WO1998041072A2

WO1998041072A2 - A power supply unit for a plug-in unit of a communications apparatus and a communications equipment using a plurality of such plug-in units

Info

Publication number: WO1998041072A2
Application number: PCT/FI1998/000224
Authority: WO
Inventors: Gösta BAARMAN
Original assignee: Nokia Telecommunications Oy
Priority date: 1997-03-13
Filing date: 1998-03-13
Publication date: 1998-09-17
Also published as: AU6402798A; FI972948A0; WO1998041072A3; FI972948A

Abstract

The invention pertains to circuit card specific power supply systems. The power supply system according to the invention comprises at least one leg (20) such that said power supply unit (10) is detachably mounted on said at least one leg (20) to provide a mounting space for other components (100) between the power supply unit and the printed circuit board (30) and said at least one leg (20) is arranged so as to provide at least one electrically conductive connection between the power supply unit (10) and the printed circuit board (30).

Description

A power supply unit for a plug-in unit of a communications apparatus and a communications equipment using a plurality of such plug-in units

The invention relates to circuit card specific power supply systems.

Most of the modern electric apparatus get their operating voltage from the public mains, in which case it usually is an AC voltage rated at 110 or 230 volts and 50 or 60 Hz. An electric apparatus typically has a power supply which converts this operating voltage into DC and/or AC voltages required by the apparatus. A power supply is often realized as a separate, closed module. The module is advantageously closed because at least mains voltage and possibly other relatively high voltages are present in the power supply which would be dangerous if the power supply were open in construction. In addition, the closed construction of the module prevents electromagnetic radiation possibly produced in the module from spreading outside the module provided that the outer casing of the module is made of an electrically conductive material. There are many different sized modular power supplies having different electrical ratings so that a designer can choose a suitable power supply according to the invention for a particular application.

In printed circuit board (pcb) assembly, prior-art power supplies are mounted on the surface of the pcb so that, being large components, they take up quite a lot of area on the pcb. Especially conventional power supply modules on circuit boards, which are soldered onto the circuit board, take up circuit board area to a considerable extent on both sides of the circuit board.

Modern communications apparatus and their construction set their own special requirements. Communications apparatus typically comprise a plurality of plug-in units which are easily replaceable for the purpose of servicing. A plug-in unit usually comprises one circuit board to which the necessary components are attached. One edge of the printed circuit board has connectors for connecting the plug-in unit to a communications apparatus, and a second edge of the pcb typically has a front panel containing indicator lights giving indication about the operation of the plug-in unit and possibly switches controlling various functions. For the communications apparatus to house as many plug-in units as possible, the plug-in units have to be relati- vely narrow, thus limiting the height of the components on a plug-in unit. A typical card spacing, i.e. distance between plug-in units, allows a maximum height of about 16 mm for the components. As the functions of communications apparatus get more complex and communications needs continually grow, the component packing densities in communications devices increase. As the component packing density increases, also the power requirements of plug-in units increase, which sets additional requirements on the power supply arrangements in plug-in units. Furthermore, the growth in component packing density also sets stricter demands on the utilization of the card area, so that the area available for power supply arrangements continually diminishes. In modern plug-in units components are typically inserted very closely on both sides of the pcb so that the utilization of the area on both sides of the pcb is essential.

A known arrangement for supplying power to a plug-in unit in a communications apparatus is to implement the plug-in unit power supply using discrete components soldered onto the printed circuit board. Such an arrangement is, however, rather poor as regards consumption of space since such a power supply is fairly big. Such a power supply also takes up a lot of area as the through conductors of discrete components and the pcb conductors connected to them reserve space on both sides of the pcb. In addition, as printed circuit boards typically are multilayer boards, the conductor lead-ins also reduce the space available to pcb conductors on the intermediate layers. Furthermore, power supplies realized using discrete components are disadvantageous because of the EMC interference caused by them.

