Abstract:
In an instant start ballast, dimming control is provided over a range of operation in which lamps driven by the ballast do not require external cathode heating. An interface circuit ( 92 ) includes a winding ( 90 ) that is inductively coupled to windings ( 68, 70 ) of an inverter circuit ( 12 ). The interface circuit ( 92 ) also includes a variable impedance in parallel with the winding ( 90 ) where the variable impedance includes a transistor ( 96 ) and a Zener diode ( 98 ). By varying an input voltage across control leads ( 94 ), the apparent inductance of the winding ( 90 ) is varied. This variance affects the switching frequency of the inverter circuit ( 12 ) affecting the frequency of a drive signal provided to the lamps. Thus the instant start ballast can be dimmed without use of multiple ballasts and/or external cathode heating.

Description:
[0001]    This application relates to currently pending U.S. application Ser. No. 11/343,335 to Nerone, et al., which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present application relates to electronic lighting. More specifically, it relates to a dimmable electronic ballast and will be described with particular reference thereto. It is to be appreciated that the present ballast can also be used in other lighting applications, and is not limited to the aforementioned application. 
         [0003]    In the past, dimmable ballast systems have typically been composed of multiple discrete ballasts. In order to achieve a lower light output, one or more of the ballasts would be shut off. Conversely, when greater light output is desired, more ballasts are activated. This approach has the drawback of only being able to produce discrete levels of light output. With each ballast only able to produce a single light output, the aggregate output is limited to what the various combinations of the ballasts present can produce. Moreover, this setup also requires multiple lamps for the same space to be lighted, resulting in an inefficient use of space. 
         [0004]    Another approach in dimmable lighting applications has been to dim a single ballast by varying the operating voltage of the ballast, that is, by varying the voltage of the high frequency signal used to power the lamp. One drawback in such a system is that as the voltage of the high frequency signal is diminished, the lamp cathodes cool down. This can lead to the lamp extinguishing, and unnecessary damage to the cathodes. To avoid this problem, such systems apply an external cathode heating. While this solves the problem of premature extinguishing, the ballast is drawing power that is not being used to power the lamp. This decreases the overall efficiency of the ballast. 
         [0005]    The present application contemplates a new and improved dimmable electronic ballast that overcomes the above-referenced problems and others. 
       BRIEF DESCRIPTION 
       [0006]    In accordance with one aspect, a dimming instant start lighting ballast circuit is provided. First and second switches receive a direct current and convert it to an alternating current and provide the alternating current to at least one lamp. A first inductive winding is connected between the gate and source of the first switch. A second inductive winding is connected between the gate and source of the second switch. A resonant portion determines an operating frequency of the ballast. An interface circuit receives an input and controls the light output of the at least one lamp. 
         [0007]    In accordance with another aspect, a method of dimming a fluorescent lamp with an instant start ballast is provided. A DC signal is provided to the ballast. The DC signal is converted into an AC signal. The AC signal is provided to power at least one lamp. The frequency of the AC signal to the at least one lamp is varied with an interface circuit. 
         [0008]    In accordance with another aspect, an interface circuit for dimming an instant start ballast is provided. A control winding interfaces with the ballast. A variable impedance in parallel with the control winding changes the apparent inductance of the control winding. Control leads for inputting a control signal that changes the conductivity of the variable impedance are included. A Zener diode provides startup protection. A rectifier converts an AC signal to a DC signal. Smoothing circuitry smoothes the DC signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a circuit diagram of a dimmable instant start electronic ballast, in accordance with the present application. 
           [0010]      FIG. 2  is a circuit diagram of one particular embodiment of the interface circuit of  FIG. 1 . 
           [0011]      FIG. 3  is a circuit diagram of a second embodiment of the interface circuit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    With reference to  FIG. 1 , a ballast circuit  10 , such as an instant start ballast, includes an inverter circuit  12  resonant circuit or network  14 , and a clamping circuit  16 . A DC voltage is supplied to the inverter  12  via a positive bus rail  18  running from a positive voltage terminal  20 . The circuit  10  completes at a common conductor  22  connected to a ground or common terminal  24 . A high frequency bus  26  is generated by the resonant circuit  14  as described in more detail below. First, second, third, through n th  lamps  28 ,  30 ,  32 ,  34  are coupled to the high frequency bus  26  via first, second, third, and n th  ballasting capacitors  36 ,  38 ,  40 ,  42 . Thus, if one lamp is removed, the others continue to operate. It is contemplated that any number of lamps can be connected to the high frequency bus  26 . E.g., lamps  28 ,  30 ,  32 ,  34  are coupled to the high frequency bus  26  via an associated ballasting capacitor  36 ,  38 ,  40 ,  42 . 
