Abstract:
A control circuit for a light emitting diode (LED) lighting system for achieving a dim-to-warm effect is provided. The control circuit includes an LED controller, a clamp circuit coupled to a set of warm correlated-color-temperature (“CCT”) LEDs, a switch coupled to a set of cool LEDs, and a feedback circuit coupled to the clamp and the switch. The LED controller is configured to control the clamp circuit to clamp current through the set of warm LEDs based on the input current, and control the switch to switch on the set of cool LEDs responsive to the input current being greater than a first threshold level and to switch off the set of cool LEDs responsive to the input current being lower than the first threshold level. The feedback circuit is configured to divert current from the set of warm LEDs to the set of cool LEDs.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/328,523 filed on Apr. 27, 2016 and European Provisional Application No. 16 173 125.2 filed on Jun. 6, 2016, the content of which is hereby incorporated by reference herein as if fully set forth. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to general lighting using light emitting diodes (LEDs) and, in particular, to a technique to cause LED light to be progressively warmer (have a lower CCT) as the LED light is dimmed by a dimmer. 
       BACKGROUND 
       [0003]    Incandescent bulbs have aesthetically pleasing lighting characteristics. For example, incandescent bulbs get progressively redder (warmer) as the user dims the light by controlling a dimmer to reduce the average current through the bulb. Although many advancements are being made in LED technology, further advancements to help achieve the quality of light typically provided by incandescent bulbs is desirable. 
       SUMMARY 
       [0004]    A control circuit for a light emitting diode (LED) lighting system for achieving a dim-to-warm effect between a minimum brightness-maximum dimming level, and a maximum brightness-minimum dimming level is provided. The control circuit includes an LED controller, a clamp circuit coupled to a set of warm correlated-color-temperature (“CCT”) LEDs, a switch coupled to a set of cool CCT LEDs, and a feedback circuit coupled to the clamp and the switch. The LED controller is configured to sense the magnitude of an adjustable input current, control the clamp circuit to clamp current through the set of warm CCT LEDs to a clamp current level based on the input current, and control the switch to switch on the set of cool CCT LEDs responsive to the input current being greater than a first threshold level and to switch off the set of cool CCT LEDs responsive to the input current being lower than the first threshold level. Responsive to the input current exceeding a second threshold level, the feedback circuit is configured to divert current from the set of warm CCT LEDs to the set of cool LEDs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates a string of warm LEDs and a string of cool LEDs, both emitting white light, and further illustrates a dim-to-warm circuit that controls the currents to each string as the input voltage varies from a minimum current to a maximum current. 
           [0006]      FIG. 2  is an example of the relative currents supplied to the warm LEDs (Iw) and the cool LEDs (Ic) over the full range of input currents. 
           [0007]      FIG. 3  illustrates various functional units in the dim-to-warm circuit of  FIG. 2 . 
           [0008]      FIG. 4  is a circuit diagram of the dim-to-warm circuit, as well as the warm LEDs and cool LEDs. 
           [0009]      FIG. 5  is a graph showing the simulated overall CCT of the lamp as the light is dimmed from the maximum to the minimum, as well as showing the ideal CCT of a halogen bulb. 
           [0010]      FIGS. 6A-6B  illustrate an embodiment of the invention, where the input currents into four dim-to-warm circuits are provided by a tapped linear driver receiving an analog dimming signal, and where four dim-to-warm circuits are used and designed to each create the same CCT at the same dimming level. 
           [0011]      FIG. 7  is a function diagram (from a data sheet) of a suitable prior art tapped linear regulator that may be used in the system of  FIG. 6 . 
       
    
    
       [0012]    Elements that are the same or similar are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0013]    In one embodiment, two series strings of LEDs are used in a lamp. The first string contains identical cool LEDs, such as GaN-based LEDs with a tuned phosphor that results in a CCT of 4000K. The second string contains identical warm LEDs, such as using the same GaN-based LED dies as the cool LEDs but using a tuned phosphor the results in a CCT of 2200K. In other embodiments, the number of strings and CCTs may be different. Both CCTs are considered white light. 
         [0014]    A power supply, such as a rectified mains voltage, is applied to one end of the two strings, and the other ends of the two strings are connected to different terminals of a dim-to-warm circuit. 
         [0015]    An adjustable analog (not PWM) current is supplied to an input of the dim-to-warm circuit, where the input current level may be adjusted by a user controlling a suitable light dimmer. 
