Linear bypass electrical circuit for driving LED strings

This application claims the benefit of U.S. Provisional Application No. 61/577,842, entitled “LINEAR BYPASS ELECTRICAL CIRCUIT FOR DRIVING LED STRINGS”, filed Dec. 20, 2011, and is hereby incorporated by reference in its entirety.

This application is related to and incorporates by reference in its entirety U.S. patent application Ser. No. 12/782,038, entitled “LAMP COLOR MATCHING AND CONTROL SYSTEMS AND METHODS”, filed May 18, 2010.

A light-emitting diode (LED) emits incoherent narrow-spectrum light when the diode is electrically biased in the forward direction of the p-n junction inside of the diode. LEDs typically have higher luminous efficacy, i.e. lumens per watt, than conventional light sources such as incandescent bulbs. LEDs are often used in battery powered or energy saving devices, and are becoming increasingly popular in higher power applications such as, for example, flashlights, area lighting, and regular household light sources.

Due to the emitting spectrum nature of the LEDs, the quality of the light is a primary consideration with the use of LEDs in higher-power applications. It is desirable to have high brightness white LED device that has better light quality, which is quantitatively measured by the color rendering index (CRI). The CRI is a measure of how true the light is as compared to an ideal or natural light source in representing the entire light spectrum. An ideal or natural light source has a high CRI of, for example, 100. Individual white LED typically has a poor CRI, in the approximate range of 70-80, because of their emitting spectral concentration. To partially remedy the problem, phosphors are utilized to convert the wavelength of the light emitted from the diode to other wavelength regions. Furthermore, LEDs with different emitting colors mixed to produce a white light better filling out the light spectrum. For example, combinations of white, amber, red, and green LEDs can provide light with CRIs at or above 90.

Combinations of LEDs having different emitting light colors may include multiple strings of LEDs having the same emitting light color. There are conventional approaches for modulating the light output from each string of LEDs having the same emitting light color. One approach is to provide a constant current source and turn the string of LEDs on and off over a particular duty cycle to change the perceived light intensity of that string. This is achieved by using switch-mode transistors switching on and off at a high frequency. The approaches are used not only to change the relative intensity of LEDs with different colors but also to raise and lower the overall intensity of the string in a manner similar to a dimming function. Although the approach provides the color control, it has significant efficiency penalties.

This approach uses a current source for each LED string and modulates the duty cycle of the LED string at a frequency imperceptible to the human eye. But, running the LEDs at their full current rating and duty cycling their outputs is typically less efficient than simply running the LEDs continuously at a lower current, because LED efficiency declines with increasing current.

Furthermore, the switching circuit introduces electromagnetic interference (EMI), whose disadvantage cannot be understated. To filter and screen the EMI, more components need to be taken into account, driving up the parts cost.

Moreover, the incandescence emission from a conventional incandescent light bulb is a black body radiation. Its emission spectrum conforms to the Planckian locus on the CIE color space. When dimming an incandescent light bulb, customers are used to the color temperature changing according to the black body radiation. Therefore, it is desirable to mimic the color temperature change conforming to the Planckian locus when dimming a light source. However, an LED barely shifts its emission spectrum when the pass-through current reduces. Therefore, when dimming a LED-based lighting fixture, a user does not observe the color shift as he is used to.

The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.

Introduced herein is an electrical circuit for driving LED strings. The electrical circuit comprises a first and a second LED strings, a first and a second transistors, at least one bypass transistor, a power supply and a controller. The first LED string includes at least one LED. The first transistor is coupled to the first LED string at a first terminal of the first transistor. The first transistor is operating in a linear mode. The second LED string includes a plurality of color strings. Each of the color strings includes at least one LED. The color strings are coupled in series. The first LED string and the second LED string are coupled in parallel. The second transistor is coupled to the second LED string at a first terminal of the second transistor. The second transistor is operating in a linear mode. The bypass transistor is coupled to one of the color strings. The bypass transistor is operating in a linear mode. A first terminal of the bypass transistor is coupled to a first terminal of the color string. The second terminal of the bypass transistor is coupled to a second terminal of the color string. The power supply is configured to provide power to the first and second LED strings. The controller is coupled to the first transistor, the second transistor and the bypass transistor at gate terminals of the transistors to control gate voltages of the transistors so that the first transistor, the second transistor and the bypass transistor are operating in the linear modes.

