1. Field of the Invention
The present invention relates to a driving device and, more particularly, to light emitting diode (LED) driving device.
2. Description of Related Art
Alternating current LED (AC-LED) module can be directly driven by utility power for thus providing the advantage of simple architecture, as shown in FIG. 1. However, it suffers a disadvantage in that, when the received AC voltage vac is being increased, the current iac flowing through AC-LED module is getting large, resulting in a droop effect that may lower the luminous efficiency. Furthermore, because the AC-LED module is sensitive to the variation of voltage, a slight change of AC voltage vac will cause a large variation in the light output. Thus, when the AC voltage is unstable, it is likely to have a flicking effect. In addition, in order to bear the peak value of the AC voltage, the AC-LED is typically made to have a high turn-on voltage by, for example, connecting several LEDs in series, which results in a low power factor and low LED efficiency.
In the known patents, U.S. Pat. No. 6,989,807 granted to Chiang discloses an “LED driving device” that receives utility power and has high power factor and high LED efficiency. As shown in FIG. 2, this LED driving device includes a bridge rectifier circuit 30, a current switching circuit 10, a plurality of LEDs and a voltage detecting circuit 20. The bridge rectifier circuit 30 receives the AC voltage of an external power supply and converts the same into a rectified voltage. The voltage detecting circuit 20 controls the current switching circuit 10 to change the number of LEDs that are turned on according to the variation of the rectified voltage, so as to improved the power factor and the LED efficiency. However, such a constant current control architecture is complicated and the control there of is difficult, resulting in that the number of devices used in the overall circuit is excessive and thus the volume becomes large and the manufacturing cost is increased.
Therefore, it is desirable to provide an improved LED driving device to mitigate and/or obviate the aforementioned problems.