WO2017202380A1

WO2017202380A1 - Integrated hybrid-type led driver

Info

Publication number: WO2017202380A1
Application number: PCT/CN2017/086104
Authority: WO
Inventors: Lisong LI; Kwok Tai Mok; Yuan Gao
Original assignee: Hong Kong University of Science and Technology
Current assignee: Hong Kong University of Science and Technology
Priority date: 2016-05-26
Filing date: 2017-05-26
Publication date: 2017-11-30
Anticipated expiration: 2018-11-26

Abstract

A light-emitting diode (LED) driver includes: a plurality of switches corresponding to a plurality of LEDs; and a controller, configured to switch between operating the LED driver in a linear mode and in a switching mode such that the LED driver is operated in the linear mode when an input voltage is relatively close to an LED voltage level and the LED driver is operated in the switching mode when the input voltage is relatively farther from the LED voltage.

Description

INTEGRATED HYBRID-TYPE LED DRIVER

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional Patent Application No. 62/392,254, filed on May 26, 2016, which is incorporated herein by reference.

TECHNICAL FIELD

Embodiments of the present invention relate to AC-powered light-emitting diode (LED) driving systems, and more particularly, to circuitry and methodology that improve the power efficiency of an LED system.

BACKGROUND

The general-purpose lighting industry has been undergoing rapid evolution relating to light-emitting diodes (LEDs) , which have been improved sharply over the last decade. LEDs hold many merits in comparison to traditional incandescent and fluorescent bulbs. These advantages include high efficacy, longer lifespan, and environmentally friendly properties. Two common types of LED drivers are switch-mode LED drivers and linear LED drivers. A switch-mode LED driver is able to provide high efficiency while driving flexible numbers of LEDs. However, switch-mode LED drivers require a bulky and expensive off-chip power inductor with induction of hundreds of microhenries (μH) and an electrolytic capacitor (E-Cap) . This increases the size and cost, while reducing the lifespan of such an LED system. Linear LED drivers, on the other hand, do not require power inductors. However, the maximum power efficiency is limited by the ratio of the LED voltage to the input voltage. If there is a big difference between the LED voltage and input voltage, a lot of power will be dissipated on the pass device (e.g., the power MOSFETS depicted in FIG. 1) , resulting in low power efficiency.

FIG. 1 is a circuit diagram depicting a conventional multiple-string linear LED driver, and FIG. 2A is a circuit diagram depicting a particular example of a conventional multiple-string linear LED driver having three strings. FIG. 2B depicts the power consumption distribution of the linear LED driver. Multiple-string linear LED drivers are widely used in mains powered general lighting applications. The LED strings are turned on sequentially according to the input voltage. For example, when V_IN is between V_LED1+V_LED2 and V_LED1+V_LED2+V_LED3, the circuits in the dashed line box are active and form a linear regulator. Since the output voltage is regulated to V_LED1+V_LED2, the extra voltage in excess of V_LED1+V_LED2 will drop on the second metal–oxide–semiconductor field-effect transistor (MOSFET) in the string (M₂) . As a result, a considerable part of the input power is dissipated on M₂ regardless of the quality of the power MOSFET. Thus, the power efficiency will be degraded, as can be seen in FIG. 2C, which is a plot depicting conduction loss caused by the voltage difference between V_IN and V_LED, regardless of the quality of the power MOSFET, in the conventional 3-string linear LED driver in FIG. 2A. Further details may be found, for example, in L. Li, Y. Gao, and P. K. T. Mok, “Amultiple-string hybrid LED driver with 97％power efficiency and 0.996 power factor, ” in 2016 IEEE Symposium on VLSI Technology, 2016, pp. 1–2, which is incorporated by reference herein.

FIG. 3 is a circuit diagram depicting a conventional single-stage switching converter-based AC LED driver and a plot showing a large switching loss due to a parasitic capacitor C_D. To ensure a small inductor current ripple, the LED driver needs a very bulky and expensive inductor, which increases the size and cost of the LED system. This driving circuit also suffers from large ripples at double-line-frequency because power factor correction (PFC) control delivers non-uniform power to the LEDs at different times in each cycle. In order to filter out this low frequency ripple, a bulky electrolytic capacitor (E-Cap) is usually employed as an energy storage device in parallel with the LEDs. As the lifespan of an E-Cap is usually much shorter than that of an LED, the lifespan of the LED system will be seriously reduced based on inclusion of the E-Cap. For example, in the switch-mode LED driver depicted in FIG. 3, with V_IN＝160V, C_D＝100pF, and a frequency f of 5MHz, the LED system has a switching loss of 6.4W.

