Patent Publication Number: US-2015061514-A1

Title: Light emitting diode driving device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a driving device, and in particular to a light emitting diode (LED) driving device. 
     2. Description of Related Art 
     Flat display device, such as a liquid crystal display apparatus, has advantages of high quality, small size and light weight, and become the mainstream of the display device. In order to achieve characteristic of low power consumption, light emitting diodes (LEDs) having advantage of small power consumption are applied to replace incandescent or fluorescent tube and provide light source for flat display device. 
     LED is a low-voltage single-conduct component driven by current, and its luminous flux is changed according to current flowing therethrough. Owing to the fact that it is mentioned above, a flat display device with uniform illumination can be achieve by providing a constant current to strings or matrix composed of LEDs. In the past, a constant-current component, such as current mirror, is applied to each LED string of LED driving circuit to stabilize current flowing therethrough, however the skeleton of the LED driving circuit is then becoming huge. Thus how to effectively stabilize flowing through all LED strings electrically connected to the LED driving device and minimize skeleton of the LED driving device becomes anxious problem for people skilled in the art. 
     SUMMARY OF THE INVENTION 
     It is an object to provide a LED driving device, the LED driving device is configured to provide a constant voltage and a constant current to LED strings such that the LED strings can be let with uniform illumination. 
     Accordingly, the LED driving device according to one aspect of the present invention is electrically connected to a voltage power source and a plurality of LED strings. The LED driving device comprises a constant-voltage outputting module, a rectifying unit, and a constant-current controlling unit. The constant-voltage outputting module is electrically connected to the voltage power source and comprises a switching unit, a resonant unit, a converting unit, and a power-balancing unit. The resonant unit is electrically connected to the switching unit. The converting unit comprises a primary winding, two second secondary windings, and a second secondary winding. The primary winding is electrically connected to the resonant unit. The power-balancing unit comprises a balancing converter comprising two balancing windings. The balancing windings are electrically connected to the primary winding. The rectifying unit is electrically connected to the power-balancing unit and the LED strings. The constant-current controlling unit is electrically connected to the second secondary winding and the LED strings. 
     In an embodiment of the present invention, the constant-current controlling unit comprises a driving component and a power-converting circuit, the driving component is electrically connected to the LED strings, the power-converting circuit is electrically connected to the second secondary winding. 
     In an embodiment of the present invention, the power-balancing unit further comprises two balancing capacitors electrically connected to the first primary winding and the rectifying unit, each balancing capacitor, each first secondary winding, and each balancing winding are electrically connected in parallel. 
     In an embodiment of the present invention, the LED driving device further comprises a controller electrically connected to the second secondary winding and the switching unit, the controller is configured to control switching frequencies of the switching unit. 
     In an embodiment of the present invention, the LED driving device further comprises a plurality of protecting capacitors, each protecting capacitor is electrically connected to each LED string in series. 
     In an embodiment of the present invention, the rectifying unit comprises two first rectifiers and two second rectifier, the first rectifiers are electrically connected to the first secondary windings and the LED strings, respectively, the second rectifier is electrically connected to the second secondary winding. 
     In an embodiment of the present invention, the LED driving device further comprises a voltage-level sensing capacitor electrically connected to the second rectifier. 
     In an embodiment of the present invention, the balancing windings comprise the same turn number. 
     In an embodiment of the present invention, the resonant unit comprises a resonant capacitor, a resonant inductor, and a magnetizing inductor, the resonant capacitor, the resonant inductor, and the magnetizing inductor are electrically connected in series, the magnetizing inductor is electrically connected to the primary winding in parallel. 
     In an embodiment of the present invention, the switching unit comprises two switching components electrically connected to the controller, the controller is configured to modulated switching frequencies of the switching components. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
       The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, may be best understood by reference to the following detailed description of the invention, which describes an exemplary embodiment of the invention, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit block diagram of a light emitting diode (LED) driving device according to the present invention. 
         FIG. 2  is a circuit diagram of the LED driving device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention will be described with reference to the drawings. 
     Reference is made to  FIG. 1 , which is a circuit block diagram of a light emitting diode (LED) driving device according to the present invention. The LED driving device  1  is electrically connected to a plurality of LED strings  3  for illuminating the LED strings  3 . The LED driving device  1  includes a constant-voltage outputting module  4 , a rectifying unit  16 , and a constant-current controlling unit  20 . The constant-voltage outputting module  4  is configured to output a constant voltage to conduct the LED strings. The constant-current controlling unit  20  is configured to output a constant current such that the LED strings  3  can be illuminated with uniform illumination. 
