GB2517015A - Transformer - Google Patents

Abstract

A planar flyback transformer comprises a low coercivity magnetic material core 11, 12 and one or more printed circuit boards 2, 3 with conductive tracks arranged to form at least a primary coil and a secondary coil, each spiralling around a portion 14 of the said core. The transformer has an output voltage rating of at least 380 V and a secondary to primary voltage ratio divided by a secondary to primary turns ratio which is greater than 7. The distances between members of the transformer may be set relative to the voltages arising between such members. The transformer may be integrally formed with a printed circuit board carrying other components. The transformer may be used in a compact battery operated instrument suitable for providing a high voltage output signal for insulation testing.

Description

TRANSFORMER

Technical Field

The present invention relates to transformers. In particular, the invention relates to relatively high voltage, planar transformers for use in power transfer. More particularly, the invention relates to step-up transformers.

Background

Embodiments herein relate in general to relatively high voltage step-up transformers of the kind that are suitable for deployment in test instruments or equipment. The test equipment may be used for testing insulation resistance in other electrical equipment or machines, for example by applying a high test voltage across insulation in the other electrical equipment or machine, If less than a threshold current due to the applied voltage is measured to flow through the insulation, the test equipment registers that the insulation test has been passed; if more than a threshold current is measured then the test equipment registers that the insulation test has been failed.

Failure is typically due to a material or mechanical breakdown of some kind in the insulation.

Known test equipment of the kind generally considered herein is intended to be relatively small, lightweight, portable and battery-powered, as exemplified by the applicant's MIT400 series of industrial maintenance insulation and continuity testers, Such test equipment typically therefore needs to be able to convert a low voltage from a battery source, for example providing 4V-12V DC, into a high voltage suitable for insulation testing. For example, to test a single-phase piece of electrical equipment, for example operating at 240V, it is usual to apply up to 500V or more to the insulation to test the electrical integrity thereof For a three-phase piece of electrical equipment, operating at 415V, it is usual to apply up to 1kV or more.

Clearly, there are large demands placed on any transformer that is used to generate from batteries the high voltages that are used for insulation testing, particularly when space for the transformer is limited (as would be the case in portable, battery operated test equipment), Commonly deployed transformers provide a means for transferring energy between input and output terminals by utilising an inductive coupling between first and second winding circuits (or "windings", or "coils"), each comprising one or more individual turns around a core. The nature of the coupling between an input (primary) winding circuit and an output (secondary) winding circuit in general allows the voltage generated at the transformer's output terminals to be either stepped up (or stepped down) with respect to the voltage applied at the input. A common method for producing winding circuits for transformers is to wind an electrical conductor, such as a length of wire, around a ferrite core. However, such transformers are bulky and hence are not optimised for small or portable electronic devices such as portable test equipment. The bulk of such transformers is often compounded through the addition of necessary insulating materials, Moreover, such transformers are also expensive to make and can be unreliable because each winding has to be wound mechanically and insulated from the other windings.

Another kind of transformer that is known has windings formed from printed circuit boards (PCB5). Such a transformer is described in JP200S 166625, which relates in particular to step-down transformers, which are designed to satisfy strict standards on safety, isolation and creepage. Such transformers may for example be suitable for use in consumer mains chargers for mobile phones or the like. These known transformers as described would not be suitable for the generation of high voltages at their output terminals, without a significant change to the respective design parameters, in particular their size, There remains therefore a demand for small scale transformers that are suitable for the field of high voltage power transfer, in particular, for the fields of electrical measuring instruments. Hence, it is an object of the present invention produce a transformer that addresses one or more of the problems identified in such known transformers.

Summary

According to a first aspect, the present invention provides a planar flyback transformer having a rated output voltage of at least 380V comprising: a core comprising a material of low coercivity; and one or more printed circuit boards comprising electrically conductive tracking arranged to form at least an input primary coil and an output secondary coil each arranged spirally around the core, wherein the secondary to primary voltage ratio divided by the secondary to primary turns ratio is greater than 7.

According to a second aspect, the present invention provides a printed circuit board comprising circuit components and at least one transformer according to the first aspect, at least one of the one or more printed circuit boards of the transformer being formed in a region of the said printed circuit board.

According to a third aspect, the present invention provides an item of electrical equipment comprising at east one printed circuit board according to the second aspect.

