Abstract:
Driver circuitry is coupled between a power supply and at least one LED in a solid-state lighting fixture, such that a non-isolated direct current (DC) path exists between the power supply and the at least one LED. The driver circuitry is configured to receive an AC input voltage and generate a driver output current for driving the at least one LED from the AC input voltage. By using driver circuitry that is non-isolated from the at least one LED in the solid-state lighting fixture, the efficiency of the driver circuitry may be increased, while simultaneously reducing the cost and complexity of the driver circuitry compared to conventional driver circuitry.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates to solid-state lighting fixtures. Specifically, the present disclosure relates to light-emitting diode (LED) based lighting fixtures including high-efficiency and high power-density driver circuitry using silicon carbide (SiC) switching components. 
       BACKGROUND 
       [0002]    Continuing advancements in solid-state lighting technologies, and specifically light-emitting diodes (LEDs), continue to result in remarkable performance improvements when compared to their incandescent and fluorescent counterparts. Generally, LED-based lighting fixtures are more efficient, last longer, are more environmentally friendly, and require less maintenance than incandescent and fluorescent lighting fixtures. Accordingly, LEDs are poised to replace conventional lighting technologies in applications such as traffic lights, automobiles, general-purpose lighting, and liquid-crystal-display (LCD) backlighting. 
         [0003]    LED lighting fixtures are driven by a linear (i.e., direct current) driver signal or a pulse-width modulated (PWM) driver signal. Since most lighting fixtures receive power from an alternating current (AC) power source, power conversion must be performed by driver circuitry in order to produce a desired light output from the LED lighting fixture. While the color of light emitted from an LED primarily depends on the composition of the material used to fabricate the LED, the light output of an LED is directly related to the current flowing through the P-N junction of the LED. Accordingly, driver circuitry capable of providing a constant current is desirable for an LED lighting fixture. 
         [0004]      FIG. 1  shows conventional driver circuitry  10  for an LED lighting fixture. For context, a power supply  12 , an electromagnetic interference (EMI) filter  14 , control circuitry  16 , and an LED light source  18  are also shown. The conventional driver circuitry  10  includes rectifier circuitry  20 , power factor correction (PFC) circuitry  22 , and DC-DC converter circuitry  24 . The rectifier circuitry  20  is a bridge rectifier including a first rectifier input node  26 A, a second rectifier input node  26 B, a rectifier output node  28 , a first rectifier diode D R1 , a second rectifier diode D R2 , a third rectifier diode D R3 , and a fourth rectifier diode D R4 . The first rectifier diode D R1  includes an anode coupled to the first rectifier input node  26 A and a cathode coupled to the rectifier output node  28 . The second rectifier diode D R2  includes an anode coupled to the second rectifier input node  26 B and a cathode coupled to the rectifier output node  28 . The third rectifier diode D R3  includes an anode coupled to ground and a cathode coupled to the first rectifier input node  26 A. The fourth rectifier diode D R4  includes an anode coupled to ground and a cathode coupled to the second rectifier input node  26 B. The first rectifier input node  26 A is coupled to a positive output of the power supply  12 , which is filtered via the EMI filter  14 . The second rectifier input node  26 B is coupled to a negative output of the power supply  12 , which is also filtered via the EMI filter  14 . 
         [0005]    The PFC circuitry  22  is a boost converter including a boost input node  30 , a boost output node  32 , a boost inductor L B , a boost switch Q B , a boost diode D B , and a boost capacitor C B . The boost inductor L B  is coupled between the boost input node  30  and an intermediary boost node  34 . The boost switch Q B  is coupled between the intermediary boost node  34  and ground. The boost diode D B  is coupled between the intermediary boost node  34  and the boost output node  32 . Finally, the boost capacitor C B  is coupled between the boost output node  32  and ground. The boost input node  30  is coupled to the rectifier output node  28  of the rectifier circuitry  20 . 
         [0006]    The DC-DC converter circuitry  24  is a flyback converter including a flyback input node  36 , a flyback output node  38 , a flyback transformer T FB , a flyback switch Q FB , a flyback diode D FB , and a flyback capacitor C FB . The flyback transformer T FB  includes a primary winding  40  coupled in series with the flyback switch Q FB  between the flyback input node  36  and ground. Further, the flyback transformer T FB  includes a secondary winding  42  coupled between an anode of the flyback diode D FB  and ground, wherein the cathode of the flyback diode D FB  is in turn coupled to the flyback output node  38 . Finally, the flyback capacitor C FB  is coupled between the flyback output node  38  and ground. The flyback input node  36  is coupled to the boost output node  32 , while the flyback output node  38  is coupled to the LED light source  18 . In some cases, an additional switch (not shown) may be coupled between the LED light source  18  and ground, such that the additional switch operates to pulse-width modulate the current through the LED light source  18  in order to generate a desired light output. 
