Abstract:
A ballast ( 100 ) includes an inverter ( 140,144,146 ) and a protection circuit that prevents excessive lamp-to-earth-ground fault current. The protection circuit includes a transformer ( 202,204,206,208,210 ) and an inverter disable circuit ( 300 ). The transformer measures a first current going out of one set of ballast output terminals ( 106,108 ) and a second current going into another set of ballast output terminals ( 206,208 ). In response to a substantial imbalance between the first current and the second current, inverter disable circuit ( 300 ) terminates inverter switching. Preferably, protection circuit further includes a restart timer circuit ( 400 ) that, following termination of inverter switching in response to a fault condition, prevents the inverter from restarting for a predetermined delay period.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast with circuitry for protecting against a lamp-to-earth-ground fault condition.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fluorescent lamps used with electronic ballasts periodically fail and require replacement. In most cases, replacement of a failed lamp is performed while AC power is still applied to the ballast; this practice is sometimes referred to as “live relamping.” Since many newer ballast designs have non-isolated outputs, the possibility exists for high frequency output current to travel from the ballast output, through the lamp, through the person replacing the lamp, to fixture ground. Because an electrical shock may be suffered under such circumstances, safety agencies such as Underwriters Laboratories now require that ballasts be tested for this condition. Thus, standards have been established for the maximum current that is allowed to flow from the ballast output through the lamp to fixture ground. For many ballasts, these standards are readily met. However, for some ballasts, such as those models which are designed to operate with higher line voltages (e.g., 277 volts) or shorter lamp lengths (e.g., 2 foot lamps), these standards can be met only by incorporating special protective circuitry in the ballast.  
           [0003]    Some ballast manufacturers have attempted to address the problem of excessive lamp-to-earth-ground current by trying to sense the high frequency leakage current that, in the event of a fault condition, flows out of the ballast output, into the grounded fixture, and back into the ballast via the ballast ground wire that is electrically connected to the fixture during ballast installation. An example of such an approach is described in U.S. Pat. No. 5,363,018. The main problem with this type of detection circuit is that this same type of leakage current normally flows even in the absence of a fault condition, and is actually quite desirable because it aids lamp ignition. Moreover, because the voltage applied to the lamps prior to ignition is much higher than voltage applied after ignition, the magnitude of this “normal” leakage current will be many times higher during the start-up mode than during the steady-state operating mode. Because the magnitude of the normal leakage current that flows into the ballast ground during normal starting conditions can be very close to the magnitude of the undesirable leakage current that flows through the body of a person who accidentally touches the ballast output and fixture ground, the prior art circuits cannot accurately discriminate between “normal” leakage current and the leakage current that occurs due to a true fault condition.  
           [0004]    What is needed, therefore, is a ballast with a protection circuit that is capable of more reliably detecting a lamp-to-earth-ground fault condition. A ballast with such a protection circuit would represent a significant advance over the prior art. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 describes a ballast with a lamp-to-earth-ground fault protection circuit, in accordance with a preferred embodiment of the present invention.  
         [0006]    [0006]FIG. 2 describes a portion of a ballast adapted to power two gas discharge lamp, in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0007]    In a preferred embodiment of the present invention, as described in FIG. 1, a ballast  100  for powering at least one gas discharge lamp  12  includes an inverter  140 , 144 , 146 , 148 , output connections  106 , 108 , 114 , 116 , and a protection circuit  202 , 204 , 206 , 208 , 210 , 300 , 400 . Preferably, ballast  100  further includes a pair of input connections adapted to receive a conventional source of alternating current (e.g., 120 VAC at 60 Hertz), a full-wave diode bridge rectifier  120 , a high frequency bypass capacitor  122 , a boost converter  130 , and a bulk capacitance  132 .  
