FI20225837A1 - LED controller and LED lamp - Google Patents

Abstract

The LED driver (10) comprises a power stage (100) which produces an operating current (lout) for controlling the LED light source (12) in a wide output voltage range (Vout), which includes at least one low load voltage range and at least one high load voltage range. The voltage detecting circuit (VF) is arranged to detect the load voltage (Vf) acting across the LED light source. The controller (110) is configured to control the operation of the power stage (100) with different load control settings depending on whether the detected load voltage is in said at least one low load voltage range or in said at least one high load voltage range when the LED driver (10) is first connected to the LED light source (12) .

Description

LED DRIVER AND LED LUMINAIRE

FIELD OF THE INVENTION

The present invention relates to LED drivers.

BACKGROUND OF THE INVENTION

Solid-State Lighting (SSL) or so-called LED lighting uses light emitting semiconductor devices, such as inorganic light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) to convert electricity into light for illumination. In the following the acronym LED is used in a broad sense that covers all kinds of light emitting semiconductor devices, such as traditional LEDs, organic LEDs (OLEDs), laser diodes, and the like.

LED is a current-driven device whose luminance or brightness is pro- portional to its forward current when the LED is forward biased. LED is forward biased when the supply voltage across the LED meets the forward voltage or for- ward knee voltage (Vf) of the LED. The LED is driven by a LED power source or LED — driver that converts an AC or DC input power to a DC output voltage and of suitable magnitude so that the light radiated by the LED or LEDs has desired brightness, color temperature, and/or possible other characteristics. The output voltage of the driver shall meet a forward voltage rating of the LED to forward bias and light the

LED. When multiple LEDs are connected in series, the Vf and thereby the required output voltage of the driver will increase according to the number of LEDs in series.

A luminaire or so-called light fixture or light fitting is the entire con- struction containing the light source (so called lamp) that provides illumination. All luminaires have a fixture body and one or more light sources. In addition to the light source, the luminaire may include a power supply, a reflector, lens, a diffuser, a 25 lamp holder (socket) and electrical connection equipment. The electrical system of

N a light fixture may provide power, control or other electrical based functions such 3 as wires, sockets, switches, drivers, connectors, circuitry, and sensors. The optical

N components may include diffusers, lenses, prismatic elements, waveguides, reflec- = tors, refractors, louvers, etc.

N 30 The light source for LED light fixtures is often called an LED array or 3 module that may refer to an assembly of LEDs (components) or LED dies (or chips) a on a printed circuit board or substrate, oftentimes with optical elements for provid-

N ing a desired pattern of light distribution. In many applications, one or more LED chips or packages are mounted within an LED module. The LED module usually has electrical interfaces to couple to the load side of a current source which is typically a LED driver.

Majority of the LED modules used in Solid-State Lighting (SSL) are hav- ing high forward voltages. With SELV (Safety Extra Low Voltage) drivers commonly

LED loads or modules are used where several LEDs or LED dies are connected in series and the total forward voltage Vf of LED modules is 9V or above. Designing a

LED driver for such applications is relatively easy. Sometimes very small light fit- tings are used for architectural purposes or creating certain mood in the space. In these cases, single LEDs may be used as a light source, which have forward voltage

Vf ofi.e. 2.5-4V. When there is a lighting control in the installation, these low volt- — age light fittings must be controllable, e.g. with a Digital Addressable Lighting In- terface (DALI), and the LED-driver must support this. DALI allows users to obtain very precise output control with a dimming range from 100% (full brightness, max- imum light intensity) down to a minimum dimming value 0.1% of the maximum light intensity. To save costs in logistics and minimize the number of LED drivers used in products, luminaire manufacturers want to use same LED driver for both single LED architectural light fittings fixtures and other LED light fixtures illumi- nating the space.

Presently there are separate very low power LED-drivers for single LED applications and then LED drivers for high voltage applications. There are also some LED drivers can support both low and high voltages, but these are expensive solutions and difficult to design. Designing a LED driver which can operate over such a wide output voltage range, e.g. from 2.5 to 55 V is expensive, difficult and time consuming. For example, designing dimming and interference management with low voltage loads at low dimming levels is difficult and costly.

Therefore, there is need for a simple and inexpensive LED driver that

N can support both low and high load voltages.

N

N BRIEF DESCRIPTION OF THE INVENTION

3 An object of the present invention is to provide a LED driver that can

N support both low and high load voltages. The object of the invention is achieved by

E 30 a LED driver and a LED luminaire as recited in the independent claims. The pre-

N ferred embodiments of the invention are disclosed in the dependent claims. & An aspect of the invention is a LED driver, comprising

N a power stage having an output connectable to a LED light source for

N provision of an operating current for driving the LED light source, a controller configured to control the operation of the power stage,

a voltage sensing circuit configured to sense a load voltage across the

LED light source, and wherein an output voltage range of the power stage comprises at least one low load voltage range and at least one high load voltage range, and the controller is configured to control the power stage with different load control settings depend- ing on whether the sensed load voltage is within the at least one low load voltage range or within the at least one high load voltage range, when the LED driver is first time connected to the LED light source.

In an embodiment, the controller is configured to select the load voltage range of the LED light source based on the sensed voltage and select load control settings associated with the selected load voltage range for controlling the power stage.

In an embodiment, the at least one low load voltage range comprises load voltages of LED light sources with a single-LED configuration, preferably load — voltages down to 2,5 V, optionally also load voltages of LED light sources with a multiple LED configuration with up to two, three or four series connected LEDs.

