Patent Publication Number: US-6657400-B2

Title: Ballast with protection circuit for preventing inverter startup during an output ground-fault condition

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 09/967,192, filed Sep. 28, 2001 and entitled “Ballast with Protection Circuit for Preventing Inverter Startup During an Output Ground-Fault Condition”, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast that includes a circuit for preventing start up of the inverter when one or more of the ballast output wires is shorted to earth ground. 
     BACKGROUND OF THE INVENTION 
     A number of existing electronic ballasts have non-isolated outputs. Such ballasts typically include circuitry for protecting the ballast inverter from damage in the event of lamp fault conditions such as lamp removal or lamp failure. 
     Occasionally, the output wiring of a ballast becomes shorted to earth ground in the lighting fixture. Such a condition can arise, for example, due to the wires becoming loose or pinched. For ballasts with non-isolated outputs, if the inverter begins to operate while an earth ground short is present at one or more of the output wires, a very large low frequency (e.g., 60 hertz) current will flow through the inverter transistors and cause them to fail. 
     Thus, a need exists for a ballast with a protection circuit that prevents the inverter from starting when an output ground-fault condition is present. A ballast with such a protection circuit would represent a significant advance over the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 describes a ballast with a half-bridge inverter and a protection circuit for preventing inverter start up during an output-to-ground fault involving a first output connection, in accordance with a first preferred embodiment of the present invention. 
     FIG. 2 describes a ballast with a half-bridge inverter and a protection circuit for preventing inverter start up during an output-to-ground fault involving the first output connection, in accordance with a second preferred embodiment of the present invention. 
     FIG. 3 describes a ballast with a half-bridge inverter and a protection circuit for preventing inverter start up during an output-to-ground fault involving a first output connection or a second output connection, in accordance with a third preferred embodiment of the present invention. 
     FIG. 4 describes a ballast with a half-bridge inverter and a protection circuit for preventing inverter start up during an output-to-ground fault involving a first output connection or a second output connection or a third output connection, in accordance with a fourth preferred embodiment of the present invention. 
     FIG. 5 describes a ballast with a full-bridge inverter and a protection circuit for preventing inverter start up during an output-to-ground fault involving a first output connection or a second output connection, in accordance with a fifth preferred embodiment of the present invention. 
     FIG. 6 describes a ballast with a with a half-bridge inverter and a protection circuit for preventing inverter start up during an output-to-ground fault involving a first output connection, in accordance with a sixth preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first preferred embodiment of the present invention is described in FIG.  1 . Ballast  10  includes a rectifier circuit  100 , an inverter  200 , an output circuit  300 , and a protection circuit  400 . 
     Rectifier circuit  100  has first and second input terminals  102 , 104  for receiving a source of conventional alternating current, such as 120 volts AC at 60 hertz, and first and second output terminals  106 , 108 . Second output terminal  108  is coupled to a circuit ground node  60 . Rectifier circuit  100  includes a full-wave diode bridge  110  and a capacitor  112 . During operation, capacitor  112  is sufficiently large (e.g., on the order of tens of microfarads) such that a substantially direct current (DC) voltage is provided between output terminals  106 , 108 . Alternatively, and as known in the prior art, a boost converter may be inserted between output terminals  106 , 108  and inverter  200  so as to provide power factor correction and other benefits, in which case capacitor  112  is selected to be relatively small (e.g., on the order of tenths of a microfarad) and the voltage between output terminals  106 , 108  is substantially unfiltered, full-wave rectified AC (i.e., “pulsating DC”). In either case, a substantially DC voltage is provided to inverter  200 . 
     Significantly, the voltage that exists between second output terminal  108  and earth ground (or, equivalently, the voltage that exists between second output terminal  108  and second input terminal  104 ; second input terminal  104  is coupled to the neutral wire of AC source  20 , which is at the same potential as earth ground) is low frequency (e.g., 60 hertz) half-wave rectified AC. 
