WO2011092606A1

WO2011092606A1 - Apparatus for enabling smooth start-up of solid-state lighting unit

Info

Publication number: WO2011092606A1
Application number: PCT/IB2011/050176
Authority: WO
Inventors: Geert Willem Van Der Veen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-02-01
Filing date: 2011-01-14
Publication date: 2011-08-04
Also published as: TW201143523A

Abstract

A device for controlling a solid-state lighting (SSL) source includes a linear regulator and a switching regulator. The linear regulator is configured to control the current through the SSL source during a start-up period. The switching regulator is configured to control the current through the SSL source following the start-up period based on a dimming level of the SSL source.

Description

APPARATUS FOR ENABLING SMOOTH START-UP OF SOLID-STATE LIGHTING UNIT

Technical Field

[0001] The present invention is directed generally to dimming of solid-state lighting units. More particularly, various inventive methods and apparatus disclosed herein contemplate selectively incorporating analog circuits during start-up of solid-state lighting units to improve dimming resolution.

Background

[0002] Digital lighting technologies, i.e. illumination based on solid-state or semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Patent Nos. 6,016,038 and 6,21 1,626.

[0003] Many lighting applications make use of dimmers. Conventional dimmers work well with incandescent (bulb and halogen) lamps. However, problems occur with other types of electronic lamps, including compact fluorescent lamp (CFL), low voltage halogen lamps using electronic transformers and solid-state lighting (SSL) lamps or units, such as LEDs and OLEDs, or other loads. Conventional dimmers typically chop a portion of each waveform (sine wave) of the mains voltage signal and pass the remainder of the waveform to the lighting fixture. A leading edge or forward-phase dimmer chops the leading edge of the voltage signal waveform. A trailing edge or reverse-phase dimmer chops the trailing edge of the voltage signal waveform.

[0004] Unlike incandescent and other resistive lighting devices which respond naturally without error to a chopped waveform produced by a dimmer, LED and other SSL units have a noticeable delay and/or flicker from when a user switches on the SSL unit to when the light actually turns on. This delay from when the physical power switch of the SSL unit is turned on to when light is first seen from the fixture may be undesirably long. The cause of this delay is the time it takes for the power converter to have sufficient voltage to start up and begin converting power from the line voltage to power the SSL unit according to the dimmer setting.

[0005] Further, lighting units or lighting sources used to light large spaces, such as studio and theater lighting, must have an extremely large dimming range or resolution to enable smooth start-up, e.g., without a visible step. Conventionally, such large dimming ranges are

implemented using filament lamps, which generally provide slow and smooth start-up behavior by nature. However, when conventional SSL units, such as lighting units using LED-based light sources, are used for theater and other large space lighting, the large dimming resolution required for smooth start-up cannot be achieved. For example, it is estimated that a dimming range of about 1 : 1 ,000,000 is required to match the dimming behavior of a filament lamp with an SSL unit in a theater setting, which cannot be achieved using a conventional pulse width modulation (PWM) dimming driver. Rather, a conventional PWM dimming driver has a practical resolution in the order of 16 bits, which provides 65536 steps, which is not sufficient to achieve smooth start-up of the SSL unit.

[0006] Notably, in this environment, the large dimming range or resolution is only needed for start-up of the SSL unit. Once the SSL unit is on, higher dim levels with lower resolutions are used. That is, after start-up, the 16-bit resolution provided by a conventional PWM dimming driver is sufficient for smooth dimming.

[0007] Thus, there is a need in the art for an SSL unit particularly suitable for illuminating large spaces and capable of slow and smooth start-up behavior, for example, when initially turned on or otherwise activated from a very low dimmer setting.

