Patent Publication Number: US-7911156-B2

Title: Thermal foldback for a lamp control device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/214,314, filed Aug. 29, 2005, which is a continuation of U.S. patent application Ser. No. 10/706,677, filed Nov. 12, 2003, now U.S. Pat. No. 6,982,528. The disclosures of each of the above-referenced applications are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to thermal foldback for a lamp control device. Specifically, this invention relates to a ballast having active thermal management and protection circuitry that allows the ballast to safely operate when a ballast over-temperature condition has been detected, allowing the ballast to safely continue to provide power to the lamp. 
     BACKGROUND OF THE INVENTION 
     Lamp ballasts are examples of lamp control devices that convert standard line voltage and frequency to a voltage and frequency suitable for driving a lamp. Usually, ballasts are one component of a lighting fixture that receives one or more fluorescent lamps. The lighting fixture may have more than one ballast. 
     Ballasts are generally designed to operate within a specified operating temperature. The maximum operating temperature of the ballast can be exceeded as the result of a number of factors, including improper matching of the ballast to the lamp(s), improper heat sinking, and inadequate ventilation of the lighting fixture. If an over-temperature condition is not remedied, then the ballast and/or lamp(s) may be damaged or destroyed. 
     Some prior art ballasts have circuitry that shuts down the ballast upon detecting an over-temperature condition. This is typically done by means of a thermal cut-out switch that senses the ballast temperature. When the switch detects an over-temperature condition, it shuts down the ballast by removing its supply voltage. If a normal ballast temperature is subsequently achieved, the switch may restore the supply voltage to the ballast. The result is lamp flickering and/or a prolonged loss of lighting. The flickering and loss of lighting can be annoying. In addition, the cause may not be apparent and might be mistaken for malfunctions in other electrical systems, such as the lighting control switches, circuit breakers, or even the wiring. 
     SUMMARY OF THE INVENTION 
     A lamp ballast has temperature sensing circuitry and control circuitry responsive to the temperature sensor that limits the output current provided by the ballast when an over-temperature condition has been detected. The control circuitry actively adjusts the output current as long as the over-temperature condition is detected so as to attempt to restore an acceptable operating temperature while continuing to operate the ballast (i.e., without shutting down the ballast). The output current is maintained at a reduced level until the sensed temperature returns to the acceptable temperature. 
     Various methods for adjusting the output current are disclosed. In one embodiment, the output current is linearly adjusted during an over-temperature condition. In another embodiment, the output current is adjusted in a step function during an over-temperature condition. In yet other embodiments, both linear and step function adjustments to output current are employed in differing combinations. In principle, the linear function may be replaced with any continuous decreasing function including linear and non-linear functions. Gradual, linear adjustment of the output current tends to provide a relatively imperceptible change in lighting intensity to a casual observer, whereas a stepwise adjustment may be used to create an obvious change so as to alert persons that a problem has been encountered and/or corrected. 
     The invention has particular application to (but is not limited to) dimming ballasts of the type that are responsive to a dimming control to dim fluorescent lamps connected to the ballast. Typically, adjustment of the dimming control alters the output current delivered by the ballast. This is carried out by altering the duty cycle, frequency or pulse width of switching signals delivered to a one or more switching transistors in the output circuit of the ballast. These switching transistors may also be referred to as output switches. An output switch is a switch, such as a transistor, whose duty cycle and/or switching frequency is varied to control the output current of the ballast. A tank in the ballast&#39;s output circuit receives the output of the switches to provide a generally sinusoidal (AC) output voltage and current to the lamp(s). The duty cycle, frequency or pulse width is controlled by a control circuit that is responsive to the output of a phase to DC converter that receives a phase controlled AC dimming signal provided by the dimming control. The output of the phase to DC converter is a DC signal having a magnitude that varies in accordance with a duty cycle value of the dimming signal. Usually, a pair of voltage clamps (high and low end clamps) is disposed in the phase to DC converter for the purpose of establishing high end and low end intensity levels. The low end clamp sets the minimum output current level of the ballast, while the high end clamp sets its maximum output current level. 
