GB2265265A

GB2265265A - Converters, inverters and power supplies

Info

Publication number: GB2265265A
Application number: GB9304964A
Authority: GB
Inventors: Rodney William Holt
Original assignee: CHEVIN ASSOCIATES Ltd
Current assignee: CHEVIN ASSOCIATES Ltd
Priority date: 1992-03-11
Filing date: 1993-03-11
Publication date: 1993-09-22
Also published as: GB9304964D0

Abstract

A power supply such as a DC/DC converter, and a method of operating the same, utilises a ferrite core transformer 12 having a primary winding 14 and at least one secondary winding 8, wound on respective formers 18, 22. One former fits within the other, but with an air gap 24 therebetween, so that air may circulate through the transformer when in use. The DC supply to the transformer is electronically switched, such as by FETs (28, 30 Fig 2), at a relatively high rate (typically 100 kHz), allowing operating at high power levels. The transformer may deliver at least g/200 kW where g is the weight of the transformer in grammes, or at least (g x f)/(20 x 10<6>) kW where f is the switching frequency of the primary supply. <IMAGE>

Description

CONVERTERS, INVERTERS AND POWER SUPPLIES This invention relates to electric converters and to methods of operating the same, which in preferred arrangements are used in inverters, power supplies and audio power amplifiers.

The background of the invention will be explained in relation to audio power amplifiers but, in its broadest aspects, the invention is applicable in many other fields which will doubtless occur to the reader.

Public address systems are often required at, for example, rock concerts to provide very high quality sound reproduction at very high sound levels in large venues e.g. stadia. Stacks of loudspeakers are supplied by racks of power amplifiers. Each amplifier may require a 2kW power supply and provide, say, 1200 watts of power in total to a small number of speakers in the stack. There may be a total of, say, 50 to 100 amplifiers in use at a large concert.

Transporting and assembling racks of amplifiers is onerous and physically demanding because of their weight. One conventional power amplifier for such use, and which is rather lighter in weight than some others, weighs approximately 80 lb. That is still very heavy to shift about in the numbers used.

There have been proposals to utilise ferrite core transformers at higher frequencies to save weight, but these have apparently been under-rated compared with the information available from the core manufacturers.

Even then, there has been a tendency for such a transformer to overheat.

It is a principal aim of the present invention to provide an electric converter which is able to have a very large power through-put and yet which'is of relatively low weight and small volume, and methods of operating such converters.

Against the above background, in accordance with a first aspect of the invention there is provided a method of operating a power supply unit for an amplifier, which power supply unit has: a ferrite-cored transformer of weight g grammes, a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to the primary winding to provide an alternating supply at said at least one secondary winding, a first former on- which said primary winding is wound; at least one second former on which said at least one secondary winding is wound; said formers being arranged one within another and with air gaps therebetween through which air may circulate; in which method the transformer is operated to deliver at least g/200 kW to a load connected to said at least one secondary winding of the transformer with the primary winding being connected to a DC current source of a sufficiently high power capacity.

The invention also provides a converter comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least g/200 kW, where g is the weight of the transformer in grammes.

In one example, we found that the winding arrangement allowed the transformer to be operated at the maximum rating published by the core manufacturer and, surprisingly well beyond the maximum, e.g. up to 1.5 times or twice the maximum power through-put published by the core manufacturers. Thus, for an audio power amplifier giving about 1200 watts output, requiring a power supply giving approximately 2 kW output, a conventional power supply would have an iron cored transformer which weighs about 40 lb. We have found that operating at a switching repetition rate of about 100 kHz, two ferrite core transformers of approximately 7 oz (220 g) each is sufficient at the core manufacturer's published maximum power throughput, but that the required power can be supplied by just one such transformer operating at that frequency.

Preferably, the total continuous output available from at least one secondary winding is at least 3g/400 kW, though it is possible to obtain as much as g/100 kW.

