COOLING ELEMENT AND METHOD FOR MANUFACTURING A COOLING ELEMENT FOR COOLING OF AT LEAST ONE ELECTRIC POWER COMPONENT

Abstract: Cooling element and method for manufacturing a cooling element for cooling of at least one electric power component, which cooling element comprises a cooling body made of copper with at least one cooling channel for a liquid coolant. The cooling channel comprises a cooling pipe, made of a low oxidising metal, which is embedded in the cooling body. During manufacturing the cooling pipe is brought into a given shape, placed in its final position in a manufacturing mould for the cooling body, the material for the cooling body is brought into the mould around the cooling pipe and the cooling pipe is embedded in the cooling body during manufacture of the cooling body. The cooling element may be arranged between semiconductor components being part of a semiconductor valve in a converter station or in a station for Static Var Compensation (SVC).

Description: COOLING ELEMENT AND METHOD FOR MANUFACTURING A COOLING ELEMENT FOR COOLING OF AT LEAST ONE ELECTRIC POWER COMPONENT TECHNICAL FIELD Present invention refers to a cooling element, a semiconductor valve comprising a cooling element and a method for manufacturing a cooling element for cooling of an electrical power component, for example a semiconductor such as a thyristor or a IGBT (Insulated Gate Bipolar Transistor) or a resistor for high power in a converter station or a station for Static Var Compensation (SVC). BACKGROUND ART Power components produce dissipation heat during operation, mostly by switching losses. The temperature of the components influence their performance and their lifetime, and may in extreme cases destroy components. For semiconductors, for example, the temperature is critical inside the component at a semiconductor crystal. If the crystal temperature exceeds a critical value, the performance of the component is deteriorated, or it is destroyed. Part of the dissipation heat is led away from the components by active cooling. Thereby the component may achieve a higher operating power and withstands higher power losses without being destroyed. The active cooling may prolong the lifetime of the components considerable at unchanged power losses. In connection with power semiconductors in, for example, a converter station cooling systems with cooling bodies of aluminium are normally installed, which cooling bodies are in direct thermal and electrical contact with the power semiconductors. The cooling bodies are provided with through-going cooling channels, through which a coolant is passed. The cooling channels may be meander-shaped or helical, in order to achieve as large a transfer surface as possible between the coolant and the cooling body itself. De-ionised water normally flows as coolant through the cooling bodies, which water takes up the dissipation heat and carries it away from the component. Known manufacturing methods for such cooling bodies are, for example, machining, casting or extruding. The amount of dissipation heat which may be carried away from the components is, at least partly, dependent on the amount of coolant flowing through. The amount of coolant may, to a certain extent, be increased by increasing the flow rate of the coolant through the cooling body. High flow rates may lead to problems with corrosion and wear of the cooling bodies. These problems are met by embedding stainless steel tubes, through which tubes the fluid flows, into the aluminium cooling body. Such an aluminium cooling body is known, for example, from the publication SE 458 652. Power semiconductors as, for example, thyristors, are built up in layers of a number of different materials, in order to achieve a good electrical contact and as low temperature stresses as possible in the material. A known thyristor element normally comprises a semiconductor crystal in form of a silicon plate with connections. On each side the silicon plate is surrounded by a buffer zone in the form of a molybdenum disc and a connection in the form of a circular plate made of copper. Finally, the thyristor element is surrounded by a tubular housing made of an insulating material, such that only the outer part of each connector plate is protruding. When manufacturing a thyristor valve, comprising a plurality of stacked thyristor elements with interlayered cooling bodies, the cooling bodies thus make direct contact with the connector plates. However, copper and aluminium have different temperature coefficients, which may lead to material stresses and contact problems between the component and the cooling body. It is desirable to increase further the lifetime and the switching capabilities of the components, respectively, by an improved cooling capacity. SUMMARY OF THE INVENTION The object of the invention is to achieve a cooling element for liquid cooling and a semiconductor valve with a cooling element, which cooling element has an improved cooling capacity. Material stresses and contact problem shall be avoided as far as possible. This is achieved by a device according to the independent claims 1 and 15 and a method according to the independent claim 10. Copper has a good specific thermal conductivity, which results in improved cooling. Copper also has a lower thermal expansion coefficient than aluminium, which results in less material stresses due to temperature changes. Finally, copper also has a low specific resistance, which gives less voltage drop across the cooling element. By using copper, a cooling body may