BI-DIRECTIONAL AC OR DC VOLTAGE REGULATOR

Abstract: The invention provides a bi-directional AC or DC voltage regulator having a controller, an input circuit and an output circuit, the input and output circuits being capacitively coupled one to the other and being symmetrical one relative to the other, wherein each circuit comprises two terminals (AC1, AC2; AC3, AC4) across which are connected a capacitor (C2a; C2b) and, in parallel with the capacitor (C2a; C2b), a series connection of an inductor (L1; L2) and a switching network (S1; S2) controlled by the controller. Each switching network (S1; S2) has two branches in anti-parallel, of which each branch permits only uni-directional current flow and comprises a switching means. Since both branches of each switching network comprise switching means, then the device may be constructed as an AC or DC regulator/transformer which retains the capacity to permit bi-directional power flow.

Description: TITLE Bi-directional AC or DC Voltage Regulator. DESCRIPTION Field of invention The present invention relates to the field of electrical power supplies, and particularly although not exclusively to a bi-directional AC or DC voltage regulator. Background art A conventional AC variable transformer (a variac) for stepping down a mains voltage, for example 230 volts AC to a reduced AC voltage, comprises AC voltage input terminals, across which is connected an inductive winding, and AC output terminals, which draw power from the winding at a selectable voltage, depending upon where a wiper blade is positioned along the winding. The wiper is typically a rotating wiper which rotates across the winding which is formed in a substantially cylindrical or ring shape. The wiper may be driven by a servo motor, in order to automatically move the wiper, thus varying the output voltage in response to a control signal . However, the conventional variac has the problems of high weight, large size and poor response time in moving the wiper blade, and produces noise which is fed back onto the mains supply and through to the output terminals. An apparatus which has been used in DC power systems to transform and regulate voltage is the uk converter. A basic topology "Cuk converter, as illustrated in Figure 1 of the drawings, has a circuit comprising input and output choke inductances LI and L2, an energy-transfer capacitor Cl , an output smoothing capacitor C2 , a diode Dl and a switching transistor Ql . This arrangement permits the DC output voltage to be stepped up or stepped down for a given input voltage depending on the proportion of time the transistor Ql conducts during a period of its operation. This ratio is known as the duty cycle of the transistor. During a first time interval when the transistor Ql is off the diode Dl is forward biased and the capacitor Cl is charged in the positive direction through the inductor LI. During a second time interval, the transistor Ql is turned on, and the capacitor Cl becomes connected across the diode D l , reverse biasing it. Thus the capacitor Cl discharges through the load and the output inductance L2 , charging the output capacitor C2 to a negative potential . The circuit operation is repeated when the transistor Ql is turned off again. The D C output voltage V out is dependent upon a number of parameters. Firstly the input voltage V in naturally effects the voltage value across the output terminals of the converter. If all other parameters are kept constant and the input voltage V in is increased, the DC output voltage of the converter will also increase. As discussed previously, the duty cycle (δ) of conduction of the transistor Q l is another parameter which effects the DC output voltage V out . A high duty cycle (δ) may yield a stepped up voltage at the output terminals, while a low duty cycle (δ) will produce an output voltage V out which is smaller in magnitude than the input voltage V in . The remaining principal parameter which controls converter performance is the converter circuit efficiency (e ) . It has been evaluated that the voltage relationship between the output signal and the input signal is as follows : Further extensions of the converter, illustrated in Fi g ures 2 to , have a similar operation to that discussed above . Figure 2 shows a Cuk converter in which the input and output choke inductors LI and L2 are coupled by a common core. There are obvious advantages in developing the converter in this way, namely, reductions in converter size, weight and component numbers, while the basic DC-to-DC conversion properties of the converter remain unchanged. Further, it has been shown that a significant reduction in ripple current magnitudes can be achieved by magnetic coupling of the choke inductances LI and L2. Figure 3 illustrates how an isolating transformer TX1 can be introduced to the 'Cuk converter to provide galvanic isolation between the