Another known solution, better than the one above, is to use a separate modular power supply. Such a modular power supply typically comprises a printed circuit board enclosed in a metal casing wherein the necessary power supply components are surface mounted on said printed circuit board. Such a modular power supply typically has legs which are taken through the printed circuit board onto which it is mounted and soldered onto said pcb. Because of the metal casing such a power supply causes considerably less EMC interference than a solution implemented with discrete components. However, such a solution also takes up pcb area as the module is mounted the bottom against the pcb so that components cannot be mounted between the module and the pcb. Furthermore, because of the legs taken through the pcb, area is also used on the other side and in the intermediate layers of the pcb. The legs taken through the pcb limit the routing of wire connections implemented in the pcb, especially the routing of wide paths. Because of this, such a modular power supply cannot be placed on an arbitrary location on the plug-in unit. Typically, the mid-area of a plug-in unit contains wide paths interconnecting components in the border areas of the plug-in unit, making the placement of such a modular power supply in the mid-area of the plug-in unit very difficult. Another problem with such a modular solution is that the heat generated by the power supply module is effectively conducted to the pcb in the plug-in unit because the bottom of the module touches the pcb. The heat conducted to the pcb may result in malfunctioning and physical damage, especially as the apparatus gets older.

Furthermore, the solutions described above have the disadvantage that power supply systems soldered to the plug-in unit are very difficult to service and replace because a modular power supply has to be unsoldered from the printed circuit board or, in the case of discrete components, one or more components have to be unsoldered. Various easily replaceable solutions have been disclosed for applications other than plug-in units of communications apparatus. For example, US Patent No. 4,251,853 discloses a construction wherein several circuit boards are placed on top of each other by means of connectors. However, solutions of this type have not been applied in plug-in units of communications apparatus because of a lack of space due to the small distances between the plug-in units.

It is an object of the invention to provide a power supply system avoiding the aforementioned disadvantages characteristic of the prior art. It is an object of the invention to provide a power supply system that takes up less mounting area on the printed circuit board than prior-art solutions. It is also an object of the invention to provide a power supply system that is easily serviced and replaced. It is a further object of the invention to reduce the deterioration of the strength and electrical characteristics of the construction caused by the warming of the power supply.

The objects of the invention are achieved by a power supply construction in which the power supply is placed on supporting or connector legs. By means of various supporting or connector legs the power supply can be mounted at a desired height from the printed circuit board so that other components can be mounted in the space between the power supply and the printed circuit board.

The power supply construction according to the invention is characterized in that it comprises in addition to a power supply unit at least one leg, said power supply unit being detachably mounted on said at least one leg to create a mounting space for other components between the power supply unit and the printed circuit board, and said at least one leg being arranged so as to provide at least one electrically conductive connection between the power supply unit and the printed circuit board. The invention also pertains to communications equipment characterized in that it comprises at least one printed circuit board onto which at least one leg is mounted for the attachment of a power supply unit, and a power supply unit is detachably attached to said at least one leg and at least two components are mounted onto said printed circuit board such that at least one of said components is between the printed circuit board and the power supply unit.

The invention will now be described in more detail with reference to the preferred embodiments presented by way of example and to the accompanying drawing whe- rein

Fig. 1 is a side view of a preferred embodiment of the invention, and

Fig. 2 shows a second preferred embodiment of the invention, and

Fig. 3 shows a preferred application of the invention, and Fig. 4 shows a structure of a preferred embodiment of the invention.

Like elements in the drawing are denoted by like reference designators.

Fig. 1 shows a preferred embodiment of the invention. A power supply 10 is raised by means of legs 20 from the surface of a printed circuit board 30 so that the space under said power supply can be used to mount other components 100. The legs 20 are preferably separate from the power supply unit 10 proper. It is, however, obvious to a person skilled in the art that the power supply unit according to the invention can also be realized with fixed legs 20.

Preferably the supporting legs serve as connectors through which the necessary electric connections between the power supply module and the printed circuit board are realized. The construction is then advantageously designed such that the connector/supporting legs are soldered or otherwise attached according to a certain pre- determined pattern to the pcb, and the power supply is pushed into the connectors so that, when necessary, it can be relatively easily removed for service purposes. Said predetermined pattern depends on what kind of connector pins or other counterparts for the connector/supporting legs the power supply module has. In addition to or instead of the push-in attachment the power supply module may be attached to the connector/supporting legs and/or printed circuit board by means of screws, snap- on locks or using some other known arrangement. Fig. 2 shows a preferred embodiment of the invention wherein a power supply unit is attached to the printed circuit board 30 not only by means of separate legs 20, but also by means of screws 40 in order to improve the mechanical strength of the construction. When the power supply is attached to the pcb, it can be conveniently re- moved and attached by screwing, e.g. in such a manner that some of the screws function as lifting screws and some as attachment screws. In the embodiment shown in Fig. 2 the legs 20 that provide electric coupling are ordinary connection strips providing mechanical and electrical contact with the connection pins 23 of the power supply unit for connecting the power supply's input and output voltages and ot- her possible signals. The embodiment of Fig. 2 also illustrates the fact that in addition to the legs 20 providing the electric connection the power supply unit 10 may also have fixed legs 21. In the example described in Fig. 2 these fixed legs have holes 22 for screws 40.

