GB2507776A

GB2507776A - Filter circuit having common mode bifilar inductor

Info

Publication number: GB2507776A
Application number: GB201220198A
Authority: GB
Inventors: Colin Arrowsmith
Original assignee: KBO Dynamics International Ltd
Current assignee: KBO Dynamics International Ltd
Priority date: 2012-11-09
Filing date: 2012-11-09
Publication date: 2014-05-14
Anticipated expiration: 2032-11-09
Also published as: GB201220198D0; GB2507776B

Abstract

A filter circuit 701 removes noise from a supply of direct current. During normal current drawing conditions, a common mode bifilar inductor 70) provides rejection of common mode noise between positive and negative input and output terminals 702-705. During transient current drawing conditions, the bifilar inductor is short circuited by the provision of two forward biased diodes 709, 710 so as to maintain output voltage. The filter can be provided as a standalone filter circuit for connection to a power supply, or can be incorporated into a power supply unit or an audio amplifier. The filter circuit may be used in audio signal amplification apparatus, for example for an electric guitar.

Description

FILTER CIRCUIT

CROSS REFERENCE TO RELATED APPLICATIONS

This application represents the first application for a patent directed towards the invention and the subject matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a filter circuit for, and a method of, filtering a supply of direct current to remove electrical noise. The present invention also extends to a power supply and audio signal amplification apparatus including such a filter.

2. Description of the Related Art

Filtering of electrical supplies is common in fields where a clean supply of current is required, particularly where mains-type, alternating current supplies are being transformed into direct current by a power supply for use by a load. Power conditioners are often used to reduce or even remove mains hum, which can be caused by stray magnetic fields and shared power lines.

This type of approach is broadly successful in reducing noise influenced by phenomena occurring on the "AC side" of a transforming power supply.

However, it is just as common for the loads being supplied by the power supply to themselves inject high-frequency noise back into the "DC side" due to their own operation. Such a scenario is often seen in personal computers, where several devices share the same supply of direct current. The electromagnetic activity of the devices within the computer, such as central processing units, random access memory and mechanical hard disk drives, can cause unacceptable levels of noise in the supply of direct current. Such a scenario can also occur in high fidelity hard disk-based music players, which store audio in digital formats on a hard disk, which is then converted into analog format by a processor for playback.

When the supply of direct current from the power supply is also used to power sensitive peripherals in such a device, such as audio amplifiers and digital-to-analog converters for example, the noise created by the other peripherals can become audible. At high gain levels, the noise can be

unacceptable.

Previous approaches to solving this problem usually involve providing two power supplies -one for noise-creating devices in a sytem, and another for powering devices less tolerant of electrical noise. However, this approach is generally expensive, and only circumvents the problem.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a filter circuit for removing electrical noise from a supply of direct current as set out in claim 1.

According to a second aspect of the present invention, there is provided a method of reducing electrical noise in a supply of direct current produced by a power supply unit as set out in claim 8.

According to a third aspect of the present invention, there is provided audio signal amplification apparatus as set out in claim 4.

According to a fourth aspect of the present invention, there is provided a power supply as set out in claim 16.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an example of a device that suffers from the aforesaid types of electrical noise; Figure 2 shows another example of a device that suffers from the aforesaid types of electrical noise; Figure 3 shows an approach to solving the electrical noise issue encountered in the examples of Figures 1 and 2; Figure 4 shows the topology of a filter circuit according the present invention; Figure 5 shows the mode of operation of the filter circuit shown in Figure 4 during normal current drawing conditions; Figure 6 shows the mode of operation of the filter circuit shown in Figure 4 during transient current drawing conditions; Figure 7 shows additional components that can improve the operation of the filter circuit shown in Figure 4; Figure 8 shows the connection of a power supply to the filter circuit of is the present invention; Figure 9 shows a power supply including the filter circuit; and Figure 10 shows an audio amplification apparatus including a power supply, an amplifier and the filter circuit.

DETAILED DESCRIPTIONOF EXAMPLE EMBODIMENTS

Figure 1 An example of a device that suffers from the aforesaid types of electrical noise issues is illustrated in Figure 1. The device shown is an audio reproduction device (101), and is often referred to as a docking station.

Docking station 101 includes a digital audio interlace socket 102, which is configured to receive a digital audio input signal from digital media player 103.

Digital audio interface socket 102 is also configured to provide digital media player 103 with power.