output and the input voltages V out and V jn . As the transformer TX1 is isolated by the two energy-transfer capacitors Cla and Clb, no DC transformer core magnetization can take place. The Cuk converter illustrated in Figure 4 has coupling of the input and output inductances LI and L2 and an isolating transformer TX1. This converter benefits from the features described above but its basic operation remains unchanged. The Cuk converters discussed so far permit only DC voltage/current transformation and allow power to flow in one direction only. In order to fully understand the invention, a further possible extension of the Cuk converter is described below, with reference to Figures 5 and 6 of the drawings. Although the converter illustrated in Figure 5 is similar to a basic topology Cuk converter and is essentially a DC regulator, the additional components, a second transistor Q2 and second diode D2, permit bi-directional operation of the converter . The controlling signals supplying the base of the transistors switch each of the transistors on and off alternately, in anti-phase with each other. During a first time interval, when the first transistor Ql is off and the second transistor Q2 is on, the first diode Dl is forward biased and the energy- ransfer capacitor Cl is charged in the positive direction through the input inductor LI . During a second time interval, when the first transistor Ql is on and the second transistor Q2 is off, the energy- ransfer capacitor Cl is connected across the first diode Dl , reverse biasing it. Therefore, the energy-transfer capacitor Cl discharges through the output load and inductance L2 , and in the process charges the output capacitance C2b to a negative potential . The circuit operation described above is similar to that of a basic topology Cuk converter. However, the converter of Figure 5 is symmetrical in respect of the inputs and the outputs, and therefore will permit power flow in either direction. As described previously, use can be made of a common core to couple the input and output choke inductances LI and L2 to reduce ripple, and/or an isolating transformer TX1 to provide galvanic isolation. Figure 6 illustrates the addition of such an isolating transformer TX1 to the circuit of Figure 5. Cuk converter technology has been used exclusively to convert a DC input voltage to a DC output voltage, and is essentially uni-directional in terms of power flow. The present Application addresses the problem of providing an apparatus which permits bi-directional power flow so as to be able to accommodate regenerative load currents. In a preferred aspect, the invention provides an AC or DC voltage regulator/converter which, while functionally analogous to conventional iron/copper AC transformers, benefits from solid state control so as to permit a reduction in weight, size and cost while improving performance when compared to conventional means . SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a bi-directional AC or DC voltage regulator having a controller, an input circuit and an output circuit, the input and output circuits being capacitively coupled one to the other and being symmetrical one relative to the other, wherein each circuit comprises two terminals across which are connected a capacitor and, in parallel with the capacitor, a series connection of an inductor and a switching network controlled by the controller; CHARACTERIZED IN THAT: each switching network has two branches in anti-parallel, wherein each branch comprises a switching means for permitting only uni-directional current flow. Preferably, an isolation transformer in conjunction with a pair of energy transfer capacitors can be inserted between the input and output circuits permitting magnetic and capacitive coupling thereof. Alternatively, the input and output circuits may be coupled capacitively only, through a single energy transfer capacitor. Where a transformer is used to couple the input circuit to the output circuit, the turns ratio may be selected so as to establish the required output voltage range for a given input voltage . In a further embodiment of the present invention, the inductor of the input circuit is magnetically coupled to the inductor of the output circuit. Preferably, each switching means effectively comprises a series connection of a transistor and an aligned diode. Preferably, the controller monitors the polarity of the input voltage so as to establish which of the switching means would, if closed, permit current to flow through the switching network. Preferably, the controller operates at high frequency the transistor in the input circuit which, if closed, would permit current flow through the switching network, and simultaneously operates the oppositely aligned transistor in the output circuit so that it is in the opposite switching state to the high frequency operated transistor in the input