With respect to FIG. 3, a buck converter LED driver can theoretically achieve 100％efficiency. However, in high voltage applications, large switching losses occur due to the parasitic drain capacitor of the power MOSFET, especially when the switching frequency is above 1MHz.

Another approach is a quasi-resonant LED driver can eliminate switching loss due to its zero-voltage-switching (ZVS) characteristics. However, the current and voltage stress on the power MOSFET will be increased several times over in quasi-resonant LED driver.

SUMMARY

In an exemplary embodiment, the invention provides a light-emitting diode (LED) driver. The LED driver includes: a plurality of switches corresponding to a plurality of LEDs； and a controller, configured to switch between operating the LED driver in a linear mode and in a switching mode such that the LED driver is operated in the linear mode when an input voltage is relatively close to an LED voltage level and the LED driver is operated in the switching mode when the input voltage is relatively farther from the LED voltage.

In another exemplary embodiment, the invention provides a light-emitting diode (LED) system. The LED system includes: a plurality of LEDs； and an LED driver. The LED driver includes: an inductor, wherein one end of the inductor corresponds to an input voltage and the other end of the inductor corresponds to an LED voltage； a plurality of switches, each of the plurality of switches corresponding to a respective LED of the plurality of LEDs； and a controller, configured to switch between operating the LED driver in a linear mode and in a switching mode such that the LED driver is operated in the linear mode when a voltage drop across the inductor is relatively close to zero and the LED driver is operated in the switching mode when the voltage drop across the inductor is relatively farther from zero.

In yet another exemplary embodiment, the invention provides a method for controlling a plurality of light-emitting diodes (LEDs) . The method includes: operating, by the controller, an LED driver in a linear mode, wherein in the linear mode, a first switch corresponding to a first LED voltage is activated while a second switch corresponding to the second voltage is deactivated； and operating, by a controller, the LED driver in a switching mode, wherein in the switching mode, the first switch corresponding to the first LED voltage and the second switch corresponding to the second LED voltage are alternately turned on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a conventional multiple-string linear LED driver.

FIG. 2A is a circuit diagram of a conventional three-string linear LED driver.

FIG. 2B depicts the power consumption distribution of the linear LED driver.

FIG. 2C is a plot depicting conduction loss caused by the voltage difference between V_IN and V_LED, regardless of the quality of the power MOSFET, in the conventional 3-string linear LED driver in FIG. 2A.

FIG. 3 is a circuit diagram of a conventional single-stage switching converter-based AC LED diver and a plot showing a large switching loss due to a parasitic capacitor C_D.

FIG. 4 is a circuit diagram depicting the structure of an integrated hybrid-type LED driver according to an exemplary embodiment of the present invention.

FIG. 5 is a circuit diagram depicting an exemplary implementation of the integrated hybrid-type LED driver depicted in FIG. 5A having three LEDs.

FIG. 6 is a plot depicting exemplary waveforms corresponding to the integrated hybrid-type LED driver depicted in FIG. 5.

FIGS. 7A-7C illustrate exemplary operation of the integrated hybrid-type LED driver depicted in FIG. 5.

FIG. 8 is a circuit diagram depicting the structure of an integrated hybrid-type LED driver according to an exemplary embodiment of the present invention.

FIG. 9 is a chip micrograph depicting an exemplary implementation of an integrated hybrid-type LED driver.

FIGS. 10A to 10D are plots showing measured waveforms for an exemplary implementation of an integrated hybrid-type LED driver resulting from a variety of voltage inputs.

FIG. 11 is a plot of power efficiency and power factor against input voltage for an exemplary implementation of an integrated hybrid-type LED driver.

