Abstract:
A method and circuit to use light-emitting diodes to emulate the dimming performance of incandescent lighting, and more particularly, to making a circuit that uses only white and deep red light-emitting diodes to achieve a coordinated-color-temperature as a function of dim level that is close to that of an incandescent light being similarly dimmed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/140,997, entitled “LIGHT EMITTING DIODE (LED) WARM ON DIM CIRCUIT,” filed on Mar. 31, 2015, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The disclosure relates to providing a method and circuit to use light-emitting diodes (LEDs) to emulate the dimming performance of incandescent lighting, and more particularly, to making a circuit that uses only white and deep red LEDs to achieve a coordinated-color-temperature (CCT) as a function of dim level that is close to that of an incandescent light being similarly dimmed. 
     BACKGROUND 
     It is frequently desirable to dim lights. Dimmers are found in many residential and commercial locations. This dimming has been done historically by removing power from the light during a certain portion of each line cycle, as is accomplished using a triac dimmer. However, it is becoming increasingly common for the dimming level to be set by a DC signal, typically 0-10V. 
     Traditionally, dimmers have been used with incandescent light bulbs. These bulbs have very particular characteristics as they dim. Not only does the light they produce get dimmer, it also becomes redder, so-called ‘warm-on-dim’. One measure of the change in color of the light is the CCT. The CCT of a light is measured by finding the closest black-body radiation emission temperature of the light, and so is measured in degrees Kelvin (K). A typical incandescent light bulb, as it is dimmed, changes from about 2800K to about 1800-2000K. The relationship between dim level, measured by average voltage applied over a line cycle, and CCT is highly non-linear. 
     An increasingly common form of lighting is the LED. These work by applying a DC current through them. The amount of light they emit is approximately proportional to the magnitude of the current. It is thus common to dim LEDs by controlling their current as a function of the dim level, which is determined by either the average or RMS voltage, or by the DC 0-10V signal. 
     However, a problem arises when dimming LED lights. A particularly common form of LED is the so-called ‘white’ LED. These can be designed to have a CCT anywhere along the black-body curve, from 6000K down to below 2500K. However, for a given LED their CCT is basically not adjustable. When the current through them is reduced, they produce less light, but the CCT is unaffected. White LEDs thus do not produce the same type of light output on a dimmer as does an incandescent light bulb. 
     The typical method of solution to this problem in an LED light is to use a variety of different colored LEDs, such as a combination of red, green and blue (RGB), and to mix their light emissions together optically. The amount of current in each may be designed to produce white light of a particular CCT, for example that of an incandescent bulb at full brightness. When used with a dimmer, the ratios of currents in the different colors is varied in such a way as to produce a desired arbitrary CCT, and in particular, one that is close to that of an incandescent being similarly dimmed. 
     However, generating and controlling the currents appropriately to the three colors of LEDs typically involves considerable circuitry, and frequently is done under software control of a microcontroller that is built in to the light. This level of complexity makes such a control expensive and large. Furthermore, the type of red LED used to achieve this performance has a very significant change in light output as a function of its temperature, even at constant current drive level. To compensate for this, the microcontroller will typically also sense the temperature of the light, applying corrections to the amount of current received by the red LED as a function of temperature. 
     Yet further, the red LEDs used also have a very significant drop-off in light output with age. After a few years of operation, their light output has become significantly reduced, and the amount of that reduction is different than that of the green or blue LEDs used together with the red to produce the white light. To compensate for this, a light sensor is sometimes built in to the light to detect the light&#39;s spectral characteristics, and adjust the red LEDs&#39; current to compensate for this aging. The light sensor can also be used to adjust the red LEDs&#39; current to achieve the desired CCT as a function of dim level. The light sensor further adds cost and size to the design of the light, and the temperature measurement and correction of the red LEDs&#39; drive current further add to the complexity of the firmware used by the microcontroller. 
     It would be desirable to have an LED system that could be dimmed in such a way as to emulate the light output of an incandescent bulb, both in brightness and in CCT, when operated on either a triac or 0-10V dimmer. It would be desirable that it would be insensitive to temperature variation and aging, and that it would be easy to control without the use of a microcontroller or a spectral sensor. It would also be desirable that it would be inexpensive, and that it not require the use of three or more different types of LEDs. 