The techniques introduced here minimize the electromagnetic interference (EMI), eliminate the need for inductors in the circuit, can operate from a standard TRIAC dimmable constant current power supply, optimize the power efficiency, and have built-in ability to adjust the intensity and the emitting light color characteristics, such as color temperature and color rendering index.

By controlling the current ratio between white and color LED strings, and bypassing some of the current from one or more color strings, the color temperature of the dimming output light can be fine-tuned to conform to the Planckian locus, mimicking the behavior of a conventional incandescent light bulb under dimming.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, not is it intended to be used to limit the scope of the claimed subject matter.

One or more embodiments of the present invention are illustrated by way of example and are not limited by the figures of the accompanying drawings, in which like references indicate similar elements.

FIG. 1 depicts a block diagram of an electrical circuit for driving LED strings according to one embodiment.

FIG. 2 depicts an electrical circuit for driving LED strings according to one embodiment.

FIG. 3 is an example of wave forms of electric currents passing through LED strings according to one embodiment.

FIG. 4 depicts an electrical circuit for controlling LED strings currents according to one embodiment.

FIG. 5 depicts an electrical circuit for bypassing LED strings currents according to one embodiment.

FIG. 6A depicts a flow diagram illustrating an example process of adjusting the bypass transistors in the electrical circuit.

FIG. 6B depicts a flow diagram illustrating an example process of adjusting the string linear mode transistors in the electrical circuit.

FIG. 7 depicts a flow diagram illustrating an example process of adjusting the power supply for the electrical circuit.

Various aspects of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these examples. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description. Although the diagrams depict components as functionally separate, such depiction is merely for illustrative purposes. It will be apparent to those skilled in the art that the components portrayed in this figure may be arbitrarily combined or divided into separate components.

The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific examples of the invention. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section.

References in this specification to “an embodiment,” “one embodiment,” or the like mean that the particular feature, structure, or characteristic being described is included in at least one embodiment of the present invention. Occurrences of such phrases in this specification do not necessarily all refer to the same embodiment.

FIG. 1 depicts a block diagram of electrical circuit 100 for driving LED strings according to one embodiment of the disclosure. The electrical circuit 100 includes an LED string 120 . The LED string 120 includes one or more color strings, e.g., 122 , 124 and 126 (collectively “color strings 122 through 126 ”). Each of the color string contains one or more LEDs. A linear mode transistor 128 is coupled directly to color string 126 in series. The electrical circuit 100 further includes an LED string 130 and a linear mode transistor 138 coupled in series. The LED string 130 contains one or more LEDs. As illustrated in FIG. 1 , the series of 122 - 124 - 126 - 128 and the series of 130 - 138 are coupled in parallel. A power supply 101 is coupled to both LED string 130 and color string 122 to supply power to the LED string 130 and color strings 122 , 124 and 126 . As shown in FIG. 1 , for each of the color strings 122 , 124 , 126 , a bypass linear mode transistor ( 112 , 114 , 116 ) is coupled to the corresponding color string ( 122 , 124 , 126 ), respectively. As shown in FIG. 1 , first and second terminals of each bypass linear mode transistor ( 112 , 114 , 116 ) are respectively coupled to the first and second terminals of each corresponding color string ( 122 , 124 , 126 ). A bypass linear mode transistor 118 is coupled to LED string 130 in parallel. The first and second terminals of bypass linear mode transistor 118 are respectively coupled to the first and second terminals of LED string 130 . A bypass linear mode transistor is a linear mode transistor used to bypass a portion or all of the current from a load. Other embodiments may have fewer or more color strings and/or bypass linear mode transistors. A controller 102 is coupled to and is configured to control the linear mode transistors 128 , 138 and bypass linear mode transistors 112 , 114 , 116 , 118 .

