Abstract:
An electronic ballast circuit (FIG.  1 ) includes a voltage-regulated DC power source and an H-bridge having a plurality of transistors (Q 5,  Q 6,  Q 7,  Q 8 ). The circuit operates a high intensity discharge lamp ( 50 ). A regulating circuit electrically connected to the transistors regulates the peak current flowing through the H-bridge by discharging a parasitic capacitance (C parasitic).

Description:
[0001]    This application claims priority to Provisional Application S/No. 60/178,005 filed on Jan. 24, 2000. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to an electronic ballast circuit and a method of operating the circuit. The electronic circuit detects and limits peak current surge, specifically peak current surge through a high intensity discharge (HID) lamp.  
         BACKGROUND AND RELATED ART  
         [0003]    An inrush of current occurs after ignition of a typical D1 HID lamp. This current nominally has a peak of 30 A. A lamp of this type, like any lamp used in automotive HID circuitry, uses an H-bridge, or Full Bridge, as a switching circuit to provide AC current to the lamp. The 30 A surge can cause unreliable operation of the H-bridge control circuitry, or failure of the H-bridge MOSFET switches.  
           [0004]    Handling high current through an electronic ballast circuitry can be accomplished in a variety of ways. One way is to utilize components, such as MOSFETs, that are rated at a given current level that is as high or higher than the maximum current through the system. One drawback of using components having a high current rating is that they are more expensive. Also, the higher rated components tend to be larger and heavier than their lower rated counterparts. In most applications, the space occupied by the components and the weight of the components should be kept to a minimum.  
           [0005]    A second way of handling the excess current is to install a component that provides for controlled inrush of “ON” current. Such components include, but are not limited to, a thyristor or pair of thyristors in an active filter-voltage step-up circuit as disclosed in U.S. Pat. No. 5,719,473 to Huber et al; an output stage designed as a step-up regulator or a blocking oscillator as disclosed in U.S. Pat. No. 5,877,614 to Huber; an oscillatory transformer as disclosed in U.S. Pat. No. 6,078,144 to Twardzik; a multivibrator with hysteresis as disclosed in EP0 757,420; or an inductor of sufficient volt-second capability. The inductor would normally be placed in series with the lamp to limit the peak current. Installing any of these components increases the overall size, complexity and cost of the system.  
         SUMMARY OF THE INVENTION  
         [0006]    It is an object of the present invention to utilize an existing lower rated MOSFET in an H-bridge circuit to limit current flowing therethrough.  
           [0007]    Another object of the present invention is to reduce the gate voltage of a MOSFET in an H-bridge during the period following lamp ignition to limit current flowing through the MOSFET.  
           [0008]    A further object of the present invention is to provide a method for limiting the current through the MOSFET using parasitic capacitance.  
           [0009]    The present invention achieves these and other objects by using at least one of the existing H-bridge MOSFET switches to limit the D1 lamp surge current to a level that is within the rating of the existing devices. The present invention makes use of a parasitic capacitance that is created by operating the circuit in the preferred embodiment. Drawing voltage from one of the resistors in the circuit in order to discharge the parasitic capacitance reduces the voltage at the gate of the MOSFET. The gate then partially closes and the current across the MOSFET is then limited. An advantage of using this approach is that by detecting and limiting the surge, the addition of external components or costly oversized switching MOSFETs can be avoided.  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    This invention may be clearly understood by reference to the attached drawing which shows an electronic ballast circuit for an HID lamp according to a preferred embodiment of the invention.  
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the disclosure and accompanying claims taken in conjunction with the above-described drawing.  
         [0012]    The preferred embodiment of this invention is illustrated in the drawing. In this embodiment, a lamp  50  is an automotive D1 HID lamp. The lamp  50  may be integral with an igniter transformer  60 . The lamp  50  and the transformer  60 , are represented by the circuit inside the dashed box  70 .  
         [0013]    The output of a ballast provides a DC voltage that is fed to an H-bridge switching circuit  30 . Any such ballast with appropriate modulation may be used. The output of the H-bridge  30  has a squarewave voltage with a frequency of approximately 500 Hz. The components of the H-bridge and the operation are described below.  