According to a fourth aspect, the present invention provides battery operated equipment for insulation testing, for delivering a test voltage of at least 380V from a battery input delivering 30V or less, said equipment comprising a transformer according to the first aspect, for stepping up the battery input vohage to the test voltage.

Hence, design of a small scale transformer is enabled that is suitable for use in high voltage scenarios, The arrangement of PCBs in accordance with aspects of the present invention removes the need for interleaving structures found in alternative known arrangements. Furthermore, the use of PCBs for the construction of the primary and secondary coils results in low manufacturing costs.

Further aspects, embodiments, features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Various embodiments will now be illustrated, by way of example only, with reference to the accompanying drawings.

Figure 1 is a schematic isometric diagram of an embodiment of a planar transformer, Figure 2 is an exploded view of various parts of the planar transformer of Figure I, Figure 3 is a diagram of a cross-section through the core of the planar transformer in direction I of Figure 1, Figure 4 is a diagram of a close up view of a portion of the cross section of Figure 3, Figure 5 is a diagram of a cross section through the core of the planar transformer in direction II of Figure 1, Figure 6 is a diagram of a close up view of a portion of the cross section of Figure 3, Figure 7 is a photograph of a part of a wider PCB on which is formed a planar transformer according to an embodiment of a planar transformer, Figure 8 is a block diagram of a planar transformer and a driving circuit for a planar transformer according to an embodiment, Figure 9 is a schematic block diagram of a magnetic core including an air gap, Figure 10 is a graph showing input current and output voltage over time of a transformer according to an embodiment, and Figure 11 is a graph showing voltage across a MOSFET in an input winding of a transformer according to an embodiment.

Detailed Description

Figure 1 shows a schematic isometric view of a step-up planar transformer 100, in accordance with exemplary embodiments of the present invention, which has a rated output voltage Vratcd of 600V (referred to hereinafter as a "600V embodiment"). Figures 2 to 7 depict the same transformer in different ways, as will be described.

The planar transformer of Figure 1 comprises a core 1, a first, upper PCB 2, and a, second, lower PCB 3 (the labels upper' and lower' are merely for reference and PCB 3 could equally be above PCB 2). The first PCB and the second PCB may be held in place by the core 1. For example, the core I may comprise two core portions 11, 12, which are secured together by means of a core clamp 13. Alternative locking means may also be used, including mechanical fastenings (such as pins or screws) or chemical adhesives. PCBs are typically substantially planar. By forming the planar transformer shown in Figure 1 from PCBs, a small size of transformer can be achieved, For example, a length of about 20 mm, a width of about 15 mm, and a thickness of about 6mm is attainable. Within that, the core has respective dimensions in mm of 14.5, 7 and 6. Even with these relatively small dimensions, transformers designed according to embodiments herein are capable of stepping up from a few volts (for example, produced from five or six standard 1.5VAA batteries installed in relatively small and/or portable equipment) to hundreds or even thousands of volts. Such transformers, which may be capable of delivering hundreds or even thousands of volts, may be driven by batteries delivering from in the region of 5V up to about 30V, although for most applications an input voltage of no more than 12V would be satisfactory.

A similar exemplary transformer to that depicted in Figure I, but with a rated output voltage Vmtcd of 1 100V (referred to hereinafter as a "1 100V embodiment"), has a core with dimensions in mm of 18, tO and 6, Other features of both the 600V embodiment and the ii OOV embodiment will be described herein, by way of example. Nevertheless, the skilled person will appreciate that embodiments of the invention are in no way limited to these particular output voltage ratings and associated design parameters.

A PCB may be formed of an insulating material, such as a non-conductive substrate, upon which a series of metal tracks are formed to provide electrically conductive pathways, or turns, around the core. The turns form primary (input) and secondary (output) coils, as will be described, Each turn may comprise one or more tracks around the core, For example, if two tracks pass around the core and are connected in series (i.e. in an end-to-end relationship passing in the same direction around the core), they form two turns, Alternatively, for example, if two tracks around the core are connected to one another in parallel, they form a single turn, effectively, of double thickness, as will be described. Each PCB, 2, 3, may comprise a number of sub- layers, 20, 30, each comprising one or more substantially co-planar tracks. Each sub-layer of a PCB may be interconnected by means of so-called vias 2], 3L A via may comprise a through-hole between the various sub-layers of a PCB that is lined with an electrical conductor so as to connect a track from at least one of the sub-layers to either a track from another of the sub-layers, or to an external electrical connection or terminal, Also shown in Figure 1 are transformer terminals, A, B, C and D. According to the present embodiment, terminals A and B form input terminals to a primary coil on the upper PCB 2 and terminals C and D form output terminals from a secondary coil on the lower PCB 3, In other embodiments, upper' and lower', primary' and secondary', as applied to the orientation of the PCBs, may be reversed.