     According to one embodiment of the invention, a ballast temperature sensor is coupled to a foldback protection circuit that dynamically adjusts the high end clamping voltage in accordance with the sensed ballast temperature when the sensed ballast temperature exceeds a threshold. The amount by which the high end clamping voltage is adjusted depends upon the difference between the sensed ballast temperature and the threshold. According to another embodiment, the high and low end clamps need not be employed to implement the invention. Instead, the foldback protection circuit may communicate with a multiplier, that in turn communicates with the control circuit. In this embodiment, the control circuit is responsive to the output of the multiplier to adjust the duty cycle, pulse width or frequency of the switching signal. 
     The invention may also be employed in connection with a non-dimming ballast in accordance with the foregoing. Particularly, a ballast temperature sensor and foldback protection are provided as above described, and the foldback protection circuit communicates with the control circuit to alter the duty cycle, pulse width or frequency of the one or more switching signals when the ballast temperature exceeds the threshold. 
     In each of the embodiments, a temperature cutoff switch may also be employed to remove the supply voltage to shut down the ballast completely (as in the prior art) if the ballast temperature exceeds a maximum temperature threshold. 
     Other features of the invention will be evident from the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a prior art non-dimming ballast. 
         FIG. 2  is a functional block diagram of a prior art dimming ballast. 
         FIG. 3  is a functional block diagram of one embodiment of the present invention as employed in connection with a dimming ballast. 
         FIG. 4   a  graphically illustrates the phase controlled output of a typical dimming control. 
         FIG. 4   b  graphically illustrates the output of a typical phase to DC converter. 
         FIG. 4   c  graphically illustrates the effect of a high and low end clamp circuit on the output of a typical phase to DC converter. 
         FIG. 5   a  graphically illustrates operation of an embodiment of the present invention to linearly adjust the ballast output current when the ballast temperature is greater than threshold T 1 . 
         FIG. 5   b  graphically illustrates operation of an embodiment of the present invention to reduce the ballast output current in a step function to a level L 1  when the ballast temperature is greater than threshold T 2 , and to increase the output current in a step function to 100% when the ballast temperature decreases to a normal temperature T 3 . 
         FIG. 5   c  graphically illustrates operation of an embodiment of the present invention to adjust the ballast output current linearly between temperature thresholds T 4  and T 5 , to reduce the ballast output current in a step function from level L 2  to level L 3  if temperature threshold T 5  is reached or exceeded, and to increase the output current in a step function to level L 4  when the ballast temperature decreases to threshold T 6 . 
         FIG. 5   d  graphically illustrates operation of an embodiment of the present invention to adjust the ballast output current in various steps for various thresholds, and to further adjust ballast output current linearly between levels L 6  and L 7  if the stepwise reductions in output current are not sufficient to restore the ballast temperature to normal. 
         FIG. 6  illustrates one circuit level implementation for the embodiment of  FIG. 3  that exhibits the output current characteristics of  FIG. 5   c.    
         FIG. 7  is a functional block diagram of another embodiment of the present invention for use in connection with a dimming ballast. 
         FIG. 8  is an output current versus temperature response for the embodiment of  FIG. 7 . 
         FIG. 9  is a functional block diagram of an embodiment of the present invention that may be employed with a non-dimming ballast. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, wherein like numerals represent like elements there is shown in  FIGS. 1 and 2  functional block diagrams of typical prior art non-dimming and dimming ballasts, respectively. Referring to  FIG. 1 , a typical non-dimming ballast includes a front end AC to DC converter  102  that converts applied line voltage  100   a, b , typically 120 volts AC, 60 Hz, to a higher voltage, typically 400 to 500 volts DC. Capacitor  104  stabilizes the high voltage output on  103   a, b  of AC to DC converter  102 . The high voltage across capacitor  104  is presented to a back end DC to AC converter  106 , which typically produces a 100 to 400 Volt AC output at 45 KHz to 80 KHz at terminals  107   a, b  to drive the load  108 , typically one or more florescent lamps. Typically, the ballast includes a thermal cut-out switch  110 . Upon detecting an over-temperature condition, the thermal cutout switch  110  removes the supply voltage at  100   a  to shut down the ballast. The supply voltage is restored if the switch detects that the ballast returns to a normal or acceptable temperature. 
