LEDs have the potential to revolutionize the efficiency, appearance, and quality of lighting. See http://www.energystar.gov/index.cfm?c=lighting.pr_what_are. The United States Department of Energy estimates that rapid adoption of LED lighting in the U.S. could provide savings of roughly $265 billion, avoid 40 new power plants, and reduce lighting electricity demand by 33% by 2027. Thus, the market for LED lighting is expected to grow significantly in the coming years compared to traditional, non-LED based lighting.
An LED emits light when a voltage exceeding a certain minimum is applied across the LED to enable current to flow through the LED. The current flowing through the LED, or forward current, must be a direct current (DC) and therefore LEDs require a DC source to drive the LEDs. Additionally, due to the particular voltage-current characteristic of an LED, small changes in the voltage applied can result in large changes in the current flowing through the LED, and hence the amount of light emitted by the LED. The disproportionate voltage-current response can make it difficult to implement functions that rely on precise current control in LED lighting applications, such as dimming.
Most LED lighting applications employ an LED driver to drive an array or multiple arrays of LEDs. The LED driver typically includes a power converter that converts the line AC into the DC source needed to drive the LED arrays. There are generally two types of power converters: AC/DC constant voltage converters, and AC/DC constant current converters. An AC/DC constant current converter, as the name suggests, takes an AC input voltage and provides a relatively constant DC output current, while an AC/DC constant voltage converter takes the AC input voltage and provides a relatively constant DC output voltage.
Because small voltage variations across an LED can produce large changes in the LED forward current, LED drivers that use an AC/DC constant voltage converter must usually include a downstream current-limiting resistor or current-regulating circuit in order to maintain the desired LED forward current. An AC/DC constant current converter, on the other hand, can ordinarily control the forward current much more precisely despite small voltage variations. As a result, AC/DC constant current converters are generally more suitable than AC/DC constant voltage converters for implementing dimming in LED lighting applications.
AC/DC constant voltage converters, however, are more commonly used and better understood than AC/DC constant current converters. This is due in part to the generally accommodating design of the AC/DC constant voltage converter topology, the lower cost resulting from wide popularity of the design, and well-established supply chains for AC/DC constant voltage converter components. Thus, there is a general preference in the lighting industry to continue using AC/DC constant voltage converters for lighting applications, including LED lighting applications.
A drawback of using an AC/DC constant voltage converter in LED lighting applications is the LED driver cannot readily provide dimming. In a typical LED lighting application, the AC/DC constant voltage converter is connected to a downstream DC/DC converter that converts the constant DC output voltage to a corresponding DC output current to drive the LED array. The problem is the AC/DC constant voltage converter will try to maintain its DC output voltage constant even during dimming, when the AC output is being decreased by the dimming controller. This constant DC output voltage causes the DC/DC converter to keep its DC output current the same, so the LED arrays do not dim. In addition, the AC/DC constant voltage converter will try to draw more current from the dimming controller in order to offset the decrease in the AC output. This increased current may cause the current rating of the dimming controller to be exceeded in some cases, potentially damaging the dimming controller over time, and may also require the transformer of the AC/DC constant voltage converter to be over designed.
Thus, a need exists for an improved way to provide dimming in LED lighting applications, and particularly for a way to control dimming in LED lighting applications that can use AC/DC constant voltage converters.