Figure 2 shows an exploded view of an example planar transformer according to embodiments of the present invention as depicted in Figure 1. In the embodiment depicted in Figure 2, the core 1, comprising core portions 11 and 12, further comprises a central core portion 14 (which is shown in Figures 3 and 5). The central core portion 14 may comprise a cylindrical cross-section (as depicted) or have some other cross-section, for example a triangular, square, or any other appropriate cross-section. In the embodiments shown in Figure 2, core portion 1] comprises a first part 14a of the central core portion 14, whereas core portion 12 comprises a second part 14b of the central core portion 14. In alternative embodiments, the entire core portion 14 may be formed by a single part (not shown) extending from one or other of the core portions ii or 12.

Indeed, one core portion 11 may have an "F" shape, with the other core portion 12 comprising a flat plate to close the arrangement. Alternatively still, the core portion may be a separate part of the core 1, which is held in place by the interlocking of the core portions 11, 12. In the PCBs 2, 3, there is defined a central hole which is profiled to receive the central core portion 14.

Each PCB 2, 3 comprises a stmcture of alternating tracked sub-layers 22a, 22b, 22c, 32a, 32b, 32c, and insulation sub-layers 23a, 23b, and 33a, 33b. In embodiments, at least one of the tracked sub-layers comprises an upper track on the top surface of the sub-layer and a lower track on the bottom surface of the sub-layer. At least one of the tracked sub-layers 22a, 22b, 22c, 32a, 32b, 32c comprises tracks which are arranged spirally, as has been explained, into one or more turns. In the embodiments shown in Figure 2, the tracks are arranged spirally around central core portion parts 14a, 14b. In some embodiments, each sub-layer comprises a single turn. In alternative embodiments, each sub layer may comprise two or more turns, which are electrically isolated. The beginning and end of each turn on one or more of the sub-layers of a PCB are each connected to vias.

Depending on how the connections between layers are arranged, two or more tracks may be connected in series or in parallel. According to the present embodiment, three tracked sub-layers 22a, 22b, 22c, of a six-layer PCB, each carry four layers of tracks, 220, 221a, 221b and 222 arranged as two sets ofturns each connected in parallel.

That is, although there are 16 tracks shown, making up only eight turns in the primary coil. Tracked layers 32a, 32b, 32c, of a second six-layer PCM in contrast cany five layers of tracks, 320a, 320b, 321a, 321b and 322 arranged in series, thereby forming 42 turns in the secondary coil.

S A I IOOV embodiment may be arranged over one six-layer PCB, having only three turns in a primary coil, for example arranged over one layer, and 35 turns in a secondary coil, for example arranged over five layers. For either of the aforementioned exemplary embodiments, the tracks are spaced apart by 0.15mm and have track widths of at least 0.15mm, which is relatively standard for known PCB production processes, meaning that production costs associated with PCBs including transformer tracks should be no higher than production costs for normal PCBs that do not have transformer tracks.

More generally, this kind of track arrangement accommodates various different arrangements. For example, each sub-layer may comprise two tracks, wherein a primary winding circuit is created by electrically connecting the first of the tracks on each sub-layer in series, and a secondary winding circuit is created by electrically connecting the second of the tracks on each sub-layer in series, Alternatively, each sub layer may comprise only one track, wherein the first and second winding circuits are created by electrically connecting the tracks by alternating tracked sub-layers. In addition, in either of these arrangements, the PCBs 2, 3, may be electrically connected to allow a greater number of sub-layers to be connected in series. Alternatively, the planar transformer may comprise only a single PCB having multiple sub-layers.

Alternatively still, the planar transformer may comprise more than two PCBs, electrically connected to allow a still greater number of sub-layers to be connected in series. In further embodiments, the primary winding circuit may be created by electrically connecting all of the tracks in the first PCB, and the secondary winding circuit may be created by electrically connecting all of the tracks in the second PCB. In some arrangements, one of the PCBs may be integrally formed as part of a larger PCB comprising a wider electrical circuit. A benefit of such an arrangement is that the tracks of the respective winding circuit may be printed on the layer(s) of the PCB during normal manufacturing of the larger PCB, thereby reducing any additional manufacturing overhead of the overall arrangement.

