Patent Publication Number: US-9906169-B1

Title: DC-AC conversion circuit having a first double ended DC pulse stage and a second AC stage

Description:
This application is a continuation of U.S. patent application Ser. No. 15/004,433, filed Jan. 22, 2016, which is a division of U.S. patent application Ser. No. 14/209,282, filed Mar. 13, 2014, which claims priority to U.S. provisional application No. 61/785,958, filed Mar. 14, 2013, the entire contents of each being hereby incorporated by reference. 
    
    
     FIELD 
     The present invention relates generally to voltage converter systems, in particular to systems adapted to convert direct-current (DC) voltages to alternating-current (AC) voltages and vice versa. 
     BACKGROUND 
     A DC-AC voltage converter is an electrical system that changes a DC voltage to an AC voltage. The converted AC voltage may have any desired voltage level, waveform and frequency with the use of appropriate transformers, switching, filtering and control circuits. DC-AC voltage converters are used in a wide range of applications, from small switching power supplies in electronic devices such as computers to large electric utility high-voltage direct current applications that transport bulk power. DC-AC voltage converters are also commonly used to supply AC power from DC sources such as solar panels or batteries. 
       FIG. 1  shows a typical prior art DC-AC voltage converter  10 , which operates at a relatively low frequency. Voltage converter  10  is relatively simple, but it suffers from significant disadvantages. A first disadvantage is cost, because it uses a low-frequency transformer  12  that requires a relatively large amount of copper for transformer windings. In recent years the cost of copper has increased, while the cost of power semiconductors has decreased. This trend is expected to continue. In addition, a low-frequency transformer has relatively low efficiency when it is configured with a relatively high winding turns ratio and is used for voltage step-up. An example of such configurations is a DC-AC voltage converter with a step-up transformer having a turns ratio of about 19:1 or more and a relatively low input voltage power source, for example about 10 to 20 volts DC. 
     SUMMARY 
     Given the foregoing, it is desirable to perform voltage conversion with a relatively high-frequency transformer driven by suitable power switching semiconductors. In one embodiment the present invention is a DC-AC voltage converter capable of operating with a relatively low DC voltage source input, such as from a battery power supply. 
     In some embodiments of the present invention the DC-AC voltage converter may be bidirectional, thereby capable of receiving an AC voltage signal and generating an output DC voltage signal. This arrangement is useful, for example, for charging a battery from an AC grid. 
     Preferably, a transformer is utilized to provide electrical isolation for DC-AC and AC-DC conversion. For example, an isolation transformer may be used between a DC voltage input (e.g., a battery) and an AC voltage output. The voltage converters of the present invention may be generally divided into several types according to the type of transformer selected. For example, the isolation transformers may be relatively low-frequency, on the order of 50/60 Hertz (Hz). Preferably, the isolation transformers are relatively high-frequency, on the order of tens or more kilohertz (kHz). 
     An aspect of the present invention is a voltage converter system that includes a first, high-frequency, DC-AC voltage converter configured to receive a first DC voltage signal and generate a first AC voltage signal. A DC link is configured to receive the first AC voltage signal and convert the first AC voltage signal to a second DC voltage signal. A second DC-AC voltage converter is configured to receive the second DC voltage signal and generate a second AC voltage signal. 
     Another aspect of the present invention is a voltage converter system that includes a DC-AC voltage converter configured to receive a DC voltage signal and generate a first, relatively high-frequency, AC voltage signal. An AC-AC voltage converter is configured to receive the first AC voltage signal and generate a second AC voltage signal. The frequency of the second AC voltage signal is preferably lower than the frequency of the first AC voltage signal. 
