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 filter 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 and 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:
a is a circuit diagram of a first embodiment of a transformer according to the invention;
b is a circuit diagram of a second embodiment of a transformer according to the invention including rectifiers and filters;
c is a circuit diagram of a third embodiment of a transformer according to the invention including rectifiers and filters;
d is a circuit diagram of a fourth embodiment of a transformer including rectifiers and filters according to the invention including rectifiers and filters;
e is a circuit diagram of a fifth embodiment of a transformer according to the invention including rectifiers and filters;
f is a circuit diagram of a sixth embodiment of a transformer according to the invention including rectifiers and filters;
g is a 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;
a is a schematic side view of a transformer having the circuit diagram of
b is a schematic end view of the transformer of
a is a schematic side view of a transformer having the circuit diagram of
b is a schematic end view of the transformer of
a is a schematic side view of a transformer having the circuit diagram of
b is a schematic end view of the transformer of
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
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·Vsc
where Vload is a total voltage across the Nsc secondary turnings 12 and Vsc is a voltage across each secondary turning 12
and
where Vloop is a voltage across the single turn low resistance loop 13, and Vpri is a voltage across each of the primary turnings 111 in parallel
therefore substituting for Vsc
For the Current Ratio from Secondary to Primary
Iloop=nsc·Iload=npc·Ipc
where Iloop is a current in the single turn low resistance loop 13, load is a current in the secondary windings 121 and Ipc is a current in each of the primary windings 111.
and
I
pri
=Npc·I
pc
where Ipri is a sum of the currents in the Npc primary turnings 111 therefore, substituting for Ipc
Thus, it can be seen that, as would be expected from a known transformer:
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
Referring to
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 30 kV 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.
c shows a circuit diagram of a transformer 103 according to a third embodiment of the invention using L1-N inductive smoothing filters 25 in an output circuit of each of the secondary coils 121, which adaptation is well known in the art.
d shows a circuit diagram of a fourth embodiment 104 with L1-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
Similarly, in
g 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 respective power supply 29, psu1 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 Vpc between each of the power supply units modules can be tolerated and the total voltage Vloop is the sum of all the individuals Vpc 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
As voltage increases what is considered a “sharp point” becomes important. Referring to
This function is plotted by line 301 in
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 load, 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
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
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 0.46 mm at 20 kHz 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
a and 4b show a system 103 where the PCBs 3 have added the LN inductors 25 shown in
a and 5b show the embodiments 105, 106 of the invention employing the circuits of
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.
Referring to the end view of
Referring to
Referring to
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
Referring to
Referring to
Referring to
Referring to
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.
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/ft2 copper clad board and 50 μM copper. The photographs of
At 10 kHz the series inductance and series resistance for each windings were as follows:
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.
Number | Date | Country | Kind |
---|---|---|---|
0706197.1 | Mar 2007 | GB | national |
This application is derived from international patent application PCT/GB2008/000980 and claims priority from GB 0706197.1 filed Mar. 29, 2007.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/GB08/00980 | 3/19/2008 | WO | 00 | 1/11/2010 |