DETAILED DESCRIPTION

As discussed in the background, a number of challenges exist with respect to existing LED systems. The power efficiency of conventional commercial LED drivers is around 80％to 90％. Therefore, there exists a need to improve power efficiency to save electricity. Further, some conventional LED drivers contain huge inductors with inductances of hundreds of microhenries (μH) , which make LED systems utilizing such LED drivers bulky and expensive. Additionally, electrolytic capacitors possess only a tenth of the lifespan of an LED, and thus reduce the lifespan and increase maintenance costs for LED systems. Therefore, improvements with respect to performance and cost for LED drivers are also needed.

Exemplary embodiments of the present invention provide an LED driving system that uses a hybrid control method which achieves very high performance and efficiency at a low cost. In an exemplary embodiment, the integrated hybrid-type LED driving system is AC powered (it will be appreciated that exemplary embodiments of the invention may be applied to any environment in which a time-varying input voltage is used, and that environments having an AC input are generally the most common) . By adaptively switching between a switching converter mode and a linear regulator mode (which effectively combines the merits of a conventional linear LED driver and a conventional switch-mode LED driver) , the integrated hybrid-type LED driving system ensures that the majority of power that is inputted into the system is delivered to the LEDs, and very high power efficiency and power factor can thus be achieved. In addition, the integrated hybrid-type LED driving system utilizes a very small inductor and does not require an electrolytic capacitor. This allows for relatively low costs, long lifespan, and low maintenance relative to conventional LED driving systems. Further, the power MOSFETs and the controller of the integrated hybrid-type LED driving system may be implemented on-chip.

Accordingly, exemplary embodiments of the invention provide improvements with respect to size, efficiency, and reliability relative to conventional LED driving systems, while achieving outstanding performance.

FIG. 4 is a circuit diagram depicting the structure of an integrated hybrid-type LED driver according to an exemplary embodiment of the present invention, which efficiently utilizes input power without large switching losses or excessive current and voltage stress. As shown in FIG. 4, an inductor L is added to the power stage of a multiple-string linear LED driver. For a certain input voltage, only two adjacent MOSFETs will be fully turned on and off alternately while other power MOSFETs remains off-state. The inductor current is sensed by a resistor R_S and fed back to the controller. Hysteretic control is used to ensure the inductor current I_L is bounded between I_LOk and I_HIk (I_LOk and I_HIk refer to the LED current boundaries in different states, with k ranging from 1 to n where n is the number of LEDs； in general I_LOk and I_HIk should increase with input voltage and I_HIn should be smaller than the LED current rating) . For example, when the input voltage V_IN is between V_LED1 and V_LED1+V_LED2, the integrated hybrid-type LED driver is in a Switching Mode where M₁ and M₂ are turned on and off alternately (in an example with a 110 V AC input, V_LED1 may be about 60V and V_LED1+V_LED2 may be about 80V) . When M₁ is on and M₂ is off, unlike a conventional linear LED driver, the extra voltage drops on the inductor and I_L ramps up from I_LO1 to I_HI1. In this way, the extra energy is stored in L. After I_L reaches I_HI1, M₂ will be turned on and M₁ will be turned off. Although V_IN is smaller than V_LED1+V_LED2, the two LED strings can still be driven with the energy previously stored in L until it is discharged to I_LO1 again. As V_IN approaches V_LED1+V_LED2, the inductor will be charged faster and discharged more slowly. I_L will eventually fail to reach I_LO1 when V_IN approximately equals V_LED1+V_LED2 (I_LED＝I_LO1) . At this point the integrated hybrid-type LED driver will enter a Linear Mode. In Linear Mode, the V_IN approximately matches with V_LED1+V_LED2. As V_IN increases, V_LED1+V_LED2 also increases (from V_LED1+V_LED2 (I_LED＝I_LO1) to V_LED1+V_LED2 (I_LED＝I_HI1) ) , and the LED current rises from I_LO1 to I_HI1. During this period, the voltage across the inductor is 0V and there is no switching behavior. It will be appreciated V_IN is described as “approximately” equaling V_LED1+V_LED2 that because there may be a small amount of voltage drop on the inductor, the power MOSFET, and R_S, such that V_IN ＝ V_LED1 + V_LED2 + I_LED × (R_ON2 + R_L + Rs) , where R_ON2 is the resistance of the MOSFET and R_L is the resistance of the inductor.