     Inverter  200  includes first and second input terminals  202 , 204 , an output terminal  206 , first and second inverter switches  210 , 220 , a drive circuit  230 , and a DC supply circuit that includes resistor  240 , capacitor  250 , capacitor  260 , diode  262 , and a zener diode  264 . First input terminal  202  is coupled to first output terminal  106  of rectifier circuit  100 . Second input terminal  204  is coupled to second output terminal  108  of rectifier circuit  100 . First inverter switch  210  is coupled between first input terminal  210  and output terminal  206 . Second inverter switch  220  is coupled between output terminal  206  and circuit ground  60 . As depicted in FIG. 1, inverter switches  210 , 220  are preferably implemented as field-effect transistors. Drive circuit  230  is coupled to inverter switches  210 , 220 , and includes a DC supply input  232 . Drive circuit  230  may be implemented using any of a number of circuits known to those skilled in the art, such as the IR2155 high-side driver integrated circuit manufactured by International Rectifier. Alternatively, although not explicitly shown or described in the drawings, drive circuit  230  may be implemented using any of a number of a self-oscillating drive arrangements known to those skilled in the art; for example, drive circuit  230  may include a diac-based start up circuit for initiating inverter operation and a feedback circuit that uses signals from output circuit  300  to provide inverter switching once the inverter begins to operate. 
     During operation, drive circuit  230  turns inverter switches  210 , 220  on and off in a substantially complementary fashion and preferably at a high frequency rate in excess of 20,000 hertz. Drive circuit  230  initially turns on when the voltage at DC supply input  232  exceeds a start up threshold (e.g., 10 volts), and remains on as long as the voltage at DC supply input  232  remains above a turn-off threshold (e.g., 8 volts). Resistor  240  and capacitor  250  are coupled to DC supply input  232  and provide energy for initially turning on drive circuit  230 . Once inverter  200  begins to operate, energy from output circuit  300  is delivered, via capacitor  260  and diode  262 , to capacitor  250  and drive circuit  230 . This low-impedance “bootstrapping” circuit supplies the operating current required by drive circuit  230  and maintains the voltage across capacitor  250  at a value (e.g., 15 volts) well above the turn-off threshold (e.g., 8 volts) of drive circuit  230 . Zener diode  264  protects drive circuit  230  from overvoltage and/or excessive power dissipation by ensuring that the voltage at DC supply input  230  does not exceed a specified level (e.g., 15 volts). 
     Output circuit  300  includes first and second output connections  302 ,  306 , a resonant inductor  320 , a resonant capacitor  330 , and a direct current (DC) blocking capacitor  340 . First and second output connections  302 , 306  are adapted for connection to a lamp load comprising at least one gas discharge lamp  30 . Resonant inductor  320  is coupled between inverter output terminal  206  and first output connection  302 . Resonant capacitor  330  is coupled between first output connection  302  and circuit ground  60 . DC blocking capacitor  340  is coupled between second output connection  306  and circuit ground  60 . During operation, resonant inductor  320  and resonant capacitor  330  function in a well-known manner as a series resonant circuit having a natural resonant frequency that is typically at or near the frequency at which inverter switches  210 , 220  are turned on and off. Output circuit  300  supplies a high voltage for igniting lamp  30 , as well as a magnitude-limited current for operating lamp  30  in a controlled manner. DC blocking capacitor  300  blocks the DC component in the inverter output voltage (which is equal to half of the rectifier output voltage) and thus prevents substantial DC components from appearing in the voltage and current provided to lamp  30  during steady-state operation. 
     Protection circuit  400  includes an input  402  coupled to inverter output  206 , and an output coupled to DC supply input  232  of drive circuit  230 . During operation, protection circuit  400  prevents inverter  200  from starting if first output connection  302  is shorted to earth ground. 
     As described in FIG. 1, in a first preferred embodiment of the present invention, protection circuit  400  includes a first resistor  420 , a second resistor  440 , an electronic switch  450 , and a third resistor  460 . First resistor  420  is coupled between input  402  and a first node  430 . Second resistor  440  is coupled between first node  430  and circuit ground  60 . Electronic switch  450  is preferably implemented as a NPN bipolar junction transistor having a base  452 , a collector  454 , and an emitter  456 . Base  452  is coupled to first node  430 . Emitter  456  is coupled to circuit ground  60 . Third resistor  460  is coupled between output  410  and the collector  454  of transistor  450 . 
