Abstract:
A DC to AC power converter is disclosed. The power converter has four power-switching devices, two diodes, a step-up and isolation transformer, a capacitor-choke filter and a controller. Two power-switching devices located on the primary side of the transformer are switched to provide alternate cycles of an ac current to the primary side of the transformer, which magnetically couples the ac current to the secondary side of the transformer. Two power-switching devices on the secondary side of the transformer are switched to alternately allow the forward and return ac currents from the secondary side of the transformer in the output path to a load connected to the output of the DC to AC power converter.

Description:
RELATED APPLICATIONS 
   This application claims priority from British patent application numbers GB0612859.9 filed Jun. 29, 2006 and GB0712536.2 filed Jun. 28, 2007, which are incorporated herein by reference. 
   FIELD OF INVENTION 
   This invention generally relates to DC to AC power converters. In particular, this invention relates to DC to AC power converters suitable for stand-alone use and for connection to a power grid. 
   BACKGROUND 
   Broadly speaking, DC to AC power converter converts DC power received at an input to AC power for outputting to a load. Typically, DC to AC power converters comprise a plurality of power switching devices, a transformer, a rectifying circuit, an intermediate DC stage and an output current shaping circuit. A plurality of power switching devices in the input path to the transformer are switched to convert the DC input voltage to an AC current, which is magnetically coupled by the transformer to the output side of the transformer. 
   Typically, the ac current in the output path of the transformer is full-wave rectified, for example using a rectifying diode bridge, to produce an intermediate DC signal in an intermediate DC stage, such as a Buck stage. The intermediate DC signal is current-shaped by switching a plurality of power switching devices in order to shape the output current to conform to a desired ac output current waveform. In such a configuration, the number of power switches is large, typically 8 to 12 or more. 
   We have appreciated the problems associated with using a large number of power switches in such a DC to AC power converter. The large number of power switching devices and the circuits necessary to control such devices decrease the overall efficiency of the circuit as each power-switching device has associated losses intrinsic to the device. Therefore, the larger the number of switching devices, the larger the losses and the lower the efficiency. 
   We have appreciated the need to reduce the number of power switching devices used in a DC to AC power converter. 
   SUMMARY 
   According to the present invention, there is provided a dc-to-ac power converter, the converter including a transformer having a primary and a secondary winding, the primary winding of said transformer being coupled to a dc input of said power converter and the secondary winding of said transformer being coupled to an ac output of said converter, and wherein the converter further comprises: a first pair of switches on said primary side of said converter, coupled between said dc input and said primary winding, to convert a dc supply from said dc input to an ac current for driving said transformer; a second pair of switches on said secondary side of said converter coupled between said secondary winding and said ac output, one in a forward path to said ac output and one in a return path from said ac output; a diode coupled across each of said secondary side switches; and a controller configured to control said primary and secondary side switches to convert a dc supply at said dc input to an ac supply at said ac output. 
   Embodiments of the present invention have the advantage of reducing the number of switches when compared to typical DC to AC power converters. Embodiments of the present invention also have an advantage in that the intermediate DC stage has been removed. 
   The present invention also provides a dc-to-ac power converter for providing an ac mains voltage power supply from a lower voltage dc input, the said power converter lacking an intermediate high voltage dc stage and comprising no more than four power switching devices, a first pair of power switching devices on a dc input side of said dc-to-ac converter and a second pair of power switching devices on an ac output side of said dc-to-ac converter. 
   The present invention further provides a system to convert a dc voltage input to an ac approximately sinusoidal current for a dc-to-ac power converter, the system comprising: a dc input with a pair of dc input terminals; a transformer having a primary winding with a tap and a second, output winding to provide said ac current; a pair of switches each coupled to one of said input terminals and to a respective end of said primary winding, said tap being connected to the other of said pair of dc input terminals; and a controller configured to control each of said pair of switches in turn during respective first and second half cycles of said approximately sinusoidal current using a pulse width modulated control such that each of said switches generates a current to approximate one of said half cycles of said approximate sinusoid; said system having an output from said output winding. 