A linear mode transistor is an electrical regulator based on a transistor, such as a field effect transistor (FET), bipolar junction transistor (BJT), metal oxide semiconductor field effect transistor (MOSFET), or insulated gate bipolar transistor (IGBT), operating in its linear dissipative mode (also sometimes referred to as linear mode, linear region, triode mode, or ohmic mode). For example, the linear mode transistor may be a metal oxide semiconductor field effect transistor (MOSFET) operating in linear mode, i.e. the gate-source voltage is higher than the threshold voltage, and the drain-source voltage is higher than the difference between the gate-source voltage and the threshold voltage. In the linear mode, the transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The relationship of the drain-source voltage and the drain current are relatively close to a linear form. A bypass linear mode transistor is a linear mode transistor used to bypass a portion or all of the current from a load. By controlling the gate voltage of the bypass linear mode transistor, the circuit can control how much current it wants to bypass from the load. A linear mode transistor is different than a switching transistor (also referred to as switch, or switcher) which is based on a transistor forced to act as an on/off switch (also referred to as saturation mode, or active mode).

There are two major types of transistors, bipolar transistors and field effect transistors. The terminals are labeled differently between these two types. A bipolar transistor has terminals labeled as base, collector, and emitter. A field effect transistor has terminals labeled as gate, source, and drain. For purposes of explanation, a transistor is commonly described herein as a field effect transistor; however, a transistor can also be any transistor including bipolar transistor. Similarly for purposes of explanation, a controlling terminal (also referred to as third terminal) of a transistor is commonly described herein as a gate terminal of a field effect transistor (a small current from base to emitter can control current between the collector and emitter terminals); however, a controlling terminal can also be a base terminal of a bipolar transistor (a voltage at the gate can control a current between source and drain). In this disclosure, a first or second terminal of the transistor means a source or drain terminal in case of a field effect transistor, or a collector or emitter terminal of a bipolar transistor.

In one embodiment, the power supply 101 is a DC power supply having a voltage output range and a current output range. During operation, power supply 101 provides power to LED string 130 and LED string 120 , which includes color strings 122 , 124 , 126 . In one embodiment, controller 102 is configured with an algorithm to control the linear mode transistors 112 , 114 , 116 , 118 , 128 and 138 . In one embodiment, the algorithm for controlling the linear mode transistors 112 , 114 , 116 , 118 , 128 , 138 can be based upon a brute force algorithm described in U.S. patent application Ser. No. 12/782,038, entitled “LAMP COLOR MATCHING AND CONTROL SYSTEMS AND METHODS”, filed May 18, 2010, where the luminous flux output of each color string is adjusted incrementally but still retaining a constant total output luminous flux from all of the LED strings. Similarly, the currents sent to each color string by the controller 102 can be adjusted incrementally while still maintaining a constant total drive current supplied to all of the LED strings. Alternatively or additionally, an algorithm can be used by the controller 102 to control linear mode transistors 112 , 114 , 116 , 118 , 128 , 138 to calibrate the LED strings' outputs. In one embodiment, the algorithm can also be based upon input received from a temperature monitor that measures the temperature near one or more of the LED strings.

In another embodiment, controller 102 may be a digital control circuit, an analog control circuit, or any circuit suitable for controlling the linear mode transistors. During operation, controller 102 executes the software to provide control signals to the linear mode transistors, which are thus controlled to route power provided by power supply 101 among the LEDs, as described further below.

