FR2835389A1 - Car electric circuit parasitic protection having electrical circuit placed isolating material/isolating support and having additional capacitive connection connecting electrical circuit node. - Google Patents

Abstract

The electrical circuit has a support (1) and a layer of isolating material (3). The circuit has conductor tracks and electronic components. There is an additional capacitive connection connecting to a node (6,8) of the electrical circuit.

Description

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DEVICE COMPRISING AN INSULATED METAL SUPPORT AND A
ELECTRICAL CIRCUIT
The present invention relates to a device comprising an insulated metal support and an electrical circuit, and finds applications, in particular, in the automotive field.

 It relates more particularly to a device comprising an electrical circuit disposed on a support known as an insulated metal substrate, or SMI. The SMI supports consist of a metal soleplate, one face of which is substantially flat and is covered with a layer of insulating material, itself covered with a layer of copper. SMI supports are commonly used for making electrical circuits in which large electrical currents can flow, which can cause local heating. The metal soleplate, in addition to its role of mechanical support, can participate in the dissipation of the heat thus generated.

 The electrical circuit includes electronic components linked together by electrical connections. According to a technology for producing electrical circuits on SMI supports, the electronic components are surface mount components (or CMS), mounted on the face of the support SM 1 which carries the layer of copper. According to this technology, the connections are made in the form of conductive tracks obtained by etching the copper layer.

 In certain applications, high frequency currents may appear in electrical connections of the electrical circuit. By high frequency is meant here a frequency greater than the frequencies typical of the signals processed in the electrical circuit, hereinafter called operating frequencies of the electrical circuit. Electric currents corresponding to these high frequencies are therefore parasitic currents which can cause malfunctions of the electric circuit, or even damage to certain electronic components. This risk affects more particularly the electrical circuits intended to implement significant electrical powers, such as voltage generators,

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 voltage converters or voltage regulators.

 Such high frequency parasitic currents can have various origins. They can in particular be induced in the connections of the electrical circuit by electromagnetic radiation coming from a source external to the circuit. They may also correspond to harmonic frequencies of a fundamental frequency involved in the operation of the electrical circuit, in particular of a switching frequency in the case of a switching circuit.

 It is known, for certain applications, to connect one or more nodes of said electrical circuit by a short circuit to a metal frame ensuring the function of electrical ground. Such short-circuit links perform one or more functions known to those skilled in the art, such as a safety function, or even a function of defining an electrical potential reference common to several electrical circuits. It is also known that the node or nodes of the electrical circuit thus connected to the electrical ground must be chosen so as to prevent possible accumulations of electrical charges at particular points of the electrical circuit. Likewise, a connection between a node of the electrical circuit and the electrical ground is generally also adapted to avoid any accumulation of electrical charges at any point of this connection.

 However, this type of short-circuit connection of a node of an electrical circuit to a determined electrical mass provides only an imperfect solution and, in particular, does not provide a specific response to the problem of current circulation high frequency parasites mentioned above.

 An object of the present invention is to provide protection against malfunctions which can be caused by high frequency parasitic currents, for an electrical circuit produced on an SMI support.

 The present invention thus provides a device comprising a substantially planar support and an electrical circuit disposed on the support, the support comprising a metal support part and a layer of insulating material which covers at least a portion of one face of the support part. metallic, the electrical circuit comprising conductive tracks and

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 electronic components interconnected by the conductive tracks, the electronic components and the conductive tracks being arranged on a face of the layer of insulating material opposite to the metal support part. According to the invention, the device further comprises an additional essentially capacitive electrical connection connecting a determined node of the electrical circuit and the metal support part.

 The essentially capacitive electrical connection which is the subject of the invention is therefore arranged in addition to the components of said electrical circuit. It therefore does not participate, nor any of the electronic components which form part thereof, in the operation of the electrical circuit as such. On the other hand, thanks to this essentially capacitive electrical connection, the metal support serves as the electrical ground of the electrical circuit for high frequency signals only. It then helps protect against malfunctions that can be caused by high frequency stray currents.

 By essentially capacitive electrical connection is meant an electrical connection whose electrical behavior, for the operating frequencies of the electrical circuit, is comparable to that of a capacitor.

 In terms of complex electrical impedance of the connection, the mathematical formalism of which is known to those skilled in the art, this means that the electrical impedance of this connection has a negative imaginary component which is much higher, in module, than the component real of this same electrical impedance, for the operating frequencies of said electrical circuit. Of course, this is not the case for short-circuit links known in the state of the art.

