EP2761654A1

EP2761654A1 - Clamping mechanism and method for applying rated force to power conversion apparatus

Info

Publication number: EP2761654A1
Application number: EP20110873228
Authority: EP
Inventors: Junfeng SHENG; Richard S. ZHANG; Fan Zhang; Mark Edward DAME; Brian Eric Lindholm; Patrick Jose Lazatin; Xiaodan ZHANG; John Wilbur CANNON
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2011-09-28
Filing date: 2011-09-28
Publication date: 2014-08-06
Also published as: WO2013044409A1; EP2761654A4; CA2848802A1; BR112014005674A2

Abstract

A clamping mechanism and a method for applying a predetermined rated force to a power conversion apparatus are provided. The power conversion apparatus (40) includes plural power switching elements (42), plural heat sinks (44), a stack frame (60), a force application mechanism (67) and a rated force pre-assembly (72). The plural heat sinks (44) are provided among the plural power switching elements (42) to form a column, the stack frame (60) is configured to maintain the plural power switching elements (42) and the plural heat sinks (44) in the column, the force application mechanism (67) is provided at a first end of the column and configured to apply a predetermined rated force on the column, and the rated force pre-assembly (72) is provided at the first end of or at a second end of the column and configured to indicate when the predetermined rated force is present. The design of the clamping mechanism can ensure that the plural power switching elements (42) and the plural heat sinks (44) are clamped with the right amount force and it also provides uniform pressure distribution over the whole contact surfaces of the power conversion apparatus (40).

Description

CLAMPING MECHANISM AND METHOD FOR APPLYING RATED FORCE TO POWER CONVERSION APPARATUS

BACKGROUND TECHNICAL FIELD

Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for applying a rated force on press-pack semiconductor devices provided in a power conversion apparatus.

DISCUSSION OF THE BACKGROUND

Press-pack semiconductor devices are in many applications powerful components that are used for controlling a flow of electrical power or convert voltage, current or frequency necessary for connecting to a motor or a generator, or interfacing with a utility grid. The press-pack semiconductor devices are used in power conversion apparatuses (e.g., power converters) for a diverse range of applications. Those applications include motor drives for oil and gas, metal, water, mining and marine industries, as well as power/frequency converters for renewable energy (wind, solar), and electric power industries. To utilize the full potential of the press-pack semiconductor devices, a proper mechanical design of the complete assembly, including power switching elements, heat sinks, busbars and other components, is required.

The current and heat conducting interfaces in a press-pack semiconductor device are designed to retain good conduction properties throughout the equipment lifetime. This is accomplished by creating a sufficient number of stable metal-to-metal connections which can efficiently conduct current from the semiconductor through the heat sink to the busbars.

For power converters with press-pack power semiconductor devices, the power semiconductor devices are stacked on top of each other under a required pressure to make electrical and thermal contacts to form an electrical circuit. The stack (power stack assembly) may have single or plural of columns comprising power semiconductor devices, heat sinks, insulators, bus bars and alike with a clamping mechanism to hold those components together. Pressure is applied to each column to assure proper electrical and thermal contact between the individual press pack modules. Power switching elements (power semiconductor devices) are the core components in a power converter or variable frequency drive for electric motors.

There are two types of packaging for power semiconductor devices. A first type of packaging includes the power switching elements provided in a press-pack form (silicon wafer(s) in hockey-puck like ceramic housing), such as an Integrated Gate Commutated Thyristor (IGCT), Insulated Gate Bipolar Transistor (IGBT), Injection- Enhanced Gate Transistor (IEGT), Thyristor (ETT or LTT), and diode in press-pack package etc. A second type of packaging includes power switching elements provided in a module form, such as IGBT, MOSFET, and diode modules. For high power medium voltage power converters, when used in applications such as oil and gas, electric power, steel mill, and offshore, the press-pack form is preferred due to its higher power density and higher power handling capability. Even more, the press- pack form is preferred for the ruggedness and benign failure condition of the power switching elements, i.e., due to the ceramic housing of the power switching elements and strong mechanical clamping force, failure of press-pack components will not lead to an arc and plasma event, unlike power switching element in a plastic module.

