Heat sinks for LED lighting: natural convection and aesthetics

The thermal path in an LED luminaire

Heat leaves an LED at the die, crosses the package and the board, crosses the interface to the heat sink and then leaves the heat sink to the air. The heat sink is only one part of that chain, but it is often the part with the most design freedom.

The board and the interface are usually fixed by the LED module supplier. The heat sink has to take the heat flux from a small footprint and spread it over a larger surface. A thick base helps spreading, and a fin field helps convection, but the two compete for mass and cost.

The luminaire enclosure changes the boundary condition. A sealed housing has no internal airflow, so the heat sink surface inside the housing is less useful than the outer surface. That is why many sealed products use the body itself as the heat sink.

Natural convection and fin orientation

In natural convection, air moves because it is heated and becomes buoyant. The fin field has to let that air enter at the bottom, rise through the channels and leave at the top. If the fins run the wrong way, or the gap is closed at one end, the flow stalls.

Vertical fins with open ends work well on a wall mounted luminaire. Horizontal fins on the top of a housing can trap a layer of hot air. A chimney shape can help, but only if the inlet and the outlet are both open.

Fin spacing is a compromise. Tight spacing adds area but raises the flow resistance. Wide spacing lowers the area but lets the boundary layer refresh. For a small natural convection part, a gap of roughly 6 to 10 mm is a common starting range, but the right value depends on fin height and the surrounding surfaces.

The mounting surface matters. A heat sink pressed against a ceiling or a wall has less access to air than one hanging in free space. Test the luminaire in the orientation it will be installed, not on a bench.

Downlights and sealed enclosures

A downlight sits in a ceiling cut out, often with insulation around it. The heat sink may be hidden above the ceiling, where the air is still. That is a harder environment than an open bench, and the product has to be rated for it.

The housing is often a die casting because it has to hold the LED board, the reflector, the driver and the trim ring. The casting can include a fin field on the back, a cable entry and a mounting spring pocket. That integration is the reason to cast rather than assemble.

A sealed downlight cannot rely on vents. The heat has to pass through the housing to the ceiling void. A powder coated surface adds a thin resistance but protects the metal. The coating thickness on the fins should be controlled.

High bay, floodlight and street lighting

A high bay or a floodlight has more room for a fin field and often uses a die cast body with a separate driver box. The thermal load is higher, and the product may be aimed in a direction that changes the fin orientation.

For a floodlight, the fins may be horizontal when the product is aimed. That reduces natural convection through the channels. A design that works on a bench at the factory may run hotter on the wall. The test orientation should match the installation.

Street lighting adds wind, rain and pollution. The heat sink has to drain water, avoid traps and resist corrosion. A cast body with a gasketed lens and a defined cable entry is common. The thermal design still has to work on a still night with no wind.

Die casting or extrusion for a luminaire

An extrusion gives straight fins with a constant section and a low tooling cost. It is efficient when the product is a linear fixture and the fin field can run the full length. The trade-off is that an extrusion cannot easily carry a boss, a curved surface or a closed end.

A die casting gives a shaped body with bosses, a trim ring, a cable entry and a fin field in one part. It suits a compact luminaire with a strong visual identity. The trade-off is tooling cost, draft on the fins and a minimum fin thickness.

Some products use both: an extruded heat sink inside a cast or moulded housing. Others use a cast body with a separate extruded fin section. The choice should follow the shape and the volume, not a preference for one process.

The visible heat sink

In a luminaire, the heat sink is often the product. The fin field can be a design feature or a dust trap. Sharp fin tips are a handling risk and a coating weak point. A radius on the tip helps both.

Colour and texture affect the look and the emissivity. A black anodised or powder coated surface hides marks and raises emissivity. A bright finish shows fingerprints and may need a protective wrap during assembly.

The trim ring and the visible edges need a consistent finish. A casting can show a parting line or an ejector mark on a visible face. Put the cosmetic surfaces on the drawing and agree a sample before production.

Finish, sealing and outdoor life

An outdoor luminaire needs a finish that survives UV, rain and salt. A conversion coating plus a powder coat is common on a cast body. The coating must not bridge the fin gaps or cover the lens seal.

The gasket groove and the lens interface need a flat, machined surface. A cast groove with draft may not give a consistent compression. For an IP rated product, the sealing face should be machined and the gasket compression specified.

Condensation is a real risk in a sealed luminaire. A breather membrane or a desiccant can help, but the thermal design should avoid cold spots where water collects. The heat sink shape can create a cold bridge if it is not considered.

What to put in the specification

Give the LED load, the driver loss, the ambient temperature, the installation orientation and the expected life. State the maximum case temperature and the test condition. Then give the mechanical envelope, the mounting and the IP requirement.

For the heat sink, specify the alloy, the process, the fin geometry at the tip, the base thickness, the interface and the finish. Add the masking for the LED contact area and any earth bond.