 An advantage of the present invention lies in the fact that the protection means used can be of particularly reduced cost compared to the price of the overall device.

 To obtain more effective protection, the electrical device can comprise several additional essentially capacitive electrical connections respectively connecting several specific nodes of the electrical circuit and the metal support part.

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 The effectiveness of the protection against malfunctions which can be caused by high frequency currents can also be increased by also using said metal support part as the electrical mass of the device. At least one short-circuit connection then connects a determined node of the electrical circuit to the metal support part, in addition to the essentially capacitive electrical connection (s) object (s) of the invention .

 The determined nodes of the electrical circuit thus connected to the metal support part can also be connected to one another by one or more electrical connections belonging to the electrical circuit. In this case, these electrical connections, the essentially capacitive electrical connections and the metal support part constitute loops for the flow of high frequency parasitic currents. These parasitic currents, or at least part of these currents, no longer pass through the electronic components of the electrical circuit, which limits the effect of these currents on these electrical components in particular, and on the electrical circuit in general.

 Optionally, two essentially capacitive links can connect the same determined node of the electrical circuit to the metal support part. They then constitute two different paths for the flow of high frequency parasitic currents, which further improves the protection obtained.

 In certain cases, specific nodes of the electrical circuit connected to the metal support part may belong to parts of conductive tracks of the electric circuit which have between them a non-zero voltage. In this case, the impedance of the essentially capacitive links between these nodes and the metal support part is adapted so as not to disturb the operation of the electrical circuit.

 In particular embodiments of the device of the invention, one can optionally also have recourse to one and / or the other of the following provisions: each additional essentially capacitive electrical connection comprises a metal screw, one end of which s '' extends with contact

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- le condensateur a une capacité comprise entre 5 et 2000 picofarads ; - le dispositif comprend en outre au moins une pièce métallique rapportée, qui fait partie d'une liaison électrique additionnelle essentiellement capacitive entre un noeud déterminé du circuit électrique et la partie de support métallique ; - ladite pièce métallique rapportée fait partie de plusieurs liaisons électriques additionnelles essentiellement capacitives entre des noeuds déterminés respectifs du circuit électrique et la partie de support métallique ; - ladite pièce métallique rapportée appuie d'une part sur un noeud déterminé du circuit électrique et d'autre part sur une première piste conductrice, cette première piste conductrice étant reliée électriquement par l'intermédiaire d'un condensateur à une seconde piste conductrice sur laquelle appuie une vis métallique dont une extrémité s'étend avec contact électrique dans la partie de support métallique, à travers la couche de matériau isolant ; - ladite pièce métallique rapportée est disposée en élévation du côté de certains au moins des composants électroniques et des pistes conductrices du circuit électrique opposé à la couche de matériau isolant ; - la partie de support métallique est constituée d'un métal choisi parmi l'aluminium, le cuivre, le fer et un alliage comprenant au moins l'un de ces deux métaux ; - la couche de matériau isolant est constituée d'un matériau choisi parmi une résine, un verre, du mica et un matériau synthétique organique. electric in the metal support part, through the layer of insulating material; each additional essentially capacitive electrical connection comprises at least one capacitor; - The capacitor is electrically connected on the one hand to a determined node of the electrical circuit, and on the other hand to a conductive track on which the metal screw bears; - the capacitor is a ceramic capacitor with surface mounting;

- the capacitor has a capacity between 5 and 2000 picofarads; - The device further comprises at least one attached metal part, which is part of an additional essentially capacitive electrical connection between a determined node of the electrical circuit and the metal support part; - Said attached metal part is part of several additional essentially capacitive electrical connections between respective determined nodes of the electrical circuit and the metal support part; - Said metal insert bears on the one hand on a determined node of the electrical circuit and on the other hand on a first conductive track, this first conductive track being electrically connected by means of a capacitor to a second conductive track on which presses a metal screw, one end of which extends with electrical contact into the metal support part, through the layer of insulating material; - Said attached metal part is arranged in elevation on the side of at least some of the electronic components and conductive tracks of the electrical circuit opposite to the layer of insulating material; - The metal support part consists of a metal chosen from aluminum, copper, iron and an alloy comprising at least one of these two metals; the layer of insulating material consists of a material chosen from a resin, a glass, mica and an organic synthetic material.