   The present invention further provides a DC-to-AC power converter, the converter including a transformer having a primary and a secondary winding, the primary winding of the transformer being coupled to a dc input of the power converter and the secondary winding of the transformer being coupled to an ac output of the converter, and wherein: a first and second switch connected to the primary winding of the transformer to convert a dc supply from the dc input to an ac current for driving the transformer; a first and second switch connected to the secondary winding of the transformer such that the first switch is in a forward path to the ac output and the second switch is in a return path to the ac output; a first and second diode coupled across the respective first and second switches connected to the secondary winding; and wherein the first switch connected to the primary winding of the transformer is controlled to provide a first half cycle of an ac voltage to the primary winding of the transformer; the second switch connected to the primary winding of the transformer is controlled to provide a second half cycle of an ac voltage to the primary winding of the transformer; and the first and second switches connected to the secondary winding of the transformer as switched to alternately conduct the first and second half cycles of the signal coupled from the primary winding of the transformer to the secondary winding of the transformer. 
   The invention still further provides a controller for controlling a DC to AC power converter, the power converter comprising a transformer having a primary winding and a secondary winding, a first and second switch connected to the primary winding of the transformer, a first switch connected to the secondary winding of the transformer in the forward path to an ac output and second switch connected to the secondary winding of the transformer in the return path to the ac output, and a first and second diode coupled across the respective first and second switches connected to the secondary winding; the controller comprising: a plurality of outputs to control each of the first and second switches connected to the primary winding of the transformer and first and second switches connected to the secondary winding of the transformer; wherein the controller controls the first and second switches connected to the primary winding of the transformer to convert a dc input to an ac current to drive the primary winding of the transformer; the controller controls the first and second switches connected to the secondary winding of the transformer to alternately conduct the first and second half cycles of the signal coupled from the primary winding of the transformer to the secondary winding of the transformer. 
   In embodiments of the present invention, the dc-to-ac power converter or system further comprises a non-electrolytic capacitor energy storage capacitor on a dc side of said converter or system. 
   Preferably, the above converter or system further comprises a boost converter coupled to the dc input to said converter or system, and wherein said non-electrolytic capacitor is coupled across an output of said boost converter. 
   The present invention also provides a dc-to-ac power converter having a dc input, a dc-to-ac conversion stage having an input coupled to said dc input and an ac output coupled to an output of said dc-to-ac conversion stage, and further comprising a non-electrolytic energy-storage capacitor coupled to said input of said dc-to-ac conversion stage. 
   Preferably, the power converter further comprises a boost converter coupled between said dc input and said conversion stage input. 
   Preferably, the power conversion stage comprises a first stage operating at a first frequency and coupled to a second stage operating at a frequency of said ac output, said first frequency having higher than said output frequency. Preferably, said first frequency is 10, 100 or 1000 times higher than said output frequency. 
   Preferably, said first stage is configured to convert a dc to an ac current. Preferably, said ac output is a single phase output. 
   The present invention also provides a dc-to-ac power converter having a dc input, a dc-to-ac conversion stage having an input coupled to said dc input and an ac output coupled to an output of said dc-to-ac conversion stage, wherein said dc-to-ac conversion stage comprises a plurality of MOS switching devices, and wherein all of said switching devices are referenced to ground when switched on. 
   The present invention also provides a dc-to-ac power converter having a dc input, a dc-to-ac conversion stage having an input coupled to said dc input and an ac output coupled to an output of said dc-to-ac conversion stage, wherein said dc-to-ac conversion stage comprises a plurality of MOS switching devices, and wherein said switching devices are driven without level shifting. 
   The present invention also provides a dc-to-ac power converter having a dc input, a dc-to-ac conversion stage having an input coupled to said dc input and an ac output coupled to an output of said dc-to-ac conversion stage further comprising a boost converter coupled between said dc input and said conversion stage input, wherein said dc-to-ac conversion stage comprises a plurality of MOS switching devices, and wherein said boost converter comprises at least one MOS switching device, and wherein said boost converter switching device is a vertical device and wherein said dc-to-ac conversion stage devices are lateral devices. 
   Preferably, said vertical device and said lateral devices are fabricated on a single integrated circuit. 
   The present invention further provides a dc-to-ac power converter having a dc input, a dc-to-ac conversion stage having an input coupled to said dc input and an ac output coupled to an output of said dc-to-ac conversion stage and further comprising an input stage coupled between said dc input and said conversion stage input, and wherein said input stage is selectably configurable between a boost converter and a buck converter. 
   The present invention still further provides a dc-to-ac power converter having a dc input, a dc-to-ac conversion stage having an input coupled to said dc input and an ac output coupled to an output of said dc-to-ac conversion stage and further comprising an input stage coupled between said dc input and said conversion stage input, and wherein said input stage is selectably configurable such that the converter is configured to operate in either an on-grid configuration or an off-grid, battery powered configuration. 
