Abstract:
A method and apparatus for driving a ballast in critical discontinuous mode (CDCM). The method provides for zero loss switching in order to maximize efficiency, and also provides controllable input and output power without the use of complex and costly feedback loops.

Description:
TECHNICAL FIELD 
     This invention relates to a ballast for lighting devices, and in particular, to an electronic ballast for discharge lamps. 
     DESCRIPTION OF THE PRIOR ART 
     The most efficient electrical lighting sources that are commonly available are gaseous, low pressure and high-pressure discharge lamps. Examples of these include high intensity discharge (HID) lamps. These types of lamps typically utilize a gas sealed within a tube, which gives off light when excited with an electrical signal. 
     Electronic ballasts typically include switching transistors and utilize high switching frequencies to convert energy into an appropriate form to drive the lamps. For HID lamps, it is common to drive the lamps with a low frequency (for example 100 Hz) square wave of current. Electronic ballasts of this type typically switch transistors off and on utilizing a duty cycle selected to adjust the power delivered to the lamp. More specifically, the power that the ballast delivers is typically determined from the duty cycle of switching transistors, wherein a greater duty cycle implies a higher amplitude driving current, which results in more power and thus a brighter light output. 
     FIG. 1 shows a prior art technology described U.S. Pat. No. 5,917,290, issued to Shen, the applicant herein. In accordance with FIG. 1, an output light source V out  is driven by an input voltage V in  through a ballast control circuit as shown. In such systems, the switching cycle of the bridge transistors M 3  and M 4  is synchronized with the input mains voltage. Switches M 1 , M 2 , M 5  and M 6  are operated at high frequency in order to shape the current drawn from the mains and to deliver a square wave of current to the lamp that is synchronized to the mains. 
     In operation, the switches M 1  through M 6  operate in conjunction with each other, with M 1 , M 2 , M 5  and M 6  operated in a high frequency pulse width modulation mode (PWM) to drive the output light source V out  In such an arrangement, the current through L 1  is regulated by the switching of transistors M 1  and M 2  in a manner such that a sinusoidal wave current is present in L 1 . The sinusoidal wave effectively follows the input voltage V in , scaled by a prescribed factor. Similarly, the current through L 2  is regulated by the switching of transistors M 5  and M 6  in a manner such that a square wave of current is present in L 2 . 
     In order to control the power drawn from the input, the current through inductor L 1  must be monitored, and the duty cycle utilized to switch transistors M 1  and M 2  appropriately adjusted through a feed-back loop in order to provide the proper amplitude. 
     FIG. 1A shows the representation of the current flowing through inductor L 1  of FIG. 1, superimposed upon the switching state of the transistor M 2 . As FIG. 1A shows, the current within inductor L 1  is substantially constant for any switching cycle, a switching cycle referring to the high frequency switching cycles of M 1  and M 2 . 
     Similarly, in order to control the power delivered to the output, the current through inductor L 2  must be monitored, and the duty cycle utilized to switch transistors M 5  and M 6  appropriately adjusted through a feedback loop in order to provide the proper amplitude. 
     Although the arrangement of the &#39;290 patent solves many of the prior art problems, there are still two problems that the &#39;290 arrangement does not solve. First, as previously indicated, the amplitude of the current flowing through L 1 , and thus of the power drawn from the input, is controlled by rapidly switching the transistors off and on. The particular sequence of switching these transistors off and on as described in the &#39;290 patent and other prior art systems results in significant power loss through the ballast. A similar situation exists with inductor L 2  and switches M 5  and M 6 . Accordingly, the arrangement is less efficient than desired. 
     Second, the technique utilized for matching the current through inductor L 1  to a specified AC voltage is a feedback loop. More specifically, the current through L 1  is monitored, fed back to an error amplifier, and the output of such error amplifier is utilized to adjust the duty cycle of the transistors in order to increase or decrease the current through L 1  to the desired value. A similar feedback loop arrangement must be utilized for the current in L 2 . The constant monitoring of the currents through L 1  and L 2  and the use of the feedback loops requires additional components (not shown in FIG. 1) which add to the cost and complexity of the ballast circuit. 
