Current split circuit for equally splitting current between parallel connected LED luminaire strings

A system and method for equally splitting the current supplied to parallel connected strings of LEDs. The system and method includes a current splitting circuit such as a mirror circuit that divides the current substantially equally between two or more parallel connected strings of LEDs. The current splitting circuit ensures that illumination levels of the strings of LEDs are uniform without requiring the strings of LEDs to be binned. The current splitting circuit also allows the strings of LEDs to be dimmed in both pulse width modulation (PWM) and continuous modes.

I. FIELD OF THE INVENTION

The present invention relates generally to lighting circuits for LED lamps. More specifically, the present invention relates to current split circuits for light emitting diodes (LEDs) used in LED luminaires.

II. BACKGROUND OF THE INVENTION

LEDs are widely used in the lighting field. In many lighting applications, it is desirable to utilize many LEDs having the same illumination level or brightness. However, achieving the same, or substantially the same, illumination level for many LEDs can be difficult and may require substantial cost and effort.

Due to practical considerations, e.g., achieving a similar illumination level, LEDs are often connected in series, i.e., in strings having multiple LEDs. Connecting the LEDs of a lighting system in series also allows the lighting system to achieve a supply voltage that can be delivered by a driver power supply unit (PSU) of the lighting system. Even so, in order to avoid exceeding the highest voltage delivered by the PSU, the serially-connected strings of LEDs are often connected in parallel. However, this configuration also has deficiencies.

Notably, the electrical and thermal parameters of LEDs (and LED strings) are not exactly (100%) identical. These differences are created by various factors including, e.g., manufacturing intolerances, inconsistencies in materials, and the like. LEDs are produced by coating a wafer or substrate with various materials through a chemical process, such as epitaxial growth, doping, and the like, to produce a semiconductor material. The semiconductor material is then sliced to create a small die. Wire bonds or other electrical connectors are then added, e.g., by coating or suspension. The assembly is then encapsulated to create a finished LED package.

Inconsistencies in the coating processes and/or dopant materials create significant inherent variations that impact the characteristics of the LEDs including, voltage, lumens, color (temperature) of the LEDs, and the like. Therefore, placing strings of LEDs in parallel does not guarantee that the current will be split equally between the individual strings of LEDs.

Generally, one string of LEDs (i.e., the “hot” channel) will have more current than the other(s), i.e., the “cold” channel(s). In cases of multiple LED strings being connected in parallel, all the strings of LEDs may have different currents and therefore different illumination levels.

Further, the hot channel will produce more heat (even at the same voltage) and will thus have a higher junction (chip) temperature than the “cold” channel(s). The higher temperature chip tends to carry an increasingly higher current (due to the negative temperature coefficient in terms of voltage-ampere (V-A)) which further increases the temperature of the chip. Correspondingly, the cold channel tends to carry increasingly less current and therefore decreases in temperature. This phenomenon renders parallel connected LED circuits unstable, particularly from a current split standpoint.

Because LEDs emit light depending on current, the performance of the unbalanced LEDs will be suboptimal. In some instances, the high temperature LED, i.e., hot channel, can pull current high enough to damage the lighting device. In this scenario, placing the strings of LEDs in parallel will not resolve the problem as the statistical sum of parameters will not eliminate the effect of the negative temperature coefficient in terms of V-A characteristics.

In recognition of the differences in electrical and thermal parameters, LED manufacturers typically provide a datasheet with the LED string that includes the nominal forward voltage (Vf) for producing white light. The nominal forward voltage for white light may be indicated as, for example, Vf=3.2V. However, the datasheet may also indicate that the forward voltage (Vf) has a minimum value, e.g., Vfmin=2.8V, and a maximum value, e.g., Vfmax=3.6V. Due to the variance of the forward voltage, i.e., the difference between Vfmin and Vfmax, the current between two parallel strings of LEDs will not typically be equal. Therefore, the two parallel strings of LEDs will not have the same illumination level or brightness.

In order to address the electrical and thermal differences, particularly the differences that impact light output and color temperature, manufacturers often group or “bin” the LEDs based on lumen, color and voltage. “Binning” allows luminaire manufacturers to select only the LEDs that meet specific and required performance ranges, e.g., voltage ranges. However, binning can be a complex process that groups the LEDs into smaller bins having tighter control of color variation or larger bins having less control of color variation.

