FIELD OF THE INVENTION

The present invention generally relates to a method for fixation of a pulsating heat pipe element to a heat spreading body. The invention also relates to a heat spreading body comprising a pulsating heat pipe and a lighting arrangement comprising the heat spreading body.

BACKGROUND OF THE INVENTION

A global need and desire for a reduced use of energy and in particular electricity has rapidly advanced the development of more energy efficient light sources. To this end light emitting diodes, LEDs, are employed in a wide range of lighting applications. As LEDs have the advantage of providing a bright light, being reasonably inexpensive and energy efficient, it is becoming increasingly attractive to use LEDs as an alternative to traditional lighting. Furthermore, LEDs have a long operational lifetime. To use LED based light sources may significantly reduce the amount of energy required to illuminate e.g. a road, a hall or a stadium.

Although LEDs are energy efficient light sources, the heat that is produced can lead to lifetime and efficiency issues. Therefore the thermal management is an important concern in LED lighting design. Therefore, so called heat sinks or heat spreaders are commonly used. The heat sinks used are designed to extract, spread and transport heat, generated by the LEDs, away from the LEDs and dissipate it to the ambient air. In high power LED applications, like stadium lighting or road lighting, the amount of heat that is produced may be in the order of hundreds of watts and the heat sinks commonly used can become bulky and heavy. The heat sinks used may include several kilos of die-casted aluminum or copper. In addition to making the heat sinks bulky they become expensive too.

For several years heat pipes or heat tubes have been used in lighting applications to save weight and volume. Commonly a heat pipe is a closed aluminum or copper tube in which a suitable fluid evaporates and condenses, making it an effective transporter of heat. Recently a new type of heat pipe technology commonly called pulsating heat pipes has been started to be used in conjunction with LED lighting. In pulsating heat pipes, one long but narrow heat pipe is commonly bended in a coil like way. The ends of the pipe are then connected forming a closed loop. The loop is filled with a fluid suitable for heat transfer. When attached to e.g. a hot base plate the fluid in the coil pulsates in a chaotic way. This is due to the capillary characteristics of the narrow pipe forming the closed loop. This results in good heat transport and heat dissipation. In order to dissipate the heat picked up by the pipe, the pipe is commonly exposed to the ambient air.

In order to establish a sufficient thermal connection between the heat pipe in question and the heat sink the heat pipe is fixed to the heat sink or a component of the heat sink such as a base plate.

Fixation of heat pipes is commonly done by clamping or soldering. For larger tubes clamping is the most common way of fixating the pipes. The pipes are then pressed into slots of the heat sink. In doing this the soft material of the pipe has to overcome surface irregularities of the slot which may result in damages to the pipe. Further, a risk with this clamping technique is that the thermal connection between the pipe and heat sink will become unpredictable and uneven. Furthermore, commonly special tools are needed for clamping and some volume of the heat sink has to be dedicated for this fixation, resulting in increased costs. In addition to the above, milling often is needed as a post-process, to achieve a smooth surface. Hence, clamping is a bulky and unreliable way of connecting heat pipes to heat sinks.

Smaller heat pipes, are commonly soldered to a heat sink or a base plate thereof. Commonly, soldering of aluminum or copper is tiresome and time consuming, which does not support mass production. Further, in case there is a coating on the pipe concerned soldering may become unsuitable or even impossible. When using pulsating heat pipes, commonly several tenfold of pipes have to be soldered individually. Further, soldering results in a non-flat surface with edges of solder, which may appear messy and is unwanted especially in outdoor applications due to corrosion. A corrosion attack may present a risk for the reliability of the system.

Hence, there is a need for an improved method for fixation of a pulsating heat pipe element to a heat spreading or heat dissipating body like a heat sink, a heat spreader or a base plate of a heat sink.

SUMMARY OF THE INVENTION According to an aspect of the invention, the above is at least partly alleviated by a method for fixation of a heat pipe element to a heat spreading body, the method comprising: providing a heat pipe element, positioning the heat pipe element in a mold, and over molding at least a portion of the heat pipe element by means of a molding material, such that the heat pipe element is fixed to the heat spreading body.

