WO1991001589A1

WO1991001589A1 - Power supply inversion

Info

Publication number: WO1991001589A1
Application number: PCT/GB1990/001083
Authority: WO
Inventors: Andrew James Thomas
Original assignee: The Secretary Of State For Defence In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland
Priority date: 1989-07-19
Filing date: 1990-07-13
Publication date: 1991-02-07
Also published as: GB8916502D0

Abstract

A power supply inverter for providing an AC output from a DC power supply. Conventional power supply inversion involves the use of bulky transformers. By using high frequency transformers which are small and light a DC power supply is converted into at least two high frequency AC outputs of different frequency. Processing and filtering the outputs followed by conversion to a bipolar waveform produces an AC power supply which is the difference of the outputs so that for example AC outputs of 50,000 Hz and 50,050 Hz produce an AC power supply of 50 Hz. Uses include back-up power supplies for computers etc. and power supplies for electric tools where mains supply is not available.

Description

Power Supply Inversion

The invention relates to power supply inversion specifically though not exclusively for the production of a domestic mains frequency supply and voltage from DC batteries.

Various inverters areknown andused toproduce amains supplyfranbatteries and other DC sources.

Prior totheinventiontheproductionofmainsfrequencyelectricity, 50 or60 Hz at 110V-240V from a DC battery supply was achieved by inverters which included transformers. Ωiese suffer fran the disadvantage of employing a large iron core which makes them bulky and expensive. They can also be inefficient ormay notyieldanaturallysinusoidal outputwaveformunderall load conditions.

UK Patent 2130,823 describes a power siψply which overcomes the necessity for large iron cored low frequency power transformers by using a resonantLC circuit to convert a DC input to a high frequency output. By connecting inverters of this sort in parallel and controlling the power at the output from each so that only the high current output is delivered allows the invention ofGB 2,130,823 toproduce apower supplywhich givesconsistently high current output even under substantial load. liiis is however not suitable for supply of alow frequency sinusoidal outputwaveformtypical of mains electricity.

Consequently there is a need to provide an inverter suitable for the production of domesticmains electricity fromaDCbattery or other suitable supply which overcomes the above disadvantages.

Accordingly there is provided amethod for producing inversion of aDCpower supply to a low frequency AC output which comprises:

(a) converting theDCpower supply toproduceat least twoACoutputs of different and relatively high frequency;

(b) processing the outputs to produce a resultantwaveformwhich is the sum and difference of the outputs;

(c) low pass filtering of the resultant waveform to remove the sum component; and

(d) converting the filtered waveform to a bipolar waveform. The conversion of the DC power supply to high frequency AC outputs can be achievedwith transformerswhichare smaller, lighterand cheaper than those used for the direct conversion of DC tomains frequency electricity and by adopting the approach of using high frequency transformers as described herein it ispossibletoproduceasinusoidaloutputsuitableforapplication as mains electricity.

Whilst the low frequencyACoutput of thepresent invention is specifically intended to beproduced fromaDC source, referenceherein to theDCsource, which is used to produce low frequency AC, may also be taken to mean high frequency AC where appropriate.

Preferablytheinversion isusedfortheproductionofmainselectricity. If requiredmulti-phaseAC output such as a threephaseoutputcanbeproduced.

WhilstanysuitableDCpowersourcecanbeusedit ispreferredthatthesource comprisesone ormoreDCbatteries. Aconvenientalternativewouldbe solar cells. TheDCpower source isthelimitingfactor tooutputandsoasbattery and solar cell efficiency along with other DC sources improves so will the available AC output..

Preferably the frequencies of theoutputsproduced fromthepower supplyare typically of the order of 50,000 Hz. Outputs at frequencies of this order have the advantage that th-sy can be be used to induce electricity from a primary to a secondary winding without the need for any ferroelectric material.

Preferably the DC power supply is used to produce two outputs of different frequencies.

Wheremainsfrequency electricity is requiredthetwooutputsarechosenwith a frequency difference of 50 Hz or as may be required. Thewaveformof the outputs is not vital and canbe square or sinewave. In thecase of apower supply for aircraft a frequency difference and output of 400 Hz. As frequency output can be crystal controlled the stability of the output frequency may be improved.

Preferably the conversion of theDCsupplytoproduce the resultantwaveform is achievedwith the aid of inductor capacitor (L C) circuits. The use of

L/Ccircuits inconjunctionwith thetransformers improvestheefficiency of converting the power supply from DC to AC.

Preferably theL/Ccircuitsareconnectedtogether inparallel. Preferably the n rber of L/C circuits is twowhere high power output is not required. Where higher power output is required then it is necessary to connect in parallel an appropriate number ofL/C circuits at the two frequencies chosen to achieve the desired output.