Each of color strings 122 , 124 , 126 has at least one LED. In some embodiments, one or more of color strings 122 , 124 , 126 may have a plurality of LEDs. Thus, color strings 122 , 124 , 126 may have a different number of LEDs. In various embodiments, the LEDs of each of color strings 122 , 124 , 126 may be placed in various physical orientations. Additionally, in some embodiments the LEDs of color strings 122 , 124 , 126 may share a similar physical orientation. Furthermore, the LEDs of each of color strings 122 , 124 , 126 may be configured to emit light in various colors.

In some embodiments, the LEDs of each of color strings 122 , 124 , 126 are electrically coupled in series. In such embodiments, given that color strings 122 , 124 , 126 themselves are coupled in series as shown in FIG. 1 and described above, the LEDs of all of color strings 122 through 126 are coupled in series. Thus, in such embodiments, the LEDs of all of color strings 122 , 124 , 126 present a high string voltage drop that is the sum of their individual voltage drops. Power supply 101 , providing power to color strings 122 , 124 , 126 as described above, may thus provide a similarly high supply voltage. In doing so, power supply 101 therefore exhibits higher efficiency. In some embodiments, some of the LEDs of one or more color strings are electrically coupled in parallel.

LED string 130 has at least one LED. In some embodiments, LED string 130 may have a plurality of LEDs. In some embodiments, the LEDs of LED string 130 may be configured to emit white light. In some embodiments, the LEDs of LED string 130 are electrically coupled in series. Thus, in such embodiments, the LEDs of LED string 130 present a string voltage drop that is the sum of their individual voltage drops. In some embodiments, the number of the LEDs of LED string 130 are arranged so that the total voltage drop of LED string 130 is substantially similar to the total voltage drop of LED string 120 . Thus, two LED strings 120 and 130 , coupled in parallel, have matched voltage drops to achieve better energy efficiency.

FIG. 2 depicts electrical circuit 200 for driving LED strings according to one embodiment of the disclosure. The electrical circuit 200 includes LED strings 220 and 230 . LED strings 220 and 230 are coupled in parallel. The LED string 220 includes multiple color strings 222 , 224 and 226 coupled in series (collectively “color strings 222 through 226 ”). Each of the color string contains one or more LEDs. Color string 222 includes LEDs D 1 , D 2 , D 3 and D 4 . LEDs D 1 and D 2 are coupled in series. LEDs D 3 and D 4 are coupled in series. The two series are coupled in parallel. Color string 224 includes LEDs D 5 , D 6 , D 7 , D 8 and D 9 coupled in series. Color string 226 includes LEDs D 10 and D 11 coupled in series. The color strings may contain LEDs emitting light having different colors. In one embodiment, LEDs D 1 , D 2 , D 3 and D 4 are red-emitting LEDs; LEDs D 5 , D 6 , D 7 , D 8 and D 9 are amber-emitting LEDs; while LEDs D 10 and D 11 are blue-emitting LEDs. LED string 230 includes LEDs D 12 , D 13 , D 14 , D 15 , D 16 , D 17 , D 18 , D 19 and D 20 . In one embodiment, all LEDs in LED string 230 are white-emitting LEDs. In some embodiments, some of the LEDs may emit light having other colors, such as cyan.

