Patent Publication Number: US-10770220-B2

Title: Planar transformer layer, assembly of layers for planar transformer, and planar transformer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to foreign European patent application No. EP 16306215.1, filed on Sep. 22, 2016, the disclosures of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a planar transformer layer, an assembly of layers for planar transformer, and a planar transformer. 
     BACKGROUND 
     Planar transformers are known whose power is limited to 2500 W at 300V, or to 1400 W at 2 kV. 
     The limiting of the power handled by a transformer involves using two to three converters each using a transformer in order to achieve a total power of 5 kW. A transformer capable of transferring 5 kW makes it possible to save on one to two converters. 
     The existing solutions are limited in power by:
         the effects of proximity in the transformer limit either the usage frequency or the accessible copper section;   the thermal resistance of the transformer limits the power which can be dissipated in the transformer;   the high output voltage entails a significant electrical insulation which is accompanied by an increase in thermal resistance; and   the interleaving of the secondary and primary windings makes it possible to increase the frequency without reducing the copper section but also entails an increase in the electrical insulation layers which entails an increase in thermal resistance.       

       FIG. 1  illustrates a planar transformer according to the prior art. The right-hand part of  FIG. 1  shows the materials, and the left-hand part shows the heat fluxes. 
     Stacked individual windings  1 , in this case three of them, are made up of several layers of copper  2 , in this case two of them. These layers of copper or electrical conductors  2  are electrically insulated from one another by an insulator or dielectric  3 . An insulating layer or dielectric layer is disposed between each of the individual windings  1 , and between the individual winding  1  at the base of the stack and a cold source on which the stack of individual windings is disposed. 
     Cooling such a transformer through the magnetic core requires the heat dissipated in the conductors to pass through the dielectric layers which insulate the electrical conductors from one another and which insulate the conductors from the magnetic core. Since the dielectric materials are generally poor thermal conductors, the thermal resistance between the hot point of the conductors and the magnetic core is high (the thermal resistances of each dielectric layer are connected in series from the hot point to the magnetic core). Furthermore, since the magnetic core is also a source of heat dissipation, it does not represent a good cold source. 
     The use of the electrical connections as cold source makes it possible to cool the electrical conductors without passing through the series of dielectric layers. When the transformer is connected to a busbar, the heat can be removed by convection. When convection is not possible, the busbar is itself electrically insulated and does not therefore represent a good cold source. 
     An increase of the output voltage of such a transformer would entail increasing the thickness of insulation and consequently increasing the thermal resistance. The increase in thermal resistance would entail reducing the power transferrable through the transformer. To maintain the transferred power, it would be necessary to increase the volume and the weight of the transformer which would pose problems of resistance to the thermomechanical environment, which would lead an acceptable limit in terms of the weight and the volume of the current designs to be exceeded. Doubling the transferred power is therefore inconceivable with the known embodiments. 
     Furthermore, such a transformer has to operate in a vacuum which prevents the cooling by convection. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to produce a transformer for transmitting an electrical power of at least 5 kW with a galvanic insulation under an output voltage of 300 V to 2 kV in order to power an ion thruster for satellite or space probe. 
     There is proposed, according to one aspect of the invention, a planar transformer layer comprising distinct electrical connections and thermal connections. 
     Thus, it is possible to significantly improve the discharging of thermal energy, and produce a planar transformer capable of transmitting an electrical power of at least 5 kW with a galvanic insulation under an output voltage of 300 V to 2 kV in order to power an ion thruster for satellite or space probe. 
     In one embodiment, a thermal connection comprises a hole. 
     Such a hole allows an element such as a screw to hold a plurality of layers together. 
     According to one embodiment, such a hole comprises an extension towards the interior of the layer. 
     Such an extension towards the interior of the layer makes it possible to maximize the exchange surface between the layer and the heat sink. 
     As a variant, a thermal connection can be comb-shaped. 
     Thus, the exchange surface between the layer and the heat sink is increased. 
     According to another aspect of the invention, there is also proposed an assembly of layers for planar transformer, comprising at least one primary planar transformer layer as previously described, and two secondary planar transformer layers without distinct electrical and thermal connections, the three layers being separated and covered by a dielectric material, except for the thermal connection or connections of the planar transformer layer as previously described. 
     Such an assembly of layers offers a minimal thermal path between the secondary layers and the primary layer, the assembly being thermally drained by the access from the primary layer to the heat sink. This assembly is particularly advantageous when the electrical insulation between secondary layers and heat sink is difficult to guarantee. 
     According to another aspect of the invention, there is also proposed a planar transformer comprising at least one assembly as previously described. 
     In one embodiment, a transformer comprises a plurality of assemblies stacked one on top of the other, in which the thermal connections of the primary layers are connected to a heat sink. 
     Thus, each assembly is individually drained. The assembly of the layers of the transformer is cooled by as many connections to the heat sink in parallel which improves the draining compared to a series connection. 
     According to one embodiment, the heat sink comprises a cold source and a dielectric part. 
     Thus, the dielectric part ensures the electrical insulation between the heat sink and the layers. By placing in the heat sink the layers requiring the lowest dielectric withstand strengths in relation to the heat sink, the choice of the dielectric is widened, authorizing the optimization of the thermal conductivity, and the thickness of dielectric separating the layer and heat sink can be minimized to maximize the thermal conductivity between layer and sink. 
     In one embodiment, the cold source is disposed on the outer part of the heat sink, surrounding the dielectric part. 
     According to one embodiment, the planar transformer further comprises a magnetic core and an associated fixing element. 
     Also proposed, according to another aspect of the invention, is an electronic energy conversion equipment item for satellite provided with at least one planar transformer as previously described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on studying a few embodiments described as nonlimiting examples and illustrated by the attached drawings in which: 
         FIG. 1  schematically illustrates a planar transformer according to the prior art; 
         FIG. 2  schematically illustrates a planar transformer according to one aspect of the invention; 
         FIGS. 3 and 4  schematically illustrate a planar transformer layer according to two aspects of the invention; 
         FIGS. 5 to 11  schematically illustrate an embodiment of a transformer according to one aspect of the invention. 
     