     In a prototype ballast configured substantially as shown in FIG. 1, the components of protection circuit  400 , and selected components of the DC supply circuit of inverter  200 , were sized as follows: 
     Resistor  240 : 220 kilohms 
     Capacitor  250 : 22 microfarads 
     Resistor  420 : 220 kilohms 
     Resistor  440 : 2.2 kilohms 
     Transistor  450 : 2N3904 
     Resistor  460 : 2.2 kilohms 
     The detailed operation of protection circuit  400  is now explained with reference to FIG. 1 as follows. When AC power is initially applied to ballast  10 , drive circuit  230  and inverter  200  are off and remain off until such time as the voltage at DC supply input  232  reaches the predetermined start up threshold (e.g., 10 volts) of drive circuit  230 . In the absence of a ground fault condition at output connection  302 , protection circuit  400  will exert no effect upon inverter start up because transistor  450  will be non-conductive prior to inverter start up. With transistor  450  off, capacitor  250  charges up via resistor  240 . Once the voltage across capacitor  250  reaches the start up threshold (e.g., 10 volts), drive circuit  230  turns on and begins to turn inverter switches  210 , 220  on and off in a periodic manner. 
     At this point, with inverter  200  operating, the voltage between inverter output  206  and circuit ground  60  varies between zero and a high DC value (i.e., the DC voltage provided between inverter input terminals  202 , 204 ) at a high frequency rate, which causes two things to occur. First, the voltage at inverter output  206  excites output circuit  300 . Consequently, bootstrapping energy is fed back from output circuit  300  to capacitor  250  and drive circuit  230  via capacitor  260  and diode  262 , thereby keeping drive circuit  230  active. Second, during those intervals when the voltage at inverter output  206  is high, sufficient voltage is developed across resistor  440  to turn on transistor  450 . Thus, transistor  450  turns on and off at a high frequency rate. However, this exerts no substantial effect on the operation of inverter  200  because, even with transistor  450  on and resistor  460  coupled to circuit ground  60 , abundant bootstrapping current is provided to maintain the voltage at DC supply input  232  well above the turn-off threshold (e.g., 8 volts) of drive circuit  230 ; for this reason, resistor  460  is sized sufficiently large (e.g., 2.2 kilohms) so as not to present so great a load upon the bootstrapping circuit. Thus, once inverter operation commences, protection circuit  400  has no effect on the continued operation of inverter  200 . 
     If, on the other hand, a ground fault condition is present at first output connection  302  prior to inverter start up, the following events occur. As previously discussed, once AC power is initially applied to ballast  10 , the voltage between circuit ground  60  and earth ground is low frequency (e.g., 60 hertz) half-wave rectified AC. More specifically, during the negative half-cycles of the voltage provided by AC source  20  (i.e., when a negative voltage exists between first input terminal  102  and second input terminal  104 ; equivalently, when a positive voltage exists between second input terminal  104  and first input terminal  102 ), the lower left-hand diode in bridge rectifier  110  is forward-biased and the voltage between earth ground (i.e., the neutral wire at the lower end of AC source  20 ) and circuit ground  60  has a positive polarity. Consequently, under a fault condition wherein first output connection  302  is connected to earth ground, a positive current flows up from earth ground, into first output connection  302 , through resonant inductor  320 , into input  402 , through resistors  420 , 440 , into circuit ground  60 , through the lower left-hand diode of bridge rectifier  102 , out of first input terminal  102 , through AC source, and back to the neutral wire of AC source  20  (which is at the same potential as earth ground). This positive current produces sufficient voltage (e.g., greater than 0.7 volts) across resistor  440  to activate transistor  450 . With transistor  450  turned on, DC supply input  232  is coupled to circuit ground  60  via resistor  460 . Because resistor  460  has a resistance (e.g., 2.2 kilohms) that is very low relative to that of resistor  240  (e.g., 220 kilohms), the voltage across capacitor  250  is limited to a low value that is less than the start up threshold of drive circuit  230 . Transistor  450  will be on during only the negative half-cycles of the AC source voltage (during the positive half-cycles of the AC source voltage, the voltage between earth ground and circuit ground  60  is negative, and thus incapable of keeping transistor  450  on), but that is still sufficient (provided that the RC time constant of resistor  240  and capacitor  250  is sufficiently large) to prevent the voltage across capacitor  250  from reaching the start up threshold. In this way, inverter  200  is prevented from starting when an earth ground fault condition is present at output connection  302  prior to inverter start up. 
