High Frequency Transformer for High Voltage Applications

This invention relates to a high frequency transformer for high voltage applications.

According to the invention there is provided a transformer comprising primary winding means comprising a first plurality of magnetic circuits each with a second plurality of turns and electrically connected in parallel or each powered with respective power supply means; and secondary winding means comprising a third plurality of magnetic circuits each with a fourth plurality of turns and electrically connected in series; wherein the primary winding means are electromagnetically coupled to the secondary winding means by single turn electrically conductive loop means. Conveniently, the primary winding means and secondary winding means are coaxial.

Advantageously, at least one of the magnetic circuits of the secondary winding means is provided with rectifier means and filter means to provide a DC output from the transformer.

Conveniently, at least one of the magnetic circuits of the secondary winding means is further provided with inductive smoothing filter means.

Advantageously, the inductive smoothing filter means is a double wound inductor.

Conveniently, the inductive smoothing iilter means for each of the third plurality of magnetic circuits have a common core.

Advantageously, the transformer further comprises inductance means in the single turn electrically conductive loop means.

Conveniently, the single turn electrically conductive loop means comprises tube means coaxial with the primary winding means and secondary winding means.

Preferably, the single turn electrically conductive loop means further comprises formed conductive end cheek means and base means. Advantageously, the formed conductive end cheek means are provided with at least one aperture for passing cooling fluid therethrough.

Conveniently, each of the first plurality of magnetic circuits, each of the plurality of second magnetic circuits, the rectifier means and the filter means are mounted on respective printed circuit board means.

Advantageously the inductive smoothing filter means is mounted on the respective printed circuit board means.

Alternatively, the primary winding means and the secondary winding means are in side-by-side relationship. Conveniently, the secondary winding means further comprises extra high tension end insulating means.

Advantageously, the secondary winding means is arranged in two groups of magnetic circuits such that a secondary voltage is tapped substantially from a centre of the secondary winding means between the two groups of magnetic circuits. Conveniently, screen means is provided between the primary winding means arid the secondary winding means.

Advantageously, the smoothing inductive filter cores are provided coaxially with, and internally of, the primary winding means.

Conveniently, the magnetic circuits of the secondary winding means are of decreasing diameter from a high voltage end to a low voltage end of the secondary winding means.

Optionally, the primary winding means are divided into a first group of magnetic circuits and a second group of magnetic circuits and the first and second group are arranged with their respective axes collinear with an axis of the secondary winding means with the first group at a first end of the secondary winding means and the second group at a second end of the secondary winding means opposed to the first end.

Advantageously, a return path of the coupling loop means is formed of a strip wider than a remaining portion of the coupling loop means.

Preferably, at least one of the primary winding means and the secondary winding means comprises single layer windings.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure Ia is a circuit diagram of a first embodiment of a transformer according to the invention; Figure Ib is a circuit diagram of a second embodiment of a transformer according to the invention including rectifiers and filters;

Figure Ic is a circuit diagram of a third embodiment of a transformer according to the invention including rectifiers and filters;

Figure Id is a circuit diagram of a fourth embodiment of a transformer including rectifiers and filters according to the invention including rectifiers and filters; Figure Ie is a circuit diagram of a fifth embodiment of a transformer according to the invention including rectifiers and filters;

Figure If is a circuit diagram of a sixth embodiment of a transformer according to the invention including rectifiers and filters;

Figure Ig isa circuit diagram of a seventh embodiment of a transformer according to the invention, wherein each of the first plurality of magnetic circuits is powered by respective power supply units;

Figure 2a is a schematic side view of a transformer having the circuit diagram of Figure Ib;

Figure 2b is a schematic end view of the transformer of Figure 2a; Figure 3 is a graph of field stress vs. radius of an inner core radius for a fixed radius of an outer core of the transformer of Figures 2a and 2b.

Figure 4a is a schematic side view of a transformer having the circuit diagram of Figure Ic;

Figure 4b is a schematic end view of the transformer of Figure 4a; Figure 5a is a schematic side view of a transformer having the circuit diagram of

Figure Ie;

Figure 5b is a schematic end view of the transformer of Figure 5 a;

Figure 6 is a photograph of windings for the transformer of Figures 5a and 5b;

Figure 7 is a circuit diagram of a seventh- embodiment of a transformer according to the invention;

Figure 8 is a schematic end view of transformer cores and windings of a first embodiment of a transformer having the circuit diagram of Figure 7;

Figure 9 is a schematic plan and end view of an eighth embodiment of the invention with a plot of voltage along the secondary windings;