Summary

[0008] The present disclosure is directed to inventive methods and apparatus for enabling dimming of an SSL unit, including during start-up of the SSL unit, for illuminating large spaces, e.g., such as studios and theaters. For example, a linear regulator is used to control current through the SSL during the start-up period and a switching regulator, e.g., including a PWM circuit, is used to control current through the SSL following the start-up period, to provide a smooth start-up and a high resolution during dimming. [0009] Generally, according to one aspect, a device for controlling an SSL unit includes a linear regulator and a switching regulator for controlling an SSL source. The linear regulator is configured to control the current through the SSL source during a start-up period. The switching regulator is configured to control the current through the SSL source following the start-up period based on a dimming level of the SSL unit.

[0010] According to another aspect, a device for selectively providing large dimming resolution of an SSL source includes a hysteretic downconverter, a linear regulator and a shunt switch. The hysteretic downconverter is connected between an input power source and the SSL source, and includes a first switch that is continually closed during a start-up period of the SSL source. The linear regulator is connected in series with the SSL source, and is configured to have a resistance that progressively decreases during the start-up period, causing corresponding increases in current through the SSL source for controlling dimming of the SSL source during the start-up period of the SSL unit. The shunt switch is connected in parallel with the SSL source and the linear regulator. The shunt switch is continuously open during the start-up period of the SSL source and is periodically closed after the start-up period to provide a pulse width modulation (PWM) signal for controlling dimming of the SSL source after the start-up period.

[0011] According to another aspect, a device for selectively providing large dimming resolution of an LED unit includes a hysteretic downconverter, a linear regulator, a shunt switch and a controller. The hysteretic downconverter is connected between an input power source and the LED unit, and includes a first switch that is continually closed during a start-up period of the LED unit. The linear regulator is connected in series between the LED unit and a ground voltage, and includes a metal-oxide-semiconductor field-effect transistor (MOSFET) configured to have a resistance that progressively decreases during the start-up period, causing

corresponding increases in current through the LED unit. The shunt switch is connected in parallel with the LED unit and the linear regulator, the shunt switch being continuously open during the start-up period of the LED unit and being periodically closed after the start-up period to provide a PWM signal for controlling dimming of the LED unit after the start-up period. The controller is configured to provide a first control signal to the hysteretic downconverter for controlling operation of the first switch, a second control signal to the linear regulator for controlling the resistance of the MOSFET, and a third control signal to the shunt switch for controlling operation of the shunt switch. [0012] As used herein for purposes of the present disclosure, the term "LED" should be understood to include any electroluminescent diode or other type of carrier injection/junction- based system that is capable of generating radiation in response to an electric signal. Thus, the term LED includes, but is not limited to, various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, and the like. In particular, the term LED refers to light emitting diodes of all types (including semi-conductor and organic light emitting diodes) that may be configured to generate radiation in one or more of the infrared spectrum, ultraviolet spectrum, and various portions of the visible spectrum (generally including radiation wavelengths from approximately 400 nanometers to approximately 700 nanometers). Some examples of LEDs include, but are not limited to, various types of infrared LEDs, ultraviolet LEDs, red LEDs, blue LEDs, green LEDs, yellow LEDs, amber LEDs, orange LEDs, and white LEDs (discussed further below). It also should be appreciated that LEDs may be configured and/or controlled to generate radiation having various bandwidths (e.g., full widths at half maximum, or FWHM) for a given spectrum (e.g., narrow bandwidth, broad bandwidth), and a variety of dominant wavelengths within a given general color categorization.

[0013] For example, one implementation of an LED configured to generate essentially white light (e.g., a white LED) may include a number of dies which respectively emit different spectra of electroluminescence that, in combination, mix to form essentially white light. In another implementation, a white light LED may be associated with a phosphor material that converts electroluminescence having a first spectrum to a different second spectrum. In one example of this implementation, electroluminescence having a relatively short wavelength and narrow bandwidth spectrum "pumps" the phosphor material, which in turn radiates longer wavelength radiation having a somewhat broader spectrum.

