WO2015036551A1

WO2015036551A1 - Controller for controlling a current regulating element of a lighting load

Info

Publication number: WO2015036551A1
Application number: PCT/EP2014/069518
Authority: WO
Inventors: Junhu LIU; Harald Josef Günther RADERMACHER; Jie Fu; Xiao Sun; Xianhui ZHANG
Original assignee: Koninklijke Philips N.V.; Philips Deutschland Gmbh
Priority date: 2013-09-13
Filing date: 2014-09-12
Publication date: 2015-03-19

Abstract

The invention describes a controller (2_1,..., 2_n) for controlling a current regulating element (T_1,..., T_n) of a lighting load (3_1,..., 3_n) having a number of light-emitting diodes (30_1,..., 30_n), which controller (2_1,..., 2_n) comprises a first input for sensing a value representative of the voltage (V_{S_1...}, V_{S_n}) at a node between the lighting load (3_1,..., 3_n) and the current regulating element (T1); a second input for sensing a value representative of the current (I_{LED_1},..., I_{LED_n}) through the lighting load (3_1,..., 3_n) and the current regulating element (T_1,..., T_n); and a control signal generator (20; OP1_1,..., OP1_n; OP2_1,..., OP2_n) for generating, on the basis of the sensed voltage and the sensed current, a control signal (T_{ON_1},..., T_{ON_n}), for controlling the current regulating element (T_1,..., T_n) to minimize a voltage drop (V_{DS_1},..., V_{DS_n}) across the current regulating element (T_1,..., T_n) and to maintain an essentially constant lumen output of the lighting load (3_1,..., 3_n). The invention further describes an LED lighting arrangement (1); the invention also describes a method of operating an LED lighting arrangement (1) comprising a number of LED circuit modules (10_1,..., 10_n).

Description

Controller for controlling a current regulating element of a lighting load

FIELD OF THE INVENTION

The invention describes a controller for controlling a current regulating element of a lighting load; an LED lighting arrangement; and a method of operating an LED lighting arrangement.

BACKGROUND OF THE INVENTION

In light-emitting diode (LED) lighting applications, the driver is a very important element. Many of the known LED lighting driver circuits are based on bulk, boost, buck and boost or fly-back designs. These drivers perform well from the point of view of constant current control (which is critical for the performance of the LEDs), and are usually associated with good driver efficiency. However, these types of design require a bulk electrolyte capacitor and a large inductor and have correspondingly large space requirements. Such driver designs are generally associated with high cost and a relatively limited driver lifetime.

As an alternative to such driver designs in LED lighting, the use of a linear switch driver has been considered. Such a driver rapidly switches a transistor switch such as a field-effect transistor (FET) or other semiconductor switch such as a bipolar junction transistor (BJT), etc., to open and close a current path through the LED(s). Such a current regulating element may also be referred to as a "linear switch". The controller or driver of such a linear switch does not need a bulk capacitor and large inductor, and can reliably provide a constant level of current through the LED(s) when the switch is closed, i.e. "on". However, the efficiency of a linear switch driver is impaired when the forward voltage over an LED or a series-connected string of LEDs does not match the supply or bus voltage. Such a mismatch is inevitable in many situations, for example whenever the operating point of one or more of the LEDs changes; when an increase or decrease in temperature affects the voltage-current characteristics of the LEDs; when the supply voltage fluctuates, etc.

Therefore, it is crucial to match the total forward voltage of an LED string comprising one or more LEDs to the supply or bus voltage in order to achieve a desired high efficiency when using a linear switch driver. The known approaches - for example as described in US 2009/0302776 Al - generally monitor the voltage drop across the transistor switch (usually a MOSFET) and then perform any necessary adjustment to the supply voltage (using a pre-regulator) to make this match the altered forward voltage. In this way, the voltage drop across the linear switch can be minimized and the amount of power dissipated by the switch can be reduced. This is an important consideration, since unnecessary power dissipation is undesirable from an environmental point of view. Furthermore, unnecessary power dissipation can significantly shorten the lifetime of a power supply, particularly a power supply such as a battery.

The above approach, namely adjusting the supply voltage to obtain a favorable efficiency, is quite complex to realize but can work well when the supply voltage (or a pre- regulator for the supply voltage) is controllable by the driver circuitry and when the LED lighting arrangement only comprises a single LED or one or more LED strings with identical voltage-current characteristics and identical forward voltages. However, such identical multi- strings are rarely the case in LED lighting, since the required effort in binning adds considerably to the overall cost of the lighting arrangement, and the forward voltage will change in any case during operation due to temperature changes etc. Furthermore, when an external DC voltage is used to feed the LED driver, the supply voltage cannot be controlled by the LED driver circuitry, unless an additional controller circuit is used as an interface to the power stage.

Therefore, it is an object of the invention to provide a more straightforward, economical and efficient way of driving LEDs, avoiding the problems mentioned above.

SUMMARY OF THE INVENTION

The object of the invention is achieved by the controller of claim 1 ; by the LED lighting arrangement of claim 8, and by the method of claim 14 of operating an LED lighting arrangement.

According to the invention, the controller - for controlling the current through a current regulating element connected in series with a lighting load comprising a number of light-emitting diodes - comprises a first input for sensing the voltage at a node between the light-emitting diodes and the current regulating element; a second input for sensing the current through the lighting load and the current regulating element; and a control signal generator for generating an output control signal, which output control signal controls the operation of that current regulating element on the basis of the sensed voltage and current such that a voltage drop across the current regulating element is minimized, and also constant lumen output of the lighting load is maintained.

