Abstract:
An arrangement wherein a plurality of LED strings are driven with a balanced drive signal, i.e. a drive signal wherein the positive side and negative side are of equal energy over time, is provided. In a preferred embodiment, the drive signal is balanced responsive to a capacitor provided between a switching network and a driving transformer. Balance of current between various LED strings is provided by a balancing transformer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/482,116 filed May 3, 2011, entitled “High Efficiency LED Driving Method”, the entire contents of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of solid state lighting, and in particular to an LED driving arrangement with a balancer and a capacitively coupled driving signal. 
     BACKGROUND OF THE INVENTION 
     Light emitting diodes (LEDs) have become very popular for use as lighting devices due to their advantages of high efficiency, long life, mechanical compactness and robustness, and low voltage operation, without limitation. Application areas include liquid crystal display (LCD) backlight, general lighting, and signage display. LEDs exhibit similar electrical characteristics to diodes, i.e. LEDs only conduct current when the forward voltage across the device reaches its conduction threshold, denoted V F , and when the forward voltage increases above V F  the current flowing through the device increases sharply. As a result a particular drive circuit has to be furnished in order to control the LED current stably. 
     The existing approach in today&#39;s market normally uses a switching type DC to DC converter, typically in a current control mode, to drive the LED lighting device. Because of the limited power capacity of a single LED device, in most applications multiple LED&#39;s are connected in series to form a LED string, and multiple such LED strings work together, typically in parallel, to produce the desired light intensity. In multiple LED string applications a DC to DC converter is normally employed to supply a DC voltage sufficient for the LED operation, however because the operating voltage of LEDs have a wide tolerance (+/−5% to +/−10%), an individual control circuit has to be deployed with each LED string to regulate its current. For simplicity, such a current regulator typically employs a linear regulation technique, wherein a power regulation device is connected in series with the LED string and the LED current is controlled by adjusting the voltage drop across the power regulating device. Unfortunately, such an approach consumes excessive power and generates excessive heat because of the power dissipation of the linear regulation devices. In some approaches a switching type DC to DC converter is provided for each LED string. Such an approach yields a high efficiency operation but the associated costs also increase dramatically. 
     What is needed, and not provided by the prior art, is an LED drive method with high operating efficiency and a low system cost, which provides a balancing function between the various LED strings of a multiple LED string luminaire. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to overcome at least some of the disadvantages of the prior art. This is provided in certain embodiments by an arrangement wherein a plurality of LED strings are driven with a balanced drive signal, i.e. a drive signal wherein the positive side and negative side are forced to be of equal energy over time. In a preferred embodiment, the drive signal is balanced responsive to a capacitor provided between a switching network and a driving transformer. Balance of current between various LED strings is provided by a balancing transformer. 
     Additional features and advantages of the invention will become apparent from the following drawings and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. 
       With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: 
         FIG. 1  illustrates a high level schematic diagram of an embodiment of a driving arrangement for four LED strings wherein the anode end of each of the LED strings are commonly coupled to the center tap of a driving transformer, and wherein the cathode ends of the LED strings are each coupled to respective ends of windings of a balancing transformer via respective unidirectional electronic valves; 
         FIG. 2  illustrates a high level schematic diagram of an embodiment of a driving arrangement for four LED strings wherein the anode end of each of the LED strings are commonly coupled to the center tap of a driving transformer, the cathode ends are each coupled to respective ends of windings of a balancing transformer, and the center taps of the balancing transformer windings are coupled to the driving transformer second winding ends via respective unidirectional electronic valves; 
         FIG. 3  illustrates a high level schematic diagram of an embodiment of a driving arrangement for two LED strings wherein the anode end of each of the LED strings are commonly coupled to the center tap of a driving transformer, the cathode ends of the LED strings are each coupled to a center tap of respective windings of a balancing transformer, and the balancing transformer winding ends are coupled to the driving transformer second winding ends via respective unidirectional electronic valves; 
