AU720276B2 - Improvements relating to inductive assemblies in electronic circuits - Google Patents

Description

WO 98/11765 PCT/NZ97/00114 -1- IMPROVEMENTS RELATING TO INDUCTIVE ASSEMBLIES IN ELECTRONIC CIRCUITS FIELD OF THE INVENTION This invention relates to inductive assemblies for electronic circuits, and particularly but not solely to assemblies having multiple windings, such as transformers and common mode chokes, which are formed on printed circuit boards (PCBs).

BACKGROUND TO THE INVENTION Transformers are widely used in power control circuits for devices such as motor speed controllers, heating and incandescent lighting controls, and computer equipment. The size, cost and electrical performance of transformer assemblies and their associated drive components are a significant consideration when designing electronic circuits of this kind.

The efficiency of manufacturing these circuits using through-hole and surface-mount components on suitable PCB substrates must also be considered.

Many designers of high frequency (approaching 1 MHz) power circuits are using planar transformers in which the traces on multiple layers of PCB provide the windings and are integrated directly with the circuit substrate. These transformers use a two-piece magnetic core structure and perhaps six or more PCB layers in some complex systems. They are generally convenient and compact to use, and require no labour in winding so are reasonably cheap. See for example the DC-DC converter transformer circuits described in Integrated PC Board Transfomnners, PCIM, July 1994. See also GB 2285892 and cited documents.

However, two-piece magnetic structures incorporate an airgap which reduces their inductance factor, and creates an appreciable disadvantage in noise suppression devices such as common mode chokes. One-piece structures such as toroids on the other.hand, can provide near ideal magnetic shapes, are cheaper and easier-t6-insulate, but must normally be mechanically wound and are not readily used in planar designs. Existing transformer assemblies which incorporate a toroidal core also generally require a separate base for mounting on a PCB.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide for alternative inductive assemblies which can make cheap and convenient use of one-piece magnetic core structures.

SUBSTITUTE SHEET (RULE 26) WO 98/11765 PCT/NZ97/00114 -2- Accordingly the invention may broadly be said to consist in an inductive assembly for an electronic circuit wherein at least one electrical winding is formed on a combination of two substrates which are joined together at substantially right angles.

The substrates are preferably soldered together to provide electrical connections between parts of each winding which are formed on respective substrates. Each turn of each winding is preferably formed to lie partly on each substrate.

In a second aspect the invention may be said to-consist in an inductive assembly for an electronic circuit wherein at least one winding of the assembly is formed on a combination of two substrates which are soldered together to provide electrical connections bewteen parts of the winding A magnetically susceptible magnetic structure such as a toroid is preferably mounted on one substrate to form a core for the winding. The other substrate preferably includes a slot or otherwise generally U shaped portion which straddles the magnetic structure.

Some example assemblies according to the invention include high frequency DC-DC converter transformers, pulse transformers, and single or three-phase common mode chokes, operating at around 1-2 MHz. The invention is less useful for relatively low frequency devices requiring large maanetising inductances.

In a third aspect the invention may broadly be said to consist in an inductive assembly for an electronic circuit wherein two or more electrical windings are formed on a combination of three or more substrates with one of the substrates providing a base to which the other substrates are joined at substantially right angles.

One or more magnetic core structures are preferably mounted on the base substrate. Each core preferably links at least two windings. Each winding of any core is preferably formed on a respective substrate which straddles the core, in combination with the base substrate.

Each substrate may include parts of several windings which have respective cores. Each substrate which is mounted on the base substrate is preferably parallel to the other substrates which are mounted on the base substrate.

In a further aspect the invention may broadly be said to consist in a transformer assembly for an electronic circuit including three substrates, two windings and a core structure, WO 98/11765 PCT/NZ97/00114 -3wherein the core is mounted on a main one of the substrates and encompassed by each of the other substrates, which are each joined to the main substrate, and wherein each winding includes one or more turns which are formed partly on the main substrate and partly on a respective other substrate.

The core preferably has a toroidal shape which is straddled by the other substrates on the main substrate. The other substrates preferably have an approximately U shaped portion through which the core passes to create a magnetic link with respective windings.

Preferably each winding includes a plurality of turns which are formed partly on the main substrate and partly on a respective other substrate with a soldered connection between those parts of each of each turn which are on separate substrates.

