LED lighting applications will begin with three basic input power levels: low power less than or equal to 20W, medium power between 20W and 50W, and high power greater than 50W applications: see Figure 1. Keep in mind that real-world applications don't fit exactly into these three divisions, but when considering LED driver solutions, these three power levels are consistent. LED applications focus on high brightness LED design.
The subject matter of this paper is ≤20W low power applications, especially replacement or retrofit of bulb-type luminaires - replacing existing luminaires and lighting fixtures. This also includes new structural lighting fixtures.
Trends in low power LED lighting
In 2010, global sales of high-brightness LEDs are estimated at US$890 million. It is estimated that from 2010 to 2015, the average annual growth rate (CAGR) will be 39%, which shows that the market potential is very large. But for LED drivers , the main trends are driver-related improvements in power, cost reduction, and long working life. The effect is the ratio of lumens to watts.
"DOE SSL" is expected to exceed the traditional technology of today and the past; Figure 2 And Figure 3 shows the trend of increasing efficacy. In terms of power efficiency, the input power is in the denominator, and the input power and the efficiency of transferring energy to the LED string are related to the LED driver solution. With a full range of LED load power and load possibilities, a single drive topology is not the best choice, but the smallest topology can be considered to meet all LED driver development needs.
Choosing the most efficient semiconductor can be used as a basis for selecting a topology, but the cost of the driver is also a constraint. The DOE SSL plan estimates today's costs as shown in Figure 4; drives account for 10% to 20% of total manufacturing costs.
This is the overall cost goal seen by end users, and this has become the most common barrier to performance improvement for LED lighting solutions. The cost targets set by the US Department of Energy at the 2011 Solid State Lighting Market Introduction Symposium are shown in Figure 5; costs are reduced by almost 50% every four years. LED driver topology selection also offers the most cost-effective solution.
The working life is also related to the reliability of the power supply. Reliability is affected by the number of components of the LED driver, the type of component used, temperature or power loss. Using the component number method, the reliability of the LED driver can be calculated and the number of components can be reduced according to the target. Reliability is also affected by operating temperature, so thermal design is also important, as is the reduction of power losses associated with LED driver components and topology control methods. The trend is to eliminate electrolytic capacitors, as well as other components such as opto-isolators, and to integrate functions into silicon control devices.
Low Power LED Driver Design Challenge
LED driver design faces the following challenges today; the listed projects will be design constraints that designers must balance, and the order will vary from company to company.
â— Shorten the development cycle;
â— Reduce costs;
â— design complexity;
â— Find power supply topologies that meet input and output voltage-current parameters, thermal design, safety regulations, and protection requirements;
â— efficiency and efficacy;
â— Meet global regulatory requirements, ie reduce power loss in LED drivers, use power factor correction (PFC) and low THD (total harmonic distortion);
â— Reliability and service life of the drive;
â— Constant current output tolerance;
â— Dimming and dimming range (phase-cut dimmer requirements, dimming rate, inrush current limit, damping circuit, voltage divider, etc.);
â— No flicker;
â— Limited printed circuit board (PCB) space or volume (height) limits;
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