     In view of the above, there exists a need in the art for a more cost effective manner of controlling a ballast to drive a lighting device. 
     SUMMARY OF THE INVENTION 
     The above and other problems of the prior art are overcome and a technical advance achieved in accordance with the present invention. An electronic ballast is driven using critical discontinuous mode (CDCM) operation of both the input and output stages. In CDCM, the current through the input inductor L 1  is switched on and off so that it ramps up and down creating a triangular type wave form during each high frequency switching cycle. The envelope of the triangle peaks outlines the desired waveform, and the actual waveform produced, after filtering, is the desired sinusoidal waveform. The output stage is also operated in CDCM such that the resulting current through inductor L 2  also has a high frequency triangular waveform. The envelope of the triangle peaks outlines the desired waveform, and the actual waveform produced after filtering is the desired square waveform. 
     By utilizing CDCM and a constant charging time, the amplitude of the input current directly follows a prescribed portion of the supply voltage. It is only necessary to measure the zero crossing of the input inductor current so that the system can maintain the triangular waveform. The prior art feedback loop which continuously monitors the input current is eliminated and a simple zero crossing sensor is utilized instead. In an enhanced embodiment, the PWM switches are switched on and off in a manner such that lossless switching occurs by timing the switching correctly. 
     In a further enhanced embodiment, a saturable transformer is added in order to monitor when the inductor current reaches zero and properly time the switching. The complexity required to monitor zero crossings in an inductor current is significantly less than that of continuously monitoring the value of an inductor current and feeding it back for an adjustment, therefore simplifying the complexity, and reducing the cost, of the circuitry. 
     Similarly, by utilizing CDCM, the amplitude of the output current directly follows the DC bus voltage with polarity alternating with the mains voltage as controlled by the switching of switches M 3  and M 4 . Again, it is only necessary to measure the zero crossings of the inductor current, thus eliminating the feedback loop. With proper switch timing, lossless switching can be achieved. 
     Further advantage and functionalities of the present invention will become apparent from review of the following detailed description and drawings of an exemplary embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a prior art ballast circuit for driving a lighting device; 
     FIG. 1A depicts a switching waveform and an inductive current waveform superimposed, the waveforms being taken from a typical prior art ballast arrangement. 
     FIG. 2 depicts the circuitry of an exemplary embodiment of the present invention. 
     FIG. 3 depicts the state of circuitry for an exemplary embodiment of the present invention during a prescribed state; 
     FIG. 4 depicts the state of the circuitry and current flow through an exemplary embodiment of the invention for a different prescribed state; 
     FIG. 5 depicts the waveform of current through the input inductor utilizing the present invention; 
     FIG. 6 depicts the voltage at a prescribed point in the circuitry as a function of time as the invention operates; 
     FIG. 7 depicts an exemplary timing diagram of a switching transistor that is part of the present invention; and 
     FIG. 8 depicts an exemplary timing diagram of the operation of a second switching transistor. 
     FIGS.  9 ( a )-( d ) depict a similar series of exemplary timing diagrams related to the output stage switches M 5  and M 6  of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 depicts an exemplary embodiment of a circuit implementing the present invention. The arrangement of FIG. 2 includes an input voltage source  200  connected in parallel with a capacitor  201 , and an inductor L 1  coupled to a saturable transformer Ls 1 . Two switching transistors M 1  and M 2  control the delivery of current from the input stage to the ballast. The saturable transformer Ls 1  serves to measure the zero crossing of the inductor current in L 1  to control the timing of the system, as described below. 
     FIG. 3 shows a slightly more detailed view of the input stage wherein each switching transistor is shown with its associated parasitic capacitances, being  301  and  302 , respectively, as indicated, and its body diodes  303  and  304 , respectively. 
     In accordance with the present invention, the capacitance and diode inherent in switching transistors M 1  and M 2  are utilized in an advantageous manner for the purpose of creating the resonant network that may be switched in a lossless manner. 