In a further aspect the invention may be said to consist in a method of forming an inductive assembly, comprising: forming base parts for one or more electrical windings on a first substrate, forming complementary parts of each electrical winding on one or more second substrates, arranging a magnetic core structure on one of the substrates proximate to the respective parts of the windings, and forming the electrical windings by bringing the first and second substrates into electrical contact around the core structure.

Preferably forming each part of the winding comprises forming a respective set of tracks on PCB substrates. Preferably the core structure is a one-piece toroid which is glued onto the base PCB. Preferably forming the winding comprises fitting lugs on the first substrate into apertures on the second substrate, and forming a solder connection between the base and complementary parts of the winding.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will be described with respect to the drawings, of which: Figures la, Ib, Ic and Id are respectively top plan, side elevation, end elevation and bottom plan views of an example transformer assembly formed on PCBs, Figure 2 is a perspective view showing an arrangement of windings for the assembly in Figure 1 with the underlying PCBs having been omitted, Figure 3 schematically shows how some geometrical characteristics of the simple transformer assembly in Figure 1 may be calculated, Figures 4a, 4b and 4c are respectively top plan, side elevation and bottom plan views of a multiple transformer assembly, Figures 5a, 5b and 5c are respectively top plan, side elevation and bottom plan WO 98/11765 PCT/NZ97/00114 -4views of a multiple output transformer assembly, Figures 6a, 6b and 6c are respectively top plan, side elevation and bottom plan views of a common mode choke assembly, and Figure 7 is a side elevation showing how circuitry associated with an inductive assembly according to the invention may be positioned on the assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings it will be appreciated that the invention is still in a prototype form and that various alternatives and modifications, and a range of components, may be contemplated in accord with the concepts summarised above. It will also be appreciated that transformer and PCB principles and techniques are taken as understood by the skilled reader without detailed explanation at this stage.

Figures la-d show a simple transformer assembly 10 in various views without accompanying circuitry. The transformer is formed on a base substrate 9 which may be a separate piece of single or double sided printed circuit board, or a small portion of a much larger board containing other circuits. One and usually two additional substrates 11 and 12, also generally pieces of PCB, are fixed to the base substrate. Primary and secondary windings 21 and 22, shown fully in Figure 2, are formed on the substrates and inductively/magnetically linked by a core structure 13. In this case the core is toroidal in shape although a wide range of one-piece and possibly multiple-piece core shapes could be used if required.

The additional substrates 11 and 12 are typically cut in U-shapes and straddle or otherwise fit over the core structure 13 into the base substrate 9. A wide range of shapes might be considered to achieve this purpose provided the core structure can be effectively encompassed by each additional substrate and the base substrate. They each have two lugs or extensions 15 and 16 which fit securely into suitably sized apertures or slots on the base substrate. The additional substrates are also placed at right angles or at least approximately perpendicular to the base substrate, and generally spaced apart parallel to each other as can be seen. Other means of fixing the additional substrates to the base substrate in other acceptable configurations can be envisaged, although lugs and apertures are preferred.

The primary and secondary windings 21 and 22 typically have between 1 and 10 turns each, and possibly up to several dozen turns, formed by metallic (copper) track patterns 17 on the substrates. Four similar turns have been shown for each winding in the figures, Wo 98/11765 PCT/NZ97/00114 as part of a DC-DC converter which is mentioned below, although the number of turns will differ depending on the nature of the transformer requirements. As can be seen particularly in Figure 2, the turns form a generally spiral winding having a shape which roughly follows the cross section of the core structure 13, although various track patterns can be envisaged in this regard. The windings have generally similar metallic terminals 24 and 25 for input and output of current. These are formed on an appropriate substrate, in this case the base substrate, although terminals on the other substrates could be more convenient in some cases.

Each of the windings 21 and 22 has a track pattern formed partly on the base substrate 9 and partly on a respective additional substrate 11 or 12. Those parts 26 and 27 of the patterns which lie on respective additional substrates 11 and 12 will usually be positioned as far from each other as possible, typically though not necessarily facing outwards as shown, to maximise creepage distances. Similarly for those parts 28 and 29 of the patterns which preferably lie on the underside of base substrate 9, to provide appropriate creepage distances between the respective turns and terminals. An electrostatic shield can be created from each additional substrate using double sided PCB, with one side containing the tracks, and the other a solid copper area.