More recently, binning has been standardized to specify a bin size that approximately correlates to with the degree of color variation experienced by commercial compact fluorescent lamp (CFL) sources. However, binning adds substantial costs to the LEDs and still fails to ensure that the current is split equally amongst parallel connected strings of LEDs.

In order to split the current between two or more strings of LEDs, other known solutions have utilized a voltage dropping circuit for each LED string. In these systems, the first LED string uses a current generator as a reference for the other LED strings. However, this solution has the disadvantage of the LED strings not being dimmable in pulse width modulation (PWM) or continuous modes.

III. SUMMARY OF THE EMBODIMENTS OF INVENTION

Given the aforementioned deficiencies, a need exists for systems and methods to split current substantially equally between two or more parallel connected strings of LEDs without the need for binning the strings of LED. It is also desirable to provide a system and method that splits the current equally between two or more parallel connected strings of LEDs and dims the current source in both PWM mode and continuous mode. It is further desirable to provide a system and method that significantly reduces the voltage required to power the system while also ensuring an equal current slit between the parallel connected strings of LEDs.

Embodiments of the present invention provide a lighting circuit including two or more strings of LEDs connected in parallel, and a current splitting circuit including two or more transistors.

In the embodiments, the lighting circuit is configured to split the current supplied by a power supply equally amongst the strings of parallel connected LEDs. The current split circuit includes a reference channel connected to a first string of LEDs and one or more copy channels connected to the other strings of LEDs. The one or more copy channels taps and divides the voltage at the reference channel to ensure that the voltage applied to each string of LEDs is the same.

In at least one aspect, the embodiments provides a lighting circuit including a power source, a first string of solid state lighting devices, a second string of solid state lighting devices, and a current split circuit. The first string of solid state lighting devices includes individual lighting devices connected in series with each other. The second string of solid state lighting devices includes individual lighting devices connected in series with each other.

The current split circuit is in communication with the first string of solid state lighting devices and the second string of solid state lighting devices. The current split circuit includes a first transistor and a second transistor. The first transistor is connected in series with the first string of solid state lighting devices. The second transistor is in communication with the first transistor and is connected in series with the second string of solid state lighting devices.

During the operation of one exemplary embodiment, the current split circuit divides a current substantially equally between the first string of solid state lighting devices and the second string of solid state lighting devices. The first string of solid state lighting devices and the second string of solid state lighting devices are illuminated at substantially the same level of brightness. The first string of solid state lighting devices and the second string of lighting devices are dimmable in both pulse width modulation mode and continuous mode.

In at least another aspect, the embodiments provide a lighting circuit including a power source, a plurality of strings of light emitting diodes, and a current split circuit. Each of the plurality of strings of light emitting diodes is connected to the power source and in parallel with each other. The current split circuit is connected to the plurality of strings of light emitting diodes.

The current split circuit includes a reference transistor and one or more copy transistors. The reference transistor is connected in series with a first of the plurality of strings of light emitting diodes. The one or more copy transistors are each connected in series with one of the plurality of strings of light emitting diodes.

The present disclosure may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The present disclosure is illustrated in the accompanying drawings, throughout which, like reference numerals may indicate corresponding or similar parts in the various figures. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the disclosure. Given the following enabling description of the drawings, the novel aspects of the present disclosure should become evident to a person of ordinary skill in the art.

V. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description is merely exemplary in nature and is not intended to limit the applications and uses disclosed herein. Further, there is no intent to be bound by any theory presented in the preceding background or summary, or the following detailed description. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility.

While embodiments of the present invention are described herein primarily in connection with LEDs, the concepts are also applicable to other types of lighting devices including solid state lighting devices. Solid state lighting devices include, for example, LEDs, organic light emitting diodes (OLEDs), semiconductor laser diodes, and the like. Similarly, while solid state lighting devices are illustrated as examples herein, the techniques and apparatuses disclosed herein are readily applied to other types of light sources, such as incandescent, halogen, other spotlight sources, and the like.