The present invention is based on the realization that by positioning the heat pipe element in a mold, and over molding at least a portion of the heat pipe element by means of a molding material the heat pipe element may be fixated to a heat spreading body in an efficient and secure manner and at the same time a reliable thermal connection between the heat spreading body and the heat pipe element may established. This method is applicable to all kinds of heat pipe elements and in particular to pulsating heat pipe elements.

It should be noted that within the context of this application the term

"pulsating heat pipe element" may be any type of pipe or element comprising a cavity and being suitable to form part of or constituting a pulsating heat pipe.

Further, a "pulsating heat pipe" is to be construed as a heat pipe so constructed that when filled with a suitable fluid and exhibited to heat, the fluid of the heat pipe will pulsate in a chaotic way, as described above. The chaotic pulsating nature originates from the capillary characteristics of the narrow pipe forming the pulsating heat pipe.

It should be noted that within the context of this application the term "heat spreading body" may be any type of body capable of spreading or dissipating heat, like a heat sink, a base plate for a heat sink, a heat spreader, a cooling flange, a radiator, a chiller or similar. Further, the heat spreading body may be made of metal, metal alloys, ceramics or any other suitable material capable of spreading or dissipating heat.

It should be noted that within the context of this application the term "mold" may be any type of element presenting a recess or cavity which is suitable to be filled with a molding material during a molding or casting process. For instance, the mold may be an element which is filled by a molding material in order to form a component or similar. The mold may also be a cavity in an element which is to be filled with a molding material.

By "molding material" is meant any material that may be poured, injected or similar into a mold in which the molding material is left to solidify, being cooled, cured or similar. Hence, the molding material may be a metal, a metal alloy, a plastics material or a chemically curable material, such as an epoxy resin. The molding material may also be a mixture of various types of material. For instance, the thermal conductivity and mechanical properties of the molding material may be tailored to suit a specific application by mixing various materials to form the molding material.

By "over molding" is meant that the element to be over molded is present in some form of mold or cast like a die cast which is subsequently filled or partly filled with a molding material, such that at least a portion of the element is covered by the molding material. Consequently, any molding technique may be employed to over mold an element, such as die casting or injection molding.

In an embodiment of the invention, the pulsating heat pipe element may form a pulsating heat pipe which may be completely over molded during the over molding. This is advantageous in that the pulsating heat pipe may be incorporated in the heat spreading body and in that a thermal connection may be established simultaneously. As the pulsating heat pipe may be incorporated in the heat spreading body the heat transfer within the heat spreading body may be enhanced.

In an embodiment of the invention, the heat spreading body may be formed during the over molding of at least a portion of the pulsating heat pipe element, which is advantageous in that the heat spreading body and the fixation of the pulsating heat pipe element may be performed simultaneously in one single process step.

According to a preferred embodiment, the method may further comprise providing a heat spreading body comprising a mold. This is advantageous in that the heat spreading body may be formed separately whereupon the pulsating heat pipe element may be fixated to the heat spreading body at a later stage.

In an embodiment of the invention, the heat spreading body may be a heat sink, which is advantageous in that an efficient heat transfer may be realized in a simple and well functioning manner.

In an embodiment of the invention, the heat spreading body may be a base plate for a heat sink. Fixation of the heat pipe element to a base plate for a heat sink brings about several advantages. For instance, a size, a material and/or a shape of a cooling flange connected to the base plate may easily be adapted to suit a particular application.

According to a second aspect of the invention, there is provided a heat spreading body comprising a pulsating heat pipe wherein the pulsating heat pipe is fixed to the heat spreading body by means of over molding at least a portion of the pulsating heat pipe. By over molding at least a portion of the pulsating heat pipe, the pulsating heat pipe may be fixated to a heat spreading body in an efficient and secure manner and at the same time a reliable thermal connection between the heat spreading body and the heat pipe element may be established. In general, features of this aspect of the invention provide similar advantages as discussed above in relation to the previous aspect of the invention.

According to third aspect of the invention, the heat spreading body as discussed above may preferably be incorporated in a lighting arrangement, further comprising a light source. The heat spreading body may be arranged in communication with the light source such that heat is transferred from the light source to the heat spreading body. By arranging the light source in communication with the heat spreading body, heat as generated by the light source may be transferred away from the light source to prevent the light source from reaching undesired temperatures, i.e. temperatures which are of a magnitude that will damage or shorten the service life of the light source. The heat as transferred to the heat spreading body may then be transferred further, e.g. to ambient air or to an additional body connected to the heat spreading body.

