Modular LED drivers shorten time to market

Technology News | June 27, 2013

By eeNews Europe

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 minimised 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 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.

Data sheets 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 & temperature
Most data sheets feature a range of charts that indicate the high efficiency of the devices (fig. 1).

Fig. 1: Efficiency relative to input voltage and load

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

Fig. 2: Derating of power loss

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.

2) EMC
In general, data sheets of LED driver ICs contain no information regarding electromagnetic compatibility. This often leads to failures in the EMC lab when it becomes apparent that the circuit emits strong interfering signals. If class B limits must be achieved, such circuits need to be redesigned with complex filters with significant and valuable time lost in EMC lab testing.
RECOM solves this problem for designers by offering modular LED drivers with integrated class A or class B filtering.

3) Dimming
Many LED driver ICs offer only PWM signal dimming. This means, of course, that it is not sufficient to have an external voltage supply (e.g. control voltage of 1V to 10V) or a potentiometer, as an auxiliary circuit is required for the PWM signal. This, in turn, leads to a more complex layout of the circuit and increases the number of components. As a result, costs go up and the system might become less stable.
In other cases, there is no real PWM input available, and the function is to be implemented by means of a CTRL pin. This means that the IC must always be fully switched on and off, as it is not possible to modulate only the output signal. This can lead to additional problems in connection with linearity. This is particularly critical between 0 and 10%, as the human eye is especially susceptible to changes in luminosity in this range, and as the colour temperature remains virtually the same, which is not the case with conventional light bulbs.

The information in data sheets often does not reflect this reality. While data sheets provide a good basis for the choice of a driver IC, they sometimes omit important information, for example on the performance limits or the EMC properties, and therefore need to be read with caution.

Reliability & service life

The reliability of a driver is directly correlated with its MTBF (Mean Time Between Failure). The MTBF can best be illustrated by means of a "bathtub curve" (fig. 3). It consists of three phases referred to as infant mortality, useful life, and end of life phase. The MTBF only covers the centre range, leaving aside early failures and end of life wear-out. The MTBF values stated in data sheets for LED drivers are normally around 600,000 hours.

Fig 3: Bathtub graph

The MTBF is calculated as follows: λDRIVER = S λCOMPONENTS

This means that the MTBF of an LED driver can only be determined if the MTBF values of all its components are known. These values can be looked up in various databases. The best known reference list is probably the Military Handbook 217, Version F (MIL HDBK 217F), which is used by most manufacturers. It is therefore possible to determine the reliability of the finished product during the design phase.

But what exactly does an MTBF of 600,000 hours mean? Does this correspond to a service life of 68 years? This would be a wrong conclusion. The MTBF indicates only the probability of a failure within a specific period of time, e.g.:

· 2% failure rate per year –> MTBF = 438,000 hours (8,760 h / 2%)

· 5% failure rate over 5 years –> MTBF = 876,000 hours (5 x 8,760 h / 5%)

In addition, one must take into account the effect of the temperature on the MTBF. The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of reaction rates. Applied to electronic devices, it clearly shows that the failure rate increases at higher temperature. The following rule of thumb applies: "increasing the temperature by 10°C reduces the expected lifetime of a device by half."

The example to the left illustrates the significant effect of the ambient temperature on the reliability of the system.

Another relevant factor for the reliability of an LED driver is the choice of the correct components, in particular of suitable capacitors. LED driver ICs are often equipped with cheap electrolytic capacitors rather than with high-quality ceramic capacitors (MLCC).

Why do electrolytic capacitors have such a huge influence on the reliability of a circuit? The electrolyte contained in the electrolytic capacitor is a liquid, which tends to evaporate over time, resulting in the failure of the component. This happens even more quickly when the capacitor is exposed to high temperatures. Electrolytic capacitors are therefore generally regarded as the component that determines and limits the total service life of the LED driver.

As we have seen, the MTBF does not accurately indicate the service life of a device, but is a good indicator of the reliability of a system or component. For discrete drivers, it is therefore crucial that components are chosen whose MTBFs are in line with each other. In the end, the overall system is as reliable as its weakest component.

In contrast to the calculated MTBF value, the "Design Lifetime" specified by RECOM is based on real tests and therefore indicates the lifetime of the driver. Unfortunately, this valuable information is rarely found in data sheets.

Certification

Another important aspect for the development of a bespoke LED driver is certification. This is a costly and time-consuming process and often delays the launch of a product.

The list below is just an excerpt of the international standards that must be met:

General safety

EN 61347-1 and EN 61347-2-13

UL 8750

CE marking

EMC:

EN 55015

EN 55022

EN 61000

EN 61547

UL 1310

RoHS Directive 2002/95/EC

REACH Regulation (EC) no. 1907/2006

EcoDesign Directive 2009/125/EC

WEEE Directive 2002/96/EC

The CE mark regulations are particularly stringent. By applying the CE mark, the manufacturer confirms that the product meets all relevant standards and regulations. While compliance is achieved easily by completing a single form, the manufacturer must be aware that he bears all responsibility in relation to warranty and liability for a product featuring the CE mark.

Special considerations with regard to AC/DC LED drivers

The design of an AC/DC LED driver is not only more complex than that of DC/DC LED drivers but must also meet more stringent requirements regarding safety. This is due to the fact that these devices are outside the SELV (safe extra-low voltage) range. This concerns, however, not only the finished product, but also the design and testing process where the grid voltage applies. Special measures need to be taken to protect users.

In addition, AC/DC LED drivers must conform to specific energy efficiency and quality regulations and directives such as the European EcoDesign Directive or the Energy Star regulations. From 25W, they must, for example, provide for a power factor correction (PFC) >0.9, which makes their design considerably more complex. This means, of course, that the development and testing equipment must also meet more stringent standards. Power analyzers, high-precision power supplies, and switching LED loads are a must. For additional functions such as TRIAC dimming, manufacturers need arbitrary generators in order to simulate accurately the signals of the various leading and trailing edge dimmers. The block diagram (fig. 5) shows the various elements that are required as standard for the testing of an AC/DC LED driver.

In conclusion, the in-house development of AC/DC LED drivers is only an option for manufacturers who have the necessary expertise. It is generally advisable to rely on a fully assembled, tested and certified product from an established supplier.

About the author:

Stephan Wegstein is VP Marketing & Sales, RECOM Electronic GmbH.