Abstract:
A current balance scheme for driving LED strings is disclosed. A power supply unit provides a driving voltage to drive a plurality of LED strings. A feedback selector is configured to select a minimum feedback signal indicative of the driving voltage to the power supply unit for regulating the driving voltage. Based on the minimum feedback signal, the driving voltage gets a minimum value to sufficiently drive the plurality of LED strings. A protection circuit is configured to protect the current balance circuit from high voltage especially when there is a shorted LED string.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority to and the benefit of Chinese Patent Application No. 201010269938.5, filed Aug. 24, 2010, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to electrical circuits, and more particularly but not exclusively to LED drivers. 
       BACKGROUND 
       [0003]    Light emitting diode (LED) has become increasingly popular as a lighting choice, and for many applications, has begun to replace conventional lamps having a filament. For example, LED is now widely used in traffic signal lights and for the back lighting of liquid crystal display (LCD) panels. 
         [0004]    The LEDs are often arranged in parallel “strings” driven by a shared voltage source, and each LED string has a plurality of LEDs connected in series. To provide consistent light output between the LED strings, each LED string is typically driven at a regulated current that is substantially equal among all of the LED strings. 
         [0005]    Although driven by currents of equal magnitude, there is often a considerable variation in the driving voltages due to variations in the forward-voltage drop of individual LED of the LED strings resulting from process variations in the fabrication and manufacturing of the LED. Dynamic variations due to changes in temperature when the LEDs are enabled and disabled may also contribute to the variation in the driving voltages with a fixed current. In view of this variation, conventional LED drivers typically provide a driving voltage that is sufficiently higher than an expected worst-case bias drop, so as to ensure proper operation of each LED string. However, as the power consumed by the LED driver and the LED strings is approximately a product of the output voltage of the LED driver and the sum of the currents of the individual LED strings, the use of an excessively high driving voltage unnecessarily increases the power consumption. Accordingly, an improved technique for driving LED strings would be advantageous. 
         [0006]      FIG. 1  schematically shows a prior art LED driver  10 . In  FIG. 1 , the LED driver  10  is realized by a power supply unit configured to supply a driving voltage to a plurality of LED strings. Each LED string is respectively coupled to an adjusting resistor R B  in series. All the adjusting resistors are coupled to the drain of an N-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The source of the N-type MOSFET is coupled to a feedback resistor Rs. The current flowing through each LED string flows through the feedback resistor Rs. The voltage across the feedback resistor Rs is fed back to a pin FB of the power supply unit to regulate the driving voltage. In addition, the power supply unit provides a dimming signal through a DIM pin to the gate of the N-type MOSFET to adjust the conductance of the NMOS, thereby regulating the current flowing through the LED strings. The value of the adjusting resistor R B  in each LED string may be adjusted to make the current in each LED string be substantially similar. However, it is difficult to precisely balance the current in each LED string by the adjusting resistors. Meanwhile, the adjusting resistors in each LED string consume lots of power especially when the current flowing through the LED strings is high. Moreover, the N-type MOSFET coupled between the adjusting resistor and the feedback resistor suffers from high voltage when there is a shorted LED string. The N-type MOSFET may be broken down when the driving voltage is too high. 
         [0007]      FIG. 2  schematically shows another prior art LED driver  20 . The same parts of  FIG. 2  with  FIG. 1  are not described for brevity. In  FIG. 2 , each LED string is respectively coupled to a current source CS. The current provided by the current sources is set by a setting resistor R SET . A voltage divider consisting of R 1  and R 2  is couple to the output of the power supply unit. The power supply unit gets a feedback signal generated by the voltage divider to regulate the driving voltage of the LED strings. The current sources in  FIG. 2  may be regulated by a dimming signal from the DIM pin of the power supply unit too, and may balance the current flowing through each LED string as well. But the power dissipation on the current sources is huge. Moreover, the current source in a shorted LED string suffers from high voltage and may be damaged thereby. 
         [0008]    The present disclosure provides a current balance scheme for driving a plurality of LED strings which is free from the shorted voltage damage and huge power dissipation, and the method thereof. 
       SUMMARY 
       [0009]    It is an object of the present disclosure to provide a current balance scheme for driving a plurality of LED strings and the method thereof. 