     It should be appreciated that protection circuit  400  does not necessarily require a true short (i.e., zero ohm impedance) between first output connection  302  and earth ground in order to prevent inverter start up. For example, with the component values discussed above, protection circuit  400  will prevent inverter start up as long as the impedance between first output connection  302  and earth ground is less than about 100,000 ohms. Given that inverter damage may occur even for earth ground faults in which there is a substantial impedance between first output connection  302  and earth ground, this added capability of protection circuit  400  is a potentially significant advantage. 
     Turning now to FIG. 2, in a second preferred embodiment of the present invention, protection circuit  400  is configured in substantially the same manner as previously described with reference to FIG. 1, except that input  402  is coupled to first output connection  302  instead of inverter output  206 . Even with this modification, the operation of protection circuit  400  remains substantially unchanged from that which was previously described. More specifically, because the voltage that exists between circuit ground  60  and earth ground is low frequency (e.g., 60 hertz) half-wave rectified AC, the impedance of resonant inductor  320  is negligible compared to that of resistor  420 . Thus, it makes no significant functional difference whether input  402  is coupled to inverter output  206  (as in FIG. 1) or first output connection  302  (as in FIG.  2 ); either way, protection circuit  400  will respond to occurrence of an earth ground fault at first output connection  302 . However, because the maximum voltage at first output connection  302  is (due to resonant voltage gain that occurs prior to ignition of lamp  30 ) substantially greater than the maximum voltage at inverter output  206 , it may be necessary to increase the voltage rating of resistor  420  accordingly if the embodiment of FIG. 2 is employed. 
     Referring now to FIG. 3, in a third preferred embodiment of the present invention, protection circuit  400 ′ includes a second input  404  and a fourth resistor  422 , in addition to the components present in protection circuit  400  in FIG.  1 . Second input  404  is coupled to second output connection  306 . Fourth resistor  422  is coupled between second input  404  and first node  430 . The addition of fourth resistor  422  allows protection circuit  400 ′ to monitor both output connections  302 , 306  and correspondingly prevent the inverter from starting if an earth ground fault is present at either (or both) of the output connections  302 , 306 . 
     Because resistor  422  is coupled, via input  404 , to DC blocking capacitor  340  (which, during operation of lamp  30 , has a large positive DC voltage across it all of the time), it is likely that transistor  450  will remain on all of the time after lamp  30  begins to operate following inverter start up. This should be contrasted with what was previously described with reference to the circuit of FIG. 1, where it was explained that transistor  450  will turn on and off at a high frequency rate (when input  402  is coupled to inverter output  206 ). Although this behavior in the circuit of FIG. 3 does not impact the desired functionality of protection circuit  400 ′ in preventing inverter start up under an output ground fault condition, it is relevant from a design standpoint because the designer must be sure that resistor  460  is large enough so as not to present an unduly large load that interferes with proper bootstrapping during normal operation of the inverter. 
     Although not explicitly shown in the drawings, it should be appreciated that first resistor  420  in FIG. 3 may alternatively be coupled to first output connection  302  rather than inverter output  206 , along the same lines as previously discussed, without substantially affecting the desired operation of protection circuit  400 ′. 
