LED reddens as it dims for soft lighting

LED Engin is to reveal details of an LED that dims like a halogen bulb, reddening as its output drops, at Light+Building in Frankfurt (15 – 20 April), writes Steve Bush.

LED reddens as it dims for soft lightingLED Engin is to reveal details of an LED that dims like a halogen bulb, reddening as its output drops, at Light+Building in Frankfurt (15 – 20 April), writes Steve Bush.

Dubbed Luxitune, its dimming characteristic is intended to match established expectations in architectural lighting.

“This is particularly useful in hotels, restaurants and bars, where the right ambience is desired,” said the firm.

LED Engin’s speciality is packing multiple LED die into a small area so that tight beams can be formed with compact optics – for a given beam angle, the minimum diameter of an optical system is proportional to the diameter of the light source.

Specifically, the firms expertise is in getting heat out of closely-packed die – more of this later.

LuxiTune takes advantage of individually addressable multiple die, and in-source mixing, to produce white light varying from 2,000K to 3,200K.

“The colour temperature moves on the black body curve,” LED Engin general manager Uwe Thomas told Electronics Weekly. “You get what you are used to when you see a halogen. You don’t get a pinkish or greenish look.”

There are actually three strings of LEDs in the emission area of the module, said Thomas, each of a different colour temperature.

Electronics for colour temperature variation and dimming are built into the 50mm diameter LuxiTune module, which requires only 24Vdc and a standard architectural 0-10V dimming signal.

There is also a remote electronics version.

“The light stays within two McAdams ellipses along the black body,” said the firm.

LED Engin’s argument for multi-die packages, and LuxiTune, is the same as that offered by multi-die module firm Bridgelux: that while LEDs offer a wide range of possibilities, many lighting designers want LED sources that match the ones they are already working with.

“They need illumination equivalent to a traditional source, like 60W or 100W light bulbs,” said LED Engin’s Thomas.

There is no single die LED that puts out 850 or 1,600 lm – although Cree’s XM-L LED, which is thought to have a 2x2mm die, is said to deliver over 700 lm of warm white light when it is driven at its maximum rated current of 3A – which is close to 10W.

With multi-die clusters on a single substrate, light output can be increased arbitrarily by adding more die.

Once the decision to go for multiple die is taken, there are two options for focussing optics – one optic per die, or one optic for the whole die cluster.

Thomas’ argument is that one-optic-per-die is always a bulky option.

As narrow beams larger diameter optics, and larger die require larger diameter optics, he said that you better have a small source if you want tight beams and narrow optics: “Our LED with 12 die is a 9x9mm ceramic substrate with 1mm2 die packed into an equivalent diameter of 4.6mm,” said Thomas.

This he compares with 20-50mm diameters from some of his competitors.

There is an additional factor here, pointed out Thomas.

If you want light concentrated on a target with little spill around the sides – as is often the case in stage and architectural lighting – then moulded collimators form better beams than simple reflectors.

The reason is, that simple torch-style reflectors focus only light that leaves the source sideways and bounces off the reflector. Light that leaves the top of the source just spreads in that part of the hemisphere not limited by the reflector – so reflectors always deliver a broad halo.

With collimators, more widely known now as TIRs (total internal reflectors), all light is controlled. Side spill is focused by the total internal reflection from the outside of the TIR, and top light is focussed by the simple refractive lens moulded in the middle – so TIRs can suppress the halo and deliver more light on target.

LED Engin’s interest is largely in low-spill application where light in concentrated in the beam centre.

“We are carving out a niche where the customer really needs punch,” said Thomas. “If you don’t have lux on target, you don’t have punch.”

As such, Thomas favours TIR optics, which are limited in size by moulding physics, and therefore need die very close together, which cannot be done without advanced thermal management.

One exception to the reflector-TIR argument is the 225 assembly from Berkshire-based Polymer Optics’ which suspends a Fresnel lens above the middle of a conventional reflector to increase light control. Hollowing the front of this large LED TIR from LED Engin allows it to be moulded without cracking, and gives it an unusual appearanceTo get its narrowest beam: +/-4° from the 4.6mm source, LED Engin makes possibly the largest TIR available (see photo) whose unusual appearance is due to hollowing out the front of the structure to allow it to cool from the mould without cracking.

With a compact collection of LED die replacing a 60 or 100W bulb “you can end up with 30W in a tiny space”, said Thomas.

To get this heat away from the die, LED Engin uses a complex ceramic tile structure based on alternating layers of metal and ceramic.

“This is not the cheapest technology,” said Thomas.

Immediately under the die is a pad of silver – the best metallic thermal conductor known – to extract heat and spread it for the first ceramic layer.

Effective heat spreading is the key to transferring heat from a die to a heatsink.

Under the first ceramic layer is another silver layer to further spread the heat.

And the ceramic is not standard aluminium oxide.

Aluminium oxide is lower cost, aluminium nitride is better,” said Thomas. “There are other manufacturers that also see aluminium nitride as the way to go.”

All told, he said, there are up to nine metal layers spreading heat within the ceramic sandwich making up an LED Engin package.

Metals, ceramics, and thicknesses are chosen to best match the coefficient of expansion of the die, the die attach material, and the glass primary lens.

“An off-the-shelf AlN substrate is certainly not matched,” said Thomas.

The die attach material is also special.

Thomas argues that solders are good for die-attach because they can be so thin compared with the attach area that the thermal conductivity of the solder is largely irrelevant – as opposed to epoxies which cannot be that thin.

He also argues that voids in the attach material are a significant factor contributing to thermal resistance, as they can be up to 30% of the attach area.

“Epoxy does not allow you to drive LED hard. We use gold-tin flux-less eutectic,” said Thomas. “Gold-tin minimises void building. Our interface has 90% coverage.”

Packing all this heat into such a small space would mean cooking the die if it was not handled well.

“LED Engin will show test data to confirm consistent performance over time, lumen output remaining within 2% over 6,000 hours at 85°C ambient temperature with very stable colour temperature,” claimed the firm.

Using its thermal technology, LED Engine makes products with 1, 4, 12, 24, and 25 1mm2 die, sourced from firms including Cree, Epi-Star and SemiLEDs.

It also produces TIRs with beam angles of +/-4, 7.5, 12, 17.5 and 23.5°.

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