An advantage of the controller according to the invention is that it can automatically adapt to any change in voltage difference between a supply voltage to the total forward voltage over those light-emitting diodes - i.e. the "LED string forward voltage" - and can correct for such a change. As mentioned above, fluctuations in the supply voltage or an increase or decrease in the LED string forward voltage over the LEDs can result in markedly inefficient operation of a prior art current regulating arrangement, which may have to dissipate excess power, thus detracting from the overall efficiency of such an LED lighting circuit. In the controller according to the invention, by ensuring that an essentially minimal voltage drop through the current regulating element is maintained, any unnecessary excess power dissipation by the current regulating element can be avoided. The controller according to the invention therefore allows for a very efficient realization of an LED lighting circuit. Furthermore, the controller according to the invention allows the operation of such an LED lighting circuit to be tailored to the actual voltage-current characteristics of the LEDs. In this way, an efficient operation can be ensured regardless of the bin origin of the LEDs used. Efficiency is one of the most important qualities of an LED driver, since it is responsible for converting various power sources into a constant current to drive the LEDs. Therefore, the driver's efficiency is directly related to the efficiency of the lighting arrangement. When an efficient driver is used, it is possible to obtain a high lumen output with relatively low power consumption, so that energy can be saved accordingly. Also, an efficient driver is

characterized by low heat dissipation, thus improving the thermal performance of the lighting arrangement.

According to the invention, the LED lighting arrangement comprises a number of LED circuit modules, wherein at least one LED circuit module comprises an LED string with a number of preferably series-connected light-emitting diodes; a current regulating element for switching or regulating a current through that LED string; and such a controller for controlling the current regulating element of that LED circuit module. The LED lighting arrangement further comprises an input for connecting to a voltage supply that provides a supply voltage to the LED circuit modules. In the context of the invention, the expression "string of LEDs" can be used to refer to a plurality of LEDs (generally connected in series), but may for simplicity also be used to refer to a single LED.

An advantage of the LED lighting arrangement according to the invention is that each LED circuit module that comprises a controller according to the invention can adapt or adjust the control of the relevant current regulating element to adapt to any fluctuations in the common supply voltage or to an increase or decrease in the total forward voltage over the LED string of the relevant LED circuit module. Therefore, even though the LED circuit modules are driven by a common voltage bus, each LED circuit module that comprises a controller according to the invention can react independently. For example, because of their different temperature characteristics, the total forward voltage over a string of red LEDs may have decreased to a greater extent - at any particular instant - compared to the total forward voltage over a string of blue or white LEDs. Without any correction, the current regulating element of the LED circuit module with the red LEDs would effectively dissipate more power owing to the higher current through the current regulating element and any resistive load in the current path. The LED lighting arrangement according to the invention ensures that each LED circuit module comprising a controller according to the invention can perform as much of an adjustment as necessary to ensure that the voltage drop across the current regulating element is minimized, resulting in maximized power efficiency.

The controller according to the invention makes it possible to adaptively control the forward voltage over an LED or LED string to match a supply voltage, so that a high converter efficacy is possible. Instead of attempting to make the supply voltage follow or match the forward voltage over the LEDs, the LED lighting arrangement according to the invention provides a method of letting the forward voltage adaptively follow the supply voltage. The LED string of each LED circuit module has its own adaptive control circuitry to regulate the LED current amplitude in order to obtain a desired forward voltage, based on the momentary supply voltage value. As a result, supply voltage and forward voltage are

'matched' well, while a minimal voltage drop across the current regulating element is ensured, and the controller can always operate with high efficiency, regardless of any fluctuations in supply voltage and/or forward voltage, and regardless of any differences between multiple LED strings.

According to the invention, the method of operating an LED lighting arrangement comprising a number of LED circuit modules - wherein at least one LED circuit module comprises an LED string with a number of light-emitting diodes and a current regulating element for switching a current through that LED string - comprises the steps of sensing the voltage at a node between the LED string and the current regulating element of that LED circuit module; sensing the current through the current regulating element of that LED circuit module; and generating an output control signal on the basis of the sensed voltage and current, which output control signal controls the operation of that current regulating element such that the voltage drop across the current regulating element is essentially minimized, and also to maintain an constant lumen output of the light-emitting diodes of that module.

An advantage of the method according to the invention is that the adjustment of the operation of the current regulating element can be carried out in a favorably straightforward manner, and can easily be used to correct the behavior of one LED circuit module independently of the behavior of any other LED circuit module. Therefore, the method according to the invention allows for a more relaxed LED binning or sorting, so that an LED lighting arrangement can be realized at a significantly lower cost.

The dependent claims and the following description disclose particularly advantageous embodiments and features of the invention. Features of the embodiments may be combined as appropriate. Features described in the context of one claim category can apply equally to another claim category.

Usually, in LED lighting circuits, the desired light output is achieved by rapidly switching the LEDs on and off. Because of the high switching speed necessary to avoid visible flicker, it is usual to employ some suitable semiconductor switch. Therefore, in the following, it may be assumed that the current regulating element of an LED circuit module comprises a linear switch such as a transistor switch realized to open and close a current path through the series-connected LEDs of that LED circuit module. Preferably, a field-effect transistor such as a MOSFET is used, since this type of transistor can be switched very rapidly and can be controlled to increase or amplify the current passing through the transistor when it is turned "on". In other words, an increase of the gate voltage to the MOSFET will result in a higher current through the MOSFET. In the following, it is assumed that a linear switch such as a MOSFET is operating as a linear amplifier instead of a simple on/off switch in the saturated area. In this way, the drain-to-source current can be adjusted.