         [0007]    In operation, an EMI-filtered AC input voltage from the power supply  12  is received at the rectifier circuitry  20 , where it is rectified to generate a rectified voltage. The rectified voltage is then received by the PFC circuitry  22 , which performs power factor correction and boosts the voltage of the signal to generate a direct current (DC) PFC voltage. The DC-DC converter circuitry  24  receives the PFC voltage and regulates a driver output current, which is used to drive the LED light source  18 . The control circuitry  16 , which may be separated into discrete PFC control circuitry, DC-DC control circuitry, and dimming control circuitry in some cases, operates the boost switch Q B  and the flyback switch Q FB  to generate a desired driver output current. While effective at generating a driver output current that is suitable for driving the LED light source  18 , the conventional driver circuitry  10  shown in  FIG. 1  generally suffers from low efficiency due to the use of a flyback converter topology for the DC-DC converter circuitry  24 . That is, the isolated nature of the flyback converter restricts the efficiency of the DC-DC converter circuitry  24 , thereby increasing the power consumption and heat production thereof. 
         [0008]    Notably, the switching components in the conventional driver circuitry  10 , (i.e., the boost switch Q B , the boost diode D B , the flyback switch Q FB , and the flyback diode D FB ) are silicon (Si) parts, which further hampers the performance of the conventional driver circuitry  10 . Specifically, because of the use of silicon (Si) switching components in the conventional driver circuitry  10 , the switching frequency and power handling capability of these components is significantly limited. Accordingly, the acceptable voltage range of the AC input voltage as well as the output voltage and current of the conventional driver circuitry  10  are likewise limited. Since the AC input voltage may vary significantly (i.e. from 208V to 480V depending on the infrastructure of the country in which the lighting fixture is deployed), the limited input voltage of the conventional driver circuitry  10  may result in the need to design separate driver circuitry for each country or region in which the driver circuitry is to be sold or used, thereby driving up the cost of manufacturing. Further, since the power handling capability of silicon (Si) devices is limited, the switching devices must be made large for high power applications, and further may produce excessive amounts of heat, resulting in lighting fixtures that are bulky or otherwise undesirable. 
         [0009]      FIG. 2  shows the conventional driver circuitry  10  wherein the DC-DC converter circuitry  24  is a half-bridge LLC converter. The DC-DC converter circuitry  24  thus includes a half-bridge input node  44 , a half-bridge output node  46 , a first half-bridge switch Q HB1 , a second half-bridge switch Q HB2 , a first half-bridge capacitor C HB1 , a half-bridge inductor L HB , a half-bridge transformer T HB , a first half-bridge diode D HB1 , a second half-bridge diode D HB2 , and a second half-bridge capacitor C HB2 . The first half-bridge switch Q HB1  is coupled between the half-bridge input node  44  and a half-bridge intermediary node  48 . The second half-bridge switch Q HB2  is coupled between the half-bridge intermediary node  48  and ground. The first half-bridge capacitor C HB1 , the half-bridge inductor L HB , and a primary winding  50  of the half-bridge transformer T HB  are coupled in series between the half-bridge intermediary node  48  and ground. A second center-tapped winding  52  of the half-bridge transformer T HB  is coupled between an anode of the first half-bridge diode D HB1  and an anode of the second half-bridge diode D HB2 , while the center-tap of the second center-tapped winding  52  is coupled to ground. The cathode of the first half-bridge diode D HB1  and the cathode of the second half-bridge diode D HB2  are each coupled to the half-bridge output node  46 . Finally, the second half-bridge capacitor C HB2  is coupled between the half-bridge output node  46  and ground. The half-bridge input node  44  is coupled to the boost output node  32 , while the half-bridge output node  46  is coupled to the LED light source  18 . 
         [0010]    The conventional driver circuitry  10  shown in  FIG. 2  functions in a substantially similar manner to the conventional driver circuitry  10  shown in  FIG. 10 , substituting the principles of operation of a flyback converter for that of an LLC half-bridge converter. Using an LLC half-bridge converter for the DC-DC converter circuitry results in an increase in the efficiency of the conventional driver circuitry  10 , however, such a performance increase comes at the expense of increased complexity, cost, and area. Further, the switching components (i.e., the boost switch Q B , the boost diode D B , the first half-bridge switch Q HB1 , the second half-bridge switch Q HB2 , the first half-bridge diode D HB1 , and the second half-bridge diode D HB2 ) are also silicon (Si) components in the conventional driver circuitry  10  shown in  FIG. 2 , which once again results in the same limits on the performance of the circuitry as discussed above with respect to  FIG. 1 . 
         [0011]    Accordingly, there is a need for compact driver circuitry for a solid-state lighting fixture that is capable of delivering a constant output current while operating efficiently over a wide range of input voltages. 
       SUMMARY 
       [0012]    The present disclosure relates to driver circuitry for solid-state lighting fixtures. In one embodiment, driver circuitry is configured to drive at least one light emitting diode (LED) in a solid-state lighting fixture at an efficiency greater than about 90% over at least a portion of an input voltage range between about 185V and 528V. The increased efficiency of the driver circuitry may reduce the operating cost and longevity of the solid-state lighting fixture. 
         [0013]    In one embodiment, circuitry includes an input node, an output node, and driver circuitry. The input node is coupled to a power supply, which is configured to deliver an alternating current (AC) input voltage. The output node is coupled to at least one LED in a solid-state lighting fixture, such that a non-isolated direct current (DC) path exists between the input node and the output node. The driver circuitry resides in the non-isolated DC path between the input node and the output node, and is configured to receive the AC input voltage from the power supply and generate a driver output current for driving the LEDs from the AC input voltage using one or more switching components in the driver circuitry. By using driver circuitry that is non-isolated from the LEDs in the solid-state lighting fixture, the efficiency of the driver circuitry may be increased, while simultaneously reducing the cost and complexity of the driver circuitry compared to conventional driver circuitry. 