In order to attain the desired efficiency of the transformer, the core comprises a material with a low coercivity. According to embodiments, the core 1 has a coercivity of 0,7,10 Aim or less. According to embodiments the core comprises a magnetic material. In some embodiments, the core comprises a ferrite material. Both the core and the conductive tracks of the PCBs 2, 3, are elecifically conductive and typically have electrical conductivity values higher than about 10 MS/m. In contrast, the insulating material of PCBs 2,3, upon which the tracking is embedded, typically has an electrical resistivity value higher than I MQm (both at temperatures of about 293 K).

It will be clear to those skilled in the art that the values mentioned above are not limited to such ranges, and these values may have to be adjusted for certain alternative materials, for example if polymer based conductors are used.

In the embodiments shown in the Figures 1 and 2, so-called through' vias are depicted, which pass through all layers of a PCB. In alternative embodiments, blind vias may be used (also referred to as buried or hidden vias). Use of blind vias would tend to increase manufacturing complexity and cost, but may result in further size reductions being attainable.

Further, it will be apparent to those skilled in the art that the core I may be formed of shapes other than as shown in Figures I and 2, and that gaps may be lefi between first and second core portions and/or between the core portions and the PCB(s), Embodiments may further comprise a layer (or layers) of electrostatic and/or electromagnetic shielding.

Figures 3 and S show enlarged views of cross sections through the embodiments of the planar transformer depicted in Figure 1, whereby to illustrate the structure of the sub-layers according to embodiments, and in particular, the structure of conductive parts and insulation parts, including the distribution of and distances between such parts. Figure 3 shows a view generally in direction 1, as shown in Figure L Figure 5 shows a view generally in direction II, as shown in Figure]. For the purposes of clarity, some areas have been left blank in order to show more clearly the position of certain parts, for example the position ofthe tracks making up the coils. It will be clear to those skilled in the art those areas that typically comprise electrically resistive material as mentioned above.

In order to reduce the size of the planar transformer, embodiments comprise arranging the sub-layers such that they are closely stacked, with no additional insulation beyond that provided by the insulation material of the PCBs. Such close stacking of sub-layers is attainable as a result of the configuration arrangements employed in some embodiments as described below. Planar transformers of such embodiments may be deployed for transferring relatively low powers, comprising outputting low currents and high voltages (when energised with respective relatively higher input currents and lower input voltages). Relatively low power in this context means less than SW and typically less than 3W, even for HOOV and 600V embodiments respectively. More generally, according to embodiments, "low current" comprises output currents less than mA and typically no more than imA, "high voltage" comprises output voltages over 380V and typically over 500 V, for example, for testing single-phase machinery, and up to as high as I kV, or even higher, for example, for testing three-phase machinery.

When power is applied to the transformer, voltages will exist between neighbouring conductive parts, such as between different conductive tracks, and between the conductive tracking and parts of the core I. If these voltage differences between neighbouring conductive parts becomes too large, the reliable operation and safety of the transformer may bejeopardised. This is of particular concern with regard to the combination of the conductive tracking and the core parts, because the core parts may have, for example, ground electrical potential, which may be the most different to the electrical potential of the various conductive tracking parts. In order to facilitate the close arrangement of sub-layers employed by embodiments of the present invention, the sub-layers of some embodiments of the planar transformer are designed to accord with a particular distance to voltage difference ratio, d'AV In Figure 3, the cross sections of the PCBs 2, 3 are shown substantially symmetrical to the central core cylinder 14, having a core cylinder axis A0. In alternative embodiments, the location of PCBs 2, 3, may be asymmetric. The portion of the PCBs 2, 3, viewable on the left hand side of Figure 3 are labelled 2a and 3a respectively, and on the right hand side are labelled 2b and 3b respectively. The cross sections of the conductive tracks are shown as having a substantially rectangular form, In alternative embodiments, the tracks may be alternative shapes, such as rounded. An example distance d is represented between a conductive track and the core 1.

