The present disclosure relates to color mixing of LEDs and providing a consistent color correlated temperature (CCT) from initial energization of the LEDs to steady-state operation.
Color-mixing is used in LED light engines to achieve better CRI (color rendering index) or efficacy or color controllability. When no color control is implemented, a light engine is configured in such a way that the required color coordinates are met under steady-state temperature operation by a combination of a fixed number of LEDs of different colors having fixed drive currents for each LED color. When the LEDs are energized, the LEDs are initially at ambient temperature and gradually heat up over time. Therefore, the CCT/color coordinates of the LEDs are not at the desired region upon startup. For example, for green/red LED mixing, the light appears to be reddish when initially turned on. After the LEDs have warmed up and under steady-state temperature operation, the reddish light diminishes because the red light decreases more with temperature increase and the light gradually reaches the targeted CCT and color coordinates. However, the reddish light output can be perceived as less desirable by some users when the LEDs are initially energized.
It is known to implement pulse width modulation (PWM) in a light engine. For example, a variable frequency shunting switch having a duty cycle modulated by the LED operating temperature adjusts the average current applied to various colored LEDs. The amount of average current is proportional to the duty cycle of the PWM. This approach can control the color of the light engine. However, this circuit configuration can be comparatively more complicated and expensive than alternative solutions. An example of a PWM control for an LED device is shown in U.S. Published Patent Application 2006/0006821 (Singer).
It is know to implement passive control by means of a positive temperature coefficient (PTC) thermistor in a light engine to shunt a portion of the current applied to the LEDs. Thus, when connected in parallel to an LED string, a portion of the current is shunted by the PTC thermistor such that, as the temperature increases, the current to the LED string increases. However, a PTC thermistor connected in parallel to the LED string will consume power (varying from several hundreds of milliwatts to several watts, depending on the resistance of the PTC thermistor) which decreases the efficiency of the light engine.
The following are also know in the prior art: U.S. Pat. No. 7,781,983 (Yu); U.S. Pat. No. 7,712,925 (Russell); U.S. Pat. No. 7,119,500 (Young); U.S. Pat. No. 4,952,949 (Uebbing); and U.S. Pat. No. 7,262,559 (Tripathi).
In one embodiment, a light engine comprises an array of LEDs, a power supply and a control circuit. The array of LEDs comprises at least one first string of first LEDs connected in series which, when energized, output light having a first wavelength range. The array of LEDs comprises at least one second string of second LEDs connected in series which, when energized, output light having a second wavelength range different from the first wavelength range. The second string is connected in series with the first string. The power supply connects to the array and is for connection to a power source for energizing the LEDs. The control circuit is connected to the array and comprises a temperature variable resistance component and a switch selectively connecting the NTC component to the array. The control circuit controls the switch as a function of a temperature circuit indicative of the temperature of at least one of the LEDs. The control circuit limits the current applied to at least some of the LEDs during initial energization of the LEDs prior to steady-state operation of the LEDs so that variations over time of a color correlated temperature (CCT) of output light of the energized array are reduced.
In one embodiment, a light engine comprises first and second strings of LEDs, a power supply and a control circuit. The first string of first LEDs is connected in series which, when energized, output light having a first wavelength range. The second string of second LEDs is connected in series which, when energized, output light having a second wavelength range different from the first wavelength range. The second string is connected in series with the first string. The power supply connected to the first and second strings for connection to a power source energizes the strings. The control circuit comprises a temperature circuit providing a temperature signal indicative of the temperature of at least one of the LEDs. The control circuit is responsive to the temperature circuit for selectively controlling a current applied to the second string via the power supply as a function of the temperature signal. The control circuit controls the current during initial energization of the LEDs prior to steady-state operation of the LEDs. As a result, variations over time of a color-correlated temperature (CCT) of the output light of the energized LEDS are reduced.
Other objects and features will be apparent and pointed out hereinafter.
