Transformers

Abstract: A planar intrinsically safe transformer comprises a single, bonded, multilayer printed circuit board comprising a plurality of layers (20a, 20b, 20c) of insulating material each carrying conductor tracks (TL2, TL3; TL4, TL5; TL6, TL7) on at least one side, an additional thin layer (22a, 22b, 22c, 22d) of insulating material between each pair of adjacent conductor tracks, and a magnetic core (12). No conductor track (TL3 to TL6) buried between two of the thicker layers (20a, 20b, 20c) extends to within a predetermined distance (t) from the edge of the board, and each of the thicker layers (20a, 20b, 20c) which carries tracks on both sides is at least as thick as said predetermined distance (t).

Description: This invention relates to transformers and is particularly concerned with planar intrinsically safe transformers. Intrinsically safe transformers must comply with various standard which are set down for intrinsically safe equipment, in order to guarantee the integrity of the isolation between the electronic circuits. En 50 020 is such a standard and it defines a need for a minimum of lmm of solid insulation between individual circuits and therefore between the windings of a transformer which straddle the circuits. There is also a need to maintain 6mm clearance through air, and 10mm over surfaces between the exposed conductors of circuits. EP-A-0267108 describes a miniaturised transformer in which a plurality of substrate boards are stacked on top of each other, with conductors on the boards, and with the boards separated from each other by insulator layers. The conductors are arranged in the shape of a spiral so as to serve as the turns of the transformer windings. However, in this known arrangement, the substrate boards and the insulation layers are all very thin and the aim is to maximise magnetic coupling. GB-A-2087656 describes another miniaturised planar transformer using an E-core format with spiral windings printed on a ceramic printed circuit board. Two or more of these stacked inside the ferrite constitute the transformer. The aim here is a rugged, compact construction. It is not concerned with creepage and clearance distances with safety considerations in mind. EP-A-0608127 describes a transformer where the winding means consists of a number of individual annular turns deposited on a flexible polyimide substrate and then stacked to form the whole winding. EP-A-0542445 describes a flat transformer having planar windings of which at least one is contained in an electrically-insulating flat box sealed everywhere except at the exit points for the lead-out wires. It is designed particularly for switched mode power supplies. WO88/09042 describes a high coupling transformer with a plurality of layers of printed circuits which carry conducting tracks. US-A-5321380 describes a low-profile printed circuit board including a substrate having at least one planar magnetic element supported thereon. A dielectric coating isolates the winding of the planar magnetic element. It is an object of the present invention to provide a planar transformer which does away with the need for conventional wire windings and bobbins, bobbins which in the case of an intrinsically safe design would have to be special. It is a further object of the invention to provide a planar transformer which uses a multilayer printed circuit board (PCB) to form a one-piece winding assembly. The transformer in accordance with the present invention requires very simple constructional techniques, thus making it easy to manufacture and avoiding the need for winding consolidation. The design also optimises the core ferrite cross-section and therefore the magnetic properties of the transformer. In accordance with the present invention there is provided a planar, intrinsically safe transformer as claimed in claim 1. The multilayer PCB itself consists of bonded layers of conductor tracks and insulators. If the insulators are made of standard PCB material, e.g. fibreglass, of sufficient thickness, then one can achieve the necessary standard for the isolation of the windings and also satisfy the criteria for the transformer to be intrinsically safe. The outermost layer of each face of the bonded assembly may comprise a conductor track or may be a layer of insulating material. The said first-mentioned layers are preferably at least lmm thick. Preferably, none of the conductor tracks extends to within lmm of the edge of the multilayer printed circuit board. The invention will now be described in more detail with reference to a number of embodiments in accordance with the invention which are given by way of example and which are shown in the accompanying drawings, in which: Fig. 1 is an exploded view of the essential parts of a transformer in accordance with the invention, using a low profile RM ferrite core; Fig. 2 is a cross-sectional view through an embodiment of transformer in accordance with the invention; Fig. 3 is a schematic sectional view through another embodiment of transformer in accordance with the invention; Fig. 4 is a cross-section through an embodiment of PCB for use in a transformer in accordance with