The shapes and tracks of the substrates 9, 11, 12 are readily formed by conventional PCB techniques. Different PCB material may be used for different substrates if required. The core 13 and additional substrates 11 and 12 are at present attached manually to the base substrate 9 using an appropriate bonding agent. Expanded end portions 23 of the tracks enable adequate electrical connections between those parts of the windings which lie on different substrates. The end portions are readily soldered together with other circuit components also by conventional techniques.

Track patterns forming base parts for each winding are first formed on one side of a substrate which may be part of a much larger circuit board supporting other components.

These are tracks 17 on substrate 9 in Figure Ic or 4c for example, with appropriate end points 23 and terminals 24 and 25. One or more magnetic core structures such as toroid 13 may then be arranged on the substrate where required, proximate to and generally though not necessarily on the other side from the tracks 17.

Complementary tracks for the windings are formed on one or more second substrates, such as tracks 17 on substrates 11 and 12 in Figure lb, or 41 and 42 in Figure 4b, once again with appropriate end points 23. It is possible to arrange the core structure on one or more of the second substrates rather than on the base substrate at this stage. The WO 98/11765 PCT/NZ97100114 -6second substrates are then brought into contact with the first substrate, to create electrical connections between end points 23 of respective parts of the windings.

It will be seen from the figures that the end points 23 of tracks on the first or base substrate are preferably formed to lie at or adjacent apertures or other means at which corresponding end points on the second or additional substrates will be fixed. Similarly end points of tracks on the second substrates need to be formed at or near lugs or other means which will pass through apertures in the first substrate. In the preferred embodiments the end points on respective substrates make contact when lugs 15 and 16 on the second substrates the pass through apertures on the first substrate. These requirements may be relaxed in assemblies where the tracks on the base substrate lie upwards. The substrates will generally be fixed together physically by a suitable bonding agent and their respective end points connected together electrically with solder.

Figure 7 indicates how additional substrates 71 and 72 may be extended in height, or possibly width, to create space for circuitry which can be connected directly to the windings. This is particularly useful when mixing through-hole and surface-mount components in a particular device, as they may be placed on different substrates. For example, surface-mount components may be placed on one of the additional substrates 71 or 72 having a fine pitch copper track pattern and thin copper layer. Leaded components may be placed on the base substrate 9 having a coarse pattern and thick copper layer designed for higher current.

Use of a one-piece toroidal core provides substantial advantages over two-piece structures such as used in planar systems. Multiple pieces must be clamped together firmly to provide satisfactorily closed structures and even so, an air gap will generally exist between each pair of pieces, allowing magnetic flux to escape, and thereby lowering the overall inductance. A toroid is nearly ideal in this respect. Further, most commercially available planar cores are uncoated and require extra work to be galvanically isolated from the windings. Toroids in contrast, are cheaply available already coated with insulating material, and-for the present purposes do not require special mounting or winding.

Use of the available toroidal cores does generally limit the number of turns in each winding however, and the maximum creepage distances which can be obtained between the windings. These factors in turn limit operation of the inductive assemblies to relatively high frequencies on the order of 1-2 MHz. Use of other one-piece core geometries in future may alleviate these problems to some extent.

WO 98/11765 PCT/NZ97/00114 -7- Figure 3 indicates a simple geometrical relationship between the dimensions of an assembly as shown in Figure 1. A minimum inner diameter ID and maximum width of winding w may be calculated from a creepage distance d required by output power and operating frequency of a proposed assembly. The calculation is made by iteratively solving the following equation: d 2 ID- 1 D 2 2 2 A DC-DC converter transformer according to Figure 1 was thereby constructed for operation at 600Vrms and 2 MHz, and a creepage distance d of 5.5mm, using a 14.5mm OD, 8.4mm ID toroidal core. The transformer was successfully used in both forward and flyback converters, and a 2 MHz modulated IGBT/MOSFET gate drive pulse transformer.

Figures 4a-c show a system of three gate drive pulse transformers 40 which have been used for a three-phase inverter, by way of further example. A pair of single sided PCBs served as the primary and secondary substrates 41 and 42 for each transformer. In one variation indicated in Figure7, extended substrates 71 and 72 were used to support three driver and receiver circuits respectively, implemented with surface mount technology.