In the embodiment depicted in Fig. 2 the power supply module has holes for attachment screws in its opposing corners. The printed circuit board has corresponding holes so that the attachment screws penetrate both the holes in the power supply module and those in the pcb and become attached to nuts on the opposite side of the pcb. Alternatively, the holes 22 in the legs 21 of the power supply module may be threaded so that the attachment screws are fastened to the threads of the legs 21 from the opposite side of the pcb, as shown in Fig. 2. The remaining two corners of the power supply can also have holes which have threads corresponding to those in the attachment screws of the power supply module. However, the pcb does not have holes at corresponding locations. The idea is that when a user wants to remove the power supply module according to the invention from the pcb, he first removes the attachment screws. Then he screws the attachment screws into the threaded holes so that the ends of the screws touch the pcb. As the user then continues to turn the screws, the ends of the screws are pressed against the surface of the pcb, and the force exerted on the threads of the threaded holes by the turning movement of the screws lifts the power supply module evenly away from the surface of the pcb. Even if the friction between the connector pins of the power supply module and the connector/supporting legs were strong, the evenly lifting effect of the screws detaches the power supply module from the pcb without the user needing to exert excessive force on the module. The risk of damaging the pcb and the power supply module is then considerably smaller than if the user tried to remove the module in some other way, say, prying with a screwdriver. The power supply system according to the invention does not significantly increase the vertical space reserved by the power supply. Even a 4-mm elevation, for example, is enough for placing ordinary surface-mounted components under the power supply unit. The thickness of an ordinary power supply unit is advantageous- ly about 10 mm, which means that the total height of the construction is about 14 to 16 mm, depending on the way of implementation of the legs and on the precise size of the components. Such a height is well within height limits defined in standards concerning conventional replaceable printed circuit boards.

The power supply according to the invention can be advantageously realized within a metal casing so that interference possibly caused by the power supply will not considerably affect components placed under it. In addition, the power supply according to the invention can be advantageously realized such that possible cooling elements and/or parts in the power supply that become hot are positioned on that surface of the power supply which faces away from the pcb lest the components mounted between the power supply and the printed circuit board become overheated because of the power supply. The gap left between the pcb and the power supply also improves air circulation around the power supply so that the power supply itself will not warm rp as much as in prior-art solutions, and nor will it warm up the pcb as much as prior-art solutions. By means of various connector or supporting legs the distance between the power supply and the pcb can be adjusted such that the components mounted under the power supply unit stay cool enough.

Using the raised power supply system it is also possible to decrease the pcb area us- ed by high-voltage connectors by concentrating the components requiring the high voltage generated by the power supply unit under the power supply. Such an embodiment adds to safety as it is then necessary to remove the power supply unit from the printed circuit board in order to get to the high-voltage components. Such an embodiment finds particular utility in mounting laser diodes as some laser diode models require a DC voltage of up to 110 V. In such an embodiment the power supply generating the high voltage also conveniently provides high-voltage protection.

Thanks to separate legs the power supply according to the invention is applicable al- so in places where the component height is limited. Instead of the legs 20 connectors can then be used which do not substantially add to the height so that the total height of the construction is determined by the height of the power supply unit. In such an embodiment, however, other components cannot be mounted between the printed circuit board and the power supply unit.

The invention is applicable in the realization of pcb-specific power supplies. The solution finds particular utility in cramped, densely packed printed circuit boards.

This is a considerable advantage as modern electronic apparatus often comprise printed circuit boards which there may be several in one apparatus, in which case they are usually mounted parallely, leaving a distance between the printed circuit boards. The invention does not limit the height of the supporting legs of the power supply module but the height can be chosen such that the power supply module with its supporting legs fits well in the space left between two adjacent printed circuit boards. If the supporting legs are separate, they can be manufactured in different heights so that one and the same power supply module can be used in the design of each particular apparatus, selecting the supporting legs according to space requirements. In that case, the power supply module can naturally be mounted on the pcb without the supporting legs, in which case it functions like a power supply of the prior art. The supporting legs of a power supply module may be single pin-like legs or elongated strips.

