1. Field of Invention
The present invention relates to a drive circuit for light emitting diode (LED), and more particularly, to an LED drive circuit for driving and connecting a plurality of serially connected LED circuits in parallel.
2. Related Art
Light emitting diodes (LEDs) are widely used in daily life in recent years, such as in displays, household appliances, vehicle electronic components, and lighting lamps. Taking the displays as an example, a conventional display includes lamp sets of three-color lights, namely, a red (R) lamp set, a green (G) lamp set, and a blue (B) lamp set. Before the lamp sets of three-color lights are assembled, it shall be determined first whether LED bulbs in the lamp set of each color have the same characteristic through the test method of first applying a same current on them, and then dividing them into lamp sets of three-color lights based on whether the intensity, color, forward bias (Vf) of light are similar. However, after the display is used for a long time, the luminance of one LED bulb in the lamp set of a certain color often becomes insufficient, which causes non-uniform luminance in the white light produced through the mixture of the R, G, and B lights of the entire color light lamp sets. Since the LED light lamp set is usually a modulated product, it is a waste of time and labor to find the defective LED bulb from so many LED bulbs.
In order to solve the above problem that the LED light lamp set cannot detect the luminance of the LED lamp sets therein in time, U.S. Patent Publication No. 7,045,974 discloses an LED optical energy detection and feedback device, which is shown as FIG. 1. FIG. 1 is a schematic view of an optical energy detection and feedback device of the conventional art. As shown in FIG. 1, the conventional optical energy detection and feedback system includes an energy sensor 20, a power/voltage converter 30, an analog/digital converter 40, an operation processor 50, and a driver set 60.
The energy sensor 20 has three phototransistors 22, 24, and 26. The power/voltage converter 30 is used to convert current signals transmitted from the phototransistors 22, 24, and 26 into voltage signals, and then output the voltage signals to the analog/digital converter 40. The analog/digital converter 40 is used to convert the analog (voltage) signals transmitted from the power/voltage converter 30 into digital signals, and then transmit the digital signals to the operation processor 50. The driver set 60 has three driving ICs 62, 64, and 66, and when a control instruction transmitted from the operation processor 50 is received, the driving ICs 62, 64, and 66 adjust the current of LED lamp sets. Moreover, the LED lamp set 70 has a red LED lamp group 72, a green LED lamp group 74, and a blue LED lamp group 76, which are respectively connected in series with a detection red LED, a detection blue LED, and a detection green LED of the same property, such that the phototransistors 22, 24, and 26 respectively detect the luminance of the light emitted from the red LED lamp group, blue LED lamp group, and green LED lamp group of the LED lamp set 70, and then convert into current values and transmit to the power/voltage converter 30. Thus, when the bulb of one color in the LED lamp set 70 is abnormal, the current signals transmitted from the phototransistors 22, 24, and 26 and received by the power/voltage converter 30 are converted, and then transmitted to the operation processor 50 by the analog/digital converter 40. Since the operation processor 50 has stored the default luminance reference values of the red LED lamp group, blue LED lamp group, and green LED lamp group, which color light LED lamp group that the abnormal lamp is in can be detected after through determination and comparison, and then the light of a uniform luminance can be emitted from the LED lamp set 70 by ordering one of the driving ICs 62, 64, and 66 to make compensation.
The LED lamp set 70 can use three detection LEDs to detect the luminance of the light of the red LED lamp group, blue LED lamp group, and green LED lamp group, and the luminance values of the LED lamp groups of three color lights are respectively transmitted to the power/voltage converter 30 through the phototransistors 22, 24, and 26. However, the luminance values of the LED lamp groups of three color lights are detected by the phototransistors 22, 24, and 26 at the same time, and then transmitted to the power/voltage converter 30 synchronously. When the number of the lamp groups of the LED lamp set 70 is increased and the number of the bulbs in the lamp group is increased, the number of the matching phototransistors is increased correspondingly, and thus, not only the forward bias (Vf) of the LED lamp set 70 will be excessively high, but also when a plurality of bulbs or bulbs in different lamp groups in the LED lamp set 70 are abnormal, the operation processor 50 cannot determine and adjust the needed luminance compensation value of the light of the LED groups.
Moreover, the forward bias (Vf) of the LED changes with the used time of the LED and the temperature during using, for example, when the temperature rises, the forward bias (Vf) will decrease. If the LED is driven at a constant voltage, when the forward bias decreases, the voltage for emitting light will increase, and the produced luminance is higher than the expected value. On the contrary, if the forward bias increases, the voltage applied on the driving IC (integrated circuitry) increases, the voltage for emitting light will decrease, and thus the produced brightness is lower, which is a challenge for the case requiring stable luminance. Furthermore, the load of the driving IC will also change with the change of the above-mentioned forward bias.