Abstract:
An apparatus and method for converting an input signal to an output AC signal in which the input voltage signal is inverted and modulated to provide an intermediate AC signal having twice the desired output frequency. The intermediate signal is then full-wave rectified and then the polarity of the rectified signal is switched every second cycle to produce the output AC signal of a desired frequency and voltage.

Description:
TECHNICAL FIELD 
       [0001]    This application relates generally to electrical power generation and electrical power generation systems. 
       BACKGROUND 
       [0002]    Limited cost-effective and weight-efficient means exist for conditioning high power electricity. Existing electronic high power commutation systems are heavy, bulky and expensive, and room for improvement therefore exists. Room for improvement also exists in power conditioning, generally. 
       SUMMARY 
       [0003]    In one aspect, there is provided a method for conditioning a power signal, the method comprising the steps of: providing an input voltage signal; providing an input modulation signal; inverting the input signal to an intermediate AC signal, including a step of using the input modulation signal to impart a desired a voltage-time function to the intermediate AC signal; full-wave rectifying the intermediate AC signal to provide a rectified DC signal, the rectified DC signal having a periodic cyclical voltage-time function; reversing a polarity of the rectified DC signal every second cycle to thereby produce an output AC signal. 
         [0004]    In another aspect, there is provided a power inverter having an output, the power inverter comprising: a push-pull switching circuit having a DC input and an intermediate AC output; a modulator for modulating the push-pull switching circuit; a full-wave rectifier having the intermediate AC output as an input and having a full-wave rectified DC output; a commutator having the full-wave rectified DC output as an input, the commutator being connected to the power inverter output; and a controller configured to cause the commutator to intermittently switch a polarity of the full-wave rectified DC output to thereby provide AC power to the power inverter output. 
         [0005]    In another aspect, there is provided an inverter comprising: means for providing an input voltage signal; means for providing a modulation signal; means for inverting the input signal to an AC intermediate signal having a desired a voltage-time function; means for rectifying the intermediate AC signal to provide a rectified DC signal having a periodic cyclical voltage-time function; and means for reversing polarity of the rectified DC signal every other cycle to thereby produce an output AC signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other aspects will become better understood with regard to the following description and accompanying drawings wherein: 
           [0007]      FIG. 1  is a schematic block diagram of one embodiment of a power inverter; 
           [0008]      FIG. 2  is an example schematic block diagram of a portion of  FIG. 1 ; 
           [0009]      FIG. 3  is a schematic of a mode of operation of  FIG. 2 ; and 
           [0010]      FIG. 4  is a flow chart showing an example of the steps of the method. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]      FIG. 1  shows an example of an inverter  10  as improved. In the example of  FIG. 1 , input direct current (DC) power source  12  is provided, for example from a rectified output of a generator connected to a gas turbine engine or from a battery, to drive a high frequency oscillator switching circuit  14 . In this example, switching circuit  14  is a push/pull type and forms a part of a type of resonant converter, as described below. Switching control for the switching circuit  14  is provided by a pulse width modulation apparatus  16 , described further below. Switching circuit  14  is connected to a high frequency (HF) transformer  18 , which can be a torroidal type transformer or other suitable type of high frequency power transformer. The transformer  18  can provide a step-up voltage from a provided input level (e.g. 28V) to a desired output level (e.g. 115V). The DC source  12 , switching circuit  14  resonant circuit components and transformer  18  assembly collectively provide a modulatable alternating current (AC) HF output. The use of a HF transformer for voltage step-up results in a compact light weight design, because of its very low winding turn count and high rate of change of flux, but any suitable transformer configuration may be used. 
         [0012]    The transformer secondary side provides its output AC to a suitable full-wave rectifier  20  to rectify the AC signal to a DC signal. Silicon carbide type diode-based rectifiers have good high frequency and high voltage performance, however any suitable rectifier arrangement may be used. A suitable commutator circuit  22 , controlled by a commutator switch control  40  as described below, connected to the output of rectifier  20  commutates the rectified DC wave into an AC output signal, such as by periodically reversing the polarity of the DC (as described further below) to provide an output AC signal at the desired output frequency. A generic H-bridge commutator circuit is suitable, but any other suitable commutation arrangement may be used, such as an SCR commutator. 