The layout of a power supply module is usually a rectangle but other shapes are possible, too. A power supply module shaped like a rectangle or straight-edged polygon may have supporting legs on two opposing sides of the module or even on all sides. If the supporting legs are elongated strips coated with an electrically conduct- ive material and if they are found on all sides of the power supply module shaped like a rectangle or straight-edged polygon, the entity formed by the supporting legs and power supply module may create under it a substantially closed space which can serve as an EMC (ElectroMagnetic Compatibility) shield for the components within that space.

Power supply modules according to the invention can be manufactured in various sizes and with various electrical ratings, so it is advantageous that identical supporting legs can be used to attach all power supply modules according to the invention to the printed circuit board. On the other hand, the power supply according to the invention can also be made modularly expandable, which means that it is possible to connect on top or to the side of a power supply module one or more power supply modules which e.g. double the allowed output current at a certain output voltage, or otherwise together with the original power supply module provide a power supply which better suits a particular purpose.

The power supply unit according to the invention is particularly well adapted to se- ries production since one type of power supply unit can be applied in many different products. In addition, the fact that the power supply is easily replaceable makes final testing in series production easier and the introduction in series production of new versions of the power supply unit becomes simpler because the new versions do not require changes in the printed circuit boards using the power supply units in question.

The power supply system according to the invention is particularly well adapted to be used in various communications equipment. Fig. 3 illustrates such an embodiment. Communications systems equipment 1 10 such as various telephone exchanges or network elements in SDH (synchronous digital hierarchy) networks typically comprise subracks 130 which include a plurality of similar circuit cards 30 each of which takes care e.g. of the transmission of data on a communications channel of a communications connection 120. In such an equipment high packing density of components, modularity and easy replaceability of circuit cards and their power supply units are significant advantages. The power supply system according to the invention which comprises a power supply unit 10 mounted on legs 20 and which facilitates the mounting of components 100 in the space between the power supply unit 10 and the pcb, considerably increases the component packing density and equipment modularity.

Fig. 4 shows a construction according to a preferred embodiment of the invention. Fig. 4 shows a power supply unit 10 comprising a printed circuit board 1 1 and components 12, 13 attached to it. The power supply unit 10 is attached to a printed circuit board 30 of a plug-in unit in a communications apparatus using surface- mounted connectors 20a, 20b such that a first part 20a of the connector is attached to the plug-in unit's circuit board, and a counterpart 20b of the connector is attached to the circuit board 11 of the power supply unit. In this embodiment, most of the components are attached to the printed circuit board 11 using surface mounting. In accordance with a preferred embodiment of the invention, all components are at- tached to the circuit board 1 1 using surface mounting in order to optimize the assembly of the power supply unit 10. In a preferred embodiment of the invention the printed circuit board 1 1 is coated with a relatively thick metal layer. The circuit board is advantageously a multilayer board which has more than one conductive layer comprising electrically conductive patterns and which has in the middle, with respect to thickness, a copper layer con- siderably thicker than the conductive layers, advantageously 0.5 mm or thicker, and in which the insulating material has good thermal conductivity. Such a pcb material is known in the prior art and can be obtained at least from one printed circuit board material supplier. In a preferred embodiment of the invention such a printed circuit board material is used in the circuit board of the power supply unit 10 of a plug-in unit in a communications apparatus wherein the circuit board also serves as a cooling board for the components 12, 13. Then it is not necessary to attach to the components separate cooling boards which is required when using ordinary pcb materials such as FR-4. If it is desired to boost the cooling effect of the printed circuit board 1 1, it is possible to mount separate cooling fins on the pcb, just like other com- ponents 12, 13, so that said cooling fins conduct heat away from the circuit board 11. Components that warm up a lot are in this embodiment mounted on that side of the power supply unit 10 which faces away from the mounting location, and only components 13 which do not warm up considerably nor generate powerful electric or magnetic fieϋs are mounted on that side of the power supply unit 10 which faces the mounting location, so that the heating effect of the power supply unit affects the components 100 in the mounting location as little as possible. Since the power supply circuits that generate a lot of heat are in this embodiment located on a circuit board which is separate from the circuit board 30 of the plug-in unit in a communications apparatus, the circuit board of the plug-in unit in a communications appara- tus warms up considerably less than in prior-art systems. The use of such a printed circuit board material in the power supply unit 10 of a plug-in unit of a communications apparatus has other advantages, too. The metal layer in the middle of the board further provides a very good ground plane and EMC shield. For that reason, in this embodiment, the components 12 and circuits 12 producing EMC noise are loca- ted on that side of the power supply unit 10 which faces away from the mounting location, thus producing minimal EMC interference on the components 100 of the mounting location. Such an arrangement makes it possible to leave the power supply unit without a casing without the EMC interference level becoming too high, which considerably simplifies the manufacture of the power supply unit and saves manu- facturing costs. All components 12, 13, 20b of a power supply unit according to such an embodiment can be surface mounted onto the power supply unit's circuit board 11, which rninimizes the number of assembly stages and thus produces considerable savings in manufacturing costs. No other attachment mechanisms, such as casing fastening screws according to the prior art, are needed to attach the components. Because of effective heat conduction and lack of a casing, thermally conductive insulating materials need not be cast between the components and the casing material in the system according to this embodiment, unlike in modular systems according to the prior art. So, the power supply unit according to this embodiment is very simple and economic to manufacture. This construction makes it possible to manufacture the power supply unit according to the invention entirely by means of machine assembly, which considerably saves manufacturing costs. This construction can be advantageously utilized in an embodiment according to Fig. 4, for example.

