Abstract:
A constant current LED driver circuit includes a unified controller operable to control start up peripheral circuits and control power output of the driver circuit. The unified controller initializes, starts driver circuit components in a predetermined order, and controls operation to prevent runaway operation, failure to start, and nuisance shut downs. Additionally, due to centralized operational condition monitoring, the controller can detect conditions that would cause unnecessary shut downs and prevent such nuisance shutdowns. The unified controller enables fast, finite control over switches of a DC-to-DC converter of the driver circuit to improve output current and voltage control, improving closed loop responsiveness and operation of the DC-to-DC converter.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to and hereby incorporates by reference in its entirety U.S. Provisional Patent Application Ser. No. 61/702,867 filed on Sep. 19, 2012 entitled “LED Driver Circuit”. 
    
    
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to direct current (DC) driver circuits. More particularly, this invention pertains to constant current DC driver circuits for light emitting diode (LED) light sources. 
     Light fixtures including a driver circuit packaged with LEDs are replacing incandescent and compact fluorescent light bulbs. However, LED light packages and fixtures need to be controlled for temperature, voltage, and current. These light fixtures may include housings in shapes other than standard incandescent bulb packages. An LED driver circuit is typically a constant current driver circuit that controls current by varying the duty cycle and/or the switching frequency of a DC-to-DC converter in the driver circuit. The DC-to-DC converter may be a flyback or buck converter. Switches in the DC-to-DC converter are controlled by a pulse width modulation (PWM) circuit to provide power from the DC-to-DC converter to the LEDs. Separately, a dimming circuit interprets variations in an input voltage to the light fixture, or a control signal to the dimming circuit, to provide a microprocessor in the driver circuit with a signal indicative of a dimming level. The microprocessor controls the PWM circuit to adjust operation of the DC-to-DC converter accordingly. 
     In operation, when the light fixture receives power, portions of the driver circuit (i.e., the PWM circuit, the microprocessor, the dimming circuit, safety circuits, etc.) initialize in an uncontrolled fashion. This can potentially lead to runaway operation, over-voltage conditions, over-current conditions, and a failure to start. Additionally, startup fault detection combined with controlled shutdowns and restarts are generally not possible without significant additional circuitry. Control of the PWM circuit by the microprocessor is granular and activation of, for example, safety circuits can cause the driver circuit to intermittently shut down entirely when no actual fault exists (e.g., when the light fixture is operating near a predetermined temperature limit). 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the present invention provide a constant current LED driver circuit includes a unified controller operable to control start-up of peripheral circuits and control power output of the driver circuit. The unified controller initializes, starts driver circuit components in a predetermined order, and controls operation to prevent runaway operation, failure to start, and nuisance shut downs. Additionally, due to centralized operational condition monitoring, the controller can detect conditions that would cause unnecessary shut downs and prevent such nuisance shutdowns. The unified controller enables fast, finite control over switches of a DC-to-DC converter of the driver circuit to improve output current and voltage control, improving closed loop responsiveness and operation of the DC-to-DC converter. 
     In one aspect, a method of providing power from an alternating current (AC) power source to a light source via a driver circuit includes receiving power at a controller of the driver circuit from the AC power source via an AC to direct current (DC) converter. The controller initializes in response to receiving power from the AC-to-DC converter. After initializing, the controller enables power to pass from the AC-to-DC converter to a peripheral circuit. After enabling power from the AC-to-DC converter to the peripheral circuit, the controller provides a drive signal to a DC-to-DC converter of the driver circuit. Providing the drive signal includes ramping up a duty cycle of the drive signal to a duty cycle corresponding to a predetermined output current of the driver circuit. The DC-to-DC converter receives the drive signal and provides power from the AC-to-DC converter to the light source as a function of the received drive signal. The controller monitors a current of the light source, a voltage of the light source, and a voltage of a low-voltage output of the AC-to-DC converter. The controller adjusts the drive signal as a function of the monitored current to the light source, monitored voltage of the light source, and the monitored voltage of the low-voltage output of the AC-to-DC converter. 
