Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F5/00—Coils
  • H01F5/02—Coils wound on non-magnetic supports, e.g. formers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F17/00—Fixed inductances of the signal type 
  • H01F17/0006—Printed inductances
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F19/00—Fixed transformers or mutual inductances of the signal type
  • H01F19/04—Transformers or mutual inductances suitable for handling frequencies considerably beyond the audio range

Definitions

• This invention relates to the design of high frequency transformers and, more particularly, to low-profile planar, or printed circuit board type, transformers that will meet the isolation safety standards set for transformer use on AC mains, such as in off-line switching power supplies.
• Switching power supplies have long been of great interest to product designers because of their compact size relative to their linear counterparts. But, it was not until the second half of the 1980's that switching power supplies (i.e., "switchers") became the power supply of choice in the design of most electronic equipment. Their increased popularity was largely due to the availability of switchers that were more compact, lighter in weight, equally as reliable, yet only slightly more expensive than linear designs of equivalent power rating.
• the low-profile planar, or printed circuit board (i.e., PCB) style of transformer is the low-profile planar, or printed circuit board (i.e., PCB) style of transformer.
• the primary windings which are a spiral of traces on a planar surface
• the secondary windings which are a different spiral of traces on a separate planar surface
• the magnetic housing is made of ferrite, Sumarium or some other composite material that is shaped as a pot-core, an R-M-core, an E-core, an I-core, etc. But, it can be almost any shape that is easy to place around the windings and effectively confines the magnetic field to the area around the windings.
• “Creepage” and “clearance” are investigated between conductors, conductors and terminals, grounded or ungrounded conductive parts, components and component leads.
• “Creepage” is defined as the shortest path between two conductive parts or between a conductive part and the grounding surface of the equipment measured along the surface of the insulation.
• “Clearance” is the shortest distance between two conductive parts as measured through air. If a barrier is interposed, the spacing is measured around the barrier, or, if the barrier consists of two or more uncemented pieces, the spacing is measured through a joint or around the barrier, whichever is least.
• PC board i.e., low-profile planar
• an object of this invention is to provide a low-profile planar transformer design and physical construction concept that easily meets the above-stated isolation requirements for use in commercial off-line switchers.
• Another object of this invention is to provide a high-frequency transformer that is useful in consumer applications where it must provide isolation from AC mains.
• Another object is to provide a transformer that is the basis for cost-effective consumer-oriented off-line switchers. It is still a further object of this invention to provide a high-frequency transformer that is inexpensive to manufacture.
• a low-profile planar type transformer having a unique bobbin design and a minimum of other pieces.
• the transformer is assembled by simply stacking all of the pieces, other than core pieces, in a sandwich-like-laminate and placing two appropriately shaped ferrite core pieces around the stack.
• the stack consists of the following layers, in the listed order: (a) a first thin dielectric spacer; (b) a first planar member containing a first winding; (c) two thin dielectric insulators; (d) a first nylon bobbin member; (e) a second planar member containing a second winding; (f) a third thin dielectric insulator; (g) a third planar member containing a third bobbin member; (h) a second nylon bobbin member; (i) two thin dielectric insulators; (j) a fourth planar member containing a fourth winding, and (k) a seventh thin dielectric insulator.
• Two E-shaped ferrite cores are placed around the stack, with the center arm of the "E" going through a hole in the middle of the stack, to magnetically couple the current in the second planar windings to the windings of the first and third planar member.
• the replacement of the classical transformer having wire wound around a sewing-style bobbin by planar windings placed inside a tray-like bobbin makes the entire assembly low profile, and being adaptable to low-cost mass-production.
• the simplicity of the construction makes the transformer very easy to assemble either by hand or by machine.
• the design assures that it will meet the isolation requirements of the safety agencies as mentioned above. More specifically, it is the design of the bobbin members which assures that compliance.
• each bobbin member comprises a flat surface (i.e., planar element) with a central aperture.
• planar element On each surface of the planar element, a wall extends around the area in which the winding will sit. Walls also extend around the central aperture, from both the top and bottom surfaces of the planar element. The walls create a tray-like arrangement and act as path extenders for the creepage and clearance measurements, while hardly effecting the profile of the transformer.
• the transformer of this invention is inexpensive to make, has a low profile, and (with proper dimensions) meets the international safety standards for electrical isolation.