In an embodiment, the at least one low load voltage range comprises a first low load volt-age range covering load voltages of LED light sources with a sin- gle-LED configuration, preferably load voltages down to 2,5 V, and a second load — voltage range covering load voltages of LED light sources with a multiple LED con- figuration up to two, three or four series connected LEDs.

In an embodiment, the at least one load voltage range includes load volt- ages of LED light sources having multiple-LED configuration up to at least five se- ries connected LEDs, preferably at least ten series connected LEDs.

In an embodiment, the load control settings include control settings of

N a dimming operation for load voltage ranges within the overall output voltage

AN range supported.

N In an embodiment the controller is configured to select the load voltage 7 range of the LED light source based on the sensed voltage and select load control

N 30 settings associated with the selected load voltage range for controlling the power

E stage.

N In an embodiment, the controller is configured to disable a dimming op- & eration or limit the dimming operation of LED driver in response to the sensed load

N voltage of the connected LED light source being within the at least one low load

N 35 voltage range, and where-in the controller is configured to enable full dimming op- eration of the LED driver in response to the sensed load voltage of the connected

LED light source being within the atleast one high load voltage range.

In an embodiment, the controller is configured to disable a dimming op- eration of the LED driver in re-sponse to the sensed load voltage of the connected

LED light source being within the first low load voltage range, and wherein the con- troller is configured to limit the dimming operation of LED driver in response to the sensed load voltage of the connected LED light source being within the second high load voltage range, and the controller is configured to enable full dimming op- eration of the LED driver in response to the sensed load voltage of the connected

LED light source being within the at least one higher load voltage range.

In an embodiment, the load control settings include preset values of the nominal load current for the load voltage ranges.

In an embodiment, the controller is configured to select the load voltage range of the LED light source based on the sensed voltage and select the preset value of the nominal load current associated with the selected load voltage range — for driving the LED light source used.

In an embodiment, the controller is configured to monitor the sensed load voltage during the operation of the LED driver, the controller being configured to detect a short circuit of one of LEDs in the LED light source if the monitored sensed load voltage drops from the initially sensed load voltage, and wherein the controller is configured to store the information on the detected short circuit in a memory of the LED driver, and/or the controller is configured to communicate the information on the detected short circuit to an external lighting controller or con- trol system, and/or the controller is configured to automatically control the power stage to drive the LED light source with a new higher nominal current so as to com- — pensatethereduction in the light level caused by the detected short circuit.

N In an embodiment, the power stage comprises a switched-mode power

O stage. 6 Another aspect of the invention is a LED luminaire, comprising a LED ? light source and a LED driver according to the first aspect.

N 30

I

=

N BRIEF DESCRIPTION OF THE DRAWINGS

& In the following the invention will be described in greater detail by

N means of exemplary embodiments with reference to the accompanying drawings,

N in which

Figure 1 is a simplified block diagram illustrating an exemplary light-

emitting diode (LED) lighting arrangement;

Figure 2A, 2B and 2C illustrate a switching frequency signal, a PWM dimming signal, and a resulting gate drive signal:

Figure 3 is a circuit diagram illustrating an exemplary load voltage sens- 5 ing circuit;

Figures 4A, 4B and 4C illustrate example of wide output voltage ranges

Vout having two, three and N load voltage ranges, respectively;

Figures 5A and 5B illustrate examples of dimming control settings for different load voltage ranges;

Figure 6A shows a graph depicting a load current versus a DALI lighting level for a normal or full DALI dimming control;

Figure 6B shows an example of a limited dimming range according to an embodiment applied to the DALI dimming control of Fig. 6A;

Figure 7 is a flow diagram illustrating an exemplary operation of the controller;

Figure 8 illustrates examples of current control settings for different load voltage ranges; and

Figure 9 is a flow diagram illustrating another exemplary operation of the controller.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a simplified block diagram illustrating an exemplary light-emit- ting diode (LED) lighting arrangement wherein a LED driver apparatus 10 has a power input connected to receive an input power, e.g. an input voltage Vin, and power output terminals LED+, LED- connectable to an LED light source 12, and the — LED driver apparatus is configured to control the amount of output power deliv-

N ered to the LED light source 12 and thus the light intensity of the LED light source.

N The LED driver apparatus 10 may be configured to supply an output (load) voltage <Q Vout required across the LED light source 12 and an output (load) current lout of

N suitable magnitude so that the light radiated by the LED light source has the desired

E 30 intensity, brightness, color temperature, and/or possible other characteristics. A

N power source 14 illustrated in Fig. 1 represents generally any power supply or & power source that is configured to and able to provide a suitable alternating cur-

N rent (AC) or direct current (DC) operating power Vin to the LED driver apparatus

N 12. Typically, the operating power source 14 may be a mains voltage supply (such as 230 VAC, 50Hz) via a power grid.

In embodiments of the invention, the LED driver apparatus 10 includes at least a power stage 100 and a control circuit, such as a controller 110, configured to control the operation of the power stage 100. The power stage 100 is configured to receive the input power, e.g. the input voltage Vin, and to convert it into the out- put (load) voltage Vout required across the load 12 and the output (load) current lout driving the LED light source 12.