An example of a power conversion apparatus with press-pack power semiconductor devices 10 is shown in Figure 1. Figure 1 shows a clamping mechanism 12 and 14 that maintains under pressure plural switching elements 16, busbars 18, and heat sinks 20. The switching elements 16 are directly connected to the busbars 18 while the heat sinks 20 directly contact the busbars 18. There are copper pole faces on top and bottom of a press-pack power semiconductor device, for electrical conduction and connection. The copper pole faces are also configured to enhance thermal conduction for dissipating heat produced due to the conduction and switching losses during normal operation of the power semiconductor device (or power switches). An alternative example of a power conversion apparatus is shown in Figure 2 in which the bus bars 18 are not provided between the switching elements 16 but rather they are mounted on a side of these elements.

The power switches are connected to form, for example, an electrical circuit of a power converter. In order for the power switching elements to operate normally, i.e., to conduct electrical current through the copper pole faces and to dissipate heat due to power switch losses from the copper pole face, a rated mechanical clamping force needs to be applied from both sides of the copper pole faces so that proper contact between the Si wafer, and the copper pole faces is achieved. To take away heat generated while the power switching elements operate, air cooled or liquid cooled heat sink(s) 20 are attached to one or both sides of the pole faces of the power switching elements 16 as shown in Figure 1. The heat sinks are facing the power switching elements either directly or through thermal conducting materials such as a metal bus bar 18. For achieving an electrical connection with the power switching elements, electrical conducting bus bars are attached directly or indirectly (e.g., through the heat sink as shown in Figure 1 ) to the pole face of the power switching element as shown in Figure 1. To form a complete electrical circuit according to the topology of the power converter, plural power switching elements, including active switches such as IGCT/IEGT and passive switches such as diode, may be placed in the stack, together with their heat sinks, bus bar and necessary insulators. Consequently, those bus bars are connected further to form the complete power circuit.

However, the clamping mechanism needs to be carefully designed to ensure that the press-pack semiconductor devices are clamped with the right amount of force and it also provides homogeneous pressure distribution over the whole contact surfaces of the power conversion apparatus. Uneven pressure will lead to deformation of the housing and internal stress between the different layers inside the power semiconductor switches causing them to fail prematurely. Achieving pressure uniformity is not always easy and the complexity should not be underestimated. Simple solutions, such as clamping the press-pack semiconductor devices between two rectangular plates 12 and 14 by bolting down the corners of the plates is not suitable for a high number of semiconductor devices and/or in multiple columns configuration.

As shown in Figure 3, it is possible to use a clamping mechanism 30 that uses a 4-rod structure with Belleville springs 31. The clamping mechanism 30 of Figure 3 includes two plates 32 and 34 that are connected to each other through four rods 36. Nuts 38 and Belleville springs 31 are used to force the plates 32 and 34 toward each other. The power switching devices 37 and heat sinks 39 are provided between the plates 32 and 34. A rated force is applied between plates 32 and 34 by appropriately tightening the nuts 38. However, it is difficult to maintain the plates 32 and 34 under a constant force by adjusting the four threaded rods 36 and corresponding nuts 38. In additions, such a mechanism does not allow an easy calibration of the applied force. Further, a heat generated by the power switching elements determines an expansion of some of the components of the power stack assembly and/or the clamping mechanism, which generates undesirable variations of the applied force.

A correct mechanical force needs to be applied to each pole face of the power switching elements. For example, for one type of IGCT, the rated mechanical force is 40,000 Nm, while for a type of diode, the rated mechanical force is 70,000 Nm. Typically, there is a narrow tolerance range of +/- 10% of the applied rated force. This tolerance needs to be guaranteed over the entire range of operating conditions and over the power converter's life cycle of the power stack assembly. An applied mechanical force less than the rated value will lead to insufficient contact between the pole faces and the Si wafer, which might lead to poor current conduction, thus higher loss, and unreliable control of the power switching element. An applied mechanical force that is higher than the rated force might crack the Si wafer and thus determine the failure or premature failure over time of the power switching element.