         [0014]    In one embodiment, the one or more switching components in the driver circuitry are silicon carbide (SiC) switching components. 
         [0015]    In one embodiment, the driver circuitry includes rectifier circuitry, power factor correction (PFC) circuitry, and DC-DC converter circuitry. The rectifier circuitry is configured to receive and rectify the AC input voltage to generate a rectified voltage. The PFC circuitry is coupled to the rectifier circuitry and configured to receive and provide PFC to the rectified voltage to generate a PFC output voltage. The DC-DC converter circuitry is coupled to the PFC circuitry and configured to receive the PFC output voltage and regulate a driver output current for driving the LEDs in the solid-state lighting fixture. 
         [0016]    In one embodiment, the power factor correction circuitry is a power factor correction (PFC) boost converter. 
         [0017]    In one embodiment, the PFC boost converter is configured to operate in a continuous conduction mode (CCM). 
         [0018]    In one embodiment, circuitry includes an input node, an output node, and driver circuitry. The input node is coupled to a power supply, which is configured to supply an AC input voltage. The output node is coupled to at least one LED in a solid-state lighting fixture. The driver circuitry is coupled between the input node and the output node and is configured to receive the AC input voltage from the power supply, and operate in a CCM to generate a driver signal for operating the LEDs from the AC input voltage signal using one or more switching components in the driver circuitry. By operating the driver circuitry in a CCM, the efficiency of the driver circuitry may be increased when compared to conventional driver circuitry. 
         [0019]    In one embodiment, the one or more switching components in the driver circuitry are silicon carbide (SiC) switching components. 
         [0020]    Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0021]    The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
           [0022]      FIG. 1  is a schematic representation of conventional driver circuitry for a solid-state lighting fixture. 
           [0023]      FIG. 2  is a schematic representation of the conventional driver circuitry shown in  FIG. 1 . 
           [0024]      FIG. 3  is a schematic representation of driver circuitry for a solid-state lighting fixture according to one embodiment of the present disclosure. 
           [0025]      FIG. 4  is a schematic representation of the driver circuitry and minimum off-time circuitry according to one embodiment of the present disclosure. 
           [0026]      FIG. 5  is a schematic representation of the driver circuitry and the minimum off-time circuitry according to an additional embodiment of the present disclosure. 
           [0027]      FIG. 6  is a schematic representation of the driver circuitry and the minimum off-time circuitry according to an additional embodiment of the present disclosure. 
           [0028]      FIG. 7  is a schematic representation of the driver circuitry and isolated shut-off control circuitry according to one embodiment of the present disclosure. 
           [0029]      FIG. 8  is a schematic representation of the driver circuitry and isolated dimming control circuitry according to one embodiment of the present disclosure. 
           [0030]      FIG. 9  is a schematic representation of the driver circuitry and the isolated dimming control circuitry according to an additional embodiment of the present disclosure. 
           [0031]      FIG. 10  is a schematic representation of the driver circuitry and occupancy control circuitry according to one embodiment of the present disclosure. 
           [0032]      FIG. 11  is an isometric view of a lighting fixture including a driver circuitry module according to one embodiment of the present disclosure. 
           [0033]      FIG. 12  is a bottom perspective view of the lighting fixture and the driver circuitry module according to one embodiment of the present disclosure. 
           [0034]      FIG. 13  is a side perspective view of the lighting fixture and the driver circuitry module according to one embodiment of the present disclosure. 
           [0035]      FIG. 14  is a top-isometric view of the lighting fixture and the driver circuitry module according to one embodiment of the present disclosure. 
           [0036]      FIG. 15  is an exploded isometric view of the driver circuitry module according to one embodiment of the present disclosure. 
           [0037]      FIG. 16  is an isometric view of the driver circuitry according to one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
         [0039]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0040]    It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0041]    Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. 
         [0042]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0043]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0044]      FIG. 3  shows driver circuitry  54  for a solid-state lighting fixture according to one embodiment of the present disclosure. The driver circuitry includes rectifier circuitry  56 , power factor correction (PFC) circuitry  58 , and DC-DC converter circuitry  60 . For context, a power supply  62 , an electromagnetic interference (EMI) filter  64 , control circuitry  66 , and an LED light source  68  are also shown. The rectifier circuitry  56  is a bridge rectifier including a first rectifier input node  70 A, a second rectifier input node  70 B, a rectifier output node  72 , a first rectifier diode D R1 , a second rectifier diode D R2 , a third rectifier diode D R3 , and a fourth rectifier diode D R4 . The first rectifier diode D R1  includes an anode coupled to the first rectifier input node  70 A and a cathode coupled to the rectifier output node  72 . The second rectifier diode D R2  includes an anode coupled to the second rectifier input node  70 B and a cathode coupled to the rectifier output node  72 . The third rectifier diode D R3  includes an anode coupled to ground and a cathode coupled to the first rectifier input node  70 A. The fourth rectifier diode D R4  includes an anode coupled to ground and a cathode coupled to the second rectifier input node  70 B. The first rectifier input node  70 A is coupled to a positive output of the power supply  62 , which is filtered via the EMI filter  64 . The second rectifier input node  70 B is coupled to a negative output of the power supply  62 , which is also filtered via the EMI filter  64 . 