• Fig. 1 is a exploded view of the preferred embodiment of a transformer assembly according to this invention
• Fig. 2A is a top plan view
• Fig. 2B is a front plan view
• Fig. 2C is a side view of the assembled transformer of Fig. 1.;
• Figs. 3A and 3B are isometric drawings of, respectively, the top and bottom of a first bobbin member for use in that transformer assembly;
• Figs. 4A and 4B are, respectively, top and bottom plan views of the first bobbin member (bobbin A) .
• Fig. 4C is a front view
• Fig. 4D is a left side view
• Fig. 4E is a side sectional view taken along the line B-B of Fig. 4A
• Fig. 4F is a sectional view taken along the line A-A of Fig. 4A;
• Figs. 5A and 5B are isometric drawings of, respectively, the top and bottom of the second bobbin member shown in Figs. 1, 2A and 2B; Figs. 6A and 6B are, respectively, top and bottom plan views of the second bobbin member.
• Fig. 6C is a front view
• Fig. 6D is a left side view
• Fig. 6E is a side sectional view taken along the line B-B of Fig. 6A
• Fig. 6F is a sectional view taken along the line A-A of Fig. 6A;
• Fig. 7 is an isometric drawing of the two bobbins (bobbin A and bobbin B) fitted together;
• Fig. 8A is a side-sectional view of bobbin A and bobbin B fitted together, taken along the line B-B of Fig. 4A and line B-B of Fig. 6;
• Fig. 8B is a front-sectional view of bobbin A and bobbin B fitted together, taken along the line A-A of Fig. 4A, and line A-A of Fig. 6A;
• Fig. 9 is a top plan view of a PC board including a transformer winding, for use as a partial secondary winding in the transformer of Fig. 1;
• Fig. 10 is a top plan view of another PC board including a transformer winding for use as a partial primary winding;
• Fig. 11 is a top plan view of a dielectric insulator for use in the transformer
• Fig. 12 is an isometric drawing of one half of the E-shaped magnetic core of the transformer
• Fig. 13 is an end view diagram illustrating an example of "clearance” and “creepage” measurements on a generic arrangement of electronic parts
• Fig. 14 is an enlarged reproduction of the view of Fig. 8B, annotated to show the creepage and clearance measurements for the transformer of the present invention.
• FIGS 1 through 12 illustrate, an exemplary embodiment of a PCB transformer according to the present invention, and its constituent elements. Selected dimensions are shown, but anyone skilled in the art will understand that many of the dimensions, and the shape, depend upon the low frequency cut-off specification of the transformer and other design factors. The indicated dimensions are for a transformer that operates between 100K-1M Hertz at 100 to 250 Watts.
• Fig. 1 depicts an exploded view of the preferred embodiment.
• the elements of the transformer are: a first thin dielectric insulator la; a first planar member (which may be a PC board, not expressly shown) containing a first planar winding 10; second and third thin dielectric insulators lb and lc under winding 10; a first insulating bobbin member 20; a second planar member (which may include a PC board, not expressly shown) containing a second planar winding 30a; a fourth thin dielectric insulator Id; a third planar member (which also may include a PC board, not expressly shown) containing a fourth planar winding 30b; a second insulating bobbin member 40; fifth and sixth thin dielectric insulators le and If; a fourth planar member (also possibly having a PC board, not expressly shown) containing a fourth planar winding 50; a seventh thin dielectric insulator lg; and two E-shaped ferrite core
• Figs. 2A-2C provide top, front and side plan views of the fully assembled transformer shown in Fig. 1.
• FIGs. 3A and 3B the top and bottom of the first bobbin member 20 (sometimes called "bobbin A") are shown in respective isometric views.
• the bobbin member is turned over, relative to its disposition in Fig. 3A.
• Bobbin member 20 is rectangular in over-all shape and has tray-like sides 23 and 24 that are perpendicular to both the top planar surface 21 and bottom planar surface 22.
• Bobbin member 20 also has a rectangular hole 25 in the middle. Hole 25 is ringed all around by walls 26 and 27 on the top and bottom. As illustrated, walls 26 and 27 are parallel to the tray sides 23 and 24 on both the top and bottom of the bobbin member.
• FIGs. 4A-4D provide top, bottom, front and left side plan views of the first bobbin member 20.
• Figs. 4E and 4F are cross-sectional views.
• the top and bottom of the second bobbin member 40 (also called “bobbin B") are shown in respective isometric views (with the bobbin member turned over in Fig. 5B, relative to its disposition in Fig. 5A) .
• Bobbin member 40 is rectangular in over-all shape and has tray-like sides 43 and 44 that are perpendicular to both the top planar surface 41 and bottom planar surface 42.
