Method and apparatus for power conversion having a four-quadrant output

A power converter (10) operating in all four voltage-current quadrants includes an input voltage source (14), an output current independent of an output voltage and a switching arrangement (12, 16, 18, and 20) enabling an output terminal (26) to be in common with an input terminal. The output voltage can be unconstrained by an input voltage from the input voltage source.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable

FIELD OF THE INVENTION

This invention relates generally to power converters, and more particularly to a method and apparatus for providing power conversion having a four-quadrant output.

BACKGROUND OF THE INVENTION

There are a variety of switching power converters capable of four-quadrant output (where the output voltage can be positive, negative or zero, and the output current can also be positive, negative or zero). The majority are derived from a Buck topology. Buck converters are limited in that their maximum output voltage must be less than their input source voltage. In the case of two buck converters operated in a bridge, the maximum output voltage must be less than twice the input source voltage. In applications where the available input source voltage is fixed, an intermediate power conversion may be necessary to step up the available input voltage to achieve the required output voltage. This increases cost and complexity.

Four-quadrant converters have been made using a topology other than Buck to avoid this output-voltage limitation. U.S. Pat. No. 4,186,437 by Slobodan M. Cuk describes a novel four quadrant converter and a boost-derived four-quadrant converter. Both of these topologies have no theoretical limitation on their maximum output voltage. In both topologies, four-quadrant operation is achieved by operating two converters in phase-opposition and taking the output differentially across the outputs of the two converters in a bridge configuration. Due to the use of two converters in a bridge configuration and the resultant differential output, neither output terminal may be common with a terminal of the input voltage source. This lack of a common terminal prevents the desirable capability of combining the outputs of two such four-quadrant converters to a single load in a bridge configuration. Further, two or more inductors are required in either topology.

There are several converters that have some of the same elements of the present invention, but fail to function in a manner that would provide a four quadrant output and/or an unconstrained output signal. For example, U.S. Pat. No. 6,222,352 to Ronald J. Lenk (“Lenk”) has three operational modes, each of which occurs for a controlled time. Lenk does this to independently control two different outputs. Each of the outputs in Lenk operates a single voltage-current output (positive voltage, positive current) as opposed to all four quadrants. Furthermore, each output in Lenk is constrained to be smaller than the input.

U.S. Pat. No. 6,429,629 to Tranh To Nguyen (“Nguyen”) discusses converters that have a center-tapped wound magnetic element or transformer with a DC input applied to the center tap and switches from each winding end to ground. The converters in Nguyen only operate in one voltage-current quadrant (positive voltage, positive current or negative voltage, negative current) as opposed to all four quadrants. Once again, the output voltage magnitude is constrained by the input voltage. Nguyen states that the converters shown inFIGS. 3A–3Jare capable of output of “any polarity and magnitude.” However, it must be understood that all of the converters in Nguyen operate inherently in one quadrant depending on the direction of the diodes or synchronous rectifiers and that the output magnitude is constrained by the input voltage and turns ratio of the transformer.

SUMMARY OF THE INVENTION

An embodiment in accordance with the present invention can provide for a four-quadrant output from a single switching converter that has no theoretical limit on its output voltage and does not require the use of two converters. Further, one of the output terminals can be common with one of the terminals of the input voltage source as the output is single-ended, allowing the desirable combining of the outputs of two of the four-quadrant converters to a single load in a bridge configuration. Further, the embodiment can just use a single inductor with one, two, or more windings.

In one aspect of embodiments in accordance with the invention, a power converter operating in all four voltage-current quadrants comprises an input voltage source, an output current independent of an output voltage and a switching arrangement enabling an output terminal to be selectively in common with an input terminal. The output voltage can be unconstrained by an input voltage from the input voltage source.

In another aspect, a power converter operating in all four voltage-current quadrants can comprise an inductor having at least two windings where at least one set of respective common terminals of the windings are in opposite phase and an input voltage source selectively coupled to the inductor and to an output terminal of the power converter such that an output current remains independent of an output voltage and the output voltage is unconstrained by an input voltage from the input voltage source.