         [0045]    The PFC circuitry  58  is a boost converter including a boost input node  74 , a boost output node  76 , a boost inductor L B , a boost switch Q B , a boost diode D B , and a boost capacitor C B . The boost inductor L B  is coupled between the boost input node  74  and an intermediary boost node  78 . The boost switch Q B  is coupled between the intermediary boost node  78  and ground. The boost diode D B  is coupled between the intermediary boost node  78  and the boost output node  76 . Finally, the boost capacitor C B  is coupled between the boost output node  76  and ground. The boost input node  74  is coupled to the rectifier output node  72  of the rectifier circuitry  56 . 
         [0046]    The DC-DC converter circuitry  60  is a buck converter including a buck input node  80 , a first buck output node  82 A, a second buck output node  82 B, a buck diode D BK , a buck switch Q BK , a buck inductor L BK , and a buck capacitor C BK . The buck diode D BK  includes an anode coupled to an intermediate buck node  84  and a cathode coupled to the buck input node  80 . The buck switch Q BK  is coupled between the intermediate buck node  84  and ground. The buck inductor L BK  is coupled between the intermediate buck node  84  and the second buck output node  82 B. Finally, the buck capacitor C BK  is coupled between the first buck output node  82 A and the second buck output node  82 B. The buck input node  80  is coupled to the boost output node  76  of the PFC circuitry  58 , while the LED light source  68  is coupled in series across the first buck output node  82 A and the second buck output node  82 B, such that an anode of a first LED in the LED light source  68  is coupled to the first buck output node  82 A, and a cathode of a second LED in the LED light source  68  is coupled to the second buck output node  82 B. In some cases, an additional switch (not shown) may be coupled between the LED light source  68  and the second buck output node  82 B, such that the additional switch is operated to pulse-width modulate the current through the LED light source  68  in order to generate a desired light output. 
         [0047]    Although only a single string of series-connected LEDs are shown in the LED light source  68 , any number of LEDs may be used for the LED light source and connected in various configurations without departing from the principles disclosed herein. For example, multiple strings of series-connected LEDs may be used for the LED light source  68  in some embodiments. In particular, the different strings of series-connected LEDs may each include LEDs configured to output a different wavelength of light, such that the light from each one of the strings of series-connected LEDs combine to generate light that is substantially white in color at a desired color temperature. 
         [0048]    Notably, the switching devices in the PFC circuitry  58  and the DC-DC converter circuitry  60  are compound semiconductor devices. As defined herein, “switching devices” include diodes and other solid-state switching devices configured to selectively provide power to a load. Specifically, the boost switch Q B , the boost diode D B , the buck diode D BK , and the buck switch Q BK  may each be silicon carbide (SiC) devices. Using silicon carbide (SiC) switching devices in the PFC circuitry  58  and the DC-DC converter circuitry  60  results in substantial performance improvements in the driver circuitry  54  when compared to conventional solutions. In particular, as a result of the use of silicon carbide (SiC) switching components in the PFC circuitry  58  and the DC-DC converter circuitry  60 , the driver circuitry  54  is able to maintain a high efficiency (e.g., greater than 90%) over a wide input voltage range (e.g., 185-528V) and further is able to maintain even higher efficiencies (e.g., greater than 94%) at one or more points in the input voltage range. Further, the driver circuitry  54  is able to sustain a total harmonic distortion (THD) less than about 20% and a power factor greater than about 0.9 for an input power equal to about 500 W. The use of silicon carbide (SiC) switching components in the PFC circuitry  58  and the DC-DC converter circuitry  60  additionally allows the PFC circuitry  58  to operate in a continuous conduction mode (CCM) and the DC-DC converter circuitry  60  to operate in a critical conduction or boundary mode of operation, each of which may further improve the performance of the driver circuitry  54  as discussed below. 
         [0049]    In one embodiment, the boost diode D B  and the buck diode D BK  are silicon carbide (SiC) Schottky diodes. In other embodiments, the boost diode D B  and the buck diode D BK  may be any suitable diode element, for example, P-N diodes or PiN diodes. The boost switch Q B  and the buck switch Q BK  may be silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs). In other embodiments, the boost switch Q B  and the buck switch Q BK  may be any suitable switching element, such as field effect transistors (FETs), insulated gate bipolar transistors (IGBTs), high electron mobility transistors (HEMTs), bipolar junction transistors (BJTs), or the like. 
         [0050]    In one embodiment, the switching devices in the PFC circuitry  58  and the DC-DC converter circuitry  60  are gallium nitride (GaN) devices. Specifically, the boost diode D B  and the buck diode D BK  may be gallium nitride (GaN) Schottky diodes. Further, the boost switch Q B  and the buck switch Q BK  may be gallium nitride (GaN) high electron mobility transistors (HEMTs). Using gallium nitride (GaN) devices may afford benefits similar to those discussed above with respect to silicon carbide. 