Another challenge for the clamping mechanism is to achieve a uniformity of the force distribution across the pole faces. Non-uniform force distribution across pole faces, for single wafer device such as IGCT, creates current crowding in a local region on the wafer. For multiple chip devices such as IEGT, the current crowding leads to non-uniform distribution of the total current among the paralleled multiple chips.

Both conditions, i.e., an unsuitable force and current crowding, may lead to device failure. Thus, it is desired to ensure an accuracy of the alignment of the clamped components in the column and to have a properly designed force distributor to distribute point-source applied mechanical forces uniformly across the pole faces of the column.

Other factors affecting the applied mechanical force in a power stack assembly with press-packed semiconductor devices are the nature of a compressible element that may be present in the column, (e.g., a Belleville spring), and the amount of heat generated from the operation of the power switching elements. The generated heat may lead to temperature variations of the various components in the column, e.g., power switching elements, heat sinks, bus bars, insulators, force distributors, etc. The temperature variations may produce a physical displacement of the components, like elongation or shrinkage. The physical displacement of the components may cause a variation of the applied force through the column to be outside the rated range for the power switching elements. The compressible element may be used to compensate the impact of the physical displacement, such that the applied force in a stack is mostly within the rated range.

Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.

SUMMARY

According to an exemplary embodiment, there is a power conversion apparatus that includes plural power switching elements; plural heat sinks provided among the plural power switching elements to form a column; a stack frame configured to sandwich the plural power switching elements and the plural heat sinks in the column; a force application mechanism provided at a first end of the column and configured to apply a predetermined rated force on the column; and at least a rated force pre-assembly provided at the first end or at a second end of the column and configured to indicate when the predetermined rated force is present.

According to still another exemplary embodiment, there is a rated force pre-assembly configured to indicate when a predetermined force is applied to plural power switching elements and plural heat sinks of a power conversion apparatus. The rated force pre-assembly includes a fixed body; a movable body configured to move into and out of the fixed body; compressible means provided between the fixed body and the movable body to bias the movable body away from the fixed body; and a stopper device extending into the fixed body and the movable body and configured to limit a motion of the movable body relative to the fixed body.

According to yet another exemplary embodiment, there is a method for calibrating a rated force pre-assembly. The method includes a step of applying a predetermined rated force on a movable body of the rated force pre-assembly; a step of positioning a stopper device inside the movable body and a fixed body of the rated force pre- assembly so that a head of the stopper device is flush with an external surface of the fixed body when a tension in a compressible means of the rated force pre-assembly is substantially equal to the predetermined rated force; and a step of removing the applied predetermined rated force.

According to yet another exemplary embodiment, there is a method for applying a predetermined rated force to a power conversion apparatus. The method includes a step of sandwiching plural power switching elements and plural heat sinks with a rated force pre-assembly and a force application mechanism of the power conversion apparatus; a step of inserting a central rod in an end plate of a stack frame to apply the predetermined rated force, wherein the end plate and another end plate sandwich the power switching elements and the plural heat sinks; a step of compressing a movable body of the rated force pre-assembly towards a fixed body of the rated force pre- assembly until a head of a stopper device is flush with an external surface of the fixed body; and a step of receiving an indication that the predetermined rated force is present in compressible means provided between the fixed body and the movable body.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

Figure 1 is a schematic diagram of a conventional power stack assembly;

Figure 2 is a schematic diagram of another conventional power assembly;

Figure 3 is a schematic diagram of a conventional clamping device applied to a power stack assembly;

Figure 4 is a schematic diagram of a power stack assembly according to an exemplary embodiment;

