Abstract:
A method of driving current from an AC power supply through a series arrangement of a varying number of strings of LEDs is disclosed, the method comprising, adding LED strings to the series arrangement in a first order during an increasing voltage part of a cycle of the AC power supply cycle and subsequently removing LED strings from the series arrangement in a second order s during a decreasing voltage part of the cycle, wherein the second order is not the reverse of the first order. A driver for operating such a method is also disclosed, as is an IC embodying such a driver.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to LED drivers and to methods of driving LEDs. 
       BACKGROUND OF THE INVENTION 
       [0002]    There is increasing interest in driving strings of LEDs directly from an alternating current (AC) power supply. An example is so-called “direct to mains” LED lighting circuits, in which an AC mains power supply is used to drive an LED lighting circuit without first being converted to a DC level. 
         [0003]    An LED may be considered as a current sink since an approximately fixed voltage is required to drive the LED, whereas its brightness varies with the current through it. A series arrangement of any given number of LEDs thus requires a minimum fixed voltage across it in order to switch it on, and thus can only operate during a part, that is to say the high-voltage part, of the mains cycle. 
         [0004]    In order to improve the efficiency of energy lighting units, it is known to split the series arrangement into separate, series-connected, strings. The upper LED string element is connected to the rectified mains. Taps between each of the lower strings are connected to current sources that sequentially switch on, such that the effective LED voltage follows the mains voltage as closely as possible. This configuration is generally known as a linear LED driver. 
         [0005]    As a result of this arrangement, a higher string will be in an on-state for more of the mains cycle than will a lower string. So, on average an upper or higher LED string is brighter and a lower LED string is less bright. It is known to compensate for this by adding more LEDs to the lower string or strings. However, although this may compensate for brightness, the ageing characteristics of the LEDs in the different strings will not be uniform, and the additional LEDs will lead to higher costs. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect there is provided a method of driving current from an AC power supply through a series arrangement of a varying number of strings of LEDs, the method comprising, adding LED strings to the series arrangement in a first order during an increasing voltage part of a cycle of the AC power supply cycle and subsequently removing LED strings from the series arrangement in a second order s during a decreasing voltage part of the cycle, wherein the second order is not the reverse of the first order. 
         [0007]    In embodiments the second order is the same as the first order. Since in such embodiments the LED string to be powered first, that is to say, illuminated first, is also the first to be switched off, that is to say, disconnected or unpowered, and the last LED string to be powered is also the last to be switched off, the strings are each illuminated for the same or a similar length of time. So their ageing will generally be the same or similar. In conventional methods, the first string to be illuminated is also the last to be switched off so is illuminated for a substantially longer period that the last illuminated string, which is also the first to be switched off. 
         [0008]    Embodiments comprise switching a short-circuit across a string of LEDs to remove that string of LEDs. A short circuit is a particularly convenient way of removing, that is to say bypassing, a string, since only one switch is required. 
         [0009]    Embodiments comprise switching between two current sources to add a string of LEDs. Alternatively, a single current source can be directed by a different route to add a string. 
         [0010]    According to another aspect there is provided a driver for driving current from an AC power supply through a series arrangement of a varying number of strings of LEDs by adding LED strings to the series arrangement during an increasing voltage part of a cycle of an AC power supply cycle and subsequently removing LED strings from the series arrangement strings of LEDs during a decreasing voltage part of the cycle, the driver comprising at least one short-circuit connection configured to be switchably connected across a one of the strings of LEDs, and at least one current source configured to be connected to the series arrangement, and being configured to add the LED strings in a first order and remove LED strings in a second order which is not the reverse of the first order. 
         [0011]    In embodiments the second order is the same as the first order. This generally provides the most balanced and thus the most optimal aging of the strings. In embodiments, the driver is configured to switch a short-circuit connection across a string of LEDs to remove that string of LEDs. In embodiments, the driver is configured to switch between two current sources adds a string of LEDs. 
         [0012]    According to a further aspect there is provided an integrated circuit comprising such a driver. 
