TECHNICAL FIELD

Embodiments presented herein relate to housing of electrical components, and particularly to an arrangement for pressure tolerant housing of an electrical component.

BACKGROUND

Packaging is essential for electrical components such as semiconductors in order to connect them electrically and protect them from environmental influence. This is in particular the case for power semiconductors that are employed as main switching elements in various applications such as frequency converters which are used to drive motors in pumps, compressors etc.

Frequency converters in the medium voltage and high power range drive electric motors by controlling the speed and torque of these machines and are a well proven equipment in the entire onshore as well as offshore platform based industry. In recent years, growing interest has been laid on installing electrical installations on the sea floor in depths from a few tens of meters to even kilometers. One main driver of this development is the oil and gas industry, but future applications are seen in subsea high-voltage, direct current or high voltage alternating current transmission and distribution systems as well as offshore power generation (such as wind energy, tidal energy, wave energy, ocean current energy).

Subsea oil and gas production employs electric equipment such as drilling motors, pumps, and compressors that are currently driven by frequency converters located on topside platforms. Electric power is provided to the subsea machinery by expensive umbilical cords. By installing frequency converters and other power electronic equipment (such as insulated-gate bipolar transistor (IGBT) power semiconductor elements) subsea, cables and topside installations could be spared and enormous cost savings could be achieved. In general terms, electric subsea installations and devices usually demand high standards regarding durability, long-term functionality and

independence during operation.

Thus, whilst currently the semiconductor module housing for the power electronic equipment is open to air, for applications in a liquefied and pressurized environment (as it is the case in a subsea converter installation at the bottom of the ocean) certain adjustments would be required in order to protect the silicon (Si) chip of the power semiconductor elements. In bringing power electronic equipment to be part of subsea applications, two general concepts currently exist.

According to a first concept the power electronic equipment stays at atmospheric pressure. This is realized in pilot plants today. This first concept allows standard electric/electronic components, known from onshore installations, to be used. However, thick walls are needed for the enclosure to withstand the pressure difference between inside and outside the tank as the tank is submerged into the ocean. Thick walls make the equipment heavy and costly. In addition, heat transfer through thick walls is not very efficient. Additionally, huge and expensive cooling units are required. According to a second concept the equipment is passively pressurized to the hydrostatic pressure level of the ambient sea water (increasing by 1 bar each 10 m, typically 100 to 300 bar for subsea installations under consideration). This is achieved by filling a thin-walled vessel with liquid of negligible

compressibility. This is still in early development. This second concept does not require any thick walls for enclosure since no pressure difference exists between inside and outside the containment. Cooling is greatly facilitated by thin walls. This requires all the components to be free of gas inclusions and compressible voids. Otherwise they may implode during pressurization and thus be destroyed. Dielectric liquid must be stable over time in order to keep its insulating behavior during the entire time of operation. Further, impurities may evolve over time in the dielectric liquid used within the thin- walled vessel. If this dielectric liquid directly touches the termination on the silicon (Si) chip the impurities risk triggering a high peak field that may destroy the Si chip. This may become a reliability issue that (only) affects the semiconductor modules after some time in operation in a liquid

environment.

Hence, there is still a need for an improved arrangement for pressure tolerant housing of an electrical component.

SUMMARY

An object of embodiments herein is to provide an improved arrangement for pressure tolerant housing of an electrical component.

A particular object is to provide an arrangement for subsea housing of electric components combining the advantages of the above presented first concept and second concept whilst still avoiding disadvantages of the above presented first concept and second concept.

According to one aspect there is presented an arrangement for pressure tolerant housing of an electrical component. The arrangement comprises a housing. The housing comprises at least one wall part, a bottom part, a top cover part, and a joining member. The arrangement comprises an electrical component, the electrical component being enclosed by the housing. The joining member is arranged to join the top cover part to the at least one wall part such that the joining member enables relative movement between the at least one wall part and the top cover part upon a relative pressure being applied to the top cover part and the bottom part. The top cover part and the bottom part are arranged to clamp the electrical component upon the relative pressure being applied.

Advantageously this provides an improved arrangement for pressure tolerant housing of an electrical component. The arrangement is particularly suitable for a power semiconductor element.

Advantageously this arrangement combines advantages of the above disclosed first concept and second concept. Making use of the outer pressure, such as subsea water pressure, for clamping the component between the top cover part and the bottom part is of advantage. A high contact pressure between the component and the top cover part and the bottom part is beneficial for the transfer of heat and electricity. Advantageously the disclosed arrangement enables to maintain the advantages of well-known hermetically closed press-pack devices with only small modifications to the housing construction.

Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating Von-Mises stress and

deformation of an upper flange under 50 MPa external hydrostatic pressure;

Figure 2 is a schematic diagram illustrating a cross-sectional view of an arrangement; Figures 3a and 3b are schematic diagrams illustrating cross-sectional view of arrangements according to embodiments;

Figure 4 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments (no pressure applied); Figure 5 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments when the housing is clamped (no pressure applied);

Figure 6 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments when outer pressure is applied;

Figure 7 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments;

Figure 8 is a schematic diagram illustrating a step-by-step assembly process of an arrangement according to embodiments; Figure 9 is a schematic diagram illustrating a three-dimensional presentation of a step-by-step assembly process of an arrangement according to embodiments;

Figure 10 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments without a hinge but with a small gap with size d;

Figure 11 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments without a hinge and with applied hydrostatic pressure;

Figure 12 is a schematic diagram illustrating a cross-sectional view of part of an arrangement according to embodiments with a simplified geometry used to simulate different gaps;

Figure 13 is a schematic diagram illustrating Von-Mises stress for different flange thickness and gap distances;

Figure 14 is a schematic diagram illustrating Von-Mises stress for adapted housing geometry with a CuNiCoiSi-alloy flange; and

Figure 15 is a schematic diagram illustrating Von-Mises stress for adapted housing geometry with a NiSiFeMn-alloyi flange. DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept 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 by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional.

The present invention relates to arrangements for pressure tolerant housing of an electrical component.

In general terms, the housing (such as a so-called "hockey puck" housing) of bipolar power semiconductor elements is hermetically sealed and thus by design suited for operating in a liquid environment. However, the housing does not withstand isotropic pressure present at the sea ground. The weak point in the housing is the flange enabling the clamping mechanism of the housing in order to establish a good dry contact. If the pressure outside the housing becomes too high, the flange starts to dent inwards and eventually cracks. This can be seen in the FEM simulation results shown in Figure 1 where the arrows indicate a pressure P. The simulations have been carried out using COMSOL Multiphysics® software. As can be noted in the figure the yield strength R _p0 .2 = 140 MPa of standard OF-Cu is exceeded at many locations. A cross-section drawing of an arrangement 20, 20a, 20b for housing an electrical component in the form of a bipolar diode is shown in Figure 2. The arrangement 20, 20a, 20b comprises an upper part 20a and a bottom part 20b. The upper part 20a comprises a top cover 21. The bottom part 20b comprises a bottom cover 25, a membrane 24, a ceramic ring 23, and a flange 22b. As can be seen in the upper part of the figure, the top cover 21 has a flange 22a with some flexible structure which allows the top cover 21 to move in vertical direction (i.e., along the y-axis) if needed. Typical thickness of the flange 22a is 0.5 mm. Simulations have shown that the flanges 22a, 22b are bent when exposed to 300 bars and more outer pressure, and can eventually break. A first possible solution to this issue could be to open the housing, thereby allowing dielectric liquid to fill the internal of the housing in order to compensate for the elevated environmental pressure. However, this will cause the hermeticity to be lost, which is in view of the very sensitive edge termination of bipolar power semiconductor devices a significant drawback. In particular, prevention of cross-contamination could be a serious issue to deal with.

A second possible solution could be to make the housing pressure-tolerant by design. Simulations have shown that a flange thickness of 2.5 mm or more is needed to withstand the pressure in a reliable way. However, this has the drawback that the housing loses its mechanical flexibility which may be needed to compensate for dimensional tolerances during clamping of the housing in a stack of housings. In turn, this may pose high demands on the mechanical design and tolerances which could make the housing expensive.

An object of the present invention is to provide an arrangement for pressure tolerant housing of an electrical component which can withstand higher outer pressure than currently available arrangements for housing an electrical component.

Figures 3a and 3b schematically illustrates embodiments of arrangements 30a, 30b for pressure tolerant housing of an electrical component. The arrangements 30a, 30b comprises a housing 31. The arrangements 30a, 30b further comprises an electrical component 32. The electrical component 32 is enclosed by the housing 31. The housing 31 comprises at least one wall part 33, a bottom part 36, and a top cover part 34a, 34b. The housing 31 further comprises a joining member 35. The joining member 35 is arranged to join the top cover part 34a, 34b to the at least one wall part 33 such that the joining member 35 enables relative movement between the at least one wall part 33 and the top cover part 34a, 34b upon a relative pressure P being applied to the top cover part 34a, 34b and the bottom part 36, where P may represent an outer hydrostatic pressure. The top cover part 34a, 34b and the bottom part 36 typically constitute the electrical terminals for connecting the electrical component to electrical power. The top cover part 34a, 34b and the bottom part 36 are arranged to clamp the electrical component 32 upon the relative pressure P being applied. With reference to the embodiment of Figure 3b, the top cover part 34a, 34b has a height such that its bottom surface (according to the orientation of Figure 3b) is adjacent the top surface of the electrical component 32. Hence, a relative pressure P applied on the top cover part 34a, 34b from outside the housing 31) will be translated to the electrical component 32 by means of the top cover part 34a, 34b. The electrical component 32 is thereby arranged to be clamped between the bottom part 36 and the top cover part 34a, 34b upon the relative pressure P being applied. Hence, this triple-layered sandwiched arrangement of the top cover part 34a, 34b, the electrical component 32, and the bottom part 36 makes use of the pressure P as an advantage for clamping the electrical component 32. This in turn may allow for reducing any clamping forced used during manufacturing of the arrangement 30b.