    
    
     In the different figures, the elements that have the same references are identical. 
     DETAILED DESCRIPTION 
       FIG. 2  represents a planar transformer according to one aspect of the invention, in which an individual winding  6  comprise one or more layers of copper  7  of which at least one  7   a  performs the thermal function. These layers of copper  7  are electrically insulated for example by a dielectric insulation  8 . In this particular case an individual winding or individual assembly  6  comprises, for example, a layer  7   a  performing the thermal function, and two others  7   b , conventional, not performing it. 
     The left-hand part of  FIG. 2  represents, by arrows, the diffusion of the thermal energy in the planar transformer by the layers  7   a , of which a part is surrounded by a dielectric  9  in proximity to a cold source  10 . Thus, a continuous thermal path, or heat sink, is created between the windings  6  and the cold source  10 . The thermal efficiency of the cold source  10  plays an important role in obtaining the final efficiency of the transformer. 
     The reduction of thermal resistance of the electrical conductors of the transformer makes it possible to significantly increase (more than double) the transferred power, despite an electrical output voltage multiplied by five, without increasing the volume occupied by the transformer. 
       FIG. 3  shows a planar transformer layer  7   a  comprising distinct electrical connections  12  and thermal connections  13 . 
     The thermal connections  13 , in this case four of them per layer  7   a , comprise a hole  14 , making it possible to fixedly hold together a plurality of layers  7   a.    
     For example, the holes  14  of the thermal connections  13  can comprise an extension  14   a  towards the interior of the layer  7   a . These extensions  14   a  make it possible to locally maximize the heat flux towards the cold source to do so given the constraint of a mechanical fixing of the transformer by means of screws. 
     As a variant, as illustrated in  FIG. 4 , the thermal connections can be comb-shaped, and thus without holes, which makes it possible to adapt to another transformer fixing means. 
     Any other type of distinct thermal connection can of course be envisaged, regardless of its shape, that makes it possible, by means of another element, to fixedly link a stacking of layers or of assemblies of layers. 
     Hereinafter in the description, in a nonlimiting manner, only thermal links  13  with holes  14  will be described. 
     The rest of the description illustrates an exemplary embodiment of the invention. 
     The winding production technology is based on flexible circuits made up of an electrical circuit on a layer encapsulated between two flexible insulation layers. 
     The windings produced are then stacked. 
     As illustrated in  FIG. 5 , in order to easily perform the assembly of a transformer, it is possible to produce an assembly comprising, for example, a planar transformer layer  7   a  comprising distinct electrical connections  12  and thermal connections  13  and two conventional planar transformer layers  7   b , directly by the manufacturer of the circuit in order to obtain an individual winding or assembly of layers. 
       FIG. 6  represents a stack of a plurality of assemblies of layers according to  FIG. 5 , which constitutes the assembly of the windings of the transformer according to an aspect of the invention. 
     In order to drain the heat flux leaving the primary turns or, in other words, the turns or layers  7   a , it is necessary to create a continuous path to the flat base of the transformer. 
     The assembly of the transformer is performed as follows. 
     As illustrated in  FIG. 7 , after having stacked assemblies of individual layers or windings  6  on a rig, the four heat-sinking placements, here disposed in proximity to the corners, are closed by means of capping pieces made of aluminium  16  and a comb of dielectric material  17 . These pieces  16  and  17  play a role of sealing and reproducibility of the stacking. Once this operation is finished, the feet of the transformer which extend the exchange to the cold plate or cold source  10 , are slipped. In effect, in the proposed assembly, there is a break in the link between the transformer and the cold source. More generally, this function could directly form part of the cold source which would have the effect of further improving the thermal efficiencies. 
     Next, as illustrated in  FIG. 8 , the four feet  16 ,  17  of a dielectric resin  18  have a good thermal conductivity. The design takes into account the voltages involved between the individual windings  6  in order to guarantee the electrical insulation. 
     Finally, ferrite cores  19  (magnetic cores) are placed around the winding made up of the stacking of the individual windings  6 . The present transformer proposes completely decoupling the heat flux from the losses by the copper  6  and from the losses by the irons  19 . Consequently, the ferrites  19  are held mechanically by a piece  20 , for example made of aluminium, also serving as a heat sink to the flat base. 
       FIG. 10  shows the cutting plane of  FIG. 8  to obtain the cross-sectional view of  FIG. 11 .