GB2089589A

GB2089589A - DC to AC Inverter

Info

Publication number: GB2089589A
Application number: GB8101090A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-01-22
Filing date: 1981-01-14
Publication date: 1982-06-23

Abstract

The inverter circuit is used in synthesising a sine wave and comprises two transformers, a major transformer (10) and a minor transformer (12), operated in effect as two separate inverters whose primary windings both work from the same DC source. The secondary windings (1-B, 12B) of the two transformers are connected in series. The primary windings are each connected by four switches in a respective bridge configuration and the switches are controlled in such a way as to produce bidirectional pulse trains at the transformers' respective secondary windings. The switches of each bridge are controlled to short circuit the respective transformer primary winding whenever the respective transformer is not producing output pulses. <IMAGE>

Description

SPECIFICATION DC to AC Inverters This invention relates to inverters, and is particularly concerned with sine wave power inverters which provide a sine wave AC output from a DC source using semiconductors as the power switching elements.

Such power inverters are used to power a wide range of types of euipment designed to run from the AC mains supply, especially when it is necessary to provide an uninterrupted power supply, or when no AC mains supply is available.

It is common practice nowadays to use allelectronic inverters, in contrast to the older electro-mechanical systems. One known technique for inverting a DC voltage is to use switching elements in combination with a transformer, but the AC output which is obtained with this basic method has a square waveform. In order to provide an AC output with a waveform which is as close as possible to the AC mains supply, it is desirable to provide a sinsusoidal AC waveform which contains as little harmonic distortion as possible, and which is closely regulated in amplitude. It is further desirable that there is efficient transmission of the AC power to the load.

Existing static power inverters, which generate a sine wave output from a DC input voltage usually employ either a constant voltage transformer or a linear amplification method of inversion.

However, the constant voltage transformer is inefficient at partial loads, it does not provide very good voltage stability, it draws a large no-load current, it requires special laminatidns and other parts for its construction, and is very heavy. The known linear amplification methods of inversion are characterised particularly bylow efficjencyin the transmission of power to the load. As the power source for a static inverter is usually a battery, it is very important to be able to achieve maximum efficiency at all loads up to full load. It is also important to be able to operate the inverter with a low standby, i.e. no-load, current input.

It is an object of the present invention to provide a DC to AC inverter which produces a stabilised, closely regulated, low distortion sine wave output from a DC input, the inverter being highly efficient at all loads, having a small no-load current drain and being relatively light in weight.

The inverter of the present invention can be constructed using standard low-cost transformer materials and assembly methods, and standard low-cost electronic components.

Broadly in accordance with one aspect of the present invention there is provided an inverter comprising two transformers, the primary winding of each being powered from a DC voltage supply and with their secondary windings connected in series, and switch means associated with each transformer's primary winding and operative in response to control means to produce from the transformers a pulse width modulated synthesised waveform.

The primary winding of each transformer is connected by four switch means to the DC supply, so that the respective transformers and their switch means constitute two separate inverters each working in what is known as the bridge mode.

Preferably, the ratios of the output voltages of the two transformer secondary windings are to

and the primary windings of the transformers are driven so that the transformer producing the higher secondary voltage produces a positive output pulse for a predetermined length of time during each of the periods 7r 7r -to - 6 3 7d 7 -to-, 3 2 7r 27r -to-, 2 3 and 2or 5rr -to- 3 6 and the other transformer produces a positive output pulse for a predetermined length of time during each of the periods 7r Oto, 6 7t 7t -to-, ~to 3 2 7r 27r -to-, 2 3 and and 57r -to 6 where the periods refer to the fundamental of the waveform across the secondary windings and the period 0 to 7r represents the positive half cycle of the output, both transformers producing corresponding negative output pulses during the succeeding half cycle.

The switch means of each transformer may also be operated so as to produce a short circuit across that transformer, in association with inverse connected diodes across each switch, so that the said transformer is short circuited whenever it is not producing output pulses.

The provision of this short circuit action means that the pulses of the respective pulse trains return to zero, thus maintaining the conditions necessary to ensure a low distortion sine wave output to the load.

In order that the invention may be more fully understood a preferred embodiment of inverter in accordance therewith will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 diagrammatically illustrates the basic synthesised waveform produced by the inverter of the present invention, the waveform being shown at maximum pulse width and before filtering; Fig. 2 diagrammatically illustrates the basic synthesised waveform produced by the inverter of the present invention with pulse width modulation applied to the waveform to produce six pulses in each positive and each negative half cycle of the waveform; Fig. 3 shows a first pulse train, being the output from the major transformer of the circuit shown in Fig. 7, and which constitutes one part of the synthesised waveform shown in Fig. 2;; Fig. 4,shows a second pulse train, being the output from the minor transformer of the circuit shown in Fig. 7, and constituting a second part of the synthesised waveform shown in Fig. 2; Fig. 5 is a timing diagram illustrating the timing of the operation of the short circuiting action associated with the major transformer of the circuit of Fig. 7; Fig. 6 is a timing diagram illustrating the operation of the short circuiting action associated with the minor transformer of the circuit shown in Fig. 7; Fig. 7 is a schematic circuit diagram of the inverter of the present invention.