Since the purpose of the controller according to the invention is to control the behavior of a linear switch, i.e. to "drive" the linear switch, the terms "controller" and "linear switch driver" may be used interchangeably in the following.

The light output of an LED lighting arrangement should preferably be maintained at an essentially constant level, for example with a certain luminous flux and a certain color temperature or color point. This is particularly relevant when an LED lighting arrangement comprises LED strings with distinct colors that are mixed to provide light with the desired color. However, it may happen that the supply voltage to the LED lighting arrangement can fluctuate. For example, an increase in the supply voltage may result in an unfavorably higher current through the LEDs strings of the lighting arrangement. Equally, for example, an increase in board or pad temperature may result in a drop in the forward voltage over the LEDs with an unfavorably lower current through the LED strings of the lighting arrangement. Without any correction, the light output of the LEDs may perceptibly alter. Furthermore, particularly in the case of an increased LED current, an increase in power dissipation through resistive components may result, thus lowering the efficiency and the lifetime of a power supply to the lighting arrangement. Therefore, in a particularly preferred embodiment of the invention, the output control signal of the controller is generated to adjust a duty cycle of the current through the linear switch. By extending or shortening the duty cycle as appropriate, the LEDs of an LED string can be activated in a controlled manner. Therefore, any fluctuations in supply voltage or forward voltage can be "corrected": such a fluctuation will result in a change in the LED current through the string (this would alter the lumen output), but here the new LED current is detected and the duty cycle is adjusted so that the lumen output is maintained at an essentially constant level, in spite of any voltage fluctuations, and without detracting from the driver efficiency.

As mentioned above, when a transistor such as a MOSFET is being used as the linear switch, it can be controlled to draw more or less current, depending on the amplitude of the gate signal. Therefore, in a particularly preferred embodiment of the invention, the output control signal is generated to adjust an amplitude of the current through the linear switch. By using these corrective steps - adjusting the duty cycle of the LED current and adjusting the amplitude of the LED current accordingly - any alteration as a result of the types of fluctuation described above can be compensated, so that the voltage drop and power loss across the linear switch are essentially minimized, and lumen output can be maintained essentially constant. In this way, the controller according to the invention can maintain voltage drop through the linear switch at a minimum in order to maximize power efficiency, and can also maintain a constant lumen output even during a fluctuation of a supply voltage to the light-emitting diodes and/or during an alteration of the forward voltage over the light- emitting diodes.

It may be assumed in the following that an LED module comprises an LED string (which generally comprises a plurality of series-connected LEDs but which may also simply comprise a single LED), a linear switch, and a current sense resistor. These elements are all connected in series between the supply voltage and ground. By opening and closing the linear switch, an LED current can flow through the LEDs. Since the LED module is connected between supply voltage and ground, the supply voltage is therefore the sum of the total forward voltage, the voltage across the linear switch, and the voltage across the current sense resistor. By choosing a suitably small resistor, this voltage drop is essentially negligible and may be disregarded. Any change in voltage drop over the linear switch (unrelated to the behavior of the controller) reflects a mismatch between the supply voltage and the LED string forward voltage. Therefore, any change between the supply voltage and the total forward voltage drop over the LEDs of an LED string can be detected by measuring or sensing the voltage at a node between the LED string and the linear switch, for example at the drain of a MOSFET. Furthermore, in such an LED arrangement, the current through the LEDs can be measured at any point in the series connection. Preferably, the LED current is measured at the "output" of the linear switch, e.g. at the source terminal of a MOSFET.

The linear switch driver according to the invention can use any suitable means for maintaining a constant average current through the linear switch. For example, it may comprise a circuit module for diverting an excess current away from the linear switch when the need arises. Similarly, it may include a current source for providing additional current when needed. However, such circuit modules may add to the complexity of the overall lighting circuit and may detract from its efficiency from the point of view of power consumption. Therefore, in a particularly preferred embodiment of the invention, the linear switch driver according to the invention comprises a means of adjusting the LED current amplitude as well as a means of adjusting the duration of the LED current. Such a linear switch driver can be realized in any of a number of ways. For example, it may be realized using analogue circuit components. In such a realization, any change between the supply voltage and the total forward voltage drop over the LEDs of an LED string can be corrected by adjusting the amplitude of the LED current through the LED string. To this end, in a preferred embodiment of the invention, the linear switch driver comprises a first operational amplifier ("op amp") with a first input terminal for sensing the voltage at the node between the LED string and the linear switch. The signal at the first input terminal of the op amp is preferably derived from a combination of the sensed voltage and a reference voltage chosen to result in a desired or optimal LED current for that LED string. A second input terminal of the op amp is preferably connected in the usual manner in a feedback loop from the op amp output. The output terminal of the op amp is also connected to a switching terminal of the linear switch, e.g. the gate of a MOSFET. The op amp is preferably chosen to adjust the amplitude of its output signal in response to a difference between the voltages at its input terminals, so that if the sensed voltage changes, the op amp output controls the linear switch to adjust the current flow through the linear switch as appropriate. In this way, the linear switch, for example a MOSFET, can be controlled to draw more (or less) current - i.e. LED current - in response to a change in the difference between the supply voltage and the LED string forward voltage. The first op amp acts to adapt the LED current amplitude based on the sensed voltage value. When the sensed voltage is higher than the drain-source voltage of the MOSFET, the first op amp generates a higher output voltage, which in turn increases the current through the MOSFET and therefore also the current through the LED string, which result in an increased forward voltage, and the voltage drop over the linear switch will decreased accordingly. In this way, the voltage drop cross the linear switch can always be maintained at a minimum.