   The present invention also provides an integrated circuit comprising at least one power switching device, a diode, and at least one second switching device connected such that in a first configuration said integrated circuit has terminals for connecting to external components including at least a coil and a capacitor to implement a boost converter and such that in a second configuration, which when said terminals are connected in the same way, implements a back converter and further comprising a controller, the controller having an input for selecting between said two configurations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, in which: 
       FIG. 1  shows a DC to AC power converter according to the present invention. 
       FIG. 2  shows a set of waveforms illustrating the operation of the DC to AC power converter. 
       FIG. 3  shows an implementation of the DC to AC power converter according to the present invention. 
       FIG. 4  shows a set of waveforms illustrating the operation of the DC to AC power converter of  FIG. 3 . 
       FIG. 5  shows an implementation of the DC to AC power converter in grid-connected operation according to the present invention. 
       FIG. 6  shows a set of waveforms illustrating the operation of the DC to AC power converter of  FIG. 5 . 
       FIG. 7  shows a setup of a DC to AC converter including a voltage amplification stage. 
       FIG. 8  shows a DC to AC converter comprising a voltage step-up circuit. 
       FIG. 9  shows a DC to AC converter comprising a voltage step-down circuit. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a basic circuit of the DC to AC power converter. The circuit shown comprises four switches (S 1 , S 2 , S 3 , S 4 ), two diodes (D 1  and D 2 ), a step-up and isolation transformer (T) and a capacitor-choke filter (C, L 2  and L 3 ). A controller C provides signals to drive the four switches and optionally has a reference input to sense an input signal. Two switches S 1  and S 2  are located at the primary side of the transformer T, which has two primary windings. One end one primary winding is connected to the other end of the other primary winding and the DC source positive rail. The other end of each primary winding is connected to a terminal of either S 1  or S 2 . The other terminals of S 1  and S 2  are connected to the ground. 
   Two further switches, S 3  and S 4 , are connected to the secondary winding of the transformer. Diodes D 1  and D 2  are connected across the switches S 3  and S 4  respectively. The cathode end of D 2  is also connected to the one end of the capacitor C and the choke L 2 . The other end of the diode is also connected to one end of the transformer secondary winding. The other end of L 2  is connected to the load. Similarly, the anode end of D 1  is connected to the other end of the transformer secondary winding. The cathode end is connected to the remaining terminal of the capacitor and the choke L 3 . The other end of the choke L 3  is connected to the load; this completes the transformer secondary circuit. 
   Principle of Operation 
     FIG. 2  illustrates the waveforms during operation of the above circuit. A voltage high indicates that a switch is closed and a low indicates otherwise. Switches S 3  and S 4  close and open alternately, thereby producing complementary waveforms as shown in  FIG. 2 . S 3  and S 4  are should not be closed together at any time. The switching frequency of S 3  and S 4  is lower than that of S 1  and S 2 . During the time that S 3  is closed, S 1  closes and opens several times, producing a train of pulses. S 2  remains open during this period. S 3  and S 1  then open. Following a short period of rest S 4  closes. S 2  then closes and opens several times to produce a train of pulses similar to that produced by S 1 . The signals controlling S 3  and S 4  can be generated using a reference similar to R in  FIG. 2 . S 3  is closed only when reference is below zero and S 4  is closed only when reference is above zero. In the case of using a half sinusoidal pulse width modulation (PWM) switching for S 1  and S 2 , a sinusoidal waveform current IR flows through the load R in  FIG. 1 . 
   Electronic Circuit Implementation 
   The switches S 1  and S 2  can be implemented using metal oxide field effect transistors, MOSFETs. Similarly the switches S 3  and S 4  can be implemented using the insulated gate bipolar transistors, IGBTs or MOSFETs.  FIG. 3  shows the circuit configuration using these devices. The controller C has been removed for the sake of clarity.  FIG. 4  shows the switching waveforms for the circuit. The signals  5  and  7  are the gate signals to the respectively numbered transistors in  FIG. 3 . The signals  2  and  3  are the gate signals to the respectively numbered transistors in  FIG. 3 . The gate signals are all low side as the reference is taken to be the source terminal of each transistor. The source terminals of all the transistors are grounded with respect to each circuit segment on which they are located. This implies that the anode of diode  8  is the ground line for transistor  7 . Similarly the anode of diode  6  is the ground line of transistor  5 . On the primary side of the transistor, the transistor source terminals are connected to the power source ground line. As shown in  FIG. 4  a current  112  flows in the load  12  when a sinusoidal PWM signal is used for control of transistors  2  and  3 . 