In embodiments of the invention, the power stage 100 comprises a con- stant-current power stage providing a constant output (load) current lout over a range of output voltages or load voltages Vout, i.e. a range of different forward volt- ages Vf of the LED light sources. The controller 110 is configured to provide control signals CTRL to the power stage 100 for adjusting the magnitude of the output volt- age or the load voltage Vout and the magnitude of the output current lout, or the load current 12. The required constant load current lout is established by feedback adjustment of the load current. The output voltage Vout of the power stage is reg- ulated to a load voltage required across the LED light source 12 used, taking into account the feedback adjustment of the load current. The exemplary LED driver apparatus 10 illustrated in Fig. 1 may comprise a load current sensing or feedback circuit CF that provides a load current feedback signal cf representative of the mag- nitude of the load current to the controller 110. The controller 110 may be config- ured, in response to the load current feedback cf, to control the power stage 100 to adjust the magnitude of the output or load current lout to a desired target load current to thus control the intensity of the LED light source 12 to the desired target intensity. For example, there may be a current sensing resistor providing a voltage representative of the magnitude of the load current, and the controller 110 may compare the sensed voltage with a reference voltage representative of desired tar-

N get load current and control the power stage 100 to minimize the error between

AN the compared voltages and thereby adjust the load current to the desired target.

N The controller 110 may comprise, for example, a digital controller or any other 7 suitable processing device, such as, for example, a microcontroller, a programma-

N 30 ble logic device (PLD), a microprocessor, an application specific integrated circuit

E (ASIC), or a field-programmable gate array (FPGA).

N In embodiments of the invention, the controller 110 may be provided & with or coupled to a solid-state memory 170 for storing operational parameters of

N the LED driver 100.

N 35 In embodiments, the LED driver apparatus 12 may comprise a control interface circuit 16, which may be coupled to, for example, a wired communication link or a wireless communication link for receiving external control commands from a lighting control device or system. The controller 110 may be operable, in response to receiving control commands via the control interface 16, for example for turning on/off the light, dimming up/down the light, configuring or updating one or more of operational parameters of the LED driver apparatus 12, e.g. those stored in the memory 170. The lighting control device may be, for example, a wall switch, a dimmer, a proximity sensor, a lighting system controller, etc. As an exam- ple, the control interface 16 may be a Digital Addressable Lighting Interface (DALI) that carries a digital signal according to a DALI protocol from a DALI controller to the LED driver apparatus 12 over a DALI bus, typically a pair of wires, or wirelessly (DALI+).

In embodiments of the invention, the LED driver apparatus 12 com- prises a switching power stage or a switched-mode power stage 100. Switched mode power supplies (SMPS) are known per se, and the configuration or practical implementation of a switched-mode power stage 100 is not essential to the present invention. Therefore, the description of the exemplary switched mode power stage 100 illustrated in FIG. 1 will be kept brief. Generally, a switched mode power stage 100 controls the flow of power from a power source 14 to a load 12 by controlling the "ON" and "OFF” duty cycle of one or more switching elements at a high switch- ing frequency in order to regulate the DC output voltage Vout across the output terminals LED+, LED- of the power stage. Output voltage Vout may also be regu- lated through variation of the switching frequency. Typically, a switching fre- quency fsw may from tens to hundreds of kHz. In embodiments, the switched mode power stage 100 comprises a switched-mode DC-DC converter that receives the input DC voltage and converts it into a pulsed output waveform, which may then

N be smoothed using an energy storage element. Examples of switched-mode DC-DC

AN converters include a buck (step-down) converter, a boost (step-up) converter, a

N buck-boost converter, a cuk converter, a flyback converter, a single-ended primary- 7 inductor converter (SEPIC), an LLC resonant converter, half-bridge converter, for-

N 30 ward converter, etc. The switching element typically comprises a semiconductor

E power switch, such as a metal oxide semiconductor field effect transistor

N (MOSFET) or a bipolar junction transistor (BJT). The energy storage element may & comprise an inductance or a capacitance or a combination of both. The DC-DC con-

N verter regulates the output voltage by switching the power switch between ON

N 35 (conducting) and OFF (non-conducting) states at high frequencies. When the power switch is in 'ON' mode, a current in the capacitor or inductor ramps up and energy is stored. When the power switch turns off the energy stored in the capaci- tor or inductor is released into the load. The switching frequency or the duty cycle of the one or more switching elements may be controlled by a gate drive signal

CTRL provided by the controller 110. The gate drive signal CTRL may be, for exam- ple, a pulse-width-modulated (PWM) signal, such that the duty cycle of the one or more switching elements is determined by relative pulse-widths of the PWM signal.

If the power source 14 is an AC mains grid, the switched-mode power stage 100 must meet several requirements regarding the frequency and the quality of the waveforms that are linked to the grid. Firstly, it may be necessary to keep the levels of Electromagnetic Interference (EMI) contents introduced into the grid low by a suitable EMI filter circuit provided at an input of the power stage 100. Further, a rectifier circuit is needed to rectify the (filtered) inputted AC mains voltage and to generate a rectified unregulated voltage. Further, a power factor correction (PFC) circuit may be needed for the rectified voltage to correct the power factor to adjust the power factor of the power stage 100 towards a power factor of one from a significantly lower power factor that results from the voltage and current being out-of-phase. The power stage 100 may comprise a single switched-mode DC-DC converter with constant output current regulation that also acts as a power factor correction (PFC). This is called a single-stage (SS) driver topology. For example, flyback converter may be used in a SS driver topology. In a so-called two-stage (TS) driver topology, which is used particularly in high-power applications, the power stage 100 may comprise two switched-mode DC-DC converters, where the first DC-