A linear mode transistor Q 1 is coupled directly to color string 226 in series. Another linear mode transistor Q 2 is coupled directly to LED string 230 in series. A power supply 201 is coupled to both LED strings 220 and 230 to supply power to the LEDs. In one embodiment, the power supply 201 has a current output from 100 mA to 2 A, with a maximum voltage of 40 volts. In some embodiments, the power supply 201 is a constant current power supply. In some other embodiments, the power supply 201 is a TRIAC dimmable constant current power supply. As shown in FIG. 2 , for each of the color strings 222 , 224 , 226 , a bypass linear mode transistor (Q 3 , Q 4 , Q 5 ) is coupled to the corresponding color string ( 222 , 224 , 226 ), respectively. The first and second terminals of each bypass linear mode transistor (Q 3 , Q 4 , Q 5 ) are respectively coupled to the first and second terminals of corresponding color string ( 222 , 224 , 226 ). A bypass linear mode transistor Q 6 is coupled to LED string 230 . The first and second terminals of bypass linear mode transistor Q 6 are respectively coupled to the first and second terminals of LED string 230 . A bypass linear mode transistor is a linear mode transistor used to bypass a portion or all of the current from a load. A controller 202 is coupled to and is configured to control the linear mode transistors Q 1 , Q 2 and bypass linear mode transistors Q 3 , Q 4 , Q 5 , Q 6 . The controller 202 may control the gate voltages of the linear mode transistors so that the linear mode transistors are operating in their linear modes. In one embodiment, controller 202 also measure the voltage drops across the linear mode transistor Q 1 and Q 2 . As shown in FIG. 2 , the controller 202 may also measure the voltages across the resistors R 1 and R 2 , respectively. The voltages across resistors R 1 , R 2 are proportional to the currents pass through LED strings 220 and 230 , respectively. In one embodiment, the linear mode transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 and Q 6 may be linear-mode transistors, such as a field effect transistor or bipolar junction transistor, in linear mode.

In one embodiment, the power supply 201 is a DC power supply having a voltage output range and a current output range. During operation, power supply 201 provides power to LED string 230 and LED string 220 , which includes color strings 222 , 224 , 226 . In one embodiment, controller 202 is configured with an algorithm to control the linear mode transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , such as the algorithms described above. In another embodiment, controller 202 may be a digital control circuit, an analog control circuit, or any circuit suitable for controlling the linear mode transistors. During operation, controller 202 executes the software to provide control signals to the linear mode transistors, which are thus controlled to route power provided by power supply 201 among the LEDs, as described further below.

The controller 202 may control the color temperature of output light by adjusting one or more bypass linear mode transistors. In one embodiment, the linear mode transistors Q 3 , Q 4 , Q 5 and Q 6 may be field effect transistors operating in the linear dissipative mode. The field effect transistors Q 3 -Q 6 provide for dimming of the LED with various emitting colors to achieve the desired light color characteristics. For example, by adjusting Q 3 to bypass more current from the red-emitting color string 220 , the color temperature increases to make the light color appear cooler. Or by adjusting Q 5 to bypass more current from the blue-emitting color string 226 , the color temperature decreases to make the light color appear warmer. Or by independently adjusting Q 3 , Q 4 , Q 5 and Q 6 to regulate the spectrum contribution from LEDs having different emitting colors, the color rendering index (CRI) can be tuned.

The numbers and the arrangement of LEDs having various emitting colors are designed to emit an output light with a mid-range color temperature, when all bypass linear mode transistors Q 3 -Q 6 are completely off. In one embodiment, the mid-range color temperature may be about 5000 Kelvin. In another embodiment, the mid-range color temperature may be about 4000 Kelvin. In yet another embodiment, the mid-range color temperature may be from about 3500 Kelvin to about 5500 Kelvin. The efficiency loss due to the dissipation on linear mode transistors is minimum in this case. When an output light of different color temperature is needed, one or more bypass linear mode transistor starts to turn partially on to dim corresponding LEDs.

FIG. 6A depicts a flow diagram illustrating an example process of adjusting the bypass transistors in the electrical circuit. At block 605 , the system receives a command for setting the color temperature of the light emitted by the LED strings in the electrical circuit. The command includes a specific color temperature for the emitted light or a range of color temperatures.

Then at block 610 , the controller references a pre-selected algorithm for determining the amount of current to send to each LED string for the specified color temperature (range). Next, at block 615 the controller adjusts the bypass transistors to adjust the current levels for each LED string.