The size of transformer necessary to deliver a particular power through-put depends on the frequency of operation. Recognising that advances in core technology may increase the frequency at which a core may be operated, and alternatively that unexpected power through-put may be obtained at lower frequencies, in accordance with another aspect of the invention there is provided a converter, and a method of operating the same, comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least (g x f) / (20 x 106) kW, where g is the weight of the transformer in grammes and f is the primary switching repetition rate.

Having discovered that a power through-put far higher than expected is possible and, again, recognising that other ways may be found of dealing with any problems of dissipating heat from the windings, preferably, the total continuous output available from said at least one secondary winding is at least (3g x f) / (40 x 106) kW, but may be as high as (g x f) / (10 x 106) kW.

Another way to view the relative size of a transformer is by the volume it occupies in a circuit.

Taking this view, in accordance with yet another aspect of the invention there is provided a converter, and à method of operating the same, comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to the primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least v/130 kW, where v is the volume in cc of a cuboid of minimum dimensions needed to contain the transformer.

In general, since we have discovered that a power through-put far higher than expected is possible and again recognising that other ways may be found of dealing with any problems of dissipating heat from the windings, preferably, the total continuous output available from said at least one secondary winding is at least 3v/260 kW, but may be as high as v/65 kW.

Since, as mentioned above, the size of transformer necessary to deliver a particular power through-put depends on the frequency of operation, and again recognising that advances in core technology may increase the frequency at which the core may be operated, and alternatively that unexpected power through-put may be obtained at lower frequencies, in accordance with another aspect of the invention there is provided a converter, and a method of operating the same, comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching at a repetition rate f a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least: (v x f) / (13 x 106) kW, where v is the volume in cc of a cuboid of minimum dimensions needed to contain the transformer.

Having discovered that a power through-put far higher than expected is possible and, again, recognising that other ways may be found of dealing with any problems of dissipating heat from the windings, preferably a total continuous power output of (3v x f) / (26 x 106) kW, may be achieved, though that output may be as high as (v x f) / (6.5 x 106) kW.

Converters utilising ferrite core transformers have been proposed in the power supplies for personal computers. Here the output is, say, 200 to 300 watts and, because a highly regulated supply is required, a specially designed integrated circuit is used to monitor the mains power supply and the unit's output voltage to control the length of a voltage pulse applied across a primary winding of the transformer.

Switching off the resulting current causes the decaying magnetic flux in the core to induce a pulse in the secondary winding(s) dependent on the length of the voltage pulse applied to the primary winding. The converters proposed for use in computer power supplies have too low a power rating to be useful in, say, high power audio amplifiers and the complexity of the voltage regulation circuit leads to too great an expense.

Against this background, yet another aspect of the invention provides a converter, comprising: a power transformer having a primary winding and at least one secondary winding; and means for switching connections between a direct current supply and said primary winding at a predetermined repetition rate, to provide an alternating supply at said at least one secondary winding; wherein the transformer has a ferrite core and there is a capacitor connected in series with said primary winding which capacitor has such value as to form a resonant circuit with the reactive components of the equivalent circuit of said transformer, the resonant frequency of said resonant circuit being approximately equal to said predetermined switching repetition rate.

The equivalent circuit of a practical, i.e. non ideal, transformer has a reactance which may contain components from leakage and other stray inductances and parasitic capacitances in both the primary and secondary circuits. The dominant effect will be that of a series inductance such that the higher the load current the greater the reactive loss. . The transformer's secondary would thus exhibit poor load regulation.

The series capacitor of this aspect of the invention forms a resonant circuit with the total reactance (largely inductance) seen at the primary of the transformer. Selection of the capacitance, or perhaps adjusting the switching frequency, produces resonance at approximately the switching frequency.

Although the output voltage of the switching devices is of square wave-form, the current in both the primary and secondary circuits of the transformer is largely sinusoidal. This is because the reactance of the series resonant circuit increases for frequencies higher or lower than the resonant frequency with the effect that harmonics of the switching frequency are rejected, whilst the fundamental passes unattenuated.