DC converter acts as a PFC and the second DC-DC converter performs output cur- rentregulation. For example, the first converter may be a boost (step-up) converter operating as a PFC, and the second converter may be a buck (step-down) converter

N regulating the LED current. Moreover, the power stage 100 may have an isolated

O topology wherein the AC input side (connected to the mains) and the output side 6 (connected to the LED light source) are galvanically isolated, typically by a power ? transformer. Alternatively, the power stage 100 may have a non-isolated topology

N 30 — withouta galvanic isolation (e.g. there is no power transformer) so that the input

E side and output side are directly electrically connected. Further, the output voltage

N may be limited for safety reasons. Safety regulations, such as Safety Electrical Low & Voltage (SELV) reguirements limit any potential to 60 V DC, or 42.4 V AC, so that

N no touchable conducting parts of the LED lighting arrangement have a voltage ex-

N 35 ceeding this limit.

In embodiments of the invention, the controller 110 is adapted to control dimming operation of the power stage 100 to change luminance or bright- ness of the LED light source. The dimming operation may comprise two stages: the controller 110 may receive a dimming request in the form of control signals or commands from an external dimming circuit (dimmer) or control device may send adimmingrequestin the form of control signals or commands, for example via the control interface 16, and the controller interprets the control signal and regulates the load current lout flowing through to the LED light source 12 to adjust the light output as requested.

Embodiments of the invention are not limited to any specific dimming techniques. Dimming arrangements used in lighting applications are typically based on two basic dimming techniques adjusting the average value of the current: an analogue or linear dimming technique and a PWM dimming technique. The pre- ferred technique depends on the dedicated lighting application. In some cases, also a combination of linear and PWM dimming technique may be applied.

In the linear dimming technique, the controller 110 may adjust the LED light output by directly modifying or controlling the load current lout through the

LED light source 12 depending on the amount of light that is desired. Thus, the dim- ming of the lighting is carried out with current control. The controller 110 may lin- early reduce the load current value from a nominal load current value that provides the full brightness, to dim the LED light source 12 as requested, for example by adjusting a target current value of the LED light source 12 (e.g. a reference voltage in a current control loop). Therefore, the linear dimming is also called a constant current reduction (CCR) dimming. With the linear dimming, 50% brightness is achieved by applying 50% of the nominal load current. The main disadvantage of thelinear dimming is that this variation in the current not only controls the amount

N of light, but it also affects the colour temperature (i.e. the peak value of the current

AN coincides with the average value). The amount of light emitted by an LED depends

N on the average value of the supplied current while the colour temperature of the 7 emitted light depends on the peak value of the supplied current.

N 30 In the PWM dimming, the amount of light from the LED light source is

E proportional to a duty cycle of a PWM controlled constant current and dimming is

N achieved by changing the duty cycle. In this way, the amount of light is controlled & by the average value of the PWM current (i.e. by the duty cycle) while the temper-

N ature of the light is defined by its peak value. With the PWM dimming, the load cur-

N 35 rentcan be changed from about 0% to 100% of the nominal current value. For 50% brightness, the nominal current is applied at a 50% duty cycle. The PWM frequency shall be high enough so that the load current is not affected by a low frequency ripple that is not filtered in the LED light source and affects the emitted light too, causing so-called flickering. It is commonly perceived that most humans cannot consciously detect light frequencies above 200 Hz. Thus, a PWM frequency higher than about 200 HZ, preferably higher than 400 Hz, still more preferably higher than 750 Hz may be sufficient so that it is filtered by the human eye and flickering is not a problem. With the PWM dimming, the load current can be changed from about 1% to 100% of the nominal current value. For 50% brightness, the nominal current is applied ata 50% duty cycle.

In the following, two exemplary PWM dimming approaches for a switched-mode power stage are illustrated. In the first approach, a PWM dimming signal is applied directly to the high frequency gate drive signal CTRL provided by the controller 110. Figure 2A illustrates a switching frequency of frequency fsw re- quired for the power stage 100 to operate the one or more switching elements in — order to supply the output voltage Vout and the nominal load current lout of suit- able magnitude to the LED light source 12. This may be the gate drive signal CTRL without dimming operation or with full brightness. Fig. 2B illustrates a PWM dim- ming signal of frequency Fdim that is much lower than the switching frequency.

The PWM dimming frequency fdim may be superimposed on the switching fre- quency sw, and the resulting gate drive signal CTRL of control frequency fctrl for the power stage 100 may be as illustrated in Fig. 2C. The switching elements of the power stage 100 are switched ON and OFF at the switching frequency fsw and the nominal load current is supplied only during ON states of the PWM dimming signal, and the nominal load current is discontinued during the OFF periods of the PWM dimming signal. The dimming is performed by reducing the average load current

N by changing the dimming control duty cycle ratio (ON/OFF) while the current re-

AN mains at the peak value during the ON periods. The second dimming approach may

N be to directly apply the PWM signal toa target current value of the LED light source ? 12 (e.g. a reference voltage in a current control loop), while the gate drive signal

N 30 CTRL from the controller 110 is not affected (e.g. it may be as illustrated in Fig. 2A).

E Thus, the dimming of the lighting is carried out with current control but now the

N reference of the nominal load current is unchanged during the ON periods of the & dimming signal and set to zero during the OFF periods of the dimming signal. Again,

N the average load current is adjusted based on the dimming control duty cycle ratio

N 35 (ON/OFF) but the current remains at the peak value during the ON periods.