 In an embodiment comprising an attached metal part, it may be particularly advantageous to bring as close as possible

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 possible this part of some of the conductive tracks of the electrical circuit, without however being in contact with them, so as to limit the sensitivity of the electrical circuit to external electromagnetic radiation. The attached metal part can still play a role, in combination with the metal support part, of an electromagnetic screen, according to the principle of a Faraday cage. This also makes it possible to reduce the electromagnetic radiation emitted by the electrical circuit itself, and to meet standardized requirements for electromagnetic compatibility (EMC).

 The electrical circuit of the device of the invention can be arbitrary.

In particular, it is in one example a switching circuit. This switching circuit can be a voltage converter of one of the following types: voltage step-down (buck), voltage step-up (boost), or mixed step-down step-up (buck-boost). Such circuits use significant electrical powers, and are the seat of the generation of high frequency spurious currents, in particular at harmonic frequencies of the switching frequency.

 Other particularities and advantages of the present invention will appear in the description below of nonlimiting exemplary embodiments, with reference to the appended drawings, in which: - Figure 1 is a perspective view of an example of SMI support ; - Figure 2 is an electrical diagram of a step-down converter; - Figure 3a is a top view of an example of installation of the voltage converter of Figure 2 on an SMI support as illustrated in Figure 1; - Figure 3b is a top view of the layout of Figure 3a equipped with electrical connections according to the present invention; - Figures 4a and 4b are sectional views showing two configurations of electrical connections according to the invention between a node of the voltage converter of Figure 2 and the sole of an SMI support on which is placed the converter.

 In the different figures, the same elements bear

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 identical references.

 A blank SMI support, that is to say before the production of conductive tracks and before the implantation of electronic components, is shown in FIG. 1. It comprises a metallic sole 1, having the shape of a plate, for example in aluminum. One of its faces, hereinafter designated by upper face, with reference to the orientation of Figure 1, is substantially planar.

This upper face is covered with a layer of electrically insulating material 3, for example mica, of substantially uniform thickness, bonded to this upper face. This insulating layer 3 is itself covered with a copper sheet 4, intended to be etched to form conductive tracks. The upper face F of the SMI support therefore corresponds, depending on the locations on this face, to the upper face of the copper foil 4 or to that of the insulating layer 3. The conductive tracks are designed to constitute electrical connections between electronic components reported on the upper face F of the support SM !. Such SMI supports are known to those skilled in the art, as are the details of their constitution and their use.

 Another face of the sole 1, hereinafter designated by the underside with reference to the orientation of FIG. 1, may include fins 2 so as to form a radiator for cooling the entire SMI support and the circuit electric carried by the support.

 FIG. 2 represents the diagram of a step-down converter with voltage step-up. In this figure, the letters D, S and G respectively designate the drain, the source and the gate of the transistors to which they relate. The converter shown has a first pair of positive 5 and negative terminals 6, as well as a second pair of positive 7 and negative terminals 8. A high voltage direct current network is connected to the positive terminal 5 and to the negative terminal 6 The voltage between terminals 5 and 6 is for example of the order of 42 volts. A low voltage direct current network is connected to terminals 7 and 8, terminal 7 being positive and terminal 8 being negative, and the voltage between these two terminals being for example of the order of 14 volts.

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 This voltage step-down converter is made up of two cells 10,20 identical to each other, each corresponding to an elementary step-up converter.

These two elementary converters are arranged in parallel between, on the one hand, the terminals 5 and 6, and on the other hand, the terminals 7 and 8. The terminals 6 and 8 are directly interconnected.

 In each of these elementary converters 10,20, a capacitor 17,27, for example of 30 microfarads, connects the terminals 5 and 6 in order to store, on the side of the 42-volt network, the electric charges transferred by this elementary converter 10, 20. Similarly, in each elementary converter 10,20, another capacitor 18,28, for example also of 30 microfarads, connects the terminals 7 and 8 in order to store, on the side of the 14-volt network, the electric charges transferred by this converter elementary 10.20. Each of the two 42-volt and 14-volt networks, for its operation, partially consumes the electric charges stored in the sets of capacitors 17.27 and 18.28 respectively.

