A balun transformer with a cooling mechanism The present invention relates to transformers capable of matching a single-ended output to a balanced input i.e. balun transformers, and particularly to cooling techniques of balun transformers for radio- frequency (RF) power applications. High power radio frequency (RF) sources are used in many applications for broadcast, communications, radars, healthcare applications, and so on and so forth. One of the important elements of RF sources, and more particularly high power RF sources is a final power amplifier. Solid state transistor based power amplifiers have smaller dimensions, better reliability, and higher efficiency compared to conventional RF power sources based on vacuum tubes like klystrons, tetrodes, and inductive output tubes. Presently the upper limit power capability of single RF power transistors is in a range of 1-1.5 kw (kilowatts) . In cases where output power requirements, i.e. the power demand, exceed power that a single transistor can deliver, a plurality of transistors may be combined to collectively meet the power demand. A convenient and useful method of combining or coupling two transistors is a technique known as the 'push-pull' schematic. In the 'push-pull' scheme, the drive is shared between a first transistor driving current through the load in one direction and a second transistor driving current through the load in the opposite direction. However, the 'push-pull' circuit is a balanced system that is it produces an output signal which is symmetrical with respect to the common ground potential of the coupled transistors, whereas typically a single-ended (that is ground referenced) output signal is re- quired. A solution to this problem is to provide a transformer between the output of the 'push-pull' pair and the load. This transformer is able to couple the balanced output to the single-ended load. Such a transformer is that referred to in the art as a 'balun' (balanced to unbalanced transformation) .

Many forms of balun, including transmission line forms, are known in the art. Presently one of the most promising design of the balun transformer, from viewpoint of manufacturing and assembling, is PCB (Printed Circuit Board) based planar transformer. The PCB balun transformers, for example as depicted in US Patent number 5061910 titled 'Balun Transform- ers' are being widely used in the art. However, these balun transformers and particularly the PCB balun transformers are disadvantaged by an intriguing problem of overheating, primarily overheating of primary and secondary conductive elements i.e. conductive tracts of the PCB acting as primary and secondary windings in the PCB balun transformers and overheating of other embedded electronics present in the conductive tracts, for example capacitors if any connected in the conductive tracts, of the PCB balun transformers. Presently, in some of the PCB balun transformers problem of overheating is addressed with increasing heat dissipation by using substrate (PCB substrate) material with high thermal conductivity. Another solution found in the art is the use of pre-matching circuit to reduce transformation ratio for the transformer and therefore reduce currents flow in the transformer. Yet another solution utilized in the art is to use air-cooled fins on the top of the transformer. However, none of these techniques effectively address the problem of overheating, and more particularly for RF power applications that operate at RF power levels higher than 1.2 kW.

Thus the object of the present technique is to provide a balun transformer with an efficient cooling system. Furthermore, it is desirable that the balun transformer with the cooling system of the present technique is compact, easy to integrate into with the balun transformer, and simple. The above objects are achieved by a balun transformer accord ^¬ ing to claim 1 of the present technique. Advantageous embodiments of the present technique are provided in dependent claims. Features of claim 1 may be combined with features of dependent claims, and features of dependent claims can be combined together.

According to an aspect of the present technique, a balun transformer for high power RF applications is presented. The balun transformer includes a substrate plate, a first and a second conductive element, a first and a second signal port, and a cooling module. The substrate plate has a first face and a second face. The first face and the second face are opposite to each other. The first conductive element is ar- ranged on the first face of the substrate plate. The second conductive element is arranged on the second face of the substrate plate. The second conductive element is transformingly coupled to the first conductive element and electrically isolated therefrom. The first signal port is electrically con- nected to the first conductive element. The second signal port is electrically connected to the second conductive element .

The cooling module includes a first heat sink, an intercon- necting means and a base plate. The first heat sink is arranged in thermal contact with the first conductive element to receive heat from the first conductive element. The interconnecting means is arranged to establish thermal contact between the first heat sink and the base plate. The base plate is configured to receive the heat from the first heat sink via the interconnecting means.

When the balun transformer of the present technique is in use, heat from the first conductive element transmits to the first heat sink and then to the interconnecting means and fi ^¬ nally to the base plate. This presents an efficient cooling technique of providing cooling in the balun transformer.

Moreover, the cooling technique and the construct is simple, easy to integrate and capable of fabrication in compact design.