Preferably, as mentioned above, the LED current should have a pulse waveform with a duty cycle to control the total lumen output of the LED string. This can be achieved in a number of ways. For example, the output of the first op amp might be an uninterrupted signal that is then converted to a pulsed signal with the same amplitude.

Alternatively, in a preferred embodiment of the invention, the first op amp is provided with an enable input that is driven by a signal that controls the duty cycle of the output of the first op amp.

To ensure an essentially constant average current over the linear switch, the linear switch driver according to the invention preferably comprises an enabling circuit for the first op amp. Preferably the enabling circuit comprises a second operational amplifier with a first input terminal for sensing the current through the linear switch (i.e. for sensing the LED current); a second input terminal connected to receive a reference current waveform; and an output terminal connected to an enable input of the first operational amplifier of the linear switch driver. This second op amp generates an output pulse to enable the first op amp. Therefore, the duration of the pulse at the output of the second op amp determines the duty cycle of the LED current. As mentioned above, the LED current has a pulse waveform, essentially alternating between zero and a certain momentary amplitude. Therefore, the reference current waveform is preferably generated with a frequency corresponding to the switching frequency of the LEDs. Preferably, the reference current waveform is a pulse- width modulated signal, with a shape that can be used to determine or adjust the duty cycle. In a preferred embodiment of the invention, the reference current waveform comprises a suitable waveform (e.g. a sawtooth waveform). When the essentially square-wave shape of the LED current is subtracted from this reference current waveform, the amplitude of the LED current will determine the point at which the enable signal for the first op amp becomes active. For example, if the amplitude of the MOSFET gate control signal has been increased, the resulting higher LED current pulse will result in a later switching time of the output of the second op amp, which is the enable signal to the first op amp, thereby decreasing the duty cycle of the MOSFET. Similarly, if the amplitude of the MOSFET gate control signal has been decreased, the resulting lower LED current pulse will result in an earlier switching time of the second op amp, thereby increasing the duty cycle of the MOSFET. Effectively, the second op amp in this circuit setup performs an adaptive "on the fly" linear switch gate control during operation of the LEDs, acting to adapt the LED current duty cycle.

To obtain the desired switching performance, i.e. the desired duty cycle and current amplitude when matching the forward voltage and supply voltage to each other, parameters such as the frequency and amplitude of the reference current waveform; the LED current minimum and maximum values; the voltage across a current sense resistor, etc. are chosen on the basis of a voltage-current characteristic ("V/I characteristic") of the LEDs of the LED string of that LED circuit module.

In an alternative realization, the linear switch driver according to the invention can be realized using a microcontroller or similar component to perform digital signal processing on the sensed voltage and current. In such an embodiment, the linear switch driver preferably comprises a first input for sampling the voltage at a node between the LED string and the linear switch; a second input for sampling the LED current of that LED circuit module; and a data processing module for processing the voltage and current samples and for generating an output control signal for that linear switch.

As mentioned above, the linear switch driver according to the invention is particularly advantageous since the current regulation adapts itself to the requirements of the LEDs of its LED module in a lighting arrangement, independently and unaffected by any other LED modules in that lighting arrangement. For example, an embodiment of an LED lighting arrangement according to the invention comprises two or more LED circuit modules, where each LED circuit module comprises a linear switch driver as described above, and wherein the reference current waveform for a linear switch driver comprises a pulse- width modulated signal with a frequency corresponding to the switching frequency of the LEDs of the LED string of that LED circuit module. For example, in an LED lighting arrangement with four LED circuit modules, each module can have an LED string with LEDs of a certain color (one module with red LEDs, one module with blue LEDs, one module with green LEDs, one module with white LEDs). The voltage-current characteristics of the different color LEDs can be accommodated by applying a "custom" reference waveform to the second op amp of each linear switch driver, so that the LED current through each independent LED string is tailored to the voltage-current characteristics of that string.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows a first embodiment of an LED lighting arrangement according to the invention;

Fig 2 shows a second embodiment of an LED lighting arrangement according to the invention;

Fig. 3 illustrates the relationship between forward voltage and LED current for two types of LED;

Fig. 4 shows an LED current as adjusted by an embodiment of a linear switch driver according to the invention;

Fig. 5 shows a prior art LED lighting arrangement.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig 1 shows a schematic representation of an embodiment of an LED lighting arrangement 1 according to the invention. Here, the LED lighting arrangement 1 comprises a plurality of LED circuit modules 10 1, ..., 10_n arranged in parallel. The LED lighting arrangement 1 comprises a supply voltage input 4 at which a supply voltage V_B is provided to the LED circuit modules 10 1, ..., 10_n. Each LED circuit module 10 1, ..., 10_n comprises an LED string 3 1, ..., 3_n with a number of series connected light- emitting diodes 30 1, ..., 30_n and a linear switch T_l,..., T_n, in this example a MOSFET T_l,..., T_n, for switching current through that LED string 3 1, ..., 3_n. The linear switch T_l,..., T_n of an LED circuit module 10 1, ... , 1 O n is most efficient when the forward voltage over the

corresponding LED string closely matches the bus or supply voltage V_B- However, the supply voltage V_B is not necessarily constant, and may fluctuate for various reasons.