     Yet another aspect of the present invention is a voltage converter system that includes a first voltage converter portion that is configured to receive a DC voltage signal and convert the DC voltage signal to pulses of DC voltage. A second voltage converter portion is configured to receive the pulses of DC voltage and convert the pulses of DC voltage to a relatively low-frequency AC voltage signal. The voltage converter system is selectably configurable as a DC-AC voltage converter or an AC-DC voltage converter. In some embodiments of the present invention the first voltage converter portion includes a Ćuk-type voltage converter and a single-ended primary inductor converter (SEPIC) voltage converter, the Ćuk-type voltage converter and the SEPIC voltage converter being electrically combined to operate cooperatively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the inventive embodiments will become apparent to those skilled in the art to which the embodiments relate from reading the specification and claims with reference to the accompanying drawings, in which: 
         FIG. 1  is an electrical schematic diagram of a typical prior art DC-AC voltage converter; 
         FIG. 2  is an electrical schematic diagram of a DC-AC voltage converter system with a DC link according to an embodiment of the present invention; 
         FIG. 3  is an electrical schematic diagram of a DC-AC voltage converter system without a DC link according to another embodiment of the present invention; 
         FIG. 4  is an electrical schematic diagram of a voltage converter configurable for operation as either a DC-AC or an AC-DC voltage converter according to yet another embodiment of the present invention; 
         FIG. 5  is an electrical schematic diagram showing details of a first portion of the voltage converter of  FIG. 4 ; 
         FIG. 6  is a graph showing the general waveform of certain electrical signals generated by the circuit of  FIG. 5 ; 
         FIG. 7  is an electrical schematic diagram showing details of a second portion of the voltage converter of  FIG. 4 ; 
         FIG. 8  is an electrical schematic diagram of a Ćuk-type voltage converter; 
         FIG. 9  is an electrical schematic diagram of a single-ended primary inductor converter voltage converter; 
         FIG. 10  is an electrical schematic diagram of the voltage converters of  FIGS. 8 and 9  electrically combined together in a new arrangement in accordance with an embodiment of the present invention, providing for a reduced total component count; 
         FIG. 11  is an electrical schematic diagram of the voltage converter of  FIG. 10  incorporating several refinements; and 
         FIG. 12  is an electrical schematic diagram of a voltage converter according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a DC-AC voltage converter system  100  having a first, high-frequency, DC-AC voltage converter  102  according to an embodiment of the present invention. First DC-AC voltage converter  102  receives at an input  103  a first DC voltage signal. A first, relatively high-frequency, AC voltage signal  104  generated by a transformer  105  of first DC-AC voltage converter  102  is supplied to a DC-link  106  that converts the first AC voltage signal to a second DC voltage signal  108 . Second DC voltage signal  108  is coupled to a second DC-AC voltage converter  110  that converts second DC voltage signal  108  to a second AC voltage signal, output AC voltage signal  112 . Output  112  may have either low-frequency components, high-frequency components, or both low- and high-frequency components. 
     An optional electrical filter  114  provides filtering of AC output voltage signal  112  to remove high-frequency components and/or limit electromagnetic interference (EMI) caused by the AC output voltage signal, resulting in a filtered AC output voltage signal  116 . For certain applications where power quality is not a significant issue (such as a motor drive, as one example) a filter  114  configured to remove high-frequency components may be omitted. 
       FIG. 3  shows a DC-AC voltage converter system  200  according to another embodiment of the present invention. A first AC voltage signal  202  generated by a DC-AC voltage converter  204  is supplied to an AC-AC voltage converter  206  that converts the first AC voltage signal to a second AC voltage signal, output AC voltage signal  208 . An electrical filter  210  provides filtering of AC output voltage signal  208  to reduce EMI caused by the AC output voltage signal, resulting in a filtered AC output voltage signal  211 . First AC voltage signal  202  is a relatively high-frequency voltage signal, while second AC voltage signal  208  is a relatively low-frequency voltage signal output from voltage converter system  200 . 
     With reference to  FIGS. 2 and 3  together, voltage converter system  100  provides relatively efficient voltage conversion, but compared to voltage converter system  200  it is more complex and more expensive to produce. However, the performance of voltage converter system  200  depends in part upon the operating conditions of a transformer  212 .  FIG. 3  shows a topology wherein transformer  212  operates under regulation, with a relatively high turns ratio. In this case efficiency of voltage converter system  200  will be less and the voltage converter system will generate a relatively high level of EMI on the AC output voltage signal  208 . Consequently, EMI filter  210  may require a number of relatively expensive components in order to be effective. 
       FIG. 4  shows a schematic diagram of a voltage converter system  300  according to yet another embodiment of the present invention. Voltage converter system  300  is configurable for operation as either a DC-AC or an AC-DC voltage converter and is suitable for low DC input voltages (e.g., on the order of about 8-16 VDC) at power levels of up to several kilowatts. Furthermore, voltage converter system  300  overcomes the disadvantages discussed above. Voltage converter system  300  may be implemented with a relatively low number of active semiconductor switches. In addition, a transformer  302  (comprising windings  302 A,  302 B) functions under extremely benign conditions (i.e., conditions favorable in that root-mean-square (RMS) current and RMS voltage are favorable for relatively low transformer losses). Finally, there is only a low level of EMI on the AC side. 