be produced, which is more effective than known cooling bodies made of aluminium. By having the same material in the component, as for example at the thyristor surface, and in the cooling body, material stresses and contact problems may be decreased. The cooling capacity may be increased considerably, which permits power components to be operated at a higher power rating. The increased cooling capacity also results in the lifetime of the component being prolonged. By means of a Hot Isostatic Pressing (HIP) method it is possible to embed a pipe of stainless steel into a copper body during pressing. Copper powder with a high purity ensures a good electrical conductivity as well as a good thermal conductivity for the finished cooling body. The material thickness of the stainless pipe shall be a low as possible in order to achieve a minimum thermal resistance in the cooling element. The hot isostatic pressing method includes pressing under protective atmosphere with a homogeneous pressure distribution. No surplus pressure acts on the pipe during the pressing process, which implies that the material thickness of the pipe may be minimised down to a few tenths of millimetres. Casting of copper bodies with embedded pipes is also possible, but here the minimum possible material thickness is about 1 mm. It is also possible to produce a cooling pipe from an other low oxidising metal having a melting temperature higher than that of the cooling body. It is also conceivable to produce a cooling body with a cooling channel and later coat the inner surface of the cooling channel with an appropriate material, for protection of the cooling body against oxidation and wear. A known thyristor element normally comprises a semiconductor crystal in form of a silicon plate with connections. On each side the silicon plate is surrounded by a buffer zone in form of a molybdenum disc and a connector plate in form of a circular plate made of copper. Silicon and copper have different thermal expansion coefficients, and therefore the buffer zone is arranged to mechanically protect the silicon plate in case of temperature changes. In addition to being good electric conductors, the connector plates must be able to effectively take up and carry away heat from the silicon plate. A connector plate is therefore normally fairly thick. Finally, the thyristor element is surrounded by a tubular housing made of an insulating material, such that only the outer part of each connector plate is protruding. When manufacturing a thyristor valve comprising a plurality of stacked thyristor elements with interlayered cooling bodies, the cooling bodies are thus in direct contact with the connector plates. An advantageous improvement of the invention is described in the dependent claim 3. An improvement of the invention comprises a cooling body with a circular elevation, which is arranged to replace the connector plate of the thyristor. The elevation is thus arranged to be inserted into the thyristor element and to directly make contact with the buffer zone in form of the molybdenum disc. The cooling body, arranged in this manner, thus eliminates the previous junction between connector plate and cooling element. Since each such junction between two components implies losses for electrical conduction as well as thermal conduction, the further improved cooling body permits a thyristor valve to be made more effecient. An improved cooling capacity may also be achieved, with the improved cooling body by arranging cooling channels in the circular elevation as well. In a further advantageous improvement of the invention the cooling body is provided with a protruding support for placing control equipment for the power semiconductor. The control equipment is also in heat-conducting contact with the cooling body, which results in an efficient cooling of the electric components included in the control equipment. Still another advantageous improvement of the invention comprises embedding a resistor for high power into a cooling body with at least one cooling pipe. An efficient cooling of the resistor is hereby achieved. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in grater detail by means of some embodiments and with reference to the accompanying figures, wherein figure la shows a cooling element according to the invention with a cooling pipe embedded in a cooling body for cooling of a power semiconductor, figure lb shows the construction of a typical thyristor for a converter valve, figure 2 shows a cooling element according to the invention with a pipe embedded in a cooling body, for cooling of a power semiconductor and cooling of an electronic control unit connected to the power semiconductor, figure 3 a shows a cooling element according to the invention for cooling of a modified thyristor, figure 3b shows a part of a valve stack with a cooling element and modified thyristors according to the invention, figure 4 shows a cooling body according to the invention with two resistors embedded therein, and figure 5 shows a cooling element according to the invention for use with modified thyristors and for insertion of a high power resistor. DESCRIPTION OF THE PREFERRED EMBODIMENTS For manufacturing of a cooling body according to the invention by a Hot Isostatic Pressing (HIP) method a mould made of steel sheet is first manufactured. A pipe of, for example, stainless steel, which is to be embedded as a cooling pipe in the cooling body, is brought into a given shape, is placed in the mould, and is fixed in its position in an appropriate