     The timing and operation of the ballast will now be described with respect to FIGS. 3 through 8. We note that only one switching cycle of the PWM signaling is shown. Additionally, note that while FIGS. 3 and 4 show only the input stage of the ballast, the output stage shown in FIG. 2 (right half of drawing) is operated in a substantially identical manner. 
     Referring to FIG. 2, when Vin is positive switch M 4  is closed. During the initial operation of a high frequency switching cycle, switch M 2  is closed. The closed switch M 2  provides a short circuit from ground to inductor L 1 . As a result, the entire voltage V in  is present across L 1 . The current through L 1  is increasing linearly during this stage. This stage is indicated graphically in the timing diagrams of FIGS. 5 through 8, where stage  1  shows the current in L 1  ramping up linearly during this time. As shown in FIG. 6, since M 2  acts effectively as a short circuit to ground, the voltage at point V X1  of FIG. 1 is zero during stage  1 . 
     During stage  2 , M 2  is opened, cutting off the path of current between V X1  and ground. Since the current iL 1  in inductor L 1  tends to keep flowing, it is split between the parasitic capacitors  301  and  302  of switches M 1  and M 2  respectively, as shown more clearly in FIG.  3 . During this stage  2 , capacitor  301  begins discharging and capacitor  302  begins charging, thereby bringing the voltage at point V X1  up to V bus    
     The timing of stage  2  is shown in FIGS. 5 through 8. Note that the time axis is divided into numbered states at the bottom of FIG.  8 . More specifically, the current iL 1  tends to remain flowing as it was prior to switch M 2  being opened. FIG. 6 shows that during stage  2 , as capacitor  301  tends to discharge and capacitor  302  tends to charge, the voltage at point V X1  ramps up from ground to V bus , substantially linearly as shown in FIG.  6 . FIG. 7 and 8 show the states of switches M 2  and M 1 , respectively, during this stage  2 . 
     At substantially the time that the voltage V x1  reaches V bus , stage  3  is entered. Stage  3  is initiated by closing M 1 , which makes the voltage across L 1  equal to V in −V bus . Since V bus  must be greater than V in  (for example, V bus  =200V for V in =120Vrms) the voltage across L 1  is now negative during stage  3 . As a result, the current iL 1  begins to decrease linearly as shown in FIG.  5 . Additionally, V x1  remains substantially constant during this decrease in current as shown in FIG.  6 . Eventually, at the end of stage  3 , the current iL 1  will reach substantially zero. The current reaching zero may be detected by a simple detector, such as Ls 1  shown in FIG.  3 . When the current iL 1  reaches zero, or even slightly negative, stage  4  is entered. 
     Stage  4  is initiated by opening the switch M 1 . This causes V x1  to decrease while the current iL 1  goes negative. When V x1  reaches its minimum value near ground, M 2  can be closed once again starting the cycle over. This timing is also illustrated in FIGS. 5-8. 
     In essence, in stage  4 , a resonant network is formed by L 1 , and capacitors  301  and  302 . The resonant network oscillates, and as the current in L 1  becomes negative, capacitor  301  is charged and capacitor  302  discharged. This causes the known V x1  to decrease from the voltage V bus  to ground. 
     In order to ensure that the switching is lossless, it is desirable that the turning on or off of either of transistors M 1  or M 2  is accomplished when there are zero volts across each switch. In the case of M 1 , this means that it should switch on when V x1  is at V bus , so that there is no voltage drop across M 1 . In the case of M 2 , this means that it should switch on when V x1  is at ground, so there is no voltage drop across M 2 . 
     We note however, that V x1  can actually reach a value of slightly different from ground, as an analysis of the circuit of FIG. 4 reveals. More specifically, as capacitors  301  and  302  charge and discharge the value reached by V x1  is actually 2V in −V bus , which could be slightly more or less than ground. However, if V x1  tends below ground, it will be clamped at ground by diode  304 . Alternatively, if V x1  only goes down near ground, but does not reach ground, then the switching should be done at the minimum value of V x1  to minimize losses. While switching at a value slightly greater than zero is not lossless, in nonetheless will minimize loss. 
