, the voltage inverter of the ballast typically does not simply stop oscillating. The unlit lamp acts essentially as an open circuit as far as the ballast is concerned, but a small amount of current still completes the circuit. Thus the lamp can be re-struck and again ramp up to its steady state operation. In the meantime however, a completely different circuit powers the auxiliary lamp while the primary lamp relights. 
   The present application presents a new and improved HID ballast inverter circuit that overcomes the above referenced problems and others. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In accordance with one aspect of the present disclosure, a lighting ballast is disclosed. The ballast includes a first inverter switch for providing power to a primary lamp during a first half-cycle of an alternating current lamp drive signal, and a second inverter switch for providing power to the primary lamp during a second half-cycle of the lamp drive signal. A high voltage multiplier portion boosts a signal applied to the primary lamp during a startup phase. A clamping portion clamps voltages within the ballast to levels that the lamp can tolerate. A resonant portion determines an operating resonant frequency of the ballast. An integrated auxiliary lamp power portion providing power to an auxiliary lamp if the primary lamp fails. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a depiction of a typical ballast inverter circuit; 
       FIG. 2  is a high voltage circuit portion included in the ballast to ignite a primary HID lamp; 
       FIG. 3  is a depiction of a ballast circuit with an auxiliary light power portion; 
       FIG. 4  is a depiction of an open circuit voltage present in the ballast; 
       FIG. 5  is a depiction of an inductor current under open circuit conditions. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a ballast circuit  10  includes an inverter circuit  12 , a resonant circuit  14 , and a clamping circuit  16 . A DC voltage is supplied to the inverter  12  via a voltage conductor  20  running from a positive voltage terminal  22  and a common conductor  24  connected to a ground or common terminal  26 . A lamp  28  is powered via lamp connectors  30 ,  32 . The lamp  28  can be any lamp that requires an operating ballast. The lamp in the illustrated embodiment is a high intensity discharge (HID) lamp. 
   The inverter  12  includes switches  34  and  36  such as MOSFETs, serially connected between conductors  20  and  24 , to excite the resonant circuit  14 . Other switches, such as IGBTs, or BJTs could also be used and are certainly contemplated. Typically, the resonant circuit  14  includes a resonant inductor  38  and a resonant capacitor network  40 ,  76 ,  78 , and  42  for setting the frequency of the resonant operation. The capacitor  42  also acts as a DC blocking capacitor, which prevents excessive DC current from flowing through the lamp  28 . A snubber capacitor  44  allows the inverter  12  to operate with zero voltage switching where the switches  34  and  36  turn ON and OFF when their corresponding drain-source voltages are zero. 
   The switches  34  and  36  cooperate to provide a square wave at a node  46  for exciting the resonant circuit  14 . Gate lines, i.e., control lines  48  and  50 , running from the switches  34  and  36 , respectively, each include a resistance,  52  and  54 , respectively. Diodes  56  and  58  are connected in parallel to the resistances  52  and  54 , respectively, making the turn-off time of the switches  34  and  36  faster than the turn-on time. Achieving unequal turn-off and turn-on times provides a time when the switches  34  and  36  are simultaneously in the non-conducting states to allow the voltage at node  46  to transition from one voltage state to another voltage state by a use of residual energy stored in the inductor  38 . 
   Gate drive circuitry, generally designated  60  and  62 , includes inductors  64  and  66 , which are both secondary transformer windings, each mutually coupled to primary winding  38 . The gate drive circuitry  60 ,  62  is used to control the operation of respective switches  34  and  36 . More particularly, the gate drive circuitry  60 ,  62  maintains the switch  34  ON for a first half of a cycle and maintains the switch  36  ON for the second half of the cycle. The square wave is generated at node  46  and is used to excite the resonant circuit  14 . Bi-directional voltage clamps  68 ,  70  are connected in parallel to inductors  64  and  66 , respectively. Each clamp  68 ,  70  includes a pair of back-to-back Zener diodes. The clamps  68 ,  70  act to clamp positive and negative excursions of gate-to-source voltage to respective limits determined by the voltage ratings of the back-to-back Zener diodes. 
   The output voltage of the inverter  12  is clamped by diodes  72  and  74  connected in series, which are a part of the clamping circuit  16 . The clamping circuit  16  limits the high voltage generated to start the lamp  28 . The clamping circuit  16  further includes capacitors  76  and  78 , which are essentially connected in series to each other, but are effectively in parallel due to the low impedance of the DC bus. Each clamping diode  72 ,  74  is connected across an associated capacitor  76 ,  78 . Prior to the lamp  28  starting, the lamp&#39;s circuit is essentially open, since an impedance of the lamp  28  is seen as a very high impedance. A high voltage across capacitor  42  is generated by a multiplier  80  (depicted in  FIG. 2 ) connected between nodes  82  and  84 . The resonant circuit  14 , which is further composed of capacitors  40 ,  42 ,  76 ,  78  and inductor  38 , is driven near its resonant frequency. As the output voltage on node  84  increases, the diodes  72  and  74  begin to clamp, preventing the voltage across capacitors  76  and  78  from changing sign and limiting the output voltage to the value that does not cause overheating of the inverter  12  components. When the diodes  72 ,  74  are clamping capacitors  76  and  78 , the resonant circuit  14  becomes composed of the capacitor  40  and the inductor  38 . Therefore, the resonance is achieved when the diodes  72  and  74  are not conducting. 
   With continuing reference to  FIG. 2  and  FIG. 1 , the multiplier circuit  80  boosts the voltage limited by the clamping circuit  16 . The multiplier  80  is connected across the capacitor  42 , achieving a starting voltage by multiplying the inverter  12  output. At the beginning of the operation, inverter  12  supplies voltage to the nodes  82  and  84 . Capacitors  90 ,  92 ,  94 ,  96 , and  98  cooperate with diodes  100 ,  102 ,  104 ,  106 ,  108 , and  110  to accumulate charge for one half of a cycle, (e.g., a positive half-cycle) while during the other half-cycle, (e.g., the negative half-cycle) the charge is discharged into capacitor  42  through node  84 . Typically, when the inverter  12  voltage is 500 V peak to peak, the voltage across nodes  82  to  84  rises to about 2 kV DC. The multiplier  80  is a low DC bias charge pump multiplier. During steady-state operation, the multiplier  80  applies only a small DC bias (about 0.25 Volts) to the lamp  28  which does not affect the lamp&#39;s operation or life. 