         [0016]    Between the minimum input current and a first input current level, the cool LED string is disconnected by a switch, and all the input current flows through the warm LED string. Therefore, the dimming solely controls the brightness of the warm LEDs up to the first input current level. The CCT output of the lamp is a constant warm temperature up to the first input current level. 
         [0017]    As the input current is adjusted above the first input current level, but below a second input current level, the switch is closed and a portion of the input current flows through the cool LED string, while current through the warm LED string is clamped to a constant current. Therefore, within this range of input currents, the dimming solely controls the brightness of the cool LEDs while the brightness of the warm LEDs stays constant. The CCT output of the lamp is a varying mixture of the two CCTs, with the CCT increasing as the input current approaches the second input current level. 
         [0018]    As the input current is adjusted above the second input current level to the maximum current, the cool LEDs remain controlled by the increasing input current, while the current to the warm LEDs is progressively reduced to zero at the maximum input current. The CCT output of the lamp thus approaches the CCT of the cool LEDs as the input current level approaches its maximum. 
         [0019]    Using this technique, the full range of CCTs, from 4000K-2200K is obtained and, since both sets of LEDs output a white light, there is a more natural combination of light from the different LEDs producing the varying CCT. Since the operation is linear (no PWM or high frequency switching), no EMI is generated and no filters are needed. Since the operation is linear, very small linear regulators can be used to create the input current, including a tapped linear regulator. 
         [0020]    In one embodiment, a tapped linear driver is used as the driver for the dim-to-warm circuit. The tapped linear regulator receives a voltage from a full wave diode bridge rectifying the AC mains voltage and successively supplies current to different segments of the two LED strings as the DC voltage varies at double the AC frequency. This results in a very compact and efficient control system. 
         [0021]      FIG. 1  illustrates one embodiment. A power supply  10  may be a rectified mains voltage, a battery, a regulator, or any other source. A series string of white-light cool LEDs  12  has its anode end coupled to the power supply  10 , and a series string of white-light warm LEDs  14  also has its anode end coupled to the power supply  10 . There may be multiple strings of each type of LED, depending on the desired maximum light output of the lamp, and the strings for each type of LED may be connected in parallel so that the strings of each type of LED are controlled identically. 
         [0022]    The cool LEDs may be conventional, commercially available, GaN-based LED dies, emitting blue light, with a suitable phosphor deposited over the die, such as a YAG phosphor. Other phosphors may be used. Such cool LEDs  12  will typically have a CCT in the range of 3000-6000K. In the example, the CCT is 4000K. 
         [0023]    The warm LEDs  14  may be conventional, commercially available, GaN-based LED dies, emitting blue light, with a suitable phosphor deposited over the die, such as a YAG phosphor plus a warmer phosphor emitting amber or red light. Other phosphors may be used. Such warm LEDs  14  will typically have a CCT in the range of 1900-2700K. In the example, the CCT is 2200K. 
         [0024]    Since the warm and cool LED dies may be the same type of die, they have the same forward voltage drops. In one embodiment, the same number of LEDs is in each of the strings so the strings have the same forward voltage drops. 
         [0025]    The relative brightnesses (luminous flux) of the cool LEDs  12  and warm LEDs  14  are determined by a dim-to-warm circuit  16 . The dim-to-warm circuit  16  may be a 3-terminal circuit that outputs the separate drive currents for the warm LEDs  14  (Iw) and the cool LEDs  12  (Ic). The input into the dim-to-warm circuit  16  is an adjustable analog current (input current Iin) from an external current source  18  that sets the overall dimming of the lamp. A low input current Iin results in a low overall brightness of the lamp that has a relatively low CCT, and a high input current Iin results in a high overall brightness of the lamp with a relatively high CCT. 
         [0026]      FIG. 2  illustrates the current Iw through the warm LEDs  14  (directly corresponding to the brightness of the warm LEDs  14 ) and the current Ic 1  or Ic 2  through the cool LEDs  12  (directly corresponding to the brightness of the cool LEDs  12 ) through the full range of input currents Iin. The current Ic 1  represents a current where the cool LEDs  12  are completely off between the minimum input current Iin(min) and an intermediate input current Iin 1 , and the current Ic 2  represents a current where the cool LEDs  12  are somewhat on between Iin(min) and Iin 1  so the CCT change is continuous throughout the entire Iin range. The dim-to-warm circuit  16  can be designed to achieve the Ic 1  or Ic 2  current curve. 