         [0013]    The inverter  12  includes analogous upper and lower, that is, first and second switches  44  and  46 , for example, two n-channel MOSFET devices (as shown), serially connected between conductors  18  and  22 , to excite the resonant circuit  14 . It is to be understood that other types of transistors, such as p-channel MOSFETs, other field effect transistors, or bipolar junction transistors may also be so configured. The high frequency bus  26  is generated by the inverter  12  and the resonant circuit  14  and includes a resonant inductor  48  and an equivalent resonant capacitance that includes the equivalence of first, second, and third capacitors  50 ,  52 ,  54  and ballasting capacitors  36 ,  38 ,  40 ,  42  which also prevent DC current from flowing through the lamps  28 ,  30 ,  32 ,  34 . Although they do contribute to the resonant circuit, the ballasting capacitors  36 ,  38 ,  40 ,  42  are primarily used as ballasting capacitors. The switches  44  and  46  cooperate to provide a square wave at a common first node  56  to excite the resonant circuit  14 . 
         [0014]    First and second gate drive circuits, generally designated  60  and  62 , respectively include first and second driving inductors  64 ,  66  that are secondary windings mutually coupled to the resonant inductor  48  to induce a voltage in the driving inductors  64 ,  66  proportional to the instantaneous rate of change of current in the resonant circuit  14 . First and second secondary inductors  68 ,  70  are serially connected to the first and second driving inductors  64 ,  66  and the gates of switches  44  and  46 . The gate drive circuits  60 ,  62  are used to control the operation of the respective upper and lower switches  44 ,  46 . More particularly, the gate drive circuits  60 ,  62  maintain the upper switch  44  “on” for a first half cycle and the lower switch  46  “on” for a second half cycle. The square wave is generated at the node  56  and is used to excite the resonant circuit. First and second bi-directional voltage clamps  71 ,  73  are connected in parallel to the secondary inductors  68 ,  70 , respectively, each including a pair of back-to-back Zener diodes. The bi-directional voltage clamps  71 ,  73  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. Each bi-directional voltage clamp  71 ,  73  cooperates with the respective first or second secondary inductor  68 ,  70  so that the phase angle between the fundamental frequency component of voltage across the resonant circuit  14  and the AC current in the resonant inductor  48  approaches zero during ignition of the lamps. 
         [0015]    Upper and lower capacitors  72 ,  74  are connected in series with the respective first and second secondary inductors  68 ,  70 . In the starting process, the capacitor  72  is charged from the voltage terminal  18 . The voltage across the capacitor  72  is initially zero, and during the starting process, the serially connected inductors  64  and  68  act essentially as a short circuit, due to the relatively long time constant for charging the capacitor  72 . When the capacitor  72  is charged to the threshold voltage of the gate-to-source voltage of the switch  44  (e.g. 2-3 Volts), the switch  44  turns ON, which results in a small bias current flowing through the switch  44 . The resulting current biases the switch  44  in a common drain, Class A amplifier configuration. This produces an amplifier of sufficient gain such that the combination of the resonant circuit  14  and the gate control circuit  60  produces a regenerative action that starts the inverter into oscillation, near the resonant frequency of the network including the capacitor  72  and the inductor  68 . The generated frequency is above the resonant frequency of the resonant circuit  14 . This produces a resonant current that lags the fundamental of the voltage produced at the common node  56 , allowing the inverter  12  to operate in the soft-switching mode prior to igniting the lamps. Thus, the inverter  12  starts operating in the linear mode and transitions into the switching Class D mode. Then, as the current builds up through the resonant circuit  14 , the voltage of the high frequency bus  26  increases to ignite the lamps, while maintaining the soft-switching mode, through ignition and into the conducting, arc mode of the lamps. 
         [0016]    During steady state operation of the ballast circuit  10 , the voltage at the common node  56 , being a square wave, is approximately one-half of the voltage of the positive terminal  20 . The bias voltage that once existed on the capacitor  72  diminishes. The frequency of operation is such that a first network  76  including the capacitor  72  and the inductor  68  and a second network  78  that includes the capacitor  74  and the inductor  70  are equivalently inductive. That is, the frequency of operation is above the resonant frequency of the identical first and second networks  76 ,  78 . This results in the proper phase shift of the gate circuit to allow the current flowing through the inductor  48  to lag the fundamental frequency of the voltage produced at the common node  56 . Thus, soft-switching of the inverter  12  is maintained during the steady-state operation. 