     In another aspect, a driver circuit is operable to provide power from an AC power source to a light source. The driver circuit includes an AC-to-DC converter, a peripheral circuit, a DC-to-DC converter, and a controller. The AC-to-DC converter includes a low-voltage output, and the AC-to-DC converter receives power from the AC power source and provides power at the low-voltage output. The peripheral circuit receives power from the AC-to-DC converter. The DC-to-DC converter provides power from the AC-to-DC converter to the light source as a function of a drive signal. The controller initializes in response to receiving power from the low-voltage output of the AC-to-DC converter. The controller further enables power to the peripheral circuit from the AC-to-DC converter after initializing. After enabling power from the AC-to-DC converter to the peripheral circuit and waiting a predetermined period of time, the controller provides the drive signal to the DC-to-DC converter. Providing the drive signal to the DC-to-DC converter includes ramping up a duty cycle of the drive signal to a duty cycle corresponding to a predetermined output current. The controller monitors a current of the light source, voltage of the light source, and a voltage of the low-voltage output of the AC-to-DC converter. The controller further adjusts the drive signal as a function of the monitored current to the light source, the voltage of the light source, and a voltage of the low-voltage output of the AC-to-DC converter. 
     In another aspect, a light fixture operable to receive power from an AC power source and provide light includes a light source, a driver circuit, and a housing. The light source provides light in response to receiving power. The housing supports the light source and the driver circuit. The driver circuit includes an AC-to-DC converter, a peripheral circuit, a DC-to-DC converter, and a controller. The AC-to-DC converter includes a low-voltage output, and the AC-to-DC converter receives power from the AC power source and provides power at the low-voltage output. The peripheral circuit receives power from the AC-to-DC converter. The DC-to-DC converter provides power from the AC-to-DC converter to the light source as a function of a drive signal. The controller initializes in response to receiving power from the low-voltage output of the AC-to-DC converter. The controller further enables power to the peripheral circuit from the AC-to-DC converter after initializing. After enabling power from the AC-to-DC converter to the peripheral circuit and waiting a predetermined period of time, the controller provides the drive signal to the DC-to-DC converter. Providing the drive signal to the DC-to-DC converter includes ramping up a duty cycle of the drive signal to a duty cycle corresponding to a predetermined output current. The controller monitors a current of the light source, a voltage of the light source, and a voltage of the low-voltage output of the AC-to-DC converter. The controller further adjusts the drive signal as a function of the monitored current to the light source, the voltage of the light source, and a voltage of the low-voltage output of the AC-to-DC converter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a light fixture. 
         FIG. 2  is block diagram and partial schematic diagram of a driver circuit. 
         FIG. 3  is a graph of light source voltage versus current during nominal operation. 
         FIG. 4  is a graph of light source voltage versus current during abnormal startup operation. 
     
    
    
     Reference will now be made in detail to optional embodiments of the invention, examples of which are illustrated in accompanying drawings. Whenever possible, the same reference numbers are used in the drawing and in the description referring to the same or like parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     To facilitate the understanding of the embodiments described herein, a number of terms are defined below. The terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a,” “an,” and “the” are not intended to refer to only a singular entity, but rather include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as set forth in the claims. 
     As used herein, “ballast” or “driver circuit” refers to any circuit for providing power (e.g., current) from a power source to a light source. Additionally, “light source” refers to one or more light emitting devices such as fluorescent lamps, high intensity discharge lamps, incandescent bulbs, and solid state light-emitting elements such as light emitting diodes (LEDs), organic light emitting diodes (OLEDs), and plasmaloids. 
     Referring to  FIGS. 1 and 2 , an LED driver circuit  102  includes an electromagnetic interference (EMI) filter  104  and an active power factor correction (PFC) input stage (i.e., AC-to-DC converter  106 ) with an isolated buck converter output stage (i.e., DC-to-DC converter  108 ). The entire output stage (i.e., DC-to-DC converter  108  and peripheral sensor circuits) is controlled and managed by a controller  110  that measures analog and pulsed signals from multiple sources and both develops appropriate power switch signals (i.e., drive signals) and manages its own power sources (e.g., reference power circuits). 