Along the length of a coil, the potential present on the coil changes. With regard to the left hand side (region X) of Figure 3, an enlarged view of which is in Figure 4, for example, the potential carried by the coil increases as the coil progresses, for example, from tracked sub-layers 32c to 32b to 32a. It will be appreciated that the direction of potential increase can be selected to be either top-to-bottom of the sub-layers or bottom-to-top of the sub-layers. In any event, it can be seen that the sub-layers are designed such that, as the potential carried by each of the turns increases, the minimum distance between conductive tracks (e.g. distance D t < distance D2 < distance D3) and the core I is increased, This is to accord th a predefined distance to voltage difference ratio, cI'AV. Hence, it can be appreciated that in an X-Y-Z view, a cone.like stmcture of spirally arranged tracks is formed (if the core is cylindrical; or a pyramid-like structure if the core has a square cross-section, and similarly for other cross-sections).

Similar relationships may be employed between any conductive parts of the overall transformer assembly, as depicted in Figure 5, regions W and Y of which are enlarged in Figure 6, showing two vias 210 (which is not visible in Figures 1 or 2), 310 (which is visible in Figure 2). For example, the relationship can relate to the distance between a via and the core (e.g. D4), a via and a track of the primary coil (e.g. D5), a via in one PCB and a track of the coil on the other PCB (eg, D6), the secondary coil and the core (eg, D7), the primary coil and the core (D8), any track on the secondary coil and the magnetic core (D9), the via and any track on the secondary coil (e.g. D10), the via and any track on the secondary coil (eg. Di t) and a via and the magnetic core (e.g. D2). More generally, the relationship may be employed between any parts of the different conductive tracks, between any conductive tracks and vias, between any vias and the core 1, between conductive tracks and the core and/or between neighbouring vias, Meanwhile, because of the slight and continuous increase of electrical potential values (i,e, a small AT), the distances between adjacent conductive track parts within a given sub-layer can remain relatively constant and relatively small. In some embodiments, to simplify design, the requirements may be met by maintaining a distance of at least 0,03 mm between adjacent conductive parts, In alternative embodiments, a distance of at least 0.05 mm in maintained between adjacent electrical ii parts. As has already been explained, for convenience of manufacture according to exemplary embodiments herein, the distance between neighbouring conductive tracks is 015mm.

Whilst these design relationships have been illustrated with reference to PCB 3, the same regimes are similarly applicable to PCB 2, as well as any other PCBs.

In some embodiments, a distance to voltage difference ratio, cL'AV, of d;'AV> 0.3 mm!kV is applied for conductive parts buried within a given sub-layer. In some embodiments, a distance to voltage difference ratio, dAV of aY2SV> 0.4 mm/ky is applied. For example, in higher voltage applications, a larger distance to voltage difference ratio may be preferred. In some embodiments, a distance to voltage difference ratio, dAV of dAV> 0.5 mmlkV is applied. In some embodiments, a distance to voltage difference ratio, dAV of d/2%V> 0.7 mm/ky is applied. In some embodiments, a distance to vollage difference ratio, cI/AT' of dAV> 1 mm/kV is applied. In some embodiments, a distance to voltage difference ratio, aYAV, of d/AV> IS 1.5 mm/kV is applied. This maybe appropriate for example, for use in applications with voltages of 500 V or above. In some embodiments, a distance to voltage difference ratio, d/AV, of dAVup to 2 mm/kY is applied. Overall, a dAV range associated with embodiments of the present invention may be in the range 0.3 mm/kY < ClAy S 2.0 mmlkV. For other conductive parts, which are not buried within the sub-layers, the d7AVmay be as high as 2,75 or even 3.0 mm/ky, The present inventors have appreciated that the nature of test equipment is such that it is possible to maintain a surprisingly low value of JAV -lower even than standards relating to safety and insulation breakdown in transformers would normally dictate -even for very high voltage differences, This is because the transformers that are used in test equipment according to the present embodiments are enclosed, so that their outputs are not accessible to users (unlike the output terminals of mains mobile telephone chargers, for instance) and are only required to operate briefly and sporadically, during test operations, compared for example to a transformer in a machine, which may be required to operate under a steady state for long periods oftime.

It has also been shown that the only insulation that is required between neighbouring tracks, and between tracks and the core, is provided naturally by the PCB, and that there is no need to add further insulation, In other words, the present inventors have appreciated that transformers according to embodiments herein can tolerate a d'zSV value that is significantly sower than normal design rules, especially safety design rules, would otherwise dictate, despite large step-up and high output voltage requirements.

