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                            LED lighting modules are a huge growth area across many application areas including automotive lighting, battery powered portable equipment and domestic, commercial and industrial lighting. The need for binning and its follow-on effect is one of very few drawbacks to utilising high brightness (HB) LEDs.Binning is currently a fact-of-life for manufacturers of LEDs. Difficult to control variables in the production process mean that LEDs need to be separated and labelled based on a number of key characteristics. While some end applications can live with the differences most can not.
                          Binning, or ‘ranking’ as it is sometimes known, takes place after LED wafer processing and usually before it is diced. During production, optics, phosphor coatings and other processes and components added to the original LED die can change the output characteristics of each individual device.Binning can typically be applied to batches of 1,000 pieces to over one million. It sees LEDs grouped according to characteristics such as luminous intensity (in accordance with CIE 127-1997, radiant intensity, dominant wavelength, centroid wavelength, colour coordinates, and forward voltage.

Colour coding LED infoDriving into the blueOrganic - grow your ownGreen chandeliersInto the black?Display TrilogyHeadlight glarePhosphorousSensing lightGrey areaAn alternative look at the huesand shades of LED technologyAs an example, consider an LED for which a low intensity bin starts at 20 lumens, and a high intensity bin that ends at 50 lumens. In this example a customer purchasing LEDs without specifying a bin code could receive product with an intensity of anything between 20 and 50 lumens. 
                            Depending on the nature and configuration of the final application and the detail of the customer’s product design, this variation may result in an unacceptable degree of discernable light variation.The colour binning of white LEDs presents the greatest challenges. The subtle differences between types of white such as cool white, neutral white and warm white can all be achieved through binning but this is a process that is quite technically difficult.Of all the colours, the market for white LEDs is the by far the biggest as it is the only colour suitable for use in automotive headlamps and the most likely to be used for buildings, architectural lighting and numerous other applications.If a customer’s design is able to accommodate LEDs from many different bins, then the simplification in supplying the product can lead to a component cost saving of up to 15 per cent. Conversely, ‘locking’ a design into LEDs from one bin usually incurs extra cost.  If a customer cannot or does not specify particular bin(s) before purchasing, then the LED manufacturer is free to ship product from any bin - including from the lowest light intensity bin. It is good advice for customers to check factory yields across the full range of bin numbers prior to finalising their design. 
                           Product design to allow use of multiple binsHB-LEDs are a very directional light source with all the light emitted falling within a hemisphere. More traditional light forms such as fluorescent tubes and incandescent bulbs on the other hand, perform very poorly in this respect. Hence the need to use filters, diffusers and lenses to project the available light in the desired direction for a given application. In theory, designers may often be able to achieve significant design and cost savings by not requiring filters, diffusers, and lenses for HB-LED based designs. However, the need to combat the problem of variations in light intensity due to LEDs from different bins being used may re-introduce the need for such components in order to ‘balance out’ and blend varying brightness levels from individual LEDs in a module.Programmable LED drivers and application software ease the design processSome LED driver manufacturers have introduced products that are able to compensate for the differences between HB-LEDs sourced from different bins. These can, in certain applications, overcome the need for filters, diffusers and lenses whilst still allowing LEDs from multiple bins to be used.The MAX16816 from Maxim Integrated Products for example is a programmable high-voltage, HB-LED driver designed for use in a wide range of applications including front and rear automotive combination lights (RCLs), neon replacement and emergency lighting applications.
                            It features on-chip non-volatile EEPROM registers that can be programmed either at the factory or by the user in the field. Amongst other parameters, this allows the LED current to be adjusted to compensate for the brightness differences between LEDs from various bins.A slightly different approach, and one adopted by Cypress Semiconductor’s EZ-color family, is to combine an HB-LED controller with embedded software that allows supported vendors’ HB-LED part number and bin codes to be inputted by the designer. The embedded software then applies the manufacturer’s bin specifications and temperature feedback algorithms to program the controller to compensate for not only the binning characteristics, but also their temperature response profile.Controllers such as those described above are typically available to drive single, four, eight or 16 HB-LED channels.As LEDs make headway into mainstream use more and diverse applications, binning will become a more high profile issue for LED manufacturers and product designers to contend with. Future advances made by individual companies in terms of minimising product variation will help soften the impact of binning on customers.
                             This coupled with methods such as using programmable drivers to overcome varying light levels and simplify product design will go a long way to mitigating some of the issues.The use of LEDs for mass, generalised applications will call for manufacturing techniques that can produce consistency of product in highly efficient ways that may ultimately not include binning.  Jamie Furness is global technology & development manager at Farnell

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