Cost-effective LED-drive ICs energize lighting revolution

Cost-effective LED-drive ICs energize lighting revolution

Efficient LEDs and drivers offer big energy savings

BY JOHN DICATO, Micronix, An IXYS Company and WARREN WILLIAMSON, W. L. Williamson & Assoc.
Santa Clara, CA
http://www.ixys.com/

Today, the efficacy of commercial LEDs is approaching that of fluorescent lighting (70 to 80 lumens/W). Fluorescent lighting is a mature technology and little improvement is likely to occur in the near future, while LED lumens/W has been increasing at 20x every 10 years, and is projected to continue to do so.

LEDs have begun to penetrate niche lighting markets such as billboard signage, architectural lighting, traffic signals, and vehicle interior lighting, indicator, tail, and headlights. Although in its infancy, according to recent DOE reports, LED lighting saved 8.3 TW/h in power-grid-connected niche applications alone in 2002: “. . .more than the equivalent output of one large electric power station.”

The potential for savings in building lighting is huge. Lighting is the second largest end-use of energy in buildings and consumes 22% of the total electrical energy generated in the U.S.—estimated to be 8.2 quadrillion BTUs annually.

At present, the relatively high initial installed cost of LED lighting is limiting its use. However, several other attributes offset the high initial cost. Among these are

Reduced energy consumption

Long operating life

Durability

Reduced heat production

Smaller package size

Safety improvements

Easy, accurate control

Furthermore, the cost of LED lighting is dropping rapidly, and the market for high-brightness LED lighting continues to expand. It is expected to exceed $8 billion in 2010, over twice the level of 2005. As high-brightness LEDs begin to offer cost savings and higher luminance performance relative to conventional light sources, precision, efficiency, and off-line capability will be the dominant features of the LED IC control solutions.

Off-line capability requires switching power supply techniques to realize the potential efficiency of high-brightness LEDs. The IC industry has responded with a variety of PWM controllers designed specifically for LED drive and control. One example driver IC, from a field of many semiconductor suppliers, is the MXHV9910 from Micronix (an IXYS company), a block diagram of which is shown in Fig. 1 . A distinguishing feature of this controller is an internal voltage regulator that operates directly from the high-voltage line, eliminating the requirement for an external low-voltage supply using voltage-dropping resistors or a high-voltage current source. The cost and board space otherwise required for these components is saved.

Fig. 1. A typical high-brightness LED driver design features simplicity, efficiency, and accurate current control.

The chip operates from 8 to 450 V and can be configured for buck, boost, or buck-boost operation. It provides current regulation and linear or PWM dimming control. High voltage capability allows it to drive hundreds of high-brightness LEDs in series. The device is capable of greater-than-90% efficiency when driving high-brightness LEDs, making possible significant reductions in energy usage for a wide range of lighting applications.

In addition to minimizing system power loss, its current regulation provides constant light output and extends LED lifetime. The device has an on-board 7.8-V regulator, which supplies the internal logic and up to 1 mA for external circuitry and it directly drives the gate of an external power MOSFET. A buck converter reference design is shown in Fig. 2 (available as an application board, MXHV9910DB3).

Fig. 2. This lighting reference design uses a 4.7-mH switching inductor and a 0.56-Ω current-sense resistor.

The 0.56-Ω current-sense resistor, R4, sets the nominal peak current at 250 mV/0.56 Ω or 446 mA—adjustable via RA1, which lowers the voltage on the LD input. A 0% to 100% PWM dimming signal can also be applied to the PWMD input.

The IXTA8N50P high-speed switch FET from IXYS used as the main switch is rated at 500 V with an RDS(on) of 0.8 Ω and a current rating of 8 A at 25°C. The inductor value (L1) is determined by the switching frequency, input voltage range, and the desired diode string voltage and current. Switching frequency is set at ~64 kHz by R1. Higher frequencies will result in higher losses in the FET, while lower frequencies require a larger value for L1.

Get more information on ICs for lighting apps at http://electronicproducts-com-develop.go-vip.net/linear.asp