AU605191B2

AU605191B2 - Improvements in or relating to the generating of ac from dc electric power

Info

Publication number: AU605191B2
Application number: AU80140/87A
Authority: AU
Inventors: Charles Cornelius Van Dongen
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-10-28
Filing date: 1987-10-26
Publication date: 1991-01-10
Anticipated expiration: 2007-10-26
Also published as: AU8014087A

Description

To The Conirnilssioiier of PRients.

'ii t FL STAMyP rC YA1.UE OF AT-;/V HW f A US TR AL IA PAT'ENTS ACT COMPLETE SPECIFICATION 6t~ 5191 OR I G I NAL (FOR OFFICE USE) ClIas s I nt CIa s s Application Number: Lodged: U

I

pp p p Complete Specification Lodged: Accepted: Pub]lished: P r ior i ty: ill p.

Related Art:

'I

.1 Name of Applicant(s): CH A RL ES. R.L I.Q 5. .VA.N D. QN. Q I Address of App] I can t(s) :j A ct ualI A 5 .OR.NE V.A N. D ON Address for Service: PATENT ATTORNEY SERVICES, 1025 Whitehorse Road, Box Hill, Victoria 3128 Complete specification for the invention entitled: IMPROVEMENTS IN OR RELATING TO THE GENERATING OF AC FROM DC ELECTRIC POWER The following statement is a full description of this invention, including the best method of performing it known to. 1I C'i k 80 140/87 S: IMPROVEMENTS IN OR RELATING TO THE SGENERATING OF AC FROM DC ELECTRIC POWER This invention relates the generation of an AC voltage waveform, e.g. of a sinusoidal or other desired waveform from an input power signal having a different waveform, particularly although not necessarily a DC voltage.

Such a process (referred to hereinafter generically as "conversion") is often required. For example where electrical apparatus or appliances which are constructed to operate from conventional mains AC supply are to be powered from a vehicle Sbattery, conversion of DC to AC is required. A suitable C*f -r' 1Ip. converter is therefore useful where no suitable mains supply is available, e.g. at camp and caravan sites.

AC generators or converters are known which produce a S quasi-square or other waveform which departs in varying degree from sine waves. Many appliances designed to operate from mains AC supply, at least 'or a time appear to operate in such a way 'ftr that they are operating normally when powered by such non sinusoidal sources. However such operation may be inefficient, noisy, and wasteful of energy, and may lead to damage to the I' a appliance and possible avoidance of manufacturer's warranties.

To alleviate such problems in the past, heavy, bulky and consequently expensive transformers and capacitors have been used to maintain the magnetisation current and/or output waveform within suitable parameters.

It is an object of the present invention to provide an AC voltage waveform generator which can be relatively small and Slight weight and which can efficiently and accurately produce an AC voltage waveform output of the desired waveform shape.

2 According to the present invention there is provided an AC voltage waveform generator for receiving an input voltage and for generating an output comprising an AC voltage waveform of a predetermined frequency and shape, the AC -tage waveform generator including a reference waveform generator for generating a reference waveform of frequency and waveform shape to be generated at the output, a relatively high frequency generator for generating a pulse train of frequency relatively high compared to the reference frequency, a pulse width modulator for modulating the width of the successive pulses of the high frequency pulse train, a high frequency switching Scircuit for receiving the input voltage and for switching through the input voltage for successive periods in response to the pulse width modulator, a smoothing circuit receiving as C C: input signals directly or indirectly from the high frequency switching circuit the switched through input voltage, the smoothing circuit having a relatively long time constant relative to the input voltage pulse duration from the high frequency switching circuit so as to effect waveform smoothing of successively received varying width pulses and thereby produce a generally continuously varying waveform, the AC ~cCC voltage waveform generator further including a commutator S circuit receiving the substantially continuous waveform from the smoothing circuit and being operative to invert successive half cycles of the continuous waveform in response to the reference waveform generator so as to thereby produce the AC voltage i waveform output, the AC voltage waveform generator further iI including a comparison network for comparing the waveform it- .X 3 3- derived from the smoothing circuit with the reference waveform, the comparison network being coupled to the pulse width modulator so as to control the operation thereof and thereby determine the duration of the successive relatively high frequency pulses at the output of the pulse width modulator, whereby the AC voltage waveform output continuously follows the phase and shape of the reference waveform.

The smoothing circuit preferably includes a discharge network receiving the continuous waveform output and supplying a voltage waveform to the commutator circuit, the discharge S network being operative to damp overshoot of the continuous o, waveform under low load conditions of the commutator circuit and S being operative to produce the desired waveform in the presence of a reactive load. The voltage waveform from the discharge network may be tapped and fed back to the comparison network so that the comparison network receives a substantially continuous IDC signal, the reference wa,.eform generator being operative to Sgenerate a reference comparison signal comprising a full wave rectified version of the desired AC voltage waveform output.