         [0014]    The discharge lamp  50  includes a conventional sealed arc tube  51  that defines a discharge space therein. The arc tube  51  may be transparent or translucent, as desired. A mercury free halide fill is included within the discharge space in a conventional manner.  
         [0015]    The discharge lamp  50  includes two electrodes sealed therein for establishing an arc discharge. For example, the arc tube  51  comprises two electrodes  53  and  54  sealed therein at opposite ends in a conventional manner. Electrode  54  is electrically connected to a connector  66  and electrode  53  is electrically connected to transformer  60  which is in turn electrically connected to connector  68  thereby electrically connecting the arc tube  51  to the H-bridge switching circuit  30 .  
         [0016]    The H-bridge switching circuit  30  contains four MOSFETs; two low-side MOSFETs Q 5  and Q 6  and two high-side MOSFETs Q 7  and Q 8 . During steady state operation, the circuit alternates between Q 6 ,Q 8  ON, Q 5 ,Q 7  OFF and Q 6 ,Q 8  OFF, Q 5 ,Q 7  ON. Since the invention relates to the time up to and immediately following ignition, the circuitry will only be described in relation to Q 6  and Q 8 , particularly the detection and control circuitry connected to the gate of Q 8 .  
         [0017]    Q 6  and Q 8  are held ON prior to, during and immediately after ignition. The drive source, phase B (PHB) for Q 8  is a switched DC source, i.e., the drive source cycles between −85 volts and ground, and has a frequency of about 500 Hz. The drive source (PHA) (not shown) for Q 7  operates in the same manner. The two drive sources (PHB, PHA) are 180 degrees out of phase and operate at about 50% power.  
         [0018]    The drawing illustrates an embodiment of a gate drive (controlling) circuit for an isolated gate device Q 8  constructed according to the present invention. In this embodiment, the isolated gate device Q 8  is an MOSFET. Those skilled in the art will understand, however, that the gate drive circuit may also be used to drive other FETs, IGBTs and MCTs. Of course, one gate drive circuit may drive one or multiple isolated gate devices Q.  
         [0019]    The gate drive circuit includes an energy storage capacitor C 28  connected to the high side of the ballast voltage output Vout and coupled in series to a resistor R 14  that is in turn coupled to the low side of the power source. C 28  is also coupled in parallel to a zener diode D 7 . Of course, in other embodiments, the diode D 7  may not be a zener diode. C 28 , R 14  and D 7  together form a regulated supply for transistor Q 11 . Q 11  and transistor Q 15  are in a totem-pole configuration connected between the C 28 /R 14  connection and the high side of the power source as shown in FIG. 1. In the illustrated embodiment, the totem-pole configuration consists of an npn transistor Q 11  serially-coupled to a first pnp transistor Q 15 . A transistor R 79  is between the emitters of Q 11 /Q 15  and the gate of Q 8 .  
         [0020]    A second pnp transistor Q 39  acts as a current source to maintain 10VDC across resistor R 75 . R 75  is connected to the high side of the power source and is used for static protection and to pull down the gate of Q 8  and thus turn OFF MOSFET Q 8 . Thus, Q 39  and resistor R 75  form a level shifting circuit. Those skilled in the art will realize, however, that the use of any driver circuit or device is well within the broad scope of the present invention.  
         [0021]    The gate drive circuit further includes resistor R 76  serially coupled between a drive signal phase (PHB) and the emitter of Q 39 . PHB turns isolated gate device Q 8  ON and OFF. As disclosed above, PHB operates 180° out of phase from a second drive signal PHA that controls Q 7 .  
         [0022]    The drive voltage Vb from drive signal phase PHB drives the gate of Q 8  to turn ON the isolated gate device Q 8 . The voltage required to maintain Q 8  in a fully ON state is between 5V and 8V. This voltage is approximately equal to the voltage drop across R 75  minus the voltage drop across the base to emitter junction of Q 11 .  
         [0023]    Due to the circuitry configuration as described above, a parasitic capacitance (C parasitic) occurs between the base and the collector of Q 39 . Those having ordinary skill in the art would normally not want the circuit configured in this manner because the occurrence of C parasitic slows the switching speed of the circuit. However, the present invention includes the previously unknown technique of using C parasitic to reduce the peak current seen by the MOSFETs as described below.  