Figure 10 is a schematic plan and end view of a ninth embodiment of the invention with a plot of voltage along the secondary windings;

Figure 11 is a schematic plan and end view of a tenth embodiment of the invention with a plot of voltage along the secondary windings; Figure 12 is a schematic plan and end view of an eleventh embodiment of the invention with a plot of voltage along the secondary windings;

Figure 13 is a schematic plan and end view of a second embodiment of a transformer having the circuit diagram of Figure 7, with a plot of voltage along the secondary windings; Figure 14 is a schematic plan and end view of a twelfth embodiment of the invention with a plot of voltage along the secondary windings;

Figure 15 is a photograph of a test example of a partially assembled transformer according to the invention; and

Figure 16 is a photograph of the test example of Figure 15 fully assembled. In the Figures, like reference numbers denote like parts.

Transformers have a primary winding and a secondary winding. Referring to a basic circuit of a transformer 101 according to the invention in Figure Ia, the primary winding 11 comprises a first plurality Npc of magnetic circuits 111 connected in parallel each with an identical number of turns n _pc ,. The secondary winding 12 comprises a second plurality Nsc of magnetic circuits 121, which need not be the same number as Npc which are connected in series, each of which has a number of turns n _sc .

All the primary and secondary magnetic circuits 111, 121 are linked by a single turn low resistance loop 13 which electromagnetically couples them together.

The compound assembly 101 has the following relationships between the primary and secondary voltages and currents.

For a voltage step up ratio from primary to secondary

Vload = Nsc-V _sc

where .Vload is a total voltage across the Nsc secondary turnings 12 and V _sc is a voltage across each secondary turning 12 "arid

_{λ n}

Vloop = V _pri - where Vloop is a voltage across the single turn low resistance loop 13, and V _pr j is a voltage across each of the primary turnings 111 in parallel therefore substituting for V _sc and thus

Vload "sc = Npc

^V pri ⁰ PC

For the current ratio from secondary to primary

Iloop = n _sc -Iload = n _pc -I _pc

where Hoop is a current in the single turn low resistance loop 13, Iload is a current in the secondary windings 121 and I _pc is a current in each of the primary windings 111. and I _pri = Npc-I _pc

where I _pr j is a sum of the currents in the Npc primary turnings 111 therefore, substituting for I _pc and thus

Iload _ "pc

Thus, it can be seen that, as would be expected from a known transformer:

Vload _ *pri Vpri ^Iload

The ratios are independent of a number Nsc of secondary windings or cores 121. Other known transformer rules apply such as impedance transformations and shunt inductances as they would for any transformer.

Thus in effect the circuit diagram of Figure Ia represents a transformer with a number of multiple magnetic circuits.

Referring to Figure Ib, a transformer 102 is illustrated similar to the transformer 101 illustrated in Figure Ia, but in which each of the Nsc secondary windings 121 is provided with a respective Br _1-0 rectifier 24 and a Q _-11 filter 23 respectively, so that the transformer 102 output is rectified to provide a DC output. This is a well known and most effective way to meet a high voltage DC requirement.

Semiconductor diodes are efficient rectifiers for such an application but are each limited to a maximum voltage of, for example, approximately 2000V. Thus for a 3OkV system at least 15 would be required. By providing an individual winding 121 for each diode 24 sharing occurs naturally, and a need for complex and lossy networks to ensure sharing is avoided.

It will be understood that a further benefit is that many of the stray capacitances that are inevitable within the transformer structure are charged to fixed DC voltages rather than being subject to alternating voltages at high frequencies. This reduces dynamic capacitance and lowers dielectric related losses. Thus an advantage of the multiple core 121 transformer described is that the transformer ideally lends itself to a multiple rectifier 24 approach. In high voltage applications, where many semiconductor devices must be used to obtain a suitable voltage rating, a risk of failure of components must be considered. With the multiple core approach of the present invention, a short circuit failure shorts out only a single core thus the system can continue to operate with the remaining cores until a suitable service interval for shut down and repair is reached. This is a very valuable characteristic in regard to reliability. With a traditional transformer, rectifier failure of a single rectifier can shut down an entire system.

Figure Ic shows a circuit diagram of a transformer 103 according to a third embodiment of the invention using Li _-N inductive smoothing filters 25 in an output circuit of each of the secondary coils 121, which adaptation is well known in the art.