         [0027]    The minimum input current Iin(min) corresponds to a maximum dimming level (least bright and most warm), and the maximum input current Iin(max) corresponds to a minimum dimming level (most bright and most cool). 
         [0028]    The following description assumes the dim-to-warm circuit  16  outputs the current Ic 1 . Between Iin(min) and Iin 1 , the dim-to-warm circuit  16  only outputs the current Iw to drive the warm LEDs  14  with a current proportional to the adjustable input current Iin, so the CCT output of the lamp is 2200K. Between Iin 1  and In 2 , the dim-to-warm circuit  16  clamps Iw so that the brightness of the warm LEDs  14  is relatively constant, while Ic 1  rises proportional to the input current Iin. Therefore, between Iin 1  and Iin 2 , the overall (perceived) CCT output of the lamp will become increasing cooler. Between Iin 2  and Iin(max), Iw ramps down, while Ic 1  still rises proportional to the input current Iin. The overall CCT of the lamp at the various dimming levels generally matches the varying CCT of a halogen lamp or incandescent bulb. 
         [0029]      FIG. 3  illustrates the overall system showing the dim-to-warm circuit  16 , the string of warm LEDs  14 , the string of cool LEDs  12 , and the dimming control adjustable current source  18  outputting Iin. 
         [0030]    At an Iin below Iin 1 , a control circuit  22  (a comparator) keeps a switch  24  off so that no current flows through the cool LEDs  12  and all the input current Iin flows through the warm LEDs  14 . 
         [0031]    When Iin exceeds Iin 1 , the control circuit  22  turns on the switch  24  so that the current Ic through the cool LEDs  12  is generally proportional to Iin. The control circuit  22  also controls a clamp circuit  26  to clamp the current Iw to a fixed level so that the brightness of the warm LEDs  14  does not change between Iin 1  and Iin 2  ( FIG. 2 ). 
         [0032]    When the input current exceeds Iin 2 , a feedback circuit  28  becomes forward biased to progressively divert some current to the left leg of the circuit, which controls the clamp  26  to progressively reduce the current Iw through the warm LEDs  14 . 
         [0033]    The resulting Iw and Ic currents in  FIG. 3  match the currents Iw and Ic 1  in  FIG. 2 . 
         [0034]      FIG. 4  is a schematic circuit diagram of the system of  FIG. 3 . The circuit of  FIG. 4  may be formed as a four-terminal packaged IC, with two of the terminals being coupled to the cathode ends of the series strings of warm and cool LEDs, a third terminal being the vdd local terminal (labeled in  FIG. 4 ), and the fourth terminal being coupled to ground. The adjustable dimming current is coupled to the anodes of the two series strings. 
         [0035]    The controllable Zener diodes U 1  and U 2  may be the TLV431 adjustable shunt regular by Diodes Inc, whose data sheet is incorporated herein by reference. The preferred adjustable shunt regulator has an 18V cathode-anode rating with a reference voltage (threshold voltage) of 1.25 V. The Zener diode symbol represents the function of the shunt regulator, even though a Zener diode is not required for the shunting. Other controllable shunt regulator circuits may be used. An input control voltage into the diode U 1  and U 2  controls the clamping voltage. Between the input currents Iin(min) and Iin 1  ( FIG. 2 ), the diode U 1  is virtually non-conducting, and the gate of the MOSFET M 1  is pulled to a high level by the pull-up resistor R 5  to turn the MOSFET M 1  on. As a result, all the input current Iin flows through the MOSFET M 1  and the warm LEDs  14 . 
         [0036]    The diode U 1 , resistors R 1 , R 5 , R 8 , and the MOSFET M 1  form a current regulator (the clamp circuit  26 ), where the gate voltage of the MOSFET M 1  determines Iw. The control terminal of the Zener diode U 1  is coupled to the top node of resistor R 1 . In the particular circuit example, when the input current Iin increases the current Iw to the point at which the voltage at the top node of resistor R 1  is at 1.25 volts, the Zener diode U 1  will conduct to clamp the gate voltage to the level required for conducting the clamped current Iw in  FIG. 2 . A reference voltage is set in the TL 431  (represented by the Zener diode U 1 ) so that a control voltage of 1.25 volts causes the Zener diode U 1  to conduct sufficiently to maintain the voltage of 1.25 at the top node of resistor R 1 . Prior to the control voltage reaching 1.25 volts, the Zener diode U 1  is off. The clamping by the Zener diode U 1  begins at Iin 1  in  FIG. 2 . Thus, between Iin 1  and In 2 , the current Iw flowing through the MOSFET M 1  will be clamped to 1.25V/R 1 . So the value of R 1  determines the location of Iin 1 . Although a particular value of 1.25 volts for the control voltage is described, any technically feasible control voltage may be used. 