         [0013]    According to a yet further aspect there is provided a driver for driving current from an AC power supply through a series arrangement of a varying number of strings of LEDs, the number of strings of LEDs being varied by adding LED strings to the series arrangement in a first order and subsequently removing LED strings from the series arrangement in a second order, the driver comprising a plurality of switches each configured to short-circuit at least one of the stings of LEDs. 
         [0014]    These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
           [0016]      FIG. 1  shows schematically an operating sequence of a conventional multi-string linear LED driver; 
           [0017]      FIG. 2  shows the operating sequence of the strings of an LED circuit according to  FIG. 1 ; 
           [0018]      FIG. 3  shows the operating sequence of the strings of an LED circuit according an embodiment; 
           [0019]      FIG. 4  shows schematically an operating sequence of a linear LED driver according to  FIG. 3 ; 
           [0020]      FIG. 5   a  shows an operating sequence of the strings of an LED circuit according to an embodiment; 
           [0021]      FIG. 5   a  shows another operating sequence of the strings of an LED circuit, according to another embodiment; 
           [0022]      FIG. 6  shows a schematic of a known LED lighting circuit; 
           [0023]      FIG. 7   a  shows a schematic of an LED lighting circuit according to embodiments; 
           [0024]      FIG. 7   b  shows a schematic of an LED lighting circuit according to other embodiments; 
           [0025]      FIG. 8  is a flowchart illustrating the control signals and control logic of embodiments. 
       
    
    
       [0026]    It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0027]      FIG. 1  shows schematically an operating sequence of a conventional multi-string linear LED driver, occurring in a conventional circuit such as that shown in  FIG. 6 . 
         [0028]      FIG. 6  shows an LED driver  60 , which may be configured as an integrated circuit, having as an input an AC signal, which may be a dimmed mains AC signal, across two terminals mains 1  and mains 2 . A lighting circuit  62  comprises a series arrangement of LEDs strings  64 ,  65  and  66  each of which may have associated with it, as will be familiar to the skilled person, a capacitor  60   a  and/or a blocking diode  60   b  to filter for mains flicker (also known as 100 Hz flicker). Although as illustrated each string ad  64 ,  65  and  66  is depicted as a single LED, it will be appreciated that typically each string comprises a series-connected group of LEDs. Typically 15-30 LEDs will be connected in series to comprise each string although the strings may comprise another number of LEDs. The string may comprise one or more so-called HV-LEDs, or high-voltage LEDs, in which a single package include two or more LED&#39;s connected in series. Normally, but not necessarily, each string comprises the same number of LEDs in series. One end of the end string  64 —that is, the highest string—is connected to the rectified mains by means of a terminal rect of the driver. A tap between each string is connected to the driver, as shown at str 1  and str 2 . The further end of the lowest string  66  is connected to the driver at connection terminal str 3 . The actual LED current is set by means of the voltage at the input pin ILED, converted to a current by resistor Rsense, An overvoltage circuit  68 , connected to an input pin OV on the driver  60 , may be provided in order to protect the LEDs against a high mains voltage. 
         [0029]    It will be noted that for convenience, only three series connected strings are shown in the circuit arrangement of  FIG. 6 ; for an increased understanding the operating sequence will be described with respect to  FIGS. 1 and 2  for the case of four series-connected strings. 