 In each elementary converter 10,20, the terminals 5 and 7 are connected together by a branch comprising the following components, connected in series, listed in the order considered from terminal 5 to terminal 7: - a step-down transistor 11, 21, for example n-channel MOS-FET (NMOS), connected by its drain D to terminal 5. Such a transistor comprises a structure diode 12,22 placed in parallel with the switch constituted by the step-down transistor 11,21 , and oriented in the passing direction from terminal 7 to terminal 5; - an inductor 13,23, for example of 12 microhenrys and of resistance 6 milliohms, connected to the source S of the step-down transistor 11,21 by an intermediate node N 1, N2; - a shunt resistor 14.24, for example of 2 milliohms, connected to the inductor 13.23 and to terminal 7.

 Each step-down step-up converter 10, 20 also includes a step-up transistor 15.25, for example MOS-FET with

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 channel n (N-MOS), connected by its drain D to the intermediate node N1, N2 and by its source S at the negative terminals 6 and 8. Such a transistor also includes a structure diode 16,26 placed in parallel with the switch constituted by the step-up transistor 15.25, and oriented in the passing direction from the negative terminals 6.8 to the intermediate node N1, N2.

 Each of the elementary step-down step-up converters 10,20 thus formed can transfer approximately 250 watts of power between the 42-volt network and the 14-volt network. For this, the lowering and raising transistors 11, 21 and 15, 25 of each converter 10, 20 are controlled by a controller, not shown, by means of opening or closing signals transmitted to their respective gates. According to a Pulse Width Modulation (PWM) control mode known to those skilled in the art, this controller controls the lowering and raising transistors 11, 21 and 15, 25 according to a common pulse frequency, corresponding for example at a frequency of 70 kilohertz. Advantageously, the pulses intended for each elementary converter 10, 20 are offset by a delay equal to half the period of the control pulses of each elementary converter.

 In a step-down mode of operation, the controller controls the gate G of the step-down transistor 11, 21 of each elementary converter 10,20 to alternately open or close this step-down transistor 11,21 according to the value of the current in the resistor shunt 14.24 of this same elementary converter. It simultaneously controls the gate G of the step-up transistor 15.25 of this elementary converter so that it is open at least during the time intervals during which the step-down transistor 11.21 is closed.

 Symmetrically, in a voltage booster operating mode, the controller controls the gate G of the booster transistor 15.25 of each elementary converter 10.20 to open or close it alternately depending on the value of the current in the 14.24 shunt resistor of this same elementary converter. It then simultaneously controls the gate G of the step-down transistor 11.21 of this elementary converter so that it is open at least during the time intervals during

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 which the step-up transistor 15.25 is closed.

 In alternative embodiments of this voltage converter circuit, the transistors 11, 21, 15, 25 of the N-MOS type can be replaced by homologous transistors of the P-MOS type. They can also be replaced by transistors of bipolar technology, without changing the function, the general operation of the converter, or the implementation of the invention as described below.

 An example of installation of the electronic components of the step-down converter of FIG. 2 on the SMI support of FIG. 1 is shown in FIG. 3a. FIG. 3a is an elevation view of the upper face F of the SMI support equipped with its components.

This face F carries copper zones obtained from the copper layer 4. These copper zones form conductive tracks which respectively constitute the terminals 5, 6 and 8 together, and 7, the intermediate nodes N1 and N2, and zones of contact 30,34, 36,37 to which we will return later. Outside these copper-colored zones, the copper layer 4 has been removed by an etching process known per se, and the face F then corresponds to the surface of the insulating material 3. The transistors 11,21, 15,25, the capacitors 17,27,
18,28, the shunt resistors 14,24 as well as the inductors 13,23 are represented diagrammatically compared to their real forms. The inductors 13, 23, which are components of cylindrical shape implanted so that their main axis is perpendicular to the plane of the face F, are here symbolically indicated by circles corresponding to their contours only. This allows the underlying conductive tracks to appear.

 Fixing supports 32, 33, 35 are respectively arranged at the level of the conductive track corresponding to the terminals 6 and 8 joined together and at the level of the two contact zones 34 and 36. The contact zones 30 and 37 come to the right of holes 31 and 38 respectively which, through the insulating layer 3, extend into the metal sole 1. These are, for example, blind holes which are tapped to receive a screw.

 Figure 3b reproduces Figure 3a and further shows the elements

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 added to the device for making the additional essentially capacitive electrical connections of the invention. A first additional essentially capacitive electrical connection is established between the conductive track corresponding to terminals 6 and 8 joined together and the metal soleplate 1.