Fig. 1 illustrates the form of unfiltered synthesised waveform which is produced by the inverter of the present invention. A complete cycle of this waveform is represented as the interval 0 to 27r along the abscissa. The maximum amplitude of the waveform is A. The amplitude of the waveform during the intervals O to - 6 and 5rr -to,r 6 is

(approximately equal to +0.268) of the maximum amplitude A.The amplitude of the waveform during the intervals 7r 7r -to- 6 3 and 27r 57r -to - 3 6 2A is ~~~~~~~ 1 +T (approximately equal to +0.732A). During the intervals n -to- 3 2 and 7r 27r -to - 2 3 the amplitude of the waveform is equal to A.

Corresponding negative amplitudes occur during the negative half cycle during the time interval from 'rto 2err.

It will be seen that the sum of 0.268 and 0.732 is 1.000. Thus the amplitude of the waveform over the periods 7r 7r -to - 32 and 7r 27r -to- 2 3 may be obtained simply by adding the amplitudes 0.268A and 0.732A. As will be described in detail hereinafter, the inverter of the present invention produces two separate pulse trains, as shown in Figs. 3 and 4, which, by summation, produce the composite waveform shown in Fig. 2. This composite waveform can then be filtered to produce a final AC output which closely approximates a sine wave.

Fig 7 shows the power inverter of the present invention in schematic form. The inverter comprises two transformers, hereinafter referred to as a major transformer and a minor transformer. The major transformer primary winding is indicated at 1 OA, the major transformer secondary winding is indicated at 1 OB, the minor transformer primary winding is indicated at 1 2A and the minor transformer secondary winding is indicated at 1 2B. The two transformer's secondary windings 1 OB and 128 are connected in series as shown.

The ends of the major transformer primary winding 1 Oa are connected by switches S1 and S2 respectively to one side of the DC input line 16, and by respective switches S4 and S3 to the other side of the DC input line 14, as shown.

Diodes D1, D2, D3 and D4 are connected in inverse parallel across respective switches S1, S2 S3 and S4 as shown This connection of the winding 1 OA is what is commonly known as a bridge connected inverter circuit.

in a similar way, minor transformer primary winding 1 2A is connected to the DC lines 16 and 14 by switches S5, S6, S7 and S8 with their respective inverse connected diodes D5, D6, D7 and D8 as shown to constitute in effect a separate bridge connected inverter circuit working from the same DC supply as the major transformer.

Although NPN transistors are shown in positions S1 to S8 of Fig. 7, other types of electronic switch may also be employed to produce the same circuit operation.

In use, switches S1 to S4 on the one hand and switches S5 to S8 on the other hand, are independently controlled such that the major and minor transformers 10 and 12 will produce in their secondary windings. 1 OB and 1 2B pulses coiresponding to amplitudes of 0.732A and 0.268A respectively of either polarity.

The output of the transformer secondary windings 1 OB and 1 2B is fed to an LC filter and thence to the AC output terminals which are connected to the load. A sensing circuit 1 8 is connected across the AC output terminals to sample the output sine wave. Information regarding the amplitude of the output sine wave is fed back from the sensing circuit 1 8 to a control circuit 20. The control circuit 20 is connected to the eight switches S1 to S8. The control circuit controls the widths of the pulses produced by the transformers, and their time sequence.The control circuit ensures that all the pulse widths are of equal time duration and applies pulse width modulation in accordance with the information from the sensing circuit 18, so providing close regulation of the final sine wave amplitude by closed loop control.

Diodes D1 to D8 provide a path for reverse direction currents to flow which occur due to stored energy in the load, in filter LC, and in the respective magnetic fluxes of the transformers 10 and 12. These diodes are commonly fitted to bridge inverters for this reason.

Some of the said diodes also enable a short circuit or "clamping" action to be produced across the respective transformer primary winding at certain times. For example the major transformer primary winding 1 OA may be short circuited by closing both S1 and S2, while S3 and S4 are kept open. Thus short circuit current may flow through either S1 and D2, or S2 and D1 as the case may be. Alternatively S1 and S2 may be kept open, and S3 and S4 closed to produce the short circuit. A similar clamping action may be obtained with winding 1 2A of the minor transformer.