In an embodiment of the balun transformer, at least a part of the first conductive element is printed on the first face of the substrate plate and/or at least a part of the second conductive element is printed on the second face of the substrate plate. This provides an advantageous embodiment of printed circuit board (PCB) based balun transformer of the present technique. This also helps in compact design of the balun transformer of the present technique.

In another embodiment of the balun transformer, a shape of the first heat sink corresponds to a shape of the first con- ductive element. Thus the first heat sink has greater thermal contact with the first conductive element and helps in providing greated heat transfer from the first conductive element to the base plate. In another embodiment of the balun transformer, the first heat sink includes at least one heat pipe arranged within the first heat sink. This provides a better way of conducting heat through the first heat sink. In another embodiment of the balun transformer, the substrate plate is sandwiched between base plate and the first heat sink. This provides a compact design of the balun transformer. Additionally the base plate acts as a support for the substrate plate with the first and the second conductive ele- ments and the heat sinks and interconnecting means of the cooling module. Moreover, relative orientation of the cooling module and the substrate plate can be rigidly fixed with the help of the base plate. In another embodiment of the balun transformer, the base plate includes active cooling means. The base plate may be, but not limited to, cooled by air flow, fluid flow, and so on and so forth. Thus the heat flux from the first conductive element to the base plate, and thus cooling of the first conductive element is facilitated.

In another embodiment of the balun transformer, the base plate is grounded. Thus the base plate can be advantageously used for grounding a desired part of the balun transformer for example for grounding the first conductive element.

In another embodiment of the balun transformer, the first heat sink is electrically connected with the base plate. Thus the requirement of grounding the first conductive element for transformer functions of the balun transformer are achieved without requiring to ground the first conductive element by any additional means.

In another embodiment of the balun transformer, the interconnecting means is an extension of the first heat sink. The first heat sink is electrically and thermally connected with the base plate via the extension of the first heat sink. By not obviating the need of using any additional means to electrically and thermally connect the first heat sink to the bottom plate, thermal resistance in the cooling module is at least partially removed. Additionally, this provides a simple and compact design of the balun transformer.

In another embodiment of the balun transformer, the first heat sink is in direct physical contact with the base plate. By not obviating the need of using any additional means to connect the first heat sink to the bottom plate, thermal re- sistance in the cooling module is at least partially removed. Additionally, this provides a simple and compact design of the balun transformer.

In another embodiment of the balun transformer, the substrate plate includes at least one slot. The slot is in form of a hole in the substrate plate. The extension of the first heat sink is in direct physical contact with the base plate going through the slot of the substrate plate. Thus the first heat sink is mechanically secured with the base plate and the substrate plate which provides a sturdy construct of the balun transformer and also helps in maintaining relative orientation and relative position of the base plate, the substrate plate and the cooling module.

In another embodiment of the balun transformer, the first heat sink is electrically insulted from the base plate. This electrically decouples the first conductive element of the balun transformer from the ground.

In another embodiment of the balun transformer, the interconnecting means includes a dielectric member and a second heat sink. The dielectric member is inserted between the second heat sink and the first heat sink such that the second heat sink is electrically insulted from the first heat sink. The first heat sink is in thermal contact with the base plate via the dielectric member and the second heat sink, respectively. This helps in achieving electrical insulation of the first heat sink from the base plate, and thus effectively the first conductive element of the balun transformer is electrically decoupled from the ground.

In another embodiment of the balun transformer, the second heat sink is in direct physical contact with the base plate. Thus heat is efficiently transferred from the second heat sink to the base plate. Moreover, direct physical contact of the second heat sink with the base plate may be used to support and/or mechanically secure the second heat sink with the base plate.

In another embodiment of the balun transformer, the substrate plate includes at least one slot. The slot is in form of a hole in the substrate plate. The second heat sink is in di- rect physical contact with the base plate going through the slot of the substrate plate. Thus the second heat sink is mechanically secured with the base plate and the substrate plate which provides a sturdy construct of the balun trans- former. Additionally, this helps in maintaining relative orientation and relative position of the base plate, the substrate plate and the cooling module. The present technique is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawing, in which:

FIG 1 schematically illustrates an exemplary embodiment of a balun transformer in accordance with the present technique,

FIG 2 schematically illustrates parts of an exemplary embodiment of the balun transformer viewed from a side without depicting cooling of the balun transformer ,

FIG 3 schematically illustrates the exemplary embodiment of the balun transformer of FIG 2 viewed from a top side,

FIG 4 schematically illustrates the exemplary embodiment of the balun transformer of FIGs 2 and 3 viewed from a bottom side opposite to the top side of FIG 3,

FIG 5 schematically illustrates another exemplary embodiment of the balun transformer, and FIG 6 schematically illustrates an exploded view of the exemplary embodiment of the balun transformer of FIG 5, in accordance with aspects of the present technique . Hereinafter, above-mentioned and other features of the present technique are described in details. Various embodiments are described with reference to the drawing, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be noted that the illustrated embodiments are intended to explain, and not to limit the invention. It may be evident that such embodiments may be practiced without these specific details.