Figure 5 is a schematic diagram of a central rod and gimble of a clamping device according to an exemplary embodiment;

Figures 6 to 9 illustrate how a rated force pre-assembly of a clamping mechanism for a power stack assembly is loaded with a predetermined rated force according to an exemplary embodiment;

Figure 10 is a flowchart of a method for loading a rated force pre-assembly with a predetermined rated force and providing the rated force pre-assembly in the power stack assembly according to an exemplary device;

Figure 1 1 is a flowchart of a method for loading a rated force pre-assembly with a predetermined rated force according to an exemplary embodiment; and

Figure 12 is a flowchart of a method for attaching a rated force pre-assembly to a clamping device according to an exemplary embodiment. DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a power conversion apparatus with press-packed semiconductor devices. However, the embodiments to be discussed next are not limited to these apparatuses, but may be applied to other systems that require the supply of a constant force for maintaining together various components during changing operating conditions.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

According to an exemplary embodiment, a power conversion apparatus with press- packed semiconductor devices includes plural power switching elements and their corresponding plural heat sinks implemented mechanically as one or more power stack assemblies that comprises of one or more columns. A power stack assembly includes a force application mechanism configured to apply a rated force to the components sandwiched in the column. The force application mechanism may include a center threaded rod configured to apply force on the column. The power stack assembly further includes a rated force pre-assembly that is configured to accurately apply the rated force on the column of the power stack assembly.

In an exemplary embodiment illustrated in Figure 4, a power stack assembly 40 with press-packed semiconductor devices is shown including a single column 41 , a stack frame 60, a force application mechanism 67 and a rated force pre-assembly 72. Figure 4 shows a single force application mechanism 67 disposed at the top of the power stack assembly 40. However, the force application mechanism 67 may be provided at the bottom of the power stack assembly 40 or at both ends of it. The column 41 includes plural power switching elements 42 and plural heat sinks 44 sandwiched between two insulators 46 and 48. The insulators 46 and 48 may be two blocks of machined insulation material such as glass polyester GP03, FR4, etc. The insulators are configured to electrically insulate the power switching elements 42 and heat sinks 44 from the stack frame and external voltage potentials.

Metal blocks 50 and 52 may be placed in contact with the insulators 46 and 48 such that the metal blocks do not electrically contact the power switching elements 42 and/or the heat sinks 44. The purpose of the metal blocks 50 and 52 is to more uniformly distribute a force received from the force application mechanism 68 to the power switching elements 42 and the heat sinks 44.

The stack frame 60 may include one or more end plates 62 and 64, and rods 70. The end plates 62 and 64 are metal blocks (e.g., aluminum or steel) that are sufficiently stiff with small deformation under pressure. Rods 70 connect the two plates 62 and 64 to each other. Two or more rods 70 may be used and they be made out of steel or stainless steel.

The force application mechanism 67 for a column in the power stack may include a central threaded rod 68 that may be screwed into the top plate 62 such that a first end 68a touches the metal block 50. The first end 68a may have a semi-spherical shape that transfer a force received from the force application mechanism 67 to the column as a point source. If a point source is used, then a force distributor, for example, the metal block 50, is used to distribute the applied force to the column in such way that the pressure over the surface of the column is uniform. Although it is called a central rod, this rod 68 does not have to be in a central position of the plate 62. The other end 68b of the central rod 68 may either extrude beyond the surface of the top plate 62 or may be recessed inside the top plate 62. Irrespective of the position of the end 68b of the central rod 68, it is desirable that an operator of the power stack assembly has access to the end 68b and can rotate this end.

