Multi-level LED driver design for efficient dimming

Although the basic LED drivers designed for general lighting applications are relatively simple, this design can become very complex when additional functions such as phase cut dimming and power factor correction are required. Non-dimming LED drivers without power factor correction typically include an off-line switching power supply that is tuned for constant current output. This is not much different from standard off-line switching power supplies and models that are common in AC-DC adapters. This design can use standard SMPS (switching power supply) circuit topologies such as buck, boost or flyback converters.

On December 3, 2009, the US Department of Energy (DOE) released the final version of the "Energy Star" specification for the integrated LED lamp project, stipulating that the power factor of LED drivers used in the United States must be better than 0.7, while industrial applications Expected to be better than 0.9. Many products in the market are still unable to meet such requirements, so the future needs to be replaced with more advanced products. There are two ways to implement power factor correction (PFC), each requiring additional circuitry at the front end of the power converter: a simple low-cost passive PFC, and a more complex active PFC.

Before delving deeper into these methods, one thing to emphasize is that in order to receive an ENERGY STAR rating, the LED driver must be dimmable.

In general, this means that the adjustability will be derived from the existing wall-type electronic dimmer based on the principle of phase-cutting. This principle was originally designed to design pure-impedance incandescent lamps. While other dimming methods, such as linear 0-10V dimming or DALI, may be desirable, they may all be limited to high-end industrial LED drivers.

So far, the application of phase-cut dimmers is quite extensive, and it is clear that LED lamps that can effectively dim will have great advantages. Since there are many low-cost dimmers based on triacs on the market, it is unrealistic to ensure that LED drivers are compatible with all categories. In particular, many dimmers use only basic designs and have very limited performance. For this reason, the ENERGY STAR program only requires LED driver manufacturers to specify in a web page which dimmers are compatible with their products.

Another requirement worth noting in the ENERGY STAR specification is that the LED's operating frequency must be greater than 150 Hz to eliminate the possibility of visible flicker. This means that the output current to the LED cannot be supplied with any large amount of ripple that is twice the linear frequency (50 Hz or 60 Hz).

In off-line applications such as office lighting, public buildings and neighborhood lighting, LED lighting is used in more and more applications and will continue to be in the next few years. In these applications, high-power LEDs will replace linear or high-power CFL fluorescent lamps, HID lamps, and incandescent lamps. These applications require an LED driver with a typical power range of 25W to 150W. In many cases, the LED load consists of a high-brightness white LED array, typically in a variety of chip packages. The DC current used to drive these loads is typically at least 1 amp. Actually there are also AC current driven LED systems, but DC systems are generally considered to provide better driving conditions for LEDs.

Galvanic isolation is required in LED lighting equipment to prevent the risk of electric shock where it can be accessed, which can occur in most cases unless an insulated mechanical system is used. This is because LED chips need to be connected to a metal heat sink unlike products such as fluorescent lighting devices that do not require insulation to achieve safety. In order to achieve good thermal conductivity, it is necessary to form a thermal barrier between the LED chip and the heat sink, so that it is not necessary to add an insulating material to meet the insulation requirement. Therefore, it is the best choice to form insulation inside the LED driver, and it also shows that the power converter topology is feasible.

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