     Turning now to FIG. 4, in a fourth preferred embodiment that is suited for a ballast that powers a lamp load comprising two lamps  30 , 32 , protection circuit  400 ″ includes three resistors  420 , 422 , 424 , each of which is coupled to a corresponding output connection  302 , 304 , 306 . More specifically, the output circuit includes first, second, and third output connections  302 , 304 , 306 . First and second output connections  302 , 304  are adapted for connection to a first lamp  30 , while second and third output connections  304 , 306  are adapted for connection to a second lamp  32 . Second output connection  304  is coupled to a junction  34  between first lamp  30  and second lamp  32 . Protection circuit  400 ″ includes first, second, and third inputs  402 , 404 , 406 , and first, fourth, and fifth resistors  420 , 422 , 424 . First input  402  is coupled to inverter output  206 . Second input  404  is coupled to second output connection  304 . Third input is coupled to third output connection  306 . First resistor  420  is coupled between first input  402  and first node  430 . Fourth resistor  422  is coupled between second input  404  and first node  430 . Finally, fifth resistor  406  is coupled between third input  406  and first node  430 . 
     In the circuit of FIG. 4, protection circuit  400 ″ monitors all three output connections  302 , 304 , 306  and correspondingly prevents the inverter from starting if an earth ground fault is present at any one (or any pair, or all three) of the output connections  302 , 304 , 306 . As previously discussed, first input  402  may alternatively be coupled to first output connection  302  (rather than inverter output  206 ) without affecting the desired operation of protection circuit  400 ″. 
     It should be appreciated that protection circuit  400 ″ may be further modified, in like fashion, to accommodate more than two lamps (i.e., more than three output connections) simply be adding additional inputs and resistors to protection circuit  400 ″. 
     Turning now to FIG. 5, in a fifth preferred embodiment of the present invention, inverter  500  is a full-bridge inverter comprising first and second input terminals  502 , 504 , first and second output terminals  506 , 508 , first, second, third, and fourth inverter switches  510 , 512 , 516 , 518 , a drive circuit  530 , and a DC supply  570 . Input terminals  502 , 504  are intended for connection to either a rectifier or a rectifier followed by a boost converter. Output terminals  506 , 508  are adapted for connection to a lamp load comprising at least one gas discharge lamp  30 . First inverter switch  510  is coupled between first input terminal  502  and second output terminal  508 . Second inverter switch  512  is coupled between second output terminal  508  and circuit ground  60 . Third inverter switch  516  is coupled between first input terminal  502  and first output terminal  506 . Fourth inverter switch  518  is coupled between first output terminal  506  and circuit ground  60 . Drive circuit  530  is coupled to each of the inverter switches  510 , 512 , 516 , 518 , and includes a DC supply input  532 . During operation, drive circuit  530  turns each opposing pair of inverter switches (i.e., switches  510 , 518  are one pair, switches  512 , 516  are the other pair) on and off in a substantially complementary fashion and preferably at a high frequency rate in excess of 20,000 hertz. Drive circuit  530  initially turns on when the voltage at DC supply input  532  exceeds a start up threshold (e.g., 10 volts), and remains on as long as the voltage at DC supply input  532  remains above a turn-off threshold (e.g., 8 volts). DC supply  570 , which is coupled to DC supply input  532 , provides energy for initiating operation of drive circuit  530  and maintaining operation of drive circuit  530  after inverter switching commences. 
     Protection circuit  600  includes a first input  602  coupled to first output terminal  506 , a second input  604  coupled to second output terminal  508 , and an output  610  coupled to DC supply input  532  of drive circuit  530 . During operation, protection circuit  600  prevents inverter  500  from starting if either one, or both, of output terminals  506 , 508  is shorted to earth ground. 
     As described in FIG. 5, protection circuit  600  includes a first resistor  620 , a second resistor  622 , a third resistor  640 , an electronic switch  650 , and a fourth resistor  660 . First resistor  620  is coupled between first input  602  and a first node  630 . Second resistor  622  is coupled between second input  604  and first node  630 . Third resistor  640  is coupled between first node  630  and circuit ground  60 . Electronic switch  650  is preferably implemented as a NPN bipolar junction transistor having a base  652 , a collector  654 , and an emitter  656 . Base  652  is coupled to first node  630 . Emitter  656  is coupled to circuit ground  60 . Fourth resistor  660  is coupled between output  610  and the collector  654  of transistor  650 . 