         [0008]    The inverter is preferably implemented as a driven half-bridge  140 , 144 , 146 , 188 . In combination with a direct-coupled series resonant output circuit  160 , 170 , the inverter supplies a high frequency (e.g., greater than 20 kilohertz) alternating current to gas discharge lamp  12  via first, second, third, and fourth output connections  106 , 108 , 114 , 116 . The inverter includes an inverter drive circuit  140  having a voltage supply input  142  for receiving a direct current (DC) supply voltage. Upon initial application of AC power to ballast  100 , capacitor  150  charges up via resistor  152 . Once the voltage across capacitor  150  reaches a predetermined startup threshold (e.g., 10 volts), inverter drive circuit  140  starts and begins to switch inverter transistors  144 , 146  on and off in a substantially complementary manner. Inverter drive circuit  140  continues to provide inverter switching as long as the voltage at voltage supply input  142  remains greater than a predetermined shutdown threshold (e.g., 8 volts), but will cease to provide inverter switching if the voltage at voltage supply input  142  falls below the predetermined shutdown threshold. During normal operation, the voltage at voltage supply input  142  is maintained well above the shutdown threshold by a “bootstrapping” circuit that includes capacitor  172 , zener diode  174 , diode  190 , and resistor  192 .  
         [0009]    First and second output connections  106 , 108  are adapted for connection to a first filament  14  of lamp  12 , while third and fourth output connections  114 , 116  are adapted for connection to a second filament  16  of lamp  12 .  
         [0010]    Protection circuit  202 , 204 , 206 , 208 , 210 , 300 , 400 , which is coupled to the inverter and the output connections, monitors a first current and a second current. The first current is defined as the absolute value of the difference between the current flowing out of first output connection  106  and the current flowing into second output connection  108 . The second current is defined as the absolute value of the difference between the current flowing out of third output connection  114  and the current flowing into fourth output connection  208 . During normal operation (i.e., when no lamp-to-earth-ground fault condition is present), the first and second currents will be substantially equal. During a fault condition, the first current will not be substantially equal to the second current. Under such a fault condition, the protection circuit will disable the inverter.  
         [0011]    The protection circuit includes a transformer T 2  and an inverter disable circuit  300 . Transformer T 2  comprises four primary windings  202 , 204 , 206 , 208  and a secondary winding  210 . First primary winding  202  is coupled in series with first output connection  106 . Second primary winding  204  is coupled in series with second output connection  108 . Third primary winding  206  is coupled in series with third output connection  114 . Fourth primary winding  208  is coupled in series with the fourth output connection  208 . Secondary winding  210  is part of inverter disable circuit  300 . Preferably, first, second, third, and fourth primary windings have the same number of wire turns (e.g., 1 turn). Secondary winding  210  has a number of wire turns (e.g., 30 turns) that is substantially greater than the number of wire turns on the primary windings. The relative orientation or polarity of the four primary windings is indicated by the dots depicted in FIG. 1.  
         [0012]    During normal operation (i.e., when no fault condition is present), the first current is substantially equal to the second current. Correspondingly, the voltages induced in first and second primary windings  202 , 204  are cancelled out by the voltages induced in third and fourth primary windings  206 , 208 . Consequently, the voltage across secondary winding  210  will be substantially zero.  
         [0013]    During a lamp-to-earth-ground fault condition, the first current will not be substantially equal to the second current because a portion of the current flowing out of output connections  106 , 108  will be diverted to earth ground and, thus, will not flow back into output connections  114 , 116 . Correspondingly, the voltages induced in first and second primary windings  202 , 204  will not be cancelled out by the voltages induced in third and fourth primary windings  206 , 208 . Consequently, a nonzero voltage will appear across secondary winding  210 . In this way, the voltage across secondary winding  210  indicates the presence of a lamp-to-earth-ground fault condition.  
         [0014]    The nonzero voltage that appears across secondary winding  210  during a fault condition is detected by the other circuitry in inverter disable circuit  300  so as to shut down the inverter. More particularly, in response to a nonzero voltage across secondary winding  210  of transformer T 2 , inverter disable circuit  300  terminates inverter switching by coupling the voltage supply input  142  of inverter drive circuit  140  to circuit ground  30 .  
         [0015]    In a preferred embodiment, as described in FIG. 1, inverter disable circuit  300  comprises the secondary winding  210  of transformer T 2 , a disable output  302 , a transistor  320 , a first resistor  304 , a diode  310 , a capacitor  316 , a second resistor  318 , and a third resistor  328 . Secondary winding  210  and first resistor  304  are each coupled between a first node  302  and circuit ground  30 . Disable output  302  is coupled to voltage supply input  142  of inverter drive circuit  140 . Transistor  320  has a base  322 , a collector  324 , and an emitter  326 . Emitter  326  is coupled to circuit ground  30 . Diode  310  is coupled between first node  302  and the base  322  of transistor  320 ; more specifically, diode  310  has an anode coupled to first node  302  and a cathode coupled to base  322 . Capacitor  316  and resistor  318  are each coupled between base  322  and circuit ground  30 . Finally, third resistor  328  is coupled between disable output  302  and emitter  324  of transistor  320 .  
