New approaches to High-Power LED Clusters Cooling
uring the operation of any light-emitting diode the transformation of electrical energy into the light and heat energy takes place. If this heat is removed from the crystal with insufficient efficiency its temperature increases and reaches some critical values, which sharply reduce the warranty lifetime. The problem of removal of the excessive heat from LED crystal becomes critically important due to the occurrence of compact, high-power (tens and hundreds of watts) LED clusters at the market. In many aspects, the further progress in the increase of specific energy characteristics of high-power light-emitting diode lamps and projectors depends on the solution of this specific problem.
The arrangement based on the rule “crystal: glass in front, heat sink behind” is implemented in the absolute majority of light-emitting diode constructions. Such arrangement completely justified itself for the previous generation of low-power light-emitting diodes. Such approach to the cooling turned out to be clearly insufficient for modern high-power light-emitting diode clusters.
The situation was partially solved at the expense of the transition to the metal core printed circuit boards (MC PCB). The heat was “dispersed” on the whole surface of the board and the area, on which it was possible to locate additional heat sinks, increased. But even this innovation is practically exhausted due to the rapid growth of specific generating capacities.
The critical analysis of existing approaches to the projection of cooling systems of lamps and other heat-emitting electronic devices allowed finding the reserves for heat removal, which have not been used by the present time, by the authors [1, 2] – these are so-called front and transit schemes. They are designated for the intensification of heat removal from the front side of the printed circuit boards MC PCB, on which the light-emitting diodes are mounted.
This term means the heat removal with the help of specifically designed cooling radiators which are located at the front side of МС PCB . The most important characteristics of this cooling system include shape, dimensions and location of front heat sinks 1 (Fig. 1).
They are determined by the configuration and dimensions of “dead” optical zones – space areas, through which the radiation 2 generated by light-emitting diode does not transmit; as a rule, this radiation 2 has the shape of cone and is numerically characterized by radiation angles. The main requirement specified for heat sinks – they must fit inside these “dead” optical zones and not prevent the transmission of light-emitting diode radiation. Introduction of the additional front heat sinks allowed increasing the total heat-dissipating area of cooling system significantly and improving its efficiency. The heat flows Q2 are added to the heat flow Q1, which is removed by the traditional heat sinks, and this fact makes it possible to reduce the operating temperature of crystal materially.
Besides the main function – cooling, the front heat sinks can simultaneously serve as the basis for the assembly of lamp optical elements (Fig. 2). These elements are designated for the light control (focusing, scattering, deflection etc.).
Such “individual” optical solutions for every light-emitting diode offer the new opportunities for the quality control of lamp emission in general. Depending on the lamp designation, one part of light-emitting diodes which are incorporated into the cluster, can be equipped, for example, with the scattering optics, the second part – with the focusing optics and the third part – with the deflectcing optics. As a result of their joint operation, the light fields can be obtained; using the traditional methods such light fields can be obtained with significant costs or cannot be obtained practically.
It is well known that the efficiency of cooling of the electronic devices, which are located on the printed circuit board, considerably depends on its spatial orientation. Vertically-oriented board is cooled more intensively by 1.2–1.5 times than the board with the horizontal orientation which is the most widespread for lamps.
It is explained by the difference of hydrodynamic conditions upon the cold external airflow of the heated boards.
With the horizontal orientation of boards (Fig. 3) the air molecules heated near the bottom side of the board 1 need to travel the way S1н to the area of mixing with the external cold air 2, which is considerably longer than the way S1в for the air heated on the top side of board. It makes the process of heat exchange slower and reduces its efficiency.
The heat exchange can be accelerated at the expense of the arrangement of through (transit) cooling . The main point of this method consists in the removal of “bottom” heated air using the shorter way S2 (Fig. 4).
For this purpose it is suggested to locate the special through inserts – heat sinks 1 with vertically-oriented aperture for the unrestricted passage of hot air over the whole surface.
At the expense of it, the way S2 to the cold air becomes shorter and the heat exchange is intensified. The specific place for location of such transit inserts on the MC PCB depends on the specific topology of electronic device mounted on it. They should not cross the switching elements and disturb their operation.
The transit heat sinks (Fig. 5) consist of two interpenetrating parts – bottom nozzle 1, which takes the warm air, and top part 2, which is represented by the classical heat sink with developed surface and aperture in the middle.
Herewith, it should be noted that at the expense of the insert height h the pressure difference, which enhances the effect of air inflow (effect of “exhaust” pipe), occurs.
It is interesting that upon the gradual reduction of insert height to the value, which is equal to the thickness of cooled board MC PCB, the insert is morphologically transformed into the aperture through the board.