     Reference is made to  FIG. 2 , which is a circuit diagram of the LED driving device according to the present invention. The LED driving device  1  is electrically connected to a voltage power source VIN and LED strings  3 . The voltage power source VIN is direct current (DC) voltage power source. The LED strings  3  include a plurality of LED components  30  electrically connected in series. In this embodiment, an amount of the LED strings  3  is, for example, three. Each LED component  30  is, for example, LED die or LED package, wherein LED die is composed of a plurality of semiconductor layers, and LED package is LED die mentioned above covered with transmissive resin or phosphor. The LED driving device  1  is configured to drive the LED strings  3  such that the LED strings  3  can be illuminated with uniform illumination. 
     The constant-voltage outputting module  4  includes a switching unit  10 , a resonant unit  12 , a converting unit  14 , and a power-balancing unit  18 . The switching unit  10  is electrically connected to the voltage power source VIN. The resonant unit is electrically connected to the switching unit  10 . The converting unit  14  is electrically connected to the resonant unit  12 . The power-balancing unit  18  is electrically connected to the converting unit  14 . The rectifying unit  16  is electrically connected to the power-balancing unit  18 , the constant-current controlling unit  20 , and the LED strings  3 . 
     The switching unit  10  is configured to switch the voltage power source into pulsating DC signal, and in this embodiment, the switching unit  10  is a half-bridge switching circuit including two switching components  100 . The switching components  100  are, for example, metal-oxide-semiconductor field-effect transistor (MOSFET). However, in the practical application, the switching unit  10  can be full-bridge switching circuit, and includes switching components such as insulated gate bipolar transistor (IGBT). The switching unit  10  is electrically connected to a controller  11 . the controller  11  is configured to control switching frequencies of the switching components  100 . 
     The resonant unit  12  is electrically connected to the switching components  100 , and receives DC pulsating signal while the switching component  100  being turned-on and turned-off. Besides, the resonant unit  12  also modulates voltage level according to the pulsating signal outputted from the switching unit  100  such that a constant voltage can be outputted therefrom. The resonant unit  12  includes a resonant capacitor  120 , a resonant inductor  122 , and a magnetizing inductor  124 . The resonant capacitor  120  is electrically connected to the resonant inductor  122  in series, and the magnetizing inductor  124  is electrically connected to a primary winding of the converting unit  12  in parallel. The magnetizing inductor  124  is also electrically connected to the resonant inductor  122 . The resonant capacitor  120  is not only used for blocking DC component of the DC pulsating signal, but constructing a resonant tank with the resonant inductor  122  and the magnetizing inductor  124 . In this embodiment, the magnetizing inductor  124  is an external inductor as shown in  FIG. 2 , however, the magnetizing-inductor  124  can also be magnetizing inductance of the converting unit  14 . 
     The converting unit  14  includes a primary winding  140 , two first secondary winding  142 , and a second secondary winding  144 . The primary winding  140  is electrically connected to the resonant unit  12  and converts electric energy into magnetic energy such that the task of stepping-up or stepping-down voltage magnitude is achieved. The first secondary windings  142  have the same turn number. The turn number of the second secondary winding  144  can be the same as the turn number of first secondary windings  144 , or different form the turn number of the first secondary winding  142 . In this embodiment, the second secondary winding  144  is a center-tapped converter. 
     The second secondary winding  144  is electrically connected to a voltage-level sensing capacitor  15 . The voltage-level sensing capacitor  15  is used for sensing outputting voltage of the second secondary winding  144 . By modulating ratio of second secondary winding  144  and the first secondary winding  142 , the voltage level of voltage outputted form the first secondary windings  142  can be sensed by sensing the voltage level of the voltage-level sensing capacitor  15 . Besides, the controller  11  is also electrically connected to the second secondary winding  144 , the controller  11  modulates levels of voltages outputted form the first secondary windings  142  and the second secondary winding  144  by detecting level of voltage outputted form the second secondary winding  144  and controlling switch frequencies of the switching component  100 . 
     The rectifying unit  16  includes two first rectifier  160  and a second rectifier  162 . The first rectifier  160  is electrically connected to the first secondary winding  142  and the LED strings  3  such that alternative current (AC) power source can be converted into DC current power source with high-frequency ripple component. In this embodiment, the first rectifier  160  is a full-bridge rectifying circuit composed of four diodes  161 , and the second rectifier  162  is a fill-bridge composed of two diodes  163 . 
