Compact Drivers Help LEDs Replace Halogens

May 22, 2007 2:46 PM
By DaFeng Weng, Product Definer, and Mehmet Nalbant, Director of Applications Engineering, Maxim Integrated Products, Sunnyvale, Calif.

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Halogen MR16 lamps are widely used in professional store and home decorative lighting applications. The power dissipation of the most commonly used halogen-based MR16s ranges from 10 W to 50 W, and their light output ranges from 150 lumens (lm) to 800 lm, which equates to an efficacy of about 15 lm/W. The lifetime of a typical halogen bulb is limited to about 2000 hours, and the filament must be protected from high levels of vibration to prevent the bulb from failing prematurely.

LED technology offers a cost-effective alternative that offers greater efficacy, longer lamp life and greater ruggedness. However, LED replacements for halogen bulbs have very different power requirements. Powering LED-based MR16 lamps demands a cost-effective drive circuit that is tailored to meet the LED’s electrical requirements, yet is small enough to fit within the unique form factor of the MR16 lamp assembly.

LED Performance

The latest generation of 5-W (4-mm × 4-mm single-chip package) and 10-W (7-mm × 7-mm four-chip package) high-power LEDs from LedEngin generate typical efficacies of 40 lm/W at 1000 mA when the junction temperature, T_J, equals 120°C. This is equivalent to a luminous output of 140 lm (I_LED = 1000 mA, T_J = 120°C) for the 5-W package and 315 lm (I_LED = 700 mA, T_J = 120°C) for the 10-W package under typical working conditions. Note that when these LEDs perform at the same brightness level as halogen bulbs, the power dissipation can be reduced by about 50%. In addition, LedEngin predicts a remarkable lumen maintenance of 90% for operation at T_J = 120°C after 100,000 hours, thus eliminating the need for bulb replacement throughout the life of the product.

For the MR16 reference design, Maxim selected the LedEngin 5-W product to demonstrate the 1000-mA drive capabilities of the MAX16820 application circuit. In most MR16 applications, the input voltage supplied to the halogen lamp is 12 Vac ±10%, so this was selected as the input voltage requirement for the drive circuit using the IC.

The MAX16820 has been specifically designed for LED driver applications targeting MR16 LEDs (among others), and is available in a 6-pin TDFN package. A 4.5-V to 28-V input voltage range, along with the ability to drive an external MOSFET, gives MAX16820-based driver circuits a broad range of LED current-drive capabilities. Additionally, its operating temperature range extends to 125^°C, allowing the MAX16820 to be safely operated in the high-temperature environment of the MR16 light fixture. While the MAX16820 can control power levels beyond 25 W, a 2-MHz switching frequency results in a small inductor and small capacitors, which allow the driver circuit to be placed in the MR16 fixture.

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