DIMMING METHOD AND CIRCUIT FOR LIGHT-EMITTING-DIODE (LED) SYSTEMS DRIVEN BY PASSIVE LED DRIVERS

摘要

This invention is related to a dimming method and a dimming circuit for light-emitting-diode (LED) assemblies driven by nominally non-dimmable passive LED drivers. An LED system usually consists of an LED driver and an LED assembly. The LED driver normally provides a current source for the LED assembly. The LED assembly consists of individual LED devices or several LED modules mounted on a heatsink. LED devices or modules can be connected in series or in parallel or a combination of both. This invention involves the combined use of a controller and circuit for providing dimming function in the LED assembly driven by a passive LED driver without closed-loop current regulation. By using one or more controllable bypass-switches across sections of the LED strings in an LED assembly, the power of one or more groups of LED devices or LED modules can be cut off in order to dim the LED system. When the voltage of LED is reduced by closing the bypass switch or switches, a uniqueness of this invention is that the per-unit increase in the output current of the passive driver must be smaller than the per-unit voltage reduction of the LED load, resulting in reduction of power consumption in the LED load for dimming purpose. The dimming control can take turns to cut off the power of different groups of LED devices or modules in different times in order to share operating burden and preserve lifetime of the LED assembly. The invention is particularly suitable for, but not restrict to, lighting applications in smart lampposts in which smart wireless communication systems are mounted on the same lampposts as the LED assemblies and dimming signals can be received wirelessly. The advantages of this invention are that the dimming function is provided in the LED assembly rather than in the LED drivers, and the dimming circuit can be linked to smart control in emerging smart lampposts.