   Modes of Operation 
   The circuit presented can be used in stand-alone mode, in which case the connected load is passive.  FIG. 5  shows an implementation of the circuit in the stand-alone mode. The controller has been removed for the sake of clarity. In the stand-alone mode the load may be purely resistive or may have capacitive and/or inductive elements. The gate signals for transistors  5  and  7  are provided by the controller in response to a reference signal input to the controller. The reference signal may, for example, be provided by onboard reference signal. The frequency of the reference signal in this mode can be adjusted to any suitable range. 
   The circuit can also be operated in grid-connected mode, in which case the load connected is active.  FIG. 7  shows an implementation of the circuit in the grid-connected mode. The controller has been removed for the sake of clarity. The gate signals for transistors  5  and  7  are provided by the controller in response to a reference signal input to the controller. The reference signal may, for example, be the grid voltage frequency signal generated by  13 . The current that flows through the grid in the case of sinusoidal PWM switching for  2  and  3  is as shown by  113 . 
   Electronic Operation of Circuit 
   The transformer core used in the circuit is gapped to allow for energy storage. In one half cycle of the grid voltage, the transistor  7  is ON and the transistor  5  is OFF. In the other half cycle the transistor  5  is ON and the transistor  7  is OFF. During the time in which the transistor  7  is ON, the transistor  3  is repeatedly switched ON and OFF at high frequency. The duty cycle for switching transistor  3  is varied during this time in order to produce a sinusoidal PWM pulse train. When the transistor  3  is ON, the connected end of the transformer primary winding is clamped to ground. Current builds up in the part of the winding as a result. This results in energy storage in the magnetic core air gap of the transformer. When transistor  3  is turned OFF, the energy stored in the air gap is released into the secondary winding of the transformer. A current therefore flows through the diode  6 , the choke  11 , the grid  13 , the choke  10 , the transistor  7  and through the secondary winding to complete the circuit. This results in power being transferred from the DC source to the load or grid. In the other grid half-cycle, switching the transistor  2  repeatedly whilst the transistor  5  is ON results in power transfer into the grid. 
   We will now describe further embodiments of the present invention. 
   The DC to AC Converter and Voltage Amplification Stage 
   The proposed DC to AC converter circuit can be used in conjunction with various voltage amplification stages to suit particular applications. The amplification stage can be included before the proposed power circuit. The block diagram of  FIG. 7  shows the generic set up. 
   The amplification stage can constitute either a step-up or step-down (or a combination of the two) circuit. Two of the commonly used amplification methods are discussed in the two sections that follow. 
   Step-up Circuit 
     FIG. 8  shows a typical voltage step-up circuit incorporated at the front of the new DC to AC converter circuit. The step-up consists of a diode D 3 , a switch S 5  and an inductor L 1  to form a boost stage. This type of circuit set up can be used for example in many applications were the voltage appearing across capacitor Cb needs to be higher than the source voltage. A high voltage across Cb can have benefits of reducing the value of Cb required (for example, from 60 μF to 3 mF for a 150-200 W converter). For low power ranges this can lead to avoiding the use of an electrolytic based capacitor for Cb. Instead, for example, a polypropylene capacitor may be used. The boost converter may also work with 3 phase current. 
   another benefit would be the improvement in efficiency of the transformer as a result of reduced step-up ratio. 
   S 5  may be, for example, a high current (5 to 10 A), low voltage switching device. The device may also be a vertical device. Preferably, the switches in the dc to ac conversion stage are lateral devices. This configuration allows the vertical and lateral devices to be fabricated on a single integrated circuit. 
   Alternatively, the boost converter may comprise two stages. The first stage operates at a first frequency and the second stage operates at a frequency of the AC output. The first frequency may be in the range of 10, 100 or 1000 times higher than that of the output frequency. 
   Step-down Circuit 
     FIG. 9  shows a typical voltage step-down circuit incorporated at the front of the new DC to AC converter circuit. The step-down consists of a diode D 3 , a switch S 5  and an inductor L 1  to form a buck stage. This type of circuit set up can be used in many applications were the voltage appearing across capacitor Cb needs to be lower than the source voltage. For example a battery can be connected in parallel with Cb to form an interruptible power supply system. The circuit can also be applied in off-grid systems in which case the grid is replaced by another load. 
   Preferably, the amplification input stage is selectively configurable between a buck and a boost stage. Furthermore, the input stage is also selectively configurable to operate in an on-grid, off-grid or battery-powered configuration. 
   The switching devices may comprise MOS switching devices. Preferably, the switches are referenced to ground when switched on. Furthermore, it is preferable that the switches are driven without level shifting. 
   No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.