         [0017]    The output voltage of the inverter  12  is clamped by serially connected clamping diodes  80 ,  82  of the clamping circuit  16  to limit high voltage generated to start the lamps  28 ,  30 ,  32 ,  34 . The clamping circuit  16  further includes the second and third capacitors  52 ,  54 , which are essentially connected in parallel to each other. Each clamping diode  80 ,  82  is connected across an associated second or third capacitor  52 ,  54 . Prior to the lamps starting, the lamps&#39; circuits are open, since impedance of each lamp  28 ,  30 ,  32 ,  34  is seen as very high impedance. The resonant circuit  14  is composed of the capacitors  36 ,  38 ,  40 ,  42 ,  50 ,  52 , and  54  and the resonant inductor  48 . The resonant circuit  14  is driven near resonance. As the output voltage at the common node  56  increases, the clamping diodes  80 ,  82  start to clamp, preventing the voltage across the second and third capacitors  52 ,  54  from changing sign and limiting the output voltage to a value that does not cause overheating of the inverter  12  components. When the clamping diodes  80 ,  82  are clamping the second and third capacitors  52 ,  54  the resonant circuit  14  becomes composed of the ballast capacitors  36 ,  38 ,  40 ,  42  and the resonant inductor  48 . That is, the resonance is achieved when the clamping diodes  80 ,  82  are not conducting. When the lamps ignite, the impedance decreases quickly. The voltage at the common node  52  decreases accordingly. The clamping diodes  80 ,  82  discontinue clamping the second and third capacitors  52 ,  54  as the ballast  10  enters steady state operation. The resonance is dictated again by the capacitors  36 ,  38 ,  40 ,  42 ,  50 ,  52 , and  54  and the resonant inductor  48 . 
         [0018]    In the manner described above, the inverter  12  provides a high frequency bus  26  at the common node  56  while maintaining the soft switching condition for switches  44 ,  46 . The inverter  12  is able to start a single lamp when the rest of the lamps are lit because there is sufficient voltage at the high frequency bus to allow for ignition. 
         [0019]    An interface inductor  90  is coupled to the inductors  68  and  70 . The interface inductor  90  provides an interface between an interface circuit  92  and the inverter  12 . With reference now to  FIG. 2 , a continuous interface circuit is provided. An input is provided to the interface circuit across control leads  94 . The external signal may be, for example, from 0 to 10 Volts. If the 10 Volts is applied, then the ballast  10  runs at 100%, whereas if a 0 Volt signal is applied, then the ballast  10  runs at the minimum value that does not require external cathode heating (about 50-60%), with dimming being continuous across the 0-10 volt input signal corresponding to 100%-50/60% of ballast operation. 
         [0020]    More specifically, the interface inductor  90  is manipulated to change its apparent inductance. This, in turn, affects the operating frequency of the ballast  10 , which is what dims the lamps, by reducing the power output to the lamps. A variable impedance is placed in parallel with the interface inductor  90  to manipulate its apparent inductance. The variable impedance is made up of a transistor  96  and a Zener diode  98 . The control leads  94  are attached across the gate and drain of the transistor  96 , controlling its conductivity, that is, its observed impedance. If no voltage is placed across the control leads  94  then the transistor  96  does not conduct and a very high impedance is seen in parallel with the interface inductor  90 . As the voltage applied to the control leads  94  increases, so does the conductivity of the transistor  96 , thereby lowering the impedance seen in parallel with the interface inductor. As the conductivity of the transistor  96  changes, so does the apparent load on the interface inductor  90 . 
         [0021]    Diodes  100 ,  102 ,  104 , and  106  form a full wave bridge rectifier for converting the AC signal provided by inductors  68  and  70  into a DC signal. A capacitor  108  provides filtering for the interface circuit  92 . A Zener diode  110  provides protection for startup purposes. Capacitor  112  and resistor  114  provide additional filtering for the interface signal. 
         [0022]    With reference now to  FIG. 3 , another embodiment of the interface circuit  92  is provided. In this embodiment, a single control lead  116  provides an input that is either on or off, which determines whether a transistor  118  is conductive or non-conductive. When the transistor  118  is conductive, then the interface circuit  92  is limited to the voltage of the Zener diode  120 , forcing the ballast  10  into is lower output state. The additional input of the interface circuit  92  can be provided from node  122  to the inverter  12  via a high frequency bus controller inductively coupled to inductors  68 ,  70 . One possible embodiment of the high frequency bus controller can be found in currently pending U.S. application Ser. No. 11/343,335 to Nerone, et al., at  FIG. 3 . Referring again to  FIG. 3  of the present application, when the transistor  118  is not conductive, no additional interface signal is provided to the ballast  10 , thus the ballast  10  runs at 100%. This embodiment provides step dimming. For example, the control lead  116  may be connected to a motion sensor. The lamps can come up to full when someone is present, but be dimmed at other times. Resistors  124  and  126  are selected to appropriately temper the voltage of the input signal from the control lead, and thus are dependent on the particular input source. Capacitor  128 , resistor  130  and resistor  132  provide additional filtering to the interface circuit. 
         [0023]    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.