     The topology of voltage converter system  300  may be divided into two portions for the purpose of explanation. A first voltage converter portion,  400  shown in  FIG. 5 , provides pulses of DC voltage regulated from 0 volts to a predetermined maximum voltage, with a generally half-sinusoidal waveform as shown in  FIG. 6 . A second voltage converter portion  500 , shown in  FIG. 7 , provides electrical isolation and conversion from the pulsed DC voltage of  FIG. 6  to a predetermined relatively low-frequency AC voltage signal including, without limitation, about 120 VAC at a frequency of about 50/60 Hz. 
     With continued reference to  FIG. 5 , this power stage is a combination of two types of power converters. The first is a Ćuk-type voltage converter  600 , shown in  FIG. 8 . The other is a single-ended primary inductor converter (SEPIC) voltage converter  700 , shown in  FIG. 9 . The operational details of these voltage converters are well-known in the art and thus will not be further elaborated upon here. Both voltage converters have a number of common features. For example, each is capable of providing an output voltage from zero to several times higher than the input voltage. In addition, both are bi-directional. 
     One important difference between the Ćuk-type voltage converter and the SEPIC-type voltage converter is that the Ćuk-type voltage converter reverses the polarity of the input voltage while the SEPIC-type voltage converter does not. With reference again to  FIG. 5 , these characteristics may be utilized to advantage, to provide an output voltage from an appropriately paired and electrically combined Ćuk-type voltage converter and SEPIC-type voltage converter that is about twice the output voltage available from each voltage converter individually, each voltage converter providing about half of output power delivered by the electrically combined voltage converters. A further advantage of this arrangement is that doubling the output voltage in this manner aids to reduce the required primary-to-secondary winding turns ratio of isolation transformer  302 . 
     With reference now to  FIGS. 8 and 9  together, switches  602 ,  702  respectively exhibit substantially the same operating characteristics. Likewise, inductors  604 ,  704  in  FIGS. 8 and 9  respectively exhibit substantially the same operating characteristics. Therefore, these components can be combined in an appropriately paired Ćuk-type voltage converter and SEPIC-type voltage converter to form the circuit  800  shown in  FIG. 10 . In  FIG. 10 , switch  802  replaces switches  602 ,  702  while inductor  804  replaces the inductors  604 ,  704 . Thus, switch  802  and inductor  804  are common to both the Ćuk-type voltage converter and the SEPIC voltage converter. This results in one less active switch and one less inductor in an appropriately paired Ćuk-type voltage converter and SEPIC-type voltage converter, thereby reducing voltage converter cost. Circuit  800  may be substituted for circuit  400  in the system of  FIG. 4 . 
     Ćuk and SEPIC voltage converters have one common disadvantage in that neither provide forward power conversion. Rather, they use passive components such as capacitors and inductors for energy storage. Consequently, the efficiency of these voltage converters depends very much on the quality factor of the aforementioned passive components. The quality factor of capacitors are generally good, but the quality factor of inductors are often less than desirable and often tend to worsen under high-current and low-voltage operating conditions. To reduce losses and increase efficiency, system  800  may be modified, replacing inductor  804  with an inductor/transformer  904 , as shown in the circuit  900  of  FIG. 11 . In this embodiment of the present invention when a switch  902  begins conducting forward power conversion will be provided by inductor/transformer  904 , thereby increasing the efficiency of system  900  in comparison to system  800  of  FIG. 10 . Circuit  900  may be substituted for circuit  400  in the system of  FIG. 4 . 
     With reference again to  FIG. 7 , voltage converter portion  500  comprises a power stage which will provide isolation between the low voltage side and the high voltage side. This topology is a series-resonant voltage converter, which is bi-directional. The power transformer  302  in this case works under substantially benign conditions, with a generally trapezoidal voltage wave form and a generally sinusoidal current wave form. The transformer  302  leakage inductance is part of the resonant inductor or, optionally, may comprise the entire resonant inductor. All these features aid to keep efficiency and the commutation frequency as high as possible. This reduces the transformer size and reduces its cost, as well as total inverter cost, reducing the cost of EMI filters if used. 