FIG 1 schematically illustrates an exemplary embodiment of a balun transformer 10, according to aspect of the present technique. The balun transformer 10 includes a substrate plate 5 with a first face 51 and an opposite second face 52, a first conductive element 3 arranged on the first face 51 and a second conductive element 4 arranged on the the second face 52, a first signal port 1 electrically connected to the first conductive element 3 and a second signal port 2 electrically connected to the second conductive element 4, and a cooling module 20. The second conductive element 4 is trans- formingly coupled to the first conductive element 3. The first conductive element 3 and the second conductive element 4 are electrically isolated from each other.

The cooling module 20 includes a first heat sink 21, an interconnecting means 23 and a base plate 29. The first heat sink 21 is arranged in thermal contact with the first conductive element 3 for receiving heat from the first conductive element 3. The interconnecting means 23 is arranged to establish thermal contact between the first heat sink 21 and the base plate 29. At least one function of the base plate 29 is to receive the heat from the first heat sink 21 via the interconnecting means 23.

FIG 1 is explained further in combination with FIG 2, 3 and 4. FIG 2, 3 and 4 schematically illustrate some parts of an exemplary embodiment of the balun transformer 10, without depicting the cooling module 20. FIG 2 schematically illustrates some parts of the balun transformer 10 from a side view; FIG 3 schematically illustrates the balun transformer 10 of FIG 2 viewed from a top view; and FIG 4 schematically illustrates the balun transformer 10 of FIGs 2 and 3 viewed from a bottom view, a view opposite to the top view of FIG 3. As depicted in FIG 2, the balun transformer 10 includes the substrate plate 5. The substrate plate 5 has a first face 51 and a second face 52. The first face 51 and the second face 52 are opposite to each other i.e. first face 51 and the second face 52 are reverse sides of each other. In one embodi- ment, the substrate plate 5 has a planar structure, as depicted in FIG 2, and first face 51 and the second face 52 are different faces of the two major faces of the plane i.e. the two planes formed by the length and breadth of the planar structure without involving the faces that form the width of the planar structure. The substrate plate 5 is electrically non-conductive and may be formed of semiconductor or electrically insulating material for example, silicon, silicon dioxide, aluminum oxide. The first conductive element 3 is arranged on the first face 51 of the substrate plate 5. The second conductive element 4 is arranged on the second face 52 of the substrate plate 5.

The first conductive element 3 and the second conductive element 4 are transformingly coupled to each other. The first conductive element 3 and second conductive element 4 are electrically isolated from each other. The term 'conductive' as used herein means conductive to RF (radio frequency) power or RF signals. It may be noted that in the present disclosure the term ' transformatively coupled' or like phrases mean ar- ranged in such a way that energy between two or more circuits or conductors or conductive elements 3 and 4 are transferred through electromagnetic induction. Thus, when RF power or signal is received by the first conductive element 3, it is conducted or propagated through the first conductive element 3 arranged on first face 51 of the substrate plate 5, and by this propagation or flow of the RF power through the first conductive element 3 a current and corresponding power flow is electromagnetically induced in the other conductive ele- ment i.e. second conductive element 4 placed on the other side i.e. second face 52 of the substrate plate 5.

Alternatively, when RF power or signal is received by the second conductive element 4, and is conducted or propagated through the second conductive element 4 arranged on second face 52 of the substrate plate 5, then by this propagation or flow of the RF power through the second conductive element 4 a current and corresponding power flow is electromagnetically induced in the other conductive element i.e. first conductive element 3 placed on the other side i.e. first face 51 of the substrate plate 5. The first conductive element 3 and/or the second conductive element 4 are arranged on their corresponding faces i.e. first and the second face 51, 52 by either attaching conductive material on the substrate plate 5, for ex- ample by soldering, or by printing a conductive material on the surface of the substrate plate 5. The technique of printing conductive material on substrate plates or wafers is well known in art of printed circuit boards and thus has not been explained herein in details for sake of brevity.