     The above description is applicable to  FIG. 2 , except that  FIG. 2  shows additional details of the back end DC to AC converter  106 , and includes circuitry  218 ,  220  and  222  that permits the ballast to respond to a dimming signal  217  from a dimming control  216 . The dimming control  216  may be any phase controlled dimming device and may be wall mountable. An example of a commercially available dimming ballast of the type of  FIG. 2  is model number FDB-T554-120-2, available from Lutron Electronics, Co., Inc., Coopersburg, Pa., the assignee of the present invention. As is known, the dimming signal is a phase controlled AC dimming signal, of the type shown in  FIG. 4   a , such that the duty cycle of the dimming signal and hence the RMS voltage of the dimming signal varies with adjustment of the dimming actuator. Dimming signal  217  drives a phase to DC converter  218  that converts the phase controlled dimming signal  217  to a DC voltage signal  219  having a magnitude that varies in accordance with a duty cycle value of the dimming signal, as graphically shown in  FIG. 4   b . It will be seen that the signal  219  generally linearly tracks the dimming signal  217 . However, clamping circuit  220  modifies this generally linear relationship as described hereinbelow. 
     The signal  219  stimulates ballast drive circuit  222  to generate at least one switching control signal  223   a, b . Note that the switching control signals  223   a, b  shown in  FIG. 2  are typical of those in the art that drive output switches in an inverter function (DC to AC) in the back-end converter  106 . An output switch is a switch whose duty cycle and/or switching frequency is varied to control the output current of the ballast. The switching control signals control the opening and closing of output switches  210 ,  211  coupled to a tank circuit  212 ,  213 . Although  FIG. 2  depicts a pair of switching control signals,  223   a, b , an equivalent function that uses only one switching signal may be used. A current sense device  228  provides an output (load) current feedback signal  226  to the ballast drive circuit  222 . The duty cycle, pulse width or frequency of the switching control signals is varied in accordance with the level of the signal  219  (subject to clamping by the circuit  220 ), and the feedback signal  226 , to determine the output voltage and current delivered by the ballast. 
     High and low end clamp circuit  220  in the phase to DC converter limits the output  219  of the phase to DC converter. The effect of the high and low end clamp circuit  220  on the phase to DC converter is graphically shown in the  FIG. 4   c . It will be seen that the high and low clamp circuit  220  clamps the upper and lower ends of the otherwise linear signal  219  at levels  400  and  401 , respectively. Thus, the high and low end clamp circuitry  220  establishes minimum and maximum dimming levels. 
     A temperature cutoff switch  110  ( FIG. 1 ) is also usually employed. All that has been described thus far is prior art. 
       FIG. 3  is a block diagram of a dimming ballast employing the present invention. In particular, the dimming ballast of  FIG. 2  is modified to include a ballast temperature sensing circuit  300  that provides a ballast temperature signal  305  to a foldback protection circuit  310 . As described below, the foldback protection circuit  310  provides an appropriate adjustment signal  315  to the high and low end clamp circuit  220 ′ to adjust the high cutoff level  400 . Functionally, clamp circuit  220 ′ is similar to clamp circuit  220  of  FIG. 2 , however, the clamp circuit  220 ′ is further responsive to adjustment signal  315 , which dynamically adjusts the high end clamp voltage (i.e. level  400 ). 
     The ballast temperature sensing circuit  300  may comprise one or more thermistors with a defined resistance to temperature coefficient characteristic, or another type of temperature sensing thermostat device or circuit. Foldback protection circuit  310  generates an adjustment signal  315  in response to comparison of temperature signal  305  to a threshold. The foldback protection circuit may provide either a linear output (using a linear response generator) or a step function output (using a step response generator), or a combination of both, if the comparison determines that an over-temperature condition exists. In principle, the exemplary linear function shown in  FIG. 3  may be replaced with any continuous function including linear and non-linear functions. For the purpose of simplicity and clarity, the linear continuous function example will be used. But, it can be appreciated that other continuous functions may equivalently be used. Regardless of the exact function used, the high end clamp level  400  is reduced from its normal operating level when the foldback protection circuit  310  indicates that an over-temperature condition exists. Reducing the high end clamp level  400  adjusts the drive signal  219 ′ to the ballast drive circuit  222  so as to alter the duty cycle, pulse width or frequency of the switching control signals  223   a, b  and hence reduce the output current provided by the ballast to load  108 . Reducing output current should, under normal circumstances, reduce the ballast temperature. Any decrease in ballast temperature is reflected in signal  315 , and the high end clamp level  400  is increased and/or restored to normal, accordingly. 
