Design Con 2015

Is an off-the-shelf LED solution better?

Stephan Wegstein, RECOM Electronic GmbH. -July 16, 2013

Data sheets of LED driver ICs are normally quite impressive. They promise high performance and a wealth of features at a low price. In practice, there are, however, many stumbling blocks that are not mentioned in the data sheet and can cause serious problems for developers. In addition, the development, testing and certification of LED drivers are extremely time-consuming and expensive undertakings. This applies in particular to AC/DC LED drivers. Ready-made and tested modules do away with the most costly development tasks, saving up to 80% of the design costs.

Modular versus discrete: general considerations

Conventional wisdom maintains that discrete LED drivers are always more cost-effective than modular solutions. This is, however, not always true. If the cost comparison is based on the BOM (bill of material) costs, LED driver modules are dearer than discrete LED drivers. This is, however, not a fair comparison, as the actual costs of a discrete solution amount to much more than the bare BOM costs. Also to be taken into account is the fact that the LED driver is normally the last component defined in the design process so that time becomes a pressing factor.

For a decision for or against modules that is based on facts, the following issues must be taken into consideration:


1) Design

Companies must ask themselves whether they actually have the engineering know-how to develop a discrete solution. A module is a plug & play device that can easily be integrated into a lighting system, thus shortening the time to market of the product. Furthermore, the development costs of a discrete solution might be considerably higher than projected, due to the possible need for re-design in the future. The overall expenses might therefore exceed the costs for fully certified LED driver modules that can be easily calculated.
2) Assembly/warehousing

With regard to assembly and warehousing, easy handling is a must. With a modular solution, manufacturers need to handle only a single component from a single supplier. Logistics costs are minimized and production is more efficient due to easy assembly. Manufacturers benefit from constant product quality and there is no need to take into account component tolerances. The risk that production comes to a halt due to the unavailability of a part (e.g. capacitor, inductor, resistor, etc.) is practically eliminated.
3) Certification

The certification of an LED driver can be a time consuming and costly process. It is not uncommon that components fail in lab tests and that the application process must be repeated in order to obtain the necessary certification. This results, of course, in additional expenses as re-design costs not only money but also significant time! When opting for modular LED drivers, certification and the associated pitfalls are not an issue.

4) Reliability

Reliability goes hand in hand with warranty and affects the image of a brand. While a company might be in a position to accurately determine the MTBF, it might not have access to the expensive systems for reliability and service life testing. Leaving aside the development process and the associated time/cost factors, a manufacturer must decide whether it wants to rely on an in-house design or feels safer with a tried and tested product from an experienced supplier.
If a company decides to go ahead with a discrete solution, the decision tends to be based on the following considerations:

  • Availability of development capacity and time
  • Large quantity of items required, e.g. for retrofits
  • Long service life of product, e.g. > 20 years

In all other cases, fully assembled, tested and certified LED drivers are the more cost-effective solution.

Datasheets and what they don't tell

Data sheets of LED driver ICs often contain an impressive list of features. On first sight, they promise a driver solution that can be implemented quickly and relatively easily. But the devil is in the detail, as the examples below show:


1) Efficiency and temperature
Most data sheets feature a range of charts that indicate the high efficiency of the devices (Fig. 1).

 

 

 

To accurately determine the performance limits of the ICs and the behavior over temperature, a more in-depth analysis is required. This can, for example, be done by means of power dissipation derating (Fig. 2).

 

 

This curve, if it is included at all, is normally found at the very back of the specifications. The relationship between power dissipation, maximum operating temperature and actual ICs power can best be explained by two examples:

a) Vin = 12V, 3x 1W LEDs at output:
Based on the efficiency characteristic (fig. 1, red curve), this arrangement results in an efficiency of 90%. This means that the power dissipation is 300mW (calculation: 3 x 1W x 10%). The power dissipation derating curve (fig. 2) indicates that the maximum operating temperature for this arrangement is around 80°C. So far, so good.


b) Vin = 24V, 5x 1W LEDs at output:

In this case, the efficiency is 92% (fig.1, yellow curve), and power dissipation is 400mW. According to fig. 2, the maximum permissible operating temperature for this application is, however, 40°C.


c) Vin = 30V, 7x 1W LEDs at output:

Here, the efficiency is even higher at 93% (fig. 1, blue curve), while power dissipation increases to 490mW. According to the derating curve (fig. 2), the IC is, however, not capable of compensating for power dissipation that exceeds 450mW. This means that the circuit would overheat. In other words, the above arrangement is not possible, although it appears to be within the basic specifications of the IC.
These examples illustrate that the basic specifications of LED driver ICs normally only list the "best case" values. The IC might therefore quickly reach its limits, even if the relevant values are within the specifications. In fairness to the manufacturers of LED driver ICs, they normally make people aware of potential limits, with statements such as "...typical application circuit driving XY LED in the following conditions …" in small print at the bottom of the page. The technical problem remains, however, the same.
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