This means that embodiments herein, with a lower value of d'Ak can be smaller than would be expected for an alternative transformer that is bound by the safety rules, Moreover, dimensions herein associated with ciAk' values herein are single dimensions between two neighbouring parts in a generally line of sight (i.e. a straight line) relationship. In contrast, in known prior art, dimensions are measured cumulatively (e.g. dimensions a+b+c in Fig.6 of JP2008 166625) between multiple parts, and distances between some parts are measured perpendicularly with respect to distances between other parts. It is noted also in JP2008 166625 that the distances are external distances rather than relating to distances buried within a sub-layer.

Embodiments of the invention may comprise a so-called gapped core, of the kind illustrated in Figure 9. As shown, at least one air gap 140 is maintained between the first part 14a of the central core portion 14 and the second part 14b of the central core portion 14. In this case, a transformer according to embodiments herein may operate and be classed as a flyback transformer. A flyback transformer is a special kind of transformer that can be designed to deliver very high voltages, for example up to 10kv or more. In a flyback transformer, the air gap(s) increase the reluctance of the magnetic circuit and allow impulse energy storage in the magnetic core in the first part of the magnetising cycle. This energy is then released in the following part of the cycle to generate an impulse of high voltage, which is captured and passed to the output of the device. Embodiments herein may conveniently include gaps because the magnetic core is split across two parts (e.g. 14a, 14b), and, for example, the central cylindrical part of the core may be shortened with the other parts remaining the initial length.

Embodiments herein may thereby acquire flyback transformer characteristics, although such characteristics depend also on the nature of the supply voltage (e.g. a high-frequency input in the region of t0kHz to 300kHz, or more specifically in the region of 20khz to 200kHz) and electronic circuitry driving transformer.

In normal transformers, the ratio of output to input voltage typically matches the ratio of secondary to primary windings. With a flyback transformer according to the present invention, however, this is not the case and such a transformer can produce a very high output voltage even with a relatively low secondary/primary turns ratio.

For example, with reference to the transformer in the accompanying drawings, which as has been described is a 600V embodiment, there are 42 secondary turns and 8 primary turns, providing a secondary/primary turns ration of 5.25. The output voltage to input voltage ratio is nearer 120, with a voltage per output turn value of 14.3 V/turn.

The secondary (output) to primary (input) voltage ratio divided by the secondary to primary turns ratio is about 19. This final value is relatively high compared to known transformers.

For a similar 1 100V embodiment in which there are 35 secondary turns and 3 primary turns, providing a secondary/primary turns ratio n of 11.7. The output voltage to input voltage ratio is nearer to 220, with a voltage per output turn value of 31.4V/turn and a secondary to primary voltage ratio divided by the secondary to primary turns ratio of about 17. This final value is also relatively high compared to known transformers.

More generally, embodiments herein may have one or more of the following parameters: a rated output voltage of greater than 500V (e.g. for a 600V embodiment) or greater than 1000V (e.g. for a 1 100V embodiment); an input voltage of greater than 5V and less than 30V or, more specifically, of between 6V and 9V; equal to or fewer than 45 secondary turns; equal to or fewer than ten primary turns; a secondary/primary turns ratio of equal to or less than 12; an output voltage to secondary turns ratio of equal to or more than IOV/turn, and a secondary to primary voltage ratio divided by the secondary to primary turns ratio of greater than seven, in some embodiments greater than ten and in other embodiments greater than 15 and even greater than 20.

According to embodiments, at least one of the PCBs of the transformer is integrally formed in or as a region of a larger PCB. For example, the PCB may support or contain many other electrical components or subsystems that form a part of a piece of electrical equipment. As has been indicated, the electrical equipment may be insulation test equipment, which is relatively small and/or portable, and it may be battery powered, which makes use of the high voltages that are delivered by the transformer. Of course, the electrical equipment may be of another kind, unrelated to insulation testing but requiring a very high output voltage. In any event, the overhead of producing the transformer, or at least the part of the transformer that is common with the larger PCB, is relatively small compared with manufacturing the entire transformer as a separate assembly.

A photograph of a portion 700 of such a larger PCB is provided in Figure 7. As can be seen, the larger PCB 700 supports various components including a planar transformer 100 according to embodiments herein. The planar transformer is seen to have a clamp 13, an upper core portion 11, a lower core portion 12 and a first (e.g. primary) PCB 2. The larger PCB 700 itself forms the second (e.g. secondary) PCB 3 of the planar transformer 100. In other embodiments, the PCB 700 may form a primary PCB of the planar transformer, The number of layers in a single multi-layer PCB according to embodiments herein may be four, six (as shown) or up to eight; and one or two PCBs may be deployed in forming such a transformer.