     The power-balancing unit  18  includes a balancing converter  180  and two balancing capacitor  182 . The balancing converter  180  includes two balancing windings  184  having the same turn number. The balancing windings  182  are electrically connected to the first secondary windings  142  and the first rectifier  160 , respectively for balancing currents flow through the balancing windings  182  such that the currents flow through the rectifying unit  16  are uniform. The balancing capacitor  182  is electrically connected to the first secondary windings  142  and the first rectifier  160 . The balancing capacitors  182  have function of stabilizing voltage. Each balancing capacitor  182 , each first secondary winding  142 , and each balancing winding  182  are electrically connected in series. 
     The constant-current controlling unit  20  includes a driving component  200  and a power-converting circuit  202 . The driving component  200  is used for detecting current flow through the LED strings  3  and feedbacks the value of current mention above into the power-converting circuit  202 . When value of current flows through the LED strings  3  is larger or smaller than a predetermined value, the power-converting circuit  202  reduces or increases level of voltage outputted form the constant-voltage outputting module  4 , such that the current flows through the LED strings  3  can be maintained at the predetermined value. In this embodiment, the power-converting circuit  202  is, for example, a boosting circuit, however the power-converting circuit  202  can also be buck converting circuit. 
     Besides, the LED driving device  1  further comprises a plurality of protecting capacitor  22 , each protecting capacitor  22  is electrically connected to each LED string  3  in parallel. Each LED string  3  has a particularly operating voltage, and the particularly operating voltage is equal to the sum of forward voltages of LEDs  30  of each LED string  3 . Owing to the operating voltage of each LED string  3  is different the others, the voltage outputted form the first secondary winding  142  of the converting unit  14  must be the same as the largest operating voltage of the operating voltages of the LED strings  3 , therefore the LED strings  3  can be driven successfully. Each protecting capacitor  22  is used for storing difference between the largest operating voltage and the operating voltage of the LED strings  3  smaller than the largest operating voltage to prevent the LEDs  30  from damaging causing by over-voltage. 
     It is noted that when the amount of the LED strings  3  is added, the amount of the first secondary winding  142 , the balancing winding  182   m  and the first rectifier  160  must be correspondingly added. In additions, two LED strings  3  electrically connected in parallel can be added when a first secondary winding  142  is added. However, only one LED string  3  can be added while a first secondary winding  142  is added. Besides, while a first secondary winding  142  is added, a balancing winding  182  and a first rectifier  160  must be correspondingly added. 
     In the practical application, the switching unit  10  switches DC signal outputted form the voltage power source VIN and received form the LED driving device  1  into pulsating DC signal. The resonant unit  12  modulates voltage outputted from the switching unit  12  according to the switching frequency of the voltage, and then sends the modulated voltage to the converting unit  14 . The converting unit  14  provides function of power conversion, the first secondary windings  142  thereof outputs a voltage for illuminating the LED strings  3 , and the second secondary winding  144  thereof outputs a voltage for calibration. 
     The balancing winding  182  of the power balancing unit  18  is configured to uniform currents conducted to the first rectifier  160  of the converting unit  16 , the balancing capacitor  182  is used for stabilizing voltage, therefore the currents flow through the LED strings  3  can be maintained in a predetermined value, and the LED strings  3  achieve uniform illumination. The driving component  200  of the constant-current controlling unit  20  is used for detecting whether currents flowing through the LED strings  3  achieve the predetermined value or not. For example, the constant-voltage module  4  must output 600 mA if current flows through each LED string  3  is 200 mA. The constant-voltage outputting module  4  maintains voltage outputted therefrom in a constant value while the voltage outputted form the constant-voltage outputting module  4  makes current flows through the LED strings  3  achieve the predetermined value. However, if the current outputted form the constant-voltage outputting module  4  and detected by the constant-current controlling unit  20  cannot achieve the predetermined value, the driving component  300  sent a signal to the power-converting circuit  202  and makes the controller  11  modulate switching frequencies of the switching component  100  to u=increase value of voltage conducted to the LED strings  3 . 
     To sum up, the LED driving device  1  according to the present invention uses the constant-voltage outputting module  4  and the constant-current controlling unit  20  to maintain values of voltage and current conduct to the LED strings  3 , such that the LED strings  3  can be illuminated with uniform illumination. Besides, the constant-voltage outputting module  4  and the constant-current controlling unit  20  of the LED driving device  1  according to the present invention can simultaneously electrically connected to multiple LED strings  3 , therefore volume of the LED driving device  1  can be reduced during to circuit skeleton of the LED driving device  1  is scaled down. 
     Although the present invention has been described with reference to the foregoing preferred embodiment, it will be understood that the invention is not limited to the details thereof. Various equivalent variations and modifications can still occur to those skilled in this art in view of the teachings of the present invention. Thus, all such variations and equivalent modifications are also embraced within the scope of the invention as defined in the appended claims.