1. Field of the Invention
The present disclosure relates to a lighting apparatus which supplies a current to a solid-state light-emitting device, and a luminaire including the lighting apparatus.
2. Description of the Related Art
As a lighting apparatus which supplies a current to a solid-state light-emitting device such as an LED (light-emitting diode), an apparatus which includes an AC/DC converter and a DC/DC converter connected to the AC/DC converter has been proposed (see, for example, Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2013-30416).
In the lighting apparatus according to Patent Literature 1, the DC/DC converter includes a chopper circuit having a switching element that switches (turns on and off repeatedly) in BCM (boundary conduction mode), an inductor, etc. It is to be noted that the BCM is an operation mode in which a switching element is turned on when a current flowing through the inductor reaches zero in switching of the DC/DC converter. In a DC/DC converter of this type, in order to keep an output current constant, an ON time which is a period of time during which an ON state of the switching element is maintained varies depending on a DC voltage provided to the DC/DC converter. Specifically, ON time Ton of the switching element is represented by Expression 1 below.Ton=2Iout·L/(Vdc−Vf)  Expression 1
Here, Iout denotes an output current of the DC/DC converter. L denotes inductance of an inductor included in the DC/DC converter. Vdc denotes a DC voltage provided to the DC/DC converter. Vf is a DC voltage (forward voltage) applied to a solid-state light-emitting device connected to an output terminal of the DC/DC converter.
As shown in the above-described Expression 1, for example, ON time Ton increases when DC voltage Vdc decreases, thereby keeping the output current Iout constant.
When ON time Ton excessively increases, however, a switching frequency decreases and enters an audible frequency band, causing sounding. For that reason, with the DC/DC converter including a switching element that operates in BCM, the ON time of the switching element is controlled such that the ON time of the switching element is kept from exceeding a predetermined upper limit.