Figures 5a-c show a transformer system 50 having a primary winding 51 and centretapped secondary winding 52 which has been used in a 3W resonant DC-DC converter, also by way of example. This employed 14.5 OD cores and 5.5mm creepage distances as before.

Figures 6a-c show a low voltage common mode choke 60 in which the windings are connected in a flux cancelling fashion around a lossy high permeability core. Reduced creepage requirements permit a double sided PCB substrate 61 to provide both windings, increasing the number of turns possible on the core and maximising the common mode inductance. Multiple phase chokes can be constructed using multiple winding concepts as shown in Figures Various inductive devices may be constructed in accord with the invention to make use of a one piece core structure. Several embodiments have been described as examples but without limiting the scope of the following claims.

Claims (18)

1. An inductive assembly for an electronic circuit wherein at least one electrical winding of the assembly is formed on a combination of two substrates which are joined together at substantially right angles.

2. An assembly according to claim 1 wherein the winding is formed partly on each of the two substrates which are soldered together to provide electrical connections between parts of the winding.

3. An assembly according to claim 1 wherein each turn of the winding is formed to lie partly on each substrate.

4. An assembly according to claim 1 wherein a one piece magnetic structure is mounted on a first one of the substrates to form a core for the winding. An assembly according to claim 4 wherein the second substrate straddles the magnetic structure.

6. An assembly according to claim 1 wherein two electrical windings are formed on the combination of two substrates.

7. An inductive assembly for an electronic circuit wherein at least one winding of the assembly is formed on a combination of two substrates which are soldered together to provide electrical connections bewteen parts of the winding,

8. An assembly for an electronic circuit wherein two or more electrical windings are formed on a combination of three or more substrates with one of the substrates providing a base to which the other substrates are joined at substantially right angles.

9. An assembly according to claim 8 wherein one or more magnetic core structures are mounted on the base substrate each linking at least two windings. An assembly according to claim 8 wherein each winding is formed partly on the base substrate and partly on a respective other substrate and encompasses a magnetic core.

11. An assembly according to claim 10 wherein at least one of the windings encompasses two magnetic cores. wo 98/11765 PCT/NZ97/00114 -9-

12. An assembly according to claim 8 wherein the other substrates are substantially parallel to one another.

13. A transformer assembly for an electronic circuit including three substrates, two windings and a core structure, wherein the core is mounted on a main one of the substrates and encompassed by each of the other substrates, which are each joined to the main substrate, and wherein each winding includes one or more turns which are formed partly on the main substrate and partly on a respective other substrate.

14. An assembly according to claim 13 wherein each the other substrates have a generally U shaped potion which is placed over the core to create a magnetic link between the windings.

15. An assembly according to claim 13 wherein the turns of each winding are formed partly on the main substrate and partly on a respective other substrate with a soldered connection between those parts of each of each turn which are on separate substrates.

16. An assembly according to claim 13 wherein the other substrates are joined approximately perpendicularly to the main substrate

17. A method of forming an inductive assembly, comprising: forming base parts for one or more electrical windings on a first substrate, forming complementary parts of each electrical winding on one or more second substrates, mounting a magnetic core structure on one of the substrates proximate to the respective parts of the windings, and forming the electrical windings by bringing the first and second substrates into electrical contact around the core structure.

18. A method according to claim 17 further comprising: forming the base parts of the electrical windings on one side of the first substrate, and attaching the core structure to the other side of the first substrate.

19. A method according to claim 17 further comprising: forming the second substrates to have generally U shaped cutout portions which fit around the core. WO 98/11765 PCT/NZ97/00114 A method according to claim 17 further comprising: forming the complementary parts of the electrical windings on separate second substrates, and attaching the separate second substrates to the first substrate spaced apart substantially in parallel to each other. A method according to claim 17 further comprising: forming the base parts of the windings to end at apertures in the first substrate, forming the complementary parts of the windings to end at respective lugs on the second substrates, and placing the lugs into the slots to form the electrical connections between the base and complementary parts of the windings.

22. A method according to claim 17 further comprising: forming a plurality of inductive assemblies having base parts of their windings on a single first substrate.