         [0051]    In operation, an EMI-filtered AC input voltage from the power supply  62  is received at the rectifier circuitry  56 , where it is rectified to generate a rectified voltage. The rectified voltage is then received by the PFC circuitry  58 , which performs power factor correction and boosts the rectified voltage to generate a direct current (DC) PFC voltage. Specifically, a boost control signal provided to the boost switch Q B  from PFC control circuitry  86  in the control circuitry  66  is modulated in order to charge the boost inductor L B  (i.e., cause the boost inductor L B  to store energy in the form of a magnetic field) while the boost switch Q B  is ON (i.e. closed), and to discharge the boost inductor L B  through the boost diode D B  and across the boost capacitor C B  when the boost switch Q B  is OFF (i.e. open). The boost capacitor C B  acts as a low-pass filter, providing a relatively constant DC output voltage (the PFC output voltage) to the DC-DC converter circuitry  60 . 
         [0052]    The particular modulation frequency and pattern of the boost control signal determines the amount of power factor correction and the magnitude of the resulting PFC output voltage generated by the PFC circuitry  58 . In one embodiment, the boost control signal is modulated in relation to the AC input voltage from the power supply  62 . That is, the boost control signal may be modulated based on the AC input voltage of the power supply  62  such that the PFC output voltage tracks the AC input voltage of the power supply  62 . Operating the PFC circuitry  58  in this manner may lead to significant improvements in the efficiency of the PFC circuitry  58  over the input voltage range. 
         [0053]    If the boost control signal is modulated such that the current through the boost inductor L B  never falls to zero, the PFC circuitry  58  is said to operate in a continuous conduction mode (CCM). Operating the PFC circuitry  58  in a continuous conduction mode is desirable for high power applications, as it reduces the conduction loss of the boost inductor L B  and the boost switch Q B  used in the PFC circuitry  58  while maintaining a required or desired output voltage. However, operating the PFC circuitry  58  in a continuous conduction mode may require the boost control signal to be modulated at a significantly higher frequency than if the PFC circuitry  58  was operated in a discontinuous conduction mode. Accordingly, operating conventional driver circuitry in a continuous conduction mode is generally impractical or impossible due to the limitations on the switching speed of the silicon (Si) switching components therein, as discussed above. Because the driver circuitry  54  shown in  FIG. 3  utilizes silicon carbide (SiC) switching components, the switching speed of the PFC circuitry  58  is not limited by the boost switch Q B  or the boost diode D B . The PFC circuitry  58  may therefore operate in a continuous conduction mode, which allows for a significant reduction in conduction power loss and possibly the size of the boost inductor L B  and the driver circuitry  54  in general. 
         [0054]    The DC-DC converter circuitry  60  receives the PFC voltage from the PFC circuitry  58  and regulates a driver output current, which is used to drive the LEDs of the LED light source  68 . Specifically, a buck control signal provided to the buck switch Q BK  from buck control circuitry  88  in the control circuitry  66  is modulated in order to charge the buck inductor L BK  (i.e., cause the buck inductor L BK  to store energy in the form of a magnetic field) while the buck switch Q BK  is ON (i.e., closed), and to discharge the buck inductor Q BK  and into the buck capacitor C BK  when the buck switch Q BK  is OFF (i.e., open). The buck capacitor C BK  acts as a low-pass filter, providing a relatively constant DC output current (the driver output current) to the LED light source  68 . 
         [0055]    The particular modulation frequency and pattern of the buck control signal determines the magnitude of the resulting driver output current generated by the DC-DC converter circuitry  60 . If the buck control signal is modulated such that the buck switch Q BK  is turned ON each time the current through the buck inductor L BK  decreases to zero the DC-DC converter circuitry  60  is said to operate in a critical conduction or boundary mode of operation. Operating in a critical conduction or boundary mode of operation is desirable because the buck switch Q BK  is turned ON when the voltage across the switch resonates to a valley, which results in lower switching loss and reverse recovery loss of the buck diode D BK . However, similar to the principles discussed above with respect to the PFC circuitry  58  operating a continuous conduction mode, operating the DC-DC converter circuitry  60  in a critical conduction or boundary mode may require the buck control signal to be modulated at a significantly higher frequency than if the DC-DC converter circuitry  60  was operated in a discontinuous conduction mode. Because the driver circuitry  54  shown in  FIG. 3  utilizes silicon carbide (SiC) switching components, the switching speed of the DC-DC converter circuitry  60  is not limited by the buck switch Q BK  or the buck diode D BK . The DC-DC converter circuitry  60  may therefore operate in a critical conduction or boundary mode, which reduces the switching losses experienced by the DC-DC converter circuitry  60  and increases the performance of the driver circuitry  54 . 
         [0056]    One issue experienced by operating the DC-DC converter circuitry  60  in a critical conduction or boundary mode is that the switching frequency of the buck switch Q BK  varies as a function of the voltage across and current through the LED light source  68 , as well as the inductance of the buck inductor L BK , and the output PFC voltage, as shown by Equation 1 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     s 
                   