the invention; Fig. 5 is a plan view of the PCB of Fig. 4; Fig. 6 is a view, on an enlarged scale, of the PCB of Figs. 4 and 5, showing a track layer and also the core; Fig. 7 is a cross-section through another embodiment of PCB for use in a transformer in accordance with the invention; Fig. 8 is a plan view of the PCB of Fig. 7; Fig. 9 is a view, on an enlarged scale, of the PCB of Figs. 7 and 8, showing a track layer and the associated RM core; Fig. 10 shows a further embodiment of transformer in accordance with the invention, together with a schematic circuit diagram; and, Fig. 11 shows yet a further embodiment of transformer, again with its circuit diagram. Referring first to Fig. 1, this shows the general construction of the planar transformer. The design is based upon the use of a multilayer printed circuit board, indicated generally at 10, and low profile RM ferrite cores, indicated generally at 12. The PCB is made as a multilayer product, with more than one layer giving more than one winding and hence forming a transformer. Unlike most planar transformers which use reduced height "E" cores, the transformers of the present invention use low profile RM cores. This style of core maintains a large winding window, sufficient to make it 3 port (i.e. 3 isolated windings) with the full 1mm circuit separation and printed circuit windings. Also, the RM core has two large openings in the ferrite which encloses the windings, through which the winds of the isolated circuits can be brought while still maintaining the necessary circuit segregation. Each of the core pieces 12 has a cylindrical stub 14 which extends into a hole 16 formed centrally through the multilayer PCB 10. The core pieces 12 are held in place in relation to the PCB 10 by a pair of securing members 18 which fit around the core pieces and snap into place thereon. Referring now to Fig. 2, this shows a section through an 8-track layer PCB set within the ferrite core 12. The PCB is here made up of three lmm thick layers of insulator 20a, 20b, 20c. Between each pair of these insulator layers and on the outer face of the outer layers 20a and 20c there is provided a thin insulator 22a, 22b, 22c, 22d. These thin layers are kept as thin as possible. Eight track layers TL1 to TL8, indicated by broken lines in Fig. 2, are provided. The track layers TL1, TL2 and TL7, TL8 arranged on each side of the thin insulators 22a and 22d respectively constitute a split primary, for low leakage inductance. The track layers TL3 to TL6 associated with the thin insulators 22b and 22c constitute buried secondaries. In the case of these buried track layers TL3 to TL6, no track extends to within a predetermined distance from the edge of the board, here 1mm from the edge of the PCB, in order to give the necessary segregation. This predetermined distance is indicated as t in Fig. 2. The tracks TL3 to TL6 forming the winding can extend up to 1mm from the edge of the PCB provided that the layers of insulation are sufficiently bonded. The relatively thick insulator layers 20a, 20b and 20c, which carry conductor tracks on both sides, have a thickness which is at least equal to said predetermined distance between the edge of the board and any buried conductor track TL3 to TL6, here 1mm. The conductor tracks TL1, TL2. TL7 and TL8 which constitute the primary can approach more closely to the edge of the board. Fig. 3 shows a modified structure, again utilising three thick insulator layers 20a, 20b and 20c and four thin insulators 22a, 22b, 22c and 22d, and with eight track layers TL1 to TL8. Here however a further insulator layer 24a, 24b is provided at the top and at the bottom respectively of the PCB. These insulator layers 24a, 24b have a minimum thickness of 0.5mm. Like most planar transformers, the printed circuit board has the windings etched as copper spirals around the central core. In the case of the embodiment shown in Fig. 3, the track layers indicated at TL1, TL2, TL7 and TL8 typically have tracks which are 0.15mm in width with 0.15mm gaps separating the turns of the spiral. The track layers indicated at TL3, TL4, TL5 and TL6 typically have 0.3mm width tracks with 0.2mm gaps. The PCB shown in Fig. 3 contains four buried vias, from TL1 to TL2, from TL3 to TL4, from TL5 to TL6 and from TL7 to TL8. Again, as in Fig. 2, the buried track layers do not extend to within the distance t from the edge of the board. Reference is now made to Figs. 4 to 6. In Fig. 4 there is shown a PCB comprising a 1mm insulator layer 26 with two outer insulator layers 28a and 28b each 0.5mm thick. Between layers 26 and 28a are two track layers TL1 and TL2 separated by a thin insulator 29a, and between insulator layers 26 and 28b are two further track layers TL3 and TL4, again separated by a thin insulator 29b. No track is provided on the outside of the outer insulator layers 28a and 28b. The PCB incorporates four buried vias (not shown). Fig. 5 shows the configuration of the PCB, with the central non-plated hole 16 for the core and with six plated holes for track connections. Fig. 6 shows a typical track 30 and indicates the orientation of the PCB relative to the RM ferrite core 12. Figs. 7 