circui . Preferably, the duty cycle of the transistors in the input and output circuits which are operated at high frequency can be varied so as to vary the actual output voltage within an output voltage range. Preferably, those transistors not operated at high frequency are held closed. THE DRAWINGS Figure 1 is a network drawing of a basic topology Cuk converter; Figure 2 is an extension of the converter shown in Figure 1 wherein the input and output choke inductances are magnetically coupled; Figure 3 is a further extension of a basic topology Cuk converter wherein an isolation transformer is introduced between the input and output terminals; Figure 4 illustrates a combination of the circuitry shown in Figures 2 and 3 ; Figure 5 shows a modification to a basic topology Cuk converter which permits bi-directional power flow; Figure 6 is an extension of the converter of Figure 5 wherein an isolating transformer is introduced between the input and output terminals; Figure 7 shows a bi-directional AC or DC voltage regulator/transformer according to the invention in which the input and output circuits are coupled through an isolating transformer; and Figure 8 shows a bi-directional AC or DC voltage regulator/transformer according to the invention with a slightly different topography geometry of the switching networks and in which the input and output circuits are directly coupled through a capacitor. DESCRIPTION OF PREFERRED EMBODIMENT Figures 1 to 6 have been described above in the 'background art' section of this Specification. Referring to Figure 7 of the drawings, a bi-directional AC or DC voltage regulator/transformer according to the invention comprises an input circuit and an output circuit which is symmetrical to the input circuit . As the regulator/transformer is fully symmetrical with regard to the input and output terminals, power may flow in either direction giving the regulator/transformer its bi-directional characteristics. As such, the input and output terminals can be interchanged. The regulator/transformer will first be described with reference to an AC input . Since the two circuits are symmetrical, it is only necessary to describe the arrangement of the components within one of the circuits . This is sufficient to develop a full understanding of the construction of the regulator/transformer . The input circuit has two terminals ACl and AC2 across which are connected a capacitor C2a and, in parallel with the capacitor C2a, a choke inductor LI serially connected to an energy-transfer capacitor Cla which in turn is connected to a winding of an isolating transformer TX1 that magnetically couples the input and output circuits. A switching network SI, comprising two diodes Dl and D2 and two transistors Ql and Q2, is connected between the inductor Ll/capacitor Cla junction and the transformer winding/terminal AC2 junction of the circuit. Firstly it is necessary to consider a period during which terminal ACl is positive with respect to terminal AC2 and load current is in phase with load voltage. Transistor Q2 and transistor Q3 are held on and therefore, in conjunction with diode D2 and diode D3 , provide bi-directional current paths. During this period transistors Ql and Q4 switch alternately at high frequency in response to a high frequency control signal from the controller. For a first time interval of this high frequency alternation, transistor Ql is off and transistor Q4 is on. During this interval diode D4 is forward biased and the energy transfer capacitors Cla and Clb charge through the choke inductor LI. During a second time interval of the high frequency alternation, the switching states of transistors Ql and Q4 are reversed. Once this occurs, the energy transfer capacitors Cla and Clb discharge, driving current through the output load via inductance L2 , and charging the output capacitor C2b. Circuit operation is repeated when transistor Ql is turned off and transistor Q4 is turned on again. Secondly it is necessary to consider a period during which terminal ACl is positive with respect to terminal AC2 and load current is out of phase with load voltage. Transistor Q2 and transistor Q3 are held on and therefore, in conjunction with diode D2 and diode D3 , provide bi-directional current paths. During this period transistors Ql and Q4 switch alternately at high frequency in response to the high frequency control signal from the controller. For a first time interval of this high frequency alternation, transistor Q4 is off and transistor Ql is on. During this interval diode Dl is forward biased and the energy transfer capacitors Cla and Clb charge through the choke inductor L2. During a second time interval of the high frequency alternation, the switching states of transistors Ql and Q4 are reversed. Once this occurs, the energy transfer capacitors Cla and Clb discharge, driving current out through the input terminals