Series of D 1 , D 2 and series of D 3 , D 4 are coupled in parallel, so that the total string voltage drop in LED string 220 is about 9 times of a single LED voltage drop. Since LED string 230 contains 9 LEDs in series, the total string voltage drop in LED string 230 is the sum of LEDs' individual voltage drops. So the total voltage drop in LED string 230 is also about 9 times of a single LED voltage drop. Thus, in such embodiments, the total string voltage drop of LED string 230 is substantially similar to the total string voltage drop of LED string 220 . Thus, two LED strings 220 and 230 , coupled in parallel, have relatively matched voltage drops to achieve better energy efficiency.

The voltage drop of individual LED varies among LEDs. Therefore, the total string voltage drop of LED string 220 may not be exactly the same as the voltage drop of LED string 230 . If there is no controlling mechanism, the difference between the parallel loads will cause most of the current to pass through one of the LED strings having higher voltage drop. This will cause poor light output performance and even LED failure due to the excessive current. In one embodiment, the controller 202 controls the operations of Q 1 and Q 2 such that at least one of Q 1 and Q 2 is fully on for most of the time. This mode of operation leads to minimum power loss while providing current apportioning between the LED strings.

FIG. 3 shows the wave forms of electric currents passing through LED strings 220 and 230 . It can be measured by the voltages across resistor R 1 and R 2 as shown in FIG. 2 . In one embodiment, the controller 202 controls the operations of Q 1 and Q 2 such that Q 1 stays fully on for a period while Q 2 stays off. Most of the current passes through the LED string 220 and R 1 at that period (marked as String 1 in FIG. 3 ). Then the controller 202 starts to turn Q 2 gradually on and Q 1 gradually off. The current starts to shift to the path through the LED string 230 and R 2 until Q 2 turns fully on and most of current passes though the LED string 230 and R 2 (marked as String 2 in FIG. 3 ). The waveforms shown in FIG. 3 are shown as a function of color temperature. Thus, as more or less contribution is needed from the white LEDs relative to the contribution of the color LEDs, the controller 202 controls the waveforms of the electric currents driving the LED strings 220 and 230 .

In one embodiment, a dimmer 203 is coupled to controller 202 as a user control. The dimmer 203 may be controlled by a user to instruct the controller 202 to adjust the light intensity or light color characteristics. In another embodiment, two user controls are provided so that user can control the light intensity and color temperature separately.

The incandescence emission from a conventional incandescent light bulb is a black body radiation. Its emission spectrum conforms to the Planckian locus on the CIE color space. When dimming an incandescent light bulb, customers are used to the color temperature change according to the black body radiation. Therefore, it is desirable to mimic the color temperature change conforming to the Planckian locus when dimming a LED based light source. By controlling the current ratio between white and color LED strings, and bypassing some of the current from one or more color strings, the color temperature of the dimming output light can be fine-tuned to conform to the Planckian locus, mimicking the behavior of a conventional incandescent light bulb under dimming.

The electric circuit 200 does not need any inductors in the circuit. Since there is no switching components or circuitry involved, electromagnetic interference (EMI) is minimized. Noise is also minimized since it is a DC circuitry. The electric circuit 200 can operate from a TRIAC dimmable constant current power supply. The circuit has optimized power efficiency and built-in ability to adjust the intensity and the emitting light color characteristics, such as color temperature and color rendering index. When the linear mode transistors are being adjusted, the circuit does not put heavier-than-normal load onto the power supply.

In some embodiments, the number of parallel LED strings may be more than two. For example, the number of LED strings may be, but not limited to, three, four, or five.