Under these conditions, the reactance of the circuit is zero and losses are confined to resistive effects in the various windings and conductors. As it is possible to use only a few turns of heavy gauge wire, the resistive looses, and the parasite capacitances are small, so that significant improvement in the load regulation can be obtained.

The resonant arrangement has a number of other significant advantages. The sinusoidal current in the transformer circuits is in phase with the output voltage of the switching arrangement, as a tuned circuit exhibits no phase difference between applied voltage and current at its resonant frequency. This has the following benefits: 1) Current in the switching devices is at (or in practical terms close to) zero at the times when the devices switch i.e. go from on to off or off to on.

This removes the need for switching devices to be abruptly driven from fully off to fully on or vice versa, because with little or no current passing about the time of switching, power dissipation during switching is reduced to a low level even if the switching is performed relatively slowly.

2) As well as a reduction in power dissipation in the switching devices, the fact that there are no abrupt current changes: a) reduces the generation of RFI; and b) reduces stresses on the switching devices.

3) The current in the secondary circuit also falls close to zero about the time when switching takes place. The rectifiers in the secondary circuit therefore fall gradually out of conduction before switching occurs, instead of experiencing a rapid transition from being forward conducting to reverse biased, with a consequent reduction in heat dissipation and RFI which would otherwise be generated by the reverse recovery transient, which is characteristic of a diode suddenly experiencing a change from forward to reverse bias.

There are some proposed forms of stabilized negative-feedback switching power supplies where a series resonant arrangement consisting of a capacitor and inductor in series with the transformer primary is employed. By selecting the values of the resonant elements and the on-time of the switching device(s) it is possible to have very little current passing through the switches at the instant of switching, and the benefits listed above are obtained. Regulation is achieved by varying the repetition frequency of the switching to take account of changing supply voltage and load condition. This is usually performed by a special I.C. The high degree of regulation which is available from these supplies is unnecessary for an audio amplifier.

In the present invention it is found possible to tune the circuit of the ferrite core transformer effectively without the addition of additional series inductance, so that the reactance of the circuit is approximately zero at the operating frequency.

In order to provide a simple arrangement for switching the supply to the primary of the transformer, in any aspect of the invention, the means for switching preferably comprises two MOSFET devices connected with their source to drain paths in series across the direct current supply, the gate of each MOSFET device being connected to its source via a secondary winding of a signal transformer, the node between one MOSFET's drain and the other MOSFET's source providing the switched supply to the primary winding of the power transformer and being connected in a feed back loop via a capacitor to provide a feed back signal to a primary winding of the signal transformer, the arrangement being such that the circuit oscillates at said predetermined frequency, the MOSFET devices switching on and off alternately in antiphase with one another.Each MOSFET device may be connected in parallel with one or more other MOSFET devices in order to achieve the required current rating.

As will be clear from the preferred application, the invention extends to an inverter comprising a converter embodying one or more aspects of the invention and means for rectifying and smoothing the output of the or each secondary.

It will also be apparent that the invention extends to a power supply, including such an inverter and means for rectifying and smoothing an alternating supply from AC mains to provide said direct current supply to the switching means, and to an audio amplifier which includes such a power supply.

By way of example only, one embodiment of the invention will now be described, with reference to the accompanying drawing, in which: Figure 1 is a circuit diagram of a power supply embodying aspects of the invention; Figure 2 is a circuit diagram of the general switching means shown in the diagram of Figure 1; and Figure 3 is a cross section through the core and windings of a physical arrangement of the transformer shown in the diagram of Figure 1.

