Although it is said that an LED can be subjected to light control more easily than a discharge lamp, it is never easy to smoothly perform deep dimming. That is, when deep dimming is performed, flickering of the LED easily occurs due to the following reasons.
As a first reason, the LED emits light even if it is deeply dimmed and little lighting current flows, and the visual sensation of a person tends to sense flickering of the LED more clearly as the amount of light decreases.
As a second reason, when the LED is lit by continuously flowing DC current, the LED becomes a light load in a deep dimming region, an operation point vibrates in relation to a rise point of a voltage-current property of the LED as a center in accordance with a slight fluctuation in output current of a converter. Consequently, conduction and block are repeated, an unstable lighting state of the LED is shown and thus flickering of the LED occurs.
By the first and second reasons, flickering of the LED easily occurs in the LED lighting device capable of deep dimming and the quality of the device is remarkably lowered.
In order to remove flickering, a circuit system is required to be adopted which strictly manages current of an LED lighting circuit and performs pulse-control by making an amplitude of current large.
On the other hand, a two-wire phase control type dimmer using a phase control element such as a triac is widely used as a dimmer for incandescent bulbs. If an LED can be subjected to light control with use of the dimmer, a low power consumption type illumination system with a light control function can be realized only by exchanging light sources without renewing existing equipment and wiring.
However, there actually exist the following problems. That is, when the LED is lit at a low-current level, self-holding current of the phase control element of the dimmer cannot be secured and thus flickering of the LED occurs. Additionally, a timer circuit for turning on the phase control element of the dimmer at a desired phase does not operate from the moment when an AC source of the LED is switched on.
An LED lighting device is known which, in order to solve the above problems, includes a dynamic dummy load, which is arranged in parallel with a converter, receives a control signal from the converter and adjusts a load in response to the control signal, and thus feeds self-holding current of a phase control element and operation current of a timer circuit of a dimmer when each of them is required.
However, in a conventional prior art, when the above solving method is performed for suppressing flickering of the LED occurring in accordance with the slight fluctuation in current in the deep dimming region, a control circuit becomes complicated, the number of circuit components is increased in accordance with this, and thus downsizing is difficult to realize and this leads to an increase in cost.
Additionally, regarding the phase control type dimmer, although the self-holding current of the phase control element and the operation current of the timer circuit of the dimmer are properly made to flow by providing the dynamic dummy load, not only does a ripple of input voltage to the LED lighting device become large by the phase control but also a firing phase easily becomes unstable at a light load in the deep dimming region. Thus, flickering of the LED is further encouraged in the deep dimming region.
The present invention aims to provide: an LED lighting device that reduces flickering, which easily occurs in the vicinity of a dimming lower limit, of an LED by a relatively simple circuit constitution; and an illumination apparatus including the LED lighting device.
USPTO Backgrounds
USPTO Backgrounds