Corresponding reference characters indicate corresponding parts throughout the drawings.
The light engine 100 also includes a control circuit 108 comprising a temperature circuit 110 providing a temperature signal 112 indicative of the temperature of at least one of the LEDs 102A-D, 104A-D. Examples of the temperature circuit 110 are noted below with regard to
The control circuit 108 includes a current limiting circuit 111 responsive to the temperature circuit 110 for selectively controlling a current applied to the second string 104 by the power supply 106. The current limiting circuit 111 operates in response to (i.e., as a function of) the temperature signal 112. In particular, the circuit 111 responds to the temperature signal 112 to control the current during initial energization of the LEDs 102, 104 prior to steady-state operation of the LEDs 102, 104. As noted below, the control circuit 108 diverts some of the current that passes though the second string 104 during start-up and prior to steady-state operation. As a result, variations over time from start-up to steady state of a color correlated temperature (CCT) of output light of the energized LEDS 102, 104 is reduced.
In one embodiment, the first string 102 of first LEDs 102A-102D emit light in the first wavelength range which includes green light and the second string 104 of second LEDs 104A-104D emit light in the second wavelength range which includes red light. As a result, the combination of red and green light appears to an observer as yellow or white light. For example, the red (e.g., amber) light may have a dominant wavelength of 625 nm which is within a red range of 590 nm to 750 nm. The green (e.g., mint) light may have a dominant wavelength of 510 nm which is within a green range of 475 nm to 570 nm. Although the illustrations herein show red and green LEDs in combination, it is contemplated that other color combinations of LEDs emitting two or more different colors may be used. For example, a string may have LEDs emitting light in three or more different wavelength ranges. In addition, additional LEDs emitting light other than red or green may be simultaneously energized with the red and green LEDs as part of the same circuit or different circuits.
In operation of
During this start-up period prior to steady state operation, a current Ia flows through the first string 102 and the current limiting circuit 111 diverts at least a proportional part of the current Ia so that less than all of the current Ia flows through the second string 104. Thus, the current limiting circuit 111 diverts current Ic so that Ia−Ic=Ib flows through the second string 104 during start-up. The temperature circuit 110 provides a temperature signal 112 to the current limiting circuit 111. The temperature signal 112 corresponds to the temperature or state of operation of the strings 102, 104. The current limiting circuit 111 is responsive to the temperature signal 112. When the LEDs have reached steady state operation, as indicated by the temperature signal 112, limiting by the current limiting circuit 111 is substantially reduced or eliminated so that all or substantially all current Ia flows through the second string 104.
In one embodiment, the temperature signal 112 is indicative of the state of operation of only the first string 102. For example, assume that the first string 102 is a green string that emits green light and the second string 104 is a red string that emits red light. At start-up, the red light from the red string would appear more dominant so that the total light output of the green and red strings would have a reddish appearance to an observer. To minimize this, the current supplied to the red string is shunted by the current limiting circuit 111 to reduce the intensity of the red light. As a result, during start-up the total light output of the green and red strings would have a yellow (mixed green and red) appearance to an observer. As the green string warms up and approaches steady state, the temperature signal 112 changes. The current limiting circuit 111 responds to the change to reduce the amount of shunted current Ic. When the temperature signal 112 indicates that the green string has reached its steady state, the current limiting circuit 111 responds to substantially or completely eliminate the amount of shunted current Ic.
Referring to
The switching circuit 209 of the light engine 200 includes a MOSFET 204 in series with at least a part of the first temperature sensitive circuit 208 for selectively providing an open circuit. The switching circuit 209 also includes a comparator 210 responsive to a second temperature sensitive circuit 212 for controlling the MOSFET 204. The second temperature circuit 212 is a part of the first temperature sensitive circuit 208. The first temperature sensitive circuit 208 comprises a positive temperature coefficient (PTC) component 202 connected in series with the MOSFET 204. The second temperature sensitive circuit comprises a voltage circuit 214 including a constant voltage source VCC and second temperature variable resistance component. As shown in
In operation of
The thermistor T1 and MOSFT 204 are selected to have properties which correspond to the properties of the first string 102. Initially, the thermistor T1 has a low resistance. As the thermistor T1 diverts current, it heats up and its resistance increases to a maximum over a period of time. Similarly, as noted below, the comparator 210 causes the drain to source resistance Rds of the MOSFET 204 to increase to a maximum over the period of time. The period of time is selected to be about the same as the period of time that it takes for the second string 104 to reach steady state operation.