via inductance LI, and charging the input capacitor C2a. Circuit operation is repeated when transistor Q4 is turned off and transistor Ql is turned on again. Thirdly it is necessary to consider a period during which terminal ACl is negative with respect to terminal AC2 and load current is in phase with load voltage. Transistor Ql and transistor Q4 are held on and therefore, in conjunction with diode Dl and diode D4 , provide bi-directional current paths. During this period transistors Q2 and Q3 switch alternately at high frequency in response to the high frequency control signal from the controller. For a first time interval of this high frequency alternation, transistor Q2 is off and transistor Q3 is on. During this interval diode D3 is forward biased and the energy transfer capacitors Cla and Clb charge through the choke inductor LI. During a second time interval of the high frequency alternation, the switching states of transistors Q2 and Q3 are reversed. Once this occurs, the energy transfer capacitors Cla and Clb discharge, driving current through the output load via inductance L2, and charging the output capacitor C2b. Circuit operation is repeated when transistor Q2 is turned off and Q3 is turned on again. Fourthly it is necessary to consider a period during which terminal ACl is negative with respect to terminal AC2 and load current is out of phase with load voltage. Transistor Q l and transistor Q4 are held on and therefore, in conjunction with diode Dl and diode D4 , provide bi-directional current paths. During this period transistors Q2 and Q3 switch alternately at high frequency in response to the high frequency control signal from the controller. For a first time interval of this high frequency alternation, transistor Q3 is off and transistor Q2 is on. During this interval diode D2 is forward biased and the energy transfer capacitors Cla and Clb charge through the choke inductor L2. During a second time interval of the high frequency alternation, the switching states of transistors Q2 and Q3 are reversed. Once this occurs, the energy transfer capacitors Cla and Clb discharge, driving current out through the input terminals via inductance LI, and charging the input capacitor C2a. Circuit operation is repeated when transistor Q3 is turned off and transistor Q2 is turned on again. The output voltage V ou across the output terminals AC3 and AC4 is dependent upon the input voltage V in and the high frequency switching duty cycle of SI and S2. Thus if the input voltage amplitude Vj_ n is sinusoidal, the output voltage V ou will follow in proportion dependent upon the high frequency switching duty cycle of SI and S2 and the turns ratio of the isolating transformer TX1. When pairs of transistors are switched in the manner described above, it is possible to transform AC voltages in proportion to the duty cycle of the control signal and the turns ratio of the transformer TX1. In addition refinement of the high frequency control signal source, which alternately controls pairs of transistors, will permit harmonic distortion correction, synthesis of harmonics and/or fast acting regulation control to compensate for voltage drops in the circuit, mains voltage fluctuations and load variance. The frequency of the control signal is preferably from one to several orders of magnitude greater than the AC input frequency and may be, for example, from 500 Hertz to 250 KHertz. It will readily be understood from the above description that exactly the same circuitry and controller will produce, from a DC input, a regulated DC output. Power can flow through the converter in either direction, whether used with an AC supply or a DC supply. In Figure 7 the two branches of each switching network SI and S2 are illustrated as a pair of series-connected transistors and diodes, Q1-D2 and Q2-D1 for example, connected at their transistor-diode junctions. This topography enables the use of integrated circuit sub-assemblies of Ql and Dl, and Q2 and D2. Functionally, however, the transistor-diode uni-directional switching means of Figure 7 is exactly the same as that of Figure 8 in which two discrete branches are shown for each switching network. In Figure 8 the input circuit and output circuit are shown as being coupled, not by the capacitors Cla and Clb and isolating transformer TX1 of Figure 7, but by a capacitor Cl which is connected between the inductor (Ll, L2) /switching network (S1;S2) junctions of both circuits; and by a direct connection of the switching network (SI; S2) /terminal (AC2 ; AC4) junctions of both circuits. The circuit of Figure 8 loses the step up/step down function of the circuit of Figure 7 which is achieved by choice of the turns ratio of the isolating transformer TX1, but still permits step up or down by control of the duty cycle and bi-directional AC or DC voltage regulation as otherwise described with reference to Figure 7.