     SUMMARY 
     In one or more embodiments, a circuit uses a number of white LEDs of approximately 3000K CCT, plus a smaller number of deep red LEDs with an emission spectrum of approximately 650 nm, or more generally ranging in wavelength from 630 nm up to the limits of visibility. It provides for controlling the current to the two types of LEDs when used with a dimmer in such a way as to emulate both the brightness and CCT of an incandescent light used on that dimmer. It also provides for being insensitive to temperature variations and aging, does not require the use of either a spectral sensor or a microcontroller, and is small and inexpensive. 
     In one or more embodiments, a current drive is provided to the white LEDs, with the current being controlled by the dim level. The current drive may be, for example, a controllable constant current sink. In another embodiment, the current drive may be a switch-mode power supply (SNIPS). The control may be either an average line voltage or an externally sourced DC voltage such as 0-10V. In an embodiment, the average line voltage is determined by an RC filter attached to the output of the bridge. In some embodiments, a current drive is provided to the deep red LEDs, with the current being controlled by the dim level. In an embodiment, the current drive for the deep red LEDs may be a SNIPS. The control used may be the same control as that used for the white LEDs. 
     In one or more embodiments, the current to the white LEDs is proportional to the dim level, so that, for example, when the dimmer is at maximum, the current to the white LEDs is maximum, and when the dimmer is at 60%, the current to the white LEDs is also 60%. The current to the deep red LEDs is also dependent on the dim level, but additionally has a saturation value and may have a minimum level. In an embodiment, the current to the deep red LEDs is saturated at maximum when the dimmer is at maximum, and the current remains saturated at maximum while the dimmer is lowered to 60%. When the dimmer is below 60%, the current to the deep red LEDs is proportional to the further drop of the dimmer level, so that, for example, when the dimmer is at 30%, the current to the deep red LEDs is at 65%, and when the dimmer is close to 0%, the current to the deep red LEDs is at a minimum level of 35%. The specific numbers used are dependent on the particular light output and number of LEDs used for both the white and the deep red LEDs. 
     As an example, at dimmer maximum, both the white and the deep red LEDs are on at maximum current. In one embodiment, the ratio of the number of white LEDs to the number of deep red LEDs may be selected such that this combination produces white light of a CCT of approximately 2700K. At 60% setting of the dimmer, the white LEDs are at 60% of their maximum current, while the deep red LEDs are still at their maximum current. This produces a white light of CCT of approximately 2630K, closely matching that of an incandescent light at 60% dim level. At 5% setting of the dimmer, the white LEDs are at 5% of their maximum current, while the deep red LEDs are at 35% of their maximum current. This produces a reddish light of CCT of approximately 1800K, close to that of an incandescent at 5% dim level. The particular CCT as a function of dim level may be designed by selecting the brightness and drive level compatible with the particular white and deep red LEDs selected. 
     Such embodiments are, for example, insensitive to variations in temperature and aging, as the deep red LEDs, unlike amber or red LEDs, have little variation in light output with either. Thus, neither a spectral sensor nor a microcontroller is required, and these embodiments can be is thus both small and inexpensive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. 
         FIG. 1  is a diagram of an LED warm-on-dim circuit controlled by the average line voltage, according to an embodiment. 
         FIG. 2  is a diagram of an LED warm-on-dim circuit controlled by a 0-10V dim signal, according to another embodiment. 
         FIG. 3  is an example of a diagram of the CCT vs. dim level of an LED warm-on-dim circuit. 
         FIG. 4  is an example of a diagram of the drive current to the white and deep red LEDs in an LED warm-on-dim circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the various embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     According to the design characteristics, a detailed description of the embodiments is given below. 