By "being arranged in communication" is meant that the light source and the heat spreading body are arranged such that heat may be transferred between the light source and the heat spreading body. Any connection, direct or indirect between the light source and the heat spreading body may be used as long as heat may be transferred between the light source and the heat spreading body. In practice one or several materials, components or layers may be present between the light source and the heat spreading body. For instance, the light source may be mounted on a substrate which in turn is mounted on or connected to the heat spreading body.

In an embodiment of the invention, the light source may be a light emitting diode, LED, which is advantageous in that a bright light may be realized using a limited amount of energy.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person will realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

Fig. 1 conceptually illustrates an arrangement of pulsating heat pipes fixed to a base plate by being partly over molded;

Fig. 2 illustrates a heat sink with fins in which heat pipes are fixed by being partly over molded;

Fig. 3 illustrates a heat sink with fins in which a heat pipe is fixed by being completely over molded;

Fig. 4 illustrates a lighting arrangement comprising a light source and a heat sink; and

Fig. 5 is a flow chart of the method according to the invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

Referring now to the drawings and to Fig. 1 in particular, there is conceptually depicted an arrangement 100 of pulsating heat pipes 102 which have been fixed to a base plate 104 by being partly covered, i.e. over molded, by the material of the base plate. Hence, a portion 102a of the pulsating heat pipes 102 are covered by the material of the base plate 104 whereas another portion 102b of the pulsating heat pipes 102 are not covered by the material of the base plate. The pulsating heat pipes 102 are fixed to the base plate 104 as the base plate 104 is being formed in a die casing process. Hence, the base plate 104 comprises a molding material in the form of a metal, which have been melted and poured into a die cast mold as the base plate 104 is being produced.

Before molding the base plate 104 and fixating the pulsating heat pipes 102, the pulsating heat pipes 102 are positioned in the die cast mold in a predetermined way. In practice a favorable positioning of the pulsating heat pipes 102 are determined based on several conditions which may vary depending on the current application. However, in positioning the pulsating heat pipes 102 considerations concerning mechanical stability and heat transfer are generally accounted for. In addition to this considerations regarding physical dimensions and weight restrictions may also come into play. The skilled person realizes that an arrangement 100 of pulsating heat pipes 102 suitable for a specific application, such as a heat generating LED board may not be suitable for another application. For instance, different LED board may generate heat distributed over their surface differently, resulting in different cooling needs. When taking this into account, the positioning of the pulsating heat pipes 102 in the base plate 104 will become different for different applications.

In molding the base plate 104 and fixating the pulsating heat pipes 102, the molding material, in this case in the form of melted metal, is poured into a die cast, in which the pulsating heat pipes 102 are prepositioned. The melted metal is then left to cool or is being actively cooled, such that the melted metal solidifies, forming the base plate 104.

Hence, the pulsating heat pipes 102 are preferably made in such a way that they may withstand the temperature of the melted metal without being damaged or reshaped. Keeping this in mind, the skilled person realizes that there are several suitable material combinations which may be successfully used when fixating the pulsating heat pipes 102 to the base plate 104 being formed. As discussed above the molding material forming the base plate 104 may be of different type. For instance, the following material combinations may advantageously be used: pipe material copper and molding material aluminum, pipe material aluminum and molding material aluminum, pipe material stainless steel and molding material aluminum, pipe material aluminum and molding material polyurethane resins/composite resins/potting materials/silicone glues/thermal plastics (e.g. based on nylon or PPS).

When over molding the heat pipes 102 by a molding material, the portions 102a of the heat pipes 102 being over molded will be closely encased by the molding material, providing a reliable thermal connection. When metal pipes are used, with a molding material in form of a metal, a thermal connection having a good thermal conductivity may be realized.