Referring to the drawings, the power supply shown in Figure 1 is used to provide positive and negative 85 volts on rails 2 and 4 with respect to a common rail 6, for a high power audio amplifier having an output rating of, for example, 1 Kilowatt or more. The audio circuits of the amplifier may be conventional and are not shown in the drawings. The voltages are obtained from a centre tapped secondary winding 8 of a power transformer 10 having a ferrite core 12 of a type known as ETD49, available from Philips. The alternating output of the secondary winding 8 is full wave rectified by a bridge arrangement of diodes D1 to D4, and smoothed by capacitors C1 and C2 each connected between a respective one of the positive and negative 85 volt rails 2 and 4 and the common rail 6.

The transformer 10 has a primary winding 14 driven by a switching circuit 16 at a high frequency e.g. 100 kHz. This is possible because of the ferrite core and enables the transformer to be very much smaller than it would need to be if it had a conventional iron core operating at 50 Hz, a feature which has not been fully utilised previously in the design of high power audio amplifiers.

Manufacturer's data is contradictory on the power through-put available from an EDT49 core, but taking the maximum of two stated values, at an operating frequency of 100 kHz, the maximum continuous throughput as published is 1 kW. At that rating, with conventional windings we experienced some overheating.

The transformer illustrated in the drawings has a special construction intended to ensure reliable operation at the manufacturer's maximum published through-put power and provides the potential for a further saving in size, or increase in output power, given the present core technology.

Thus. given the illustrated construction, we were able to obtain a through-put of 1 kW, 1.5 kW and even 2 kW and more operating at a frequency of 100 kHz, without any over-heating in the core or windings.

The construction is illustrated in the cross section of Figure 3. The ferrite core 12 is conventional and suitable for operation at 100 kHz.

the secondary 8 winding, which has 18 turns and a centre tap, is wound on a bobbin 18 which is placed on the central limb 20 of the core. The primary winding 14, which has 18 turns, is wound on a plastics former 22 spaced from the secondary winding 8 by an air gap 24 through which air can circulate to cool both the secondary winding 8 and the inside of the primary winding 14. The positions of the primary and secondary windings are interchangeable. With this construction we have been able to draw approximately twice as much power from the secondary winding than would be expected from a conventionally wound transformer based on the core manufacturer's published data.

The core utilised in the illustrated transformer was an EDT49 available from Philips. Based on that core the transformer weighed 220 g and a cuboid of minimum dimensions necessary to contain it has the dimensions 55mm x 55mm x 43mm, i.e. approximately 130 cc. The ability to use the transformer to supply 1.5 kW and 2 kW implies the possibility of reducing the volume of the cuboid of minimum dimensions necessary to contain the transformer to 0.75 or 0.5 respectively of the volume of the transformer illustrated and the weight of the transformer to 0.75 or 0.5 respectively of the weight of the transformer illustrated, to supply the same power.

Improvements in core technology are expected to reduce the weight and the volume of the cuboid of minimum dimensions necessary to contain the transformer, in inverse ratio to the frequency, so that doubling the frequency is expected to reduce the dimensions of the transformer to half what is now required using an operating frequency of 100 kHz.

The transformer will inevitably have a small amount of leakage inductance which would cause the voltage obtained at the output rails 2 and 4 to fall when the load current increases. In the circuit illustrated, a capacitor C3 is connected in series with the primary winding 14 and has a value which, with the leakage inductance, is resonant at a frequency equal to the repetition rate of the switching circuit 16, i.e.

100 kHz. Utilising an ETD49 transformer core wound as described, the leakage inductance provided series resonance at 100 kHz with a 0.4 pF capacitor. Provided the series resonant circuit is correctly tuned to the repetition rate of the switching circuit, the reactance of the circuit (at that frequency) is zero. The resistance of the 18 turn winding is very low, and the circuit is very selective so, although driven by a switching circuit, the current is close to sinusoidal and in phase with the fundamental of the output voltage of the switching circuit. The result is that at the moment the switching devices in the switching circuit operate, i.e. go conducting or non-conducting, the current is at or close to zero. This reduces stress and heating in the switching devices and reduces radio frequency interference.