Internally, the digital audio input signal is processed and amplified by docking station 101, whereupon it is reproduced by loudspeakers 104 and 105.

S Figure 2 A further example of a device that suffers from electrical noise issues is illustrated in Figure 2. The device shown is a digital audio workstation (201), comprising a personal computer 202, a digital mixing console 203, a visual display unit 204 and a pair of audio monitors 205 and 206. Digital audio io workstation 201 allows a sound engineer to edit audio using digital mixing console 203, which controls digital audio editing software running on personal computer 202. Audio being edited, such as vocals or guitar tracks, can be heard using monitors 205 and 206, which are powered by a sound card (not shown), including audio amplifiers, installed in personal computer 202.

Figure3 In both of the examples illustrated in Figure 1 and Figure 2, a power supply unit is used to receive alternating current from a mains supply, transform it into direct current, and supply said direct current to the peripherals in the exemplary devices.

Figure 3 illustrates, in the form of a block diagram, an approach to solving the electrical noise issue encountered in the examples of the previous Figures. After transforming alternating current from a mains supply, a power supply unit 301 provides a source of direct current, which includes high-frequency electrical noise. The source of direct current is utilised by an audio source 302, which, linking back to the examplesof Figures 1 and 2, could in an embodiment be the combination of a storage device and a processor operating under program control to provide, in the form of an audio signal, a source of audio that can be amplified. These components are generally the source of the high-frequency electrical noise found in the source of direct current. In one other example, the audio signal itself could be received from an electric guitar or another type of electronic musical instrument.

The audio signal itself is then supplied to an amplifier 303, which, by drawing current from power supply unit 301, and thereby acting as a load, amplifies the audio signal, whereupon it can be supplied to a loudspeaker 304.

io In the present embodiment, a filter circuit 305 is provided in the current path from power supply unit 301 to amplifier 303. Filter circuit 305 is configured to remove the electrical noise from the supply of power to amplifier 303. The topology and mode of operation of filter circuit 305 will be detailed with reference to Figures 4 to 7.

In another embodiment, power supply unit 301 and filter circuit 305 provide, in combination, an improved power supply -this embodiment will be expanded upon with reference to Figure 9. Additionally, in another embodiment, power supply unit 301, amplifier303 and filter circuit 305 provide, in combination, improved audio amplification apparatus -this embodiment will be expanded upon with reference to Figure 10.

Figure 4 Filter circuit 305 is illustrated in detail in Figure 4.

In the embodiment shown in the Figure, filter circuit 305 comprises a positive input terminal 401 and a negative input terminal 402, arranged such that they may be connected to a power supply of the known type. An input voltage VIN is developed across the input terminals, which could be 12 volts DC.

Additionally, filter circuit 305 includes positive output terminal 403 and a negative output terminal 404 to allow connection to a load, for example amplifier 303 shown in Figure 3. An output voltage VOUT is developed across the output terminals.

In order to solve the problem of reducing the high-frequency noise in the supply of direct current, a common mode bifilar inductor 405 is provided, having a first winding 406 connected between the positive input terminal 401 and the positive output terminal 403, and a second winding 407 connected between the negative input terminal 403 and the negative output terminal 404.

In one embodiment of the present invention, the core of common mode bifilar inductor 405 is a ferrite core, but alternative suitable materials may be used.

The advantage of using an inductor is the tendency of its electrical reactance to increase in proportion to the frequency of a signal passing therethrough. Additionally, due to the bifilar winding of the common mode bifilar inductor 405, any noise that is common to both windings is cancelled in the core of the inductor, thereby reducing noise by around 60 decibels.

The higher an inductor's inductance, the higher its reactance, and therefore its ability to reject high-frequency noise. However, the higher inductance generally means a higher DC resistance, which, when considering the already-transformed supply of direct current from a power supply, can lead to a relatively substantial drop in output voltage.

In normal current drawing conditions by a toad, however, a drop in output voltage generally does not occur. Nevertheless, when considering an audio amplifier as the load, for example, transient peaks of current draw can occur that cause the current drawn from a power supply to equal, and in some cases exceed 5 amps. In an example where the power supply normally provides an input voltage VIN to filter circuit 305 of 12 volts DC, the transient current drawing conditions can lead to a short (of the order of less than 1 millisecond), but large depression of the output voltage VIN to 8.5 volts DC.