Further embodiments of the disclosed arrangements 30a, 30b will now be disclosed.

The housing 31 may be hermetically closed. For example, the housing 31 may be a hermetic and pressure- and liquid-tolerant (hockey puck-like) press- pack.

The arrangement 30a, 30b may be configured such that pressure inside the housing 31 is closer to atmospheric pressure than to the environmental pressure outside the housing 31. Particularly, the arrangement 30a, 30b may be configured for subsea use. For example, the housing 31 may be configured such that pressure inside the housing 31 is below 20 bar virtually regardless of the pressure outside the housing 31. Preferably, the pressure inside the housing 31 is below 20 even if the outer pressure is 300 bar. In a subsea environment the pressure outside the housing 31 may be in the region 300 bar, or more - depending on the depth in the subsea environment the arrangement 30a, 30b is placed for deployment. One way to enable the pressure inside the housing 31 maintain at atmospheric pressure regardless of pressure outside the housing 31 is to fill the housing 31 with a gas, which is compressible. One example of such a gas is nitrogen gas (N ₂ ). In more detail, the gas would be filled on-shore under atmospheric pressure conditions. Once the arrangement reaches the bottom of the ocean in a converter tank, the joining member would be lightly engaged, resulting in a small volume reduction of the housing which would then cause a pressure increase in the gas. However, since the overall housing structure is robust against

hydrostatic pressure, no further changes of pressure inside the housing will occur.

There are different types of joining members 35 that can be arranged to join the top cover part 34a, 34b to the at least one wall part 33. According to one embodiment the joining member 35 is configured to be mechanically moved upon the relative pressure P (as caused by an outer hydrostatic pressure) being applied. For example, the joining member 35 may be a hinge 35a. The hinge 35a may thus be mechanically moved upon the relative pressure P being applied. In this respect, the hinge 35a may comprise a shaft extending between two heads, wherein the heads are arranged in respective sockets of the top cover part 34a, 34b and the at least one wall part 33. This type of hinge 35a is illustrated in Figures 4 to 9.

According to one embodiment the joining member 35 is configured to be mechanically deformed upon the relative pressure P being applied. For example, the joining member 35 may be a flange 35b. The flange 35b may thus be mechanically deformed upon the relative pressure P being applied. In this respect, the flange 35b may be provided as a washer (a thin disk-shaped plate with a through-hole in the middle of the plate) extending between the top cover part 34a, 34b and the at least one wall part 33. This type of flange 35b is illustrated in Figures 4 to 12.

The housing 31 may have a flange structure with only a sealing part (as defined by top flange 35b). This may enable easy assembly of the hinge structure by means two-piece flange (top and bottom). For example, the flange 35b may be arranged to hermetically close the housing 31. The joining member 35 may comprise both a hinge 35a and a flange 35b. Hence, the housing 31 may have a flange structure with separate moving part (hinge 35a) and sealing part (top flange 35b). The housing 31 may comprise a single joining member 35 or a plurality of joining members 35. The plurality of joining members 35may be interconnected by a flexible structure. For example, the joining member 35 may comprise a plurality of hinges 35a and a single flange 35b. Hence the hinges 35a can be individual or interconnected with a flexible structure. There are different materials that are suitable for the joining member 35. For example, the joining member 35 may be made from a metal, such as copper or steel.

The housing 31 may take different shapes. For example, the top cover part 34a, 34b may have a circular shape. For example, the bottom part 36 may have a circular shape. For example, the housing 31 may have a circular symmetric shape in view of a longitudinal axis (y-axis) of the housing 31. For example, the at least one wall part 33 may have a metal part 33a and a ceramic part 33b. The ceramic part may be a ceramic ring.