         [0013]    Referring still to  FIG. 1 , the output of the commutator circuit  22  can be connected if desired to an HF filter  24  to remove undesired residual switching frequency components introduced by the high frequency switching, and thus eliminate any ripple remaining in the signal, before providing the signal at an output  26  to the load  28 . Any suitable filtering approach, or lack thereof, may be employed. 
         [0014]    As mentioned, inverter  10  includes modulation apparatus  16 , which is connected for amplitude envelope modulation of switching circuit  14 , as will be described in more detail below, and is connected to control commutator circuit  22  as well. Modulation apparatus  16  includes a pulse width modulator circuit  30  fed by a full-wave modulation generator  32 , which may comprise a reference wave generator  34  and a full-wave rectifier  36  or a suitable digitally simulated equivalent. The reference wave generator  34  provides a reference signal representative of the selected waveform and frequency desired for output  26 , which may be any suitable waveform and frequency, but for the purposes of this example is selected to be a relatively low frequency sine wave (say, 60 Hz or 400 Hz). In order to obtain a high fidelity output  26 , the reference wave generator  34  output can be provided to one input of a differencing error amplifier  38 , while an output current and or voltage feedback signal  42  is provided to the other input of the differencing error amplifier  38 . The error amplifier  38  determines the difference or error (if any) between the reference and the output signal feedback and, from this, generates a suitable error waveform modified from the “pure” input wave provided by generator  24  which is provided to the pulse width modulator  30  in an action arranged to remove the “error” in the output voltage or current from the output  26  waveform. The action of the pulse width modulation arrangement is to provide a 0% to 100% variable HF input amplitude to the transformer circuit, which in turn becomes a 0% to 100% variable DC voltage output from the full wave rectifier circuit. The commutation control  22  remains at a fixed frequency and period duty cycle of 50%, however the phase may be corrected from time to time as required, using the feedback signal  42 . In this way, any nonlinearities in the inverter  10  can be minimized if not eliminated altogether. 
         [0015]    Referring now to  FIG. 2 , an example of switching circuit, transformer and pulse width modulator circuit  30  are shown in more detail. As mentioned, switching circuit  14  comprises a resonant circuit having inductors L 1  and L 2 , and capacitors C 1  &amp; C 2 . These components in conjunction with the transformer T 1  work together as a resonant converter. The example pulse width modulator circuit  30  includes HF voltage sources V 1  and V 2 , resistors R 4  and R 5  respectively connected in series to transistors Q 1  and Q 2 , and rectifier  20  (shown with diodes D 1  to D 4 ). 
         [0016]    Referring to  FIG. 3 , an example switching arrangement is schematically depicted over time. Signal A represents the system operating frequency or clock signal, which is the HF switching frequency and is generated within the pulse width modulator circuit  30 . Signals B 1 , B 2  represent the drive signal provided to transistors Q 1 , Q 2 , respectively, to obtain approximately  10 % of transformer output voltage, while signals C 1 , C 2  represent the drive signal provided to transistors Q 1 , Q 2 , respectively, to obtain approximately 100% of transformer T 1  output voltage. Within this envelope, the resonant circuit is driven and thus controlled to modulate the output of the transformer T 1 , according to a suitable control pattern, as described below. 
         [0017]    The frequency of the switching may performed at the resonant frequency of the circuit made up from the circuit components supplied, however since the pulse width driving the power switching transistors is variable the input power to the resonant circuit, and as such the output voltage from the HF transformer is variable and is proportional to the pulse duration (relative to the frequency of operation). This effect may be used to modulate the output power of the present system, as described further below. 