Still in another application, the central rod 68 has a gimbal structure 69 as shown in Figure 5. Figure 5 shows that the gimbal structure 69 has a support element 69a that is configured to screw into the central rod 68. The support element 69a is configured to hold a ball 69b that partially is provided inside the support element 69a and partially outside. The ball 69b touches the metal block (force distributor) 50 to transfer the force from the force application mechanism 67. Figure 4 also shows that the central rod 68 has threads 68c. Thus, the central rod 68 may be moved relative to the plate 62, either upwards or downwards to a desired position to apply a desired force. Other elements, as insulators and bus bars may be present in the column but are not discussed here as they are known in the art. The gimbal structure 69 may be provided to the other force distributor 52 or to both force distributors 50 and 52.

An advantage of the gimbal structure is to uniformly distribute the pressure across surfaces of the components in the power stack assembly, and thus the column, making the power stack assembly less sensitive to parallelism and alignment of force application from top and bottom plates. In one application, the force distributor may have a surface that mirrors a surface of the ball 69b. For example, Figure 5 shows such a surface 50a.

Figure 4 also shows a rated force pre-assembly 72 that is provided between the metal block 52 and the bottom plate 64. The rated force pre-assembly 72 is configured, as discussed next, to ensure that a rated force is reached during an assembly step of the power stack assembly, and it had gone through a calibration step on its own prior to the assembly step. The rated force is applied on the power switching elements and heat sinks of the power conversion apparatus. The rated force pre-assembly 72 is now discussed with reference to Figures 6-9.

According to an exemplary embodiment illustrated in Figure 6, the rated force pre- assembly 72 may include a movable body 74 and a fixed body 76 configured to receive the movable body 74. Compressible means 78 (e.g., Belleville springs) are provided between the movable body 74 and the fixed body 76 so that the movable body 74 is biased to move away from the fixed body 76. The fixed and movable bodies may have shoulders 76a and 74a for accommodating the compressible means 78 and also for preventing the fixed body to move along -Z direction relative to the lower plate 64. It is noted that the figures show the fixed body 76 provided at the lower plate 64. However, the fixed body 76 may be provided at the top plate 62 or at both plates. The compressible means 78 may include one or more Belleville springs. In one application, an even number of Belleville springs may be used. For example, Figures 6-9 shows four Belleville springs. To minimize a rubbing action between the Belleville springs and the surfaces of the fixed and the movable bodies, the Belleville springs 78a-d are provided in pairs, each pair 78a-b having a Belleville spring 78a facing another Belleville spring 78b such that the outside diameters of these two springs 78a and 78b are in direct contact. This configuration also minimizes inaccurate forces to be applied due to the friction between the Belleville springs and surface of end plates or force distributor. A stopper device 80 is provided for limiting a movement of the movable body 74 along axis Z. The stopper device 80 may be screwed or fixedly attached (e.g., welded) to the movable body 74.

In one application, the stopper device 80 is a screw that is screwed into the movable body 74 up to a desired location. Considering that Figure 6 shows the stopper device 80 located at the desired position and assuming that the rated force pre-assembly 72 is not yet attached to the column 41, it is noted that a head 80a of the stopper device 80 is not flush with a surface 82 of the fixed body 76, thus forming a clearance Dl . Further, it is noted that a tail 80b of the stopper device 80 does not touch an inside surface 84 of the movable body 74, thus forming a clearance D2. These clearances Dl and D2 are intentional, and have predetermined values as discussed next.

Next it is discussed how the rated force F is applied to the rated force pre-assembly. According to an exemplary embodiment shown in Figure 7, the fixed body 76 is fixedly provided in the lower plate 64 (or as discussed above, in the upper plate 62 or in both of them) and the movable body 74 is pressed by a force providing mechanism 88 towards the fixed body 76 until the tension in the compressible means 78 is substantially equal to the rated force F. As the rated force may be large, the force providing mechanism 88 may be a hydraulic press.

When the rated force F is achieved, the stopper device 80 is screwed or unscrewed into the movable body 74 so that the head 80a is flush with the surface 82 of the fixed body 76, as shown in Figure 7. At this stage the clearance Dl is zero. To be able to move the stopper device 80 up or down along the Z direction, an operator can access the head 80a of the stopper device 80 from beneath the lower plate 64. Also, the fixed body 76 is so shaped that access to the head 80a of the stopper device 80 is possible. Figures 6 and 7 show one possibility for permitting access to the stopper device but other approaches may be used. This calibration step of the rated force pre-assembly 72 is performed before providing the rated force pre-assembly into the column.