An exemplary planar flyback transformer driving circuit according to embodiments herein is illustrated in the diagram in Figure 8. As shown, the planar transformer 100 has a primary winding Li, across inputs A and B, and a secondary winding [2, across outputs C and D, Input A and input B of the planar transformer are connected to an input circuit in which input A is connected to a voltage supply V and input B is connected to the drain of a MOSFET Ti, of which the gate is connected to a clock or control voltage Yci and the source is connected to ground via an optional current sense resistance Rsense, Outputs C and D of the planar transformer are connected to an output circuit, which (when in use) is connected across a load Rload.

The output circuit comprises a capacitor C_out across the outputs, in parallel with the load R load, and a diode D_out in series between the output C and the capacitor C_out.

The input and output circuits, comprising at least T, R sense, Dout and C_out, form driving circuitry of the planar transformer and, typically, reside on a larger PCB 700, of the kind illustrated in Figure 7. Many other forms and arrangements of driving circuit are of course possible, for example using discrete andlor integrated components to suit any desired configuration.

A planar flyback transformer driving circuit according to embodiments herein ensures that the secondary winding has a fast response time and, in particular, a relatively short discharge time. For example, for a ÔOOV embodiment, the discharge time according to an embodiment is in the region of I IOns, whereas the discharge time associated with a 1 100V embodiment may be as low as 88ns. More generally, for embodiments herein, discharge times will typically be equal to or less than 200ns. The low discharge time is attainable in part because of the low number of turns in the primary and secondary coils, which leads to reduced inductance. Such a rapid discharge time is important, so that a relatively low voltage applied to the input can generate a relatively high voltage at the output. The skilled person will appreciate that the output diode D_out must have a response time commensurate with the required response time of the secondary winding.

The graph in Figure 10 illustrates a cycle Cl of a first waveform A showing the current in the primary winding and of a second waveform B showing the voltage across the secondary winding of a I bOY embodiment. On the graph, the x-axis has a scale of 2s per major division and the y-axis has a scale of 5OmV per major division. An operating frequency of the circuit is 90 kFIz, whereby each cycle takes about 12i.ts. At some instant the MOSFET Ti is switched ON, which causes the current in the primary winding to rise in a ramp-like fashion. When enough energy is stored, the MOSFET Ti is switched OFF, The secondary winding takes over the stored energy and delivers it through the diode D_out to the output capacitor C_out and the load Rload.

A discharge time of the secondary winding is measured at around 200ns, but depending on how the time is measured can in some embodiments be as high as 300ns (for example due to oscillations caused by parasitic (unwanted) parameters of the transistor, transformer and diode operating at such high frequencies). It will be seen that the ramp time is about 25 times longer than the discharge time for the exemplary circuit design.

A discharge time of 200ns is equivalent to a frequency component of 5 MHz, which is relatively high compared with known high-voltage flyback transformers that would typically not be used above 1 MHz due to the limiting influence of the parasitic factors. While low-voltage non-flyback converters can be easily driven at 1MHz speed, high-voltage transformers usually have high parasitic capacitance due to many turns, which is so deleterious that higher frequencies, for example above 50kHz, are not typically attainable.

The graph in Figure II illustrates the voltage across the input MOSFET Ti. On the graph, the x-axis has a scale of 4jis per major division and the y-axis has a scale of 20V per major division. When the MOSFET is ON the voltage across it is relatively low -this is during the ramp phase of the graph in Figure 10. At the moment of turn S OFF, the secondary voltage rises (see the graph in Figure 10) and, at the same time, the primary voltage also rises, according to the secondary to primary turn ratio. As can be seen, the peak voltage is up to 120V. Such a high voltage is caused by the output voltage pulling-up' the input voltage across the transformer as dictated by the turns ratio of the transformer. Consequently, despite the input voltage due to the batteries typically delivering no more than 12V, and probably closer to 5V, it is important to ensure that the MOSFET Ti is rated to cope with such high voltage spikes. For example, it would be advisable to rate the MOSFET Ti at above t2OV, for example up to 200V or even up to 600V according to embodiments herein. Because of the transformer action resulting from the turns ratio a typical snubber circuit would not be used. Instead, according to embodiments herein, a high-rated MOSFET is incorporated in order to facilitate appropriate functionality of the whole transformer circuit.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It will also be seen that by following one or more of the aforementioned embodiments, it may be possible to forego the operating insulation typically added to transformers (e.g. to coil formers or bobbins) after or during their manufacture, It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

