Abstract:
A power converter circuit converts an AC line signal to a DC signal for powering an organic light emitting diode. The circuit uses only capacitive elements to limit current to the LED. Inductive and resistive elements are not included in the circuit to limit current. The absence of inductive components eliminates electromagnetic interference generated by the circuit and avoids circuit components magnetically coupling to one another. The circuit includes complementary MOSFET switches that alternately conduct to convert the AC line voltage into a DC current for powering the LED.

Description:
BACKGROUND 
       [0001]    The present application relates to artificial lighting systems. It finds particular application when used in conjunction with organic light emitting diodes (OLEDs) and will be described with particular reference thereto. It is to be understood, however, that it is also applicable to applications that use silicon based diodes, or other inorganic semiconductors, and is not necessarily limited to the aforementioned application. 
         [0002]    Incandescent lamps have found wide range success in the lighting industry, meeting a wide range of commercial demands. The incandescent lamp, however, does have several drawbacks that make it unsuitable for certain applications. For instance, the filament generates a substantial amount of heat, and the vacuum chamber that encapsulates the filament occupies a substantial amount of space. Additionally, incandescent filaments can be easily damaged, as they are sensitive to shock and vibration. Thus, it is impractical or impossible to use incandescent lamps for certain applications. 
         [0003]    Light emitting diodes (LEDs) have demonstrated that they are a viable alternative to incandescent lamps in conventional roles, and can fulfill additional lighting roles where incandescent lamps cannot. LEDs have a robust, compact structure that makes them ideal for applications with space constraints, and applications prone to mechanical shock and vibration. 
         [0004]    With such a stress in the industry on space savings, (e.g. cell phones, PDAs, laptops, flat panel displays, and the like) smaller is better. Even though the LEDs themselves are small, durable, and dissipate relatively little power, additional space is required by circuitry that supports the LED. Typically, the source current is much greater than the operating current of the LED. To limit the current to the LED, resistors are typically used to limit the current by dissipating energy, also generating heat. Resultantly, additional space is needed to provide adequate cooling for the circuit components. 
         [0005]    Also typical is the use of planar magnetics in conventional LED supporting circuitry. A planar magnetic implementation uses a high frequency power converter that causes electromagnetic interference. Bulky filters are then used to suppress the interference generated by the power converter. Both the inductors used in the planar magnetics and the filters add bulk to the LED supporting circuitry. 
       BRIEF DESCRIPTION 
       [0006]    In accordance with one aspect, a power converter circuit for supplying power to a light emitting diode is provided. A voltage source supplies power to the circuit. A first switch is connected to the voltage source. A second switch is also connected to the voltage source. The second switch is in parallel to the first switch with respect to the voltage source. A first capacitor stores charge during a conductive state of the first switch. A second capacitor stores charge during a conductive state of the second switch. At least one light emitting diode illuminates upon receiving forward current from at least one of the first and second switches. 
         [0007]    In accordance with another aspect, an AC to DC converter for limiting current to a light emitting diode consisting of only non-inductive components is provided. A voltage source provides power to the converter. A first switch is connected to the voltage source. A second switch is also connected to the voltage source. The second switch is in parallel to the first switch with respect to the voltage source. A first capacitor stores charge during a conductive state of the first switch. A second capacitor stores charge during a conductive state of the second switch. At least one light emitting diode illuminates upon receiving forward current from at least one of the first and second switches. 
         [0008]    In accordance with another aspect, a method of limiting current to a light emitting diode using only non-inductive components is provided. First and second switches are placed in a parallel relationship with respect to a voltage source for providing current to a light emitting diode. A first current limiting capacitor is connected between the first switch and the voltage source. A second current limiting capacitor is connected between the second switch and the voltage source. The current across the light emitting diode is sensed with a current sensing resistor 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  depicts a circuit diagram of an OLED power converter in accordance with the present application; 
           [0010]      FIG. 2  is a voltage comparison over time of signals across transistor switches, in accordance with the present application; 
           [0011]      FIG. 3  is a graph depicting current across a light emitting diode with respect to time in accordance with the present application; 
           [0012]      FIG. 4  is a graph depicting voltage across the light emitting diode with respect to time in accordance with the present application. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    With reference to  FIG. 1 , an exemplary AC to DC power converter for supplying power to an LED is depicted. A voltage source  10  supplies an AC signal that powers an LED  12 . In one embodiment, the voltage source  10  is a typical line voltage, such as a 60 Hz 120 V rms signal, and the LED is an organic LED (OLED). Of course, sources with different values could be used without departing from the scope of the claims. Operation at the power line frequency gives the added advantage of producing no interference within FCC regulatory bands. 
         [0014]    A first switch  14  and a second switch  16  are connected to the source  10  and are in parallel with one another with respect to the source  10 . The switches  14 ,  16  act as selective gates for a traditional charge pump circuit. The switches  14 ,  16  are conventional power MOSFETs in one embodiment. A thin profile, D-pack MOSFET can be used, and possibly mounted to the back of a circuit board to save room. The switches  14 ,  16  are turned on as soon as a negative drain-source voltage is sensed. In effect, the transistor switches  14 ,  16  act as diodes in the sense that they respond to a positive forward voltage. Instead of utilizing the intrinsic body diode of the transistor, however, the channel of the transistor is used, improving efficiency and reducing the conduction loss. 
         [0015]    A control circuit  18  senses the drain-source voltage of the second transistor switch  16 . In the depicted embodiment, the transistor switches  14 ,  16  are enhancement-type MOSFETs. An n-channel enhancement-type MOSFET conducts whenever a control signal is applied to its gate. When the control circuit  18  senses a negative drain-source voltage across the second transistor switch  16 , the control circuit applies a control signal to the gate of the second transistor switch  16 . This causes the channel of the second transistor switch  16  to conduct current. The control circuit is able to regulate the current through an error amplifier that senses the output current via a small sense resistor  19  placed in series with the LED  12 . In one embodiment, the control circuit  18  is embodied in a low-power integrated circuit that operates at 1-1000 gW per channel, such as a micropower logic circuit. 
         [0016]    A voltage comparator  20  similarly controls the first transistor  14 . When a negative drain-source voltage is sensed across the first transistor switch  14 , the voltage comparator  20  applies a control signal to the gate of the first transistor switch  14 , turning it conductive. Due to the circuit layout, the transistor switches  14 ,  16  alternate periods of conduction. The voltage comparator  20  is the slave in a master-slave relationship with the control circuit  18 . The voltage comparator  20  merely responds to the voltages it senses based on the operation of the second switch  16 . The control circuit  18  does not allow the switches  14 ,  16  to both be conductive during the same period of time. This can be seen in  FIG. 2 . Signal  21   a  is the voltage across the first transistor switch  14 , and signal  21   b  is a complementary voltage across the second transistor switch  16 . As mentioned previously, when one switch is conductive, the other switch is non-conductive. In one embodiment, the voltage comparator  20 , like the control circuit  18  is embodied in a micropower logic circuit. 
         [0017]    With reference again to  FIG. 1 , energy is stored in a first current limiting capacitor  22  and a second current limiting capacitor  24 . By varying the conduction times of the switches  14 ,  16 , the stored energy is delivered to an output node  26 . As their names suggest, the current limiting capacitors  22 ,  24  limit the current flow across the light emitting diode  12 . The current (I 0 ) seen at the output node  26  is determined by the reactance of the current limiting capacitors  22 ,  24 . The current I 0  is shared by the light emitting diode  12  and a storage capacitor  28 . When the switch  14  is not conductive, energy stored in the storage capacitor  28  can be discharged to power the light emitting diode  12 . In one embodiment, the value of the storage capacitor  28  is much greater than the values of the current limiting capacitors  22 ,  24 . In one embodiment, the capacitors  22 ,  24 ,  28  are ceramic capacitors. Ceramic capacitors are thin and highly heat tolerant, so they make good candidates for inclusion in a circuit where one of the objects is to make it as thin as possible. 
         [0018]    In one embodiment, the desired output current (I 0 ) is 300 mA. Knowing that desired value, and the rms value of the input voltage V in , the necessary impedance X c  (in Ohms) of the current limiting capacitors  22 ,  24  can be found by the relationship: 
         [0000]    
       