The embodiment according to Fig. 4 has the additional advantage that because of the surface-mounted connectors 20a, 20b the wiring to the power supply unit 10 of the printed circuit board 30 in the plug-in unit can be realized in the topmost conductive layer of the plug-in unit's printed circuit board 30 so that the layout of the other conductive layers in the plug-in unit's printed circuit board 30 is independent of the location of the power supply unit on the plug-in unit's printed circuit board 30. In particular, the component layout on that side of the plug-in unit's printed circuit board which is opposite to the power supply unit 10 can be realized independently of the power supply unit so that the power supply unit 10 will not waste component mounting area on the opposite side. In addition, since the wiring to the power supply unit can be realized on the topmost layer of the plug-in unit's printed circuit board 30, the routing of wide data paths in the intermediate layers of the plug-in unit's printed circuit board 30 can be realized independently of the power supply unit so that in the system according to Fig. 4 the power supply unit can be positioned on the plug-in unit's printed circuit board more freely than in the prior art, yet not disturbing important data paths. Since there are on the plug-in unit side of the power supply unit only substantially non-heating components 13 that do not generate considerable EMC noise, such as circuits processing control signals, it is also possible to mount on the plug-in unit's printed circuit board 30, between the plug-in unit and the power supply unit, sensitive components such as memory circuits. Since the wiring to the power supply unit can be concentrated on the topmost layer of the plug-in unit's printed circuit board 30, the intermediate layers of the plug-in unit's printed circuit board 30 can be freely utilized for the wiring of components on the power supply unit side of the plug-in unit so that the mounting density of these components can be maximized without the power supply unit's wiring causing considerable limitations to the realization of the routing of the wiring of various components, unlike in systems according to the prior art. In accordance with a preferred embodiment of the invention, the power supply unit of a plug-in unit of a communications apparatus can feed other plug-in units, too. Then all plug-in units need not have power supply units, which saves manufacturing costs of the communications apparatus. In such an embodiment the voltages genera- ted by the power supply unit for the other plug-in units are brought to the plug-in umts through the plug-in unit interfaces of the communications apparatus.

The modular power supply system according to the invention has the additional advantage that the system utilizes the space available better than prior-art systems by utilizing the volume between plug-in units. Modular power supply units according to the prior art simply aim at a construction as low as possible but the system according to the invention gains considerable advantage from the novel idea that in plug-in units of communications apparatus it is more advantageous to aim to utilize the volume between plug-in units as fully as possible and not just to minimize the height of the construction.

The power supply of the construction according to the invention is easily replaced or removed e.g. for the purpose of servicing the power supply or components under it. Thus the invention makes the replacement and servicing of the power supply unit considerably easier.