         [0030]      FIG. 1  shows the line voltage  10  of an AC power supply which may for instance be a mains voltage. One half-cycle of the supply is split into 9 regions,  0 ,  1 ,  2  . . .  8 . During the first region  0 , starting at the zero crossing of the supply, the line voltage is insufficient to drive an LED string. At the end of this region, the line voltage reaches the first value V 1 , which is sufficient to drive a first string of LEDs. At this moment the string is connected to the mains by means of the tap str 1 . Across region  1 , current is provided to the first string  64 , and as the line voltage rises, the remainder of the voltage is dropped elsewhere in the circuit. At the end of region  1 , the line voltage has reached a second value V 2 , typically V 2 =2×V 1 ; the line voltage is now sufficient to drive current through two strings,  64  and  65 , in series. The current supply to str 1  is then switched off and, instead, current is supplied to the second tap str 2 . During the next region  2 , both first and second strings  64  and  65  are powered. Once the voltage is sufficient to power three strings, the current supply to tap str 2  is switched off and the current is applied to the next tap next tap across region  3 , and so on until all the strings are connected, as shown in  FIG. 1  at region  4 . As the line voltage starts to fall it becomes insufficient to power all the strings, so the strings are sequentially disconnected in the inverse order to that which they are connected, by supplying current to the taps in the opposite, that is to say the reverse, order to that in which it was supplied during the increasing voltage part of the cycle. In  FIG. 1  this is illustrated by region  5  having  3  strings connected, region  6  having  2  strings connected and region  7  having just  1  string connected. Once the voltage is insufficient to support even one string, the current source is switched off. 
         [0031]      FIG. 2  shows the operating sequence of the strings of an LED circuit according to  FIG. 1 . This figure shows time passing from left to right, and for each of the regions R, numbered  0  to  8  discussed above, which of the strings S  64 ,  65 ,  66  and  6 X ( 6 X not being shown in  FIG. 6 ) are powered. A powered string is depicted by a shaded area and an unpowered string is depicted by an un-shaded area. It is clear from this figure that string  64  is illuminated for a greater proportion of time than any of the other strings. The difference is most extreme with respect to the lowest string  6 X. 
         [0032]    According to embodiments of the invention, a different sequence or order is used to remove the strings from the series arrangement relative to that shown in  FIG. 2 .  FIG. 3  shows the operating sequence of the strings of an LED circuit according to example embodiment. In the example shown in  FIG. 3 , the series arrangement of strings is of three strings S, labelled as  64 ,  65  and  66 . In this embodiment, the strings are disconnected or unpowered in the same sequence that they were connected or powered. That is to say, whereas during the regions  1 ,  2  and  3 , whilst the line voltage is increasing, the sequence of connection is such that string  64 , then both  64  and  65 , then all of  64 ,  65  and  66  are powered, during the decreasing part of the line voltage, the strings are disconnected or removed in the same order, such that during region  4  strings  65  and  66  are powered but string  64  is not powered and then during region  5 , just string  66  is powered, whilst string  64  and  65  are not powered. It will be apparent that, according to this embodiment, each of the strings is powered for the same fraction of the mains half-cycle. A high uniformity of illumination may thereby be achieved; it may also be possible to achieve a relatively uniform ageing of the LEDs themselves. 
         [0033]      FIG. 4  shows schematically an operating sequence of a linear LED driver according an example embodiment. The figure depicts a mains line voltage  42 , and the current  464 ,  465 , and  466  to each of the LEDs strings  64 ,  65 , and  66  respectively, over a half cycle. Finally, the figure shows at  44  the operation of the current source itself, in the case that a single current source is used throughout. In embodiments, it may be preferable to use different current sources over different parts of the cycle, as will be discussed in more detail below. 
         [0034]      FIG. 5  shows two arrangements wherein there are five strings  51 ,  52  . . .  55  respectively and the AC half-cycle is split into  11  regions  0 ,  1  . . .  10 . In  FIG. 5   a  is shown the operating sequence of an arrangement for  5  strings which is equivalent to that shown in  FIG. 3  for  3  strings; at  FIG. 5   b  is shown an operating sequence of the strings of an LED circuit according to another embodiment. Whereas the operating sequence shown in  FIG. 5   a  may be considered as a “first on, first off” sequence, that shown in  FIG. 5   b  is more complex. The arrangement of  FIG. 5   b  has in common with that of  FIG. 5   a , that the number of strings in the series arrangement is sequentially increased from 0 to 5 during the increasing part of the line voltage; however, in order to add one to the total number of strings, instead of strings being added one at a time (first  51 , then  52 , then  53 , etc.) as in the example of  FIG. 5   a , in  FIG. 5   b  one or more strings may disconnected or removed, provided that at the same time the same number of strings, plus one more, are connected or added. Thus, whilst just string  53  is powered during the region  1 , at the transition between region  1  and region  2 , string  53  is disconnected and both strings  51  and  52  are connected. Then, at the transition between regions  2  and  3 , strings  51  and  52  are both disconnected and strings  53 ,  54  and  55  are all connected. 
