Abstract:
A light emitting diode (LED) light source, LED driver circuitry and methods for controlling the brightness of an LED light source are presented. In some embodiments, an LED driver control circuit receives a dimming command signal to dim the LED light source, modulates a continuous direct current (DC) level to dim the LED light source, and determines that a predetermined threshold level has been reached. At this time, the process includes initiating a fixed pulse width generator (PWG) control signal having a fixed duty cycle, automatically adjusting the LED current amplitude to its nominal current level, and decreasing the current amplitude while the fixed PWG control signal is active to achieve commanded lower dimming of the LED light source.

Description:
BACKGROUND 
       [0001]    Light-emitting diode (LED) light sources are becoming more common in the marketplace. Originally used to replace conventional incandescent, fluorescent, or halogen lamps, and the like in homes, LED light sources are now becoming more commonly used in products such as automobiles. Their increased use is not surprising as LEDs are typically more efficient than conventional incandescent light bulbs and the like, and have longer operational lives. 
         [0002]    In some implementations, LED light sources include a plurality of light-emitting diodes provided in a suitable housing. LEDs are current-dependent components, and thus in order to illuminate LEDs properly an LED driver control device (an LED driver) is typically coupled between an alternating-current (AC) source and the LED light source to regulate the power supplied to the LED light source. The LED driver may regulate the current supplied to the LED light source to a specific peak current value, or regulate the voltage provided to the LED light source to a particular value, or may regulate both the current and the voltage. 
         [0003]    Many different LED driver configurations are known, and some provide power by using pulse-width modulation (PWM). Some driver circuit designs switch an LED power supply unit on and off using a pulse duration modulator to control the mean light output of the LEDs. Thus, PWM signals may be used to alter the brightness and color of LEDs. 
         [0004]    The light output of an LED is proportional to the current flowing through it, and thus methods have been developed to control the current delivered to an LED light source. For GaInN type LEDs, a typical load current is about 350 milliamps (mA) at a forward operating voltage of between three and four volts (3V-4V), which corresponds to about a one watt (1 W) power rating. At this power rating, this type of LED provides about 100 lumens per watt which is significantly more efficient than conventional light sources. For example, incandescent lamps typically provide 10 to 20 lumens per watt and fluorescent lamps, 60 to 90 lumens per watt. 
         [0005]    LED light sources usually include a plurality of individual LEDs that may be arranged in both a series and parallel relationship. Thus, a plurality of LEDs may be arranged in a series string and a number of series strings may be arranged in parallel to achieve a particular desired light output. 
         [0006]    LED light sources are typically rated to be driven via either a current load control technique or a voltage load control technique. An LED light source that is rated for the current load control technique is also characterized by a rated current (for example, 350 mA) to which the peak magnitude of the current through the LED light source is regulated to ensure that the LED light source is illuminated to the appropriate intensity and color. In contrast, an LED light source that is rated for the voltage load control technique is characterized by a rated voltage (for example, 15 V) to which the voltage across the LED light source should be regulated to ensure proper operation of the LED light source. Typically, each string of LEDs in an LED light source rated for the voltage load control technique includes a current balance regulation element to ensure that each of the parallel legs has the same impedance so that the same current is drawn in each parallel string. 
         [0007]    It is also known that the light output of an LED light source can be dimmed by using a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. PWM dimming can be used for LED light sources that are controlled in either a current or voltage load control mode. In PWM dimming, a pulsed signal with a varying duty cycle may be supplied to the LED light source. 
         [0008]    If an LED light source is being controlled using the current load control technique, the peak current supplied to the LED light source is kept constant during an On-time of the duty cycle of the pulsed signal. But as the duty cycle of the pulsed signal varies, the average current supplied to the LED light source also varies to vary the intensity of the light output of the LED light source. 
         [0009]    If the LED light source is being controlled using the voltage load control technique, the voltage supplied to the LED light source is kept constant during the On-time of the duty cycle of the pulsed signal in order to achieve the desired target voltage level, and the duty cycle of the load voltage is varied in order to adjust the intensity of the light output. 
         [0010]    Constant current reduction dimming is typically only used when an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current is continuously provided to the LED light source, however, the DC magnitude of the current provided to the LED light source is varied to thus adjust the intensity of the light output. 
         [0011]    There remains a need in the art for an energy-efficient and simple LED driver circuit to control dimming of an LED light source. with reduced component count. 