The acronym LED as used herein refers generally to all types of light emitting semiconductor devices, such as traditional LEDs, organic LEDs (OLEDs), graphene LEDs, laser diodes, and the like.

The exemplary LED light source 12 is shown as a plurality of LEDs con- nected in series. i.e a LED string, but may comprise a single LED, or a plurality of

LEDs connected in parallel, or a plurality of LED strings connected in parallel, or a suitable combination thereof, depending on the particular lighting system. The out- put voltage Vout of the driver apparatus 10 shall meet a forward voltage rating Vf of the LED light source 12 to forward bias and light the LED or LEDs. Typically, a forward voltage Vf of a single LED may be in a range from 2,5 V to 4 V. Thus, the required output voltage Vout of the LED driver apparatus 10 for a LED light source with a single LED (or single LEDs in parallel) may be as low as 2,5 V. When multiple

LEDs are connected in series, the total Vf of the LED light source 12 and thereby the required output voltage Vout of the LED driver apparatus 10 will increase ac- cording to the number N of LEDs in series. For example, series connection of 10

LEDs may require an output voltage Vout of 25 V to 40 V. The number of parallel connected single LEDs or LED strings multiply the output current lout required for the LED light source 12.

Although a majority of the LED modules used in Solid-State Lighting (SSL) have several LEDs or LED dies are connected in series and the forward volt- age Vfis 9V or above, sometimes very small light fittings with single LEDs may be used for architectural purposes or creating certain mood in the space. To save costs in logistics and minimize the number of LED drivers used in products, luminaire manufacturers want to use same LED driver for both single LED architectural light fittings fixtures and other LED light fixtures illuminating the space.

Presently there are separate very low power LED-drivers for single LED

N applications and then LED drivers for high voltage applications. There are also

AN some LED drivers can support both low and high voltages, but these are expensive

N solutions and difficult to design. Designing a LED driver which can operate over 7 such a wide output voltage range, e.g. from 2.5 to 55 V or above is expensive, diffi-

N 30 cultand time consuming. For example, designing dimming and interference man-

E agement with low voltage loads at low dimming levels is difficult and costly.

N According to an aspect of the invention a LED driver comprises a power & stage having an operating current output connectable to a LED light source for pro-

N vision of an operating current for driving the LED light source, a controller config-

N 35 ured to control the operation of the power stage, and a voltage sensing circuit con- figured to sense a load voltage across the LED light source, and wherein the controller is configured to detect the load voltage of the LED light source and select different load control settings or operating parameters for the LED driver depend- ing on whether the detected load voltage is within a first lower load voltage range or within at least one higher load voltage range, when the LED driver is first time connected to the LED light source. Thereby, the same LED driver having a wide output voltage range can be used for both single LED applications with a low load voltage (e.g. 2,5) and multiple LED applications with high load voltages (e.g. up to 60 V and higher) by controlling the LED load in a different manner depending on the detected load voltage of a LED module or array used. The operation of the — driver can be adapted to be different for different load voltage ranges within the wide output voltage range simply by setting different load control settings or op- eration parameters for the different load voltage ranges. The controller may allow operation similar to that of conventional high-voltage drivers, when the detected load voltage is within a higher load voltage range and change the operation within lower load voltage range or ranges. No complicated and expensive circuit designs are needed as in conventional LED drivers that can support both low and high load voltages and operate in the same way over the whole output voltage range.

An exemplary output voltage range Vout from about e.g. 2,5 V to a max- imum Vout Max (e.g. 40 V, 55V, 60V, ..., 400 V) is illustrated in Fig. 4A. In embodi- ments of the invention, the lower end of the output voltage range may in practice be set by the lowest forward voltage Vf of single LED light sources used. The maxi- mum Vout depends on the lighting application in question. The wide output voltage range is subdivided into a low load voltage range 42 that may include output volt- ages from 2,5 V to a load voltage threshold th2, and into a high load voltage range 44 that may include output voltages above the threshold th2 up to the maximum

N Vout Max. In exemplary embodiments, the load threshold th2 may be e.g about 8-

AN 10 V or more, so that LED light sources 12 with a single LED or a series connection

N of two LEDs may be included within the low load voltage range 42 and LED light ? sources 12 with a series connection of three or more LEDs may be included within

N 30 — the high voltage range 44. However, it shall be appreciated that the threshold th2

E can vary depending on a specific application. The threshold th2 may be lower the

N 8 V, e.g. it may be egual to a threshold th1 discussed below. The threshold th2 may & be higher than 10 V, particularly when the maximum output voltage Vout Max is

N high, e.g. over 60 V. As another example, the threshold th2 may be about 15% to

N 35 20%0ofthe maximum output voltage Vout Max.

In the example illustrated in Fi. 4B, the exemplary output voltage range

Vout from about e.g. 2,5 V to a maximum Vout Max (may be similar to the one de- scribed with reference to Fig. 4A) is subdivided into two low load voltage ranges 42-1 and 42-2, and into a high load voltage range 44. The first low load voltage range 42-1 may include output voltages from 2,5 V to a load voltage threshold th1, and the second low load voltage range 42-2 may include the output voltages above the threshold th1 up to a threshold th2. In exemplary embodiments, the load threshold th1 may be e.g about 4-6 V, so that LED light sources 12 with a single LED will be included within the first low load voltage range 42-1. The threshold th2 may be similar to the threshold th2 discussed with reference to Fig. 4A, so that, so that

LED light sources 12 with a series connection of two LEDs may be included within the second low load voltage range 42-2 and LED light sources 12 with a series con- nection of three or more LEDs may be included within the high voltage range 44.