Such a scenario, when considering an audio amplifier as the load, manifests itself as the audible level of audio dropping substantially.

Thus, as shown in Figure 3, in order to alleviate this issue, a first diode 408 and a second diode 409 are provided -each of which is connected forward biased and in parallel with first winding 406 and second winding 407 respectively. In an embodiment of the present invention, the diodes are Schottky diodes, and thus they have a very small voltage drop across them, and have a very fast switching action. Indeed, Schottky diodes possess the property of having zero recovery time, i.e. when they switch from conducting to non-conducting operation due to there being no charge carrier depletion at their metal-semiconductor junction.

FigureS The mode of operation of filter circuit 305, including common mode bifilar inductor 405 and first diode 408 and second diode 409, is illustrated in Figures 5 and 6.

Figure 5 shows the flow of current through filter circuit 305 during normal current drawing conditions, e.g. those conditions where a load is not running at maximum power, or drawing very large amounts of current over a short space of time. Current flow is shown by the solid line in the Figure. In this type of operation, the input voltage VIN developed across input terminals 401 and 402 is substantially the same as the output voltage VouT developed across output terminals 403 and 404, i.e. there is little voltage drop across common mode bifilar inductor 405.

Figure 6 As mentioned previously, in an embodiment of the present invention, first diode 408 and second diode 409 are Schottky diodes. The nominal voltage drop across this type of diode is 0.1 volts (as opposed to 0.6 volts for standard silicon diodes), and thus if the voltage drop across common mode bifilar inductor 405 exceeds 0.1 volts -which may be considered a transient condition -the diodes become forward biased.

The flow of current during such a transient condition, where the difference between the input voltage VIN and the output voltage VOUT will io exceed 0.1 volts, is shown in Figure 6. As can be seen in the Figure, due to first diode 408 and second diode 409 becoming forward biased, current flows through them rather than through common mode bifilar inductor 405, i.e. short circuiting of the inductor takes place. As discussed previously, any effects caused by the resistance of common mode bifilar inductor 405 at high currents is negated during this type of high current draw transient conditions. Thus, output voltage across the output terminals is substantially maintained by the short circuiting.

It is worth noting that, whilst it is apparent that any noise in the supply of direct current will not be filtered out when current flows through the diodes, in the example where the load is an audio amplifier, the audio itself, being amplified to high levels, will in effect mask the noise, making it inaudible.

Thus, it can be seen that the present invention provides a method of reducing the electrical noise present in a supply of direct current, comprising removing common mode noise during normal current drawing conditions using a common mode bifilar inductor, and short-cirbuiting the bifilar wound inductor so as to substantially maintain the voltage supplied to a load during transient current drawing conditions.

Figure 7 Whilst the circuit topology illustrated in Figures 4, 5 and 6 provides good DC performance whilst maintaining the ability to remove common mode noise from a supply of direct current, the present invention also provides, in other embodiments, additional circuit elements to improve functional operation.

Referring now to Figure 7, a filter circuit 701 similar in construction to filter circuit 305 is shown. It includes a positive input terminal 702 and a negative input terminal 703, a positive output terminal 704 and a negative output terminal 705, a common mode bifilar inductor 706, having a first winding 707 and a second winding 708, and a first diode 709 and a second diode 710, both of which are forward biased. Each of these components in filter circuit 701 is arranged in the same way as the components making up filter circuit 305. Again, in an embodiment, the core of common mode bifilar inductor 706 is a ferrite core, and in an embodiment, first diode* 709 and is second diode 710 are Schottky diodes.

Filter circuit 701 differs by the inclusion, in one!mbod1m1t of current buffering capacitor connected across output terminals, having a low effective series resistance. In the embodiment shown in Figure 7, the current buffering capacitor is an electrolytic capacitor 711. The provision of electrolytic capacitor 711 has the advantage of lowering the output impedance of the filter circuit, and acts as a current reservoir so as to allow the supply of over 10 amps of current to a load during transient current drawing conditions, if required. In an embodiment, electrolytic capacitor 711 has a capacitance of 2200 microfarads, although its exact capacitance can be altered to meet the requirements of the filter circuit's application.