manner. The mould is subsequently filled with copper powder. The mould with the copper powder and the objects to be embedded, are placed in a press suitable for the HIP process. The HIP press is closed and evacuated. The whole HIP process takes place under a protective atmosphere, for example argon gas. The press is filled with the argon. During a few hours, the pressure and the temperature are increased in the press, until a temperature of about 750-950° C and a pressure of about 1000 bar are reached. The pressure and the temperature are maintained for a few hours. Subsequently, the pressure and the temperature are allowed to drop for a few hours, until normal temperature and normal pressure are reached again. During the HIP process the copper powder is sintered into a homogeneous copper body. After the HTP-process the mould is treated such that the mould is removed from the copper body. When manufacturing a cooling body according to the invention by casting, the embedded stainless pipe is enclosed by a protective gas to ensure that the pipe retains its stainless properties. It is also possible to produce a cooling body without embedding a cooling pipe when manufacturing the body, but with a cooling channel, the surfaces of which are subsequently coated with a corrosion resisting material. A suitable material should have a good resistance to wear and should have low oxidising characteristics. In the subsequent text the term cooling pipe shall also comprise a cooling channel. The final shape of the cooling body is achieved by machining the copper body in a manner known per se. The cooling body is provided, at least on the surfaces which will adjoin the power components, with a layer of low oxidising metal, which metal has good thermal conductivity as well as good electrical conductivity. Surfaces, which will be in conducting contact with the cooled components, are smoothened, for example by polishing. End pieces with connectors for coolant piping, such as de-ionised water, are arranged at the cooling body in a manner known per se. Figure la shows a cooling element 1, according to the invention, in perspective. The cooling element 1 is intended to make contact with at least one semiconductor element in a semiconductor valve in a converter station. A semiconductor valve comprises at least one valve stack with a plurality of semiconductors, for example thyristors, in series connection. Each of the respective thyristors may produce power losses in the range of 1-6 kW. Each cooling element 1 is placed either between two semiconductor elements or adjoining a semiconductor element on the bottom, or on the top, of the valve stack. An electric current from one semiconductor element to a following semiconductor element flows through the cooling element 1. The cooling element 1 has a substantially flat parallelepipedic cooling body 11 made of copper. A cooling pipe 12, made of stainless steel, is embedded in the cooling body 11 and extends in the cooling body 11 from one narrow side 13 to an opposite narrow side 14. The cooling pipe 12 is bent so as to adopt a spiral-like shape, and thus has a relatively large surface in heat-conducting contact with the surrounding cooling body 11. Two substantially parallelepipedic end pieces 15, made of stainless steel, and with the same cross section as the cooling body 11, are arranged at the two narrow sides 13, 14. The end pieces 15 each have through openings 16, which are in connection with the cooling pipe 12 on the sides facing the cooling body, and which, on the opposite sides, comprise an arrangement for connection of a pipe for supply and discharge of coolant. Such pipes are electrically insulating, for taking up a potential difference between the individual cooling elements 1 and between the cooling element 1 and ground potential, respectively. The coolant is normally de-ionised water with a high electrical resistance. At a first base surface 18 of the parallelepipedic cooling body 11 a circular elevation 20 is formed, which rises a few mm from the rest of the cooling body 11. The elevation has a diameter corresponding to the diameter of a connector plate at a thyristor element 21, according to figure lb, and is after assembly in conductive contact with the thyristor element 21. The surface of the elevation 20 is polished and shaped to fit a corresponding surface of the thyristor element 2. On the underside of the cooling body 11 , in figure la, a corresponding elevation (hidden), directed in the opposite direction, is arranged. Figure lb shows the composition of a known thyristor element 21 in cross section. A known thyristor element 21 usually comprises a semiconductor crystal, in form of a silicon plate 24, with connectors. On each side the silicon plate 24 is surrounded by a buffer zone 23 a, 23b, in the form of a molybdenum disc, and a connector plate 22a, 22b, in the form of a circular plate, made of copper. Silicon and copper have different expansion coefficients, and therefore the buffer zones 23a, 23b are arranged to mechanically protect the silicon plate 24 during temperature changes. A connector plate 22 serves as a contact for electrical connection, and its surface serves as cooling surface to make contact with a cooling element. The thyristor element 21 is encapsulated, for protection of the silicon plate 24, and it is surrounded by a tubular, insulating housing 25, made of ceramics, such that only the outer part of the respective connector plate 22a, 22b protrudes. Figure 2 shows a further advantageous embodiment of a