     The detailed operation of protection circuit  600  is substantially similar to that which was previously described with reference to the other preferred embodiments disclosed herein. 
     As previously discussed with reference to FIG. 1, resistor  240  and capacitor  250  function as a start up circuit for initially turning on drive circuit  230 . In those applications where resistor  240  and capacitor  250  have suitably large values (e.g., 220 kilohms and 22 microfarads, respectively), the arrangement of FIG. 1 works well. If, however, resistor  240  and/or capacitor  250  are substantially lowered in value (e.g., to 120 kilohms and 2.2 microfarads, respectively) in order to accommodate “low-line” operation where the AC line voltage is considerably lower than its nominal value (e.g., 90 volts instead of the nominal 120 volts), it is possible that the inverter will start even if an output fault is present. More particularly, as previously discussed, when an output fault is present, transistor  450  will be on only during the negative half cycles of the AC line voltage. However, with resistor  240  coupled to a source of full-wave rectified AC voltage, capacitor  250  will be allowed to charge up during the positive half cycles when transistor  450  is off. If the RC time constant of resistor  240  and capacitor  250  is very short (i.e., small enough to allow the voltage across capacitor  250  to reach the start up threshold of 10 volts during one positive half-cycle), the inverter may momentarily start even if an output fault is present. The possibility of this occurring becomes even greater when operating under a “high line” condition where the AC line voltage may exceed its nominal value by as much as twenty percent (e.g., 144 volts instead of the nominal 120 volts). Although increasing the resistance of resistor  240  and/or the capacitance of capacitor  250  may solve the problem, that is not a feasible design option; for example, the resistance of resistor  240  must be low enough to ensure normal inverter start up under low-line conditions. 
     In order to properly solve this problem, and thereby ensure that the inverter does not start up when a fault is present at the ballast output, the start up circuit may be modified by changing the connection of the start up resistor. More specifically, in a sixth preferred embodiment as described in FIG. 6, start up resistor  242  is coupled to the second input terminal  104  of rectifier circuit  100  (as opposed to the arrangement in FIG. 1, in which start up resistor  240  is coupled to the first output terminal  106 ). Because the voltage between input terminal  104  and circuit ground  60  is half-wave rectified AC that is substantially in phase with the voltage that activates transistor  450  when an output fault is present, resistor  242  will supply charging current to capacitor  250  only during the same half of the AC line cycle as the fault signal. Thus, when transistor  450  is off, no charging current is provided to capacitor  250 , and when transistor  450  is on, charging current flows through resistor  242  but capacitor  250  is prevented from charging up. In this way, inverter start up is prevented under a fault condition, even if the RC time constant of resistor  242  and capacitor  250  is very short. 
     In a prototype ballast configured substantially as shown in FIG. 6, the components of protection circuit  400 , and selected components of the DC supply circuit of inverter  200 ′, were sized as follows: 
     Resistor  242 : 120 kilohms 
     Capacitor  250 : 2.2 microfarads 
     Resistor  420 : 200 kilohms 
     Resistor  440 : 10 kilohms 
     Transistor  450 : 2N3904 
     Resistor  460 : 4.7 kilohms 
     The modified start up circuit described in FIG. 6 is equally applicable to the embodiments previously described with reference to FIGS. 2-5. 
     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. For example, although the preferred embodiments disclosed herein describe inverters  200 , 500  as a driven-type inverter, it should be understood that inverter need not be a driven-type inverter, and that protection circuits  400 ,  400 ′,  400 ″ may be used in conjunction with a self-oscillating type inverter (e.g., to prevent triggering of a diac in a diac-based inverter starting circuit). As another example, although all of the preferred embodiments disclosed herein relate to a discrete circuit implementation of protection circuits  400 ,  400 ′,  400 ″, it should be appreciated that each protection circuit may alternatively by realized using a non-discrete means, such as a microcontroller or custom integrated circuit along with peripheral components that is programmed or configured to provide the input/output functionality of protection circuits  400 ,  400 ′,  400 ″ as described herein.