Claims: CLAIMS 1. A bi-directional AC or DC voltage regulator having a controller, an input circuit and an output circuit, the input and output circuits being capacitively coupled one to the other and being symmetrical one relative to the other, wherein each circuit comprises two terminals (ACl, AC2 ; AC3 , AC4) across which are connected a capacitor (C2a; C2b) and, in parallel with the capacitor (C2a; C2b) , a series connection of an inductor (LI; L2) and a switching network (SI; S2) controlled by the controller; CHARACTERIZED IN THAT: each switching network (SI; S2) has two branches in anti-parallel, wherein each branch comprises a switching means for permitting only uni-directional current flow. 2. A bi-directional AC or DC voltage regulator according to claim 1, wherein the capacitive coupling of the input and output circuits is provided by: a) a capacitor (Cl) which is connected between the inductor (LI; L2) /switching network (SI; S2) junctions of both circuits, and; b) the connection of the switching network (SI ;S2) /terminal (AC2 ; AC4) junctions of both circuits. 3. A bi-directional AC or DC voltage regulator according to claim 1, wherein the capacitive coupling of the two circuits comprises: a serial network, comprising a capacitor (Cla; Clb) and a winding of an isolating transformer (TX1) , connected in parallel across the switching network (SI; S2) of each circuit, such that each circuit is connected to a different winding of the isolating transformer (TX1) , wherein the capacitor (Cla; Clb) terminal of each serial network is connected to the inductor (LI; L2) /switching network (SI; S2) junction and the winding terminal of each serial network is connected to the switching network (SI; S2) /terminal (AC2; AC4) junction. 4. A bi-directional AC or DC voltage regulator according to claim 3, wherein: the turns ratio of the transformer (TX1) is selected so as to establish the required output voltage range for a given input voltage. 5. A bi-directional AC or DC voltage regulator according to any preceding claim, wherein: the inductor (LI) of the input circuit is magnetically coupled to the inductor (L2) of the output circuit. 6. A bi-directional AC or DC voltage regulator according to any preceding claim, wherein: each switching means comprises a series connection of a transistor (Ql, Q2 ; Q3 , Q4) and an aligned diode (D2, Dl; D4, D3) . 7. A bi-directional AC or DC voltage regulator according to any preceding claim, wherein: the controller monitors the polarity of the input voltage so as to establish which pair of the switching means (Q1,D2 or Q2,D1 in the input circuit, and Q4,D3 or Q3,D4 in the output circuit) would, if closed, permit current to flow through the respective switching networks (SI and S2) . 8. A bi-directional AC or DC voltage regulator according to any preceding claim, wherein: the controller operates at high frequency the switching means (Q1,D2 or Q2,D1) in the input circuit which, if closed, would permit current flow through the switching network (SI) , and simultaneously operates the oppositely aligned switching means (Q4,D3 or Q3,D4) in the output circuit so that it is in the opposite switching state to the high frequency operated switching means (Q1,D2 or Q2,D1) in the input circuit. 9. A bi-directional AC or DC voltage regulator according to claim 8, wherein: the duty cycle of the switching means (Q1,D2 or Q2 ,D 1 ) in the input circuit and the switching means (Q4,D3 or Q3 ,D 4 ) in the output circuit operated at high frequency can be varied so as to vary the actual output voltage within an output voltage range. 10 . A bi-directional AC or DC voltage regulator according to claim 8 or claim 9, wherein: the switching means (Q2,D1 or Q1,D2) in the input circuit and the switching means (Q4,D3 or Q3,D4) in the output circuit which are not operated by the controller at high frequency are held closed.

Inventors: GREENWOOD SIMON RICHARD, SOAR STEPHEN, GREENWOOD, SIMON, RICHARD, SOAR, STEPHEN
Applicants: GREENWOOD SIMON RICHARD, SOAR STEPHEN, GREENWOOD, SIMON, RICHARD, SOAR, STEPHEN
Application year: 1999
Main classification: H02M3
IPC codes: H02M3, H02M5