         FIG. 4  illustrates a high level schematic diagram of an embodiment of a driving arrangement for four LED strings wherein the cathode ends of a first two of the LED strings are commonly coupled to a first end of the second winding of a driving transformer, the cathode ends of a second two of the LED strings are commonly coupled to a second end of the second winding of the driving transformer, and the anode ends of the LED strings are each coupled to respective ends of windings of a balancing transformer; and 
         FIG. 5  illustrates a high level schematic diagram of an embodiment of a driving arrangement for two LED strings wherein the cathode end of each of the LED strings are commonly coupled to the center tap of a driving transformer, the anode ends of the LED strings are each coupled to a center tap of respective windings of a balancing transformer, and the balancing transformer winding ends are coupled to the driving transformer second winding ends via respective unidirectional electronic valves. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
       FIG. 1  illustrates a high level schematic diagram of an embodiment of a driving arrangement  10  comprising: a switching control circuit  20 ; a switching bridge  30  comprising a first electronically controlled switch Q 1  and a second electronically controlled switch Q 2 ; a DC blocking capacitor CX; a driving transformer TX comprising a first winding TXF magnetically coupled to a second winding TXS; first, second, third and fourth LED strings  40 ; a balancing transformer BX comprising a first winding BXF magnetically coupled to a second winding BXS; a first, second, third and fourth smoothing capacitors CS; and a first, second, third and fourth unidirectional electronic valve  50 . First and second electronically controlled switches Q 1 , Q 2  are illustrated without limitation as NMOSFETs, however this is not meant to be limiting in any way. Switching bridge  30  is illustrated as a half bridge, however this is not meant to be limiting in any way, and in particular embodiment a full bridge is implemented without exceeding the scope. 
     A first output of switching control circuit  20 , denoted VG 1 , is coupled to the control input of first electronically controlled switch Q 1  of switching bridge  30 , and a second output of switching control circuit  20 , denoted VG 2 , is coupled to the control input of second electronically controlled switch Q 2  of switching bridge  30 . The drain of first electronically controlled switch Q 1  is coupled to a source of electrical power, denoted V+, and the source of first electronically controlled switch Q 1  is coupled to drain of second electronically controlled switch Q 2  and to a first end of DC blocking capacitor CX. The common node of the source of first electronically controlled switch Q 1 , the drain of second electronically controlled switch Q 2 , and the first end of DC blocking capacitor CX is denoted node  35 . The second end of DC blocking capacitor CX is coupled to a first end of first winding TXF, and a second end of first winding TXF is coupled to the source of second electronically controlled switch Q 2 , and to the return of the source of electrical power, denoted V−. 
     A center tap of second winding TXS is coupled to the anode end of each of the LED strings  40  and to a first end of each of the smoothing capacitors CS. The cathode end of each of the LED strings  40  is coupled to a second end of a respective smoothing capacitor CS and to the anode of a respective unidirectional electronic valve  50 . The cathode of a first unidirectional electronic valve is coupled to a first end of first winding BXF, the cathode of a second unidirectional electronic valve  50  is coupled to a second end of first winding BXF, the cathode of a third unidirectional electronic valve  50  is coupled to a first end of second winding BXS, and the cathode of a fourth unidirectional electronic valve  50  is coupled to a second end of second winding BXS. A center tap of first winding BXF is coupled to a first end of second winding TXS, and a center tap of second winding BXS is coupled to a second end of second winding TXS. 
     In operation, and as will be described further below, driving arrangement  10  provides a balanced current for 4 LED strings  40  with a single balancing transformer BX. The 4 LED strings  40  are configured with a common anode structure. The balancing transformer BX has two center tapped windings, each of the two windings BXF and BXS having the same number of turns. The center taps of BXF, BXS and TXS are each preferably arranged such that an equal number of turns are exhibited between the center tap and the respective opposing ends of the winding. 
     Switching control circuit  20  is arranged to alternately close first electronically controlled switch Q 1  and second electronically controlled switch Q 2  so as to provide a switching cycle having a first period during which electrical energy is output from second winding TXS with a first polarity and a second period during which electrical energy is output from second winding TXS with a second polarity, the second polarity opposite the first polarity. 