     A voltage converter  1000  is shown in  FIG. 12  according to yet another embodiment of the present invention. Like voltage converters  800  and  900 , voltage converter  1000  may be substituted for circuit  400  in the system of  FIG. 4 . 
     Voltage converter  1000  includes a first inductor  1002  and a second inductor  1004  connected in series, the first and second inductors each having an input and an output. A first capacitor  1006  is electrically intermediate the first and second inductors  1002 ,  1004 , a first terminal of the first capacitor being electrically connected to the output of the first inductor and a second terminal of the first capacitor being electrically connected to the input of the second inductor. A third inductor  1008  and a fourth inductor  1010  are connected in series, the third and fourth inductors each having an input and an output. A second capacitor  1012  is electrically intermediate the third and fourth inductors  1008 ,  1010 , a first terminal of the second capacitor being electrically connected to the output of the third inductor and a second terminal of the second capacitor being electrically connected to the input of the fourth inductor. A first switch  1014  is coupled between the input of the first inductor  1002  and the output of the third inductor  1008 . A second switch  1016  is coupled between the output of the first inductor  1002  and the input of the third inductor  1008 . A rectifier  1018  is arranged such that an anode of the rectifier is electrically connected to the second terminal of the first capacitor  1006 , a cathode of the rectifier being electrically coupled to the second terminal of the second capacitor  1012 . A third switch  1020  is electrically connected in parallel with the rectifier  1018 . Voltage converter  1000  is configured to receive a DC voltage signal at the inputs of the first and third inductors  1002 ,  1008  and to generate an AC voltage signal at the outputs of the second and fourth inductors  1004 ,  1010 . 
     Voltage converter system  1000  may further include third capacitor  1022 , the third capacitor being electrically intermediate the second and fourth inductors  1004 ,  1010 . A first terminal of third capacitor  1022  is electrically connected to the output of the second inductor  1004  and a second terminal of the third capacitor is electrically connected to the output of the fourth inductor  1010 . 
     The foregoing configuration of voltage converter system  1000  has the advantage of relatively low inductor current and a low switch current, similar to the embodiment of  FIG. 5 , since there are two input inductors ( 1002  and  1008 ) rather than the single input inductor of the previously-described configurations, and also has a low number of switches similar to the embodiment of  FIG. 10 . It should be noted that voltage converter system  1000  has more input current ripple compared to the embodiment of  FIG. 5 , as half of the input current is discontinuous because it flows through the switches, it is important in this embodiment that the switches switch synchronously to eliminate voltage transients across the switches and losses. 
     Inductors  1002 ,  1008  of voltage converter system  1000  may optionally be coupled magnetically to allow current balancing to occur. The current in inductor  1008  and switch  1014 , and in inductor  1002  and switch  1016 , may not necessarily ramp up identically as these inductor-switch pairs are independent of one another. However, when switches  1014 ,  1016  are opened the current flows in a complete circuit through the output (i.e., “a” and “b” of  FIG. 12 ) so the current in inductors  1002 ,  1008  must be substantially the same. Any error will result in the energy being dumped in the switches  1014 ,  1016  until the currents are substantially the same. If the windings  1002 ,  1008  are coupled the energy can transfer between the windings until the currents are substantially the same rather than the energy being lost. 
     In some embodiments of the present invention certain inductors of voltage converter system  1000  may be wound upon a common core. For example, inductors  1002 ,  1008  may be wound upon a common core. Similarly, inductors  1004 ,  1010  may be wound upon a common core. Winding the inductors upon a common core may provide certain advantages, such as a reduction in the overall size of the inductors. 
     One skilled in the art will appreciate that any suitable electronic components may be utilized for the circuits shown in the accompanying figures and described herein. For example, the switches may be any suitable types of power switching components including, without limitation, semiconductors such as bipolar junction transistors, field effect transistors and thyristors. Likewise, the diodes, capacitors, inductors and transformers shown in the accompanying figures may be any suitable types and values for a particular realization of the circuitry. 
     In addition, the circuits shown in the accompanying figures are simplified for purposes of explanation and are not intended to be limiting in any way. Accordingly, the circuits may include any suitable number and type of ancillary components including, without limitation, biasing, feedback and filtering components and circuitry as well as analog and/or digital monitoring, feedback and control circuitry. 
     While this invention has been shown and described with respect to a detailed embodiment thereof, it will be understood by those skilled in the art that changes in form and detail thereof may be made without departing from the scope of the claims of the invention.