After applying the rated force F to the movable body 74 and having the head 80a of the stopper device 80 flush with the surface 82 of the fixed body 76, the hydraulic press 88 may be removed. At this point, as shown in Figure 8, the movable body 74 is displaced along the positive direction of axis Z until the head 80a of the stopper device touches shoulder 76b of the fixed body 76. During this relaxation step, the stopper device 80, by being screwed into the movable body 74 with threads 80c, moves along the Z direction under the tension existing in the compressible means 78. However, the tension in the compressible means 78 is now less than the rated force F due to the displacement D3 of the movable body 74.

After this relaxation step, the rated force pre-assembly 72 is provided in the press- packed semiconductor apparatus 40 as shown in Figure 9. For simplicity, only column 40, metal plates 50 and 52, end plates 62 and 64 and the central rod 68 are illustrated individually, all other components being symbolized by reference numeral 40. In this embodiment, the central rod 68 is screwed in, along the negative direction of the Z axis, so that the metal blocks 50 and 52 compress the column 41. At the same time, the metal block 52 presses on the movable body 74 so that the entire movable body moves along the negative direction of the Z axis towards the fixed body 76. This motion is stopped when the head 80a of the stopper device 80 is flush with surface 82 of the fixed body 76.

During a verification step, the tension present in the compressible means 78 need to be determined. If the tension is equal to the rated force F, the rated force pre- assembly has been correctly calibrated and installed in the column. In one exemplary embodiment, the operator may feel with his hand or may visually determine when the head 80a is flush with surface 82. In another application, a scale may be used to determine when the two elements (80a and 82) are flush with each other. In still another application, a sensor 92 may be provided to determine when the two elements are flush.

When the head 80a becomes flush with surface 82 of the fixed body 76, the tension in the compressible means 78 is substantially equal to the rated force F due to the calibration shown in Figure 6. Thus, the force application mechanism 68 needs to not apply more force.

As discussed above, by screwing the central rod 68 into the end plate 62 (see Figure 4), the movable body 74 moves down until the rated force F is substantially achieved in the compressible means 78 (when the head 80a is flush with surface 82). This tension is also present on surfaces of the power switching elements and the heat sinks of the press-packed semiconductor apparatus 40, thus achieving the goal of applying a desired rated force. In an exemplary embodiment, more than one rated force pre- assembly 72 may be used at a bottom or a top of the press-packed semiconductor apparatus 40. Also, it is noted that the embodiments discussed with regard to Figures 4-9 show only one column. However, it is possible to have multiple columns in the power conversion apparatus.

According to an exemplary embodiment illustrated in Figure 10, a method for applying a rated force to the power conversion apparatus 40 is now discussed. In step 1000, a rated force pre-assembly is provided inside a power stack assembly 40. The rated force pre-assembly may be the one shown in the embodiments of Figures 6 to 9. In step 1002 which is the calibration phase, a desired rated force is applied to a movable body of the rated force pre-assembly until a tension in a compressible means 78 is substantially equal to the desired rated force. In step 1004, a stopper device is positioned inside the movable body of the rated force pre-assembly so that a head of the stopper device is flush with an external surface of a fixed body of the rated force pre-assembly. In step 1006, the applied force is removed and in step 1008 the rated force pre-assembly is inserted into the power stack assembly. It is noted that steps 1000 to 1006 are part of a calibration method in which the desired rated force is applied to the rated force pre-assembly. These steps may be performed independent of the steps discussed next.