To the extent possible, the transit inserts must be evenly located on the board.
Selection of the Material
for Heat Sink Production
The material designated for the production of front and transit heat sinks must meet, at least, two requirements:
It should have the heat conductivity l which is required and efficient for the supply of such heat amount, which can practically “collect” the ambient air under the conditions of natural cooling. Calculations and multiple experiments refer the materials with the value of l, which is higher than 7–10 wt/mK, to them.
This material should be treated using the technologies which ensure high accuracy comparable with the accuracy of production of cluster frames and have the reasonable cost under the conditions of mass production.
The heat-conducting (heat-scattering) plastics, which have recently occurred at the market, meet all these requirements .
Having the heat-scattering capability at the natural cooling, which is similar for aluminum (at the level of 90–95%), the products made of such plastics, which are analogous to the metal products by shape and dimensions, are lighter by almost 2 times and their production cost is lower than the production cost of aluminum products by 3–5 times. Coefficient of thermal linear expansion of these plastics has low (5–10 ppm/°C) and close to metals (10–20 ppm/°C) values. Therefore, with their conjugation in the structures the minimum thermal stresses and effects of part warping will occur.
In Fig. 6 the results of experimental validation of the efficiency of suggested front cooling are specified by the example of the operation of light-emitting diode of XML type with the power of 10 watts produced by CREE firm, fixed on the aluminum board 50 × 50 mm with the thickness of 2 mm. The temperature of the hottest point of assembly – under crystal was measured using the thermocouple. The module pin heat sinks M50 made of heat-scattering polymer composite “TEPLOSTOK T6-E5–7” with the heat conductivity of 6 wt/m K were used.
In figures the temperatures of several assembly variants are specified: without heat sinks (а), with “classic” heat sink (b), with front heat sink (с), with “classic” and front heat sinks (d).
Given results prove the efficiency of the application of front cooling of light-emitting diodes in the role of additional and independent cooling system.
The various cooling schemes were studied for the experimental validation of the efficiency of transit cooling by the example of light-emitting diode cluster with the capacity of 25 watts (No Brand) and dimensions 20 × 20 mm located on the aluminum substrate with the dimensions 40 × 40 × 1 mm (Fig. 7). The cluster was fixed by heat-conducting paste on aluminum plate with the dimensions 165 × 165 × 1.8 mm. The pin module heat sinks M50 with the dimensions 50 × 50 × 25 mm made of heat-scattering polymer composite “TEPLOSTOK” with the heat conductivity of not less than 7 wt/mK were used in the role of cooling radiators.
The transit cooling was provided by the modification of construction of bottom and top heat sinks at the expense of introduction of the system consisting of 76 apertures with the diameter of 5.2 mm drilled through the feet of bottom and top heat sinks and basic aluminum plate. As a result, the efficient height h of inserts-heat sinks was 12 mm. The total area of ventilating apertures in this cooling system was 16 cm 2 (6% of the cooled area).
Efficiency of the studied cooling systems was evaluated by the difference of ambient air temperatures Tair and the hottest point of construction Тmax, by which the temperature of the frame of LED cluster is usually understood. This value is directly connected with the thermal resistance of cooling system – the lower it is, the more efficient the general cooling system operation is.
The temperature Tmax was measured by the thermocouple inside the aperture with the diameter of 1 mm drilled in the center of aluminum plate directly under the cluster plant base plate. The temperature values were registered at the steady-state thermal conditions of assembly, as a rule, in 1.5–2 hours after the experiment start.
Horizontally-oriented construction of the traditional heat design was used as the basic cooling scheme: heat sinks on the top, light-emitting diode crystals on the bottom, and they are protected by the transparent plafond (item 2 in Table 1). In this case the temperature, which is maximum for all compared constructions, was registered – 80 °C (difference 55 °C).
Obtained results confirmed the efficiency of the introduction of transit cooling to any cooling schemes of electronic devices, which are known at the present time. Notwithstanding the used cooling scheme, the additional introduction of transit cooling elements, which in essence requires the minimum additional expenditures, was accompanied by the decrease of the cluster operating temperature Тmax.
As it is shown in Table 2, the greatest effect of crystal temperature decrease is achieved in case of combined application of two additional cooling systems – front heat sinks and transit cooling. In this case, the practical 25% improvement of cooling efficiency of studied LED cluster is observed.
The pictures of the actual constructions of light-emitting diode lamps of different manufacturers with front cooling system, which is produced from heat-scattering plastics, are given in Fig. 8.
Results of the studies of front cooling of high-power light-emitting diode clusters were reported for the first time in Japan at the International Conference “LED Japan Conference & Expo – Strategies in Light Japan 2014”  and had positive feedback.