The NTC component 216, e.g., NTC thermistor T2, provides a temperature signal 213 to the comparator 210. The temperature signal is the voltage drop across thermistor T2 caused by the fixed voltage VCC applied to thermistor T2. Initially, the resistance of thermistor T2 is high so the voltage applied to the negative input of the comparator 210 is much less than VCC and much less that the fixed voltage applied to the positive input to the comparator 210 by voltage divider resistors R1 and R2. As a result, the initial output of the comparator 210 is high resulting in the voltage applied to the gate of the MOSFET 210 to be high. This high gate voltage causes the drain to source resistance Rds of the MOSFET 204 to be low. The initially low resistance of the thermistor T1 and the initially low Rds resistance of the MOSFET 204 limits the current applied to string 104 by shunting or conducting current Ic.
As the thermistor T2 conducts current and increases in temperature, its resistance decreases so that the voltage applied to the negative input of the comparator 210 increases and approaches VCC. This increase results in an decrease in the output voltage of the comparator 210 applied to the gate of the MOSFET 204. As the gate voltage decreases, the drain to source resistance Rds of the MOSFET 204 increases so that the MOSFET conducts less current. Simultaneously, the resistance of the thermistor T1 increases as it conducts current so that the thermistor T1 also conducts less current. Thus, as the circuit continues to operate and approach steady state, the thermistor T1 and MOSFET 204 increases the resistance to reduce the amount of current shunted from string 104.
The voltage drop VT2 across NTC thermistor T2 is equal to VCC minus the voltage drop VR3 across resistor R3 (VR3), i.e., VT2=VCC−VR3. Since VR3=VCC*R3/(RT2+R3), as the resistance RT2 across NTC thermistor T2 decreases, VR3 increases and VT2=VCC−VR3 decreases. Thus, as the circuit continues to operate and approach steady state, the resistance RT2 of NTC thermistor T2 decreases causing the voltage drop across T2 to decrease. Thus, the voltage applied to the negative input of the comparator 210 becomes higher than the fixed voltage applied to the positive input of the comparator 210. This causes the comparator output to be reduced causing Rds to increase. As the circuit continues to operate and approach steady state, the resistance of PTC thermistor T1 increases. As the circuit continues to operate and approach steady state, the increased resistance of PTC thermistor T1 and the increased resistance of the Rds of the MOSFET 204 discontinues any current limiting or shunting so that full current Ia is applied to the string 104. Thus, any losses due to the thermistor T1 are essentially eliminated.
The period of time it takes for thermistor T1 to reach its maximum resistance, for Rds to reach its maximum resistance and for the thermistor T2 to reach its minimum resistance is selected to be about the same as the period of time that it takes for the second string 104 to reach steady state.
The temperature signal 213 corresponds to the temperature or state of operation of string 102. The comparator 210 is responsive to the temperature signal 213.
Essentially, the comparator 210 compares the voltage drops across thermistor T2 and resistor R1. As the thermistor T2 resistance decreases, the voltage drop across T2 decreases. This will result in an increase in the output of the comparator 204 and of the drain to source resistance of the MOSFET, forcing more current to go through the second string 104. At a certain point in time, thermistor T1 reaches its maximum and the MOSFET will be fully off (an open circuit), so that substantially all the current Ia will go through the second string 104. This point in time is selected to correspond to about the time when the first string 102 reaches steady state.