• Bobbin member 40 also has a rectangular hole 45 in the middle. Hole 45 is ringed all around by walls 46 and 47 on the top and bottom. As illustrated, walls 46 and 47 are parallel to the tray sides 43 and 44 on both the top and bottom of the bobbin member. If the first bobbin member takes on a different configuration, corresponding changes would be made in the second bobbin member.
• Bobbin members 20 and 40 are similar, but not necessarily identical, parts. Upwardly-depending wall 46, .100" high and .020" thick, around hole 45 of bobbin member 40 is dimensioned to fit tightly inside the downwardly depending wall 27, .100" high and .020" thick, of bobbin member 20.
• the bobbin members are preferably molded, but they may also be machined. While various insulating materials can be used, nylon has been found to work well.
• Figs. 6A-6D provide top, bottom, front and left side views of second bobbin member 40.
• Figs. 6E and 6F are cross-sectional views of bobbin 40.
• Fig. 7 is an isometric view of the two bobbin members showing how they fit tightly together. The "bottoms" of the bobbin members face each other.
• Figs. 8A and 8B respectively show a front cross-sectional view and left side cross-sectional view of the two bobbin members fitted together.
• Fig. 9 shows both the first planar winding 10 and fourth planar winding 50 on the respective first planar member 11 and fourth planar member 51.
• each planar member (11 and 51) contains the conductor pattern (i.e., windings 10 and 50) for half of the secondary winding.
• the secondary winding is completed by wiring windings 10 and 50 in series.
• windings 10 and 50 are identical in this example but they may, in general, be different.
• Planar windings 10 and 50 are .030" from any edge of planar members 11 and 51, respectively, that is positioned within the perimeters of bobbins 20 and 40.
• Fig. 10 shows the top view of planar members 30a and 30b, and 31a and 31b.
• Planar members 30a and 30b are sized and shaped to fit into the space within the "tray" of bobbin member 20.
• Planar members 30a and 30b can have spiralling conductor traces, or some other wiring pattern, that carries transformer current.
• windings 31a and 31b are wired in series as one continuous primary winding of the transformer.
• the spiral traces of windings 31a and 31b carry the AC mains current of this transformer.
• the traces are of sufficient gauge to handle that current, and are within the area bounded by the dotted lines 33a and 33b, so they are no closer than .020" to any edge of the planar member (e.g., PCB substrate) that is within the perimeters of bobbins 20 and 40.
• Fig. 11 shows the thin insulating spacers la, lb, lc and Id, le, If, and lg. They may be stamped out of dielectric material (e.g., mylar or polyemide) that is .005" + .001", so they are .004" thick or thicker.
• the seven spacers la, lb, lc, Id, le, If and lg typically have the same outside dimensions and central hole pattern as planar members 11, 30a, 30b and 51.
• One spacer is placed on top of planar member 11, one on top of planar member 51 to insulate them from the core, while the others are used to easily meet the 3-ply specification for primary winding-to-SELV winding insulation.
• Fig. 12 is an isometric drawing of one of the two identical "E"-shaped ferrite core members 70a and 70b used in this embodiment.
• the central projection is .250" wide, while each end projection is .125" wide.
• the lengths of the three projections (71, 72 and 73) of the core members are .250" from the top surface, such that the cores 70a and 70b fit snuggly around the bobbin members, planar elements and spacers of the assembly with their E-projections contacting each other.
• the two core members may be glued together.
• “creepage” is defined as the shortest path between two conductive parts or between a conductive part and the grounding surface of the equipment measured along the surface of the insulation. It is important to note that creepage is measured along the surface of the insulation between two conductors. Fig. 13 defines the paths 91 and 92 along which the creepage measurement would be made in two different situations.
• “Clearance” is a similar measurement of conductor-to-conductor separation, but it is made through air, along the shortest path between conductors.
• “Clearance” is the shortest distance between two conductive parts as measured through air, as in path 94.
• a barrier is interposed (e.g., 90)
• the spacing is measured around the barrier, as in path 95. If a barrier between conductors consists of two or more uncemented pieces, the spacing is measured through a joint or around the barrier, whichever is least.
• the transformer can be assembled by the following exemplary steps: First, planar member (PM) 31a (which is not expressly shown, to avoid unnecessary obfuscation, but which carries winding 30a) is placed on the bottom side 22 of bobbin member 20. The lip 27 around hole 25 in bobbin 20, locates the PM and holds it in place. Next, a thin dielectric insulator Id is placed over PM 31a, then PM 31b (which also is not expressly shown, to avoid unnecessary obfuscation, but which carries winding 30b) is placed on top of it.