Figure Id shows a circuit diagram of a fourth embodiment 104 with Li-N double wound inductors 26 in place of the inductive smoothing filters 25, which further aid in reduction of stray capacitance effects described earlier herein, as portions of the individual choke structures 26 have DC voltages between the windings and not alternating voltage. In Figure Ie a circuit diagram of a fifth embodiment 105 is illustrated which uses a common magnetic core 27 through all of the inductive smoothing filter single wound windings 28 similar to those of the third embodiment, and this is further discussed later herein in relation to assembly techniques.

Similarly, in Figure If a circuit diagram of a sixth embodiment 106 is illustrated which uses a common magnetic core 27 through all of the inductive smoothing filter double wound windings 29 similar to those of the fourth embodiment.

Figure Ig is a circuit diagram of a seventh embodiment 107 in which instead of connecting all the primaries 111 in parallel each primary winding is powered by a reespsective power supply 29, psul to psuN. Each power supply unit 29 is suitably designed so that the power supply units behaves as a relatively low impedance to the loop and the remaining power supplies make up a required power input, so that overall performance of the system is not adversely affected if one power supply unit fails.

Alternatively, a spare power supply unit 29 is installed in the system and powered up only when one of the other power supply units fails.

All the power supply units are operated with a symmetrical AC output with a suitable phase to add, operate with a common output current, and are synchronised. Small voltage variations in V _pc between each of the power supply units modules can be tolerated and the total voltage V| _OO p is the sum of all the individuals V _pc outputs. The power supply units all operate in parallel from a DC link with all positive poles . connected in parallel and all negative poles in parallel.

Many alternative implementations of power supply unit can be employed which are well known to those skilled in the art.

A realisation of the circuit diagram shown in Figure Ib of the second embodiment 102 of the invention is shown, in Figures 2a and 2b, in which primary toroids 111 at low vnitaee are located coaxially with, and at least partially within, secondary toroids 121 operated at high voltage, as illustrated in Example 1 herein. A key advantage of this system

is that voltage insulation requirements are between structures that have large diameters and reasonably smooth surfaces. This is ideal because as voltages rise it is important to avoid sharp points that cause significantly sharp field enhancements that can lead to failure by voltage breakdown. As voltage increases what is considered a "sharp point" becomes important.

Referring to Figure 3, Epk is a peak electric field, V = 100 kV is a voltage between the inner toroid 111 of outer diameter r and the outer toroid 121 of inner diameter R = 100 mm

V := 100 R := 100

r := l ,2.. 9O

This function is plotted by line 301 in Figure 3 for a constant radius R = 100 mm of the larger toroid 121. It will be noted that for very small radius r of the smaller toroid 111, although a distance between the toroids is larger than for larger values of r, the electric field rises to quite high values. Not unexpectedly if r tends towards R, then the electric field again rises as the surfaces become closer. For the given value of R = 100 mm a minimum electric field stress occurs when r = 37 mm and the resultant field stress of 2.72 kV/mm is only just below a breakdown value for dry air of around 3 kV/mm at normal temperature and pressure. A key advantage of the invention is an ability, for a given voltage, to select radii r and R for electric field minimisation.

Sizes of the toroidal cores 111, 121 are preferably selected such that a required winding for both the primary cores 111 and secondary cores 121 can be achieved with a single layer winding. This is most desirable but is not essential and multi-layer windings could be used. An advantage of a single layer winding is that eddy current losses in the wires are minimised as layers can otherwise compound eddy current loss to a very high degree. Also as a voltage on an individual core is low, the winding can be placed directly on a plastic finish that is usually found on the toroid core. The apparatus requires no further complex insulation systems between the core and the winding. Also with a single . layer winding the actual winding operation of putting a single layer winding on a toroid is one of the simplest and lowest cost processes in transformer winding.

A choice of materials for the core, an operating frequency, and individual core dimensions are determined by calculations for each individual core based upon Iload, and

Vloop. In this regard standard methods of calculation, assuming each individual toroid has a single turn winding, are applied. It may be expected that the benefits of the apparatus will be relevant when the core material is ferrite or nanocrystalline material.

Referring again to Figures 2a and 2b, there is illustrated assembly 102 with five individual secondary toroids 121 as an example but another number, even as many as 30 or 40 toroids could be used.

Each secondary coil 121 is mounted on a printed circuit board 3 on which the Brn rectifiers 24 and the CN filter capacitors 23 are also mounted.

There is a small space between individual circuit boards 3 for voltage isolation. As the secondary coils 121 are connected in series small connector systems 6 can be used so that the PCB's can be plugged together to ease assembly. The loop voltage is much lower and so the primary coils 111 can be closer together than the secondary coils, maybe even touching. A radial space 8 between the primary coils 111 and the secondary coils 121 is shown in the end view of Figure 2b and can be designed for a required voltage hold off as detailed earlier herein.