[0014] It should also be understood that the term LED does not limit the physical and/or electrical package type of an LED. For example, as discussed above, an LED may refer to a single light emitting device having multiple dies that are configured to respectively emit different spectra of radiation (e.g., that may or may not be individually controllable). Also, an LED may be associated with a phosphor that is considered as an integral part of the LED (e.g., some types of white LEDs). In general, the term LED may refer to packaged LEDs, non-packaged LEDs, surface mount LEDs, chip-on-board LEDs, T-package mount LEDs, radial package LEDs, power package LEDs, LEDs including some type of encasement and/or optical element (e.g., a diffusing lens), etc.

[0015] The term "light source" should be understood to refer to any one or more of a variety of radiation sources, including, but not limited to, LED-based sources (including one or more LEDs as defined above), incandescent sources (e.g., filament lamps, halogen lamps), fluorescent sources, phosphorescent sources, and high-intensity discharge sources (e.g., sodium vapor, mercury vapor, and metal halide lamps).

[0016] The term "lighting fixture" is used herein to refer to an implementation or arrangement of one or more lighting units in a particular form factor, assembly, or package. The term

"lighting unit" is used herein to refer to an apparatus including one or more light sources of same or different types. A given lighting unit may have any one of a variety of mounting

arrangements for the light source(s), enclosure/housing arrangements and shapes, and/or electrical and mechanical connection configurations. Additionally, a given lighting unit optionally may be associated with (e.g., include, be coupled to and/or packaged together with) various other components (e.g., control circuitry) relating to the operation of the light source(s). An "LED-based lighting unit" refers to a lighting unit that includes one or more LED-based light sources as discussed above, alone or in combination with other non LED-based light sources. A "multi- channel" lighting unit refers to an LED-based or non LED-based lighting unit that includes at least two light sources configured to respectively generate different spectrums of radiation, wherein each different source spectrum may be referred to as a "channel" of the multichannel lighting unit.

[0017] The term "controller" is used herein generally to describe various apparatus relating to the operation of one or more light sources. A controller can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A

"processor" is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. A controller may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

[0018] In various implementations, a processor or controller may be associated with one or more storage media (generically referred to herein as "memory," e.g., volatile and non-volatile computer memory such as random access memory (RAM), programmable read-only memory (PROM), electrically programmable read-only memory (EPROM), electrically erasable and programmable read only memory (EEPROM), floppy disks, compact disks, optical disks, magnetic tape, etc.). In some implementations, the storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform at least some of the functions discussed herein. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present invention discussed herein. The terms "program" or "computer program" are used herein in a generic sense to refer to any type of computer code (e.g., software or microcode) that can be employed to program one or more processors or controllers.

[0019] It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein. Brief Description of the Drawings

[0020] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

[0021] FIG. 1 is a block diagram illustrating a solid-state lighting unit, according to a representative embodiment.

[0022] FIG. 2 is a circuit diagram illustrating a solid-state lighting unit, according to a representative embodiment.

[0023] FIG. 3 is a graph showing current of a solid-state lighting unit over time, according to a representative embodiment.

Detailed Description

[0024] Applicants have recognized and appreciated that it would be beneficial to have a solid- state lighting fixture for large spaces, such as a theater lighting unit, that has sufficiently large resolution to enable smooth start-up, e.g., increase of intensity without a visible step. In view of the foregoing, various embodiments and implementations of the present invention are directed to a solid-state lighting fixture or system that includes an analog start-up circuit for initial powering-up period, thus enabling a large dimming range or large dimming resolution. The solid-state lighting system includes a downconverter circuit (e.g., hysteretic downconverter) and shunt switch combined with an analog startup circuit (e.g., linear regulator) in series with a solid- state load, such as an LED string, for creating a smooth startup and high resolution during dimming.

[0025] For example, solid-state lighting units required for lighting particularly large spaces, such as Studio, Stage and Television (SSTV) lighting systems, for example, need extremely large dimming ranges or resolutions to enable a smooth start-up. According to various embodiments, the extreme dimming ranges are enabled by combining a PWM dimmable current source with a linear regulator. The linear regulator is only used during a startup period of the system, during which the light source is taken from essentially fully dimmed (e.g., 99 to 100 percent dimmed) to within a normal operation range (e.g., 0 to 99 percent dimmed). After the start-up period, a switch or transistor in the linear regulator is driven fully on, resulting in low losses, and a PWM dimmable current source is enabled for controlling subsequent dimming levels of the system within the normal operation range.