In step 1010, a central rod of a force application mechanism of the power conversion apparatus is screwed in so that a force is applied to the elements of the apparatus. At the same time, the movable body of the rated force pre-assembly is compressed towards the fixed body until the head of the stopper device is flush with a surface of the fixed body. In step 1012, it is determined that the head of the stopper device is flush with the surface of the fixed body and no further force is applied at the force application mechanism. At this time, the rated force is achieved in the compressible means of the power conversion apparatus.

Some of the steps noted above are now discussed in more details. During the calibration phase of the rated force pre-assembly, the rated force pre-assembly is placed in a force provider mechanism, such as a hydraulic press. When the rated force pre-assembly is compressed, the compressible means is compressed and the movable body slide inside the fixed body. When the rated force is reached, the stopper device may be adjusted until its surface is leveled with a preset surface of the pre-assembly such as an end surface of the fixed body.

During the apparatus assembly phase, i.e., when the rated force pre-assembly is attached to the column with the stack frame, the fixed body is attached to the lower or upper plate of the stack frame. The central rod is mounted on the upper plate (or lower plate) of the stack frame. Then, all necessary components of the column are placed and aligned, such as power switching elements, heat sinks, force distributors, bus bars and insulators. These elements are provided between the rated force pre- assembly and the force application mechanism to form the column. Finally, the rated force is provided by the central rod until the compressible means reaches its rated force and the head of the stopper device reaches its rated position, e.g., a surface of the screw in the fixed body is flush with a surface of the fixed body.

According to an exemplary embodiment illustrated in Figure 11 , there is a method for calibrating a rated force pre-assembly to be used in a power stack assembly. The method includes a step 1 100 of applying a desired rated force on a movable body of the rated force pre-assembly; a step 1102 of positioning a stopper device inside the movable body and a fixed body of the rated force pre-assembly so that a head of the stopper device is flush with an external surface of the fixed body when a tension in a compressible means of the power stack assembly is substantially equal to the desired rated force; and a step 1 104 of removing the applied desired rated force.

According to another exemplary embodiment illustrated in Figure 12, there is a method for applying a desired rated force to a power conversion apparatus. The method includes a step 1200 of inserting a rated force pre-assembly into a stack frame of the apparatus; a step 1202 of inserting a central rod in an end plate of a force application mechanism to apply the desired rated force, wherein the end plate and another end plate sandwich a column of the power conversion apparatus; a step 1204 of compressing the movable body towards the fixed body until the head of the stopper device is flush with the external surface of the fixed body; and a step 1206 of receiving an indication that the desired rated force is present in compressible means provided between the fixed body and the movable body.

One or more of the above discussed embodiments advantageously provides a more accurate rated force on various components provided in a column of a power conversion apparatus. Also, one or more of the devices of the exemplary embodiments may maintain the rated force during changing operations of the apparatus. The force in the column is accurately achieved without the need of using a measuring instrument. The calibration of the rated force pre-assembly is performed prior to being inserted into the column, which is easier and more accurate. A friction present in a conventional device between a single washer and a contact surface is reduced by using a plural number of washers in the compressible means. For example, an even number of springs in the compressible means may be advantageous.

The disclosed exemplary embodiments provide a power conversion apparatus and a method for applying a desired rated force to press-packed semiconductor devices. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

Claims

WHAT IS CLAIMED IS:

1. A power conversion apparatus (40) comprising: plural power switching elements (42); plural heat sinks (44) provided among the plural power switching elements (42) to form a column (41); a stack frame (60) configured to sandwich the plural power switching elements (42) and the plural heat sinks (44) in the column; a force application mechanism (67) provided at a first end of the column (41) and configured to apply a predetermined rated force on the column (41); and at least a rated force pre-assembly (72) provided at the first end or at a second end of the column (41) and configured to indicate when the predetermined rated force is present.

2. The power conversion apparatus of Claim 1 , wherein the rated force pre- assembly (72) comprises: a fixed body (76); a movable body (74) configured to move into and out of the fixed body (76); and a stopper device (80) configured to keep together the fixed body (76) and the movable body (74).