In another example, when the input voltage V_IN is less than V_LED1, M₁ is on and M₂ through M_n are off. In another example, when the input voltage V_IN is greater than V_LED1+V_LED2 but less than V_LED1+V_LED2+V_LED3, the LED driver switches between M₂ and M₃ being on. Thus, it will be appreciated that with increasing voltage, the LED driver operates in the following modes of operation: (1) a linear mode with a first switch M₁ on； (2) a switching mode where the first switch M₁ and a second switch M₂ are alternately on； (3) a linear mode with the second switch M₂ on； (4) a switching mode with the second switch M₂ and a third switch M₃ alternately on； (5) a linear mode with the third switch M₃ on； and so on up to an n^th switch M_n corresponding to an n^th LED.

It will be appreciated that to ensure safe operation, exemplary embodiments may be configured such that V_LED1+V_LED2+...+V_LEDn is larger than the maximum V_IN for the system.

In Linear Mode, the input voltage almost equals the LED voltage except for some minimal drop on the power MOSFET and R_S. Therefore, the power loss is very small compared with the output power. In Switching Mode, only one of the two active power MOSFETs has switching loss, and it is equal to 1/2×C_Dk×V_LED (k+1) ²×f, where C_Dk is the parasitic drain capacitance of the power MOSFETs and f is the switching frequency.

Compared with the switching loss of the buck converter, which is 1/2×C_D×V_IN ²×f, the power loss for an integrated hybrid-type LED driver in accordance with exemplary embodiments of the invention is roughly reduced by K² times, where K is the number of LEDs that are on. The inductor value is also reduced by about K times assuming they operate at the same frequency, because the voltage swing across L in the proposed driver is V_LED (k+1) other than V_IN in the buck converter.

FIG. 5 is a circuit diagram depicting an exemplary implementation of the integrated hybrid-type LED driver shown in FIG. 4 having three LEDs. The integrated hybrid-type LED driver depicted in FIG. 5 operates in a similar manner as discussed above with respect to FIG. 4.

Table 1 below depicts power stages corresponding to the three-string integrated hybrid-type LED driver shown in FIG. 5, showing which power MOSFET (s) should be active (not OFF) in different voltage ranges. For example, in state 010, V_G1 ＝ Q and V_G2 ＝ NQ, meaning that M₁ and M₂ are alternately turned on.

Table 1

FIG. 6 includes plots depicting exemplary waveforms corresponding to the integrated hybrid-type LED driver depicted in FIG. 5. With an AC input voltage, V_IN is a sinusoidal wave as shown. V_X is the voltage of the anode of LED1. If M₁ is on, V_X equals V_LED1. If M₂ is on and M₁ is off, V_X equals V_LED1+V_LED2. If M₃ is on and M₁ and M₂ is off, V_X equals V_LED1+V_LED2+V_LED3. Thus, V_X is the voltage of the LED (s) that is/are on, namely output voltage. V_G1, V_G2, V_G3 are the gate control voltages of M₁, M₂ and M₃, respectively. If V_G1 is high, M₁ is turned on. I_L is the current that flows through the inductor. If V_IN is larger than V_X, I_L will ramp up and if V_IN is smaller than V_X, I_L will go down. Therefore, when V_IN is between V_LED1 and V_LED1+V_LED2, I_L will go up and down by alternately turning on M₁ and M₂. Alternately turning on M₁ and M₂ allows the difference between input voltage (V_IN) and output voltage (V_X) to be stored and used to drive an additional LED once enough energy is stored (when V_IN is smaller than V_LED1+V_LED2, V_IN alone is not enough to drive both LED1 and LED2 without the help of the energy stored in the inductor) .

As shown in the V_IN, V_X plot of FIG. 6, even while V_IN is smaller than V_LED1+V_LED2, the two LED strings can still be driven with the energy previously stored in L until it is discharged to I_LO1 again in the Switching Mode, and V_IN matches with V_LED1+V_LED2 in Linear Mode. As shown in the V_G1, V_G2, V_G3 plot of FIG. 6, for a certain input voltage, only two adjacent power MOSFETs will be fully turned on and off alternately while other power MOSFETs remain in the off state. And as shown in the I_L plot of FIG. 6, hysteretic control ensures that the inductor current I_L is bounded between I_LOk and I_HIk

FIG. 7A illustrates the integrated hybrid-type LED driver depicted in FIG. 5 in Switching Mode under conditions where V_LED1 < V_IN < V_LED1 + V_LED2, L is being charged, and M₁ is on while M₂ is off. Under these conditions, M₁ is fully turned on with a minimum voltage drop on M₁, and extra energy is stored in L.