In the examples illustrated in Figs. 4A and 4B there is only one “high load voltage range” 44 but embodiments of the invention are not limited to one. In — the example illustrated in Fig. 4C, the exemplary output voltage range Vout from about e.g. 2,5 V to a maximum Vout Max (may be similar to the one described with reference to Fig. 4A) is subdivided into two low load voltage ranges 42-1 and 42-2 (e.g. similar to Fig. 4B) and into a plurality (N) of high load voltage ranges 44-1 to 44-N, wherein N=2, 3, .... The subdivision into multiple ranges allows using more accurate or optimal load control settings for each narrower range, which may sim- plify designing and improve the overall performance of a LED driver having a very wide output voltage range. In the example of Fig. 4C, the first high load voltage range 44-1 may include output voltages from above the threshold th2 up to a threshold th3, the second high load voltage range 4421 may include output volt- ages from above the threshold th3 up to a threshold th4, etc. The number N of the

N load voltage ranges, the threshold values and/or the widths of the load voltage

O ranges may vary depending on a specific application. 6 Referring to Fig. 1, aload voltage sensing circuit is generally presented 7 by a voltage feedback block VF in the power stage 100 of the exemplary LED driver

N 30 apparatus 10. The load voltage sensing or feedback circuit VF may provide a load

E voltage feedback signal vf representative of the magnitude of the load voltage

N Vout/Vf to the controller 110. An exemplary load voltage sensing circuit VF is il- & lustrated in Fig. 3. A voltage divider formed by resistors R1 and R2 is connected

N between the high-end output terminal LED+ and a ground potential to form a

N 35 sensed load voltage vfrepresentative of the magnitude of the load voltage Vout. As another example, the load voltage sensing circuit VF may comprise a voltage divider formed by resistors R1 and R2 is connected between the low-end output terminal LED- and a ground potential to form a sensed load voltage vf representa- tive of the magnitude of the load voltage Vout. The controller 110 may be config- ured to compare the load voltage feedback signal, e.g. the sensed load voltage vf, with one or more predetermined thresholds (e.g. thresholds th1, th2, th4 illus- trated in Figs. 4A, 4B, and 4C) or at least two load voltage ranges (e.g. load voltages ranges 42, 42-1, 42-2, 44, 44-1, 44-2,..., 44-N illustrated in Figs. 4A, 4B, and 4C) to detect the load voltage or load voltage range used by the LED light source 12, when the light source 12 is connected to the driver 10. For example, the controller 110 — may detect that the LED light source 12 has a load voltage Vout within a lowest load voltage range 42-1, if the sensed load voltage is below or equal to a first lowest predetermined threshold th1, that the LED light source 12 has a load voltage Vout belonging to a second higher load voltage range 42-2 if the sensed load voltage is above the first threshold th1 and below or equal to a next higher threshold th2, etc.

The controller 110 may then select predetermined load control settings of the de- tected load voltage range to automatically adapt the operation of the LED driver 12 to the light source 12 being used.

In embodiments, the load control settings include different control set- tings of a dimming operation for different load voltage ranges within the wide out- putvoltage range supported. In embodiments, control settings may define a dim- ming operation with a full dimming range atleast for the highestload voltage range, and control settings may disable the dimming operation or define a limited dim- ming operation with a limited dimming range at least for lowestload voltage range.

At low dimming levels and low load voltages the load current lout of the LED driver 10 involves a high-frequency ripple having an amplitude that may be 30 % or more

N of the current amplitude, which may cause instability of the power stage 100 and

AN unwanted fluctuation of the load current. A switched-mode power stage, as a high-

N frequency switching device, is by nature a significant potential source of instability. ? For a LED driver, the stability is particularly significant, as any unwanted fluctua-

N 30 tions in current are rapidly visible as unwanted fluctuations in the light generated

E by the LED. Limiting the lowest dimming levels out of the dimming range, or disa- 5 bling the dimming, at the low load voltages simplify designing of the LED driver x having a wide output voltage range. Moreover, especially with single LED applica-

N tions the low dimming levels or dimming at all may not be a relevant or a required

N 35 feature, because the light output of a single LED is already low or the architectural or otherwise special nature of single LED applications.

Figs. 5A and 5B illustrate examples of dimming control settings for dif- ferent load voltage ranges 42 and 44 of Fig. 4A, and 42-1, 42-2 and 44 of Fig. 4B, respectively. In Figs. 5A and 5B, the control settings associated with the high load voltage range 44 will provide a normal or full dimming operation, such as a dim- mingrange from about 1% to about 100% ofa of the nominal (highest) load current value (100% corresponds to full brightness). In Figs. 5A and 5B, the control settings associated with the low load voltage range 42 or 42-1 may disable a dimming op- eration so that no dimming is possible but the LED light source 12 has full 100% brightness. As another example, the control settings associated with the low load voltage range 42 may limit the dimming operation or dimming range at least at the lower end of dimming range, eg. dimming may be possible only down to about 10% of the nominal current. Optionally, also the upper end of the dimming range may to be less than 100%. In an embodiment, control settings for a lower limit, optionally also upper limit, of a dimming range may comprise a minimum load current, op- tionally also maximum load current allowed in the dimming operation. Fig. 6A shows a graph depicting a load current versus a DALI lighting level for a normal or full DALI dimming control, wherein 1 means 0.1% of full brightness, 254 means full brightness, and other values being logarithmically interpolated, giving a 2.77% in- crease in current per step. Fig. 6B shows an example of a limited dimming range 62 (dotted line) according to an embodiment applied to the DALI dimming control.