cooling element 3 according to the invention. The cooling element 3 has a cooling body 31, which is substantially parallelepipedic and provided with a protruding support 33. A cooling pipe 32 is arranged substantially in a number of winding turns around a core of stainless steel, not shown in the figure, which core is embedded into the cooling body 31. Large sections of the cooling pipe 32 are thereby arranged in the vicinity of a first base surface 34 and a second base surface 35 of the cooling body 31. A part 36 of the cooling pipe 32 is arranged in the protruding support 33. The cooling body 31 has a first side 37 and a second side 38, respectively, which are parallel to each other. The surfaces of the first and second sides each have a connection opening 39, 40. The connection openings 39, 40 are separated and comprise a common longitudinal axis. Inside the cooling body 31 a first end 41 and a second end 42 of the cooling pipe 32 are associated with each one of the connection openings 39, 40. At the first side 37 and the second side 38 of the cooling body, two substantially parallelepipedic end pieces 43 are arranged. The end pieces 43 each have a through opening 44 as an extension of the connection openings 39, 40. The openings 44 are threaded for connection pipes for supply and discharge of coolant, in a manner known per se. The protruding support 33 is arranged for fixing of control equipment (not shown) for a power semiconductor. The control equipment may be attached to the console 33 by means of screws, arranged in openings 45 at the console 33. Components included in the control equipment, which are not at the same electric potential as the cooling element 3, must be electrically insulated from the cooling body 31. Figure 3a shows another advantageous embodiment of a cooling element 5 according to the invention. A cooling body 51 according to this embodiment has a base plate. The base plate has substantially the shape of a rectangle, with two adjacent corners, rounded off in to a semi-circle. On the base plate a first circular elevation 53 is arranged with a diameter smaller than that of the base plate. The elevation 53 is arranged to be in good thermal and electrical contact with a modified thyristor element 26 according to figure 3b. The elevation 53 has a height which is equal to or larger than the height of the connector plate 22 in a thyristor element 21 according to figure lb and is arranged to replace the connector plate 22. At the underside of the cooling body 51 in figure 3a a corresponding elevation, showing in the opposite direction (hidden) is arranged. A cooling pipe (not shown) is arranged in the cooling body, substantially in the elevation 53. Connectors 55 for coolant piping are arranged at a second half 52 of the base 51. Figure 3b shows, in cross section, a cooling element 5 according to figure 3a, between two thyristor elements 26 belonging to a semiconductor valve with a plurality of thyristors. A thyristor element 26 is modified with respect to a thyristor element 21, according to figure lb. Both thyristor elements are constructed in substantially the same manner, with the exception that the modified thyristor element 26 does not comprise any connector plates 22. The elevations 53 at the cooling element 5 are instead arranged to adjoin directly a buffer zone 23 at a thyristor element 26. The cooling body 5 with its respective elevations 53 is inserted into one thyristor element 26, in the figure shown above the cooling element 5, and into one thyristor element 26, in the figure shown below the cooling element 5, respectively. A further advantageous embodiment of a cooling body 6 according to the invention, is shown in figure 4. Two cylindrical thyristors 63 for high power and with an outer housing made of a suitable metal 64, for example stainless steel, are embedded in a cylindrical cooling body 61. Two cooling pipes 62 are embedded in parallel with the resistors. On one outer side of the cooling body 61, connection openings 65 for connecting of coolant piping are arranged. The resistors 63 may be used, for example, as voltage dividing resistors, connected in series at a semiconductor position in a converter station. Figure 5 shows still another embodiment of a cooling element 7 with a cooling body 71 according to the invention. The cooling element 7 is intended for cooling of modified thyristors 26 according to figure 3 b and is substantially parallelepipedic. At a base surface a circular elevation 77 is formed, arranged to fit mechanically to a modified thyristor 26. At a first narrow side a protruding support 72 is arranged, to which control equipment (not shown) for thyristors may be attached. At a second narrow side 73, opposite to the first narrow side the cooling body 71 is provided with a gap 74, arranged over the whole width of the cooling body 71. In the cooling body 71, parallel to the second narrow side 73, the gap 74 is widened to form a cylindrical groove 76, intended to take up a resistor, not shown in the figure. At the outer edge of the second narrow side a plurality of through-openings 75, transversely of the slot 74, are arranged. The holes 75 are intended to take up fixing means such as, for example, screws. By means of the fixing means the resistor may be fixed in its position in the cooling element 7. At a third narrow side of the cooling body 71 connection openings 78 for connection of coolant piping to the cooling element 7 are arranged. The invention is not limited to the embodiments described above. Other advantageous embodiments are thus possible within the scope of the invention. For example, a cooling pipe may be made of other metal than stainless steel. A suitable metal has low oxidising characteristics and a melting point higher than the process temperatures reached in a hot isostatic pressing process. Conceivable materials are, for example: nickel, silver or alloys with said characteristics. A cooling pipe in a cooling body 5 according to figure 5 may be wound around a resistor inside the cooling body. One or more resistors may be embedded in a cooling element for thyristor cooling. The cooling elements, which are exemplified in the embodiments for cooling of thyristor elements, may be formed for use with, for example, GTO elements (Gate Turn-Off Thyristor) or IGBT elements (Insulated Gate Bipolar Transistor)