       SUMMARY OF THE INVENTION 
       [0012]    In an embodiment, a light emitting diode (LED) light source includes at least one LED and a driver control circuit for controlling the brightness of the LED. The driver control circuit includes a power circuit operable to receive input power (either an alternating current (AC) or a direct current (DC)) and to generate a direct current to power the at least one LED, and at least one of a comparator or an amplifier having a current sense signal input and a dimmed current reference command signal input. The comparator or amplifier generates a control signal output that controls the power circuit to dim the at least one LED by decreasing the amplitude of LED current when the control signal is above a predetermined threshold. The driver control circuit also includes a fixed pulse width generator (PWG) having an output connected to the power circuit, wherein the fixed PWG is configured to operate with a predetermined fixed duty cycle, and a comparator circuit having a predetermined threshold signal input and a dimming command signal input. When the dimming command signal is above the predetermined threshold the comparator circuit disables the fixed PWG to dim the at least one LED. In addition, the comparator circuit enables the fixed PWG when the dimming command signal decreases below the predetermined threshold level resulting in the power converter circuit being controlled On-Off with the predetermined fixed duty cycle. The current amplitude level is automatically adjusted in response to the dimming command signal and the activation of the PWG in order to further dim the at least one LED. 
         [0013]    Advantageously, the driver control circuitry may include an averager circuit or a low-pass filter having an output connected to the comparator or amplifier circuit, wherein the averager circuit or low-pass filter operates to extract the average value of the sensed load current. By so doing, the control loop automatically adjusts the current amplitude level in response to the activation of the PWG in order to further dim the at least one LED. Beneficially, in some implementations, the power circuit may further include an integrated circuit (IC) for controlling of a boost, buck, buck-boost, SEPIC, hysteretic, or flyback-type topology. In addition, in some advantageous embodiments the fixed PWG component is one of a fixed 555 timer circuit or a ripple counter circuit, and the power converter circuit is controlled On-Off within a frequency range of 100 Hertz to 2 Kilohertz. 
         [0014]    A method for controlling brightness of an LED light source is also described. In some embodiments, the process includes receiving, by an LED driver control circuit, a dimming command signal to dim an LED light source, and then modulating a continuous direct current (DC) level to dim the LED light source. When it is determined that a predetermined threshold level has been reached, the process includes initiating a fixed pulse width generator (PWG) control signal having a fixed duty cycle, automatically adjusting a current amplitude to its nominal current level, and decreasing the current amplitude while the fixed PWG control signal is active to achieve commanded lower dimming of the LED light source. 
         [0015]    In some beneficial embodiments, the predetermined threshold level comprises a percentage of a dimming range, which may be ten percent. In addition, the duty cycle of the fixed PWG control signal may advantageously be a percentage of a peak current, and in some embodiments the duty cycle is twenty-percent. The process may advantageously also include determining that the dimming command signal is greater than the threshold level, and then modulating the continuous direct current (DC) level to dim the LED light source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Features and advantages of some embodiments, and the manner in which the same are accomplished, will become more readily apparent with reference to the following detailed description taken in conjunction with the accompanying drawings, which illustrate exemplary embodiments (not necessarily drawn to scale), wherein: 
           [0017]      FIG. 1A  is a schematic block diagram of a prior art amplitude modulated dimming circuit; 
           [0018]      FIG. 1B  is a schematic block diagram of a prior art pulse-width modulated (PWM) modulated dimming circuit; 
           [0019]      FIG. 2A  is a schematic block diagram of an LED dimming circuit in accordance with aspects of the present invention; 
           [0020]      FIG. 2B  is a schematic block diagram of an LED dimming circuit in accordance with aspects of another embodiment of the present invention; and 
           [0021]      FIG. 3  is a flowchart illustrating a dimming method in accordance with aspects of the present invention. 
       
    
    
       [0022]    Like reference numbers in the drawings indicate the same or similar elements. 
       DETAILED DESCRIPTION 
       [0023]    The inventor recognized that a need exists for an energy-efficient and simple LED driver circuit to control dimming of an LED light source down to dimming levels below ten percent, wherein such an LED driver circuit has a reduced component count as compared to prior art configurations. Embodiments of such are described hereinbelow with reference to  FIG. 2A ,  FIG. 2B , and  FIG. 3 . 