to 9 show an alternative design. Here there are three 1mm thick insulator layers 32a, 32b, 32c, with eight track layers TL1 to TL8 arranged in pairs and separated each by a thin insulator 33a, 33b, 33c and 33d. This embodiment of PCB has two buried vias and two blind vias. Fig. 8 shows the configuration of the PCB with its different configuration of plated connection holes 34. Here the PCB is generally L-shaped. A typical track 36 is indicated in Fig. 9, together with the outline of the associated RM ferrite core 12. It is to be noted that in each of the embodiments the conductor track terminations 34 are positioned as far apart as possible from each other and from the ferrite core. If the ferrite does not form one of the isolated circuits, it can be electrically floating and then the outermost insulation of the multilayer PCB need only be 0.5mm thick, as in Figs. 3 and 4. Likewise, with such an arrangement, the predetermined distance between the edge of the board and any buried conductor track can be reduced, so that the winding tracks can run up to for example 0.5mm from the edge of the PCB. Close coupled windings can be formed by adjacent layers if they are not isolated, but a better arrangement is for two tracks to be wound together to form a bifilar winding, and even a centre tap configuration. In a conventional transformer close coupling is achieved by the use of bifilar wire, that is two wires wound together. The planar equivalent which can be used in the embodiments of the present invention is to have the PCB spiral tracks etched as a concentric pair with two buried vias. This principle can be extended to produce centre-tapped windings by laying out a bifilar winding on the PCB and connecting the terminations in an appropriate manner. Figs. 10 and 11 show assembled transformers, each of a different configuration but embodying the principles described above. Again, low profile RM ferrite cores 12 are used. In Fig. 10 the ferrite is equally segregated from the primary and from the secondary, as shown in the circuit diagram. In Fig. 11 the ferrite is not isolated from the primary, but the two secondaries are fully segregated, again as can be seen from the circuit diagram. This and many other variations may suggest themselves to those skilled in the art within the scope of the invention disclosed herein.

Claims: A planar, intrinsically safe transformer comprising: a single, bonded, multilayer printed circuit board (10) comprising a plurality of first layers (20a, 20b, 20c; 26, 28a, 28b; 32a, 32b, 32c) of insulating material each having conductor tracks (TL1 to TL8) on at least one side and at least one of said layers having conductor tracks on both sides, said tracks (TL1 to TL8) forming at least part of a transformer winding; an additional layer (22a, 22b, 22c, 22d; 29a, 29b; 33a, 33b, 33c, 33d) of insulating material constituting part of the printed circuit board between each pair of adjacent conductor tracks; and a magnetic core (12) enclosing at least a part of each of the transformer windings; characterised in that: (i) the or each said additional layer (22a, 22b, 22c, 22d; 29a, 29b; 33a, 33b, 33c, 33d) of insulating material is co-extensive with the first layers (20a, 20b, 20c; 26, 28a, 28b; 32a, 32b, 32c); (ii) no conductor track (TL3 to TL6) buried between two of said first-mentioned layers extends to within 0.5mm from the edge of the board; (iii) said additional layers (22a, 22b, 22c, 22d; 29a, 29b; 33a, 33b, 33c, 33d) of insulating material are substantially thinner than the first layers (20a, 20b, 20c; 26, 28a, 28b; 32a, 32b, 32c); and (iv) the or each of said first layers (20a, 20b, 20c; 26; 32a, 32b, 32c) which has conductor tracks on both sides has a thickness of at least equal to the distance between the edge of the board and any buried conductor track (TL3 to TL6). A planar transformer as claimed in claim 1, characterised in that the outermost layer on each face of the multilayer printed circuit board (10) comprises conductor tracks (TL1, TL8). A planar transformer as claimed in claim 1, characterised in that the outermost layer on each face of the multilayer printed circuit board (10) is a layer (24a, 24b; 28a, 28b) of insulating material. A planar transformer as claimed in any preceding claim, characterised in that the said first layers of insulating material are each at least lmm thick. A planar transformer as claimed in any preceding claim, characterised in that none of the buried conductor tracks extends to within lmm of the edge of the multilayer printed circuit board (10). A planar transformer as claimed in claim 1, characterised in that the multilayer printed circuit board (10) is faced with layers (24a, 24b; 28a, 28b) of insulating material which are about half the thickness of said first layers (20a, 20b, 20c; 26). A planar transformer as claimed in any preceding claim, characterised in that the magnetic core (12) comprises two RM ferrite core parts.

Inventors: CONBOY PETER D, SIMPSON ROGER R, CONBOY, PETER D., SIMPSON, ROGER R.
Applicants: MTL INSTR GROUP PLC, THE MTL INSTRUMENTS GROUP PLC
Application year: 1995
Main classification: H01F27
IPC codes: H01F27, H01F17