FIG. 7B illustrates the integrated hybrid-type LED driver depicted in FIG. 5 in Switching Mode under conditions where V_LED1 < V_IN < V_LED1 + V_LED2, L is being discharged, and M₂ is on while M₁ is off. Under these conditions, energy previously stored in L is released to drive an additional LED, allowing the extra energy discussed above with respect to FIG. 7A to be efficiently used.

FIG. 7C illustrates the integrated hybrid-type LED driver depicted in FIG. 5 in Linear Mode under conditions where V_IN ≈ V_LED1 + V_LED2. Under these conditions, all of the input power goes to the LEDs, allowing high efficiency to be achieved.

FIG. 8 is a circuit diagram depicting the structure of an integrated hybrid-type LED driver, having six LEDs in a string (i.e., six branches) , according to an exemplary embodiment of the present invention. In an exemplary implementation, the power MOSFETs and the controller are implemented on-chip. In operation, the sensed inductor current, represented by a voltage V_S, is compared with V_HI and V_LO and the outputs of the comparators are sent into an RS latch, whose outputs are Q and NQ. For a certain input voltage, V_Gk is connected to Q, V_G, k+1 is connected to NQ and other power MOSFETs are off . When NQ＝1, the inductor is discharged, and V_S will reach V_LO, turning S from 0 to 1. However, as V_IN keeps increasing, V_S will eventually fail to reach V_LO. Instead, V_S will reach V_HI, turning R from 0 to 1. When NQ＝1 and R＝1, the 3-bit bidirectional counter will count up by 1, and when Q＝1 and S＝1, the 3-bit bidirectional counter will count down by 1. As a result, A₂A₁A₀ can be used to indicate V_IN and select the two power MOSFETs that are suitable for the current input voltage. A₂A₁A₀ also controls V_HI and V_LO to increase or decrease with V_IN such that a high power factor (PF) is achieved.

It will be appreciated that the control logic provided by the RS latch, the 3-bit bidirectional counter, the reference voltage selector, and the gate voltage selector depicted in FIG. 7 is merely exemplary, and that other implementations of control circuits, controllers, integrated circuits (ICs) , etc., whether analog or digital, may be used to implement exemplary embodiments of an integrated hybrid-type LED driver.

It will further be appreciated that while the examples discussed herein correspond to a 3-string LED driver and a 6-string LED driver, exemplary embodiments of the invention are not limited thereto and may be utilized in connection with driving any number of LEDs.

FIG. 9 is a chip micrograph depicting an exemplary implementation of an integrated hybrid-type LED driver for general lighting applications. The exemplary integrated hybrid-type LED driver was fabricated with a 0.35μm 120V high voltage CMOS process, and utilizes a 6.8μH inductor and 9 LEDs, each of the LEDs having a voltage of about 20V. By selecting a suitable process, additional implementations of the present invention may also be implemented in higher voltage applications—e.g. 220VAC.

The exemplary integrated hybrid-type LED driver depicted in FIG. 9 was demonstrated as achieving 97％power efficiency and 0.996 power factor with a 120V_AC 60Hz input.

FIGS. 10A to 10D are plots showing measured waveforms for the exemplary integrated hybrid-type LED driver depicted in FIG. 9 resulting from a variety of voltage inputs. FIG. 10A, 10B, 10C, and 10D show the measured waveforms of V_IN, V_X and I_L under 100/110/120V_AC inputs. In Linear Mode, V_X follows V_IN, as shown on the left side of FIG. 10A. In Switching Mode, V_X switches above and below V_IN, and I_L ramps up and down between a hysteretic window of about 100mA, as shown on the right side of FIG. 10A. FIGS. 10B and 10C show the measured waveforms under 100V_AC and 120V_AC input, respectively. FIG. 10D shows the waveforms of I_L as well as A₂, A₁, and A₀ under 110V_AC input. In this exemplary implementation, the inductor value was 6.8μH (relatively small when compared to inductors used in conventional switch-mode LED drivers (e.g., 5.5mH, 470 μH, 400μH) ) , the maximum switching frequency was approximately 5MHz, and the supply voltage for the controller was 5.5V. Further, I_L had a similar shape to V_IN, indicating that a good power factor was achieved.