       FIG. 1  is a diagram of an LED warm-on-dim circuit  100  controlled by the average line voltage, according to an embodiment. As shown in  FIG. 1 , the AC line  110  is rectified by a diode bridge  120 . The average line voltage may be determined by a filter  130 , if a DC dimming signal is not present. The filter  130  consists of two resistors  131  and  132  connected between the output of the diode bridge  120  and ground that divide down the rectified voltage, and a capacitor  133  that averages the divided down voltage. In an embodiment, the time constant of the filter  130  may be at least several times the period of the AC line  110 . 
     Also attached to the output of the diode bridge  120  is a set of white LEDs  150 . These may be a series string or parallel combination of LEDs  150 , or a parallel set of series strings of white LEDs  150 . The set of white LEDs  150  have their current controlled by a controllable current sink  160 . The controllable current sink  160 , for example, includes a transistor  161 , a current sense resistor  162 , a control opamp circuit  163  and a shunt reference  164 . The transistor  161  passes current from the set of white LEDs  150 , through the current sense resistor  162  to ground. The current sense resistor  162  produces a voltage proportional to the current from the set of white LEDs  150 . The control opamp circuit  163  has as inputs the voltage from the current sense resistor  162  and the voltage from the filter  130 . The output of the control opamp circuit  163  produces a voltage that controls the shunt reference  164 . In operation, the output of the opamp circuit  163  is equal to a fixed gain, times the difference in voltage between the current sense resistor  162  and the filter  130 . The gain of the opamp circuit  163  is set high. This forces the voltage across the current sense resistor  162 , and thus the current through the white LEDs  150 , to be nearly equal to the voltage from the filter  130 , and thus to the average line voltage. 
     Also attached to the output of the diode bridge  120  is a set of deep red LEDs  170 . This may be a single LED  170  or a series string or parallel combination of LEDs  170 , or a parallel set of series strings of deep red LEDs  170 . The set of deep red LEDs  170  have their current controlled by a controllable SMPS  180 . The controllable SMPS  180 , for example, may be buck-derived, consisting of a controller  181 , an inductor  182 , a transistor  183 , a current sense resistor  184 , a rectifier diode  185  and a control opamp circuit  186 . The controller  181  turns on the transistor  183 . With the transistor  183  on, the current in the inductor  182  increases. The current in the inductor  182  comes from the deep red LED  170 , goes through the transistor  183  and thence through the current sense resistor  184  to ground. The current sense resistor  184 , in response to the current, develops a voltage proportional to the inductor  182  and deep red LED  170  current. The controller  181  compares the voltage developed across the current sense resistor  184  with a reference voltage. When the voltage developed across the current sense resistor  184  is equal to the reference voltage, the controller  181  turns off the transistor  183 . With the transistor  183  off, the current in the inductor  182  goes through the rectifier diode  185  and decreases. The entire cycle then repeats, either at a constant frequency or with constant off-time, or with other known control schemes. The result is that to a first approximation, the current in the inductor  182  and the deep red LEDs  170  is constant. 
     The reference voltage for the controller  181  is generated by the control opamp circuit  186 . The control opamp circuit  186  has as inputs the voltage from the current sense resistor  184  and the voltage from the filter  130 . The output of the control opamp circuit  186  produces a voltage that generates or controls the reference voltage for the controller  181 . The output of the control opamp circuit  186  has a saturation value and may have a minimum level. In some instances, the output of the opamp circuit  186  may be saturated at its maximum voltage when the filter  130  voltage is at maximum, and this output remains saturated at maximum until the filter  130  voltage lowers to a particular value, such as 60% of maximum. When the filter  130  voltage is lower than this particular value, such as below 60%, the output of the opamp circuit  186  may be proportional to the further drop of the filter  130  voltage. In some instances, when the filter  130  voltage reaches zero or close to zero, the output of the opamp circuit  186  may remain at some minimum level, such as 35%. 
     In operation, the output of the opamp circuit  186  is equal to a fixed gain, times the voltage of the filter  130  plus a reference voltage. The gain may be less than one, in which case it may be formed in part by a resistor divider, not shown. As long as the filter  130  voltage is above a particular value, such as above 60%, the sum of the filter  130  voltage plus the reference voltage is large enough that the output of the opamp circuit  186  is at its maximum. In an embodiment, this maximum is set by the power supply voltage of the opamp circuit  186 . This causes the SMPS  180  to produce maximum current through the set of deep red LEDs  170 . When the filter  130  voltage is linearly decreased below this particular value, the output of the opamp circuit  186  linearly decreases. This causes the SMPS  180  to produce linearly less current through the set of deep red LEDs  170 . 
