POWER


Constant voltage & current modes in one AC-DC LED driver?


LED application designers face many challenges when creating a new lighting application. Lighting parameters such as brightness, lumens, chromaticity, heat dissipation, refraction, and reflection all come into play. The electrical specifications of the application are just as important and the selection of power source can dramatically affect the overall ‘fit, form, and function’ of the end application.


At the heart of a lighting application is the basic need to supply a single LED or string of LEDs with a constant current source. The benefit of a constant current source is stability over time, defeating ambient temperature variations, line voltage variations, and reducing the effects of LED ageing, all while providing the best possible lighting parameters.


For luminary designers, meeting this basic need can become quite challenging with more complex configurations of LEDs. In the case of an application that requires very high lumens and LED stings of 16 or more, a LED driver would have to produce around 60 VDC or more. This adds safety considerations which in turn, adds another level of complexity to the overall design. The designer has a choice of solutions in this case: a) apply a matrix configuration of both


Vout v. lout characteristics of dual mode LED drivers example using AMEPR60-24250AZ


Vout (VDC) Over Voltage Protection 26.0


24.7 24.0


Constant voltage Constant current 12.0 Foldback 0.0 0.1 2.5


Protection (c) lout (A)


Constant Current Mode (b)


Constant Voltage Mode (a)


series and parallel LEDs, or b) connect LEDs in series with the addition of current stabilizing circuitry and connecting these LED strings in parallel. For a matrix approach, a constant current driver is preferred, while a constant voltage driver is the better choice when connecting LEDs strings in parallel. The question is then, how to support the design process while both LED configurations are still being considered, while avoiding the expense and time associated with sourcing two types of LED drivers? The ideal solution is a dual mode LED driver, which is a relatively new concept for LED driver manufacturers and design engineers. Such LED drivers are at the forefront of driver technology. The sophistication of these driver designs are simplifying the process and providing cost savings for both luminary designers and manufactures alike.


Such dual mode drivers function as a constant voltage driver when the loading current is between zero and the normal maximum output current. Alternatively, when a load resistance results in output current higher than the driver’s maximum, it will operate in constant current mode. The output current will be the


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maximum for each particular model of driver. The load voltage will depend on the load resistance and will follow the principles of Ohm’s law. Ingeniously, the switching between modes is automatic based upon load resistance. The flexibility of such a dual design simplifies the early design stages for luminary designers, and further streamlines the ‘parts bin’ for off the shelf driver selection, regardless of mode of operation required.


Aimtec has taken the design of dual mode LED drivers one step further with a line of drivers that operate at a bone chilling -55⁰C (an industry first), while remaining small in form factor to support end product minimization and are available in output power ranges of 90W to 150W. The entire line of Aimtec’s dual mode LED drivers extend from 30W to 150W, with minimum operating temperature ranges of -40 to +85⁰C, for use in virtually all indoor and outdoor applications. Core parameters include: wide input voltage range (90-305VAC/47- 440Hz); waterproof IP68 cases; efficient rates up to 91% efficiency, and power factor correction of up to 0.98.


focus magazine - issue 12


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