[0026] FIG. 1 is a block diagram illustrating a solid-state lighting unit, according to one representative embodiment. Referring to FIG. 1, in one embodiment, SSL unit 100 includes switching regulator 105 and linear regulator 140, which operate a solid-state light source, indicated as representative LED string 130. The switching regulator 105 includes hysteretic downconverter 1 10 and shunt switch circuit 120. The hysteretic downconverter 105 and the shunt switch circuit 120 are combined in series with the LED string 130 and the linear regulator 140 for creating a smooth startup and a high resolution during dimming. A controller 150 controls the hysteretic downconverter 110, the shunt switch circuit 120 and the linear regulator 140 using respective control signals.

[0027] During a start-up period, the SSL unit 100 operates in a linear mode, in which the resistance of the linear regulator 140 gradually decreases from a very high resistance value at the beginning of the start-up period to a very low resistance value at the end of the start-up period (corresponding to the beginning of a normal operation period). Accordingly, the current through the LED string 130 gradually increases in response to the decreasing resistance of the linear regulator 140 until the current through the LED string 130 reaches the control value, which is the maximum current setting of the hysteretic buck. Subsequently, during the normal operation period, the SSL unit 100 operates in a switching mode, in which the shunt switch circuit 120 is selectively activated to generate a PWM signal for controlling the current through the LED string 130. The duty cycle of the PWM signal may be adjusted via the controller 150 to accommodate variations in dimming levels during the normal operation period.

[0028] FIG. 2 is a circuit diagram illustrating a solid-state lighting unit, according to a representative embodiment. FIG. 3 is a graph showing current provided by a solid-state lighting unit over time, according to a representative embodiment. In particular, FIG. 3 depicts current I_LED flowing through LED string 230 of FIG. 2, as discussed below, where a start-up period is indicated by time tO through time tl .

[0029] Referring to FIG. 2, SSL unit 200 includes switching regulator 205 and linear regulator 240, which operate an SSL source, indicated by the representative LED string 230. As discussed above with reference to FIG. 1, the switching regulator 205 includes hysteretic downconverter 210 and shunt switch circuit 220. The LED string 230 includes one or more LEDs, indicated by representative LEDs 231 and 232, connected in series between the switching regulator 205 and the linear regulator 240. A controller 250 controls the hysteretic

downconverter 210, the shunt switch circuit 220 and the linear regulator 240 through selective activation and control of control signals CTL_HDC, CTL_ss and CTL_LR, respectively, as discussed below. For the sake of clarity, FIG. 2 does not show various supporting circuitry, such as protection circuits, supply circuits, filtering circuits, and the like.

[0030] In the depicted embodiment, the hysteretic downconverter 210 includes first switch 21 1 connected between voltage source 210 and first node Nl , and inductor 214 connected between first node Nl and second node N2, which corresponds to inputs of the LED string 230 and the shunt switch circuit 220. The hysteretic downconverter 210 may also include a filter capacitor (not shown) between node N2 and node N3. The voltage source 201 provides input voltage VIN (e.g., about 24V or 48V) for powering the SSL unit 200. The first switch 21 1 may be a field-effect transistor (FET), such as such as a metal-oxide-semiconductor field-effect transistor (MOSFET) or a gallium arsenide field-effect transistor (GaAsFET), for example. Of course, various other types of switches and/or transistors may be implemented without departing from the scope of the present teachings.

[0031] The hysteretic downconverter 210 also includes resistor 212 and diode 215. The resistor 212 has a fixed resistance and is connected between third node N3 and fourth node N4, which is connected to ground voltage 202. The diode 215 has an anode connected to fourth node N4 and a cathode connected to first node Nl .