       FIGS. 5   a - 5   d  graphically illustrate various examples of adjusting the output current during an over-temperature condition. These examples are not exhaustive and other functions or combinations of functions may be employed. 
     In the example of  FIG. 5   a , output current is adjusted linearly when the ballast temperature exceeds threshold T 1 . If the ballast temperature exceeds T 1 , the foldback protection circuit  310  provides a limiting input to the high end clamp portion of the clamp circuit  220 ′ so as to linearly reduce the high end clamp level  400 , such that the output current may be reduced linearly from 100% to a preselected minimum. The temperature T 1  may be preset by selecting the appropriate thresholds in the foldback protection circuit  310  as described in greater detail below. During the over-temperature condition, the output current can be dynamically adjusted in the linear region  510  until the ballast temperature stabilizes and is permitted to be restored to normal. Since fluorescent lamps are often operated in the saturation region of the lamp (where an incremental change in lamp current may not produce a corresponding change in light intensity), the linear adjustment of the output current may be such that the resulting change in intensity is relatively imperceptible to a casual observer. For example, a 40% reduction in output current (when the lamp is saturated) may produce only a 10% reduction in perceived intensity. 
     The embodiment of the invention of  FIG. 3  limits the output current of the load to the linear region  510  even if the output current is less than the maximum (100%) value. For example, referring to  FIG. 5   a , the dimming control signal  217  may be set to operate the lamp load  108  at, for example, 80% of the maximum load current. If the temperature rises to above a temperature value T 1 , a linear limiting response is not activated until the temperature reaches a value of T 1 *. At that value, linear current limiting may occur which will limit the output current to the linear region  510 . This allows the maximum (100%) linear limiting profile to be utilized even if the original setting of the lamp was less than 100% load current. As the current limiting action of the invention allows the temperature to fall, the lamp load current will once again return to the originally set 80% level as long as the dimmer control signal  217  is unchanged. 
     In the example of  FIG. 5   b , output current may be reduced in a step function when the ballast temperature exceeds threshold T 2 . If the ballast temperature exceeds T 2 , then the foldback protection circuit  310  provides a limiting input to the high end portion of the clamp  220 ′ so as to step down the high end clamp level  400 ; this results in an immediate step down in supplied output current from 100% to level L 1 . Once the ballast temperature returns to an acceptable operating temperature T 3 , the foldback protection circuit  310  allows the output current to immediately return to 100%, again as a step function. Notice that recovery temperature T 3  is lower than T 2 . Thus, the foldback protection circuit  310  exhibits hysteresis. The use of hysteresis helps to prevent oscillation about T 2  when the ballast is recovering from a higher temperature. The abrupt changes in output current may result in obvious changes in light intensity so as to alert persons that a problem has been encountered and/or corrected. 
     In the example of  FIG. 5   c , both linear and step function adjustments in output current are employed. For ballast temperatures between T 4  and T 5 , there is linear adjustment of the output current between 100% and level L 2 . However, if the ballast temperature exceeds T 5 , then there is an immediate step down in supplied output current from level L 2  to level L 3 . If the ballast temperature returns to an acceptable operating temperature T 6 , the foldback protection circuit  310  allows the output current to return to level L 4 , again as a step function, and the output current is again dynamically adjusted in a linear manner. Notice that recovery temperature T 6  is lower than T 5 . Thus, the foldback protection circuit  310  exhibits hysteresis, again preventing oscillation about T 5 . The linear adjustment of the output current between 100% and L 2  may be such that the resulting change in lamp intensity is relatively imperceptible to a casual observer, whereas the abrupt changes in output current between L 2  and L 3  may be such that they result in obvious changes in light intensity so as to alert persons that a problem has been encountered and/or corrected. 