FIG 3 and FIG 4 depict a view of first face 51 and a view of second face 52, respectively. The balun transformer 10 includes a first signal port 1 connected to the first conductive element 3 and thus RF power received by the first signal port 1 propagates or flows to the first conductive element 3, or vice versa any electromagnetically induced current in the first conductive element 3 propagates or flows to the first signal port 1 and is able to leave the balun transformer 10 from the first signal port 1. Furthermore, the balun trans- former 10 includes a second signal port 2 connected to the second conductive element 4 and thus any electromagnetically induced current in the second conductive element 4 propagates or flows to the second signal port 2 and is able to leave the balun transformer 10 from the second signal port 2, and vice versa any RF power received by the second signal port 2 propagates or flows to the second conductive element 4. In one embodiment of the balun transformer 10, the first signal port 1 is a balanced signal port and the second signal port 2 is a single-ended signal port, as depicted in FIGs 3 and 4. Thus, when the balun transformer 10 is used to match balanced input to single-ended output, the first conductive element 3 functions as primary winding of the balun transformer 10, and the second conductive element 4 functions as secondary winding of the balun transformer 10. Alternatively, in another embodiment (not shown) of the balun transformer 10, the first signal port 1 is a single-ended signal port and the second signal port 2 is a balanced signal port. Thus, the first conductive element 3 is capable of functioning as secondary winding of the balun transformer 10, and the second conductive element 4 capable of functioning as primary winding of the balun transformer 10.

The first conductive element 3 includes a ground point 6 which is connected to ground directly or via one or more ca- pacitors. Optionally, the second conductive element 4 includes a ground point 7 which is connected to the ground or may be have no connections and left open. The area or region 8 on the first face 51 i.e. associated with the first conductive element 3 or the primary winding 3 can be used for ca- pacitor placement to optimize transformer behavior of the balun transformer 10.

For the purposes of explanation only, and not as a limitation to the present technique, hereinafter in the present disclo- sure the embodiment of the balun transformer 10 for using to match balanced input to single-ended output has been discussed, i.e. the embodiment in which the first conductive element 3 functions as primary winding of the balun transformer 10, and the second conductive element 4 functions as second- ary winding of the balun transformer 10.

Referring again to FIG 1, the balun transformer 10 includes the cooling module 20. The cooling module 20 includes a first heat sink 21. The first heat sink 21 is solid structure made of materials that are suitable for using as heat sinks. Such materials that are suitable for using as heat sinks are well known in the art of cooling electronic devices. Some examples of such material include, but not limited to, copper, aluminum, and so on and so forth. The first heat sink 21 has a substantial mass compared to the first conductive element 3. The first heat sink 21 has a substantial thickness i.e. dimensional parameter in the direction perpendicular to the first face 51. The first heat sink 21 is not a printed layer. The first heat sink 2 is not a painted layer. The first heat sink 21 is arranged in thermal contact with the first conductive element 3. The first heat sink 21 may have a shape and size suitable for establishing an optimal thermal contact with the first conductive element 3. For example, as depicted in FIG 1, the first heat sink 21 has a ^¾ C shaped structure formed from a solid rectangular parallelepiped bent to form the ^λ C shape. The ⁴ C shaped first heat sink 21 may be aligned or arranged on the first conductive element 3 in such a way that the opening in the shape ⁽ C' is arranged over a region 8. The region 8 may be used for further attaching electrical components, such as capacitors, to the first conductive element 3.

In one embodiment of the balun transformer 10, a heat pipe (not shown) is present within the first heat sink 21 to facilitate better flow of heat through the first heat sink 21. The heat pipe may have a shape similar to the shape of the first heat sink 21 or alternatively, the heat pipe positioned inside the first heat sink 21 may have a shape different from the shape of the first heat sink 21.