[0032] Operation of the first switch 21 1 is controlled by an output of operational amplifier 218, which is configured to compare the voltage of a (feedback) signal at third node N3 and the analog control signal CTLHDC output by the controller 250. For example, the control signal CTLHDC may be a predetermined analog reference voltage indicating the average current through the LED. Of course, the hysteretic downconverter 210 includes circuitry around operational amplifier 218 for generating the hysteresis, as would be apparent to one of ordinary skill in the art, although this circuitry is not shown in FIG. 2 for simplicity. When the voltage at third node N3 is the same as the reference voltage of the control signal CTLHDC, the first switch 21 1 is opened (e.g., the corresponding transistor is turned off), temporarily removing the voltage source 201 from the LED string 230, resulting in a slow reduction of the current ILED through the LED string 230 via the diode 215, as shown by a ripple effect of the current I_LED beginning at times tl and t4 of FIG. 3. The ripple effect may occur at a frequency of about 100kHz, and the difference between the high levels (e.g., at times tl , t4) and the low levels (e.g., at times t2, t5) of the ripple effect may be about 100mA, for example. The first switch 21 1 is initially cycled between closed and opened states after the start-up (ending at time tl) of the LED string 230, as well as throughout normal operation (an example of which begins at time t4) of the LED string 230.

[0033] The shunt switch circuit 220 includes second switch 221 connected between second node N2 and third node N3, so that it is connected in parallel with the LED string 230 and the linear regulator 240. Like the first switch 21 1 , the second switch 221 may be a FET, such as such as a MOSFET, a GaAsFET or the like, for example, although various other types of switches and/or transistors may be implemented without departing from the scope of the present teachings. Operation of the second switch 221 is controlled by the digital control signal CTLss output by the controller 250. In particular, the control signal CTLss has high and low signal levels, where the high signal level causes the second switch 221 to close (e.g., turning on the corresponding transistor) and the low level causes the second switch 221 to open (e.g., turning off the corresponding transistor).

[0034] Operation of the second switch 221 provides the duty cycle of a PWM signal, which drives the LED string 230 in accordance with the dimming level set by the dimmer (not shown) after the start-up. For example, the PWM signal has a high duty cycle in response to a high dimmer setting (e.g., providing a low amount of dimming), and the PWM signal has a low duty cycle in response to a low dimmer sitting (e.g., providing a high amount of dimming), as determined by the controller 250. FIG. 3 depicts an illustrative PWM signal responsive to operation of the second switch 221 , where the pulse width PW over period T beginning at time t4 indicates the duty cycle.

[0035] The linear regulator 240 includes transistor 241 and measuring shunt resistor 242, connected in series between the LED string 230 and third node N3. The transistor 241 may be a MOSFET, for example, although the transistor 241 may be implemented using various other types of transistors and/or other types of programmable resistors, without departing from the scope of the present teachings. For example, the transistor 241 may be a different type of current source or programmable resistor with a normal transistor. The measuring shunt resistor 242 has a fixed resistance. Assuming for purposes of illustration that the transistor 241 is a MOSFET, the transistor 241 includes a drain connected to an output of the LED string 230, a source connected to the measuring shunt resistor 242 and a gate connected to an output of operational amplifier 248.

[0036] In the depicted embodiment, the operational amplifier 248 outputs a feedback signal to the gate of the transistor 241 , discussed below, based on comparison of the voltage of the analog control signal CTL_LR provided by the controller 250 and the voltage at the source of the transistor 241. The feedback signal thereby dynamically adjusts the resistance of the transistor 241 (e.g., by changing the amount that the transistor 241 is turned on) during the start-up period. In an alternative embodiment, the linear regulator 240 does not include the operational amplifier 248, and the resistance of the transistor 241 is dynamically adjusted directly in response to the control signal CTL_LR provided by the controller 250. Also, in various embodiments, the measuring shunt resistor 242 may be shared with the hysteretic downconverter 210.