         [0024]      FIG. 1A  is a schematic circuit diagram illustrating a conventional driver control system  100  with amplitude modulation (AM) dimming control for an LED light source  102  that includes a plurality of LEDs in a series configuration. Amplitude modulated dimming is also known in the industry as linear dimming or continuous dimming. A power circuit  104 , which may include an integrated circuit (IC) for control of a boost, buck, buck-boost, SEPIC, hysteretic, or flyback power topologies and the like, operates to control the amplitude of a DC (direct current) load current through the series of LEDs  102 . A comparator/amplifier circuit  106  operates to monitor the LED current (by utilizing the LED current sense feedback input  110 ) and to produce an appropriate error voltage signal  112  (that may be compensated in order to achieve loop stability) that is fed to the power circuit  104  for LED current regulation. A dimming function can be implemented by simply mixing a dimming command signal  108  (which may be generated by a dimming control circuit, not shown) with the current reference  111  in order to obtain a dimmed current reference signal  116 . When the dimmed current reference signal  116  falls below ten percent (10%) of the full load level, then the load current supplied to the LED light source  102  is amplitude-modulated to a direct current (DC) level that is too small to avoid flickering and significant color change of the LEDs of the LED light source  102 . These are sub-optimal results which are to be avoided in most LED lighting situations. 
         [0025]      FIG. 1B  shows another conventional dimming circuit diagram  120  that utilizes a pulse width modulated (PWM) control function. In this case the amplitude of the current is not varied as a function of a dimming command; rather, the dimming command is provided directly to the power circuit  104  in the form of a PWM signal  122 . Thus, the current reference  124  is fed directly into the comparator/amplifier circuit  106 , which again operates to monitor the LED current and to produce an appropriate error voltage signal  112 . The power circuit responds to the PWM signal  122  by allowing current to flow in the LED string  102  during the “On” level of the PWM signal, and by inhibiting current from flowing in the LED string during the “Off” level of the PWM signal. 
         [0026]      FIG. 2A  is a schematic circuit diagram of an LED driver control system  200  according to an embodiment of the invention. In some embodiments, the LED light source  202  includes a plurality of LEDs in series. In this implementation, the system includes a power circuit  204  (which may include an integrated circuit (IC) for control of a boost, buck, buck-boost, SEPIC, hysteretic, or flyback power topologies and the like) which operates to control the direct current (DC) through the series of LEDs  202 . As shown, the output of a comparator/amplifier circuit  206  is connected to the power circuit  204  along with the output of a fixed pulse-width generator (PWG) circuit  208 . As used herein, a “comparator/amplifier circuit” may be a comparator, an amplifier, or both. The inputs to the comparator/amplifier circuit  206  include a current sense signal  210 , and a command signal  212  that is derived from a dimmed current reference signal  211  in a manner that is dependent on whether the PWG circuit  208  is activated or not activated. The activation of the PWG circuit  208  itself depends on whether the dimming command signal  213  is higher than a predetermined threshold level  216 . In some embodiments, for example, a predetermined threshold level of twenty percent (20%) of the dimming command signal may be selected to initiate operation of the fixed PWG  208 . Thus, during operation of the LED driver control system  200 , as long as the dimming command signal is above 20%, the comparator  214  operates to disable the operation of the fixed PWG circuit  208 . In such a case, the LED driver control system  200  operates in the same manner as described above with regard to the LED driver  100  of  FIG. 1A . 
         [0027]    However, as the dimming command signal diminishes so that it reaches the predetermined threshold level of 20%, then the power circuit  204  is controlled to be “On” and “Off” with a fixed 20% duty-cycle at a frequency ranging between one hundred hertz (100 Hz) to two kilohertz (2 kHz). It should be understood that this duty cycle is fixed, and that it is either activated or not activated. Thus, no feedback mechanism and/or no modulation mechanism is/are required. 
         [0028]    Referring again to  FIG. 2A , the main loop, consisting of comparator/amplifier  206  and power circuit  204 , stays in control by automatically adjusting the average load current to 20% of the full load value. This is accomplished by increasing the value of the dimmed reference signal  211  by a factor equal to the inverse of the fixed duty cycle selected for the PWG  208 . For instance, if the selected PWG duty cycle is 20%, the dimming current reference signal  211  is automatically increased by a factor of five because one divided by 20% equals five (1/0.2=5). Accordingly, when the command signal  212  decreases to ten percent (10%) of the full load level, flicker-free operation of the LED light source  202  is still maintained, while obtaining 2% dimming (10% amplitude reduction and 20% duty ratio for a total average output current of 2%). 
         [0029]    Accordingly, instead of utilizing amplitude control to achieve higher level dimming of an LED light source and then using PWM control for lower level dimming of the LED light source (as taught by the prior art), the present LED driver control system  200  imposes a fixed PWG regime (fixed duty cycle) when the dimming command falls below a predetermined threshold value (in the above described example, the threshold value is 20%, but other choices are possible and/or permissible). Such operation can be achieved with a simple circuit rather than by using a full PWM modulator, to save cost. For example, an extremely simple timing circuit, such as a fixed 555-type timer circuit or a ripple counter circuit, could be used to implement the fixed PWG component  208  of  FIG. 2A . 
