Circuit Magazine publishes Oxman Lighting article on LED thermal management

We are proud to be featured as technical experts in September's issue of Circuit Magazine.

A NEW APPROACH TO THE THERMAL MANAGEMENT OF LEDS

The importance of thermal management

LEDs generate both visible light and heat when current passes through them. That heat must be dissipated because the reliability and performance characteristics of an LED depend on an acceptably low operating temperature. If that temperature gets too high, it will negatively influence brightness, efficiency and light colour, reduce service life and cause premature LED failure.

Good thermal management is therefore a vital consideration when developing effective and well-performing LED systems.

Modes of heat transfer in LED systems

Heat is transferred from the LED to the cooling agents by a process of thermal energy exchange. The modes by which this energy exchange can take place are conduction, convection and radiation.

• Conduction is the transfer of heat through macroscopically stationary matter by direct contact. Microscopically, higher temperatures cause atoms and molecules to accrue a higher kinetic energy. Thermal energy is transferred as neighbouring molecules and atoms vibrate against each other. The effect is the flow of heat from the source to areas of lower temperature and lower molecular kinetic energy. Rate of heat transfer improves with bigger temperature differences, shorter transfer distances and larger contact areas. In an LED system, conduction occurs when heat is transferred from the base of the LED to, and through, a heatsink or heat exchanger.
• Convection is the transfer of heat through movement in liquids and gases. The increased kinetic energy induced by heat causes adjacent liquid and gas molecules to expand, become less dense, rise away from the heat source and be replaced by denser, cooler molecules. This creates a cooling convective current. Larger surface areas, faster flows and higher temperature differences increase the rate of heat transfer. In an LED system, convection occurs where the surface of the heatsink or heat exchanger contacts the ambient cooling air.
• Thermal radiation is the emission of electromagnetic waves from an object. Any object can emit or absorb thermal radiation. Radiation can travel through a vacuum and will transfer its energy to any solid or liquid surface that it encounters. Net heat transfer due to radiation is relatively minor in an LED system.

The shortcomings of finned heatsinks

LED systems such as downlights traditionally use finned heatsinks for cooling. They conduct heat away from the LED and transfer it to the surrounding air by convection. Conduction within the heatsink is inefficient due to its thickness and thermal resistance. Heat gets stored internally, a temperature gradient forms from the base of the LED to the heatsink surface and cooling performance is impaired.

Measurements on conventional downlight heatsinks confirm that there is an unwanted temperature gradient of up to 20°C from the base of the LED to the tip of the cooling fin. A temperature gradient of 10°C reduces the cooling effect by about 20%. Finned heatsinks compensate for these inefficiencies through appropriately larger sizing. The reduced cooling effect necessitates a larger cooling surface in order to reliably achieve the total required heat removal.

The finned surface will work well to dissipate heat when the convective currents run along the fins but will work less well when they are forced across them. The effectiveness of finned heatsinks thus varies depending on the orientation of the fins relative to the convective air flow they generate.

Cooling with the Total Surface Heat Exchanger^TM

Oxman Lighting has replaced finned heatsinks with its patented Total Surface Heat Exchanger^TM concept. “Our concept has done away with inefficient heatsinks and takes full advantage of the available modes of heat transfer to dramatically improve efficiency and performance in LED systems”, says Philip Oechsle, CEO of Oxman Lighting.  

The Total Surface Heat Exchanger^TM needs considerably less material to achieve the same amount of heat removal as a conventional heat sink.  Lights that use the Total Surface Heat Exchanger^TM therefore achieve an extremely low ratio of mass to luminous output. This is a feature that environmentalists and sustainability advocates are rightly excited about.

The Total Surface Heat Exchanger^TM transfers heat in the shortest and most efficient way possible from the LED to all available cooling media. This is inherently more effective than heat removal processes which rely solely on voluminous finned heatsinks. The Total Surface Heat Exchanger^TM thus increases the rate of heat removal and overcomes the thermal storage and temperature gradient issues suffered by conventional heatsinks.

When applied to a downlight, the Total Surface Heat Exchanger^TMmakes direct contact with the mounting surface totake full advantage of conductive heat dissipation into the ceiling material. This aspect of the heat removal process utilises the highest possible temperature difference to drive cooling, since the ceiling surface is the coolest area of a down light installation.

The Total Surface Heat Exchanger^TM utilises the convective mode of heat transfer into the illuminated space as the lower part of it faces into the room. Convective cooling currents therefore surround the entire heat exchanger and highly efficient convective cooling is achieved regardless of its orientation.

Light fittings that use the Total Surface Heat Exchanger^TM can have higher outputs, slimmer profiles and longer lifetimes, while using significantly less raw material. For example, downlights using finned heatsink technology can weigh more than five times as much as those that use the Total Surface Heat Exchanger^TM. With the sheer volume of downlights being manufactured and installed every year, Total Surface Heat Exchanger^TM technology promises to be a huge win for the environment. The combination of low material usage and ultra-long lifetimes makes downlights using the Total Surface Heat Exchanger^TM some of the greenest and best performing on the market. 

The Total Surface Heat Exchanger^TM has led to downlight designs that are ultra slim and compact, meaning shallow roof cavities are no longer an issue and retrofits can easily be performed without needing to laboriously re-size existing mounting holes.

The ability of the Total Surface Heat Exchanger^TM to achieve heat removal in every direction means that lights which use it for cooling can be continuously operated even if one of its heat removal paths is obstructed. Thus, downlights with the Total Surface Heat Exchanger^TM can safely operate continuously (24/7), even while covered with insulation (IC-4 rating) at ambient temperatures of up to 45°C.

The Total Surface Heat Exchanger^TM offers benefits to installers, consumers and the environment. It can be effectively applied to improve the performance and environmental characteristics of not only downlights, but also floodlights and spotlights to achieve;

• Higher luminous flux (i.e. brighter light output),
• Lower whole of life costs and
• Optimum environmental sustainability.

Oxman Lighting DL1

The Oxman Lighting DL1 downlight has been developed around the Total Surface Heat Exchanger^TM concept and confirms the benefits of this cooling concept:

The Total Surface Heat Exchanger^TM offers benefits to installers, consumers and the environment. It allows for high quality illumination while achieving low whole of life costs and optimum environmental sustainability.

Ready to purchase? You can purchase Oxman's ground breaking, DL1 downlights here.