     When the PFC (AC-to-DC) converter  106  becomes active, the output control section of the PFC converter  106  becomes energized, activating the controller  110  by providing power to the controller  110  via a low voltage output  112  of the PFC converter  106 . After the controller  110  initializes (i.e., stabilizes) itself, the controller  110  enables power to peripheral circuits  120 . This will establish a stable and well-defined voltage reference for the controller&#39;s control loops. This will also make power available for the peripheral circuit  120  (e.g., dimming interface and circuitry to transfer input power switch signals such as gate drive isolation transformer  114 ). 
     In one embodiment, the light fixture  100  receives AC power from the AC power source  200  and provides illumination. In one embodiment, the AC power source  200  may be line power (e.g., 115 V at 60 Hz). The light fixture  100  includes a light source  300 , the driver circuit  102 , and a housing  150 . The light source  300  provides illumination in response to receiving power from the driver circuit  102 . In one embodiment, the light source  300  includes a string of series connected LEDs. The housing  150  supports the driver circuit  102  and the light source  300 . The housing  150  may be compact as in the case where the light fixture  100  is an incandescent bulb replacement. Optionally, the housing  150  may include additional components such as light diffusers and decorative surroundings. 
     The driver circuit  102  provides power from the AC power source  200  to the light source  300 . The driver circuit  102  includes the PFC (AC-to-DC) converter  106 , a peripheral circuit  120 , and a DC-to-DC converter  108 . The AC-to-DC converter  106  includes a low voltage output  112 . The AC-to-DC converter  106  receives power from the AC power source  200  and provides power at the low voltage output  112 . In one embodiment, the AC-to-DC converter provides active power factor correction. The AC-to-DC converter  106  also includes a high voltage DC output  404  (Vbulk). The high voltage DC output  404  provides power to the DC-to-DC converter  108 . 
     The peripheral circuit  120  receives power from the AC-to-DC converter  106 . Depending on the configuration of the peripheral circuit  120 , the peripheral circuit  120  may provide various sensor inputs or reference voltages to the controller  110 . In one embodiment, the peripheral circuit  120  includes a gate drive circuit  114 , a reference circuit  162 , a dimming interface  160 , signal amplifiers, and buffers. The reference circuit  162  (i.e., reference voltage generator) receives power from the AC-to-DC converter  106  and provides a reference voltage to the controller  110 . The gate drive circuit  114  receives the drive signal from the controller  114 , isolates the drive signal via a gate drive isolation transformer, and provides the isolated drive signal to the DC-to-DC converter  108 . In one embodiment, the DC-to-DC converter  108  may be a transformer  402  and is configured as an isolated DC-to-DC converter  108  such that the output of the driver circuit  102  is fully isolated from the input of the driver circuit  102  (i.e., from the power source  200 ). The DC-to-DC converter  108  provides power from the AC-to-DC converter  106  to the light source  300  as a function of a drive signal provided by the controller  110 . 
     In operation, the controller  110  initializes in response to receiving power from the low voltage output  112  of the AC-to-DC converter  106 . The controller  110  enables power to components of the peripheral circuit  120  from the AC-to-DC converter  106  in a predetermined order after initializing. In one embodiment, the controller  110  enables and disables power from the AC-to-DC converter  106  to the peripheral circuit  120  by controlling one or more switches  500 . A predetermined period of time after enabling power to the peripheral circuit  120 , the controller  110  provides the drive signal to the DC-to-DC converter  108 . Providing the drive signal to the DC-to-DC converter  108  includes ramping up a duty cycle of the drive signal to a duty cycle corresponding to a predetermined nominal output current. This predetermined output current corresponds to a default brightness level (e.g., full brightness). The controller  110  monitors a current of the light source  300 , a voltage of the light source  300 , and a voltage of the low voltage output  112  of the AC-to-DC converter  106 . The controller  110  adjusts the drive signal as a function of the monitored current of the light source  300 , the voltage of the light source  300 , and the voltage of the low voltage output  112  of the AC-to-DC converter  106 . The controller  110  thus reduces the duty cycle of the drive signal such that the voltage does not exceed a predetermined maximum light source voltage and the current does not exceed a predetermined maximum light source current. When the voltage of the low voltage output  112  of the AC-to-DC converter  106  falls below a predetermined minimum voltage, the controller  110  determines that power to the light fixture  100  has been shut off and reduces the duty cycle of the drive signal to zero. The controller  110  also disables power from the AC-to-DC converter  106  to components in the peripheral circuit  120  in a predetermined order. 