In a further embodiment, and due to the inherent inductance of the electrolytic capacitor, one or two low-inductance capacitors are provided to improve transient response. In one embodiment, a low-inductance capacitor 712 is connected across input terminals 702; in another embodiment, a low-inductance capacitor 713 is connected across output terminals 704 and 705; and in a further embodiment, both low-inductance capacitors 712 and 713 are provided. In an example embodiment, the low-inductance capacitors are disc ceramic capacitors, each having a capacitance of 100 nanofarads. Such capacitors have, for practical purposes, almost zero inductance, and therefore mitigate against the tendency of electrolytic capacitor 711, which has a relatively large inductance, to be reluctant to respond to fast current draw transients.

In a further embodiment, the positive input terminal 602 and negative input terminal 703 each comprise three pins connected in parallel (pins 702A, 702B and 702C, and pins 703A, 703B and 703C respectively), so as to allow connection of a six-pin power supply connector. Such a connector is commonly available on personal computer AIX-type power supply units, and is normally used for powering PCI Express® peripherals. The advantage of using this type of input is that the plurality of cables from the power supply unit to the six-pin connector has a lower effective resistance, which further enables an improvement in the DC performance of filter circuit 701.

Figure 8 As noted previously, in one aspect of the present invention, there is provided a filter circuit for removing electrical noise from a supply of direct current. A physical arrangement of apparatus according to this aspect is illustrated in Figure 8.

In this embodiment of the present invention, the power supply and filter circuit are physically separate items. Referring to the Figure, the power supply is an ATX power supply unit 801 and includes, in this example, an lEG 60320 Cl 3 connector 802, for receiving a supply of mains alternating current via a cable having an IEC 60320 C14 termination. A positive terminal and a negative terminal are present in this connector, along with an earth contact. In alternative embodiments, however, power supply 801 may instead include a connector of another type, which may not include an earth contact.

Converted and transformed electricity is supplied as direct current via a cable 803, which is terminated by a six-pin connector 804. Three of the pins are positive pins (805A, 8058 and 805C), and three are negative pins (806A, 806B and 806C).

The filter circuit is enclosed within a housing 807, with the filter circuit enclosed therein corresponding to that illustrated in Figure 7 (i.e. filter circuit 701). Referring again to Figure 8, a six-pin power port 808 is provided, with three positive terminals (809A, 809B and 809C), and three are negative terminals (810A, BlOB and 810C), each of which are suitable for receiving power from six-pin connector 804.

A positive output terminal 811 and a negative output terminal 812 are provided to allow connection of the filter to a load.

Thus, according to the embodiment shown in the Figure, there is provided a filter circuit for removing electrical noise from a supply of direct current that provides common mode noise rejection, and that substantially maintains output voltage between the output terminals.

Figure 9 In another aspect, the present invention provides an improved power supply, capable of mitigating against noise introduced by connected peripherals.

Referring to Figure 9, a power supply 901 is shown in. schematic form, and comprises a positive input terminal 902 and a negative input terminal 903 for receiving a source of alternating current. In addition, power supply 901 includes a positive output terminal 904 and a negative output terminal 905 for connection to a load. A first current path 906 connects the positive input and output terminals, and a second current path 907 connects the negative input and output terminals.

In order to produce a supply of direct current, power supply 901 also comprises a transformer circuit 908. As will be known by those skilled in the art, transformer circuit 908 provides functionality to rectify the incoming alternating current to produce direct current and reduce its voltage. Additional functionality may be provided in the case of a switched-mode power supply, where the output of the transformer circuit is isolated from the input, requiring initial rectification, filtering, switching and then secondary rectification to produce direct current.

Direct current in the two current paths is then filtered by a filter circuit 909, corresponding to the filter circuits described previously with reference to Figure 4 to 7. In this case, the first winding of the bifilar wound inductor forms part of the first current path 806, whilst the second winding of the bifilar wound inductor forms part of the second current path 907.

Filtered direct current is thus developed, and is thus able to be drawn by a load connected to the power supply at output terminals 904 and 905.

Figure 10 In another aspect, the present invention provides an improved audio signal amplification apparatus.

An audio signal amplification apparatus 1001 is illustrated in Figure 10, having a positive input terminal 1002 and a negative input terminal 1003 for receiving a source of alternating current. A power supply unit 1004 of a normal type is provided, possibly an ATX power supply unit, and transforms the alternating current into direct current.

Power is drawn by an amplifier 1005 via a first current path 1006 and a second current path 1007, so as to allow amplification of an audio signal received at audio input terminal 1008. Such an audio signal could be received from an electric guitar, for instance. Amplified audio is provided at an audio output terminal 1009, which can be connected to a loudspeaker for example.