Mains input to the power supply is rectified by rectifiers (not shown) and smoothed by a capacitor Cl to provide 340 volt direct current on a rail 26 of the switching circuit shown in Figure 2. Two complimentary N-channel MOSFETs 28 and 30, e.g. IRF740, are connected in a push pull arrangement with their source to drain paths in series across the 340 volt and zero volt rails in order to switch connections between the rectified and smoothed mains supply and the winding of the transformer. In practice, each MOSFET 28 and 30 may be two or moe devices connected in parallel, depending on the current load required.In operation, at a steady state the average voltage across the capacitor C4 is equal to half that on rail 26 so that the node 32 connecting the drain of MOSFET 28 to the source of the MOSFET 30 provides AC to the primary winding 14 of the transformer 10.

The source of each MOSFET is connected to its gate by a series circuit comprising a respective secondary winding 34 or 36 of a signal transformer 38 and a respective resistor R1 or R2. The inductances of the secondary windings 34 and 36 are chosen to form a resonant circuit with the gate capacitance of the respective MOSFETs at 100 kHz. Referring to the dots shown by the secondary windings, it will be seen that there is coupling between the two circuits that ensures that the MOSFETs switch on and off in anti-phase. The resistors Rl and R2 are used in multiple MOSFET arrangements, to eliminate parasitic oscillations.

Back-to-back zener diodes Z1 to Z4 protect the MOSFETs against excessive gate to source voltages. The circuit is maintained in oscillation by a positive feedback signal provided from the node 32 via the transformer's primary winding 40 and series resistor R3 and capacitor C4. The resistor R3 determines the gate drive currents which in turn determine the switching speed of the MOSFETS.

As will be appreciated in the power supply illustrated, the combination of the power transformer 10 and the switching circuit constitutes a converter which converts DC from the rectified and smoothed mains supply to AC at the secondary winding of the transformer. The combination of the power transformer, the switching circuit 16, the diode bridge Dl to D4, and the capacitors Cl and C2, constitutes an inverter, receiving 340 DC rectified and smoothed from the mains and providing positive and negative 85 volt outputs on rails 2 and 4.

Using two ETD49 transformers connected in parallel, a power supply was constructed capable of supplying a 1.2 Kilowatt audio amplifier.

Claims

1. A method of operating a power supply unit for an amplifier, which power supply unit has: a ferrite-cored transformer of weight g grammes, a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to the primary winding to provide an alternating supply at said at least one secondary winding, a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound; said formers being arranged one within another and with air gaps therebetween through which air may circulate; in which method the transformer is operated to deliver at least g/200 kW to a load connected to said at least one secondary winding of the transformer with the primary winding being connected to a DC current source of a sufficiently high power capacity.

2. A method of operating a power supply unit for an amplifier, which power supply unit has: a ferrite-cored transformer of weight g grammes, a primary winding and at least one secondary winding; means for repeatedly switching at a frequency f a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding, a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound; said formers being arranged one within another and with air gaps therebetween through which air may circulate; in which method the transformer is operated to deliver at least (g x f) / (20 x 106) kW to a load connected to said at least one secondary winding of the transformer with the primary winding being connected to a DC current source of a sufficiently high power capacity.

3. A method of operating a power supply unit for an amplifier, which power supply unit has: a ferrite-cored transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound; the formers being arranged one within another and with air gaps therebetween; through which air may circulate; in which method the transformer is operated to deliver at least v/130 kW to a load connected to said at least one secondary winding of the transformer with the primary winding being connected to a DC current source of a sufficiently high power capacity, where v is the volume in cc of a cuboid of the minimum dimensions needed to contain the transformer.

4. A method of operating a power supply unit for an amplifier, which power supply unit has: a ferrite-cored transformer having a primary winding and at least one secondary winding; means for repeatedly switching at a frequency f a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, the formers being arranged one within another and with air gaps therebetween through which air may circulate; in which method the transformer is operated to deliver at least (v x f) / (13 x 106) kW to a load connected to said at least one secondary winding of the transformer with the primary winding being connected to a DC current source of a sufficiently high power capacity, where v is the volume in cc of a cuboid of the minimum dimensions needed to contain the transformer.