Control circuit 20 produces this clamping action across the respective transformers whenever they are not producing output pulses.

This clamping action enables current to continue to flow through one or both transformers, as the case may be, while still not causing the prefiltered output waveform to depart from the waveform shape shown in Fig. 2. In this way one can maintain the conditions necessary to ensure a low distortion sine wave output to the load.

Without the clamping action, the reactance of the LC filter, any reactance of the load, and the magnetic fluxes of the transformers would all tend to make the waveform depart from that shown in Fig. 2 to a considerable degree, which would then result in a high distortion of the output waveform to the load.

Figs. 5 and 6 are timing diagrams illustrating when these respective clamping actions are "on" or "off", the indication "on" denoting that the short circuit is applied to the respective transformer. At certain times both transformers are short circuited together.

The proportion of the time during any 7r 6 interval that a given pulse is produced may be varied by the control circuit 20 in order to obtain the major and minor secondary waveforms shown in Figs. 3 and 4 respectively which when summed produce the composite waveform shown in Fig. 2 which is applied to the LC filter.

The peak amplitude of the sine wave which is synthesised in this way can be varied from zero to a maximum of approximately 1.023A (when the pulse width is at its maximum.) Moreover, if all the pulse widths are the same, then the composite synthesised waveform of Fig. 2 will include no harmonics lower than the 1 1 th for any pulse width from 7d Oto- 6 This synthesised waveform is therefore easily filtered to produce a high quality sine wave output with very low total harmonic distortion. In practice, it has been found that a sine wave output with a total harmonic distortion of 3% to 5% can readily be obtained at any unity power factor load.

The switching elements used in the inverter of the present invention may be any suitable type of semi-conductor device. They may be field effect transistors, thyristors, or Darlington transistors, the latter being either compounded from separate transistors or in monolithic form. Any switching element may be a combination of two or more switching elements connected in parallel and working together.

Although in the foregoing description reference has been made to the clamping or short circuiting action operating on the primary windings of the transformers, this action may be arranged to operate on a tertiary winding, or on the secondary winding, i.e. the output winding, of one or both transformers.

In a modified embodiment of the inverter, one of the transformers, together with its associated switching elements may be replaced by two or more transformers with associated switching elements, with the secondary windings of the replacement transformers connected in series so that the total output voltage at any time from the series-connected secondaries is equal to the secondary output voltage of the replaced transformer.

Although NPN transistors are shown for the switches in Fig. 7, the inverter may also be constructed using PNP transistor switches.

The power inverter of the present invention provides a very closely stabiiised sine wave output at a stable frequency, with both output voltage and frequency being substantially unaffected by load or input voltage changes.

Additionally, the inverter has very low total harmonic distortion, transmits AC power to the load with high efficiency, is relatively light in weight, and may be constructed using standard low-cost components and assembly methods.

The inverter of the present invention may also form part of an AC to AC converter, and may also be used in the construction of a three-phase DC to AC inverter, or a three-phase AC to AC converter.

Claims

Claims

1. An inverter circuit for use in synthesising a sine wave, comprising two transformers whose primary windings are each arranged to be connected by four switching means in a bridge configuration across a DC source and whose secondary windings are connected in series, with either two of the four said switching means associated with each transformer and being connected to the same side of the DC supply producing a short circuit across the respective primary winding when the other two of the four said switching means are open and thus disconnect the said primary winding from the other side of the DC supply, with control means for controlling the operating sequence and periods of said switching means, such that the output voltage across the series connected secondary windings comprises a series of positive pulses of varying amplitude alternating with a series of negative pulses of similarly varying amplitude wherein for the period (0 to 7c) of each of the alternating series of positive and negative pulses appearing across said secondary windings higher amplitude pulses from one transformer are produced for a part of each of the intervals 7d 7r to; 6 3 7d 7r ~ to; 3 2 7r 27r to; 2 3 2rr 57r -to - 3 6 and lower amplitude pulses of the same polarity are produced by the other transformer over the same part of each of the intervals 7r O to --; 6 7r 7r -to-.

3 2 to 7r 27r to; 2 3 57r to 7r.

6

2. An inverter circuit according to claim 1, wherein the ratio of the amplitudes of the output pulses from the secondary windings of the two transformers is substantially

3. An inverter circuit according to claim 1 or claim 2, wherein the short circuiting action across each transformer is produced by placing a short circuit across a tertiary winding, or the secondary winding, of the respective transformer instead of by the said switching means associated with the primary windings.

4. An inverter circuit according to any preceding claim, wherein one or both of said two transformers are formed by two separate transformers having their secondary windings connected in series.

5. An inverter circuit according to any preceding claim, wherein said DC source is derived by rectifying an AC source.

6. An inverter circuit substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.