It comprises a capacitor 41 disposed between the track constituting the combined terminals 6 and 8 and the contact zone 30, and a metal screw 42 screwed into the tapped hole 31 so as to press on the contact zone 30. The metal screw 42 is visible in Figure 4a which shows this first electrical connection in section along a plane perpendicular to the face F.

1/ (Cliaison. Ffonctionnement) > > 2. TT. Zcaract > > 1/ (Cliaison. FHF) OÙ Chanson est la capacité du condensateur 41, Fonctionnement est la fréquence de fonctionnement du circuit électrique considéré, à savoir, dans le cas présent, la fréquence de découpage du convertisseur de tension, Zcaract est le module d'une impédance caractéristique du circuit électrique, et FHF est la fréquence minimale des courants parasites à haute fréquence contre lesquels une protection est recherchée. The capacitor 41 must be chosen so as to satisfy the following double digital inequality:

1 / (Linkage. Operation)>> 2. TT. Zcaract>> 1 / (Cliaison. FHF) WHERE Song is the capacitance of the capacitor 41, Operation is the operating frequency of the electrical circuit considered, namely, in this case, the switching frequency of the voltage converter, Zcaract is the modulus of a characteristic electrical circuit impedance, and FHF is the minimum frequency of high frequency spurious currents against which protection is sought.

 For the switching voltage converter in Figure 2, Operation = 70 kilohertz and FHF = 1 gigahertz. The impedance corresponding to each inductor 13,23 constitutes an impedance characteristic of the electrical circuit. So, in a manner known per se, for the previously mentioned numerical values: Zcaract = (12 microhenrys). 2. TT. Operation. A Song capacity = 20 picofarads can therefore be chosen for capacitor 41.

 Second and third additional essentially capacitive electrical connections still connect the conductive track corresponding to terminals 6 and 8 together and the metal sole 1. An attached metal part 43, for example in the shape of a U upside down, is placed above certain electronic components and part of the conductive tracks of the electrical circuit. It is fixed at the level of the fixing supports 32, 33, 35 so as to be in electrical contact with the zones

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 copper 6.8, 34.36 which respectively surround the fixing supports. The second additional electrical connection comprises this part 43, a capacitor 45 disposed between the contact zones 34 and 37, and by a screw 44 pressing on the contact zone 37 and screwed into the threaded hole 38 formed in the metal soleplate 1. The FIG. 4b represents this additional electrical connection along a vertical plane perpendicular to the face F. The third additional electrical connection, of structure similar to that of the second additional electrical connection, also comprises the part 43 and the screw 44, as well as a capacitor 46 disposed between the contact zones 36 and 37. The capacitors 45 and 46 must also satisfy the double digital inequality mentioned above for the capacitor 41, and may also each have a capacity of 20 picofarads, for example.

 Advantageously, the part 43 has a shape adapted to be close, without however being in electrical contact with them, conductive tracks and components capable of generating electromagnetic radiation, or of being disturbed by parasitic radiation external to the circuit. In the example shown in FIG. 3a, it thus partially covers the step-up transistor 25 and the track corresponding to the intermediate node N2. This precaution partially alleviates the fact that the SMI supports only have one level of conductive tracks, and not two levels which would allow superimposing connections corresponding to a forward direction and a return direction of the same electric currents.

 The various elements of the second and third essentially capacitive electrical connections described above operate in the following order within these electrical connections, starting from the conductive track corresponding to terminals 6 and 8 together, up to the metal soleplate 1: the part 43, the contact area 34, respectively 36, the capacitor 45, respectively 46, the contact area 37, and the screw 44. Depending on the layout constraints of the various components on the SMI support, this order can be modified without that the role of the electrical connections concerned be changed. Another possible order of the different elements within an essentially capacitive electrical connection according to the invention is also listed again, starting from the conductive track corresponding to the terminals 6.

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 and 8 together, up to the metal sole 1: a capacitor connected to the conductive track corresponding to terminals 6 and 8 together, a first contact zone to which the capacitor is connected, an attached metal part similar to part 43 and which presses on this first contact zone, a second contact zone on which the metal insert also presses, and a screw making electrical contact between the second contact zone and the metal soleplate.