                   = 
                   
                     
                       
                         V 
                         LED 
                       
                       
                         2 
                          
                         
                             
                         
                          
                         
                           I 
                           LED 
                         
                          
                         
                           L 
                           BK 
                         
                       
                     
                      
                     
                       ( 
                       
                         1 
                         - 
                         
                           
                             V 
                             LED 
                           
                           
                             V 
                             B 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where V LED  is the voltage across the LED light source  68 , I LED  is the current through the LED light source  68 , L BK  represents the inductance of the buck inductor L BK , and V B  is the PFC output voltage. Assuming V LED =300V, V B =800V, and L BK =1 mH, the switching frequency of the DC-DC converter circuitry  60  increases by a factor of 10 from 89 kHz to 890 kHz when the current through the LED light source I LED  is reduced from 1.05 A to 0.105 A. An extremely high switching frequency (e.g., 890 kHz) will generally exceed the frequency limit of the buck control circuitry  88 , and further may also cause high switching loss even for the silicon carbide (SiC) buck switch Q BK . This switching loss is exacerbated when the PFC output voltage V B  is high and the voltage V LED  across the LED light source  68  is low, since the voltage across the buck switch Q BK  is approximately equal to V B −2V LED  at the moment the buck switch Q BK  is turned ON. Accordingly, the switching frequency f s  of the buck switch Q BK  should be limited to a practical value in some applications (e.g., below 500 kHz). 
         [0057]      FIG. 4  therefore shows the driver circuitry  54  and minimum off time (MOT) circuitry  90  according to one embodiment of the present disclosure. The MOT circuitry  90  is coupled to the buck control circuitry  88  in the control circuitry  66 , and is configured to ensure that the buck switch Q BK  remains OFF for a minimum amount of time between switching cycles of the buck switch Q BK  in order to prevent excessive switching loss in the DC-DC converter circuitry  60 . In one embodiment, the minimum off time is set to 2.5 μs, thereby limiting the maximum switching frequency to &lt;˜400 kHz (taking into account the turn-on time of the circuitry). 
         [0058]    The MOT circuitry  90  includes a MOT input node  92 , a MOT output node  94 , three MOT diodes D MOT1 -D MOT3 , a MOT zener diode D ZMOT , four MOT resistors R MOT1 -R MOT4 , two MOT capacitors C MOT1  and C MOT2 , and an MOT inductor L MOT . Notably, the MOT inductor L MOT  is an auxiliary winding of the buck inductor L BK , such that the MOT inductor L MOT  and the buck inductor L BK  are electromagnetically coupled. A first MOT diode D MOT1  is coupled in series with a first MOT resistor R MOT1  between the MOT input node  92  and a first MOT intermediate node  96 , such that the first MOT diode D MOT1  includes an anode coupled to the MOT input node  92  and a cathode coupled to a first MOT resistor R MOT1 . A first MOT capacitor C MOT1  and a second MOT resistor R MOT2  are coupled in parallel between the first MOT intermediate node  96  and a second MOT intermediary node  98 . A second MOT diode D MOT2 , a third MOT resistor R MOT3 , and a second MOT capacitor C MOT2  are coupled in parallel between the second MOT intermediary node  98  and ground, such that an anode of the second MOT diode D MOT2  is coupled to ground and a cathode of the second MOT intermediary node  98  is coupled to the second MOT intermediary node  98 . A third MOT diode D MOT3  is coupled between the second MOT intermediary node  98  and the MOT output node  94 , such that an anode of the third MOT diode D MOT3  is coupled to the second MOT intermediary node  98  and a cathode of the third MOT diode D MOT3  is coupled to the MOT output node  94 . Finally, the MOT zener diode D ZMOT , a fourth MOT resistor R MOT4 , and the MOT inductor L MOT  are coupled in series between the MOT output node  94  and ground, such that a cathode of the MOT zener diode D ZMOT  is coupled to the MOT output node  94  and an anode of the MOT zener diode D ZMOT  is coupled to the fourth MOT resistor R MOT4 , which is in turn coupled to ground through the MOT inductor L MOT . The MOT input node  92  is configured to receive the buck control signal from the buck control circuitry  88 . The MOT output node  94  is coupled to an input of the buck control circuitry  88 . 
         [0059]    In operation, the buck control signal is received at the MOT input node  92 . When the buck control signal is high (i.e., when the buck switch Q BK  is turned ON), the second MOT capacitor C MOT2  is charged through the first MOT capacitor C MOT1  and the second MOT resistor R MOT2 . Further, the MOT inductor L MOT  will begin to store energy coupled from the buck inductor L BK , and current will flow from the MOT inductor L MOT  through the fourth MOT resistor R MOT4  and the third MOT diode D MOT1 . The MOT zener diode D ZMOT  is used to clamp the voltage at the MOT output node  94 . The first MOT resistor R MOT1  is used to limit the peak charging current delivered to the second MOT capacitor C MOT2  and to protect the first MOT diode D MOT1  as well as the MOT zener diode D ZMOT . When the buck control signal is low (i.e., when the buck switch Q BK  is turned OFF), the voltage across the second MOT capacitor C MOT2  begins to decay. Further, the voltage across the MOT inductor L MOT  also begins to decay. When both the voltage across the second MOT capacitor C MOT2  and the voltage across the MOT inductor L MOT  drop to zero, the voltage at the MOT output node  94  will similarly drop to zero. In response to the voltage at the MOT output node  94  dropping to zero, the buck control circuitry  88  will start the cycle again, turning ON the buck switch Q BK . In other words, the buck control circuitry  88  will not turn the buck switch Q BK  back ON until the voltage at the MOT output node  94  drops to zero. The time for the voltage at the MOT output node  94  to drop to zero therefore determines the minimum off time of the buck switch Q BK . Accordingly, the minimum off time of the buck switch Q BK  may be limited in order to prevent switching losses from high switching frequencies in the DC-DC converter circuitry  60 . 
         [0060]      FIG. 5  shows the driver circuitry  54  and the MOT circuitry  90  according to an additional embodiment of the present disclosure. The MOT circuitry  90  shown in  FIG. 5  is substantially similar to that shown in  FIG. 4 , except that the second MOT diode D MOT2  and the third MOT diode D MOT3  are replaced with a MOT transistor Q MOT  and a fifth MOT resistor R MOT5 . The MOT transistor Q MOT  includes a base contact (B) coupled to the second MOT intermediary node  98 , a collector contact (C) coupled to the MOT output node  94 , and an emitter contact (E) coupled to a supply voltage (V cc ) through the fifth MOT resistor R MOT5 . 