Claims: CLAIMS 1. Cooling element (1, 3, 5, 6, 7) for cooling of at least one electronic power component (22, 26, 63), which cooling element comprises a cooling body (11,31,51,61, 71) with at least one cooling channel (12, 32, 62) for a liquid coolant, characterised i n that the cooling body consists of copper and the cooling channel comprises a cooling pipe, made of low oxidising metal which is embedded in the cooling body. 2. Cooling element according to claim 1, characterised in that the cooling pipe consists of stainless steel. 3. Cooling element according to claim 1 or 2, wherein one electronic power component is in the form of a power semiconductor (26) characterised in that the power semiconductor comprises a silicon plate (24) which is in electrical contact with a buffer zone (23a, 23b) surrounded by a insulating housing (25) and the cooling body comprises a elevation, (53, 77) arranged to make thermally and electrically conducting contact with the buffer zone. 4. Cooling element according to claim 3, wherein one electronic power component is in the form of a power semiconductor, characterised in that the cooling body comprises a protruding support (33, 72) for placing of control equipment for the power semiconductor. 5. Cooling element according to claim 4, c h a r a c t e r i s e d in that a part (36) of the cooling channel is arranged in said protruding support. 6. Cooling element according to claim 1 or 2, wherein at least one electronic power component is in the form of a cylindrical resistor, characterised in that the cooling body (71) comprises at least one cylindrical groove (76) in which the resistor may be inserted. 7. Cooling element according to any of the claims 1 to 2, wherein at least one electronic power component is in the form of a resistor (63) with a metallic housing (64), characterised in that the resistor is embedded in the cooling body (71). 8. Cooling element according to claim 1 or 2, wherein at least one electronic power component is in the form of a power semiconductor, characterised in that the cooling body (71) comprises at least one cylindrical groove (76) in which a power resistor (63) may be inserted. 9. Cooling element according to claim 1 or 2, wherein at least one electronic power component is in the form of a power semiconductor, characterised in that a power resistor (63) with a metallic housing (64) is embedded in the cooling body. 10. Method for manufacturing a cooling element (1, 3, 5, 6, 7) for liquid cooling of electronic power components (22, 26, 63), with at least one cooling pipe (12, 32, 62), in a cooling body (II, 31, 51, 61,71), characterised in - that the cooling pipe, made of low oxidising metal, is brought into a given shape, - that the cooling pipe is placed in its final position in a manufacturing mould for the cooling body - that the material for the cooling body, which is made of copper, is brought into the mould around the cooling pipe and, - that the cooling pipe during the manufacture of the cooling body is embedded in the cooling body. 11. Method according to claim 10, characterised in that the cooling body is cast. 12. Method according to claim 10, characterised in that the cooling body is sintered from copper powder, preferably according to a hot isostatic pressing method. 13. Method according to any of the claims 10 to 12, characterised in that the cooling body is provided, at least at its surface (20, 53, 77) which makes contact with the power component, with a layer of electrically well conducting and low oxidising metal. 14. Method according to any of the claims 10 to 13, characterised in that at least one resistor(63) with an outer casing (64) of a suitable material is embedded, together with the cooling pipe, in the cooling body during manufacturing of the cooling body. 15. Semiconductor valve being part of a converter station or a station for static var compensation with a plurality of semiconductor components (22, 26) with cooling elements (1, 3, 5, 7), arranged between the semiconductor components, whereby each cooling body comprises at least one cooling channel characterised in that the cooling elements comprise cooling bodies (11,31,51,71) made of copper and at least one cooling pipe made of a low oxidising metal is embedded in the cooling body. 16. Semiconductor valve according to claim 15, characterised in that the cooling bodies and the semiconductor components are designed according to any of the claims 1 to 9. 17. Semiconductor valve according to claim 15 or 16, characterised in that the cooling bodies are produced according to any of claims 10 to 14.

Inventors: HEDBLAD PER OLOV, HEDBLAD, PER, OLOV
Applicants: ABB AB
Application year: 1999
Main classification: H01L23
IPC codes: B22D19, H05K7