S 2' The commutator circuit preferably includes a plurality of electronic switches arranged to switch the polarity of the t continuous waveform output at the desired zero crossing points I determined by the reference waveform generator.

The generator may further include a half cycle sync pulse generator responsive to the reference waveform generator and operative to generate switching signals at every half cycle of ,the continuous waveform, the electronic switches of the commutator being responsive to the switching signals to switch the polarity of the input waveform.

*r The input voltage may comprise a DC input, the relatively high frequency switching circuit comprising a high frequency inverter operative in response to the pulse width modulator to invert the input DC signal for successively varying periods of time determined by the pulse width modulator, the inverter output being fed to a rectifier for producing a substantially DC pulse train of successively varying pulse widths, the DC pulse train being supplied to the smoothing circuit which is operable to produce a substantially continuous DC waveform of a shape generally corresponding to a full wave rectified output AC voltage waveform. The output of the high frequency inverter may be fed via a transformer to the rectifier so that the rectifier fr.

receives input signals of an amplitude generally corresponding to the desired amplitude of the AC voltage waveform output.

Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the a4 2T invention.

In the drawing there is shown a block diagram of an AC voltage waveform generator according to a possible and preferred embodiment of the present invention.

The illustrated generator includes a pulse width modulator 1 which is responsive to high frequency square wave oscillator 2 so as to produce output pulses on line 30 having a fundamental frequency equal to the frequency of oscillator 2. The duty 4*4 .4 44p.

a i aYit -i nI *1 11

I

i:E .1 cycle of the pulse width modulator 1 is controlled by the output of comparator 3 of the comparison network 31.

A control signal on line 32 supplied by comparator 3 is derived by comparing the output of a discharge network 4 via an output voltage detector 5 included in the comparison network 31 with a low amplitude standard or reference comparison signal on line 33, The comparison signal on line 33 is generated by a rectified sinewave generator 6 which may include an independent generator which is operative to generate a waveform corresponding in shape to the desired waveform output of the discharge network 4, The output of the pulse width modulator 1 controls a high frequency power inverter 7 which is supplied with an input power signal, illustrated as a DC input applied to terminals 8, e.g.

from a battery or the like, The output of the high frequency inverter 7 will comprise a succession of pulses of constant amplitude but with successive pulses varying in width, the width of each pulse being substantially proportional to the control signal on line 32 as generated by the comparator 3 at that particular instant, The voltage pulses at the output of the inverter 7 will therefore be of constant sign. Assuming this is positive, and that the desired output wave function V=f(t) is such that its half-cycles are symmetrical about V=O, it is advantageous to provide a reference signal on line 33 V VR If(t) I which will correspond in form to the aforementioned function converted from the pulses and which appear at the take-off point for the feedback, ie. at the output of discharge network 4.

:P

-i 8- i Typically the desired output will be a sinusoidal wave of frequency 50 c.p.s. Corresponding to each period of 0.02 seconds, the oscillator 2 may generate 1000 pulses i.e. be operating at 50KHz. The time intervals between the beginnings of successive pulses from the pulse width modulator 1 may be constant (in this example 2 x 10 5 seconds) but the pulse widths will fluctuate according to a cycle of 50 per second.

The narrowest pulses correspond to zero amplitude reference signal on line 33 and the widest pulses to maximum amplitude reference signal. Thus if V V 0 sin nt, the narrowest pulses will be at T 0,11 T/2n etc. and the widest at T I/2n, S311/2n, 5J/2n etc.

The output of the high frequency inverter is supplied to main transformer 9. The high frequency AC square wave produced at the output of transformer 9 is then converted to a high frequency DC pulse train by rectifier The DC pulse train from rectifier 10 is applied to a smoothing circuit which is shown as an averaging filter 11 which in turn is connected to discharge network 4 to produce a continuous waveform. The discharge network 4 is operative to 1 ensure that under low or no load conditions, the output from filter 11 falls at a rate sufficient to maintain good linearity at the trailing edges of the waveform.

The feedback desirably operates to render the discharge waveform from network 4 identical with the reference signal on line 33 from generator 6. Conversion to the corresponding "full wave" full AC sine wave) is achieved by alternately switch ing the switches (SI, S4) and (S2, S3) on or off respectively at the "zero crossings" of the rectified sine waveform at the output of discharge network 4.

1 1 '7

C

i-- Control signals for switches Sl, S4, S2 and S3 may be derived from the half-cycle synchronisation generator 16 which i produces a logic change of state at each zero crossing of the arroerteebratiatrc1ead 0-esecivl ,fle ieu sine wave generator 6. Switches Sl and S4 are operated by activators 17 and 18 respectively. The switches S2 and S3 are operated by activators 19 and 20 respectively which receive inverted control signals from the half cycle synchronisation generator 16 via the logic state inverter 21.

Practical implementation of the scheme could use devices such as a multi-layered transformer to perform functions of items 17, 18, 19, 20, and 21.