         [0037]    The resistors R 6 , R 7  and a second adjustable Zener diode U 2  (another TL 431 ) behave as a comparator which monitors the gate voltage of MOSFET M 1 . Before the current Iw through resistor R 1  reaches the clamp current, the Zener diode U 1  draws minimum current. Resistor R 5  is connected to a certain fixed voltage set by a Zener diode D 1  (and filtered by capacitor C 1 ) and pulls the gate of MOSFET M 1  high, where the gate voltage is equal to (R 6 +R 7 )/(R 5 +R 6 +R 7 ) multiplied by the voltage set by the Zener diode D 1 . When the current through MOSFET M 1  reaches the clamp current of the regulator (at Iin 1 ), the Zener diode U 1  (the TL 431 ) conducts to pull the gate voltage to the required level to clamp the current through MOSFET M 1 . This lowers the voltage at the resistive divider formed of resistors R 6  and R 7 , and the divided voltage lowers the control voltage into the controllable Zener diode U 2  (a TL 431 ) to below its threshold voltage to cause the Zener diode U 2  to act as an open circuit. By doing so, resistor R 4  pulls the gate voltage of the MOSFET M 2  (the switch  24  in  FIG. 3 ) high, which turns on the MOSFET M 2  at the input current Iin 1 . As the change of gate voltage is relatively large before and after the current through resistor R 1  reaches the clamp current, this circuit is rather insensitive to the spread of the internal reference threshold voltage of the TL 431  adjustable shunt regulator. More specifically, if one tries to design a fixed turn-on threshold of MOSFET M 2  to match the internal reference voltage of the TL 431  adjustable shunt regulator, mismatch can occur due to the spread of the reference voltage. With the techniques provided herein, the M 2  turn-on threshold does not try to follow the absolute value of the internal reference voltage of the TL 431  adjustable shunt regulator and is thus insensitive to that spread. 
         [0038]    Capacitor C 2  and resistor R 10  form a compensation network for maintaining closed-loop stability. 
         [0039]    The operation at the input current Iin 2  will now be described. Resistor R 3  and Schottky diode D 2  form the feedback circuit  28  in  FIG. 3 . As soon as the source voltage of MOSFET M 2  is higher than the source voltage of MOSFET M 1  by the forward voltage of the Schottky diode D 2 , some current will be diverted through resistors R 3  and R 1 . The current through resistor R 1  now consists of currents from both the resistor R 3  and MOSFET M 1 . This is the knee point at Iin 2  in  FIG. 2  and the onset of the roll off of the current Iw in MOSFET M 1 . The added current through resistor R 1  causes the Zener diode U 1  to further reduce the gate voltage of the MOSFET M 1  to maintain the voltage at the top node of resistor R 1  to 1.25 volts. A larger resistor R 2  moves Iin 2  to the left on the x axis. The slope of the roll-off is determined by the resistor R 3 . The higher the value of the resistor R 3 , the less steep the slope. The Zener diodes U 1  and U 2  and the resistors R 6 , R 7 , R 4 , and R 2  perform functionality of the control circuit  22  (also referred to as an “LED controller”). More specifically, the control circuit  22 , controls the switch  24  (the MOSFET M 2 ) to allow or disallow current flow through the cool LEDs  12  and controls the clamp circuit  26  (the current regulator including Zener diode U 1 , resistors R 1 , R 5 , R 8 , and MOSFET M 1 ) to clamp current through the warm LEDs  14 , as specified above. Note that although the control circuit  22  and the clamp  26  are described as including certain components of the circuit shown in  FIG. 4 , in at least some respects, the boundary between control circuit  22  and clamp circuit  26  is not perfectly delineated. For example, although resistors R 6  and R 7  are described as being part of the control circuit  22  and resistor R 5  is described as being part of the clamp circuit  26 , these resistors cooperate to perform functions of both the control circuit  22  and the clamp circuit  26 . Those of skill in the art will recognize that the various elements illustrated in  FIG. 4  could be grouped in different ways to correspond to the elements of  FIG. 3 . 
         [0040]    Resistor R 9 , diode D 1 , and capacitor C 1  form a voltage buffer. It makes sure that the gate voltages of both MOSFETs are within their limit and the result of the resistive divider (R 5 , R 6 , R 7 ) is predictable. 