FIG. 1illustrates a lighting circuit in accordance with embodiments of the present invention. InFIG. 1, a lighting circuit100includes two LED strings110,112connected in parallel. The LEDs strings110,112are powered by a constant voltage generator120via a resistor122. The LED strings110,112include individual LEDs110(a-j) and112(a-j), respectively. The individual LED110(a-j) and LED112(a-j) of each LED string110,112, respectively, are connected in series. The LED strings110,112are connected in parallel via a current split circuit130.

The current split circuit130includes two bipolar transistors including, reference transistor132and copy transistor134, arranged as a current mirror. The reference transistor132and the LED string110form a reference channel110,132. The copy transistor134and the LED string112form a copy channel112,134. The copy transistor134taps the current at the reference transistor132such that the current is split substantially equally between the reference channel110,132and the copy channel112,134, i.e., less than approximately a 5% difference in current. The current split circuit130thereby allows for uniform illumination of LED strings110,112without the need for binning the LED strings for forward voltage.

Uniform illumination provides that substantially all the LEDs of the LED strings110,112, i.e., 90-95% or more of all LEDs, are illuminated at substantially the same illumination levels. As used herein, for example, the term substantially the same illumination levels implies less than 2% variation of illumination such that any differences in illumination levels are not perceptible by the human eye. Eliminating the necessity to bin the LED strings110,112with forward voltage thereby substantially reduces the cost associated with manufacturing LED luminaires. Further, because the lighting circuit110does not utilize a voltage drop circuit, it allows the LED strings110,112to be dimmed in both continuous and PWM modes.

While the invention has been described in terms of a current split circuit including bipolar transistors that form a mirror circuit, other current split circuits are envisioned that include other transistor types, e.g., field effect transistors (FETs) or insulated gate bipolar transistors (IGBTs), without departing from the disclosure. It is also noted that while the invention has been described in terms of two strings of LEDs connected in parallel, other embodiments are envisioned that include multiple strings of LEDs connected in parallel, e.g., three, four, or five strings of LEDs, and the like, without departing from the disclosure.

Further, while the invention has been described in terms of a single set of LEDs connected in parallel, embodiments are envisioned having multiple sets of parallel connected LEDs that are connected in series to provide a modular configuration, without departing from the disclosure. Further still, either a constant voltage generator or a constant current generator may be used to power the two or more parallel-connected LED strings without departing from the disclosure.

FIG. 2illustrates a lighting circuit in accordance with an alternative embodiment of the present invention. As illustrated inFIG. 2, the lighting circuit200includes multiple LED strings, e.g.,210,212,214,216, connected in parallel. The lighting circuit200has a similar function as the lighting circuit100, as discussed above. The lighting circuit200has particular utility to further divide the current in applications that require a high number of LEDs and where there is a need to lower the system voltage.

While the embodiment illustrated inFIG. 2depicts four LED strings, the use of multiple LED strings, e.g., 6, 8, 10, or more strings, is envisioned without departing from the spirit and scope of the present invention. The LED strings210,212,214,216are powered by a constant current generator220. The LED strings210,212,214and216, respectively, include individual LEDs210a-c,212a-c,214a-c, and216a-c. The LED strings210,212,214and216are connected in parallel via a current split circuit230.

The current split circuit230includes multiple bipolar transistors including reference transistor232and copy transistors234,236and238, arranged as a current mirror. The reference transistor232and the LED string210form a reference channel210,232.

A first copy transistor234and the LED string212form a first copy channel. A second copy transistor236and the LED string214form a second copy channel. A final copy transistor238and the LED string216form a final copy channel. The copy transistors234,236and238tap the current at the reference transistor232such that the current is split substantially equally between the reference channel210,232, first copy channels212,234, second copy channel214,236, and final copy channel216,238.

Thus, the current split circuit230substantially reduces the overall system voltage by dividing the voltage output by constant current generator220multiple times. The current split circuit230also allows for uniform illumination of the multiple LED strings210,212,214and216without the need for binning the LED strings for forward voltage. Additionally, the current split circuit230allows the multiple LED strings210,212,214and216to be dimmed in both continuous and PWM modes.

Alternative embodiments, examples, and modifications which would still be encompassed by the disclosure may be made by those skilled in the art, particularly in light of the foregoing teachings. Further, it should be understood that the terminology used to describe the disclosure is intended to be in the nature of words of description rather than of limitation.

Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described above can be configured without departing from the scope and spirit of the disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the disclosure may be practiced other than as specifically described herein.