Furthermore, the forward voltage V_F ι, .. . , V_{F n} over an LED string 3 1, ..., 3_n can also change for various reasons, for example when the LED junction temperature increases or decreases. Such fluctuations would result in a change in the voltage Vs i, . .. , Vs__n between the LED string 3 1 , . . . , 3_n and the linear switch T_l , . .. , T_n, and accordingly may result in an unfavorable increase in the power dissipated by the linear switch T_l,.„, T_n when that voltage Vs _l, . .. , Vs _n increases. To correct for such any difference between forward voltage Vp _i , VF_J and bus voltage VB, an LED circuit module 10 1 , . . . , 10_n of the LED lighting arrangement 1 uses a linear switch driver 2_1 , . . . , 2_n that can detect the difference between the forward voltage Vp __\, Vp _n and bus voltage VB, and reacts adaptively by adjusting the LED current ILED_I, · · · , ILED_JI- TO ensure an essentially minimum voltage drop across the linear switch T_l,.„, T_n, and also a constant light output even when the LED current ILED_I, . . . , ILED_J is increased or decreased, the linear switch driver 2_1 , . . . , 2_n adjusts the duty cycle of the LED current ILED_I, · · · , ILED_J-

For example, if a higher voltage Vs i, . .. , Vs _n is sensed, the controller 2_1 , . . . , 2_n will react by generating a control signal TON_I, · · · , TON_J that increases the LED current ILED_I, · · · , ILED_J through that LED string 3 1 , . . . , 3_n, which in turn increases the LED string forward voltage Vp __\, Vp _n; This has the effect of decreasing the voltage drop across the linear switch T_l,..., T_n, thereby keeping this at a favorable minimum and achieving a favorably low power loss through the linear switch.

In the following, the equations use the reference numbers and subscripts of the first LED circuit module as an example, but apply to each LED circuit module:

Because of the series arrangement of an LED circuit module 10_1 , the supply voltage can be expressed as:

V_B = V_{F 1} + V_{S 1} + V_{RS 1} ( 1) In equation ( 1), VB is the power rail voltage (also called supply voltage or bus voltage) for the LED lighting arrangement 1 ; VFJ is the voltage drop across LED string 3_1 ; Vs _l is the drain-to- source voltage drop across the linear switch T_l, which is working in amplifying mode. VRS i is the voltage cross the current sense resistor Rs i, and can be expressed as: RS_I — Rs_i^■ ILED_I (2) R_s l is chosen to be small in value, preferably 0.5 Ω or less. With this very small value for the current sense resistor Rs i, V_RS_₁ is essentially negligible and can be disregarded. Therefore, the voltage drop across the linear switch T_l can be expressed as V_{S 1} * V_B - V_{F 1} (3)

The voltage Vs_₁ across the linear switch T_l is ultimately determined by the supply voltage V_B, the total forward voltage drop V_F_₁ of the series-connected LEDs and their V/I characteristic:

V_{F 1} = m- V_f (4) where m is the number of LEDs 30_1 in the string 3_1, and V_f is the forward voltage of one of those LEDs 30_lfor the momentary operating point.

The linear switch driver 2_1 senses the voltage Vs i over the switch T_l and uses it to adjust the amplitude of the LED current I_LED_I, thus compensating for any increase or decrease in LED string forward voltage V_F_₁ to maintain a minimum Vs i . The linear switch driver 2_1 also senses the amplitude of the LED current I_LED_I at the 'output' of the switch T_l, and uses this to adjust the duty cycle of the LED current I_LED_I- This ensures an essentially constant lumen output for that LED string 3_1.

The linear switch driver 2_1 of an LED circuit module 10_1 can be realized in the form of a microcontroller 2_1 which can sample a voltage Vs i at a node between the LED string 3_1 and the linear switch T_l in order to detect a mismatch or difference between the forward voltage V_F ι and bus voltage V_B. The microcontroller 2_1 is also realized to sample the LED current I_LED_I- data processing module 20 of the microcontroller 2_lis realized to process the data to determine any corrective steps necessary. In such an embodiment, the microcontroller 2_1 can react to a difference between the forward voltage V_FI ^and bus voltage V_B by adjusting the amplitude and duty of a control signal T_ON_I to the linear switch T_l. For example, to correct for an increase in the forward voltage over the LED string 3_1 of a first LED circuit module 10_1, the linear switch driver 2_1 increases the amplitude of the control signal T_ON by an appropriate amount so that the current I_LED_I through that LED string 3_1 is also increased to essentially minimize the voltage drop Vs ι across the linear switch T_l, and also shortens the "ON" time of the control signal T_QN_I by an appropriate amount so that the average current through the linear switch Tl_l remains essentially constant.

An embodiment using discrete components is shown in Fig. 2. Again, for the sake of clarity, only the suffixes relating to one circuit module 10_1 are used, but it will be understood that the explanation applies to all modules 10 1, ..., 10_n. The voltage Vs i at the node between LED string 3_1 and transistor T_l is effectively determined by the total forward voltage over the LED string 3_1. For example, when the LED string 3_1 comprises essentially identical LEDs 30_1, the total forward voltage V_F ι over that LED string 3_1 is simply the sum of the forward voltages of each LED 30_1.