LEGEND

1 core 11 core upper portion 12 core lower portion 13 clamp 14 central core portion (e.g. cylinder) 14a,b central core portion parts air gap between central core portion parts 2 first PCB 2a,b visible cross sections of the first PCB sub-layer structure 21 via 210 via cross section 22a,b,c tracked sub-layers 23a,b insulation sub-layers 220 track cross sections on 22a, 4 turns 221a,b upper and lower track cross sections on 22b, 4 turns 222 track cross sections on 22c, 4 turns 3 second PCB 3a,b visible cross sections of the second PCB sub-layer strncture 31 via 310 via cross section 32a,b,c tracked sub-layers 33a,b insulation sub-layers 320a,b upper and lower track cross sections on 32a, 7 and 8 turns 321a,b upper and lower track cross sections on 32b, 9 turns 322 track cross sections on 32c, 9 turns 700 wider PCB AU central core portion axis I cross section plane and direction of view as applied for FIG. 3 II cross section plane and direction of view as applied for FIG. 4

Claims (2)

1. Claims 1 A planar flyback transformer having an rated output voltage of at least 380V comprising: a core comprising a material of low coercivity; and one or more printed circuit boards comprising electrically conductive tracking arranged to form at least an input primary coil and an output secondary coil each arranged spirally around the core, wherein the secondary to primary voltage ratio divided by the secondary to primary turns ratio is greater than 7.
2. 2. A transformer according to claim I, wherein the electrically conductive tracking is arranged to have a distance to voltage difference ratio aYAV 2.0 mm/ky between the electrically conductive tracking and the core.

   3, A transformer according to either preceding claim, wherein the neighbouring electrically conductive parts thereof are each arranged to have a spacing according to said ratio dAV 4. A transformer according to any preceding claim, wherein at least one of the one or more printed circuit boards comprises two sub-layers which are interconnected by means of a conductive via.5. A transformer according to any preceding claim, wherein conductive tracking on a first of the two sub-layers is arranged such that there is a first separation distance between the electrical tracking and the core, and conductive tracking arranged on a second of the two sub-layers is arranged such that there is a second separation distance between the electrical tracking and the core, the first separation distance being larger than the second separation distance and the transformer being configured such that the magnitude of the potential present on the electrical tracking of the first sub-layer is greater than the magnitude of the potential present on the electrical tracking of the second sub-layer during operation of the transformer, whereby said ratio dAV is maintained for each sub-layer.6. A transformer according to any preceding claim, wherein at least one of the one or more printed circuit boards is integrally formed within a larger printed circuit board comprising a wider electrical circuit.7, A transformer according to any preceding claim, adapted to deliver output voltages greater than 500 V, wherein a ratio dAFT of tess than 1.5 mm/ky is maintained between the electrical tracking and the core.8. A transformer according to any preceding claim, adapted to deliver voltages greater than 500 V, wherein a ratio J2SVof less than 2.5 mm/ky is maintained between the electrical tracking and the core.9. A transformer according to any preceding claims, wherein the core comprises a central portion with an axis that is substantially perpendicular to the one or more printed circuit boards.10. A transformer according to any preceding claims, comprising a central portion, having at least one air gap between an upper portion thereof and a lower portion thereof.11. A printed circuit board comprising circuit components and at least one transformer according to any preceding claim, at least one of the one or more printed circuit boards of the transformer being formed in a region of the said printed circuit board.12. A printed circuit board according to claim 11, wherein the transformer is driven by a driving circuit and the printed circuit board supports the components of the driving circuit on one or more other regions of the printed circuit board.13. An item of electrical equipment comprising at least one printed circuit board as claimed in claim ii or claim 12.14. An item of electrical equipment according to claim 13 powered by batteries.15. An item of electrical equipment according to claim 14, wherein the batteries deliver 30 volts or less.16, Battery operated equipment for insulation testing, for delivering a test voltage of at least 380V from a battery input delivering 30V or less, said equipment comprising a transformer according to any one of claims to 0, for stepping up the battery input voltage to the test voltage.17. A transformer substantially as hereinbefore described and/or with reference to the accompanying drawings.18. A printed circuit board substantially as hereinbefore described and/or with reference to the accompanying drawings.