         [0016]    In a prototype ballast configured substantially as shown in FIG. 1, inverter disable circuit  300  was implemented with the following component values:  
         [0017]    Resistor  304 : 100 kilohms  
         [0018]    Diode  310 : 1N4148  
         [0019]    Capacitor  316 : 22 micofarads  
         [0020]    Resistor  318 : 2.2 kilohms  
         [0021]    Transistor  320 : Q2N3904  
         [0022]    Resistor  328 : 10 ohms  
         [0023]    As previously described, it is preferred that transformer T 2  be implemented with one turn on each of the four primary windings  202 , 204 , 206 , 208 , and with thirty ( 30 ) turns on secondary winding  210 .  
         [0024]    During normal operation (i.e., when no fault condition is present), the voltage across secondary winding  210  is approximately zero. Consequently, little or no voltage is provided at the base  322  of transistor  320 , so transistor  320  is off. Accordingly, in the absence of a fault condition, inverter disable circuit  300  does not affect the normal operation of inverter drive circuit  140 .  
         [0025]    If a lamp-to-earth-ground fault condition occurs, a nonzero voltage will develop across secondary winding  210 . The nonzero voltage across secondary winding  310  is peak-detected by diode  310  and capacitor  316 , which causes transistor  320  to turn on. With transistor  320  turned on, resistor  328  is connected between voltage supply input  142  and circuit ground  30 . Because resistor  328  has a very low resistance (e.g., 10 ohms), it quickly discharges capacitor  150 , in spite of the fact that appreciable current continues to be supplied to capacitor  150  from bootstrap power source  172 , 174  via diode  190  and resistor  192 . Consequently, the voltage at voltage supply input  142  rapidly falls below the level necessary to keep inverter drive circuit  140  operating, and inverter switching ceases.  
         [0026]    Preferably, the protection circuit further includes a restart timer circuit  400  for keeping the inverter disabled for a predetermined restart period following detection of lamp-to-earth-ground fault condition. Without restart timer circuit ( 400 ), the inverter will automatically restart after a brief delay period (e.g., on the order of 100-200 milliseconds) after being disabled by inverter disable circuit  300 . In order to ensure that the average rms fault current will be well within safety requirements, it is desirable that the delay period be increased considerably (e.g., to about 1.5 seconds). Restart timer circuit  300  provides such an increased delay.  
         [0027]    In a preferred embodiment, as described in FIG. 1, restart timer circuit  400  comprises a restart input  402 , a restart output  404 , a transistor  418 , a series combination of a diode  406  and a resistor  408 , a capacitor  412 , a second resistor  414 , a third resistor  416 , and a fourth resistor  426 . Restart input  402  is coupled to the bootstrap power source  172 , 174  of the inverter. Restart output  404  is coupled to voltage supply input  142  of inverter drive circuit  140 . Transistor  418  has a collector  422 , an emitter  424 , and a base  420 . Emitter  424  is coupled to circuit ground  30 . The series combination of diode  406  and resistor  408  is coupled between restart input  402  and a second node  410 ; more specifically, diode  406  has an anode coupled to restart input  402  and a cathode coupled to resistor  408 , wherein resistor  408  is coupled to second node  410 . Capacitor  412  is coupled between second node  410  and circuit ground  30 . Second resistor  414  is coupled between second node  410  and base  420  of transistor  418 . Third resistor  416  is coupled between base  420  and circuit ground  30 . Finally, fourth resistor  426  is coupled between restart output  404  and collector  422  of transistor  418 .  
         [0028]    In a prototype ballast configured substantially as shown in FIG. 1, restart timer circuit  400  was implemented with the following component values:  
         [0029]    Diode  406 : 1N4148  
         [0030]    Resistor  408 : 4.7 kilohms  
         [0031]    Capacitor  412 : 10 micofarads  
         [0032]    Resistor  414 : 100 kilohms  
         [0033]    Resistor  416 : 22 kilohms  
         [0034]    Transistor  418 : Q2N3904  
         [0035]    Resistor  426 : 3.3 kilohms  
         [0036]    The detailed operation of restart timer circuit  400  is now described with reference to FIG. 1 as follows.  
