GB2469124A

GB2469124A - Isolated power supply for an amplifier

Info

Publication number: GB2469124A
Application number: GB0905842A
Authority: GB
Inventors: Martin Hannen
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2009-04-03
Filing date: 2009-04-03
Publication date: 2010-10-06
Anticipated expiration: 2029-04-03
Also published as: GB2469124B; GB0905842D0

Abstract

A power supply for an audio amplifier has a sub-audio frequency-switched power filter between both poles of a DC input and both respective poles of a DC output. A capacitor is charged from the supply and then discharged into the load. Continuous power may be supplied to the load with two capacitors C1, C2 alternately switched so that one capacitor charges whilst the other supplies the load. Therefore there is no direct connection between the power supply and load, preventing audio signals reaching the load, and also preventing signals generated by the load circuit from reaching the dc input. The power supply is suitable for use in an aircraft.

Description

POWER SUPPLY

This invention relates to a power supply suitable for an audio or video amplifier, and to an amplifier including a power supply.

A problem on aircraft is that many different items of electronic equipment are powered from common power sources, which introduces a risk that any intelligible modulation over the power lines can inadvertently be transmitted by another piece of equipment, such as a navigation beacon, thus compromising encryption security. In modem communications there are recognised standard specifications for the separation of clear and secure signals. Some of these standards require extremely low levels of signal on power supplies for power amplifiers.

Previous attempts to suppress intelligible signals from power supplies in power amplifiers have involved active tracking constant current filter techniques, but these have been found to be inadequate. Accordingly, the present invention is aimed at improving suppression further, and it is applicable to audio amplifiers although it can also be applied equally to the prevention of data from being modulated onto the power lines. It can also be applied to different types of signal amplifier such as video amplifiers.

The present invention provide a power supply for an audio amplifier, comprising a sub-audio frequency-switched power filter between both poles of a DC input and both respective poles of a DC output.

Preferably, the power filter comprises at least first and second capacitors each connected across both the DC input poles and the DC output poles by ganged switches controlled by switching means such that at one phase of a sub-audio frequency switching cycle the first capacitor and the second capacitor are connected across 4 4 respectively the DC input and the DC output poles, and, at another phase of the said switching cycle, they are connected respectively across the DC output and DC input poles, whereby to provide continuous capacitor discharge to the DC output from one or other of the capacitors.

The invention also provides a method of filtering audio frequency signals between input and output poles of a DC power supply, by accumulating charge from the input poles in isolation from both the output poles, and then discharging the accumulated charge at the output poles in isolation from both the input poles.

Preferably, the said isolated charge accumulation and discharge is effected separately in plural out-of-phase processes, whereby to provide continuous discharge to the output poles.

The typical frequency range of audio signals is 300Hz to 6kHz base band, but modulated products containing audio content may, in certain embodiments of the invention, need to be suppressed. The range of sub-audio frequencies of the switching is preferably in the range 50Hz to 300Hz, more preferably 50Hz to 150Hz, and preferably about 100Hz.

The filter of the invention operates on both polarities of the power supply, as it is important to isolate both sides. By switching at sub-audio frequency, the current demand profile is averaged over a switching cycle and is not related to any audio component.

In order that the invention may be better understood, preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: -Figure 1 shows a power filter embodying the invention; Figure 2 shows an alternative form of power filter embodying the invention; Figure 3 shows an audio amplifier including a power filter and embodying the invention; Figure 4 shows an alternative circuit for a power supply embodying the invention; and Figure 5 shows a further alternative embodiment of the invention.

If electronic equipment were powered entirely from an internal battery, then there would be no opportunity for an extraneous signal to enter through the power supply or for signals generated by the load circuit to exit through the power supply.

However, as illustrated in Figure 1 of the drawings, if instead of a battery the equipment were supplied from an aircraft DC power supply, then there would be a danger of such extraneous audio signals being supplied across a load R, which may for example be an audio amplifier. In accordance with this embodiment of the invention, the load R is supplied from a capacitor C which has previously been charged from the aircraft power supply, thereby avoiding any direct connection between the power supply and the external power lines. In turn, this minimises the risk of extraneous audio signals reaching the load.