[0037] Generally, the controller 250 controls the SSL unit 200 to operate in a linear mode during start-up (from time tO to time tl), and to operate in a switching mode during normal operation (after time tl) in accordance with the level of dimming set by the dimmer (not shown), using the control signals CTLRDC, CTLSS and CTL_LR. For example, the controller 250 may receive digital values from an A/D converter (not shown) and determine the level of dimming based on the digital values, and output control signals CTLRDC, CTLSS and CTL_LR to the hysteretic downconverter 210, the shunt switch circuit 220 and the linear regulator 240, respectively.

[0038] The controller 250 may be constructed of any combination of hardware, firmware or software architectures, as discussed above, without departing from the scope of the present teachings. Also, in various embodiments, the controller 250 may include its own memory (e.g., nonvolatile memory) for storing software/firmware executable code that allows it to perform the various functions of the SSL unit 200. For example, the executable code may include code for identifying a start-up, for setting, generating and outputting control signals CTLRDC, CTLSS and/or CTLLR, for enabling adjustment of dimmer setting levels, and the like. Alternatively, the executable code may be stored in designated memory locations within separate ROM and/or RAM. The ROM may include any number, type and combination of tangible computer readable storage media, such as PROM, EPROM, EEPROM, and the like. In various embodiments, the controller 250 may implemented as a microcontroller, ASIC, FPGA, microprocessor, such as an ARM7 32-bit reduced instruction set computer (RISC) microprocessor, or the like.

[0039] Operation of the SSL unit 200 is now described with reference to FIGs. 2 and 3. Startup of the SSL unit 200 is detected by the controller 250 by receiving an "on" command or a certain dimming level while in "off or "standby" state, indicating that the SSL unit 200 is fully dimmed or nearly fully dimmed, such that the SSL unit 200 is operating in the linear mode. During the start-up period (time tO to time tl in FIG. 3), the first switch 21 1 of the hysteretic downconverter 210 is continuously closed (e.g., the corresponding transistor is turned on) in response to the output of the operational amplifier 218.

[0040] More particularly, the operational amplifier 218 may receive the control signal

CTLHDC from the controller 250 and a feedback signal from the linear regulator 240, and output a comparison signal to the first switch 21 1. The control signal CTLHDC may be a predetermined reference voltage, for example, which is compared to the voltage at third node N3. Until the voltage at third node N3 is the same as the voltage of the control signal CTL_HDC, the output of the operational amplifier 218 keeps the first switch 21 1 closed. When the switch 21 1 is closed, the LED string 230 is connected to the input voltage source 201 , building current I_L in the inductor 214 and current ILED through the LEDs 231 and 232. Meanwhile, the second switch 221 of the shunt switch circuit 220 is continuously opened (e.g., the corresponding transistor is turned off) during the start-up period in response to the control signal CTLss from the controller 250.

[0041] At this time, the linear regulator 240 is set to a very high resistance, which restricts current flow and ensures that only a very small current I_LED (e.g., about ΙΟΟμΑ) is able flow through the LEDs 231 and 232. That is, the transistor 241 of the linear regulator 240 is only slightly turned on in response to an output of the operational amplifier 248. More particularly, in the depicted embodiment, the operational amplifier 248 may receive the control signal CTLLR from the controller 250 and a feedback signal from the source of the transistor 241 , and output a comparison signal to the gate of the transistor 241. The control signal CTLLR may be a predetermined reference voltage, for example, which is compared to the voltage at the source of the transistor 241. Gradually, the resistance of the transistor 241 is lowered, thus increasing the current ILED through the LEDs 231 and 232, until the linear regulator 240 is set to its lowest resistance (e.g., the transistor 241 is fully on), which corresponds to time tl in FIG. 3. For example, during the start-up period, the value of the control signal CRL_LR may gradually increase, which value is compared to the voltage at the source of the transistor 241 , which also increases due to the control action of the operational amplifier 248. Thus, the gradual decrease in the resistance of the transistor 241 and corresponding increase in the current ILED continues until the current I_LED reaches the control value for the hysteretic downconverter 210, e.g., as determined by the operational amplifier 248.