     During the first period, when the end of second winding TXS coupled to the center tap of first winding BXF is negative in relation to the center tap of second winding TXS, current flows through the two LED strings  40  coupled to the respective ends of first winding BXF. During the second period, when the end of second winding TXS coupled to the center tap of second winding BXS is negative in relation to the center tap of second winding TXS, current flows through the two LED strings  40  coupled to the respective ends of second winding BXS. The current through the two LED strings  40  conducting during the first period are forced to be equal by the balancing effect of the two winding halves of first winding BXF, and current through the two LED strings  40  conducting during the second period are forced to be equal by the balancing effect of the two winding halves of second winding BXS. DC blocking capacitor CX ensures that the current flowing through first winding TXF, and hence transferred to second winding TXS, during each of the two periods is equal, because DC blocking capacitor CX does not couple DC current in steady state. In the event that the average operating voltage of the two LED strings  40  coupled to first winding BXF is different than the average operating voltage of the two LED strings  40  coupled to second winding BXS, a DC bias will automatically develop across DC blocking capacitor CX to offset the average operating voltage difference. The DC bias acts to maintain an equal total current for each of the two string groups, i.e. the first group comprising two LED strings  40  coupled to first winding BXF and the second group comprising two LED strings  40  coupled to second winding BXS. 
     To further clarify and illustrate this relationship, we denote the current through the two LED strings  40  coupled to first winding BXF, respectively, as I LED1  and I LED2 . We further denote the current through the two LED strings  40  coupled to second winding BXS, respectively, as I LED3  and I LED4 . This results in the following relations.
 
 I   LED1   +I   LED2   =I   LED3   +I   LED4  (Responsive to  CX )  EQ. 1
 
 I   LED1   =I   LED2   , I   LED3   =I   LED4  (Responsive to  BX )  EQ. 2
 
And as result of EQ. 1 and EQ. 2: I LED1 =I LED2 =I LED3 =I LED4  
 
     Smoothing capacitors CS are each connected in parallel with a respective one of LED strings  40  to smooth out any ripple current and maintain the associated LED current to be nearly a constant direct current. Unidirectional electronic valves  50  are arranged to block any reverse voltage to LED strings  40  and further prevent bleeding of current between respective smoothing capacitors CS. 
       FIG. 2  illustrates a high level schematic diagram of an embodiment of a driving arrangement  100  for four LED strings  40 , wherein the anode end of each LED string  40  is commonly coupled to the center tap of second winding TXS of driving transformer TX, the cathode ends of the various LED strings  40  are each coupled to respective ends of windings of balancing transformer BX, and the center taps of the balancing transformer windings, BXS and BXF, are coupled to driving transformer second winding TXS via respective unidirectional electronic valves  50 . Driving arrangement  100  is a simplified version of driving arrangement  10 , wherein LED strings  40  are allowed to operate with a rippled current, and thus smoothing capacitors CS are not supplied and only a single unidirectional electronic valve  50  is required for each two LED strings  40 . 
     In some further detail, the center tap of second winding TXS is commonly coupled to the anode end of each of the four LED strings  40 . The cathode end of first LED string  40  is coupled to a first end of first winding BXF; the cathode end of second LED string  40  is coupled to a second end of first winding BXF; the cathode end of third LED string  40  is coupled to a first end of second winding BXS; and the cathode end of fourth LED string  40  is coupled to a second end of second winding BXS. The center tap of first winding BXF is coupled via a respective unidirectional electronic valve  50  to a first end of second winding TXS and the center tap of second winding BXS is coupled via a respective unidirectional electronic valve  50  to a second end of second winding TXS. Switching control circuit  20  is not shown for simplicity, and the connections of switching bridge  30 , DC blocking capacitor CX and first winding TXF are as described above in relation to driving arrangement  10 . 
     The operation of driving arrangement  100  is in all respects similar to the operation of driving arrangement  10 , and thus in the interest of brevity will not be further detailed. 