The pulsating heat pipes 102 used in the disclosed arrangement 100 may be produced of a single pipe being bent into a coil like shape comprising a number of bends. For instance the pipe may be bent to form 250 bends forming the pulsating heat pipes 102. The pulsating heat pipes 102 may thus also be referred to as a pulsating heat pipe 102 as a single pipe forms all bends as illustrated in Fig. 1. On the other hand, several pipes may be used to form the pulsating heat pipes 102, e.g. by forming several closed loops of pulsating heat pipes 102. Depending on the application and designer choice, any number of pipes may be used to form the pulsating heat pipes 102. For instance, according to some embodiments, two pipes may be bent to form a specific number of bends constituting the pulsating heat pipes 102. According to other embodiments, 4, 8, 100 or 250 pipes may be used to form the pulsating heat pipes 102.

In order to function as pulsating heat pipes 102, the pipes used are filled with a suitable fluid which is used to transfer heat within the pulsating heat pipes 102. In

transferring heat, the fluid is evaporated when taking up heat and consecutively condensed when emitting heat. However, heat may also be transferred by the fluid without changing its state of aggregation. Several fluids may be used in realizing the pulsating heat pipes. For instance, the following fluids may successfully be used: ammonia, HCFC, CFC, alcohols, glycol and water. The skilled person realizes that other fluids may be used successfully. In fact any commercially available refrigerant may be used.

The base plate formed 104 presents a smooth surface after being formed, as no clamps, or soldering are needed in order to fixate the pulsating heat pipes 102. This means in practice that heat generating elements like LED-boards or similar may be mounted or connected directly to the base plate 104 without any after treatment of the surface.

Now referring to Fig. 2, a heat spreading body in form of a heat sink 204 in which pulsating heat pipes 202 are fixed by being partly over molded is depicted. The heat sink 204 is in the form of a die casted metal element. Further, the heat sink 204 comprises fins 206 in order to increase the contact area with the ambient air. The increased contact area results in an increased heat transfer between the heat sink 204 and the ambient air.

The portions of the pulsating heat pipes 202 not being covered by the material of the heat sink 204 is used to dissipate heat into the ambient air or to a remotely located body (not shown) connected to the pulsating heat pipes 202. Hence, the pulsating heat pipes 202 may be used to transfer heat from the heat sink 204 to a remote location where the heat is transferred further to e.g. a second heat sink before being transferred into e.g. the ambient air.

Now referring to Fig. 3, a heat spreading body in form of a heat sink 304 in which a pulsating heat pipe 302 is fixed by being completely over molded is depicted. The heat sink 304 is in the form of a die casted metal element. Further, the heat sink 304 comprises fins 306 in order to increase the contact area with the ambient air. The increased contact area results in an increased heat transfer between the heat sink 304 and e.g. the ambient air.

The pulsating heat pipe 302 is completely over molded by the material of the heat sink 304 meaning that the pulsating heat pipe 302 may be used to transfer heat within the heat sink 304. The pulsating heat pipe 302 may thus be used to counteract local temperature increases in case of a local heat source. In other words, the internal heat transfer of the heat sink 304 may be increased by completely over molding a pulsating heat pipe 302.

When producing the heat sink 304, the pulsating heat pipe 302 may first be positioned in the mold used; consecutively the mold is filled with a molding material in e.g. the form of melted metal. In the depicted case, the pulsating heat pipe 302 will have to be filled with a fluid and sealed before being completely over molded. However, the skilled person realizes that it for instance would be possible to over mold the pulsating heat pipe 302 but leave a passage to the pulsating heat pipe 302 free from molding material. The pulsating heat pipe 302 may hence be filled with a fluid and sealed after having been in principle completely over molded. Doing so will also enable refilling of the pulsating heat pipe 302.

Now referring to Fig. 4, when a heat spreading body 404 is used in conjunction with at a lighting arrangement 400, the heat spreading body 404 may be used to transfer heat generated from a light source 408 to e.g. the ambient air. As described above, the heat spreading body 404 may be embodied in many forms, e.g. as a base plate for a heat sink 104, as a heat sink 204, 304, 404 or a heat spreader without departing from the scope of the invention. In the embodiment of Fig. 4 a heat spreading body 404 in form of a heat sink 404 is shown. The heat sink 404 comprises a pulsating heat pipe 402 and is provided with fins 406. Further, the heat spreading body 404 is arranged in communication with the light sources 408. In practice, when the light source 408 being a LED light source, the LED is typically mounted on a substrate (not shown). The substrate may then be brought into contact with the heat spreading body 404 to establish an indirect communication between the light source 408 and the heat spreading body 404 as is shown in Fig 4.