The control settings may include a minimum DALI level (minimum brightness) al- lowed and optionally a maximum DALI level (maximum brightness) allowed, or al- ternatively a minimum load current Imin (minimum brightness) allowed and op- tionally a maximum load current Imax (maximum brightness) allowed, both result- ingin the limited dimming range 62.

N An exemplary operation of the controller 110 isnow described with ref-

AN erence to Fig. 7. Let us assume that the LED driver 12 have the load voltage ranges,

N thresholds and load control settings for dimming as discussed above with refer- 7 ence to Figs 4B and 5B. First, the controller 110 starts the LED driver 10 with a new

N 30 LED light source 12 being connected to the output of the LED driver (step 71). The

E controller 110 controls the power stage 100 to drive the LED light source with a

N preset nominal value of a load current lout (step 72). The voltage sensing circuit & VB senses a load voltage Vout affecting across the LED light source 12 and provides

N a load voltage feedback signal vf representative of the magnitude of the sensed load

N 35 voltage Vout to the controller 110 (step 73). The controller 110 selects the load voltage range and the load control settings to the LED light source 12 based on the sensed load voltage Vout as follows: if the sensed load voltage Vout is below or equal to the load voltage threshold th1 in step 74, select the load voltage range 42-1 for the LED light source 12 and disable the dimming according to the control setting associated with the range 42-1 (step 75), and if not, proceed to step 76, if the sensed load voltage Vout is above the threshold th1 and below or equal to the next higher load voltage threshold th2 in step 76, select the load volt- age range 42-2 for the LED light source 12 and apply the limited dimming range according to the control setting associated with the range 42-2 (step 77), and if not, select the load voltage range 44 for the LED light source 12 and apply the limited dimming range according to the control setting associated with the range 44 (step 78), and continue the LED driver operation with the selected load voltage range the selected dimming settings (step 79).

In embodiments, the load control settings for the load voltage ranges may include preset values of the nominal output current lout stored in the memory 120 of the driver 10. Each load voltage range within the wide output voltage range supported by the LED driver 10 is associated with one of the preset nominal cur- rents. When first connected to a LED light source 12, the controller 110 detects the load voltage Vout and thereby the load voltage range of the LED light source 12 used. The controller 110 may start the LED driver 10 with the lowest load current configured for the LED driver so as to avoid excessive load currents to the unknown

LED light source 12, and the controller may increase the load current until the load voltage is properly detected. Then the controller 110 sets the preset nominal out- put current associated with the detected load voltage range as the nominal current

N for the LED light source 12. Thereby, the LED driver 10 is able to automatically

AN select the correct nominal current for the light source being used. In this manner,

N a luminaire manufacturer manufacturing luminaires with different LED configura- 7 tions may use a single LED driver type and preconfigure and store nominal currents

N 30 matching each LED configuration into the LED driver 10 which then automatically

E adapts the operation to the LED configuration used.

N Fig. 8 illustrates an example of load voltage ranges 42-1, 42-2 , 44-1, 44- & 2, and 44-N of Fig. 4C with associated preset nominal currents 11, 12, 13, 14, ..., IN,

N respectively, as control. N = 2, .., etc. Similarly, the two load voltage ranges 42 and

N 35 43 of Fig. 4A may be associated with preset nominal currents 11 and 12, or the three load voltage ranges 42-1, 42-2 and 43 of Fig. 4B may be associated with preset nominal currents 11, 12 and 13, for example. In some cases, the preset nominal cur- rents of two voltage ranges may be equal, such as 13=14.

Another exemplary operation of the controller 110 is now described with reference to Fig. 9. Let us assume that the LED driver 12 have the load voltage ranges, thresholds and associated nominal currents as discussed above with refer- ence to Figs 4C and 8. First, the controller 110 starts the LED driver 10 with a new

LED light source 12 being connected to the output of the LED driver (step 91). The controller 110 may control the LED driver 10 to drive the LED light source 12 with the lowest load current configured for the LED driver 12 (step 92). The voltage sensing circuit VB senses a load voltage Vout affecting across the LED light source 12 and provides a load voltage feedback signal vf representative of the magnitude of the sensed load voltage Vout to the controller 110 (step 93). The controller 110 selects the load voltage range and the load control settings to the LED light source 12 based on the sensed load voltage Vout as follows: if the sensed load voltage Vout is below or equal to the load voltage threshold th1 in step 94, select the load voltage range 42-1 for the LED light source 12 and the nominal load current 11 associated with the range 42-1 (step 95), and if not, proceed to step 96, if the sensed load voltage Vout is above the threshold th1 and below or equal to the next higher load voltage threshold th2 in step 96, select the load volt- age range 42-2 for the LED light source 12 and the nominal load current 11 associ- ated with the range 42-2 (step 97), and if not, check through the following load voltage ranges one by one until a matching load voltage range is found and selected, or until the last load voltage range 44-N is reached and selected together with its associated nominal current IN