     In one embodiment, the controller  110  further monitors temperature of the driver circuit  102  via a temperature sensor  130  in the driver circuit  102 . The temperature sensor  130  provides a temperature signal indicative of a sensed temperature of the driver circuit  102  to the controller  102 . The controller  110  adjusts the drive signal by reducing the duty cycle of the drive signal as a function of the monitored temperature, reducing the duty cycle as a function of the voltage of the light source  300 , and reducing the duty cycle of the drive signal as a function of the current of the light source  300 . 
     Referring to  FIG. 3 , during normal operation, as the controller  110  increases the duty cycle of the drive signal, the voltage and current of the light source  300  stay between a normal maximum nominal impedance  302  and a normal minimum impedance  303  as the controller  110  ramps up the duty cycle of the drive signal during startup. Following startup, the controller  110  maintains the voltage and current of the light source  300  within a voltage and current box defined by a minimum operating voltage  330 , a maximum operating voltage  312 , a minimum operating current  310 , and a maximum operating current  314 . The controller  110  varies the duty cycle and, under certain conditions, the frequency of the drive signal to maintain driver circuit output within the minimum and maximum voltage and current. 
     Referring to  FIG. 4 , when a failure condition representing an open circuit light source  300  is present, the voltage of the light source increases to the maximum operational voltage  312  before the current reaches the minimum operational current  310  as shown by open circuit impedance line  320 . When a failure condition representing a shorted light source  300  is present, the current of the light source  300  increases to the maximum operational current  314  before the voltage of the light source  300  reaches the minimum operating voltage  330  as shown by short circuit impedance line  322 . In either the open circuit light source condition or the short circuit light source condition, the controller  110  reduces the duty cycle of the drive signal to zero and periodically attempts to restart operation (i.e., ramp up the duty cycle of the light source from zero to the default duty cycle). 
     Referring again to  FIGS. 1 and 2 , in one embodiment, the peripheral circuit  120  includes a dimming circuit or dimming interface. The dimming interface  160  receives a dimming input and provides a dimming signal to the controller  110  indicative of the received dimming input. The controller  110  adjusts the drive signal by setting the duty cycle of the drive signal as a function of the dimming signal and the current of the light source. The duty cycle is initially set to a duty cycle corresponding to a predetermined current of the light source indicated by the dimming signal, and the duty cycle is adjusted by the controller  110  to achieve the current corresponding to the desired current. The current is generally proportional to the brightness or dimming level of light output by the light source  300 . 
     It will be understood by those of skill in the art that information and signals may be represented using any of a variety of different technologies and techniques (e.g., data, instructions, commands, information, signals, bits, symbols, and chips may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof). Likewise, the various illustrative logical blocks, modules, circuits, and algorithm steps described herein may be implemented as electronic hardware, computer software, or combinations of both, depending on the application and functionality. Moreover, the various logical blocks, modules, and circuits described herein may be implemented or performed with a general purpose processor (e.g., microprocessor, conventional processor, controller, microcontroller, state machine or combination of computing devices), a digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Similarly, steps of a method or process described herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. Although embodiments of the present invention have been described in detail, it will be understood by those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims. 
     A controller, processor, computing device, client computing device or computer, such as described herein, may be configured to include at least one or more processors or processing units and a system memory. The controller may also include at least some form of computer readable media. By way of example and not limitation, computer readable media may include computer storage media and communication media. Computer readable storage media may include volatile and nonvolatile, removable and non-removable media implemented in any method or technology that enables storage of information, such as computer readable instructions, data structures, program modules, or other data. Communication media may embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Those skilled in the art should be familiar with the modulated data signal, which has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Combinations of any of the above are also included within the scope of computer readable media. 
     This written description uses examples to disclose the invention and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     It will be understood that the particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention may be employed in various embodiments without departing from the scope of the invention. Those of ordinary skill in the art will recognize numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims. 
     All of the compositions and/or methods disclosed and claimed herein may be made and/or executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of the embodiments included herein, it will be apparent to those of ordinary skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful LED DRIVER CIRCUIT WITH UNIFIED CONTROLLER it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.