In the following a method 500 for fixation of a pulsating heat pipe element or elements 102, 202, 302, 402 to a heat spreading body 104, 204, 304, 404 will described schematically with reference to Fig. 5, showing exemplifying steps of the method 500.

In a first step 502, a pulsating heat pipe element 102, 202, 302, 402 is provided. As discussed above the pulsating heat pipe element may be a portion of a pulsating heat pipe 102, 202, 302, 402 or may constitute the complete pulsating heat pipe 102, 202, 302, 402. In other words, a pipe element or pipe elements may be provided which later on will be assembled with additional pipe elements to form a pulsating heat pipe 102, 202, 302, 402. Correspondingly, a pipe element already formed to constitute a pulsating heat pipe 102, 202, 302, 402 may be provided without departing from the scope of the invention.

In a second step 504, the previously provided pulsating heat pipe element or elements 102, 202, 302, 402 is/are positioned in a mold. As discussed above, the pulsating heat pipe element or elements 102, 202, 302, 402 used are positioned in predetermined way, taking e.g. mechanical stability and thermal conductivity issues into account. Hence, the pulsating heat pipe element or elements 102, 202, 302, 402 may be positioned such that the positioning will suit the needs of the application in question.

In a third step 506, at least a portion of the pulsating heat pipe element or elements 102, 202, 302, 402 is/are over molded using a molding material. By over molding the pulsating heat pipe element or elements 102, 202, 302, 402 the pulsating heat pipe element or elements 102, 202, 302, 402 becomes fixed to the heat spreading body 104, 204, 304, 404. At the same time, a thermal connection between the pulsating heat pipe element or elements 102, 202, 302, 402 and the heat spreading body 104, 204, 304, 304 is established. Hence, heat may efficiently be transferred between the pulsating heat pipe element or elements 102, 202, 302, 402 and the heat spreading body 104, 204, 304, 404 once the pulsating heat pipe element or elements 102, 202, 302, 402 has been over molded and consequently fixed the heat spreading body 104, 204, 304, 404.

It is to be noted, as described above, that only a portion of the pulsating heat pipe element or elements 102, 202, 302, 402 may be over molded. Correspondingly, as also described above, the pulsating heat pipe element or elements 102, 202, 302, 402 may be completely over molded, i.e. completely covered by the molding material. Further, when using a plurality of pulsating heat pipe elements 102, 202, 302, 402 some pulsating heat pipe elements 102, 202, 302, 402 may be partly over molded and other pulsating heat pipe elements 102, 202, 302, 402 may be completely over molded without departing from the scope of the invention.

Above it has been presented how the base plate 104 and the heat sinks 204, 304, 404 may be formed by die casting and how the pulsating heat pipes 102, 202, 302, 402 may be fixed to the base plate 104 or heat sink 204, 304, 404 concerned simultaneously. In other words, it has been disclosed how a single process step is used to both form the base plate 104 or the heat sinks 204, 304, 404 and to fixate the pulsating heat pipes 102, 202, 302, 402.

In addition to the above technique using a single process step, the base plate 104 or the heat sinks 204, 304, 404 may be prefabricated using e.g. a die casting process or an injecting molding process. The base plate 104 or the heat sinks 204, 304, 404 may then be employed with a mold in form of a recess, a cavity or similar. The pulsating heat pipes 102, 202, 302, 402 may then be fixed to the base plate 104 or the heat sinks 204, 304, 404 by being positioned in the mold of the base plate 104 or the heat sinks 204, 304, 404 and in the following being at least partly over molded by a molding material. In other words, the base plate 104 or the heat sinks 204, 304, 404 are produced in a separate process step and consecutively the pulsating heat pipes 102, 202, 302, 404 are fixated to the base plate 104 or the heat sinks 204, 304, 404 by over molding the pulsating heat pipes 102, 202, 302, 402 using the mold of the base plate 104 or the heat sinks 204, 304, 404. The over molding of the pulsating heat pipes 102, 202, 302, 402 may be performed using a die casting process, an injecting molding process or similar.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.