Electrical noise developed in the first current path 1006 and second current path 1007 is filtered by filter circuit 1010, located between power supply unit 1004 and amplifier 100& In this embodiment, filter circuit 1010 corresponds to the filter circuits described previously with reference to Figure 4 to 7. The first winding of the bifilar wound inductor forms part of the first current path 1006, whilst the second winding of the bifilar wound inductor forms part of the second current path 1007.

Claims

Claims What we claim is: 1. A filter circuit for removing electrical noise from a supply of direct current, which filter circuit comprises: positive and negative input terminals for connection to a power supply, positive and negative output terminals for connection to a load; a common mode bifilar inductor having a first winding between the positive input and output terminals, and a second winding between the negative input and output terminals, to provide common mode noise rejection; and a first forward biased diode in parallel with said first winding and a second forward biased diode in parallel with said second winding to substantially maintain output voltage between the output terminals.
2. The filter circuit of claim 1, wherein the core of said bifilar inductor is a ferrite core.
3. The filter circuit of claim I or claim 2, wherein said first forward biased diode is a first Schottky diode and said second forward biased diode is a second Schottky diode.
4. The filter circuit of any one of claims I to 3, further comprising a current buffering capacitor connected across the output terminals.
5. The filter circuit of claim 4, further comprising a first low inductance capacitor connected across the output terminals.
6. The filter circuit of claim 4 or claim 5 including a second low inductance capacitor connected across the input terminals.s 7. The filter circuit of any one of claims 1 to 5, wherein the positive and negative input terminals each comprise three pins connected in parallel, so as to allow connection of a six-pin power supply connector.8. A method of reducing electrical noise in a supply of direct current io produced by a power supply unit, in which a load draws direct current from said power supply unit via a first current path and a second current path, said method comprising: during normal current drawing conditions, removing common-mode noise in the first current path and a second current path using a bifilar wound inductor; and during transient current drawing conditions, short-circuiting the bifilar wound inductor so as to substantially maintain the voltage supplied to the load.9. The method of claim 8, in which said bifilar wound inductor is provided with a ferrite core to remove high frequency common mode noise.10. The method of claim 8 or claim 9, wherein the short-circuiting of the bifilar wound inductor is achieved by use of two forward biased Schottky diodes, each one of which is placed in parallel to a respective winding of the bifilar wound inductor.11. The method of any one of claims 8 to 10, further comprising improving transient current supply to the load by providing a buffering capacitor to act as a current reservoir.12. The method of claim 11, further comprising improving transient response by providing a first low inductance capacitor across the first current path and the second current path prior to the bifilar wound inductor 13. The method of claim 11 or claim 12, further comprising improving transient response by providing a second low inductance capacitor across the first current path and the second current path after to the bifilar wound inductor.14. Audio signal amplification apparatus, having a power supply unit and an amplifier for amplifying an audio signal, in which current is drawn by the amplifier from the power supply via first and second current paths, further comprising: a common-mode bifilar inductor having a first winding forming part of said first current path and a second winding in series with said second current path, to provide common-mode noise rejection; and a first forward biased diode in parallel with said first winding and a second forward biased diode in parallel with said second winding to substantially maintain voltage between said first current path and said second current path.15. The audio signal amplification apparatus, wherein the audio signal is received from an electric guitar.16. A power supply comprising: positive and negative input terminals for receiving a source of alternating current, and positive and negative output terminals for connection to a load, with a first current path connecting thepositive input and output terminals, and a second current path connecting the negative input and output terminals; a conversion circuit configured to convert said alternating current into direct current; a common mode bifilar inductor to provide common mode rejection, having a first winding forming part of the first current path and a second winding forming part of the second current path, and a first forward biased diode in parallel with said first winding and a second forward biased diode in parallel with said second winding to substantially maintain output voltage between the positive and negative output terminals.17. The power supply of claim 16, wherein the core of said bifilar inductor is a ferrite core.18. The power supply of claim 16 or claim 17, wherein said first forward biased diode is a first Schottky diode and said second forward biased diode is a second Schottky diode.19. The power supply of any one of claims 16 to 18, further comprising a current buffering capacitor connected across the first and second output terminals.20. The power supply of claim 19, further comprising a first low inductance capacitor connected across the first and second output terminals.21. The power supply of claim 19 or