     In the example of  FIG. 5   d , a series of step functions is employed to adjust the output current between temperatures T 7  and T 8 . Particularly, there is a step-wise decrease in output current from 100% to level L 5  at T 7  and another step-wise decrease in output current from level L 5  to level L 6  at T 8 . Upon a temperature decrease and recovery, there is a step-wise increase in output current from level L 6  to level L 5  at T 11 , and another step-wise increase in output current from level L 5  to 100% at T 12  (each step function thus employing hysteresis to prevent oscillation about T 7  and T 8 ). Between ballast temperatures of T 9  and T 10 , however, linear adjustment of the output current, between levels L 6  and L 7 , is employed. Once again, step and linear response generators (described below) in the foldback protection circuitry  310  of  FIG. 3  allow the setting of thresholds for the various temperature settings. One or more of the step-wise adjustments in output current may result in obvious changes in light intensity, whereas the linear adjustment may be relatively imperceptible. 
     In each of the examples, a thermal cutout switch may be employed, as illustrated at  110  in  FIG. 1 , to remove the supply voltage and shut down the ballast if a substantial over-temperature condition is detected. 
       FIG. 6  illustrates one circuit level implementation of selected portions of the  FIG. 3  embodiment. The foldback protection circuit  310  includes a linear response generator  610  and a step response generator  620 . The adjustment signal  315  drives the output stage  660  of the phase to DC converter  218 ′ via the high end clamp  630  of the clamp circuit  220 ′. A low end clamp  640  is also shown. 
     Temperature sensing circuit  300  may be an integrated circuit device that exhibits an increasing voltage output with increasing temperature. The temperature sensing circuit  300  feeds the linear response generator  610  and the step response generator  620 . The step response generator  620  is in parallel with the linear response generator  610  and both act in a temperature dependent manner to produce the adjustment signal  315 . 
     The temperature threshold of the linear response generator  610  is set by voltage divider R 3 , R 4 , and the temperature threshold of the step response generator  620  is set by voltage divider R 1 , R 2 . The hysteresis characteristic of the step response generator  620  is achieved by means of feedback, as is well known in the art. 
     The threshold of low end clamp  640  is set via a voltage divider labeled simply VDIV 1 . The phase controlled dimming signal  217  is provided to one input of a comparator  650 . The other input of comparator  650  receives a voltage from a voltage divider labeled VDIV 2 . The output stage  660  of the phase to DC converter  218 ′ provides the control signal  219 ′. 
     Those skilled in the art will appreciate that the temperature thresholds of the linear and step response generators  610 ,  620  may be set such that the foldback protection circuit  310  exhibits either a linear function followed by a step function (See  FIG. 5   c ), or the reverse. Sequential step functions may be achieved by utilizing two step response generators  620  (See steps L 5  and L 6  of  FIG. 5   d ). Likewise, sequential linear responses may be achieved by replacing the step response generator  620  with another linear response generator  610 . If only a linear function ( FIG. 5   a ) or only a step function ( FIG. 5   b ) is desired, only the appropriate response generator is employed. The foldback protection circuit  310  may be designed to produce more than two types of functions, e.g., with the addition of another parallel stage. For example the function of  FIG. 5   d  may be obtained with the introduction of another step response generator  620  to the foldback protection circuit, and by setting the proper temperature thresholds. 
       FIG. 7  is a block diagram of a dimming ballast according to another embodiment of the invention. Again, the dimming ballast of  FIG. 2  is modified to include a ballast temperature sensing circuit  300  that provides a ballast temperature signal  305  to a foldback protection circuit  310 . The foldback protection circuit  310 ′ produces, as before, an adjustment signal  315 ′ to modify the response of the DC to AC back end  106  in an over-temperature condition. Nominally, the phase controlled dimming signal  217  from the dimming control  216 , and the output of the high and low end clamps  220 , act to produce the control signal  219  that is used, for example, in the dimming ballast of  FIG. 2 . However, in the configuration of  FIG. 7 , the control signal  219  and the adjustment signal  315 ′ are combined via multiplier  700 . The resulting product signal  701  is used to drive the ballast drive circuit  222 ′ in conjunction with feedback signal  226 . It should be noted that ballast drive circuit  222 ′ performs the same function as the ballast drive circuit  222  of  FIG. 3  except that ballast drive circuit  222 ′ may have a differently scaled input as described hereinbelow. 
