LEDs have a limited life, typically up to about 50,000 hours. An LED's life span is considered to be the point at which the output intensity drops below 70%, as shown in the following graph. Before this point is reached there will however be a gradual reduction in efficiency. In addition to producing light, LEDs also generate heat. This heat energy needs to be transmitted away from the active LED chip as the structure can be adversely affected by a sustained high temperature. Overheating the LEDs can cause total failure or accelerated ageing.
Good and efficient thermal management assists by reducing the heat build-up in and around the LEDs, thus significantly prolonging their life. However this requires more thermal transmitting material and development effort. Low-cost light designs often pay little or no attention to thermal management. The cost savings are achieved by reducing the raw materials used and also by simplifying the manufacturing process. Unfortunately this has a detrimental effect on the quality of the illumination product and has significant reliability implications especially if the lights are operated close to their maximum current rating.
The majority of LEDs used in vision applications are resin encapsulated single junction devices. The heat generated within the junction is conducted away via the resin and the metallic contacts. Therefore, the air surrounding the LEDs and the PCB to which they are attached heat up. By reducing the use of raw materials, the housing often becomes a purely structural shell, offering little or no physical contact with the LEDs which leads to very poor heat dissipation. By attempting to reduce the cost of the manufacturing process, important features, such as machined fins on the housing which increase the surface area, the addition of thermal bonding between the PCB and the housing, or the use of graded batches of diodes, are neglected.
Quality illumination products designed for machine vision applications are designed with thermal management in mind from the outset: By manufacturing the LED housings from solid aluminium, heat can be dissipated away from the LEDs. This can be further enhanced by the addition of a thermally conductive layer that allows direct conduction between the PCB and the housing. Fins can be machined onto the housing to increase the surface area, which allows greater heat transfer into the surrounding air. By using graded high quality diodes, the total efficiency of each LED making up the illuminator can be matched, ensuring a more consistent illumination and the reduction of hot LED components which stress the thermal management and cause early failure due to localised excessive heat generation.
Advanced thermal management techniques
There is a limit to the maximum amount of heat that can be transferred away from the LEDs using only passive techniques. For applications where the heat from an LED becomes a serious problem, other techniques are required. These active techniques are particularly applicable to high intensity line light illumination systems, and the following examples show the various methods that can be employed.
The diagram shows a simple form of line light with no cooling. By changing the simple folded metal housing for an extruded aluminium design, the generated heat can more easily be transferred due to aluminium's good thermal transmission properties and its increased surface area.
Further improvements can be made to the design by mounting the unit on a substantial metal platform. In this case the LED array needs to heat up a large volume of metal before any damage is done to the LEDs or any reduction in the performance occurs. We can further improve the situation by adding fins to the metal extrusion which increases the surface area and hence the heat transfers into the surrounding air.
In situations where either the ambient temperature or the total amount of heat generated by the LEDs is very high, we can increase the total amount of heat transferred away from the unit by forcing the air over it. The addition of a fan also has the added advantage of removing any concentrations of hot still air (hot spots), which would otherwise cause increases in temperature.
There is a limit to the amount of heat that can be transferred away from the unit in a given time using air alone. Liquid coolants offer a more efficient method of heat transfer, as they provide better heat conduction than air. As is shown in the drawing, the coolant can be directed very close to the LEDs to maximize the energy transfer. This can be achieved using a self-contained pump which can also be situated some distance away from the light itself.
If necessary a refrigeration unit can be included into the coolant system. Using active cooling, care has to be taken that no condensing water is generated on the housing that would cause irreversible damage to the illuminator.
There are some manufacturing and inspection environments where forced air or liquid cooling are forbidden due to safety regulations. In these environments the only active cooling technology that can be employed is thermo electric cooling (TEC) using Peltier elements. When a current is passed through a TEC device, a temperature differential is created. This effect transfers heat from one side of the TEC to the other. One of the most important features of this technique is that the exact amount of cooling can be tightly controlled by varying the current. TEC cooling is especially suitable to reach a predefined local temperature. Additional active cooling is necessary to discharge heat from the complete system.