         [0010]    In accomplishing the above and other objects, there has been provided, in accordance with an embodiment of the present disclosure, a LED driver, comprising: a power supply unit having an input terminal, a feedback terminal, and an output terminal, wherein the input terminal is configured to receive an input signal, the feedback terminal is configured to receive a minimum feedback signal, and wherein based on the input signal and the minimum feedback signal, the power supply unit generates a driving voltage at the output terminal to drive a plurality of LED strings, wherein the plurality of LED strings comprises N LED strings coupled in parallel, and wherein N is a natural number, and each LED string comprises an upper end and a lower end; a current balancing circuit having an input terminal and N output terminals, wherein N is a natural number, and wherein the input terminal is configured to receive a reference voltage, and each output terminal is respectively coupled to the lower end of each LED string to provide a regulated current to each LED string; and a feedback selector having N input terminals and an output terminal, wherein N is a natural number, and wherein each input terminal is respectively coupled to the lower end of each LED string to receive a feedback signal indicative of a voltage drop of each LED string, and based on the feedback signals, the feedback selector generates the minimum feedback signal at the output terminal. 
         [0011]    In addition, there has been provided, in accordance with an embodiment of the present disclosure, A LED driver, comprising: a power supply unit circuit for generating a driving signal to drive a plurality of LED strings in response to an input signal and a minimum feedback signal; a current balancing circuit for providing a regulated current signal to the plurality of LED strings in response to a reference voltage; and a feedback selector circuit for generating the minimum feedback signal in response to a plurality of feedback signals. 
         [0012]    Furthermore, there has been provided, in accordance with an embodiment of the present invention, a method of driving a plurality of LED strings, comprising: receiving an input signal; deriving feedback signals indicative of voltage drops of each LED string; generating a minimum feedback signal by selecting a feedback signal with minimum magnitude from the feedback signals; providing a regulated current signal to each LED string, wherein the current of each LED string is substantially similar; and generating a driving voltage in response to the input signal and the minimum feedback signal, the driving voltage being used to drive the plurality of LED strings. 
         [0013]    These and other features of the present disclosure will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  schematically shows a prior art LED driver  10 . 
           [0015]      FIG. 2  schematically shows a prior art LED driver  20 . 
           [0016]      FIG. 3  schematically shows a LED driver  30  in accordance with an embodiment of the present disclosure. 
           [0017]      FIG. 4  schematically shows a current source  40  in accordance with an embodiment of the present disclosure. 
           [0018]      FIG. 5  schematically shows a LED driver  50  in accordance with an embodiment of the present disclosure. 
           [0019]      FIG. 6  shows a schematic flowchart  60  of the method of controlling a LED driver in accordance with en embodiment of the present disclosure. 
       
    
    
       [0020]    The use of the same reference label in different drawings indicates the same of like components. 
       DETAILED DESCRIPTION 
       [0021]    In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the disclosure. Persons of ordinary skill in the art will recognize, however, that the disclosure may be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the disclosure. 
         [0022]      FIG. 3  schematically shows a LED driver  30  in accordance with an embodiment of the present disclosure. In the example of  FIG. 3 , the LED driver  30  comprises: a power supply unit  302  having an input terminal, a feedback terminal, and an output terminal, wherein the input terminal is configured to receive an input signal, the feedback terminal is configured to receive a minimum feedback signal, and wherein based on the input signal and the minimum feedback signal, the power supply unit  302  generates a driving voltage at the output terminal to drive a plurality of LED strings, wherein the plurality of LED strings comprises N LED strings coupled in parallel, wherein N is a natural number, and each LED string comprises an upper end and a lower end; a current balancing circuit  301  having an input terminal and N output terminals, wherein N is a natural number, and wherein the input terminal is configured to receive a reference voltage, and each output terminal is respectively coupled to the lower end of each LED string to provide a regulated current to each LED string; and a feedback selector  303  having N input terminals and an output terminal, wherein N is a natural number, and wherein each input terminal is respectively coupled to the lower end of each LED string to receive a feedback signal indicative of a voltage drop of each LED string, and based on the feedback signals, the feedback selector  303  generates the minimum feedback signal at the output terminal. 
         [0023]    In  FIG. 3 , the power supply unit  302  regulates the driving voltage based on the minimum feedback signal, wherein the driving voltage thereby has the minimum value to sufficiently drive the plurality of LED strings. The power supply unit may comprise any kinds of voltage regulator. The current flowing through the LED strings is determined by the current balancing circuit  301  which may be an integrated circuit or several discrete components. 
         [0024]    In one embodiment, the current balancing circuit  301  comprises: a plurality of current sources, wherein each single current source CS is coupled to the lower end of each LED string, to supply and regulate the current of each LED string, and wherein the voltage across each current source CS is coupled to the feedback selector  303  as the feedback signal; and a current setting resistor RSET having a first terminal and a second terminal, wherein the first terminal is coupled to every current source CS and the second terminal is connected to ground, and wherein the current supplied by the current source CS is adjusted by changing the value of the current setting resistor R SET . 
