Multi-die packages are currently the 'flavour of the month' amongst the high power led manufacturers.

Certainly for both manufacturers and designers chasing the title of 'highest lumen output' device, this approach gives more lumens from a single source than they have ever had before.

We should not down play the achievement of companies such as Cree and Seoul Semiconductor in pushing forward the packaging technology that allows them to do this, and there indeed applications where high output single sources can outperform other schemes.

However designers should be aware that these multi-chip devices are not necessarily the best approach to general purpose illumination requiring high lumen output.

Multiple single die power LEDs potentially offer a better solution, particularly in applications such as street lighting.

The two biggest problems that face anyone designing high power LED luminaries is how to get rid of the heat and how to direct the light to where you want it.

Unfortunately multi-chip LEDs are more difficult to use than multiple single die solutions in both of these respects.

Firstly taking four or more dies and putting them in to a single package means that the heat dissipated by the dies are concentrated in to a small area and it is the footprint of the package that becomes the limiting factor in how well the heat can be removed.

Concentrating the heat dissipation in to a small area of the heat sink inevitably means higher LED package temperatures compared with multiple single dies.

The higher package temperature leads to higher die temperatures and the inevitable consequence of this is lower efficiency.

Whereas the problem with heat dissipation is recognised by many in the industry, the problems that they present to the optical design are generally not appreciated.

Increasing the source size from typically 1x1mm in a single die package to 2x2 mm or more in a multi-die package means that for a given beam divergence the size of the output aperture needs to also double, as does the focal length.

The result is an 8x increase in the volume of the optics.

Comparing this with four single die-packages that just require four standard optics puts the multi-die packages at a factor of two disadvantage.

A further disadvantage of the multi-die packages is the non-uniform nature of the source. The gaps between the dies are black areas which when projected by the optics lead to non-uniformity in the output beam.

In architectural applications this can lead to ugly patterns projected on to walls or ceilings.

In wide area illumination it can lead to dips in the output which mean that minimum Lux levels are not maintained. Good optical design is required to overcome this problem and Carclo are demonstrating a prototype optic for Cree's four die MC-E power LED that overcomes this particular problem.

It is certain that multi-die packages are part of the future of high power LEDs. In car headlights for example where four dies arranged in a line match the desired light output pattern they are certainly the best solution.

They are also suited to MR16 halogen bulb replacements where footprint space is limited.

But for many other applications requiring high lumen output their lower efficiency and larger more complex optics requirement make the multiple single die package approach the better option.