         [0035]    Although the switching arrangement required for the operating sequence of  FIG. 5   a  is more complex than that for the operating sequence of  FIG. 5   a , it may be possible to utilise a fewer number of switches, and so such a more complex switching arrangement may be more suited for some embodiments. 
         [0036]      FIG. 7   a  shows a schematic of an LED lighting circuit  70   a  according to embodiments; the circuit comprises an arrangement of three series-connected strings  64 ,  65  and  66 . One end of string  64  is connected to the mains by means of connection rect, the other ends of the strings are connected to a driver  72  by means of taps str 1  and str 2 , and lower connection str 3 . An AC power supply, which may be as shown a dimmed mains input, is provided at terminals mains 1  and mains 2 , and rectified by rectifier  71 . The rectifier may be internal to the driver  72 . The driver may include a current mirror (or 1:N current divider)  73  which acts as a current source, for providing a current lled to the LED strings through lower connection str 3 . The driver further includes short-circuits across each of the LEDs strings  64 ,  65  and  66 , the short-circuits being switchable by means of switches  74 ,  75  and  76  respectively. The switches  74 ,  75  and  76  are controlled by a controller  78 , which may be a digital controller having outputs std 1 _short, str 2 _short, and str 3 _short. 
         [0037]    In operation, in order to remove or disconnect a particular string, say string  65  for definiteness, that string is short-circuited by closing the respective switch, in this case is switch  75 . When no strings are to be connected, for instance at region  0 , the current source is disabled or switched off. 
         [0038]    It will be appreciated that, in order to implement the switching arrangement depicted in  FIG. 5   b , there is no requirement to have a short circuit across each individual string; rather, for some combinations of strings such as strings  5152  shown, it is sufficient to provide a single switchable short-circuit which short-circuits both strings together. The switch which enables and disables the short circuit must be able to withstand the voltage across not just one string but to strings in this case, and so a high-voltage switch is required. This may be traded against the reduced number of switches required. 
         [0039]      FIG. 7   b  shows a schematic of an LED lighting circuit  70   a  according to other embodiments; this circuit is generally similar to that shown in  FIG. 7   a  and so the corresponding features will not be described further. However, in this case, the driver includes short-circuits across only two of the LEDs strings  64 ,  65  but not across the lowest string  66 . In this case, current source  73  can act as a current generator to any one of three current sources cs 1 , cs 2  and cs 3 , which can be supplied to str 1 , str 2  and str 3  respectively. The skilled person will appreciate that the configuration of current source shown is illustrative only, and that other configurations for supplying one or more separately controllable current from the AC power supply are equally possible and within the scope of embodiments. 
         [0040]    Use of separately controllable current sources cs 1 , cs 2  and cs 3  as shown in  FIG. 7   b  provides for relatively simple implementations: in the rising part of the mains voltage an extra string can be added when the (mains voltage−Vcurrent_source) equals this string voltage. The LED voltage is not constant so it is difficult to calculate this point. However when every string has its own current source this will go automatically (as will be further discussed below). 
         [0041]      FIG. 8  is a flowchart illustrating the control signals and control good logic of embodiments. The process starts at “start”  810 , provided that Vrect&lt;Vled, all the LED strings remain switched off, whilst it is polled at  816  whether the mains voltage Vrect is greater than an LED voltage Vled, that is to say, whether 
         [0000]      Vrect&gt;1*Vled. 
         [0042]    Once the mains line voltage exceeds Vled, a first LED string, string 1 , is switched on at  818 , through a first current source. It is then polled at  820  whether the line voltage Vrect exceeds twice the LED voltage Vled, that is to say, whether 
         [0000]      Vrect&gt;2*Vled. 