       FIG. 3  illustrates a high level schematic diagram of an embodiment of a driving arrangement  200  having two LED strings  40 . Switching control circuit  20  is not shown for simplicity, and the connections of switching bridge  30 , DC blocking capacitor CX and first winding TXF are as described above in relation to driving arrangement  10 . The anode end of each of the LED strings  40  are commonly coupled to the center tap of second winding TXS of driving transformer TX. The cathode end of a first LED string  40  is coupled to a center tap of first winding BXF of balancing transformer BX, and the cathode end of a second LED string  40  is coupled to a center tap of second winding BXS of balancing transformer BX. The ends of first winding BXF are each coupled via a respective unidirectional electronic valve  50  to respective ends of second winding TXS of driving transformer TX and respective ends of second winding BXF are each coupled via a respective unidirectional electronic valve  50  to respective ends of second winding TXS of driving transformer TX. 
     Each winding of balancing transformer BX thus drives a single LED string  40 . The LED strings  40  each conduct in both half cycles and therefore the ripple current frequency is twice that of the switching frequency of Q 1  and Q 2 . Opposing halves of first winding BXF conduct during the respective first and second periods generated by switching control circuit  20  and opposing halves of second winding BXS conduct during the respective first and second periods generated by switching control circuit  20  (not shown). Therefore the core of balancer transformer BX experiences an AC excitation. The connection polarity of balancer windings BXF and BXS is such so as to always keep the magnetization force generated by the current of the two LED strings  40  in opposite directions, and by such magnetization force the current of the two LED strings  40  are forced to be equal. 
     Driving arrangements  10 ,  100  and  200  illustrate a common anode structure for LED strings  40 , however this is not meant to be limiting in any way, as will be further illustrated below. 
       FIG. 4  illustrates a high level schematic diagram of an embodiment of a driving arrangement  300  exhibiting four LED strings  40 . Switching control circuit  20  is not shown for simplicity, and the connections of switching bridge  30 , DC blocking capacitor CX and first winding TXF are as described above in relation to driving arrangement  10 . The cathode ends of a first two LED strings  40  are commonly coupled to a first end of second winding TXS of driving transformer TX via a common respective unidirectional electronic valve  50  and the cathode ends of a second two LED strings  40  are commonly coupled to a second end of second winding TXS of driving transformer TX via a common respective unidirectional electronic valve  50 . The anode end of first LED string  40  is coupled to a first end of first winding BXF of balancing transformer BS; the anode end of second LED string  40  is coupled to a second end of first winding BXF of balancing transformer BS; the anode end of third LED string  40  is coupled to a first end of second winding BXS of balancing transformer BS; and the anode end of fourth LED string  40  is coupled to a second end of second winding BXS of balancing transformer BS. The center taps of each of first winding BXF and second winding BXS are commonly coupled to the center tap of second winding TXS of driving transformer TX. 
     The operation of driving arrangement  300  is in all respects similar to the operation of driving arrangement  100 , with first and second LED  40  providing illumination during one of the first and second periods, and the third and fourth LED  40  providing illumination during the other of the first and second periods, and in the interest of brevity will not be detailed further. 
       FIG. 5  illustrates a high level schematic diagram of an embodiment of a driving arrangement  400  for two LED strings  40  wherein the cathode end of each of the LED strings  40  are commonly coupled to the center tap of second winding TXS of driving transformer TX. Switching control circuit  20  is not shown for simplicity, and the connections of switching bridge  30 , DC blocking capacitor CX and first winding TXF are as described above in relation to driving arrangement  10 . The anode end of first LED string  40  is coupled to the center tap of first winding BXF of balancing transformer BX and the anode end of second LED string  40  is coupled to the center tap of second winding BXS of balancing transformer BX. A first end of first winding BXF is coupled via a respective unidirectional electronic valve  50  to a first end of second winding TXS of driving transformer TX; a second end of first winding BXF is coupled via a respective unidirectional electronic valve  50  to a second end of second winding TXS of driving transformer TX; a first end of second winding BXS is coupled via a respective unidirectional electronic valve  50  to a first end of second winding TXS of driving transformer TX; and a second end of second winding BXS is coupled via a respective unidirectional electronic valve  50  to a second end of second winding TXS of driving transformer TX. 
     The operation of driving arrangement  400  are in all respects identical with the operation of driving arrangement  200 , with the appropriate changes in polarity as required, and thus in the interest of brevity will not be further detailed. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein. 
     All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.