5. A method of operating a power supply unit for an amplifier, substantially as hereinbefore described, with reference to the accompanying drawings.

6. A converter comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least g/200 kW, where g is the weight of the transformer in grammes.

7. A converter as claimed in Claim 6, in which the total continuous output available from said at least one secondary winding is at least 3g/400 kW.

8. A converter as claimed in Claim 7, wherein the total continuous output available from said at least one secondary winding is at least g/l00 kW.

9. A converter comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least (g x f) / (20 x 106) kW, where g is the weight of the transformer in grammes and f is the primary switching repetition rate.

10. A converter as claimed in Claim 9, wherein the total continuous output available from said at least one secondary winding is (3g x f) / (40 x 106) kW.

11. A converter as claimed in Claim 10, wherein the total continuous output available from said at least one secondary winding is (g x f) / (10 x 106) kW.

12. A converter comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching a direct current supply to the primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least v/130 kW, where v is the volume in cc of a cuboid of minimum dimensions needed to contain the transformer.

13. A converter as claimed in Claim 12, wherein the total continuous output available from said at least one secondary winding is 3v/260 kW.

14. A converter as claimed in Claim 13, wherein the total continuous output available from said at least one secondary winding is v/65 kW.

15. A converter comprising: a transformer having a primary winding and at least one secondary winding; means for repeatedly switching at a repetition rate f a direct current supply to said primary winding to provide an alternating supply at said at least one secondary winding; a first former on which said primary winding is wound; at least one second former on which said at least one secondary winding is wound, said formers being arranged one within another and with air gaps therebetween through which to allow air to circulate; and wherein the transformer has a ferrite core and the total continuous output available from said at least one secondary winding is at least: (v x f) / (13 x 106) kW, where v is the volume in cc of a cuboid of minimum dimensions needed to contain the transformer.

16. A converter as claimed in Claim 15, wherein the total continuous output available from said at least one secondary winding is (3v x f) / (26 x 106) kW.

17. A converter as claimed in Claim 16, wherein the total continuous output available from said at least one secondary winding is (v x f) / (6.5 x 106) kW.

18. A converter comprising: a power transformer having a primary winding and at least one secondary winding; and means for switching connections between a direct current supply and said primary winding at a predetermined repetition rate, to provide an alternating supply at said at least one secondary winding; wherein the transformer has a ferrite core and there is a capacitor connected in series with said primary winding which capacitor has such value as to form a resonant circuit with the reactive components of the equivalent circuit of said transformer, the resonant frequency of said resonant circuit being approximately equal to said predetermined switching repetition rate.

19. A converter as claimed in Claim 18, wherein the means for switching comprises two MOSFET devices connected with their source-to-drain paths in series across the direct current supply, the gate of each MOSFET device being connected to its source via a secondary winding of a signal transformer, the node between one MOSFET device's drain and the other .MOSFET device's source providing the switched DC supply to said primary winding of the power transformer and being connected in a feed back loop via a capacitor to provide a feed back signal to a primary winding of said signal transformer, the arrangement being such that the circuit oscillates at said predetermined frequency with said MOSFET devices switching on and off alternately in antiphase with one another.

20. A converter according to Claim 18 or Claim 19 and incorporating the features of any one of Claims 6 to 17.

21. An inverter comprising a converter as claimed in any one of Claims 6 to 20, together with means for rectifying and smoothing the output of the or each secondary winding.

22. A power supply, including an inverter as claimed in Claim 21, in combination with means for rectifying and smoothing an alternating electric supply from AC mains to provide said direct current supply to said switching means.

23. An audio amplifier, including a power supply as claimed in Claim 22.

24. A power supply as claimed in Claim 22 and substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.