         [0030]    Accordingly, flicker-free dimming down to 2% of the LED light source nominal brightness can be achieved through the use of an inexpensive and simple fixed PWG regime and with the use of only one control mechanism (current amplitude), rather than by utilizing two control mechanisms as used by previous art. 
         [0031]      FIG. 2B  is a schematic circuit diagram of an LED driver control system  250  according to another embodiment. In this implementation, the system includes a power circuit  204  (which may include an integrated circuit (IC) for control of a boost, buck, buck-boost. SEPIC, hysteretic, or flyback power topologies and the like) which operates to control the direct current (DC) through the series of LEDs  202 . As shown, the output of a comparator/amplifier circuit  206  is connected to the power circuit  204  along with the output of a fixed pulse-width generator (PWG) circuit  208 . The inputs to the comparator/amplifier circuit  206  include a current sense signal  210 , and a dimmed current reference signal  212 . The averager circuit  252  (which, in its simplest form, may be a low pass filter (LPF)) operates to extract the average value of the LED current. The dimming command signal  213  is input to a comparator circuit  214  along with a pre-selected or predetermined PWG operation threshold signal  216 . In an example, a predetermined threshold level of twenty percent (20%) of the dimming command signal may be selected to initiate operation of the fixed PWG  208 . Thus, during operation of the LED driver control system  250 , as long as the dimming command signal is above 20%, the comparator  214  operates to disable the operation of the fixed PWG circuit  208 . In such a case, the LED driver control system  250  operates in the same manner as the LED driver  100  of  FIG. 1A . 
         [0032]    However, as the dimming command signal diminishes so that it reaches the predetermined threshold level of 20%, then the power circuit  204  is controlled to be “On” and “Off” with a fixed 20% duty-cycle at a frequency ranging between one hundred hertz (100 Hz) to two kilohertz (2 kHz). Thus, this duty cycle is fixed, as it is either activated or not activated. Thus, no feedback mechanism and/or no modulation mechanism is/are required. Accordingly, due to the presence of averager circuit  252 , in order for the main loop (which consists of the comparator/amplifier circuit  206  and the power circuit  204  and the averager circuit  252 ) to stay in control when the fixed duty cycle PWG is activated, the current amplitude level during the “On” time is automatically adjusted by the control loop so that the load current average is 20% of the full load level. Again, flicker-free dimming down to 2% of the LED light source nominal brightness can be achieved through the use of an inexpensive and simple fixed PWG regime and with the use of only one control mechanism (current amplitude), rather than by utilizing two control mechanisms as used by previous art. 
         [0033]      FIG. 3  is a flowchart of a process  300  for controlling the brightness of an LED light source according to an embodiment. For example, a dimming command signal may be received  302  by an LED driver control circuit. If such command is above (not less than or equal to) a predetermined threshold level in step  304 , then the process branches to step  305  wherein amplitude modulation of the DC current level continues until the appropriate level of dimness is achieved (in some embodiments, a person may wish to downwardly adjust the brightness level of the LED light source, to make it less bright, and thus he or she could operate a dimming switch or other dimming control circuitry which in turn transmits such a dimming command.) But if in step  304  the dimming command equals to or is lower than the predetermined threshold level, then the LED driver control circuit initiates  306  operation with a fixed duty cycle by using a fixed pulse width (PW) control signal having a fixed duty cycle. Next, the LED driver control circuit automatically adjusts  308  the load current amplitude to the required value (nominal current level) that will achieve the desired average current through the LED string. The process branches back to set  302  or restarts when a new dimming command is received. 
         [0034]    Advantageously, the described LED driver control systems  200  and  250 , and the process  300 , may allow the LED light source dimming mechanism to never drive the LEDs with an instantaneous load current that is so low that flickering could result, and furthermore may prevent any significant color shift in the LEDs. Moreover, dimming operation is typically accomplished utilizing only amplitude control, and thus can be without the use of two control mechanisms (one for current amplitude control and the other for PWM control). Yet further, dimming can beneficially be accomplished with fewer components which may result in increased circuit reliability and lower cost. 
         [0035]    The above description and/or the accompanying drawings are not meant to imply a fixed order or sequence of steps for any process referred to herein; rather any process may be performed in any order that is practicable, including but not limited to simultaneous performance of steps indicated as sequential. 
         [0036]    Although the present invention has been described in connection with specific exemplary embodiments, it should be understood that various changes, substitutions, and alterations apparent to those skilled in the art can be made to the disclosed embodiments without departing from the spirit and scope of the invention as set forth in the appended claims.