         [0037]    During normal operation (i.e., when no fault condition is present), capacitor  412  remains charged, via bootstrap power source  172 , 174  and the series combination of diode  406  and resistor  408 , at a voltage of approximately 15 volts. A portion of the voltage across capacitor  412  is applied (via resistors  414 , 416 ) to transistor  418 , which turns on and connects restart output  404  (and thus voltage supply input  142  of inverter drive circuit  140 ) to circuit ground  30  via resistor  426 . When the inverter is operating normally, the loading introduced by having voltage supply input  142  connected to circuit ground  30  via resistor  426  has no effect because resistor  426  is selected to be suitably large (e.g., 3.3 kilohms) and bootstrap power source  172 , 174  (which supplies operating current to inverter drive circuit  140  via diode  190  and resistor  192 ) is a low impedance current source that is more than capable of supplying the additional current required by the introduction of resistor  426  while the inverter is operating. Thus, during normal conditions, restart timer circuit  400  does not affect the operation of the inverter.  
         [0038]    When inverter drive circuit  140  is shut down by inverter disable circuit  300  in response to fault condition, the connection of resistor  426  between voltage supply input  142  and circuit ground  30  will prevent drive circuit  300  from restarting for as long as the voltage across capacitor  412  is sufficient to keep transistor  418  turned on. More specifically, with resistor  426  present, capacitor  150  will be prevented from charging up (via resistor  152 ) to a level sufficient (e.g., 10 volts, which is the typical turn-on threshold of inverter drive circuit  140 ) to restart inverter drive circuit  140 . With inverter drive circuit  140  disabled, bootstrap power source  172 , 174  no longer supplies current to capacitor  412 , so the voltage across capacitor  412  will begin to decrease at a rate determined by the capacitance of capacitor  412  and the resistances of resistors  414 , 416 . Once the voltage across capacitor  412  falls below a certain level (e.g., a few volts), transistor  418  will turn off and allow capacitor  150  to charge up (via startup resistor  152 ) to a level sufficient (e.g., 10 volts) to restart inverter drive circuit  140 . If a lamp-to-earth-ground fault condition is still present, inverter disable circuit  300  will promptly shut down the inverter once again, and the aforementioned cycle will repeat itself for as long as a fault condition is present.  
         [0039]    It is preferred that capacitor  412  and resistors  414 , 416  be sized such that transistor  418  will remain on for about 1.5 seconds after inverter drive circuit  300  is disabled in response to a fault condition; in a prototype ballast configured substantially as shown in FIG. 1, the preferred restart delay of about 1.5 seconds was achieved with capacitor  412  set at 10 microfarads, resistor  414  set at 100 kilohms, and resistors  416  set at 22 kilohms. Although the inverter will be allowed to restart every 1.5 seconds even if an uncorrected fault condition remains present, the duty cycle (and, thus, the resulting rms value of the ground fault current) will be quite low because the inverter will be promptly shut down by inverter disable circuit  300 .  
         [0040]    Although the ballast  100  described in FIG. 1 has been shown as operating a single lamp  12 , it should be appreciated that the principles of the present invention are readily extended to a ballast that operates multiple lamps connected in series. For example, as described in FIG. 2, the circuitry detailed in FIG. 1 may be adapted to a ballast for powering two lamps  12 , 22  simply by adding an additional filament winding  164  (on transformer T1), an additional current-limiting capacitor  184 , and additional output connections  110 , 112 . As illustrated in FIG. 2, output connections  110 , 112  are coupled to both the second filament of lamp  12  and a first filament of lamp  22 . Output connections  114 , 116  are coupled to a second filament of lamp  22 . Along similar lines, ballast  100  may be further adapted to power three of four series-connected lamps. For each additional lamp, an additional filament winding, current-limiting capacitor, and pair of output connections is required.  
         [0041]    Although the present invention has been described with reference to certain preferred embodiments, numerous modifications and variations can be made by those skilled in the art without departing from the novel spirit and scope of this invention.