As mentioned earlier, the first heat sink 21 is in thermal contact with the first conductive element 3 i.e. in this exemplary embodiment of FIG 1 in thermal contact with the primary winding. It may be noted that in the present disclosure the term ^v in thermal contact' and like phrases mean a direct physical or indirect i.e. through other intermediate direct physical contacts between the first heat sink 21 and the first conductive element 3 which allow transfer of thermal energy, primarily through conduction of heat, from the first conductive element 3 to the first heat sink 21. In one embodiment of the balun transformer 10, the first heat sink 21 is arranged in thermal contact with the first conductive element 3 by direct physical contact of the first heat sink 21 and the first conductive element 3 i.e. the thermal contact be- tween the first conductive element 3 and first heat sink 21 is realized by direct physical contact of a surface (not shown) of the first conductive element 3 with a wall (not shown) or surface (now shown) of the first heat sink 21. The first heat sink 21 may be attached to the first conductive element 3 by soldering, pasting, or clamping or any other suitable means of attachment.

In the balun transformer 10, the heat from the first conductive element finally flows to the base plate 29 flowing seri- ally through the first heat sink 21 and the interconnecting mean 23. In one embodiment of the balun transformer 10, the substrate plate 5 is sandwiched between base plate 29 and the first heat sink 21, as depicted in FIGs, especially FIG 2. The base plate 29 may include active cooling means 28 (shown in FIG 6) . The active cooling means 28 may be, but not limited to, coolant air flow passages, fluid coolant flow passage, and so on and so forth.

In one embodiment of the balun transformer 10, the base plate 29 is made of electrically conducting material, for example copper block. The base plate 29 is electrically grounded. Optionally, the first heat sink 21 is electrically connected with the base plate 29. This effectively means that the first conductive element 3 is grounded through interconnections via the first heat sink 21 and the base plate 29. This is

achieved in one exemplary embodiment by using an extension 22 of the first heat sink 21. The extension 22 forms an integral single structural unit with the first heat sink 21, as shown in FIG 1. The interconnecting means 23 of the cooling module is realized by the extension 22. Thus, the first heat sink 21 is electrically and thermally connected with the base plate 29 via the extension 22 of the first heat sink 21, as depict- ed in FIG 1 which shows a direct physical contact of the extension 22 with the base plate 29. The extension 22 in this embodiment is formed of an electrically and thermally conducting material such as copper. To realize direct physical contact of the extension 22 of the first heat sink 21 with the base plate 29, one or more slots 54 in the substrate plate 5 is used (though not depicted in FIG 1, slot 54 in this embodiment is same as slot 54 depicted in FIG 6) . The slot 54 is a hole or an opening or a tunnel in the substrate plate 54 such that extension 22 goes through the slot 54 from the first face 51 to the second face 52 of the substrate plate 5, and is further extended to achieve direct physical contact with the base plate 29. Now referring to FIG 5 in combination with FIG 6, another exemplary embodiment of the balun transformer 10 has been explained hereinafter. In this embodiment of the balun transformer 10, the first heat sink 21 is electrically insulted from the base plate 29.

One way of realizing the electrical decoupling or electrical insulation of the first heat sink 21 from the base plate 29, is explained hereinafter. In this embodiment, as depicted in FIG 5 and FIG 6, of the balun transformer 10, the intercon- necting means 23 includes a dielectric member 24 and a second heat sink 25. The dielectric member 24 is inserted between the second heat sink 25 and the first heat sink 21 such that the second heat sink 25 is electrically insulted from the first heat sink 21. The dielectric member 24 may be, but not limited to, a dielectric plate made of ceramic or other dielectric material. The second heat sink 25 may have a shape which allows the physical contact of the second heat sink 25 with the dielectric member 24 as well as with the base plate 29, for example, as depicted in FIGs 5 and 6, the second heat sink 25 may be shaped like the mathematical symbol 'π'. The first heat sink 21 is in thermal contact by direct physical contact with the dielectric member 24, the dielectric member 24 is in thermal contact by direct physical contact with the second heat sink 25, and the second heat sink 25 is in thermal contact by direct physical contact with the base plate 29. As depicted in FIG 6, in a related embodiment of the balun transformer 10, the substrate plate 5 includes at least one slot 54. The slot 54 is a hole or an opening or a tunnel in the substrate plate 54 such that second heat sink 25 goes through the slot 54 from the first face 51 to the second face 52 of the substrate plate 5, and is further extended to achieve direct physical contact with the base plate 29. The second heat sink 25 may be secured to the base plate 29 by fixing means (not shown) such as by using screws. While the present technique has been described in detail with reference to certain embodiments, it should be appreciated that the present technique is not limited to those precise embodiments. Rather, in view of the present disclosure which describes exemplary modes for practicing the invention, many modifications and variations would present themselves, to those skilled in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.