 The same embodiments of additional essentially capacitive electrical connections as those described previously in the case of a step-down converter can be used identically in the case of a step-down converter or in the case of a converter. voltage booster. The production of these additional electrical connections is independent of the nature of the electrical circuit produced on an SMI support, and can therefore be implemented in a similar manner for any electrical circuit.

 In the example presented in FIGS. 3a, 3b, 4a and 4b, the node of the electrical circuit connected to the metal soleplate 1 of the SMI support corresponds to the electrical mass of this circuit, materialized by the terminals 6 and 8 together. In other devices comprising electrical circuits produced on SMI supports and additional essentially capacitive electrical connections according to the present invention, these connections can connect to the metal soleplate of the SMI support connections of the electrical circuit which do not necessarily correspond to a terminal electrical ground, using appropriate capacitors within these links. These capacitors must in particular be chosen so as to respect digital inequalities similar to those introduced in the preceding description. Certain devices produced according to the present invention may also include first additional essentially capacitive electrical connections which connect an electrical ground terminal of the electrical circuit of this device to the sole of the SMI support, and second additional essentially capacitive electrical connections which connect to this same sole of the nodes of the same electrical circuit not corresponding to the electrical ground, such as the intermediate nodes N1 and N2 of the example

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 described above. For this, the value of the capacitors of these links must be suitably chosen in relation to the operating frequencies of the circuit.

Claims (15)

CLAIMS 1. Device comprising a substantially flat support and an electrical circuit arranged on the support, the support comprising a metal support part (1) and a layer of insulating material (3) which covers at least a portion of one face of the metal support part (1), the electrical circuit comprising conductive tracks and electronic components connected together by the conductive tracks, the electronic components and the conductive tracks being arranged on one side of the layer of insulating material (3) opposite to the metal support part (1), the device further comprising an additional essentially capacitive electrical connection connecting a determined node (6,8) of the electrical circuit and the metal support part (1). 2. Electrical device according to claim 1, comprising several additional essentially capacitive electrical connections respectively connecting several specific nodes of the electrical circuit and the metal support part (1). 3. Device according to claim 1 or claim 2, wherein each additional essentially capacitive electrical connection comprises a metal screw (42,44), one end of which extends with electrical contact in the metal support part (1), through the layer of insulating material (3). 4. Device according to any one of the preceding claims, in which each additional essentially capacitive electrical connection comprises at least one capacitor (41,45,46).  5. Device according to claim 3 and claim 4, wherein the capacitor (41) is electrically connected on the one hand to a determined node (6,8) of the electrical circuit, and on the other hand to a conductive track (30 ) on which the metal screw (42) rests.  6. Device according to claim 4, in which the capacitor (41, 45.46) is a surface mount ceramic capacitor. <Desc / Clms Page number 16> 7. Device according to any one of claims 4 to 6, wherein the capacitor (41,45, 46) has a capacity between 5 and 2000 picofarads. 8. Device according to any one of the preceding claims, further comprising at least one attached metal part (43), which is part of an additional essentially capacitive electrical connection between a determined node (6,8) of the electrical circuit and the metal support part (1). 9. Device according to claim 8, wherein said metal insert (43) is part of several additional essentially capacitive electrical connections between respective determined nodes (6,8) of the electrical circuit and the metal support part (1). 10. Device according to claim 8 or claim 9, wherein said metal insert (43) rests on the one hand on a determined node (6,8) of the electrical circuit and on the other hand on a first conductive track (34 , 36), this first conductive track (34,36) being electrically connected via a capacitor (45,46) to a second conductive track (37) on which a metal screw (44) rests, one end of which s 'extends with electrical contact in the metal support part (1), through the layer of insulating material (3).  11. Device according to any one of claims 8 to 10, in which said metal insert (43) is arranged in elevation on the side of at least some of the electronic components and conductive tracks of the electrical circuit opposite to the layer of insulating material. (3).  12. Device according to any one of the preceding claims, in which the electrical circuit is a switching circuit.  13. Device according to claim 12, in which the switching circuit comprises a step-down step-up circuit.  14. Device according to any one of the preceding claims, in which the metal support part (1) consists of a metal chosen from aluminum, copper, iron and an alloy comprising at least one of <Desc / Clms Page number 17>  these two metals. 15. Device according to any one of the preceding claims, in which the layer of insulating material (3) consists of a material chosen from a resin, a glass, mica and an organic synthetic material.