Here, the linear switch driver 2_1 of the LED circuit module 10_1 comprises a first op amp OPl_l with a first input resulting from a comparison between the sensed voltage Vs _l and a reference voltage V_REF_I (the reference voltage can be different for each LED circuit module, and is determined by the types of LEDs used, on their V/I characteristics, and on the number of LEDs in each string). The first op amp OPl_l has a second input that is derived from its output voltage. The current sense resistor R_s _₁ of the LED circuit module

10_1 is chosen to be very small, so that the voltage drop over that resistor can be regarded as negligible. Therefore, the voltage Vs i sensed by the linear switch driver 2_1 is essentially the difference between the bus voltage V_B and the forward voltage V_F_₁ over the LED string 3_1 of that LED circuit module 10_1.

The first op amp OPl_l of the LED circuit module 10_1 delivers an output signal T_ON_I whose amplitude is determined by the difference between its inputs, and which is used to control the MOSFET T_l. Since the MOSFET T_l acts as an amplifier, and since it is arranged in the current path of the LED string of its LED circuit module 10_1, the output signal T_ON_I effectively actuates its switch T_l as well as determining the amplitude of the LED current I_LED_I °f that LED circuit module 10_1. This embodiment also includes a second op amp OP2_l that compares the LED current I_LED_I to a pulse- width modulated input waveform PWM_ref_l whose frequency determines the duty cycle of the LEDs 30_1 of that LED circuit module 10_1. The diagram shows that the node between switch T_l and current sense resistor Rs i is connected to the "-" input of the second op amp OP2_l

(indicated by the labels " 1" in the first LED circuit module and "n" in the nth circuit module) so that the LED current I_LED_I °f ^an LED circuit module 10_1 is fed into the second op amp OP2_l of that module 10_1.

In this embodiment, the pulse- width modulated input waveform PWM_ref_l comprises a sawtooth waveform. The output of this op amp OP2_l is used to enable the first op amp OPl_l. Effectively, the linear switch driver 2_1 adjusts the LED current amplitude to compensate for a change in the sensed voltage Vs i, and the LED current amplitude is used to adjust the LED current duty cycle accordingly. This "self -regulation" ensures that an increase in LED current amplitude is tied to a shorter duty "ON" time, and a decrease in LED current amplitude is tied to a longer duty "ON" time. In this way, the average current through the switch T_l is maintained at an essentially constant level, the power dissipated by the linear switch T_l is minimized, and the lumen output of the LED string 3_1 remains essentially constant.

To obtain the desired switching performance for an LED string 3_1 of an LED circuit module 10_1, i.e. to obtain the desired duty cycle and current amplitude when matching the forward voltage V_F_₁ and supply voltage V_B to each other, parameters such as the frequency and amplitude of the reference current waveform PWM_ref_l, the minimum and maximum values of the LED current I_LED_I, the value of the current sense resistor Rs i etc. are chosen on the basis of a voltage-current characteristic of the LEDs 30_1 of the LED string 3_1 of that LED circuit module 10_1. Of course, the same applies to each LED circuit module 10 1, 1 O n in the LED lighting arrangement 1.

For example, for a string of 12 white LEDs with a forward current operating range of 100 mA to 1000 mA and a switching frequency of 1000 Hz, the maximum amplitude of the reference waveform is chosen to be 500 mV. The current sense resistor is chosen to have a value of 0.5 Ω. The voltage across the linear switch should be 1.0 V. In one exemplary scenario, the supply voltage is initially 39.4 V, and the linear switch driver has regulated the duty cycle and current amplitude so that the LED current is 400 mA with a duty cycle of 0.6 or 60% of the total switching period.

In the event of a supply voltage increase from 39.4 V to 41.8 V, if the LED current is kept unchanged at 400mA, the voltage drop across the linear switch will increase by 4.4 V, which will result in 1.76 Watt (4.4 x 0.4) extra power lost or dissipated by the linear switch. To solve this problem, the controller controls the linear switch such that the LED current is increased to 800 mA, which in turns increases the forward voltage across such an exemplary LED from 3.2 V to 3.4 V. Hence, the total forward voltage over the LED string increases from 38.4 V to 40.8 V, which ensures that the voltage drop over the linear switch is maintained essentially at about 1.0 V in spite of the supply voltage alteration. However, this adjustment on its own would result in an undesirable perceptible change in lumen output from that string, as well as increased power dissipation by the linear switch of that LED circuit module. To correct this, the linear switch driver reduces the duty cycle of the LED current to 0.3 or 30% to ensure that the lumen output of the LED string remains essentially constant. The average current through the linear switch also remains essentially constant, since 400 niA multiplied by 0.6 is the same as 800 mA multiplied by 0.3.

The diagram indicates various other discrete components in the circuit portion relating to the first op amp OP 1 1, ..., OP I n, in one of various commonly used

configurations. Component values have not been specified here for the sake of clarity. The skilled person will be familiar with the commonly used op amp configurations, and will also be able to choose suitable values for the discrete components.

Fig. 3 illustrates the relationship between forward voltage [V] and LED current [mA] for two types of LED. A first curve CI shows an exemplary V/I characteristic for white or blue LEDs, and the second curve C2 shows an exemplary V/I characteristic for green LEDs. As the diagram shows, for the same forward voltage of about 3.5 V, a green LED is associated with a lower LED current (about 400 mA) than a white or blue LED (about 900 mA). Furthermore, each color LED reacts differently to a change in forward voltage. For example, when the forward voltage increases from 3.0 V to 3.5 V, the current through the white LEDs increases more sharply than the current through the green LEDs. In a lighting arrangement with strings of white LEDs arranged in parallel with strings of green LEDs, and a common voltage rail, separate current regulation must be applied for each LED color to ensure that the LEDs deliver the desired light output. Such "custom" current regulation is achieved by the linear switch driver in the LED lighting arrangement of the invention described in Figs. 2 and 3 above, and Fig. 4 shows the effect of such current regulation.

