LED driving device

[0001] 1. Field of the Invention

[0002] The present invention relates to a driving device, and more particularly it pertains to a LED driving device capable of improving the power factor and efficiency.

[0003] 2. Description of the Related Art

[0004] Diode, a semi-conductor element, works like a switch, has played an important role in electronic system.

[0005] There are many kinds of diode around our life. One kind of diode can be lighting when being energized. This kind of diode we call it Light Emitting Diode (LED).

[0006] The application of LED is quite wide. High bright LED is widely used for traffic light, vehicle indicating light, and braking light. Full-color LED display, composed of red LED, green LED, and blue LED, is also used for stadium and street advertisement, such as the larg LED display at outside of Nasdaq marketing center in Times Square, New York city. The 20-foot high screen, composed of 19,000,000 high bright LEDs, is the largest one in the world.

[0007] LED has become an indispensable lighting device today because cell phone and portable electronic products are getting more popular. Experts believe that the LED will replace most light source in the near future due to its advantage of space-less, high lighting speed, and long lifetime.

[0008] LED is so widely used that many kinds of driving devices and chips are worked out. FIG. 1A is a diagram showing a current versus voltage relationship of a LED. The relation of the voltage and the current can be represented by an exponential function and the relation is similar to an ordinary diode. When the forward voltage is less than some value Vb, only very small current flows through the LED. When the voltage exceeds some value Vb, the current would raise sharply. The sharp current is forward current of LED. Said value Vb we call it barrier voltage. The Vb value is between 1.5V and 3.5V usually. The semiconductor material and doping level decides the barrier voltage Vb. Besides, the wave-length of the light emitted from a LED also depends on the kind of material, for example, red Led is composed of GaAsP.

[0009] The LED light output luminous intensity is proportional to LED current for most operating value of LED current, but the approximation usually over-estimates light output at high current value. A typical curve is shown in FIG. 1B. Actually, the driving devices are designed to provide a constant current for stabilizing light emitted and extending the life of LED.

[0010] FIG. 1C is a waveform diagram of voltage and current for explaining the behavior of a LED. If we use AC power source to energize the LED, the light will be emitted during the interval T within the positive part of the AC power source because the voltage level of the AC power source is higher than the barrier voltage of the LED. We could couple a bridge rectifier to AC power source for taking the advantage of AC power source. By applying a bridge rectifier, the negative part of AC power source will be converted to positive. FIG. 2A shows the circuit diagram of bridge rectifier. To get a stable voltage supply, a filtering capacitor can be coupled to. FIG. 2B shows the waveform diagram of voltage and FIG. 2C shows the waveform diagram of current related to FIG. 2B. Further, a constant current circuit is added to keep the constant luminance and color of light emitted. FIG. 2D shows the waveform diagram of current in above situation.

[0011] It is important that if there is a capacitor or an inductor in the circuit, the current and voltage will be non-synchronous. FIG. 2E is a waveform diagram of current lagging behind voltage waveform. If the voltage Eac is represented as Em Sin ωt, the current Iac is represented as Im Sin(ωt−Θ). In above situation, the product of voltage and current is not always positive. The power value is calculated as VmIm Cos Θ/2 and it is less than the power of the voltage and current in the same phase VmIm/2. The Cos Θ was called power factor.

[0012] There are many patents about LED driving device. For example, U.S. Pat. No. 5,936,599, “AC POWERED LIGHT EMITTING DIODE ARRAY CIRCUITS FOR USE IN TRAFFIC SIGNAL DISPLAYS”, FIG. 3 shows its circuit diagram. According to this diagram, we see a LED array including a number of series connected polarized LEDs D_pair are energized by an AC voltage source coupled to an inductor. Each polarized LEDs including two parallel connected oppositely polarized LEDs. The inductor is taken the place of the resistor which is used to limit the current. The inductor will limit the current with less power loss than the resistor does. To further reduce power loss, a capacitor is coupled to LED array. The capacitor has to be tuned to match up the inductor and the frequency of the AC voltage source. However, the power loss is improved but the power factor is reduced for the existence of the capacitor and inductor. Another disadvantage is that the capacitor and inductor have to be tuned with the frequency of AC voltage source. Besides, the number of LEDs in LED array is dependent on the voltage level of the AC voltage source.

[0013] U.S. Pat. No. 5,457,450, “LED TRAFFIC SIGNAL LIGHT WITH AUTOMATIC LOW-LINE VOLTAGE COMPENSATING CIRCUIT”, its circuit diagram is shown as FIG. 4. To avoid the overall light intensity dropping down, a mid-voltage compensation circuit 82 and a low-voltage compensation circuit 84 has been designed. Although this patent can avoid light intensity from dropping down without increasing the power consumption, but driving LEDs with increased current will shorten the life of LEDs.

[0014] In addition, a stable voltage source is always used to driving LEDs for increasing the lighting time of LED and a filtering capacitor is used for this purpose. The disadvantages for the existence of capacitor are lowering the power factor and the capacitor has to be changed with the frequency of power source. Besides, the life and stability of capacitor is affected by temperature very heavily so that it makes the driving device unstable.

[0015] For overcoming said defects described above, an object of the present invention is to provide an LED driving device in which the LEDs can be driven by the positive part of power source directly.

[0016] Another object of the present invention is to provide a LED driving device in which the power factor can be improved.

[0017] The other object of the present invention is to provide a LED driving device in which the greater number of LED can be lighted in comparison with conventional LED driving device supplied with the same power source.