There are different materials of which the top cover part 34a, 34b may be made from. For example, the top cover part may be made from a rigid material. Thereby, deformation of the housing 31 as caused by the relative pressure P being applied will only be constituted by movement and/or deformation of the joining member 35. In other words, according to such an embodiment the top cover part 34a, 34b will not deform upon the relative pressure P being applied; neither will the bottom part 36, nor the at least one wall part 33.

There are different types of electrical components 32 that could be comprised in the housing 31. For example, the electrical component 32 may comprise a silicon wafer. Further, the bottom part 36 and the top cover part 34a, 34b may be made from a metal and act as electrical poles for the electrical component 32.

Further aspects of the disclosed arrangement will now be disclosed.

In general terms, a hinge 35a is mechanically strong enough to sustain the relative pressure but and also ensures flexibility. Since it may be challenging to fabricate a hinge 35a which is hermetically sealed, explicit hermeticity may be provided by the joining member 35 forming an additional lid in the form of a flange 35b, as shown in Figure 4. Figure 4 shows the pressure-less case where no pressure is applied to the housing 31 and hence all parts of the housing 31 are in their default positions.

In all the following figures only the top parts of the housing 31 are shown. The bottom part 36, the electrical component 32 and the bottom part of a ceramic ring on which the electrical component 32 is provided are not shown.

As noted above, the the bottom part 36 and the top cover part 34a, 34b may be made from a metal and act as electrical poles for the electrical component 32. Therefore, as soon as the arrangement is clamped into a stack, the top cover part 34a, 34b will eventually move in vertical direction in order to make sure that a good electrical contact to the silicon wafer is established. This situation is shown in Figure 5. If pressure (P) is applied from the outside, the top cover part 34a, 34b will be bent towards the inner side of the housing 31. However, this will come to an end when the lid touches the joining member 35, which is strong enough to sustain the pressure as shown in Figure 6 (where the joining member 35 is provided as a flange 35b and a hinge 35a). In order to make this work, the hinge 35a may be made of copper or a hard material such as steel etc. As can be seen in Figure 7 the side wall 33 of the housing 31 may be divided into two parts and the top part may be combined with the flange 35b.

Figure 8 outlines an assembly process of the arrangements 30a, 30b by means of a sequence of cross-sectional illustrations of an arrangement. In a first step (step 1; left-most part of Figure 8), the pole piece is placed using a suitable alignment fixture. Then, the hinge 35a is placed as shown in the middle part of Figure 8 (step 2). Finally (step 3; right-most part of Figure 8) the top part of the flange 35b is placed and connected to the pole piece and the bottom part of the flange 35b by means of cold welding or plasma welding. A single welding step can be applied. A corresponding perspective view of the assembly process of the arrangements 30a, 30b is given in Figure 9·

As noted above, the housing 31 may comprise a plurality of joining members 35. The joining member 35 may thus comprise a number of individual hinges 35a which may be distributed equally around the top cover part 34a, 34b. These can be individual hinges 35a, or hinges 35a which are interconnected by a flexible structure. The exact size of, and the distance between, the hinges 35a can be adjusted according to the needs of the application. In general terms, the concept of having joining members 35 in the form of hinges 35a and flanges 35b as described above is deduced from the idea to separate the package functionality of pressure and liquid protection from each other. In general terms, the hinge 35a provides pressure protection while the flange 35b ensures liquid tightness. For example, a hinge 35a may be placed there such that a flange 35b is pressed towards the hinge 35a (that produces the pressure in a gas inside the housing 31) but not further more. Thus, in one embodiment the flange 35b protects mainly against the surrounding liquid from entering the housing 31 and the hinge 35a protects mainly against the external pressure. Moreover, this concept still allows the upper pole piece (i.e., the top cover part 34a, 34b) to move in order to clamp the electrical component 32 (such as a silicon wafer) in between the upper and the bottom pole pieces, (i.e., between the top cover part 34a, 34b and the bottom part 36). The bottom part 36 may be firmly attached to a ceramic ring with an extended metal structure. Thus no flexibility may be granted between the ceramic ring and the bottom part 36. However, in this way the hermeticity is ensured and the clamping of the electrical component 32 can nonetheless be performed by the top cover part 34a, 34b. Another way to keep the flexibility of the top cover part 34a, 34b and have a hermetic and pressure tolerant arrangement without having a hinge 35a will be described next. The top cover part 34a, 34b may be extended and a metal piece forming a flange 35b may be brazed onto the ceramic ring to level the height where the flange 35b is provided between the top cover part 34a, 34b and the at least one wall part 33, as illustrated in Figure 10.