The loop 13 that links all the cores is formed by a central conductive tube 4 passing axially through the primary and secondary coils and by formed conductive end cheeks and base 5. The system can be immersed in a fluid for cooling and for voltage hold off enhancement requirements by known methods.

Where the system is to be used at high frequencies where current penetration depth will be low, for example, approximately 0.46mm at 2OkHz in copper, the use of a thin walled tube 4 and a flat structure with a high surface area for the end cheeks and base is most appropriate. For the central conductor 4 multiple small tubes and/or a slot down the length of the tube may also improve current distribution and lower AC resistance of the tube, which is desirable.

Apertures 29 may be located in the end cheek 5 so that, as shown in Figure 2a, fluid flow 9 can be directed thorough the' assembly if required for cooling the transformer. Figures 4a and 4b show a system 103 where the PCBs 3 have added the LN inductors

25 shown in Figures Ic and Id.

Figures 5a and 5b show the embodiments 105, 106 of the invention employing the circuits of Figures Ie and If in which the coils 111, 121 axe wound not as toroids but in a conventional form. Then a suitably insulated core, which may be made from an assembly of toroids, one inside the other, may be used as a common magnetic circuit. By placing toroids one inside the other a very large area magnetic circuit can be produced which is ideal for a high current filter application. This type of magnetic circuit is best used with ferrite or iron powder cores that use a particulate form of magnetic material rather than strip wound cores. A photograph of components of the embodiments 105, 106 is shown in Figure 6. The core 61 shown in the centre of the photograph and the right-hand core 62 each show a wire (winding) 611, 621 respectively passing through the toroid — this is a normal way of using such a core. By placing a group of the smaller cores 62 inside a group of the larger cores 61 a long magnetic core 63 of large area can be made. The wire (winding) 631 passes around an outer diameter of the outer core 61. This arrangement has a large air gap and thus a low μ — most desirable for a DC power supply filter choke. There is no theoretical limit to the number of cores which may be placed inside each other and the height of the stack. This approach is much lower in cost than having a large cylindrical block of ferrite or iron powder manufactured.

As will be known to those skilled in the art, in any transformer, coupling between the primary and secondary is incomplete and this transformer is no exception. However, the imperfection, or leakage inductance as it is usually known, is of a similar order to that obtained with a conventional transformer.

In some transformers it is desirable to have a deliberate leakage inductance and with the transformer of the invention this can be introduced in a most effective manner. Figure 7 shows a circuit diagram of a seventh embodiment 107 of the invention with a deliberate introduction of an inductance 71 in the loop as L_Loop. Voltages in the loop across the primary Vp_Loop and the secondary Vs_loop that were virtually identical in the basic design are now therefore different and calculations are made by using different values of VJLoop for the primary and secondary coils 111, 121.

Referring to the end view of Figure 8, realisation of LJLoop 71 can be made in a transformer in which the primary and secondary windings are coaxial by inserting magnetic material 81 in a cylindrical space 8 between the primary and secondary coils 111,

121. The insulation spaces 8 must be rated for the voltage hold off required or the magnetic material 81. may._be_of_a type of ferrite that has_high hold off voltage capabilities. Certain

nickel zinc ferrites can reliably hold off voltage up to 2kV/mm and so a transformer with leakage inductance using the magnetic material as both the high voltage insulator and the magnetic core can be made.

Referring to Figure 9, it will be understood that the coils 111, 121 do not have to be coaxial but may be side by side as shown in an eighth embodiment 108 of the invention.

For a high voltage output, which is the case for all of the systems considered here, a linear gradient over the coils will result in the high voltage at one end so that an eht insulator 91 is required. This increases the loop length. It will be noted that the primary coils 111, in this and following descriptions the term "coil" implies a toroid with a winding, have a same diameter; with an eht system the primary coils need less voltage clearance than otherwise.

Referring to Figure 10, for higher current applications in a ninth embodiment 109 two groups of secondary coils 121 may be used by arranging the groups so that a high voltage is in a centre of the two groups as shown in the accompanying graph. By using two groups of coils 121 for the secondary winding the high voltage point can be brought out at the centre. This significantly simplifies the eht insulation design and results in a smaller, i.e. shorter, secondary system than in the previous embodiment 108. The parallel secondary coils 121 also increase current capability.

For these embodiments the primary loop tube 4 may have a larger diameter than in previously described embodiments to reduce copper loss if required.