   For a variety of reasons, sometimes, during steady state operation of the ballast  10 , the lamp  28  will drop out, that is, temporarily extinguish. In typical lighting applications, backup lights are provided with separate control circuitry and/or power sources. With reference to  FIG. 3 , an auxiliary lamp  120  is provided in the ballast  10 .  FIG. 3  depicts the ballast  10  of  FIG. 1 , with the inverter  12  represented by simple switches  12   a  and  12   b,  and with control and power circuitry added. Remaining like components are indicated with like reference numerals. Standard power factor correction circuitry  121  regulates an input from a power source (not shown) such as a 200-227 Volt line signal to the ballast. 
   The auxiliary lamp  120  in the illustrated embodiment is a 150 W, 120 V quartz lamp, but other lamps are also contemplated. The lamp  120  could also be a compact fluorescent lamp (CFL). In a CFL embodiment, the ballast  10  may include an extra capacitor to boost the voltage to start up the CFL. In other embodiments, the auxiliary lamp  120  could be an incandescent lamp, a halogen lamp, or other known lamp. With some additional rectification, the lamp  120  could even be a series of LEDs. The nominal light output of the auxiliary lamp can be 10% of the HID lamp&#39;s  28  output. Industry standards require that the backup lighting have at least approximately 1% of the lumen output of the primary light. Thus, in an industrial lighting setting, an auxiliary lamp that is about 10% as bright as the primary lamp can be included in one of every ten fixtures, yielding the net 1% of standard lumen output. 
   The inverter  12 , before lamp ignition, is operating to supply the multiplier  80  with charge for breaking down the lamp  28 . In this instance, and in other instances when the lamp is not lit (e.g., following a lamp drop out) the inverter  12  still oscillates with a low quiescent power. With reference to  FIG. 4 , the waveform  122  represents the AC component of the voltage seen at the node a connecting capacitor  40  and inductor  38  when the lamp  28  is not lit. In  FIG. 5 , the waveform  124  depicts the current flowing through the inductor  38  when the lamp  28  is not lit. Apropos, this low quiescent voltage is used to power the auxiliary lamp  120 , while the HID lamp  28  is cooling down. The auxiliary lamp  120  and capacitor  126  form an additional output portion that is in parallel with the HID lamp  28  so that the auxiliary lamp  120  can be powered by the same signal that is being boosted by the multiplier  80  to re-strike the lamp  28 . 
   With continuing attention to  FIG. 3 , auxiliary lamp  120  is connected in series with the capacitor  126 . The nominal value of the capacitor  126  depends on the auxiliary lamp  126  and the voltage applied across the lamps  28 ,  120 , V ab . The impedance of the auxiliary lamp  120  is known and the desired operating voltage is known. In one embodiment, the auxiliary lamp  120  is a 150 Watt, 120 Volt quartz lamp. Thus, the determination of the value of capacitor  126  includes those factors. The capacitor  126  also limits the current through the auxiliary lamp  120 . The capacitor  126  can be selected to run the auxiliary lamp at a reduced voltage, thus extending the life of the auxiliary lamp. Since the auxiliary lamp  120  may burn out in applications where the power equipment is less reliable, increasing the life of the auxiliary lamp can be of great benefit to a consumer. 
   Once the HID lamp  28  is re-ignited, the auxiliary lamp  120  does not have to extinguish immediately. In one embodiment, the ballast  10  senses the power across the HID lamp  28  and cuts out the auxiliary lamp  120  by opening switch  127  when the HID lamp re-achieves about 30%-70% of its potential lumen output. In the illustrated embodiment, the ballast  10  can cut out the auxiliary lamp  120  when the HID lamp  28  reaches approximately 50% of its potential lumen output. As shown in  FIG. 3 , a circuit power monitor  128  receives a current measurement from a current sense resistor R that measures the current flowing through the DC bus which is proportional to the total output power of the inverter, including the HID lamp  28  and the auxiliary lamp  120 . Box  130  is an interface circuit that applies the buffered vco signal to the gates of the switches  34 ,  36 . 
   Power for the monitor  128  is provided by the ballast  10  via power control circuitry  132 . From the detected current, and the DC bus voltage, the power sensor  128  can calculate the average power being applied to the lamp  28 . As the lamp  28  ramps back up to steady state operation, the current provided increases. That is, current is proportional to power. Once the power sensor  128  senses that the HID lamp  28  is running at about 50% power, (i.e. 50% lumen output) then the auxiliary lamp  120  switches off. The switch  127  that turns off the auxiliary lamp could be a BJT, MOSFET or IGBT. The switch  127  is driven by a comparator that senses when the bus power is less than a predetermined level. This would indicate that the HID lamp has extinguished. At that point, the comparator turns on the switch. When the power rises above a predetermined level, the comparator turns off the switch. 
   The signal that drives the switch  127  can be designed to turn off the auxiliary lamp  120  at any desired level. One possibility is to have the total power delivered to the HID lamp  28  and the auxiliary lamp  120  limited to the nominal power setting of the inverter. For example, the auxiliary lamp  120  could be switched off when the bus power reaches its nominal value. If the auxiliary lamp  120  is a 150 W halogen lamp and the HID lamp  28  is a 400 W lamp, this would mean that the switch  127  would extinguish the auxiliary lamp  120  when the HID lamp reaches 250 Watts. 
   Generally, the ballast is very cost effective. As has been shown, the auxiliary lamp  120  accommodating circuitry can be added to present ballast arrangements with very little modification. Thus, it can be included in every ballast produced, and not limited to the 10% of ballasts, as mentioned above. 
   While it is to be understood the described circuit may be implemented using a variety of components with different component values, provided below is a listing for one particular embodiment when the components have the following values: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Reference Character 
               Component 
             