         [0018]    How the average AC amplitude is affected by the pulse width is somewhat similar to the effect a buck regulator has on the average DC content of the resultant pulse—except that in the present case the pulse width modulation causes the AC input to the transformer stage to be modulatable (i.e. of a variable amplitude), and therefore the resultant system output is fully variable power. The AC amplitude in the present approach is affected by the pulse width during switching, by affecting the Fourier content of the fundamental switching frequency (and its harmonics), and since the resonant circuitry substantially allows only the fundamental frequency current to flow in the transformer, the AC amplitude input to the transformer at the resonant frequency is thus variable by means of the pulse width modulation. Although resonant converters generally only provide a fixed output voltage at variable load currents, provided herein is a resonant converter that has a modulatable output voltage and which is capable of delivering a variable current to the load. 
         [0019]    Referring to  FIG. 1 , in use, a high frequency AC signal (i.e. greater than 20 kHz) is provided by the switching circuit  14  switching the input DC signal at a relatively high frequency AC current (i.e. preferably greater than 100 kHz) to transformer  18  (reference step  100  in  FIG. 4 ). 
         [0020]    Pulse width modulator  30  of modulation apparatus  16  provides amplitude envelope modulation, as described above, of the HF AC signal provided by switching circuit  14 , based on the selected reference low frequency modulation signal provided by the full-wave modulation generator  32 , such that the modulated AC signal has an amplitude envelope which varies at twice the reference low frequency modulation signal (reference steps  110  and  120  in  FIG. 4 ), and thus the HF filtered output of the full-wave rectifier  20  has a voltage-time function which appears as a full-wave rectified pattern at twice the desired frequency of AC output  26 . 
         [0021]    In this example, transformer  18  transforms the voltage of the modulated AC signal to a desired stepped-up output level. The stepped-up modulated AC signal is then rectified back to DC by rectifier  20 , thus providing the mentioned voltage-time function which appears as a full-wave rectified pattern at twice the desired frequency of AC output  26  (reference step  130  in  FIG. 4 ). The commutator circuit  22 , with switch timing control  40  provided by modulation apparatus  16 , commutates the rectified DC, based on the reference modulation frequency, such that the full-wave rectified modulated DC signal is converted, preferably in this case by switching polarity at every cycle, such that the average DC output signal becomes, in effect, an AC output voltage (reference step  140  in  FIG. 4 ) alternating at the desired output frequency (say, 60 Hz or 400 Hz in this example), which is then filtered by HF filter  24  to remove unwanted residual components, leaving only the desired low frequency AC output signal  26  which may then be provided to a suitable load  28 . The output voltage and current can be monitored by modulation apparatus  16 , and compared to a reference signal in the modulation apparatus  16 , as described above, such that instantaneous corrections are made as necessary to the modulation signal to maintain the desired output waveform. 
         [0022]    As can be appreciated, the above-described arrangement allows one to provide a lightweight constant or variable speed, constant or variable frequency power conversion device. Output frequency may be varied simply by varying the modulation reference input. The frequency of the source high frequency AC remains at or near the resonant frequency, although the device may be operated to provide any desired output. The device therefore can offer a flexible fully variable output frequency and voltage device which also has a relatively low cost to produce. The selected input and modulation frequencies are left to the discretion of the designer, in view of the teachings above. The use of a high frequency transformer arrangement for voltage transformation results in an apparatus to be physically small and lightweight at a given power level relative to a transformer operating at a lower frequency, although as mentioned any suitable transformer arrangement may be used. 
         [0023]    Applicant&#39;s co-pending application Ser. No. 11/533,548, filed Sep. 20, 2006, is, in its entirety, hereby fully incorporated by reference into this application. 
         [0024]    Modifications to what is described herein will be apparent to those skilled in the art. For example, the amplitude modulation signal for the switching circuit  14  may be synthesized digitally. The use of input and output filters is optional. Any suitable modulation patterns may be used, and a variety of output waveforms may be generated. Although a sample resonant converter arrangement is described and depicted, other suitable arrangements are available within the scope of the teachings herein. As well, although this disclosure addresses a single phase system as examples, any number of such circuits can be used together to form multi-phase configurations where required. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. It will further be understood that it is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the various aspects and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth, and as follows in the scope of the appended claims.