     A still further enhancement will ensure that the voltage V x1  always returns to zero at the time of switching. In this regard, it is possible to remain in stage  3 , shown in FIGS. 5-8, until the current iL 1  goes slightly negative before switching to stage  4 , thus ensuring that there is always a time when V x1  goes to zero, and thus effectuating lossless switching. 
     The output stage of the ballast, which is comprised largely of switches M 5  and M 6 , operates in a similar manner to that described for the input stage above. FIG. 9, plots (a)-(d), show the timing diagrams related to the operation of switches M 5  and M 6 , as well as the related voltages V X3  and current iL 2  depicted in FIG.  2 . As will be appreciated from the foregoing, the operation and timing of the output stage of the ballast is similar to that of the input stage. FIGS.  9 ( a )-( d ) correspond to FIGS. 5-8, respectively. 
     In operation, during the state indicated as ( 1 ) in FIG. 9, M 5  is closed and M 6  is open. The voltage across L 2  is V bus -V out . As a result, the current iL 2  increases linearly. After a prescribed time ( 1 ), M 5  is turned off (i.e. opened), which results in the output stage entering state ( 2 ) of FIG.  9 . In state  2 , since M 5  is opened, the current iL 2  splits between the parasitic capacitors present within M 5  and M 6 . As was described with respect to the input portion of the ballast, the M 6  parasitic capacitance discharges while the M 5  parasitic capacitance charges. This causes V X3  to drop, as shown in FIG.  9 ( b ), towards ground. When V X3  reaches at or near ground, the body diode within M 6  will clamp V X3  to ground and switch M 6  may be turned on. The turning on of switch M 6  at this stage is substantially lossless, since the dropping of V X3  to ground means there is no voltage drop across M 6  at the end of state ( 2 ). Switching M 6  closed at that point results in substantially lossless switching. It is noted that even if the switching takes place at a point when V X3  is not exactly at ground, this simply means that the switching will involve some minor loss, rather than be totally lossless. However, even so, most of the advantage of the present invention will still be achieved. 
     Continuing with the timing diagram of FIG. 9, when state ( 3 ) is entered, M 6  is closed in a substantially lossless manner. The current iL 2  begins decreasing linearly as shown in FIG.  9 ( a ). When iL 2  reaches zero (or very slightly negative), M 6  is opened and the circuit enters state ( 4 ). 
     In state ( 4 ), M 6  is opened. During state ( 4 ), the current iL 2  is slightly negative, causing the parasitic capacitance in M 6  to charge while the parasitic capacitance in M 5  discharges. As a result, the voltage V X3  climbs to V bus  as shown. In actuality, V X3  will reach a peak value of twice V out , which may be greater or less than V bus . If twice V out  is greater than V bus , the body diode of M 5  will clamp V X3  to V bus . If V X3  does not reach V bus , then M 5  can be switched on at the peak value of V X3 , thus minimizing losses. 
     It is also noted, that in a manner similar to that described with respect to the input portion of the ballast, it is possible to allow iL 2  to go negative before entering state ( 4 ). If the switching occurs after iL 2  becomes negative, then additional energy will be stored in L 2 , which will ensure that Vx 2  always reaches Vbus, and that M 5  can always be switched in a substantially lossless manner. 
     It can be appreciated that this is a symmetrical circuit in the sense that the output stage switches M 5  and M 6  operate substantially identically to the input stage switches M 1  and M 2 . Furthermore, when V in  is negative, switch M 3  is on and M 4  is off, and the circuit operates similarly as described. In that case, the roles of M 1  and M 2  are reversed as well, and the polarity of iL 1  is thus reversed. Similarly, the roles of M 5  and M 6  are reversed and the polarity of iL 2  is reversed. Given these polarity differences, the circuit operates substantially identically as it does for the case V in &gt;0, as described above. 
     While the above describes the preferred embodiment of the invention various other modifications and additions will be apparent to those of skill in the art. Such modifications are intended to be covered by the following claims.