FIG. 11 is a plot of power efficiency and power factor against input voltage for the exemplary integrated hybrid-type LED driver depicted in FIG. 9. As shown in FIG. 11, for an exemplary implementation with 120V_AC, 60Hz input and 20W output power, the peak power efficiency was measured to be 97％and the peak power factor was measured to be 0.996. The measured peak efficiency for the exemplary embodiment was 98.2％when the power loss of the bridge rectifier was excluded.

From these results, it can be seen that exemplary embodiments of the invention are able to provide integrated hybrid-type LED drivers that achieve higher power factor and higher efficiency than conventional LED drivers, while utilizing a relatively small inductor compared to conventional switch-mode LED drivers.

The hybrid LED driver operates in a switching mode or a linear mode based on whether there is a voltage drop on the inductor (e.g., a voltage difference between V_IN and V_X as discussed above) . V_X, which is the “LED voltage, ” is equal to V_LED1 when M₁ is on and equal to V_LED1+V_LED2 when M₂ is on. When V_IN is between V_LED1 and V_LED1+V_LED2, the hybrid LED driver is in the switching mode, with M₁ and M₂ alternately being turned on, which causes the voltage drop on the inductor to alternately be positive and negative and the inductor current to go up and down. The switching frequency depends on the voltage drop on the inductor (V_IN-V_X) , namely the voltage difference between the input voltage and the LED voltage. When V_IN-V_X is large, the switching frequency is high (e.g., ～MHz range) and the hybrid LED driver is operating in the “switching mode. ” As the input voltage approaches the LED voltage (e.g., V_LED1 or V_LED1+V_LED2) , V_IN-V_X becomes very small, resulting in slow changes with respect to the inductor current. Since the frequency of an AC power line may be 50Hz or 60Hz, when the inductor current is going up or down in this frequency range, the foregoing “switching” behavior is no longer observed. Instead, the change of inductor current is synchronized with the change of the input voltage, and the hybrid LED driver is operating in the “linear mode. ”

In the switching mode, a switching frequency in the ～MHz range may correspond to a voltage drop on the inductor (V_IN-V_X) being about 10 V. In the linear mode, the voltage drop on the inductor (V_IN-V_X) is very small, and the input voltage is very close to the LED voltage. Since there will be some voltage difference due to the resistance of the inductor, the on-resistance of the MOSFET and the sense resistor Rs, there is not a fixed exact value for the voltage drop on the inductor (V_IN-V_X) at which the hybrid LED driver changes between the switching mode and the linear mode. By adjusting the window for the inductor current (bounded by I_LOk and I_HIk) , the point at which the changing between the switching mode and the linear mode may be configured to determine the percentage of switching mode and linear mode operations of the LED driver so as to achieve a desired tradeoff among the power efficiency, peak current in LED, and power factor. With a wider window for the inductor current, the percentage of linear mode operation will increase, resulting in higher power efficiency, higher peak current in LED, and lower power factor.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B” ) is to be construed to mean one item selected from the listed items (Aor B) or any combination of two or more of the listed items (Aand B) , unless otherwise indicated herein or clearly contradicted by context. The terms “comprising, ” “having, ” “including, ” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to, ” ) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as” ) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

A light-emitting diode (LED) driver, comprising:

a plurality of switches corresponding to a plurality of LEDs； and

a controller, configured to switch between operating the LED driver in a linear mode and in a switching mode such that the LED driver is operated in the linear mode when an input voltage is relatively close to an LED voltage level and the LED driver is operated in the switching mode when the input voltage is relatively farther from the LED voltage.
The LED driver according to claim 1, wherein in the linear mode, the controller keeps a switch corresponding to the LED voltage on.
The LED driver according to claim 1, wherein in the switching mode, the controller alternately turns on a first switch corresponding to a first LED voltage and a second switch corresponding to a second LED voltage .
The LED driver according to claim 3, wherein while the first switch is on in the switching mode, an inductor of the LED driver is charged while a first LED is on and a second LED is off, and while the second switch is activated in the switching mode, the inductor of the LED is discharged while both the first and second LEDs are on.
The LED driver according to claim 1, wherein the controller is configured to determine when to switch between operating the LED driver in the linear mode and in the switching mode based on a sensed inductor current.
The LED driver according to claim 5, wherein the sensed inductor current is represented by a voltage across a sense resistor.
The LED driver according to claim 1, wherein the plurality of switches includes a first switch corresponding to turning a first LED on, a second switch corresponding to turning the first LED and a second LED on, and a third switch corresponding to turning the first LED, the second LED, and a third LED on；

