Patent Publication Number: US-2005122062-A1

Title: Light emitting diode driving circuit

Description:
BACKGROUND OF THE INVENTION  
      (a) Field of the Invention  
      The invention relates to a light emitting diode (LED) driving circuit, and more particularly, to an LED driving circuit characterized that, when a power supply passes through a current control circuit, using charging and discharging of a capacitor and parallel equivalent resistance current-limiting characteristics of a capacitor and a resistor, a rectification circuit converts an alternating current into a direct current that is outputted to an LED array, thereby providing the LED array with a stable quota driving current.  
      (b) Description of the Prior Art  
      With reference to  FIG. 1  showing a prior light emitting diode (LED) driving circuit, wherein an alternating current first is inputted from a power supply A. After stepping-down or stepping-up using an adaptor B, the alternating current is passed through a rectification circuit D, a wave-filter circuit D and a regulator circuit E, and converted into a direct current further inputted into an LED array G, thereby conducting and illuminating the LED array G.  
      However, a common adaptor B is not only bulky in volume but also heavy in weight, and is consisted of coils. Therefore, the adaptor B easily incurs magnetic fields that interfere with normal operations of other circuits. In addition, an LED driving circuit has fixed output voltages usually being 5-volt or 12-volt. When using the adaptor B, a large amount of circuits is used for outputting the direct current F, and production costs are thus likely increased to unnecessarily add loadings upon manufacturers.  
      Therefore, it is a vital task of the invention as how to overcome the aforesaid technical drawbacks.  
     SUMMARY OF THE INVENTION  
      Referring to  FIG. 2 , a light emitting diode (LED) driving circuit according to the invention comprises a power supply A, a rectification circuit C, an LED array G and a current control circuit H.  
      The structure according to the invention is characterized that, when the power supply A passes through the current control circuit H, using charging and discharging of a capacitor and parallel equivalent resistance current-limiting characteristics of a capacitor and a resistor, the rectification circuit C converts an alternating current into a direct current F that is outputted to the LED array G, thereby providing the LED array G with a stable quota driving current.  
      Referring to  FIG. 3 , a resistor  12  has two effects. One of the effects is for micro tuning resistance in conjunction with a resistor I 1  and a capacitor J 1 , and hence a value of the resistor  12  is relatively small. When a power supply receptacle is at a bouncing state, if a voltage of a transient state and an input voltage are opposite, an instantaneous current passing through capacitors J 1  and J 2  may surge to a very large value. At this point, the other effect of the resistor  12  is for serving as a current-limiting buffer for the instantaneous current, so as to prevent the capacitors J 1  and J 2  from damaging.  
      The LED array G operates in a series connection and is capable of controlling illumination difference within a specific range, and uneven luminance is not resulted from different forward bias and differences in resistance matching regarding to manufacturing process thereof.  
      Discussions on alternating current and direct current shall be given below.  
      1. Alternating Current (Mains Fequency 50/60 Hz)  
      The resistor I 1  and the capacitor J 1  are for impedance matching in order to control an operating current of an LED L and inputted via the LED array G. Because of transient effects of capacitance caused by incision angle of the mains frequency, the capacitor J 1  serves as a discharging loop via the resistor I 1 , and the capacitor J 2  serves as a discharging loop (utilized for illuminating the LED array G) via the LED array G. Hence, influences that the transient effects have on the circuit are eliminated.  
      The mains frequency of 60 Hz becomes half-waves of 120 Hz after passing through a bridge full-wave rectifier. Without taking effects of the capacitor J 2  into consideration beforehand, conductance angle positive half-cycle is θ≦positive half-cycle conductance angle≦(180°−θ). Conductance ratio of each cycle is (180°−20)/180°, and conductance cycle is 1/120 seconds per cycle (a symmetrical half-wave of 90° is used to consider a square root of an average voltage 110V, where θ=sin −1 (total forward bias/110) and 0°≦θ≦90°), and a total value of forward bias is equal to a sum of forward bias of the diodes K 1  and K 4 , and the LED array G. At this point, by adding capacitance charging-discharging effects of the capacitor J 2 , the conductance θ is increased. Thus, conductance ratio of each cycle is enlarged for improving glittering of the LED array G. When the operation is performed in reverse for negative half-cycles, the outcomes are also the same.  
      2. Direct Current (Linear Power Supplier or Switch-type Power Supplier)  