         [0061]    In operation, the buck control signal is received at the MOT input node  92 . When the buck control signal is high (i.e., when the buck switch Q BK  is turned ON), the second MOT capacitor C MOT2  is charged through the first MOT capacitor C MOT1  and the second MOT resistor R MOT2 , thereby placing a charge at the gate contact (G) of the MOT transistor Q MOT . Further, the MOT inductor L MOT  will begin to store energy coupled from the buck inductor L BK , and current will flow from the MOT inductor L MOT  through the fourth MOT resistor R MOT4 . If the voltage across the MOT inductor L MOT  is greater than the charge across the second MOT capacitor C MOT2 , the MOT transistor Q MOT  will remain OFF, and the voltage across the MOT inductor L MOT  will hold the MOT output node  94  high. If the voltage across the MOT inductor L MOT  is less than the voltage across the second MOT capacitor C MOT2 , the MOT transistor Q MOT  will turn ON and provide a voltage suitable to continue to hold the MOT output node  94  high. When the buck control signal is low (i.e., when the buck switch Q BK  is turned OFF), the voltage across the second MOT capacitor C MOT2  begins to decay. Further, the voltage across the MOT inductor L MOT  also begins to decay. Since either the voltage across the second MOT capacitor C MOT2  or the voltage across the MOT inductor L MOT  are suitable to hold the MOT output node  94  high, both of the voltages must drop to zero before the MOT output node  94  will similarly drop to zero. As discussed above, the buck control circuitry  88  will not turn the buck switch Q BK  back ON until the voltage at the MOT output node  94  drops to zero. Accordingly, the minimum off time of the buck switch Q BK  may be limited in order to prevent switching losses from high switching frequencies in the DC-DC converter circuitry  60 . 
         [0062]      FIG. 6  shows the driver circuitry  54  and the MOT circuitry  90  according to an additional embodiment of the present disclosure. The MOT circuitry  90  includes a MOT inductor L MOT , and a MOT resistor R MOT . Notably, the buck control circuitry  88 , which may be a microcontroller, is configured to limit the OFF time of the buck switch Q BK  based on feedback provided by the MOT circuitry  90  as well as additional measurements in this embodiment. The MOT inductor L MOT  and the MOT resistor R MOT  are coupled in series between an input of the buck control circuitry  88  and ground. Similar to the embodiments discussed above, the MOT inductor L MOT  is an auxiliary winding of the buck inductor L BK , such that the MOT inductor L MOT  and the buck inductor L BK  are electromagnetically coupled. The buck control circuitry  88  may have further inputs to receive the current I LED  through the LED light source  68 , the current I QBK  through the buck switch Q BK , and a dimming control signal DIM indicating a desired level of light output from the LED light source  68 . At full load (i.e., when the dimming control signal DIM indicates that the LED light source  68  is to be driven at full intensity), the buck control circuitry  88  monitors the voltage across the MOT inductor L MOT  and turns the buck switch Q BK  ON only after the voltage across the MOT inductor L MOT  has fallen to zero. When the current I LED  through the LED light source  68  is reduced (i.e., when the dimming control signal DIM indicates that the LED light source  68  should be driven below full intensity), the switching frequency of the DC-DC converter circuitry  60  begins to increase. Accordingly, the buck control circuitry  88  increases the time that the buck switch Q BK  remains OFF between switching cycles proportionally with the amount of dimming, thereby reducing the switching losses of the DC-DC converter circuitry  60 . 
         [0063]      FIG. 7  shows the driver circuitry  54  and isolated shut-off control (SOC) circuitry  100  according to one embodiment of the present disclosure. The isolated SOC circuitry  100  may supply a signal to the PFC control circuitry  86  and/or the buck control circuitry  88  in order to instruct the PFC control circuitry  86  and/or the buck control circuitry  88  to turn OFF. The isolated SOC circuitry  100  may include a first SOC input node  102 A, a second SOC input node  102 B, an SOC output node  104 , an SOC zener diode D ZSOC , an SOC optocoupler U SOC , a first SOC resistor R SOC1 , and a second SOC resistor R SOC2 . The SOC optocoupler U SOC  may include an optocoupler LED D OC  and an optocoupler photosensitive transistor Q OC . The SOC zener diode D ZSOC , the first SOC resistor R SOC1 , and the optocoupler LED D OC  may be coupled in series between the first SOC input node  102 A and the second SOC input node  102 B, such that the first SOC resistor R SOC1  is coupled between the anodes of the SOC zener diode D ZSOC  and the optocoupler LED D OC , a cathode of the SOC zener diode D ZSOC  is coupled to the first SOC input node  102 A, and a cathode of the optocoupler LED D OC  is coupled to the second SOC input node  102 B. The optocoupler photosensitive transistor Q OC  includes a collector contact (C) coupled to the SOC output node  104  and an emitter contact (E) coupled to ground. Finally, the second SOC resistor R SOC2  is coupled between a supply voltage V CC  and the SOC output node  104 . 
         [0064]    In operation, when an external control voltage, which may be supplied, for example, by a light switch or a dimming triac, applied across the first SOC input node  102 A and the second SOC input node  102 B is higher than the zener voltage of the SOC zener diode D ZSOC , the SOC zener diode D ZSOC  begins to conduct, sending a current through the optocoupler LED D OC , thereby turning on the optocoupler photosensitive transistor Q OC  and pulling the SOC output node  104  to ground. In this embodiment, when the PFC control circuitry  86  and the buck control circuitry  88  receive a high signal at the SOC output node  104 , the PFC circuitry  58  and the DC-DC converter circuitry  60  are left ON. However, the PFC circuitry  58  and the DC-DC converter circuitry  60  are disabled when a low signal (e.g., ground) is placed at the SOC output node  104 . Using the SOC optocoupler U SOC  allows the PFC control circuitry  86  and the buck control circuitry  88  to remain isolated from the control signals used to turn the PFC circuitry  58  and the DC-DC converter circuitry  60  OFF. Accordingly, noise may be reduced in the driver circuitry  54 . 
         [0065]      FIG. 8  shows the driver circuitry  54  and isolated dimming control circuitry  106  according to one embodiment of the present disclosure. The dimming control circuitry  106  may include a first dimming control input node  108 A, a second dimming control input node  108 B, a dimming control output node  110 , a dimming control microcontroller  112 , a first dimming control resistor R DC1 , a second dimming control resistor R DC2 , and a dimming control optocoupler U DC . The dimming control optocoupler U DC  may include an optocoupler LED D OC  and an optocoupler photosensitive transistor Q OC . The dimming control microcontroller  112  may be coupled to the first dimming control input node  108 A and the second dimming control input node  108 B. The first dimming control resistor R DC1  and the optocoupler LED D OC  may be coupled between the an input of the dimming control microcontroller  112  and a negative bias voltage (V BIAS− ), such that an anode of the optocoupler LED D OC  is coupled to the first dimming control resistor R DC1 , which is in turn coupled to the input of the dimming control microcontroller  112 , and a cathode of the optocoupler LED D OC  is coupled to the negative bias voltage (V BIAS− ). The optocoupler photosensitive diode Q OC  may include a collector contact (C) coupled to the dimming control output node  110  and an emitter contact (E) coupled to ground. Finally, the second dimming control resistor R DC2  may be coupled between a positive bias voltage (V BIAS+ ) and the dimming control output node  110 . 