FIG. 4 depicts electrical circuit 400 for controlling LED strings currents according to one embodiment of the disclosure. Linear mode FETs 428 and 438 are the linear mode transistors controlling the current balance between LED strings, similar to Q 1 and Q 2 shown in FIG. 2 . In one embodiment, controller 402 is a processor sending out control signals for the FETs 428 and 438 . The control signals are amplified by operational amplifiers (op-amps) 427 and 437 . The amplified control signals are fed to control the FETs 428 and 429 operating in their linear modes. Resistors 426 and 436 are coupled between the FETs and ground to measure the current in each LED string. Op-amps 429 and 439 amplify the current measurement signals. The amplified signals are converted by analog-to-digital (ND) converters 453 , 454 and then fed back to the processor 402 . By measuring the voltages across resistors 426 and 436 , processor knows how much current is running down each LED string. Based on the feedback information, processor 402 adjusts the current ratio between LED strings by sending out the control signals. The control signals are converted by digital-to-analog (D/A) converters 451 , 452 , amplified by op-amps 427 , 437 , and then fed to FETs 428 , 429 controlling the FETs 428 and 429 operating in their linear modes. In one embodiment, FETs are operating according to processor 402 , similar in way shown in FIG. 3 . In some embodiments, D/A converters 451 , 452 share the same component or circuit. In some embodiments, ND converters 453 , 454 share the same component or circuit. In some embodiments, D/A converters 451 , 452 and ND converters 453 , 454 share the same component or circuit.

FIG. 6B depicts a flow diagram illustrating an example process of adjusting the string linear mode transistors in the electrical circuit. At block 650 , the controller measures the voltage across each resistor R 1 , R 2 that is in series with the LED strings. Based on the measured voltages, at block 655 the controller adjusts the current balance between the LED strings so that the emitted light remains near the Planckian locus.

FIG. 5 depicts electrical circuit 500 for bypassing LED string currents according to one embodiment of the disclosure. The LED string contains LEDs 521 , 522 , 523 , 524 , 525 , 526 , 527 , 528 , 529 and 530 . In one embodiment, red-emitting LEDs 521 and 522 forms a color string 520 . In case of adjusting the color characteristics of the output light, current through color string 520 may be adjusted to adjust the red emission contribution to the output spectrum. Processor 502 's control signal is amplified by op-amp 517 and fed to FET 512 . FET 512 is operating in linear mode to control how much current is bypassed from color string 520 . LEDs 521 , 522 are dimmed in a way according to the portion of current has been bypassed. Diodes 551 , 552 , 553 and 554 are introduced to further dissipate the heat generated by the bypass current.

In one embodiment, the LEDs in the LED string are divided into color strings that each color string contains LEDs emitting the same or similar color. Each color string may be complemented a bypass circuit, similar to the one shown in FIG. 5 .

In one embodiment, a temperature monitor 560 is coupled to processor 502 . Through temperature monitor 560 , processor 502 measures the temperature in proximity of heat dissipation location. Then processor 502 feeds out a signal back to the power supply (not shown in FIG. 5 ), and requests the power supply to reduce current. Thus, the power supply reduces the current it supplies, and the heat dissipation can be reduced accordingly to avoid overheating situations.

FIG. 7 depicts a flow diagram illustrating an example process of adjusting the power supply for the electrical circuit. At block 705 , the system measures the temperature near the strings of LEDs using one or more temperature sensors. Then at decision block 710 , the controller determines if the temperature is too hot. The temperature is deemed too hot if it rises above a pre-determined threshold value. If the temperature is not too hot (block 710 —No), the system returns to block 705 to monitor the temperature measured by the sensor(s).

If the temperature is too hot (block 710 —Yes), at block 715 , the controller sends a signal to the power supply to reduce the output current being supplied to the LED strings. Then at block 720 , the power supply reduces its output current. The process returns to block 705 to monitor the temperature measured by the sensor(s).

The foregoing description of various embodiments of the claimed subject matter has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art. Embodiments were chosen and described in order to best describe the principles of the invention and its practical application, thereby enabling others skilled in the relevant art to understand the claimed subject matter, the various embodiments and with various modifications that are suited to the particular use contemplated.

The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments.

While the above description describes certain embodiments of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its implementation details, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed embodiments, but also all equivalent ways of practicing or implementing the invention under the claims.