      Using principle of superposition, a power composition region is divided into direct current and high-frequency harmonic waves, which shall be individually discussed. The direct-current resistance I 1  serves for impedance matching to control an input current entering the LED array G to maintain approximately to an LED quota current. The capacitors J 1  and J 2  are regarded as open circuits and are not taken into consideration. When positively connected, a current loop passes through the diodes K 1  and K 4 . When negatively connected, the current loop passes through the diodes K 2  and K 3 . Thus, the direct-current power is conducted at all times.  
      As for the high-frequency harmonic waves, because impedances of the capacitors J 1  and J 2  are frequency functions, an impedance of the resistor I 1  is much higher than that of the capacitor J 1  at high frequencies. In parallel circuits when the impedance difference is above ten times, lowest impedance is used for calculation and highest impedance is neglected. When the power supply is at a positive loop, the loop passes through the capacitor J 1 , the diode K 1 , the capacitor J 2  and the diode K 4 . When the power supply is at a negative loop, the loop passes through the capacitor J 1 , the diode K 2 , the capacitor J 2  and the diode K 3 . In addition, forward bias of the diodes K 1 , K 2 , K 3  and K 4  is extremely small and can be disregarded. Then, a loop load ratio of the capacitor J 1  and the capacitor J 2  is dependent on capacitance of the capacitor J 1  and the capacitor J 2 . From the above, a continuous current i according to the voltage at the capacitor is C (dVc/dt). When charging the capacitor and the i is positively discharged, i is negative. The capacitor J 1  becomes a discharging loop via the resistor I 1 , and the capacitor J 2  becomes a discharging loop via the LED array G. The capacitor J 1  is capable of absorbing partial energy of the high-frequency harmonic wave component, so as to avoid energy of the high-frequency harmonic wave component being entirely born by the capacitor J 2 .  
      By adding the two aforesaid effects, the result is an equivalent outcome of the direct current power supply.  
      Wherein, the power A comes from the circuit without using alternating elements such as an adaptor, so that the power supply directly inputs an alternating current or a direct current for increasing usage conveniences. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a block diagram of a prior LED driving circuit.  
       FIG. 2  shows a circuit block diagram according to the invention.  
       FIG. 3  shows a circuit in an embodiment according to the invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 3  showing an LED driving circuit in an embodiment according to the invention, during a positive half-cycle, the current outputted from the power supply A first passes through the resistor I 1  and the capacitor J 1  of the current control circuit H, and then passes through the resistor I 2 . Using micro tuning of the serially connected resistor I 2 , the current is reduced. In the rectification circuit C, for that the diode K 1  is conducted in a forward direction and the diode K 3  is not conducted in a reverse direction, the current is directly inputted into the LED array G and returned to the power supply A after passing though the diode K 4 , thereby conducting the illuminating diodes L at the LED array G for illumination. Furthermore, the capacitor J 2  is connected before the LED array G, and hence it is favored that pulses and surges produced be absorbed when the power supply is instable.  
      During a negative half-cycle, for that the diode K 2  is conducted in a forward direction and the diode K 4  is not conducted in a reverse direction, the current is directly inputted into the LED array G and returned to the power supply A after passing though the diode K 3 , thereby conducting the illuminating diodes L at the LED array G for illumination. Furthermore, the capacitor J 2  is connected before the LED array G, and hence it is favored that pulses and surges produced be absorbed when the power supply A is instable.  
      To make the novelty and practicability of the invention more distinguishing, the present invention is compared with the prior invention.  
      Shortcomings of the prior invention: 
          1. An adaptor is necessarily used to likely incur electromagnetic interference.     2. It is essential to use an alternating current as a power input instead of using a direct current as well.     3. A large amount of electronic components is used with high production costs produced.        

      Excellence of the present invention: 
          1. Minimal circuit elements are used to offer lower production costs and easy manufacturing processes.     2. Alternating elements such as an adaptor is eliminated, and a direct current can be directly inputted.     3. No electromagnetic interference is caused.     4. The invention provides novelty and practicability.     5. The invention offers industrial competitiveness.        

      It is of course to be understood that the embodiment described herein is merely illustrative of the principles of the invention and that a wide variety of modifications thereto may be effected by persons skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.