Fig. 4 shows a first LED current II through an LED string, as governed by the control signal to a linear switch when driven by a linear switch driver according to the invention, and a second LED current 12 through the same LED string. The current has a switching cycle duration D. The first LED current II has lower amplitude and its duty cycle TON_ II is longer, and may have been generated in response to an increase in the supply voltage or a decrease in the forward voltage over the LED string. Either or both of these 'events' called for a downward adjustment of the amplitude of the current through the LED string to ensure an essentially minimal voltage drop across the linear switch. In order to maintain constant lumen output for that LED string, the "on" time or duty cycle TON_ II was simultaneously extended.

The second LED current 12 has a higher amplitude and a shorter duty cycle TON_ 12, and may have been adapted in response to a decrease in the supply voltage or an increase in the forward voltage. Either or both of these 'events' called for an upward adjustment of the amplitude of the current through the LED string. Again, to ensure an essentially minimal voltage drop across the linear switch and to maintain constant lumen output for that LED string, the duty cycle TON_ 12 of the higher LED current 12 was shortened.

Fig. 5 shows a prior art approach. Here, the lighting arrangement 5 comprises several LED strings 50_A, 50_B, 50_C, each with its own linear switch driver 52. An op amp 51 is used to sense a minimum voltage Vmin at a linear switch driver, to compare this to a reference voltage Vref, and to generate a control signal to a pre-regulator 53. On the basis of this control signal, the pre-regulator 53 adjusts an input bus voltage V_B to obtain a regulated bus voltage V_{B reg} in an attempt to match it to the forward voltages over the LED strings 50_A, 50_B, 50_C in order to obtain an acceptable converter efficiency. This approach only works well when the LED strings 50_A, 50_B, 50_C have very closely matched forward voltages, which is only achievable when considerable effort and expense is invested in the binning and sorting of LEDs. Furthermore, this approach is only possible if the pre-regulator 53 is accessible for control by the driver of the lighting arrangement 5, which is not always the case.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. The invention could be used in any lighting application area where a linear switch driver is preferred for a cost effective and compact LED driver design. It is particularly suited for use in an application such as a tube LED (TLED) for a better correlated color temperature (CCT), and color mixing hue lamps.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. list of reference signs

1 LED lighting arrangement

2_l, ..., 2_n linear switch driver

3_1, 3_n LED strings

30 1, 30_n light-emitting diodes

10 1, 10_n LED circuit module

5 prior art lighting arrangment

50_A LED string

51 op amp

52 linear switch driver

53 pre-regulator

ILED_I, ILEDJ LED current

V_F_i, V_F_„ LED string forward voltage

V_B bus voltage

V_s_i, V_s__n switch voltage

T_l, ..., T_n linear switch

ToN_l, ToN_n linear switch gate signal

Rs 1, Rs n current sense resistor

OPl l, ..., ΟΡΙ η op amp

OP2 1, ..., OP2_n op amp

OP2_en_l enable signal

OP2_en_n enable signal

PWM_ ref_l reference signal

PWM_ref_n reference signal

C1, C2 V/I characteristic

D switching period

T0N_I1, TON_I2 duty cycle

Vmin voltage at linear switch driver

Vref reference voltage

VB_reg regulated bus voltage

Claims

CLAIMS :

1. A controller (2 1, ... , 2_n) for controlling a current regulating element

(T l,..., T_n) of a lighting load (3 1, ..., 3_n) comprising a number of light-emitting diodes (30 1, ..., 3 O n), which controller (2 1, ..., 2_n) comprises

a first input for sensing a voltage (Vs i, . .. , Vs _n) at a node between the lighting load (3 1, ..., 3_n) and the current regulating element (Tl);

a second input for sensing a current (I_LED_I, · · · , I_LEDJ) through the lighting load (3 1, ... , 3_n) and the current regulating element (T l , ... , T_n);

characterized by

a control signal generator (20; OP 1 1 , ... , OP I n; OP2 1 , ... , OP2_n) for generating, on the basis of the sensed voltage and the sensed current, a control signal (T_0N_I, . .. , To_N_n) for controlling the current regulating element (T l, ..., T_n) to minimize a voltage drop (V_DS_I, · · · , Vos_n) across the current regulating element (T_l,.„, T_n) and to maintain an essentially constant lumen output of the lighting load (3 1, ..., 3_n).

2. A controller according to claim 1, wherein the output control signal (T_ON_I, · · · ,

ToN_n) is generated to adjust a duty cycle (Dl, D2) of the current (I_LED_I, · · · , I_LEDJ) through the current regulating element (T_l,.„, T_n).

3. A controller according to claim 1 or claim 2, wherein the output control signal (TON_I, · · · , ToNji) is generated to adjust an amplitude of the current (ILED_I, · · · , ILED_J) through the current regulating element (T_l,„, T_n)

4. A controller according to any of the preceding claims, realized to maintain the essentially constant average current through the current regulating element (T l, ..., T_n) even during a fluctuation of a supply voltage (V_B) to the light-emitting diodes (30 1, ..., 30_n) and/or during an alteration of the forward voltage (V_F i, . . . , V_{F n}) over the light- emitting diodes (30 1, 30_n).

5. A controller according to any of the preceding claims, comprising a first operational amplifier (OP 1 1, ..., OP I n) with

an input terminal for sensing the voltage (Vs ι, ..., Vs _n) at the node between the LED string (30 1, ..., 3 O n) and the current regulating element (T_l, ..., T_n); and - an output terminal connected to a switching terminal of the current regulating element (T_l , ... , T_n).

