Improved flexibility for LED drivers

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Features

LED drivers, essential to the performance of any LED lighting, need to be sufficiently flexible to adapt to different conditions. Markus Rademacher of Tridonic explains how new technologies are helping

Readers will clearly be aware that at the heart of every LED luminaire is an LED driver and that the most important parameter of the LED driver is the output current. Essentially, in an LED circuit, mains voltage is converted to a constant voltage and the LED driver then converts this to a constant current. This current determines the operating temperature and therefore the lifetime of the luminaire.

 

Intelligent devices that have separate communication interfaces, such as DALI and DSI, are able to automatically set the output current with a high degree of flexibility. Less sophisticated devices without such an interface lack this flexibility. This is because it is difficult to deal with resistance plugs or setting DIP switches in partially or fully automated production lines.

As a result, with these devices the current has always had to be manually configured – until recently that is. Now, new technology enables drivers with manually adjustable output current (also known as fixed-output drivers) to be automatically configured. This offers maximum flexibility for the growing number of projects that are making use of LED lighting.

At the heart of this technology is a programming unit that enables configuration data to be modulated digitally on the mains cable. Thus it is possible to simply connect it as a digital communication interface between the mains cable and the driver providing a quick, easy and flexible way to automate configuration.

One element of this flexibility is that the parameters can be selected either directly on the programmer or in appropriate configuration software. In the latter case, the configuration software allows additional parameters to be set – such as a constant light output function – as long as the driver supports this function. Very importantly, successful programming is indicated by both the luminaire and the configuration software.

If required, it is still possible to set the current manually via resistance plugs.

The benefits of digital data transfer
This technology offers significant benefits compared to conventional manual current setting methods. Transmitting the data in digital form is quick and, because it does not rely on manual steps, it is less susceptible to error. It can also be easily integrated in automated processes.

Another benefit is the cost aspect of implementing this interface because it exploits the fact that a DC recognition function is already integrated in the LED drivers. Consequently this function can be implemented without additional costs for the products.

There are also benefits in relation to the production process and how this minimises the risk of errors in the products, thereby reducing problems on site and the likelihood of delays in the project schedule caused by such errors.

The key aspect here is that the luminaire-specific parameter sets can be created, stored and re-used in the form of scripts. This means that if the development department of a luminaire manufacturer creates different scripts and verifies them, they will then be available on a drive, clearly identifiable and tested. Thus there is a clear separation in terms of time and personnel between the creation of parameters and their transfer to the relevant drivers.

These tested scripts can then be copied to the programmer as part of the production process, eliminating the need for a PC at the assembly and testing station on a production line. It is possible to configure the drivers either before the luminaire is assembled or later in the completed luminaire. Throughput times can also be reduced because up to five drivers can be configured simultaneously.

A software modules specially developed for this programme also enables the driver configuration to be integrated easily into existing software-controlled test systems. During the safety and function tests the luminaires are connected to the mains anyway, and the programmer is simply connected between the luminaire and the mains.

To configure the luminaires, for example, the appropriate configuration scripts can be retrieved automatically from the database and transferred to the driver. The programmer receives the parameters to be configured via a USB interface, buffers them if necessary, and forwards them to the LED driver. To preclude later manipulation during operation a lock command can be set during configuration. This ensures that current programming can no longer be changed via the mains cable.

These are clear benefits to the manufacturer but specifiers and installers also benefit because this procedure guarantees that incorrect parameters will not be used. In addition, easier on site configuration of either complete luminaires or individual components helps to simplify installation and commissioning for contractors as well as reducing project times. This is a major contrast to the older methods of having to manually configure the driver, as well as needing to keep a stock of resistance plugs.

Data transfer
As noted above all of the information transferred by the programmer is in digital form, using a self-contained data block that is referred to as a ‘frame’. This comprises a total of 18 half-waves, each representing a digital value. Presence of a trailing edge corresponds to a value of "1" or "TRUE"; absence of a trailing edge corresponds to "0" or "FALSE".

A frame is subdivided into individual elements and contains partial information such as the start and end points of the frame or the value of a particular parameter. For data transfer, the frames are modulated by the programmer on the mains cable. The driver decodes the transferred signals to obtain the parameter values and information on their validity.

There are also safeguards to ensure correct data transfer, using several projects simultaneously, including a parity check and a checksum calculation. The parity check works with a special error bit which detects the "flipping" of a bit during transfer. There is also a checksum calculation in which the different values of a frame are mathematically checked against one another.

The result is also transferred to the driver, enabling it to check the data for correctness. If the driver finds a discrepancy it will ignore this frame and immediately switch off. This disconnection is detected by the programmer and signalled accordingly.

A further element of quality assurance is introduced by automatically performing power measurement through load modulation – but only if the parity check and checksum calculation have been completed successfully. This power measurement procedure involves the driver changing the current through the connected LED module in a predefined sequence. Evaluation of the measured power ensures that the data has been correctly received and stored. This visual feedback and an appropriate message at the programmer and via the USB interface indicate successful completion of data transfer.

These different tests ensure correct configuration with the programmer. In addition, use of the phase angle (= the duration of the trailing edge) increases fault tolerance and improves immunity.

Summary
The ability to configure LED drivers without requiring an additional interface, enabling configuration via the mains cable, allows digital configuration without a separate communications interface. This is clearly an improvement on the manual methods described above in terms of fault tolerance, safety and flexibility.

The only requirement, which specifiers should be aware if they are to take advantage of this technology, is that the drivers must be compatible with the programmer. There are already a number of suitably compatible drivers on the market so the opportunities to deliver these benefits to projects are there for the taking.

Video explaining this technology: https://www.youtube.com/watch?v=Jw8euS2PZf4



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