Small modifications to the mechanical arrangement can be made as shown in Figures 11 and 12. This is because clearance between the primary coils 111 is usually much less than a required clearance between the secondary coils 121.

Referring to Figure 11, an apparatus realising the coupling loop 13 may have a design with a linear conductor with the return path formed by a wider, possibly grounded, strip 131. Supporting primary side power electronics may be mounted on an opposite side of the strip from the windings. Water (or air) cooling this strip allows it to form a central mechanical interface for a power electronics system of the converter.

Referring to Figure 12, the embodiment 110 of Figure 11 can be adapted to arrangements like that of the eighth embodiment 108 of Figure 9. This changes an aspect ratio of the apparatus. A wider strip 132 may help reduce resistance slightly, but requires more joins. Connection methods are important with the primary structure and as

power/frequency contours rise this may become a weak link. However it now becomes possible to fit a screen 122 between the primary and secondary coil systems 111, 121.

Referring to Figure 13, the circuit shown in Figure 7 can alternatively be realised by fitting cores 71 on the primary loop, which fit inside the primary coils 111. This produces a same equivalent circuit as in Figure 7, but would not increase loop length. The relatively low inductance at the loop level could, in a wide number of circumstances, be achieved by the use of low Ur cores slipped over a part of the loop.

Referring to Figure 14, in an eleventh embodiment of the invention, any of the preceding embodiments can be adapted to use an arrangement where the secondary coils 121 increase in diameter as the voltage gradient increases, or reduce as the voltage gradient decreases. However, while core loss could be reduced such an embodiment would require manufacture of a larger number of different size coils.

In all of the foregoing non-coaxial embodiments the secondary coils 121 may be directly connected in series or each one may have a rectifier system, as with the coaxial embodiments. This rectifier system may be a bridge or voltage multiplying arrangement if so desired.

Instead of a single tube, the loop 13 may be formed from a number of smaller tubes so that a surface area of the loop is increased thus desirably reducing AC resistance effects.

The primary coils 111 can be arranged in series and/or parallel groupings as required. A convenient concept is to put all the primary coils in parallel and use the same winding as the secondary coils - thus reducing parts variance and increasing the quantities of the coils of a same type which are manufactured by a factor of two.

For higher power complete modules, including control and primary side power electronics, may be paralleled. By timing the triggers to each of the paralleled modules a high ripple frequency may be obtained. Thus higher power systems may be operated with low stored energy and thus avoid the need for an energy diverter, commonly known as a crowbar.

As described, the use of a secondary side smoothing inductor may also be employed.

Some embodiments require, and some embodiments may benefit from, this inductor. The inductor is normally referred to as a choke input filter. If such an arrangement is required then the arrangement can be readily adapted into the approach described herein. Each

secondary coil and rectifier would have an associated smaller inductor system, these circuit elements would be series connected.

This does not preclude the use instead of a single larger inductor. However, as the frequency is increased a larger inductor may prove challenging to build with adequately low stray capacitance. The use of smaller multiple inductors reduces the dynamic capacitance. The potential quantity build advantage suggested for the coil assemblies equally applies to the output inductors.

One important aspect of an output inductor is that, with certain topologies, the peak voltage can exceed the average output voltage by some significant degree. This makes the eht design more challenging.

Example 1

A transformer was constructed with two magnetic circuits mounted coaxially one inside the other. An outer core TX87/54/14 was wound with 73 turns of ptfe covered 19/0.2 mm silver plated copper wire. The inner core was TN36/23/15 wound with 34 turns of the same wire. The system was interconnected with a fabricated single turn structure using 1 oz/ft ² copper clad board and 50 μM copper. The photographs of Figures 15 and 16 indicate the assembly technique.

Figure 15 shows a location of the cores before final fitting of the single turn coupling system. Figure 16 shows the final assembly. The unit was tested using low level RF bridges.

At 10 kHz the series inductance and series resistance for each windings were as follows:

Outer Ls = 10.23 mH Rs = 15.76 R Inner Ls = 2.3I mH Rs = 4.41 R

The ratio based on square root of the inductance ratio is 2.104,. the value based upon the turns is 2.147 which is well within measurement error.

The leakage inductance was checked on one coil with the other coil terminals shorted.

The results confirm that mounting the inner coil within the outer coil produces a device that works and that the large radius of the structure results in a construction technique most suited to high voltage applications. It is also possible to control leakage inductance between primary and secondary by introduction of magnetic material in the space between the inner and outer coil. Thus there could be three separate magnetic circuits mounted concentrically.

If mounted in fluid the construction lends itself to forced convection past the coils by forcing fluid down the center.