         
           
             
               X 
               c 
             
             = 
             
               
                 
                   2 
                 
                  
                 
                   V 
                   in 
                 
               
               
                 π 
                  
                 
                     
                 
                  
                 
                   I 
                   0 
                 
               
             
           
         
       
     
         [0019]    Then, since capacitance is related to impedance by the frequency f of the signal, the necessary capacitance values of the current limiting capacitors  22 ,  24  can be found by the following relationship if the frequency is known. 
         [0000]    
       
         
           
             C 
             = 
             
               1 
               
                 2 
                  
                 
                     
                 
                  
                 π 
                  
                 
                     
                 
                  
                 
                   fX 
                   c 
                 
               
             
           
         
       
     
         [0020]    Thus, in one embodiment, where the voltage source  10  is a typical line voltage of 120 V at 60 Hz, and the desired output current is 300 mA, the approximate value of the capacitors is 14.73 μF. Using that capacitance value for the current limiting capacitors  22 ,  24 , the current signal  30  as shown in  FIG. 3  results across the OLED  12 , and the voltage  32  as shown in  FIG. 4  results across the OLED  12 . 
         [0021]    It is to be noted that the output current I 0  is regulated without the use of any inductive or resistive circuit components. The absence of inductors means that there will be no electromagnetic interference generated, and thus obviates the need for electromagnetic shielding, filtration, or compensation. Further, there is no need to worry about magnetic coupling between components. This simplifies the converter. Since an inductor is typically bulky compared to other circuit elements, eliminating them also saves space and reduces the physical profile of the circuit once it is mounted to a circuit board. In one embodiment, the power converter can be contained in a space with a vertical profile of less than 10 mm. 
         [0022]    As noted above, current limiting resistive components have also been eliminated. Typically, resistors can be used to limit current by dissipating energy in the form of heat. Resistive components are typically the first components to fail due to overheating. Thus, by eliminating current-limiting resistive components, the profile of the converter can be further reduced because not as much space is needed to allow for cooling. The elimination of the inductive and resistive components reduces size and increases reliability and life of the power converter. 
         [0023]    The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.