The resultant waveform of the L/C circuits which are controlled at two different frequencies for example at 50,000 and 50,050 Hz is the sum and difference of the output frequencies.

Preferably each AC output is passed through a rectifier such as a diode.

Although each of the two output frequencies produced normally passes from a primary tocorresponding secondarywinding it ispossibletopasstheoutputs of the primarywindings to a single secondary winding in which case only one rectifier is necessary.

The low pass filtering of the resultant waveformwhich smooths thewaveform can be achieved by various means but preferably by either a capacitor in parallel or by an inductor in series or a combination of both.

The smoothedwaveformoutput is anenvelope of the difference in frequencyof thedifferent output frequencies for example50 Hz where just twooutputsare produced of 50,000 and 50,050 Hz.

The conversion of the smoothedwaveform to a bipolarwaveform is preferably achieved by a polarity steering device. Preferably the polarity steering device is either a low frequency thyristor or triac switching network.

Preferably triacs or thyristors are arranged to switch states in synchrony with the voltage minima. The voltage minima are preferably detected by a circuit either in parallel or series.

As theL/C system ismost efficient at resonance, it isprobablethat thetrue resonant frequency of theI Cnetwork is an average of the frequencies and in the case of theoutputs50,000 and50,050Hzmidway, at50,025Hz. Havingthe resonant frequency midway between the two output frequencies has the desirable effect of producing waveforms of equal amplitude.

This method provides a useful means for producing a domestic mains supply frequency from a DC power supply and by connecting devicesmade according to this method in parallel it is possible to produce amulti-phase supply, for example by connecting three devices to be sequentially out of phase by 120 degrees a three-phase mains supply can be produced.

According to a further aspect of the invention there isprovided an inverter for producing a low frequency AC power signal supply from a DC batterywhich comprises:

(a) 2 or more high frequency inverters for producing AC output signals having a f equency difference equal to the required lew frequency;

(b) a lew pass filter for smoothing the AC output signals to produce an envelope of the frequency difference; and

(c) passing the smoothed waveform to a low frequency polarity steering device to convert to a bipolar waveform.

Preferably the frequency supplies are high frequency supplies. Preferably thehigh frequency inverters aretransformerswhicharepreferablyconnected to L/C circuits which are tuned to resonate at the required frequencies.

Typically the required frequencies are 50,000 and 50,050 Hz which gives a frequency difference of 50 Hz.

The sum and difference of the output frequencies is preferably rectified.

The recification is preferably achieved by diodes.

The low pass filter for smoothing the AC output frequencies is preferably provided by a capacitor placed in parallel with the output and an inductor placed in series.

Preferably the polarity steering device comprises semiconductor devices; preferably using a lew frequency th ristor or triac switching network. Alternatively any suitable switching device which can be arranged in the required configuration could be used.

The apparatuswhich is conveniently sizedmay be used for supplyofdomestic mains electricity from a DC power supply.

Theapparatus couldbeusedeffectivelyforanon-interruptablepower supply to provide mains type electricity in the event mains failure.

The inventionwill nowbedescribedbyway ofexaπpieonly andwith reference to the accompanying Diagrams of which:

Figure 1 shows two high frequency supplies with L/C circuits and waveform smoothing components; Figure 2 shows a graph of time against voltage for the waveform produced at V in Figure 1; Figure 3 shows a graph of the smoothed waveform of Figure 2; and Figure 4 shows an overall view of a circuit for producing inversion and which includes a silicon controlled rectifier network and detection block.

A DC reserve battery power supply (not shown) is used to produce two high frequency power supplies of 50,000 and 50,050 Hz via the high frequency transformers (1) and (2) in Figure 1. Each high frequency power supply is passed through a respective L/C circuit (3) and (4), which comprises capacitors (3a) and (4a) which areinparallel connectionwith inductors (3b) and (4b), andwhich are designed tobe in resonancewith the frequency of the output of transformers (1) and (2). The inductors (3b) and (4b) form the primary windings with inductors (5) and (6) being secondarywindings. The outputfromtheinductors (5) and (6) ispassedthroughrespectivediodes (5a) and (6a) toprovide rectificationand the resultantwaveformatVas is shown in Figure 2.

Theresultantwaveformsubsequently smoothedbyaninductor (7) inseriesand capacitor (8) placed in parallel which act as a lew pass filter. The smoothing results in an output waveform as shown in Figure 3 which is an envelope of the difference in frequency of 50 Hz.

The envelope waveform is converted to a bipolar waveform by a system of silicon controlled rectifiers (9,10,11 and12) whichare thyristors as shown in Figure 4 and which are controlled by circuit block (13).