With reference to Figure 2, the use of two capacitors, Cl and C2, allows for continuous power to be supplied to the equipment, and prevents signals generated by the load circuit from existing through the power supply connections, one capacitor being charged from the aircraft power supply whilst the other capacitor is powering the equipment. Each capacitor is connected across both poles of the DC input and both poles of the DC output. The capacitors are connected through ganged switches controlled by switching means (not shown in Figure 2) such that at one phase of a sub-audio frequency switching cycle the first capacitor and the second capacitor are connected across respectively the DC input and the DC output poles and, at another phase of the switching cycle, they are connected respectively across the DC output and the DC input po1es.

In the embodiment shown in Figure 3, the four switches shown diagrammatically in Figure 2 are implemented as switched capacitors controlled by a timer/oscillator circuit which generates a square wave at sub-audio frequency. Power, for example from an aircraft power supply, is connected to a power supply unit 31 at bus terminals for zero volts and 28 volts. The audio-suppressed power output is connected to power an amplifier AMP within an audio amplifier circuit 32 which receives audio input across bus terminals "audio in" and "audio ret" and generates amplified audio output at corresponding loudspeaker output bus terminals "loudspeaker out" and "loudspeaker ret" which is earthed.

The input power is surge protected by a surge suppression device and associated circuitry, to limit input current and to protect the circuit from voltage surges. This input circuitry incorporates a series diode D to protect against reverse polarity power supply connection. It includes a current limiter and a voltage limiter as shown. A common-mode choke Li and capacitors are also included for switching noise suppression.

Transient voltage suppressors are connected between each input pole and earth, for lightning protection. The lightning ground G is separate from the ground plane for the remaining circuitry and is connected to a chassis of the equipment via one of its mounting screws. The ground plane of the equipment is floating within this unit.

The timer/oscillator circuit is supplied from the input power supply through a circuit which regulates the input voltage at 5 volts, and it generates a square wave to drive the switched capacitors which are ganged as shown. The switching frequency is at approximately 100Hz. In this example, the switched capacitors comprise four solid state double-pole-single-throw relays in a bi-phase sequence such that the supply input is alternately connected to one of two reservoir capacitors Cl, C2. The reservoir capacitor that is not connected to the input provides the power supply output. On alternate phases, the input charging and output supply capacitors are exchanged. The switching times of the switched capacitors are slower to make than to break the connection, so galvanic isolation between input and output is assured.

The series input current limiter is included to prevent any large inrush current on the first charging up of the capacitors at switch on. Likewise, connection of the output during operation could supply a large current, and so current limiting is included on the output.

The audio amplifier circuit 32 comprises further transient suppressors connected to each input terminal, and the input terminals are coupled to the amplifier AMP by way of a transformer L2 and a gain adjust potentiometer P, with AC coupling through a capacitor C. Between the amplifier AMP and the output buses there is output conditioning circuitry.

An alternative configuration of power supply filter R is shown in the power supply of Figure 4. This operates in a similar manner to the circuit of Figure 3, and specific values for the electronic components are illustrated. Optically isolated devices are used for the switched capacitor filter active switch elements. For simplicity, fixed resistors are used to limit the maximum current flowing through the switches during the charging and discharging of the capacitors, although the printed circuit board (not shown) also has provision for peak current clipping which would be a more power efficient alternative. The relay switches Si and S4 are switched at 100Hz in anti-phase to the switches S2 and S3 as shown. Low frequency switch transients are removed by a simple LC filter, and all input and output connections have EMC and lightning protection. The EMC protection of connections comprises adding RF filtering and lightening suppression components as close as possible to the external connections where they enter and exit the equipment in order to prevent externally-applied high power RF (such as might be experienced by flying close to a radar antenna) from upsetting operation of the equipment or internally generated RF signals existing from the equipment. The power supply is used for an audio amplifier (not shown in Figure 4 but similar to that of Figure 3), based around a class AB amplifier integrated circuit.

Alternative solid-state relays could be substituted for those illustrated. It is not essential that solid-state relays be used, although this does remove the need for complicated biasing arrangements of non-opto-isolated switches.

With reference to Figure 5, a more detailed circuit arrangement for a power supply embodying the invention is shown. The switched capacitor filter R corresponds to that of Figure 4, within the output stage 53 of the power supply circuit. The circuit component 51 downstream of the switching noise suppression choke LI comprises voltage and current limiting circuitry. Circuit 52, powered from the input power supply, is an implementation of the timer/oscillator circuitry with a timer clock module CM for controlling the capacitors' switching. Outputs Si to S4 provide the control signals for the switches Si toS4 in circuit portion 53 as shown.