     As before, in normal operation, dimming control  216  acts to deliver a phase controlled dimming signal  217  to the phase to DC converter  218 . The phase to DC converter  218  provides an input  219  to the multiplier  700 . The other multiplier input is the adjustment signal  315 ′. 
     Under normal temperature conditions, the multiplier  700  is influenced only by the signal  219  because the adjustment signal  315 ′ is scaled to represent a multiplier of 1.0. Functionally, adjustment signal  315 ′ is similar to  315  of  FIG. 3  except for the effect of scaling. Under over-temperature conditions, the foldback protection circuit  310 ′ scales the adjustment signal  315 ′ to represent a multiplier of less than 1.0. The product of the multiplication of the signal  219  and the adjustment signal  315 ′ will therefore be less than 1.0 and will thus scale back the drive signal  701 , thus decreasing the output current to load  108 . 
       FIG. 8  illustrates the response of output current versus temperature for the embodiment of  FIG. 7 . As in the response shown in  FIG. 5   a , at 100% of load current, the current limiting function may be linearly decreasing beyond a temperature T 1 . However, in contrast to  FIG. 5   a , the response of the embodiment of  FIG. 7  at lower initial current settings is more immediate. In the multiplier embodiment of  FIG. 7 , current limiting begins once the threshold temperature of T 1  is reached. For example, the operating current of the lamp  108  may be set to be at a level lower than maximum, say at 80%, via dimmer control signal  217  which results in an input signal  219  to multiplier  700 . Assuming that the temperature rises to a level of T 1 , the multiplier input signal  315 ′ would immediately begin to decrease to a level below 1.0 thus producing a reduced output for the drive signal  701 . Therefore, the 100% current limiting response profile  810  is different from the 80% current limiting response profile  820  beyond threshold temperature T 1 . 
     It can be appreciated by one of skill in the art that the multiplier  700  may be implemented as either an analog or a digital multiplier. Accordingly, the drive signals for the multiplier input would be correspondingly analog or digital in nature to accommodate the type of multiplier  700  utilized. 
       FIG. 9  illustrates application of the invention to a non-dimming ballast, e.g., of the type of  FIG. 2 , which does not employ high end and low end clamp circuitry or a phase to DC converter. As before, there is provided a ballast temperature sensing circuit  300  that provides a ballast temperature signal  305  to a foldback protection circuit  310 ″. The foldback protection circuit  310 ′ provides an adjustment signal  315 ″ to ballast drive circuit  222 . Instead of adjusting the level of a high end clamp, the adjustment signal  315 ″ is provided directly to ballast drive circuit  222 . Otherwise the foregoing description of the function and operation of  FIG. 3 , and the examples of  FIGS. 5   a - 5   d , are applicable. 
     The circuitry described herein for implementing the invention is preferably packaged with, or encapsulated within, the ballast itself, although such circuitry could be separately packaged from, or remote from, the ballast. 
       FIG. 10  illustrates a light fixture  1000  having a ballast  1010  that employs the present invention. The circuitry for implementing the invention can be integral with or packaged within, or external to, the ballast. 
     It will be apparent to those skilled in the art that various modifications and variations may be made in the apparatus and method of the present invention without departing from the spirit or scope of the invention. For example, although a linearly decreasing function is disclosed as one possible embodiment for implementation of current limiting, other continuously decreasing functions, even non-linear decreasing functions, may be used as a current limiting mechanism without departing from the spirit of the invention. Thus, it is intended that the present invention encompass modifications and variations of this invention provided those modifications and variations come within the scope of the appended claims and equivalents thereof.