• Bobbin member 40 is placed over PCB 31b, onto bobbin 20, with the hole 45 and lip 47 of bobbin member 40 fitting tightly inside the hole 25 and lip 27 of bobbin member 20.
• windings 30a and 30b are sandwiched between bobbin members 20 and 40 with the connection points 32a and 32b (i.e., solder leads) of those windings projecting out of the left end of the tightly fitted bobbins (see Fig. 1).
• connection points 32a and 32b i.e., solder leads
• two dielectric insulators lc and lb are placed on top of bobbin member 20, then PM 11 (with winding 10) is placed on the outer surface of the sandwich formed by the top of tray 21 of bobbin member 20.
• PM 51 with winding 50
• PM's 11 and 51 have the connection points 12 and 52 (i.e., solder leads) of windings 10 and 50 projecting out of the right end of the bobbin trays (see Fig. 1).
• the two E-shaped ferrite core members 70a and 70b are now placed around the entire sandwich so that their middle projections fit snuggly into the hole (26, 46) in the middle of the PM-bobbin sandwich.
• the core-PM-bobbin-sandwich can be pressure-fit together, or, for anti-tampering purposes, a conventional industrial glue may be placed on the mating surfaces of the core members, and pressure applied while the glue cures.
• the proper leads on windings 10 and 50 are soldered together to join the two halves of the secondary into one continuous winding. Or, the leads can be soldered to put the winding 10 in parallel with winding 50.
• the proper leads on windings 31a and 31b are also soldered together to join the two halves of the primary in series. Other windings (on the same or other PM's) and spacers can be added as desired.
• the height of the exemplary low profile transformer described above is approximately .500". Earlier in the text there is an outline of the three critical specifications that any transformer must meet to be useful in consumer applications.
• the first specification requires that the insulation from primary winding-to-SELV winding be either .080" as a single layer or three layers of at least .004" each.
• Fig. 14 shows two insulators (i.e., spacers) of .005" + .001" each, and bobbin A of .020" to .025", thus, complying with the 3-ply requirement.
• the second specification requires that the creepage and clearance between the primary and secondary be at least .240".
• the earlier discussion of Fig. 13 showed how creepage and clearance are measured in general. Referring to Fig.
• path 101 shows the creepage and the clearance, between the primary and secondary through the center hole, which is the path of the worst case (i.e., minimum) creepage and clearance in this transformer.
• Creepage and clearance path 101 starts at point A, the outermost extent of the etch on PM 31b, which is manufactured to be no closer than .030" from the edge of the PM in this embodiment.
• Path 101 proceeds under wall 27, which is .020" thick, to point B.
• Path 100 is the same as Path 101 from point A to point E. This path demonstrates the minimum creepage and clearance path from the core to the primary winding on PM 30a. So
• the resulting package can easily meet all isolation requirements and still be a very low profile, and extremely compact transformer.
• a low-profile planar transformer that can be manufactured easily and inexpensively, and be used successfully as a line transformer in off-line switching power supplies that operate at MegaHertz frequencies.
• bobbin members which are round or oblong or some other shape, with similarly shaped PM's, windings and spacers, rather than rectangular elements. Still another variation would be to use only two PM's with two bobbin members. Alternately, the transformer could also be constructed with more than two bobbins in a multi-cavity type of construction. Many other variations on this invention can be made by using different combinations of magnetic elements that are shaped as E-cores, I-cores, R-cores, Pot-cores, and so forth. Other variations can exist specifically for making high voltage transformers, or isolation transformers that do not have to meet the UL/VDE/CSA specifications. Accordingly the invention is defined not by the illustrative embodiment, but only by the following claims and their equivalents.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Multimedia (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Coils Or Transformers For Communication (AREA)

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• 1991
  • 1991-03-18 DE DE69117403T patent/DE69117403T2/de not_active Expired - Lifetime
  • 1991-03-18 AU AU76650/91A patent/AU7665091A/en not_active Abandoned
  • 1991-03-18 DE DE9114783U patent/DE9114783U1/de not_active Expired - Lifetime
  • 1991-03-18 EP EP91907531A patent/EP0476114B1/de not_active Expired - Lifetime
  • 1991-03-18 WO PCT/US1991/001801 patent/WO1991015861A1/en active IP Right Grant
• 1998
  • 1998-06-26 HK HK98106877A patent/HK1007829A1/xx not_active IP Right Cessation

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