claim 20, including a second low inductance capacitor connected across the first and second input terminals 22. A filter circuit substantially as shown in, and described with reference to, the accompanying drawings.Amendments to the claims have been made as follows: Claims What we claim is: 1. A filter circuit for removing electrical noise from a supply of direct current, which filter circuit comprises: positive and negative input terminals for connection to a power supply, positive and negative output terminals for connection to a load; a common mode bifilar inductor having a first winding between the positive input and output terminals, and a second winding between the negative input and output terminals, to provide common mode noise rejection; :" and a first forward biased diode in parallel with said first winding and a second forward biased diode in parallel with said second winding to substantially maintain output voltage between the output terminals.2. The filter circuit of claim 1, wherein the core of said bifilar inductor is a ferrite core.3. The filter circuit of claim 1 or claim 2, wherein said first forward biased diode is a first Schottky diode and said second forward biased diode is a second Schottky diode.4. The filter circuit of any one of claims 1 to 3, further comprising a current buffering capacitor connected across the output terminals.5. The filter circuit of claim 4, further comprising a first low inductance capacitor connected across the output terminals.6. The filter circuit of claim 4 or claim 5, including a second low inductance capacitor connected across the input terminals.
7. The filter circuit of any one of claims 1 to 5, wherein the positive and negative input terminals each comprise three pins connected in parallel, so as to allow connection of a sixTpin power supply connector.
8. A method of reducing electrical noise in a supply of direct current **: 10 produced by a power supply unit, in which a load draws direct current from said power supply unit via a first current path and a second current path, said method comprising: S...during normal current drawing conditions, removing common-mode noise in the first current path and a second current path using a bifilar inductor: * .. * S * anduring transient current drawing conditions, short-circuiting the bifilar inductor by use of two forward biased diodes, each one of which is placed in parallel to a respective winding of the bifilar inductor, so as to substantially maintain the voltage supplied to the load.
9. The method of claim 8, in which said bifilar inductor is provided with a ferrite core to remove high frequency common mode noise.
10. The method of claim 8 or claim 9, in which the two forward biased diodes are Schottky diodes.I11. The method of any one of claims 8 to 10, further comprising improving transient current supply to the load by providing a buffering capacitor to act as a current reservoir.12. The method of claim 11, further comprising improving transient response by providing a first low inductance capacitor across the first current path and the second current path prior to the bifilar inductor.13. The method of claim 11 or claim 12, further comprising improving transient response by providing a second low inductance capacitor across the first current path and the second current path after to the bifilar inductor. 0*** * .14. Audio signal amplification apparatus, having a power supply unit and an amplifier for amplifying an audio signal, in which current is drawn by the amplifier from the power supply via first and second current paths, further comprising: a common-mode bifilar inductor having a first winding forming part of said first current path and a second winding in series with said second current path, to provide common-mode noise rejection; and a first forward biased diode in parallel with said first winding and a second forward biased diode in parallel with said second winding to substantially maintain voltage between said first current path and said second current path.15. The audio signal amplification apparatus of claim 14, wherein the audio signal is received from an electric guitar.16. A power supply comprising: positive and negative input terminals for receiving a source of alternating current, and positive and negative output terminals for connection to a load, with a first current path connecting the positive input and output terminals, and a second current path connecting the negative input and output terminals; a conversion circuit configured to convert said alternating current into direct current; a common mode bifilar inductor to provide common mode rejection, having a first winding forming part of the first current path and a second winding forming part of the second current path, and a first forward biased diode in parallel with said first winding and a second forward biased diode in parallel with said second winding to substantially maintain output voltage between the positive and negative output terminals.S *Se.17. The power supply of claim 16, wherein the core of said bifilar inductor is a ferrite core.18. The power supply of claim 16 or claim 17, wherein said first forward biased diode is a first Schottky diode and said second forward biased diode is a second Schottky diode.19. The power supply of any one of claims 16 to 18, further comprising a current buffering capacitor connected across the first and second output terminals.20. The power supply of claim 19, further comprising a first low inductance capacitor connected across the first and second output terminals.21. The power supply of claim 19 or claim 20, including a second low inductance capacitor connected across the first and second input terminals.22. A filter circuit substantially as shown in, and described with reference to, the accompanying drawings. * * . * . *.,. * S. *S S *5 * S.'..S