         [0066]    In operation, the dimming control microcontroller  112  receives an external control voltage applied across the first dimming control input node  108 A and the second dimming control input node  108 B, for example, from a dimming triac or other dimming control interface. The dimming control microcontroller  112  then generates a pulse-width modulated (PWM) dimming control signal with a duty cycle proportional to the control voltage across the first dimming control resistor R DC1  and the optocoupler LED D oe . The PWM dimming control signal activates the optocoupler photosensitive transistor Q OC , which results in the PWM dimming control signal being placed at the dimming control output node  110 . In one embodiment, the dimming control circuitry  106  monitors one or more voltages or currents in the driver circuitry  54  and uses the measurements as feedback for adjusting the PWM dimming control signal. In response to the PWM dimming control signal, the PFC control circuitry  86  and the buck control circuitry  88  supply the LED light source  68  with a voltage and/or current that is proportional to the duty cycle of the PWM dimming control signal. Accordingly, the dimming control microcontroller  112  may maintain a desired amount of light output from the LED light source  68 . The PWM dimming control signal may be delivered to the PFC control circuitry  86 , the buck control circuitry  88 , or both, where it may be used to modulate the PFC control signal and/or the buck control signal, respectively in order to control the voltage across the LED light source  68  and/or the current through the LED light source  68 . 
         [0067]      FIG. 9  shows the driver circuitry  54  and the isolated dimming control circuitry  106  according to an additional embodiment of the present disclosure. The dimming control circuitry  106  shown in  FIG. 9  is substantially similar to that shown in  FIG. 8 , but further includes a low-pass filter  114  coupled to the dimming control output node  110 . The low-pass filter  114  includes a low-pass resistor R LP  and a low-pass capacitor C LP , which average the PWM dimming control signal into a linear dimming control signal. The linear dimming control signal may be delivered to the PFC control circuitry  86 , the buck control circuitry  88 , or both, where it may be used to modulate the PFC control signal and/or the buck control signal, respectively in order to control the voltage across the LED light source  68  and the current through the LED light source  68 . 
         [0068]      FIG. 10  shows the driver circuitry  54  and an occupancy control module  116  according to one embodiment of the present disclosure. The occupancy control module  116  includes an occupancy control switch SW OC  and an occupancy control sensor  118 . The occupancy control switch SW OC  may be coupled between the negative output of the power supply  62  and the EMI filter  64 . Further, the occupancy control module  116  may be coupled to the dimming control circuitry  106  via a first control voltage output node  120 A and a second control voltage output node  120 B. The occupancy control sensor  118  may detect the presence or absence of people in a given area. In response to a lack of people in the area detected by the occupancy control sensor  118 , the occupancy control sensor  118  may open the occupancy control switch SW OC , thereby cutting power to the driver circuitry  54  and thus the LED light source  68 . Alternatively, the occupancy control sensor  118  may send a control voltage to the dimming control circuitry  106  instructing the dimming control circuitry  106  to dim the LED light source  68  to a predetermined level. Accordingly, the LED light source  68  may only provide light output when a person is physically in the vicinity of the light source, thereby saving energy. 
         [0069]      FIGS. 11 through 14  show an exemplary lighting fixture  122  incorporating the driver circuitry  54  according to one embodiment of the present disclosure. The lighting fixture  122  includes an outer housing  124 , a mounting apparatus  126 , an occupancy module housing  128 , and a heatsink  130 . The driver circuitry  54  is located within a driver circuitry module  132 , which is inserted into a top cavity  134  located in the top of the outer housing  124  of the lighting fixture  122 . Notably, the driver circuitry  54  described herein may be retro-fitted into a pre-existing lighting fixture  122 , such as the Edge High Output series lighting fixtures manufactured by Cree, Inc. of Durham, N.C. The outer housing  124  of the lighting fixture  122  may include more than one top cavity  134  in order to accept a number of driver circuitry modules  132 . However, since the driver circuitry  54  discussed above utilizes silicon carbide (SiC) switching components, the power handling capability of multiple driver circuitry modules  132  may be accomplished by a single driver circuitry module  132 , thereby saving not only space in the lighting fixture  122 , but also expense. In many applications, the added expense of the silicon carbide (SiC) switching components utilized in the driver circuitry  54  is more than compensated for by the reduction in the overall number of components in the driver circuitry module  132 . The occupancy module housing  128  may be mounted on a bottom surface of the lighting fixture  122  alongside the LED light source  68 . The LED light source  68  may be mounted such that the LEDs are thermally coupled to the heatsink  130 , which may include a plurality of fins configured to disperse heat away from the LED light source  68  towards the top of the lighting fixture  122 . 
         [0070]      FIGS. 15 and 16  show details of the driver circuitry module  132  according to one embodiment of the present disclosure. The driver circuitry module  132  includes a mounting plate  136 , a number of driver circuitry enclosures  138 , a contact substrate  140 , and a number of electrical contacts  142 . The driver circuitry enclosures  138  may each include the driver circuitry  54  shown above with respect to  FIGS. 3 through 10 . Each one of the driver circuitry enclosures  138  may be thermally coupled to the driver circuitry  54  therein in order to provide adequate heat dissipation and ensure the longevity of the driver circuitry  54 , and further may be coupled to the mounting plate  136 . The contact substrate  140  may be mounted on top of the driver circuitry enclosures  138  such that the necessary electrical interconnects between the driver circuitry  54  and the contact substrate  140  are made. Finally, the electrical contacts  142  may be mounted on the contact substrate  140  such that the desired contacts to the driver circuitry  54  are made available for use by the lighting fixture  122 . 
         [0071]    Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.