When silicon controlled rectifiers (9) and (10) areconducting, circuitwire

(14) will be positive with respect to circuit wire (15) and when silicon controlled rectifiers (11) and (12) areconducting circuitwire (15) will be positive with respect to circuit wire (14).

The switching states of the silicon controlled rectifiers (9,10,11 and 12) arechangedinsynchronywiththevoltageminima, asdetectedbycircuitblock

(13).

The output is similar to a domestic mains frequency and voltage supply.

Whilstthenumberofapplicationsoftheinverter isnotlimitedpossibleuses include back-up power supplies especially for computers andpower supplies for tools.

Claims

1. AmethodforproducinginversionofaDCpowersupplytoalowfrequencyAC output which comprises:

(a) converting theDCpower supplytoproduceatleast twoACoutputs of different and relatively high frequency;

(d) converting the filtered waveform to a bipolar waveform.

2. AmethodforproducinginversionofaDCpowersupplyasclaimedinclaim1 wherein the inversion is used for the production of mains electricity.

3. AmethodforproducinginversionofaDCpowersupplyasclaimedinclaim1 or 2 wherein the supply comprises one or more DC batteries.

4. Amethodforproducing inversionof aDCpowersupplyasclaimedinanyof theprevious claimswherein thefrequenciesoftheoutputsproducedfromthe power supply are typically of the order of 50,000 Hz.

5. Amethod forproducing inversionof aDCpower supplyasclaimedinanyof thepreviousclaimswhereintheDCpowersupplyisusedtoproducetwooutputs of different frequencies.

6. Amethod forproducing inversionofaDCpower supplyasclaimedinanyof the previous claims wherein the conversion of the DC supply to produce the resultant waveform is achieved with the aid of inductor/capacitor (LC) circuits.

7. AmethodforproducinginversionofaDCpowersupplyasclaimedinanyof the previous claims wherein the L/C circuits are connected together in parallel.

8. Amethodforproducing inversion ofaDCpowersupplyasclaimedinanyof thepreviousclaimswhereinthenumberofL/Ccircuitsistwowherehighpower output is not required.

9. Amethodforproducing inversionofaDCpower supplyasclaimedinanyof claims 1 to7 whereinwherehigherpower output is required then thedesired output is achieved by connecting in parallel an appropriate number of L/C circuits at the two frequencies chosen.

10. Amethod forproducing inversionof aDCpower supplyasclaimed inanyof theprevious claimswhereineachACoutput ispassedthrougharectifiersuch as a diode.

11. Amethodforproducing inversion of aDCpower supplyasclaimed inanyof the previous claims wherein lowpass filtering of the resultantwaveform is achieved by either a capacitor in parallel or by an inductor in series or a combination of both.

12. Amethod forproducing inversion of aDCpower supplyas claimed inanyof the previous claims wherein the conversion of the smoothed waveform to a bipolar waveform is achieved by a polarity steering device.

13. Amethod for producing inversion of aDCpower supplyasclaimed inclaim

12whereinthepolaritysteeringdeviceiseitheralowfrequencythyristoror triac switching network.

14. Amethod for producing inversion ofaDCpower supplyasclaimed inclaim

13whereinthetriacsorthyristorsarearrangedtoswitchstatesinsynchrony with the voltage minima.

15. Amethod for producing inversion ofaDCpower supplyasclaimed inclaim

14 wherein thevoltageminimaaredetectedbyacircuiteither inparallel or series.

16. An inverter forproducing alowfrequencyACpower signal supplyfromaDC battery which comprises:

(a) 2 or more high frequency inverters for producing AC output signals having a frequency difference equal to the required lew frequency;

17. An inverter asclaimed inclaim16whereinthefrequencysuppliesarehigh frequency supplies.

18. An inverter as claimed in claim 16 or 17 wherein the high frequency inverters are transformers which are connected to LC circuits which are tuned to resonate at the required frequencies.

19. An inverter as claimed in any one of claims 16 to18 wherein the sumand difference of the output frequencies is rectified.

20. An inverter as claimed in any one of claims 16 to 19 wherein the recification is preferably achieved by diodes.

21. An inverter as claimed in any one of claims 16 to 20 wherein thelewpass filter for smoothing the AC output frequencies is provided by a capacitor placed in parallel with the output and an inductor placed in series.

22. An inverter as claimed in anyone ofclaims16 to21whereinthepolarity steering device comprises semiconductor devices.

23. An inverterasclaimed inclaim22whereinthepolaritysteeringdeviceis a low frequency thyristor or triac switching network.

24. An inverter substantiallyasherein described andwith reference to the accompanying drawings.