In other embodiments of the invention, multiphase filters could replace the two-phase filters described above. For example, poly-phase switching could be achieved using random sequences that fold all frequencies uniformly across the full audio band.

Also, it is possible that the addition of extra filter sections in series would increase the isolation.

Further, in the embodiment of Figure 5, the circuit 52 uses a fixed frequency clock in clock module CM to control the rate at which the capacitor filter switches between charge and discharge cycles. Use of a non-fixed clock frequency increases complexity but may be beneficial in some circumstances. A continuously varying clock frequency would have the effect of spreading the spectrum of any low frequency modulations produced around the clock frequency and thus rendering them even more difficult to detect.

The residual emissions become more difficult to detect and decipher primarily because the total energy is spread across a range of frequencies, further reducing their peak level but also because frequency dither makes the clocking frequency more difficult to either identify or to lock onto.

Two methods of dithering the clock frequency are presented but many other clock frequency-spreading regimes are possible.

in a first variation the timer clock CM of the circuit of Figure 5 is modulated with a linearly varying (triangular shaped) waveform modulating the nominal 100Hz clock at a rate of approximately 0.25Hz. The modulating signal is produced by a standard triangle generator circuit consisting of two operational amplifiers, one configured as an integrator and the other as a comparator. This output modulates the CM oscillator via a resistor network on the control voltage input.

In a second variation, the timer clock CM is replaced with a pseudo-randomly varying modulation, which spreads the clock over a range of +/-10% from its centre frequency.

A pseudo-random variation is chosen, as it is easier to generate than a truly randomized frequency variation although the latter would be superior. A spread spectrum waveform is produced at a nominal 50kHz by an integrated circuit and a binary divider divides the output of this down to a 195Hz centre frequency. The output of this drives the bases of the transistors of the original circuit.

Claims

CLAIMS: 1. A power supply for an audio amplifier, comprising a sub-audio frequency-switched power filter between both poles of a DC input and both respective poles of a DC output.
2. A power supply according to Claim 1, in which the power filter comprises at least first and second capacitors each connected across both the DC input poles and the DC output poles by ganged switches controlled by switching means such that at one phase of a sub-audio frequency switching cycle the first capacitor and the second capacitor are connected across respectively the DC input and the DC output poles, and, at another phase of the said switching cycle, they are connected respectively across the DC output and DC input poles, whereby to provide continuous capacitor discharge to the DC output from one or other of the capacitors.
3. A power supply according to Claim 2, in which the switches are within relays driven by the switching means.
4. A power supply according to any preceding claim, in which the switching means comprises a sub-audio frequency timing circuit powered from the DC input.
5. A power supply according to Claim 4, in which the timing circuit is configured to generate a square wave to operate the switches.
6. A power supply according to any preceding claim, in which the sub-audio frequency is in the range of 50 Hz to 300 Hz.
7. A power supply according to Claim 6, in which the sub-audio frequency is between 50 Hz and 150 Hz.
8. A power supply according to Claim 7, in which the sub-audio frequency is about Hz.
9. A power supply according to any preceding claim, comprising a surge suppression circuit, for limiting current, in series between at least one of the DC input poles and the switches.
10. A power supply according to any preceding claim, comprising means for suppressing a transient voltages, connected across the DC input poles.
11. An audio amplifier comprising a power supply according to any preceding claim and a signal amplifier circuit arranged to receive power from the power supply.
12. An audio amplifier according to Claim 11, configured to provide an audio output signal in a predetermined frequency range, wherein the power supply is such as to isolate the audio amplifier circuit from any extraneous audio signals on the DC input poles in the predetermined frequency range.
13. An audio amplifier according Claim 12, wherein the predetermined frequency range is substantially 300 Hz to 6 kHz.
14. A power supply substantially as described herein with reference to the accompanying drawings.
15. An audio or video amplifier or other signal amplifier, substantially as described herein with reference to the accompanying drawings.
16. A method of filtering audio frequency signals between input and output poles of a DC power supply, by accumulating charge from the input poles in isolation from both the output poles, and then discharging the accumulated charge at the output poles in isolation from both the input poles.
17. A method according to Claim 16, in which the said isolated charge accumulation and discharge is effected separately in plural out-of-phase processes, whereby to provide continuous discharge to the output poles.
18. A method of filtering audio signals in a power supply substantially as herein described with reference to the accompanying drawings.