         [0043]    Once the mains line voltage exceeds 2*Vled, a second LED string, string 2 , is switched on through a second current source at  822 , such that both the first and second LED strings are on. At the same time the first current source is switched off. It is then polled at  824  whether the line voltage Vrect exceeds three times the LED voltage Vled, that is to say, whether 
         [0000]      Vrect&gt;3*Vled. 
         [0044]    Once the mains line voltage exceeds 3*Vled, a second LED string, string 3 , is switched on through a third current source at  826 , such that first, second and third LED strings are on. At the same time the second current source is switched off. 
         [0045]    In the specific embodiment described in  FIG. 8 , there are only three LED strings so at this stage of the mains cycle all the strings are on; in other embodiments in which there are more strings, the sequence continues as described above until all the strings are on. 
         [0046]    The control sequence continues, by polling, at  828 , whether the line voltage falls to less than three times the LED voltage, that is to say Vrect&lt;3* Vled. If this condition is found to be true, then a short-circuit switch thy 1  is closed, at  830 , to switch off string 1 , whilst string 2  and string  3  remain on. It remains that the current is applied through the third current source. 
         [0047]    Similarly, it is then polled, at  832 , whether the line voltage falls to less than twice the LED voltage, that is to say Vrect&lt;2*Vled. If this condition is found to be true, then a second short-circuit switch thy 2  is closed, at  834 , to switch off string 2 , whilst string  3  remains on short-circuit switch thy 1  remains closed so string 1  remains switched off. It remains that the current is applied through the third current source. 
         [0048]    It is then polled, at  836 , whether Vrect&lt;Vled. Once this is true the third string is switched off at  838 , which in embodiments may be by switching the third current source off. In practice, since Vrect is now less the Vled, the string will normally switch off automatically, as there is insufficient voltage across it for it to operate. In other embodiments the third string may be short-circuited through a further short-circuit switch thy 3 . 
         [0049]    Once the line voltage Vrect falls below the second reference voltage Vref 2 , shown at  840 , the short-circuit switches will open, since there is insufficient voltage or current to maintain them closed. Thereafter, at  842 , all the LED strings are off and the short-circuit switches are open. The cycle then restarts at  812 . 
         [0050]    Whereas in other embodiments a single current source is used to power any combination of strings, in embodiments such as that described with reference to  FIG. 8 , separate current sources are as additional strings are added. Such embodiments are convenient to implement, since, in the rising part of the mains voltage an extra string can be added when the (mains voltage−Vcurrent_source) equals this string voltage. But the LED voltage is not exactly constant, but can vary with operating temperature, operating current etc., so it is difficult to calculate this point. However when every string—or rather, every combination of strings (that is to say string 1 , string  1 +string  2 , and string 1 +string  2 +string  3 ) has its own current source this will happen automatically. In contrast, if opening of previously-closed short-circuit switches is used to increase the number of strings, it is necessary to estimate what is the best time—or equivalently, the best voltage—to open the switch. That is more complicated. Nonetheless, during the rising slope of the mains it is sufficient to observe that the voltage across the current source approached zero, and with that information the timing of opening a switch could be determined. 
         [0051]    It will be realised that the various currents and voltages associated with  FIG. 8 . are shown in  FIG. 4 . The mains line voltage is shown at  42 . At  464 ,  465  and  466  are shown, respectively, the currents through string 1 , string 2  and string 3 . It should be noted that the figure is simplified in that the rise in current from it&#39;s off-state to its on-state is not shown in the figure. In general, due to the operating characteristics of LED strings, this rise will be relatively sharp. 
         [0052]    It will be appreciated that as used herein, the term “disconnect” is not limited to physical isolation, by for instance, including an air gap into the circuit, but includes being bypassed by for instance providing an alternative, short-circuit path. Similarly, it will be apparent that “remove” is not be interpreted so as to require physical removal or displacement, but includes logical removal, for instance by providing a short-circuit or other bypass route. 
         [0053]    From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of linear LED drivers, and which may be used instead of, or in addition to, features already described herein. 
         [0054]    Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
         [0055]    Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.