[0042] At this time, the SSL unit 200 begins operating in the switching mode or normal operation (following time tl of FIG. 3). The first switch 21 1 is opened (e.g., the corresponding transistor is turned off), disconnecting the input voltage source 201 from the LED string 230. The first switch 21 1 then periodically switches between the opened and closed states, in response to feedback from the LED string 230 via the operational amplifier 218, thus providing the ripple effect discussed above. In an alternative embodiment, the switch 21 1 may be configured to operate in linear mode during the startup period, which would result in similar functionality with respect to gradual increasing the curing I_LED- Such a configuration requires the addition of a complex gate drive circuit or level shifter (not shown).

[0043] During normal operation, the current ILED is controlled by the hysteretic

downconverter 210, and (PWM-) dimming is achieved by PWM switching of the switch 212 of the shunt switch circuit 220. That is, the shunt switch circuit 220 generates and outputs a variable PWM signal to the LED string 230 in response to the control signal CTLss from the controller 250. In the depicted embodiment, the second switch 221 is repeatedly closed and opened to provide a square wave PWM signal to the LED string 230. As shown in FIG.3, for example, the shunt switch 221 is closed at time t3 and opened at time t4, etc., thus periodically shorting the LEDs 231 and 232. The shunt switch 221 thereby generates a PWM signal having a duty cycle indicated by pulse width PW within period T, under control of the controller 250.

[0044] Of course, the duty cycle of the PWM signal may vary, depending on the level of the dimmer setting determined by the controller 250. For example, the second switch 221 generates a PWM signal having longer pulse widths (i.e., longer duty cycles) in response to higher dimmer settings, and shorter pulse widths (i.e., shorter duty cycles) in response to lower dimmer settings. Thus, current is increased to the LED string 230 in response to larger pulse widths, and decreased in response to short pulse widths. Of course, other types of control signals and methods of controlling the LED string 230 may be incorporated within the scope of the present teachings.

[0045] During the transition from high to low resistance, the losses in the linear regulator 240 are relatively high. For example, the start-up from time tO to time tl may take about 100ms and is not repeated quickly, so the high losses in the linear regulator 240 will not result in a large increase in temperature in such a short time. Also, because the linear regulator 240 is set to the lowest resistance following the start-up (e.g., following time tl), low losses are achieved during normal operation of the SSL unit 200. In various embodiments, the set point of current for the linear regulator 240 can be controlled by the controller 250, for example, from a digital-to-analog converter (not shown) or a filtered high resolution PWM signal. It can also be controlled in an analog manner, for instance by a voltage on a capacitor (not shown), which is gradually charged by a current source.

[0046] The values of the various components of FIG. 2, such as the input voltage V_IN, the inductor 214, the resistors 212 and 242, and the transistor 241, may vary to provide unique benefits for any particular situation or to meet application specific design requirements of various implementations, as would be apparent to one skilled in the art.

[0047] Various embodiments may be implemented in a driver for a solid-state SSTV lighting system, for example. The SSTV driver may be used to power an SSL light engine designed for studios, theater or other large spaces.

[0048] While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0049] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0050] The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0051] The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and/or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and/or B", when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0052] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of."

[0053] As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0054] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited. Also, reference numerals, if any, are provided in the claims merely for convenience and are not to be read in any way as limiting.

[0055] In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding,"

"composed of," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of and "consisting essentially of shall be closed or semi-closed transitional phrases, respectively.

Claims

1. A device for controlling a solid-state lighting (SSL) source, the device comprising: a linear regulator (130, 230) configured to control current through the SSL source during a start-up period; and

a switching regulator (105, 205) configured to control the current through the SSL source following the start-up period, based on a dimming level.

2. The device of claim 1 , wherein the linear regulator comprises a programmable resistor (241) connected in series with the SSL source and configured to decrease in resistance during the start-up period, increasing the current through the SSL source.

3. The device of claim 2, wherein the programmable resistor comprises a metal-oxide- semiconductor field-effect transistor (MOSFET).