The gap (d) between the top cover part 34a, 34b and the at least one wall part 33 may be decreased such that only a thin slit remains with a distance d as shown in Figure 10. The flange 35b may be soldered on top of the top cover part 34a, 34b and the at least one wall part 33 to hermetically seal the inner part of the housing 31. In this altered packaging concept no hinge 35a is required, since the slit is made as small as possible. Hence the flange 35b becomes pressure tolerant because it is pressed onto the top cover part 34a, 34b and the at least one wall part 33 by the outer hydrostatic pressure (P) as indicated in Figure 11. The thickness of the flange 35b and the slit size between the top cover part 34a, 34b and the at least one wall part 33 may be determined such that the housing 31 can endure subsea pressure conditions up to a few hundred bars without destroying the top cover part 34a, 34b.

The arrangement shown in Figure 11 was simulated using a FEM simulation software. In order to gain a better understanding a simplified geometry of the arrangement as shown in Figure 12 was used. The highest von-Mises stress values observed on the top flange 35b were plotted in the graph shown in Figure 13. The graph of Figure 13 shows the von-Mises stress for different flange 35b thicknesses and gap distances. It can be seen that that the flange thickness has a significant impact on the von- Mises stress. The same effect is also observed when the gap is decreased. The simulations were performed with a flange 35b made of copper. However, the trend is true for harder materials as well that might have slightly different absolute stress values. Nevertheless the yield strength of two type of materials which typically may be used, have been considered are standard OF-Cu (yield strength R _p0 . ₂ = 140 MPa), and copper alloy K57 (CuNiCoiSi

From Figure 13 it can be conceived that a K57 structure may be designed such that the yield strength Rpo.2 is not exceeded. At a flange thickness of

0.75mm the hydrostatic pressure of 500 bar (50 MPa) cannot bend such a flange 35b if the gap between the top cover part 34a, 34b and the at least one wall part 33 is smaller than 3.6 mm, which is in the range of the state-of-the- art distance between these two elements of the housing 31.

Another geometry was simulated that is close to the proposed geometry of the arrangement in Figure 10 with a gap of only 0.5 mm between the top cover part 34a, 34b and the at least one wall part 33. Figure 14 and Figure 15 show the results with a 0.75 mm thick flange 35b made out of CuNiCoiSi and NiSiFeMn (Vacodil), respectively. The colour bar scales with respect to the yield stress of the material (Vacodil: 300 MPa).

Figure 14 shows that only small parts of the flange 35b exceed the yield stress of 810 MPa. Therefore this geometry with a strong K57 copper alloy flange material would be suitable to produce an arrangement for pressure tolerant housing of an electrical component 32. A gap of 0.5mm might be critical because of the thermal expansion of the at least one wall part 33. However, as the graph in Figure 13 indicates, larger gaps above imm should be enough, such that K57 withstands the hydrostatic pressure up to 500 bars even though it would be pressed onto the at least one wall part 33. In Figure 15 the same geometry was simulated with a Vacodil flange

(NiSiFeMn-alloy). The von Mises stress is shown with the colour bar that extends to 300 MPa, which is the yield strength of this material. The maximum stress present in the flange 35b by applying 500 bar hydrostatic pressure is around sooMPa. Hence the flange 35b would deform very rapidly towards the at least one wall part 33. From a stability point of view, the K57- alloy shows superior behaviour against hydrostatic pressure.

One or several of the above described arrangements for pressure tolerant housing of an electrical component is typically arranged inside a subsea unit. The subsea unit is filled with dielectric liquid and pressure compensated. Pressure compensation of the subsea unit can be achieved by including a pressure compensator which by means of e.g. a membrane or a baffle transfers the prevailing outer water pressure to the dielectric liquid inside the subsea unit. This means that the pressure of the dielectric liquid inside the subsea unit essentially equals the hydrostatic water pressure surrounding the subsea unit. The pressure inside the arrangement for pressure tolerant housing of an electrical component is however, as has been described, much closer to atmospheric pressure.

By way of example, the subsea unit may be deployed at the seabed 3000 m below the sea surface where the hydrostatic pressure is approximately 300 bar. The pressure of the dielectric liquid inside the subsea unit is then approximately 300 bar and the pressure of the gas inside the arrangement for pressure tolerant housing of an electrical component is approximately 10 bar, at least not more than 20 bar. Importantly, this disclosure describes how subsea water pressure is made use of to clamp a component, typically a power semiconductor element, between a top cover part and a bottom part. Thereby the transfer of electric power and heat between the component and the top cover part and the bottom part is greatly improved, and the arrangement need not be designed to be able to apply all the required clamping force in itself. This way, the component within the arrangement for pressure tolerant housing of an electrical component may actually function better when under a high hydrostatic pressure than at atmospheric pressure.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.