With the illustrated generator, weight and cost can be considerably lower than that of a conventional low frequency Sswitching inverter, especially if the high frequency square wave oscillator 2 operates at a frequency substantially higher than the desired frequency of the AC power output 22 and 23. Use of high frequencies means particular savings can be effected in the size weight and cost of transformer and other components.

Various output AC wave shapes can be generated by using a different reference wave shape generator in place of the rectified sine wave generator 6. The averaging or integrating effect of the smoothing circuit enables the output voltage to reproduce with considerable fidelity the shape of the reference 3 voltage function.

Although the invention has been particularly described by reference to sine waves and other waves which are symmetrical about the V 0 axis, it may be possible to modify the circuitry to produce other periodic output functions.

Ilt has been found that a converter of the kind illustrated in one practical implementation can supply in excess of 300 watts of AC sine wave, full mains power from an ordinary 12 volt accumulator or car battery. Typically, the weight of the converter can be reduced to as little as 2 kilograms and may be incorporated in a compact housing. Various other practical realisations with other power outputs and input voltages may be possible, and it is envisaged that the weight of the converter may be reduced to about 10 to 15% of prior converters.

It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiment(s) of the invention as herein described without departing from the spirit and scope of the invention.

Claims

1. An AC voltage waveform generator for receiving an input voltage and for generating an output comprising an AC voltage waveform of a predetermined frequency and shape, the AC voltage waveform generator including a reference waveform generator for generating a reference waveform of frequency and waveform shape to be generated at the output, a relatively high frequency generator for generating a pulse train of frequency relatively high compared to the reference frequency, a pulse width modulator for modulating the width of the successive pulses of the high frequency pulse train, a high frequency switching circuit for receiving the input voltage and for switching through the input voltage for successive periods in response to the pulse width modulator, a smoothing circuit receiving as input signals directly or indirectly from the high frequency switching circuit the switched through input voltage, the smoothing circuit having a relatively long time constant relative to the input voltage pulse duration from the high frequency switching circuit so as to effect waveform smoothing of successively received varying width pulses and thereby produce a generally continuously varying waveform, the AC voltage waveform generator further including a commutator circuit receiving the substantially continuous waveform from the smoothing circuit and being operative to invert successive half cycles of the continuous waveform in response to the reference waveform generator so as to thereby produce the AC voltage waveform output, the AC voltage waveform generator further including a comparison network for comparing the waveform derived from the smoothing circuit with the reference waveform, A A l--i I the comparison networ k being coupled to the pulse width modulator so as to control the operation thereof and thereby determine the duration of the successive relatively high frequency pulses at the output of the pulse width modulator, whereby the AC voltage waveform output continuously follows the phase and shape of the reference waveform.

2. A generator as claimed in Claim 1 and wherein the smoothing circuit includes a discharge network receiving the continuous waveform output and supplying a voltage waveform to the commutator circuit, the discharge network being operative to damp overshoot of the continuous waveform under low load conditions of the commutator circuit and being operative to produce the desired waveform in the presence of a reactive load.

3. A generator as claimed in Claim 1 or Claim 2 wherein the commutator circuit includes a plurality of electronic switches arranged to switch the polarity of the continuous waveform output at the desired zero crossing points determined by the reference waveform generator.

4. A generator a3 claimed in Claim 3 and further including a half cycle sync pulse generator responsive to the reference waveform generator and operative to generate switching signals at every half cycle of the continuous waveform, the electronic Sswitches of the commutator being responsive to the switching signals to switch the polarity of the input waveform.

5. A generator as claimed in any one of the preceding claims wherein the input voltage comprises a DC input, the relatively high frequency switching circuit comprising a high frequency inverter operative in response to the pulse width modulator to invert the input DC signal for successively varying periods of 11 input signals directly or indirectly from the high frequency /2 time determined by the pulse width modulator, the inverter output being fed to a rectifier for producing a substantially DC pulse train of successively varying pulse widths, the DC pulse train being supplied to the smoothing circuit which is operable to produce a substantially continuous DC waveform of a shape generally corresponding to a full wave rectified output AC voltage waveform.

6. A generator as claimed in Claim 5 wherein the output of the high frequency inverter is fed via a traiisformer to the rectifier so that the rectifier receives input signals of an amplitude generally corresponding to the desired amplitude of the AC voltage waveform output.

7. A grener tor as claimed in Claim 2 or in any preceding claim appended to Claim 2 wherein the voltage waveform from the i- 15 discharge network is tapped and fed back to the comparison network so that the comparison network receives a substantially continuous DC signal, the reference waveform generator being operative to generate a reference comparison signal comprising a full wave rectified version of the desired AC voltage wavefcrm 2U output.

8. An AC voltage waveform generator substantially as herein before described with particular reference to the accompanying drawing. Dated this 21st day of September, 1990 PATENT ATTORNEY SERVICES Attorneys for CHARLES CORNELIUS VAN DONGEN 0 12 n t 1 1 1