         [0041]    If it is not desired to completely turn off the cool LEDs  12  at an input current below Iin 1 , the MOSFET M 2  can be controlled to roll off between Iin(min) and Iin 1 , as shown by the Ic 2  line in  FIG. 2 . This can be done by connecting a resistor between the nodes vcs 2  and vs 2  as a leakage path in parallel with the MOSFET M 2 . 
         [0042]      FIG. 5  illustrates how the resulting CCT output  34  of the lamp is virtually identical to the ideal CCT of a halogen bulb while dimming between 100% and about 10% (minimum dimming). 
         [0043]    The inventive system requires no high frequency filters and can be made very compact and inexpensively. It can be used with any type of dimming circuit that adjusts the analog input current. 
         [0044]      FIG. 6A  shows the use of the dim-to-warm circuit  16  with a tapped linear LED driver  40 . Tapped linear LED drivers that operate from an AC mains voltage are well known and commercially available. The driver  40  may be a MAP9010 AC LED driver  40  by 
         [0045]    MagnaChip or other suitable driver. 
         [0046]    The driver  40  receives a rectified AC signal from a full wave diode bridge  42 . The AC signal may be a mains voltage  44 . A fuse  45  (represented by a resistor symbol) protects the circuit from overcurrents, a capacitor  46  smooths transients, and a transient suppressor  48  limits spikes. The driver  40  senses the increasing and decreasing levels of the incoming DC signal and successively applies currents to its four outputs IOUT 0 -IOUT 3 , as shown in  FIG. 6B . Only one current is output on any of the four output terminals at a time, so that, at a low DC voltage level that just exceeds the forward voltage of a first group of series LEDs, only IOUT 0  outputs a current to energize the first group of LEDs. At near the highest DC voltage level, which exceeds the forward voltage of the entire string of LEDs, only IOUT 3  outputs a current to energize the entire string. The diodes  49  ensure that all currents only flow into the driver  40 . The analog driving currents are controlled by a control signal  50 , such as from a user-controlled dimmer. 
         [0047]    The first group of LEDs on the left side is on the most since those LEDs turn on when the DC voltage rises above the forward voltage of the first group of LEDs, and the fourth group of LEDs on the right side is on the least since those LEDs are only turned on when the DC voltage is near the highest level. The currents progressively increase from IOUT 0 -IOUT 3  to reduce perceptible flicker as the number of energized LEDs constantly changes with the changing DC level. Although only one cool LED  12  and one warm LED  14  are shown in each group, there may be more LEDs in each group. 
         [0048]    As a result of the currents IOUT 0 -IOUT 3  being different at the same dimming level, the combination of the currents Ic and Iw to the cool LEDs  12  and warm LEDs  14  is adjusted for each of the dim-to-warm circuits  16 A- 16 D so that the CCT of each group of LEDs at every dimming level is matched to avoid the CCT of the lamp fluctuating each cycle. Matching the CCT at each dimming level is done by adjusting the values of the resistors R 1 , R 2 , and R 3  ( FIG. 4 ). For example, for the dim-to-warm circuit  16 A receiving the IOUT 0  current (the lowest) for a particular dimming level where the cool LEDs and warm LEDs are on at the same time, the dim-to-warm circuit  16 A applies the same ratio of currents Ic and Iw to the cool LEDs and warm LEDs as the dim-to-warm circuit  16 D receiving the IOUT 3  current (highest). One skilled in the art can easily select the values of R 1 , R 2 , and R 3  to maintain equal CCTs for each of the dim-to-warm circuits  16 A- 16 D at any of the dimming levels. 
         [0049]      FIG. 7  illustrates the functional units in the MAP9010 driver reproduced from its data sheet. The MOSFETs  60  are controlled to successively supply the desired currents at the outputs IOUT 0 -IOUT 3  as the rectified DC voltage varies during the AC cycles. An analog dimming signal is applied to the terminal RDIM to control the currents at the outputs IOUT 0 -IOUT 3 . The operation is further described in the data sheet, incorporated herein by reference. 
         [0050]    The dim-to-warm circuit  16  described above may be a simple 3-terminal IC that can be used with conventional LED drivers that provide a variable current for dimming. The dim-to-warm circuit  16  requires no high frequency filtering components (e.g., large capacitors or inductors) so it is easily mounted on a printed circuit board with the LEDs. No microprocessor is needed. 
         [0051]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.