wherein the first switch corresponds to a first LED voltage, the second switch corresponds to a second LED voltage, and the third switch corresponds to a third LED voltage.
The LED driver according to claim 7, wherein the controller is configured to operate the LED driver in a first linear mode corresponding to the first switch being turned on, a first switching mode corresponding to the first and second switches being alternately turned on, a second linear mode corresponding to the second switch being turned on, a second switching mode corresponding to the second and third switches being alternately turned on, and a third linear mode corresponding to the third switch being turned on.
The LED driver according to claim 8, wherein the controller is configured to operate the LED driver in the first, second, or third linear mode when the input voltage is approximately equal to the first, second or third LED voltage, respectively.
A light-emitting diode (LED) system, comprising:

a plurality of LEDs； and

an LED driver, comprising:

an inductor, wherein one end of the inductor corresponds to an input voltage and the other end of the inductor corresponds to an LED voltage；

a plurality of switches, each of the plurality of switches corresponding to a respective LED of the plurality of LEDs； and

a controller, configured to switch between operating the LED driver in a linear mode and in a switching mode such that the LED driver is operated in the linear mode when a voltage drop across the inductor is relatively close to zero and the LED driver is operated in the switching mode when the voltage drop across the inductor is relatively farther from zero.
The LED system according to claim 10, wherein in the linear mode, the controller keeps a switch corresponding to the LED voltage activated.
The LED system according to claim 10, wherein in the switching mode, the controller alternately activates a first switch corresponding to a first LED voltage and a second switch corresponding to a second LED voltage.
The LED system according to claim 12, wherein while the first switch is activated in the switching mode, an inductor of the LED driver is charged while a first LED is on and a second LED is off, and while the second switch is activated in the switching mode, the inductor of the LED is discharged while both the first and second LEDs are on.
The LED system according to claim 10, wherein the controller is configured to determine when to switch between operating the LED driver in the linear mode and in the switching mode based on a sensed inductor current.
The LED system according to claim 14, wherein the sensed inductor current is represented by a voltage across a sense resistor.
The LED system according to claim 10, wherein the plurality of switches includes

a first switch corresponding to turning a first LED on, a second switch corresponding to turning the first LED and a second LED on, and a third switch corresponding to turning the first LED, the second LED, and a third LED on；

wherein the first switch corresponds to a first LED voltage, the second switch corresponds to a second LED voltage, and the third switch corresponds to a third LED voltage.
The LED system according to claim 16, wherein the controller is configured to operate the LED driver in a first linear mode corresponding to the first switch being turned on, a first switching mode corresponding to the first and second switches being alternately turned on, a second linear mode corresponding to the second switch being turned on, a second switching mode corresponding to the second and third switches being alternately turned on, and a third linear mode corresponding to the third switch being turned on.
The LED system according to claim 17, wherein the controller is configured to operate the LED driver in the first, second, or third linear mode when the input voltage is approximately equal to the first, second or third LED voltage, respectively.
A method for controlling a plurality of light-emitting diodes (LEDs) , comprising:

operating, by the controller, an LED driver in a linear mode, wherein in the linear mode, a first switch corresponding to a first LED voltage is activated while a second switch corresponding to the second voltage is deactivated； and

operating, by a controller, the LED driver in a switching mode, wherein in the switching mode, the first switch corresponding to the first LED voltage and the second switch corresponding to the second LED voltage are alternately turned on.
The method according to claim 19, wherein while the first switch is activated in the switching mode, an inductor of the LED driver is charged while a first LED is on and a second LED is off, and while the second switch is activated in the switching mode, the inductor of the LED is discharged while both the first and second LEDs are on.