Alternatively, if a different switch such as a transistor switch is used instead of the MOSFET 204, the thermistor T1 may be selected to have properties which correspond to the properties of the first string 102. Alternatively, thermistor T2 may be replaced by a PTC thermistor. In this embodiment, the PTC thermistor is connected to the positive input of the comparator 210 and the resistance budge R1, R2 is connected to the negative V input.
As a specific embodiment, consider the first string 102 to be green LEDs and the second string 104 to be red LEDs. Thermistors T1 and T2 and MOSFET 204 are selected so that the shunted current Ic varies with temperature in such a way that the light emitted from the first green string 102 and the second red string 104 are balanced to maintain a consistent CCT/color coordinates over the operating temperature. Both the green LEDs and the red LEDs become relatively less bright with increasing temperature. However, the green LED output decreases at a slower rate less than the red LED output, resulting in an increase of the percentage of green light in the total light output of the circuit. As a result, as the circuit continues to warm up and reach steady state, the percentage of green light in the total light output increases. Simultaneously, less current is shunted from string 104 so that the red LEDs also become relatively brighter. This maintains a balance in the light output between the green and red LEDs to maintain consistent CCT/color coordinates as the circuit warms up. The second temperature sensitive circuit 212 including thermistor T2 and associated components are selected such that when the light engine temperature reaches a threshold value (the steady-state operating temperature), the comparator 210 changes state, resulting in the MOSFET 204 turning off and the shunting current Ic going to zero.
The temperature sensitive circuit 308 of the light engine 300 comprises the PTC thermistor 302 connected between the first and second strings and a voltage circuit, such as a resistive array 312. The switching circuit 309 includes a MOSFET 304 in series with the PTC thermistor 302 for selectively providing an open circuit. The switching circuit 309 also includes a comparator 310 responsive to the voltage circuit 312 for controlling the MOSFET 304. The resistive array 312 is connected to inputs of the comparator 310. Thus, in this light engine 300 the control circuit 108 comprises the PTC thermistor 302 and the MOSFET 304 in series with the PTC thermistor 302 responsive to the comparator 310 controlling the switch. The PTC thermistor 302 and the MOSFET 304 are in parallel with the second string 104.
In operation,
The thermistor T1 is selected to have properties which correspond to the properties of the first string 102. In particular, as the thermistor T1 diverts current, it heats up and its resistance increases to a maximum rate over a period of time. The period of time is selected to be about the same as the period of time that it takes for the second string 104 to reach steady state.
The PTC component 302, e.g., thermistor T1, and resistor R4 provide a temperature signal 313 to the comparator 310. The temperature signal 313 corresponds to the temperature or state of operation of string 102. The comparator 310 is responsive to the temperature signal 313.
As thermistor T1 heats up and increases in resistance, the voltage drop across thermistor T1 increases so less voltage is applied to the positive input of the comparator 310 via divider resistors R5 and R6. When the applied voltage is less than the fixed voltage applied to the negative input of comparator 310 from resistor R4 and diode 314, the comparator output goes low to open MOSFET 304 and increase Rds to a maximum. The time when the applied voltage is greater than the fixed voltage corresponds to the time when the LEDs of string 102 have reached steady state operation. Thus, shunting by the thermistor T1 and MOSFET 304 is eliminated by the high resistance of thermistor T1 and by the high Rds of MOSFET 304 which essentially open-circuits any shunting or limiting. Any losses due to thermistor T1 are essentially eliminated
In summary, referring to
In some configurations of
In one embodiment, the comparators 210, 310 may be an operational amplifier, such as a general purpose op amp with an input voltage rating of ±15. A linear amplifier, UA741, made by TI may be used as the comparator.