6. A controller according to claim 5, comprising a second operational amplifier (OP2 1, ...,OP2_n)with

- a first input terminal for sensing the current (I_LED_I, · · · , I_LEDJ) through the current regulating element (T_l , ... , T_n);

a second input terminal connected to receive a reference current waveform (PWM_ref_l, PWM_ref_n); and

an output terminal connected to an enable input (OP l en l , ... , OP l en n) of the first operational amplifier (OP 1 1, ..., OP I n) of the controller (2 1, ..., 2_n).

7. A controller according to any of claims 1 to 4, which controller (2 1, ..., 2_n) comprises

a first input for sampling the voltage (Vsi, ... , Vs_n) at a node between the lighting load (3 1, ... , 3_n) and the current regulating element (T_l , ... , T_n);

a second input for sampling the LED current (I_LED_I, · · ·, I_LEDJ) of that lighting load (3 1, 3_n); and

a data processing module (20) for processing the voltage and current samples and for generating an output control signal (T_ON_I, · · ·, _ONJ) for that current regulating element (T_ 1 , ... , T_n) .

8. An LED lighting arrangement (1) comprising a number of LED circuit modules (10 1, ..., 10_n), wherein at least one LED circuit module (10 1, ..., 10_n) comprises

- a lighting load (3 1, ..., 3_n) with a number of light-emitting diodes (30_1,

...,30_n);

a current regulating element ( T_l,.„, T_n) for regulating a current (I_LED_I, · · ·, I_LEDJ) through that lighting load (3 1, ..., 3_n); and

a controller (2 1, ..., 2_n) according to any of claims 1 to 7 for controlling the current regulating element (T_l,..., T_n) of that LED circuit module (10 1, ..., 10_n);

which LED lighting arrangement (1) further comprises a supply voltage input for providing a supply voltage (V_B) to the LED circuit modules (10 1, ..., 10_n).

9. An LED lighting arrangement according to claim 8, wherein the current regulating element (T_l,..., T_n) of an LED circuit module (10 1, ..., 10_n) comprises a transistor switch (T_l,.„, T_n) realized to open and close a current path through the number of light-emitting diodes (30 1, ..., 3 O n) of that LED circuit module (10 1, ..., 10_n).

10. An LED lighting arrangement according to claim 8 or claim 9, wherein each of a plurality of LED circuit modules (10 1, ..., 10_n) comprises a controller (2 1, ..., 2_n) according to claim 6, and wherein the reference current waveform (PWM ref l, ...,

PWM ref n) for a controller (2 1, ..., 2_n) comprises a pulse- width modulated signal (PWM_ref_l, ..., PWM_ref_n) with a frequency corresponding to the switching frequency of the light-emitting diodes (30 1, ..., 3 O n) of that LED circuit module (10 1, ..., 10_n).

11. An LED lighting arrangement according to claim 10, wherein the amplitude of the reference current waveform (PWM ref l, ..., PWM ref n) of an LED circuit module (10 1, ..., 10_n) is chosen on the basis of an operating characteristic of the light-emitting diodes (30 1, ..., 30_n) of that LED circuit module (10 1, ..., 10_n).

12. An LED lighting arrangement according to claim 11, wherein the frequency of the reference current waveform (PWM_ref_l, ..., PWM ref n) of an LED circuit module (10 1, ..., 10_n) are chosen on the basis of a switching frequency of the light-emitting diodes (30 1, ..., 30_n) of that LED circuit module (10 1, ..., 10_n).

13. An LED lighting arrangement according to claim 12, comprising a plurality of LED circuit modules (10 1, ..., 10_n), of which at least two emit different colored light.

14. A method of operating an LED lighting arrangement (1) comprising a number of LED circuit modules (10 1, ..., 10_n), wherein at least one LED circuit module (10 1, ..., 10_n) comprises an LED string (3 1, ..., 3_n) with a number of 1 light-emitting diodes (30_1, ..., 3 O n) and a current regulating element (T_l,..., T_n) for switching a current (I_LED_I, · · · , I_LEDJ) through that LED string (3 1, ..., 3_n); which method comprises the steps of sensing the voltage (V_s i, ... , V_{s n}) at a node between the LED string (3_1 , ... , 3_n) and the current regulating element (T_l,..., T_n) of that LED circuit module (10 1, ..., 10_n);

sensing the current (ILED_I, · · ·, ILEDJ through the current regulating element ( T_ 1 , ... , T_n) of that LED circuit module (10 1, ... , 1 O n); and

generating an output control signal (T₀N_I, · · ·, T₀N_n) on the basis of the sensed voltage (Vs i, ..., Vs _n) and current (ILED_I, · · ·, ILEDJX which output control signal (TON_I, · · ·, o_Nji) controls the operation of that current regulating element (T_l,.„, T_n) to minimize a voltage drop (VDS_I, · · ·, Vos_n) across the current regulating element (T_l,.„, T_n) and to maintain an essentially constant lumen output of the lighting load (3 1, ..., 3_n).

15. A method according to claim 14, wherein the output control signal (TON_I, · ·

To_N_n) for the current regulating element (T_l,..., T_n) of an LED circuit module (10 1, 10_n) is generated to adjust a duty cycle (Dl, D2) of the current (ILED_I, · · ·, ILEDJ) through the current regulating element (T_l,.„, T_n).