A new generation of LED drivers makes it easier to design sophisticated LED luminaires

Author : Thierry Suzanne, Future Lighting Solutions

05 February 2013

A new generation of intelligent LED drivers promises to make the implementation of LED lighting power systems much simpler, quicker and more flexible.

Every LED-based luminaire requires a driver circuit; many require a means of controlling the light as well.

Electronics design expertise is required to develop in-house a custom driver circuit using semiconductor components such as DC-DC converters, linear regulators, PFC (Power Factor Correction) controllers and microcontrollers.

So many luminaire manufacturers have chosen to use driver modules, or power bricks. These complete units accept a high-voltage mains input, and provide a constant-current output set at levels commonly required in LED lighting applications.

Typically, however, these driver modules provide either no control functions, or capability limited to basic analogue (1-10V) or digital (DALI) dimming.

When luminaire manufacturers choose to use LEDs, however, they are leaving the conventional and simple world of incandescent lighting: implementation of LED systems is more complex, for a variety of reasons, and this complexity often calls for sophisticated control, to provide for safe operation, long life and useful features.

For instance, all LEDs suffer from lumen depreciation over time. If a luminaire needs to maintain a constant intensity over its lifespan, the drive current supplied to it must gradually rise over time.

LEDs also provide the ability to graduate light output, so that the same light engine can provide a different intensity in different luminaires. By providing adjustable power outputs, luminaire manufacturers can use the same basic LED and driver design in multiple end products, reducing inventory requirements, supply chain complexity and costs. This also makes it easier for manufacturers to adapt to the rapid evolution of LED technology.

Another useful feature of LED luminaires is that they are precisely dimmable. In street lighting or commercial office lighting, for instance, sophisticated dimming schemes that dim or turn off lights at certain times of day, or when people are not present, can help to save large amounts of energy.

All three cases above require sophisticated control functions that are beyond the capability of conventional LED driver modules. Until now, therefore, luminaire manufacturers have normally required both an LED driver module and a separate control circuit. To design this separate control circuit, the OEM has had to acquire knowledge of simple embedded system design, because it is typically implemented through application software running on an 8-bit microcontroller.

Now a new generation of intelligent LED driver systems combines the driver module and control circuit into a single unit. This has three important advantages:  making the system hardware simpler, smaller and cheaper; making system design and implementation simpler. No longer writing code to a microcontroller, the luminaire manufacturer now simply configures the luminaire in a simple Graphical User Interface (GUI) ; making product designs and usage more flexible, since configurations can be altered at the factory, on installation, or while operating in the field.

New intelligent LED drivers
Features to look out for in this new generation of intelligent LED drivers include:
? Programmable output current, which allows the manufacturer to program the output current of the driver in, for instance, 50mA steps, or via an external resistor
? Flexible dimming: customised dimming schemes responding to a variety of inputs such as ambient light levels, time, presence of people
? Constant light output: increase the drive current proportionately to the depreciation in light output from the LEDs over time
? Support for 1-10V and DALI control protocols
? Adjustable start-up duration, to avoid the risk of excessive inrush current when the luminaire is switched on, which could damage the LEDs
? Thermal protection: automatically decrease drive current when the temperature at the LEDs exceeds a set threshold
? Operating information read-outs: logged data on operating time, power consumption, output current and so on

Two companies making interesting examples of the new combined driver/controller devices are Philips Lighting and Roal Electronics. Both have succeeded in developing a graphical programming interface that is intuitive and easy to use.

The Philips smart driver offering is its Xitanium line of programmable modules, together with its MultiOne configurator tool (www.philips.com/multione). These programmable drivers enable the user to control light output levels, use pre-set dimming protocols and set system specifications both in the factory and in installed units.

The MultiOne Configurator offers an intuitive, PC-based programming interface for configuring settings and dimming schemes. A range of Xitanium indoor and outdoor drivers and Philips motion sensors are compatible with the MultiOne platform. The driver modules are programmed directly by the PC via a USB-to-DALI connector cable.

As well as providing an intuitive interface for programming sophisticated dimming schemes at the luminaire, MultiOne-compatible drivers also support useful diagnostic functions (see Figure 2). Operating data is logged by the driver to enable maintenance engineers to check that the luminaire is performing according to its specifications.

A competing product family, called Ozone, is available from driver manufacturer Roal Electronics (www.roallivingenergy.com/products/more/ozone_programmable_led_ballasts).

As of November 2012, the Ozone driver (see Figure 3) has four models, all rated for 70W maximum output power. Roal has produced high-performance LED driver modules for many years, and so it is no surprise that the Ozone range boasts impressive technical features such as a high power factor of >0.9 , total harmonic distortion of less than 20% and a typical efficiency of more than 90% from a compact design. Ozone modules also integrate an auxiliary 5V output for driving an external fan, and offer an optional remote gear kit.

But the new benefit offered by this driver family is the Ozone programming tool, which enables the user to easily implement sophisticated dimming controls while eliminating the need for any additional control electronics in the fixture.

The software enables users to schedule up to five timed dimming levels (see Figure 4). The software is intuitive and easy to use – setting the dimming percentages and timings is as easy as changing a PC’s display settings in Microsoft Windows.

The programming software is hosted on a PC, which also stores saved settings. An easy to use GUI enables the customer to set the following LED driver parameters: output current, fade time, DALI or PWM function, adjustable dimming function and constant light function (which automatically adjusts the output current so as to produce the same light output for the entire life of the LED lamp).These settings are saved to the Ozone programming device via a USB interface; the programming device then downloads the settings to an Ozone driver module via a three-wire programming cable (see Figure 5).

A microcontroller-based design, the Ozone LED driver family is both intelligent and flexible: a single unit may be used in many different fixtures, which helps manufacturers to reduce inventory risk and cost. With just four models, the Ozone family supports applications requiring from 20Vdc to 195Vdc, and drive currents from 350mA to 2,600mA (see Table 1).

BoM cost savings
Luminaire design engineers can use a Xitanium or Ozone driver today to achieve immediate savings in system Bill-of-Materials (BoM) cost. As stated above, LEDs suffer from lumen depreciation over time; the rate of depreciation depends on factors such as the current at which the LEDs are driven, and their operating temperature. 

It is common practice, therefore, for luminaire designers to over-specify the LEDs for a new luminaire in order to allow for depreciation, ensuring that at the end of the fixture’s lifespan it is still producing the minimum flux. Typically this results in a luminaire containing 10-20% more LEDs than required to meet the minimum flux target when new. 

The use of a Xitanium or Ozone smart LED driver offers a better alternative. Using the lumen depreciation curve provided by the LED manufacturer, the design engineer can simply programme the driver module to increase the drive current gradually, in order to maintain a constant light output over the life of the luminaire. This then requires fewer LEDs, which saves space and reduces BoM cost. 

The diagnostic features in smart LED drivers also offer operating cost savings, since routine maintenance visits can be replaced by remote examination of logged operating data.

The features in smart LED drivers offer great potential to LED luminaire designers to reduce cost and energy consumption while simplifying the design process.

Over time, luminaire manufacturers might also find that the use of smart LED drivers offers an even greater benefit: the opportunity to develop a future-proof platform from which multiple product variants can be derived, each tuned to the requirements of a different customer or application. Compared to today, this will enable the manufacturer to either reduce the high costs associated with building custom products from scratch, or to improve the appeal of standard products by fine-tuning their features and specifications in response to different customers’ or applications’ requirements.

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