         [0024]    In order to reduce the peak current seen by Q 8 , the current in the H-bridge needs to be reduced. This current reduction is accomplished by discharging C parasitic. The current to discharge C parasitic comes from Q 39 . Since the discharge current is from the same current source (Q 39 ) used to maintain 10 VDC across R 75 , the current through R 75  is necessarily reduced. When the current through R 75  is reduced, the voltage drop across R 75  is proportionately reduced. As stated above, the voltage at the gate of Q 8  is approximately the voltage drop across R 75  minus the voltage drop across the base-to-emitter junction of Q 11 . Therefore, reducing the voltage across R 75  corresponds to a voltage reduction at the gate of Q 8 . The voltage at the gate of Q 8  goes down to between 2 V-4 V. This voltage drop increases the impedance through the MOSFET channel, reducing the maximum current through Q 8  to about 20 Amps.  
         [0025]    The lamp  50  is initially OFF and remains OFF until the voltage at the secondary side of the transformer  60  is sufficient to ignite the lamp  50 . The ballast normally outputs approximately −400VDC. During ignition another approximately −600VDC is outputted and added to the primary side of the transformer  60 . The combined approximate −1000VD is presented to the primary side of the transformer  60 . The secondary side of the transformer  60  has an output voltage of about 23,000 V, which is sufficient to ignite the lamp.  
         [0026]    The ignition sequence is from zero to five microseconds in duration. Upon ignition, the lamp  50  begins to “glow”, but is not yet “arcing”. Therefore the current is not yet flowing and there is a very high, nearly infinite resistance. The lamp needs to be “arcing” to allow current to flow. The about −400VDC across Vout supplies the energy to initiate current flow and establish “arcing”. Once the lamp is “arcing”, the resistance of the lamp drops to about 10 ohms and a surge of current begins flowing through the circuit. In a time span of approximately 13 microseconds, the current changes from zero to a maximum of about 20 Amps due to the operation of the control circuit as described above.  
         [0027]    As the current increases toward its peak value, the about −400VDC at Vout decreases toward zero volts. This resultant voltage drop creates a charge capacitance, C parasitic, due to the configuration of the circuitry as stated above. Since transistor Q 39  maintains 10VDC across resistor R 75 , the voltage initially across C parasitic would be −390VDC. This voltage is also decreasing toward zero volts with the increase in current.  
         [0028]    C parasitic has been charged so that the positive side is at ground and the negative side is at the upper terminal of R 75 . For C parasitic to discharge, current has to flow through C parasitic from the R 75  side to the ground side of C parasitic. The current used to discharge C parasitic comes from Q 39 . Since this current would normally be used to maintain the 10VDC across R 75 , by using some of the current to discharge C parasitic, the current through and hence the voltage across R 75 , is reduced. As a result, the voltage at the gate of Q 8  goes down to between 2 to 4 V. Reducing the voltage at the gate of Q 8  increases the impedance of Q 8  and the resulting voltage drop at the gate of Q 8  results in a drain to source voltage increase from 0 to 150 V, which limits the lamp peak current to 20 A, which is smaller than the 30A normally found in such devices. This means that the MOSFETs, and thus the device, can be smaller.  
         [0029]    The current limiting occurs during the first phase B cycle. Once the lamp reaches a steady state, phase B and phase A alternate so that either Q 6  and Q 8  are ON, or Q 5  and Q 7  are ON. The voltage through the system is maintained so that raw lamp power consumption is 35 watts. 35 watts of power translates into a nominal Vout of approximately 85 volts, although the voltage may be anywhere in the range of 60 to 100 volts.  
         [0030]    The circuit could be reconfigured to force the circuit to operate in the opposite direction, wherein the peak limiting occurs during the first phase A cycle. External circuitry can be added and the microprocessor controlling the main power output can also be used to force the circuit to operate in the opposite direction.  
         [0031]    The embodiment that has been described herein is set forth here by way of illustration, but not of limitation. It is apparent that other embodiments that will be readily apparent to those skilled in the art may be made without departing materially from the spirit and scope of this invention.