4. The device of claim 3, wherein the MOSFET comprises a drain connected to the SSL source, a source connected to a measuring shunt resistor and a gate connected to a gate control signal, the gate control signal incrementally turning on the MOSFET to increase the resistance.

5. The device of claim 4, wherein the linear regulator further comprises an operational amplifier (248) comprising a first input for receiving a reference signal from a controller (250), a second input for receive a source voltage from the source of the MOSFET, and an output for outputting the gate control signal.

6. The device of claim 1 , wherein the switching regulator provides a pulse width modulation (PWM) signal to control dimming of the SSL source following the start-up period.

7. The device of claim 6, further comprising:

a controller configured to set a duty cycle of the PWM signal in response to the level of dimming provided by a dimmer.

8. The device of claim 6, wherein the switching regulator comprises:

a hysteretic downconverter connected between an input power source and the SSL source; and

a shunt switch circuit connected in parallel with the SSL source and the linear regulator.

9. The device of claim 8, wherein the hysteretic downconverter comprises a switch and an inductor connected in series between input power source and the SSL source, and a diode connected between a ground voltage and the inductor.

10. The device of claim 9, wherein the hysteretic downconverter switch is continually closed and the shunt switch is continually open during the start-up period.

1 1. The device of claim 9, wherein the shunt switch circuit is selectively opened and closed to generate the PWM signal following the start-up period to control the dimming of the SSL source.

12. The device of claim 9, wherein the hysteretic downconverter switch is selectively opened and closed to generate a current ripple through the SSL source following the start-up period.

13. The device of claim 1, wherein the SSL source comprises at least one light- emitting diode (LED).

14. A device for selectively providing large dimming resolution of a solid-state lighting (SSL) source, the device comprising:

a hysteretic downconverter connected between an input power source and the SSL source, the hysteretic downconverter comprising a first switch that is continually closed during a start-up period of the SSL source;

a linear regulator connected in series with the SSL source, the linear regulator being configured to have a resistance that progressively decreases during the start-up period, causing corresponding increases in current through the SSL source for controlling dimming of the SSL source during the start-up period of the SSL source;

a shunt switch connected in parallel with the SSL source and the linear regulator, the shunt switch being continuously open during the start-up period of the SSL source and being periodically closed after the start-up period to provide a pulse width modulation (PWM) signal for controlling dimming of the SSL source after the start-up period; and

a controller configured to provide reference control signals to the hysteretic

downconverter and the linear regulator, respectively.

15. The device of claim 14, wherein the start-up period ends when the current through the SSL source increases to a control value.

16. The device of claim 14, wherein the controller is further configured to control a duty cycle of the PWM signal generated by the shunt switch.

17. The device of claim 14, wherein the hysteretic downconverter further comprises: an inductor connected in series between the first switch and the SSL source, and a diode connected between a ground voltage and the inductor; and

an operational amplifier configured to compare a voltage of the linear regulator with a voltage of a control signal provided by the controller, and to control the first switch to open when the voltage of the linear regulator matches the voltage of the control signal.

18. A device for selectively providing large dimming resolution of a light-emitting diode (LED) unit, the device comprising:

a hysteretic downconverter connected between an input power source and the LED unit, the hysteretic downconverter comprising a first switch that is continually closed during a start-up period of the LED unit;

a linear regulator connected in series between the LED unit and a ground voltage, the linear regulator comprising a metal-oxide-semiconductor field-effect transistor (MOSFET) configured to have a resistance that progressively decreases during the start-up period, causing corresponding increases in current through the LED unit;

a shunt switch connected in parallel with the LED unit and the linear regulator, the shunt switch being continuously open during the start-up period of the LED unit and being periodically closed after the start-up period to provide a pulse width modulation (PWM) signal for controlling dimming of the LED unit after the start-up period; and

a controller configured to provide a first control signal to the hysteretic downconverter for controlling operation of the first switch, a second control signal to the linear regulator for controlling the resistance of the MOSFET, and a third control signal to the shunt switch for controlling operation of the shunt switch.