[0018] These and other objects, features and advantages of the present invention will become more apparent from the following description and the appended claims, taken in connection with the accompanying drawings in which preferred embodiment of the present invention are shown by way of illustrative example.

[0033] FIG. 5A is a circuit diagram of the first embodiment of the present invention. It comprises a LED string, a voltage detecting circuit 20 and a current switching circuit 10. The power source Vs can be any kind of input voltage source. Said voltage detecting circuit 20 is used for detecting the voltage level of the power source Vs and said current switching circuit 10 including grounded current controlling unit I₁, I₂, I₃, . . . , I_(n−1), and I_n.

[0034] Said LED string connected in parallel across the power source Vs is composed of series connected LED sets D1, D2, D3, . . . , Dn−1, and Dn. Each of said LED sets is composed of a LED or at least two LEDs in any electric configuration.

[0035] The current controlling unit I₁, I₂, I₃, . . . , I_(n−1), I_n is coupled to one of the LED sets. The voltage detecting circuit 20 detects the voltage level of power source and sends a signal to said current switching circuit 10 and said current switching circuit 10 is automatically activated to enable the current controlling unit I₁, I₂, I₃, . . . , I_(n−1), I_n and electrically rearrange said configuration of LED sets with a predetermined current value.

[0036] The present invention can be supplied by any kind of power source without being coupled to a filter capacitor. In order to increase the lighting time of LED, a bridge rectifier circuit 30 can be used to convert the negative part of the power source Vs.

[0037] The Voltage detecting circuit 20 and current switching circuit 10 of the present invention can be supplied by a DC voltage derived from said power source Vs or derived form another power source.

[0038] For a AC power source, the voltage detecting circuit 20 is designed that when the voltage of power source exceeds the barrier voltage Vth1 of LED set, the voltage detecting circuit 20 will only enable current controlling unit I1. Vth1 is equal to or higher than the sum of the barrier value of total LEDs, If the LEDs in the LED set are the same, then, Vth1 is equal to or higher than n*Vb. At this time, the current path is power source Vs, LED set D1, and current controlling unit I1, and ground. When the voltage of power source raises to Vth1+Vth2, if the LED set D2 and LED set D1 are identical, then Vth2=Vth1=Vth, i.e. when voltage detecting circuit 20 detects the voltage of power source Vs exceeding 2*Vth, the voltage detecting circuit 20 will disable I1 and only enable current controlling unit I2. The new current path is power source Vs, LED set D1, LED set D2, and current controlling unit I2, and ground. When the power source is raised, the controlling unit I2 is disabled and I3 is enabled and so on. In conclusion, only one current controlling unit will be enabled at any time. To keep the brightness, the current of the current controlling unit is designed to a constant value.

[0039] Said current controlling unit can be accomplished by any current controlling circuit. It can be designed to be a constant current source or a limited current source.

[0040] In practice, the LEDs included in the LED set can be different color from each other and could emit any color or intensity light. The simplest method is using red LED string, green LED string and blue LED string to combine. FIG. 5A shows a circuit diagram of the first embodiment of the present invention.

[0041] We change the original LED sets D₁, D₂, D₃, . . . , D_(n−1), D_n of a LED string to a LED array including three LED strings. Red LED sets D_a1, D_a2, D_a3, . . . , D_a(n−1), D_an; green LED sets D_b1, D_b2, D_b3, . . . , D_b(n−1), D_bn; and blue LED sets D_c1, D_c2, D_c3, . . . , D_c(n−1), D_cn. The current controlling unit I_a1, I_a2, . . . I_a(n−1), I_an, I_b1, I_b2, . . . I_b(n−1), I_bn, I_c1, I_c2, . . . I_c(n−1), I_cn are controlled by current switching circuit 11, 12, 13 separately.

The number of strings can be more than three and the LEDs in the LED sets of different string may not be the same.

[0042] FIG. 6 is a waveform diagram of power source voltage, current and drop voltage on LEDs when supplied by an AC power source. There are five LED sets for this diagram. Regardless of the power loss due to stray resistors and capacitors, because there is no capacitor in the present invention, the output power for the power source is the product of the voltage area of power source and I_Led. The voltage across all the energized LED is a step shape and the power is the product of area of the step shape and I_Led. The difference between these two powers is the power loss and the power loss is equal to the area of the shadow. The difference in voltage between the power source and the across voltage of total energized LEDs will drop on the current controlling unit. FIG. 7 is a waveform diagram of power source voltage, current and drop voltage on a LED when supplied by any kind of power source. The output power and power of all LEDs are the same as above.

[0043] Mentioned above is an ideal situation. In practice, the lowest voltage to make current controlling unit work with a predetermined value has to be overcome. The voltage level of voltage detecting circuit must be higher. The voltage across the current controlling unit we represent it as Vd. Vd is smaller than the barrier voltage of LED set. The current of current controlling unit is about 100 mA and the lowest value of vd will be designed less than 0.1V.

[0044] It is important that the predetermined current value of any string can be adjusted for fit to various situations. It is the same that the voltage level of the voltage detecting circuit also can be adjusted.

[0045] While the present invention has been described with reference to the illustrative embodiment, this description is not intended to be construed in a limited sense. Various modifications of the illustrative embodiment of the invention such as the different accomplished circuit for the voltage detecting circuit and current switching circuit will be apparent to those skilled in the art with reference to this description. It is therefore completed that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.