     As a result of these two control systems, the controllable current sink  160  and the SMPS  180 , the set of white LEDs  150  and the set of deep red LEDs  170  together produce light that, both at full brightness and when dimmed, emulates the brightness and CCT of an incandescent light when it is respectively at full brightness or dimmed. 
       FIG. 2  is a diagram of an LED warm-on-dim circuit  200  controlled by a 0-10V dim signal  140 , according to an embodiment. As shown in  FIG. 2 , the 0-10V dim signal  140  now forms the reference voltage for both the opamp  163  controlling the brightness of the white LEDs  150 , and also for the opamp  186  controlling the brightness of the deep red LEDs  170 . The 0-10V dim signal  140  may be, for example, divided down by a resistor divided, not shown, to form a signal of comparable amplitude to the voltage on the current sense resistors  162  and  184 . 
       FIG. 3  is an example of a diagram of the CCT vs. dim level of an LED warm-on-dim circuit  100 . As shown in  FIG. 3 , the x-axis  310  shows the average of the line voltage of the AC line  110  rectified by a diode bridge  120 . The x-axis  310  is marked in terms of percentage of full voltage. As the line voltage is dimmed, the average decreases, corresponding to more leftwards positions on the x-axis  310 . The y-axis  320  shows the CCT of the LED warm-on-dim circuit  100 . The y-axis  320  is marked in terms of degrees Kelvin (K). 
     At the maximum setting of the dimmer, shown on the far right of the x-axis  310 , the CCT is at approximately 2800K. As the setting of the dimmer is decreased, the CCT approximately linearly decreases, until the setting is approximately 40%, at which point the CCT has decreased to approximately 2600K. As the dimmer is further decreased, the CCT begins to drop more quickly. By the time it reaches 5%, the CCT has dropped to approximately 1800K. This CCT vs. dim level graph  300  closely matches that of an incandescent light on a dimmer. 
       FIG. 4  is an example of a diagram  400  of the drive current to the white LEDs  150  and a diagram  410  of the drive current to the deep red LEDs  170  in an LED warm-on-dim circuit  100  or in an LED warm-on-dim circuit  200 . As shown in  FIG. 4 , the x-axis  420  of the diagram  400  of the drive current to the white LEDs  150  shows the average of the line voltage of the AC line  110  rectified by a diode bridge  120 . The x-axis  420  is marked in terms of percentage of full voltage. The y-axis  430  shows the percentage of maximum current of the white LEDs  150  of the LED warm-on-dim circuit  100  or of the LED warm-on-dim circuit  200 . At the maximum setting of the dimmer, shown on the far right of the x-axis  420 , the current is approximately 100% of maximum. As the setting of the dimmer is decreased, the current approximately linearly decreases, down to approximately zero at a dimmer setting at or close to zero percent of full line voltage. 
     The x-axis  440  of the diagram  410  of the drive current to the deep red LEDs  170  shows the average of the line voltage of the AC line  110  rectified by a diode bridge  120 . The x-axis  440  is marked in terms of percentage of full voltage. The y-axis  450  shows the percentage of maximum current of the deep red LEDs  170  of the LED warm-on-dim circuit  100  or of the LED warm-on-dim circuit  200 . At the maximum setting of the dimmer, shown on the far right of the x-axis  440 , the current is approximately 100% of maximum. As the setting of the dimmer is decreased down to approximately 60%, the current remains at approximately 100% of maximum. As the setting of the dimmer is further decreased beyond 60%, the current approximately linearly decreases. When the dimmer is decreased to almost 0%, the current is reduced to approximately 35%. 
     It will be apparent to those skilled in the art that various modifications and variation can be made to the disclosed embodiments. In view of the foregoing, it is intended that the disclosure cover modifications and variations of the disclosed embodiments.