In one embodiment, the NTC component comprises an NTC thermistor 410 and the switch comprises a MOSFET 416 connected in parallel to the NTC component 410. The MOSFET 416 is controlled by a temperature circuit. Circuits similar to the temperature sensitive circuits 208, 308 and comparators 210, 310, shown in
The light engine 400 has at least a third string 412 of third LEDs 412A-C connected in series which, when energized, output light have the first wavelength range and a fourth string 414 of fourth LEDs 414A-B connected in series which, when energized, output light have the second wavelength range. The fourth string 414 is connected in series with the third string 412 and the third and fourth strings connected in parallel to the first 402 and second strings 404. Additional strings such as strings 422, 424 may be connected in parallel with the other strings.
The control circuit 406 comprises the NTC component 410 connected in series with the first string 402 and in series with second string 404 for selectively reducing the current applied to the first and second strings. The first string 402 has fewer LEDs than the third string 412 and the second string 404 has more LEDs than the fourth string 414 so that, as illustrated in
In operation of
As a specific example regarding
In the first string, the mint LEDs 402 are the subordinate contributors and the red LEDs 404 are the primary contributors. In the other strings, the mint LEDs, 412, 422-1, . . . , 422-N are the primary contributors and the red LEDs, 414, 424-1, . . . , 424-N are the subordinate contributors. An NTC thermistor 410 is connected in series with strings 402 and 404. Without any compensation, as the red and green LEDs of strings 402, 404, 412, 414, 422, 424 warm up, the red light output from the red LEDS decreases at a greater rate than the decrease in green light output from green LEDs, so that it will appear that the CCT of total output light is shifting from red to green.
In contrast, according to the embodiment of
In contrast, solid lines 504, 604 show the temperature shifts of an LED string, such as string 102,104 or strings 402-424 with limiting as noted above in
As shown in
It is contemplated that there could be other configurations that do not use thermistors and instead use other electronic devices with temperature dependent variables to realize the temperature dependent limiting functions noted above.
The order of execution of the operations in embodiments described herein is not essential, unless otherwise specified. Operations may be performed in any order, unless otherwise specified, and embodiments may include additional or fewer operations than those disclosed. For example, it is contemplated that performing a particular operation before, contemporaneously with, or after another operation is within the scope of aspects of the claims.
When introducing elements of aspects or the embodiments thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
Not all the components described may be required. Some embodiments may include additional components. Variations in the arrangement of the components may be made without departing from the scope of the claims. Additional, different or fewer components may be provided, and components may be combined or implemented by several components.
The above description illustrates by way of example and not by way of limitation. This description enables one skilled in the art to make and use the disclosure, and describes several embodiments and variations, including what is presently believed to be the best mode of carrying out the disclosure. The disclosure is not limited in its application to the details of construction and the arrangement of components in the description or illustrated in the drawings. The disclosure is capable of other embodiments and of being practiced differently. The terminology used herein should not be regarded as limiting. Having described aspects in detail, it is apparent that modifications are possible without departing from the scope of aspects as defined in the claims. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
The following is a representative, non-limiting list of the reference numerals noted above.
Number | Name | Date | Kind |
---|---|---|---|
4952949 | Uebbing | Aug 1990 | A |
5627472 | Ofer et al. | May 1997 | A |
7119500 | Young | Oct 2006 | B2 |
7262559 | Tripathi et al. | Aug 2007 | B2 |
7712925 | Russell | May 2010 | B2 |
7781983 | Yu et al. | Aug 2010 | B1 |
7863829 | Sayers et al. | Jan 2011 | B2 |
20060006821 | Singer et al. | Jan 2006 | A1 |
20110068701 | van de Ven et al. | Mar 2011 | A1 |
20120032612 | Antony et al. | Feb 2012 | A1 |
20120248995 | Bakk et al. | Oct 2012 | A1 |
Number | Date | Country |
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2011054547 | May 2011 | WO |
2011067177 | Jun 2011 | WO |
2011098334 | Aug 2011 | WO |
2011101282 | Aug 2011 | WO |
Number | Date | Country | |
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20130271018 A1 | Oct 2013 | US |