             
                 
                 
             
           
           
             
                 
               Switch 34 
               20NMD50 
             
             
                 
               Switch 36 
               20NMD50 
             
             
                 
               Inductor 38 
               90 μH 
             
             
                 
               Capacitor 40 
               22 nF, 630 V 
             
             
                 
               Capacitor 42 
               33 nF, 2 kV 
             
             
                 
               Capacitor 44 
               680 pF, 500 V 
             
             
                 
               Resistor 52 
               100 Ω 
             
             
                 
               Resistor 54 
               100 Ω 
             
             
                 
               Diode 56 
               1N4148 
             
             
                 
               Diode 58 
               1N4148 
             
             
                 
               Inductor 64 
               1 mH 
             
             
                 
               Inductor 66 
               1 mH 
             
             
                 
               Diode Clamp 68 
               1N4739 
             
             
                 
               Diode Clamp 70 
               1N4739 
             
             
                 
               Diode 72 
               8ETH06S 
             
             
                 
               Diode 74 
               8ETH06S 
             
             
                 
               Capacitor 76 
               1 nF, 500 V 
             
             
                 
               Capacitor 78 
               1 nF, 500 V 
             
             
                 
               Capacitors 90, 92, 94, 96, 98 
               150 pF, 2 kV 
             
             
                 
               Diodes 100, 102, 104, 106, 
               1 kV 
             
             
                 
               108, 110 
             
             
                 
               Switch 127 
               FCD7N60 
             
             
                 
                 
             
           
        
       
     
   
   The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.