N in step 98 and

AN continue the LED driver operation with the selected load voltage range

N and the selected nominal current (step 99). 7 In embodiments, the controller 110 may monitor the sensed load volt-

N 30 age during the operation of the LED driver 10, and the controller may detect a short

E circuit of one of LEDs D1-DN in the LED light source 12 if the sensed load voltage 5 drops by about a forward voltage of a single LED from the initial load voltage de- x tected. The controller 110 may store information on the detected short circuit int

N the memory 120 and/or provide the information to an external lighting controller

N 35 or control system, for example over the control interface 16, e.g. by means of a DALI message. Thereby, necessary maintenance or service operations may be initiated or scheduled, such as replacing the LED light source. In embodiments, as the short circuit of a LED also causes reduction in the light level emitted from the LED light source 12, the controller 110 may be configured to automatically control the power stage to drive the LED light source with a new higher nominal current so as to com- pensate the reduction in the light level. Preset new nominal currents to be used upon detection of a short circuit may be stored in the memory of the LED driver.

The description and the related figures are only intended to illustrate the principles of the present invention by means of examples. Various alternative embodiments, variations and changes are obvious to a person skilled in the art on the basis of this description. The present invention is not intended to be limited to the examples described herein but the invention may vary within the scope and spirit of the appended claims.

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Claims (14)

1. A LED driver, comprising a power stage having an output connectable to a LED light source for provision of an operating current for driving the LED light source, a controller configured to control the operation of the power stage, a voltage sensing circuit configured to sense a load voltage across the LED light source, and wherein an output voltage range of the power stage comprises at least one low load voltage range and at least one high load voltage range, and the controller is configured to control the power stage with different load control settings depend- ing on whether the sensed load voltage is within the atleast one low load voltage range or within the at least one high load voltage range, when the LED driver is first time connected to the LED light source.

2. The LED driver as claimed in claim 1, wherein the controller is con- — figured to select the load voltage range of the LED light source based on the sensed voltage and select load control settings associated with the selected load voltage range for controlling the power stage.

3. The LED driver as claimed in claim 1 or 2, wherein the at least one low load voltage range comprises load voltages of LED light sources with a single- LED configuration, preferably load voltages down to 2,5 V, optionally also load volt- ages of LED light sources with a multiple LED configuration with up to two, three or four series connected LEDs.

4. The LED driver as claimed in any one of claims 1-3, wherein the at least one low load voltage range comprises a first low load voltage range covering load voltages of LED light sources with a single-LED configuration, preferably load N voltages down to 2,5 V, and a second load voltage range covering load voltages of O LED light sources with a multiple LED configuration up to two, three or four series o» connected LEDs.

7

5. The LED driver as claimed in any one of claims 1-4, wherein the at N 30 leastone load voltage range includes load voltages of LED light sources having mul- E tiple-LED configuration up to at least five series connected LEDs, preferably at least 5 ten series connected LEDs. LO

6. The LED driver as claimed in any one of claims 1-5, wherein the load N control settings include control settings of a dimming operation for load voltage N 35 ranges within the overall utput voltage range supported.

7. The LED driver as claimed in claim 6, wherein the controller is configured to select the load voltage range of the LED light source based on the sensed voltage and select load control settings associated with the selected load voltage range for controlling the power stage.

8. The LED driver as claimed in any one of claims 1-7, wherein the con- trolleris configured to disable a dimming operation or limit the dimming operation of LED driver in response to the sensed load voltage of the connected LED light source being within the at least one low load voltage range, and wherein the con- troller is configured to enable full dimming operation of the LED driver in response to the sensed load voltage of the connected LED light source being within the at least one high load voltage range.

9. The LED driver as claimed in any one of claims 1-8, wherein the con- troller is configured to disable a dimming operation of the LED driver in response to the sensed load voltage of the connected LED light source being within the first low load voltage range, and wherein the controller is configured to limit the dim- ming operation of LED driver in response to the sensed load voltage of the con- nected LED light source being within the second high load voltage range, and the controller is configured to enable full dimming operation of the LED driver in re- sponse to the sensed load voltage of the connected LED light source being within the at least one higher load voltage range.

10. The LED driver as claimed in any one of claims 1-9, wherein the load control settings include preset values of the nominal load current for the load volt- age ranges.

11. The LED driver as claimed in claim 10, wherein the controller is con- figured to select the load voltage range of the LED light source based on the sensed — voltage and select the preset value of the nominal load current associated with the N selected load voltage range for driving the LED light source used. N

12. The LED driver as claimed in any one of claims 1-11, wherein the N controller is configured to monitor the sensed load voltage during the operation of 7 the LED driver, the controller being configured to detect a short circuit of one of N 30 LEDs in the LED light source if the monitored sensed load voltage drops from the E initially sensed load voltage, and wherein the controller is configured to store the N information on the detected short circuit in a memory of the LED driver, and/or & the controller is configured to communicate the information on the detected short N circuit to an external lighting controller or control system, and/or the controller is N 35 configured to automatically control the power stage to drive the LED light source with a new higher nominal current so as to compensate the reduction in the light level caused by the detected short circuit.

13. The LED driver as claimed in any one of claims 1-12, wherein the power stage comprises a switched-mode power stage.

14. A LED luminaire, comprising a LED light source and a LED driver as claimed in any one of claims 1-13. N N O N o I N N I Ac a N ™ 00 LO N N O N