         [0025]    The feedback signals indicative of the driving voltage are all coupled to the feedback selector  303 . In the example of  FIG. 3 , the feedback selector  303  is configured to provide the minimum feedback signal by selecting a feedback signal with minimum magnitude from all feedback signals, so as to get a minimum driving voltage which is sufficient to drive all of the LED strings. The minimum driving voltage decreases the power dissipation caused by the excessively high driving voltage which is used to ensure proper operation of each LED string. 
         [0026]    In one embodiment, the power supply unit  302  further comprises a dimming output terminal configured to provide a dimming signal to each current source. The dimming signal may be a pulse width modulation (PWM) signal. The function of the dimming signal is described with referring to  FIG. 4 . 
         [0027]      FIG. 4  schematically shows a current source  40  in accordance with an embodiment of the present disclosure. Persons of ordinary skill in the art should know that the current source CS used in the current balancing circuit  301  of  FIG. 3  may be realized in other ways. In one embodiment, the current source  40  comprises: an amplifier EA having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal is configured to receive the reference voltage V REF , the second input terminal is coupled to a current adjusting resistor Rs to receive a voltage drop of the current adjusting resistor Rs, and based on the reference signal V REF  and the voltage drop of the current adjusting resistor Rs, the amplifier EA provides an amplified signal at the output terminal; the current adjusting resistor Rs coupled between the second input terminal of the amplifier EA and ground; and a transistor Q 1  having a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the conjunction of the amplifier EA and the current adjusting resistor Rs, the control terminal is coupled to the output terminal of the amplifier EA to receive the amplified signal, and the second terminal is coupled to the lower end of the corresponding LED string to provide a regulated current to each LED string. 
         [0028]    Due to the “short” characteristic of the amplifier EA, the voltage across the adjusting resistor Rs is clamped to the reference voltage V REF  by the amplifier EA, so that the current flowing through the adjusting resistor Rs is fixed to V REF /Rs. The current supplied by each current source may be regulated by changing the adjusting resistor Rs if the reference voltage V REF  is fixed. The reference voltage V REF  of all current sources is supplied by one voltage source to eliminate the variance. In one embodiment, the reference voltage V REF  is the voltage across the setting resistor R SET  in  FIG. 3 . A current source I SET  supplies current to the setting resistor R SET . Persons or ordinary skill in the art should know that the current source I SET  may be provided by the power supply unit  302 . The voltage across the setting resistor R SET  could be regulated by modifying the current source I SET  or the setting resistor R SET . The dimming signal generated by the power supply unit is used to control the amplifier EA of the current source  40 , and thereby regulates the current of the current source. 
         [0029]      FIG. 5  schematically shows a LED driver  50  in accordance with an embodiment of the present disclosure. Compared to the example in  FIG. 3 , the LED driver  50  in  FIG. 5  further comprises a protection circuit  304  coupled between the lower end of the LED strings and the current balancing circuit  301 , to protect the current balancing circuit  301  from over voltage. 
         [0030]    In one embodiment, the protection circuit  304  comprises N transistors, wherein each transistor T has a first terminal, a second terminal and a control terminal, and wherein the control terminal of the transistor is coupled to a predetermined voltage V G , the first terminal of each transistor is respectively coupled to the lower end of each LED string, and the second terminal of the transistor T is coupled to the current balancing circuit  301 , and further wherein each transistor T has a breakdown voltage higher than the driving voltage. Persons of ordinary skill in the art should know that the transistor T may be realized by MOSFET, triode, JFET and other semiconductor devices. 
         [0031]    The operation of the LED driver  50  is similar to the operation of the LED driver  30 . The protection circuit protects the current balancing circuit from high voltage which maybe the driving voltage when the LED string coupled to this current balancing circuit is shorted. The breakdown voltage of the transistor used in the protection circuit is chosen to be higher than the driving voltage so as to suffer the most of the driving voltage when the LED string is shorted. Thus the current source CS is protected. 
         [0032]    Furthermore, the present disclosure discloses a method of controlling a LED driver. Referring to  FIG. 6 , a schematic flowchart  60  of the method is shown in accordance with an embodiment of the present disclosure. The method comprises: step  601 , receiving an input signal; step  602 , deriving feedback signals indicative of voltage, drops of each LED string; step  603 , generating a minimum feedback signal by selecting a feedback signal with minimum magnitude from the feedback signals; step  604 , providing regulated current to each LED string, wherein the current of each LED string is substantially similar; and step  605 , generating a driving voltage in response to the input signal and the minimum feedback signal, the driving voltage being used to drive the plurality of LED strings. 
         [0033]    An effective technique for getting the feedback signal from the primary side of the switching mode power supply has been disclosed. While specific embodiments of the present disclosure have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.