3. The power conversion apparatus of Claim 1 , wherein the column includes plural columns.

4. The power conversion apparatus of Claim 1, further comprising: a force distributor provided between the force application mechanism or the at least a rated force pre-assembly and the column, wherein the force application mechanism includes at least a point-source rod configured to apply the predetermined force to the force distributor.

5. The power conversion apparatus of Claim 1 , wherein the column further comprises: insulators and bus bars provided between the rated force pre-assembly and the force application mechanism.

6. The power conversion apparatus of Claim 1, wherein the rated force pre- assembly (72) comprises: compressible means (78) provided between the fixed body and the movable body to bias the movable body away from the fixed body and to limit a force variance on the column and the stack frame due to temperature expansion of elements of the column and the stack frame.

7. The power conversion apparatus of Claim 6, wherein the compressible means include an even number of Belleville spring.

8. The power conversion apparatus of Claim 6, wherein the rated force pre- assembly further comprises: the stopper device (80) extending into the fixed body and the movable body and configured to limit a motion of the movable body relative to the fixed body.

9. The power conversion apparatus of Claim 6, wherein a tension in the compressible means is substantially equal to the predetermined rated force when the head of the stopper device is flush with an external surface (82) of the fixed body.

10. A rated force pre-assembly (72) configured to indicate when a predetermined force is applied to plural power switching elements (42) and plural heat sinks (44) of a power conversion apparatus (40), the rated force pre-assembly (72) comprising: a fixed body (76); a movable body (74) configured to move into and out of the fixed body (76); compressible means (78) provided between the fixed body (76) and the movable body (74) to bias the movable body (74) away from the fixed body (74); and a stopper device (80) extending into the fixed body (76) and the movable body (74) and configured to limit a motion of the movable body (74) relative to the fixed body (76).

1 1. The rated force pre-assembly of Claim 10, wherein the compressible means include an even number of springs or washers.

12. The rated force pre-assembly of Claim 10, wherein the stopper device has a head (80a) configured to stay inside the fixed body and a tail (80b) configured to screw into the movable body.

13. The rated force pre-assembly of Claim 10, wherein a tension in the compressible means is substantially equal to the predetermined rated force when the head of the stopper device is flush with an external surface (82) of the fixed body.

14. The rated force pre-assembly of Claim 10, wherein the compressible means is a Belleville spring.

15. A method for calibrating a rated force pre-assembly, the method comprising: applying a predetermined rated force on a movable body of the rated force pre- assembly; positioning a stopper device inside the movable body and a fixed body of the rated force pre-assembly so that a head of the stopper device is flush with an external surface of the fixed body when a tension in a compressible means of the rated force pre-assembly is substantially equal to the predetermined rated force; and removing the applied predetermined rated force.

16. The method of Claim 15, further comprising: inserting the rated force pre-assembly into a stack frame of a power conversion apparatus.

17. The method of Claim 16, further comprising: inserting a central rod in an end plate of the stack frame to apply the predetermined rated force, wherein the end plate and another end plate sandwich components of the power conversion apparatus.

18. The method of Claim 17, further comprising: compressing the movable body towards the fixed body until the head of the stopper device is flush with the external surface of the fixed body.

19. The method of Claim 18, further comprising: receiving an indication that the predetermined rated force is present in compressible means provided between the fixed body and the movable body.

20. A method for applying a predetermined rated force to a power conversion apparatus, the method comprising: sandwiching plural power switching elements and plural heat sinks with a rated force pre-assembly and a force application mechanism of the power conversion apparatus; inserting a central rod in an end plate of a stack frame to apply the predetermined rated force, wherein the end plate and another end plate sandwich the power switching elements and the plural heat sinks; compressing a movable body of the rated force pre-assembly towards a fixed body of the rated force pre-assembly until a head of a stopper device is flush with an external surface of the fixed body; and receiving an indication that the predetermined rated force is present in compressible means provided between the fixed body and the movable body.