The invention has considerable advantages as the power supply system according to the invention takes up considerably less pcb mounting area than prior-art solutions and, additionally, causes considerably less electromagnetic and thermal strain on the pcb than prior-art solutions. In addition, safety factors can be better taken into account in the design of the inventional system, and assembly and service in the in- ventional system are easier than in prior-art solutions. Suitability for mass production is also better than in the prior art because of improved testability and updatability of different power supply versions, among other things.

Above the invention was described referring to some of its preferred embodiments but it is obvious that the invention can be modified in many different ways in accordance with the inventional idea defined by the claims set forth below.

Claims

1. A plug-in unit of a communications apparatus, characterized in that it comprises

- a power supplv unit (10) to feed electrical power to the printed circuit board (30) of the plug-in unit,

- at least one connector (20a) surface mounted on the printed circuit board of the plug-in unit,

- at least one connector (20b) surface mounted on the printed circuit board (11) of the power supply unit so that said connectors (20a, 20b) mechanically attach said power supply unit to the printed circuit board of the plug-in unit and provide electrically conductive connections between the printed circuit board (30) of the plug-in unit and the printed circuit board (1 1) of the power supply unit,

- components ( 100) attached to the printed circuit board of the plug-in unit, and

- components (12, 13) attached to the printed circuit board of the power supply unit such that at least one component (100) attached to the printed circuit board of the plug-in unit is between the printed circuit board of the plug-in unit and the printed circuit board (1 1) of the power supply unit and at least one component (13) attached to the printed circuit board of the power supply unit is between the printed circuit board of the plug-in unit and the printed circuit board (1 1) of the power supply unit.

2. The plug-in unit of claim 1, characterized in that said printed circuit board (11) of the power supply unit is arranged so as to serve as a cooling board for said components attached to the printed circuit board of the power supply unit.

3. The plug-in unit of claim 1, characterized in that at least one of said components attached to the printed circuit board of the power supply unit is a cooling ele- ment.

4. The plug-in unit of claim 1, characterized in that all components of said power supply unit are attached to the power supply unit using surface mounting.

5. A power supply unit for a plug-in unit of a communications apparatus, characterized in that it comprises - a printed circuit board (11) which comprises more than one conductive layer containing electrically conductive patterns to realize conductive connections between components attached to the printed circuit board,

- at least one surface-mounted connector (20b) for attaching the power supply unit to the plug-in unit, and - a plurality of components (12, 13) surface mounted to said printed circuit board (11) to realize functions in the power supply unit.

6. The power supply unit of claim 5, characterized in that said printed circuit board (11) is arranged so as to serve as a cooling board for said components at- tached to the printed circuit board.

7. The power supply unit of claim 6, characterized in that said printed circuit board (1 1) comprises at least one metal layer considerably thicker than said conductive layers in order to improve thermal conductivity.

8. The power supply unit of claim 5, characterized in that at least one of said several surface-mounted components (12, 13) is a cooling element.

9. The power supply unit of claim 5, characterized in that all components (12, 13, 20b) of the power supply unit are attached to the printed circuit board of the power supply unit using surface mounting.

10. A communications equipment (110) comprising a plurality of plug-in units (30), characterized in that at least one connector (20, 20a) is mounted on the printed circuit board of at least one plug-in unit for the attachment of a power supply unit (10), and a power supply unit (10) is detachably attached to said at least one connector, and a plurality of components (100) are mounted on said printed circuit board such that at least one of said components is located between the printed cir- cuit board (30) and the power supply unit (10).

11. The communications equipment of claim 10, characterized in that said power supply unit comprises

- a printed circuit board (11) comprising more than one conductive layer containing electrically conductive patterns to realize conductive connections between compo- nents attached to the printed circuit board,

- at least one surface-mounted connector (20b) for attaching the power supply unit to the plug-in unit, and

- a plurality of components (12, 13) surface mounted to said printed circuit board (11) to realize functions in the power supply unit.

12. The communications equipment of claim 1 1, characterized in that said printed circuit board (1 1) of the power supply unit is arranged so as to serve as a cooling board for said components attached to the printed circuit board of the power supply unit.

13. The communications equipment of claim 11, characterized in that at least one of said components attached to the printed circuit board of the power supply unit is a cooling element.

14. The communications equipment of claim 11, characterized in that all compo- nents of the power supply unit are attached to the power supply unit using surface mounting.