In yet another aspect, a method of power conversion comprises the steps of selectively converting an input signal to an output signal operating in any one of four voltage-current quadrants and selectively coupling at least one output terminal with an input terminal, where an output current is independent of an output voltage and the output voltage is unconstrained by an input voltage of the input signal.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring toFIG. 1, an electrical schematic block diagram shows the elements of an embodiment of a power converter10in accordance with the present invention. An inductor22has two windings (having terminals1–2and3–4) preferably having an equal number of turns and preferably being wound such that they are tightly-coupled magnetically, for example, in a bifilar manner. The windings are interconnected as shown, with the common terminals2,3being of opposite phase as indicated by the dots. The common terminal of the two windings preferably connects to one terminal of a voltage source14such as DC voltage source (Vin), in this case, to its negative terminal. Terminal1of the inductor22connects to bidirectional switch16(S1). The other terminal of S1connects to the positive terminal of voltage source14. In a like manner, terminal4of the inductor22connects to one terminal of bidirectional switch18(S2). The other terminal of S2connects to the positive terminal of voltage source14. One terminal of a third bidirectional switch20(S3), connects to terminal1of the inductor. Its other terminal connects to the positive output26of the converter. One terminal of a capacitor24connects to this positive output terminal. The other terminal of capacitor24connects to the negative output28or negative terminal of voltage source14which is also the negative output terminal of the converter. Capacitor24serves to filter the pulsating output current that flows when switch S3is on, reducing output-voltage ripple at the switching frequency.

A Pulse Modulator circuit12generates signals that control bidirectional switches, in this instance, S1through S3. These signals are such that S3is on when neither S1nor S2is on and S1and S2may not be on at the same time. The switches can be operated in a cyclical sequence that preferably occurs at a constant frequency. The on-time of bidirectional switch S3is preferably constant and must be less than the period of the sequence, that is, less than the reciprocal of its frequency. The sum of on-times of S1and S2is this period less the on-time of S3. Note, the pulse modulator circuit can be embodied using digital techniques and software. Also, the pulse modulator circuit and bidirectional switches can be embodied in integrated circuits, either combined or separately.

The relationships between the on times are stated below mathematically:
Ts1+Ts2+Ts3=K1  (1)
Ts3=K2  (2)
K2<K1  (3)
Where Ts(a) is the on-time of switch (a) and K1and K2are constant times, the switching period and the on-time of switch S3respectively.

An Output-voltage Control Signal is input to the Pulse Modulator that varies the on-times Ts1and Ts2of S1and S2respectively according to the above rules. Thus, if the on-time of S1is increased in response to the input signal, the on-time of S2must decrease by the same amount because the sum of all switch on-times is constant. The relationship between the input and output voltages Vin, Vout may be derived from the principle that, in the steady-state where there is no net change in inductor current over a switching cycle, the volt-second integral across an inductor winding1–2or3–4must be zero:
Vin×Ts2−Vin×Ts1−Vout×Ts3=0  (4)
where Vin and Vout have the reference polarities shown inFIG. 1.

The first two terms of (4) differ in sign because the winding1–2connected to switch S1is in phase opposition to the winding3–4connected to S2.

After algebraic manipulation:
Vout=Vin×[(Ts2−Ts1)/Ts3]  (5)
Note that the equations above correspond to an embodiment having windings with equal turns. Embodiments of the present invention can have windings with unequal turns and still operate fundamentally the same as described, except that the equations describing the operation would change. Having windings with equal turns is a special case of a more general one with unequal turns.

Three cases are of particular interest:A. Where Ts1=0, Vout.=Vin×(Ts2/Ts3). This is the maximum positive output voltage, which is larger than Vin where Ts2>Ts3.B. Where Ts2=0, Vout=Vin×(−Ts1/Ts3). This is the maximum negative output voltage, which has an absolute value larger than Vin where Ts1>Ts3.C. Where Ts1=Ts2, Vout=0.

From these three cases it can be seen that the output voltage may be positive, negative, or zero, indeed, it make take on any value between the positive and negative limits determined in (A) and (B) above. Further, it can be seen that, as the absolute value of the time ratio in the brackets of equation (5) approaches infinity, the absolute value of the output voltage Vout also approaches infinity for any nonzero value of Vin, thus, the output voltage is not limited by the input voltage Vin.

The operation of the invention in limiting cases (A) and (B) may be understood with reference to conventional Buck-boost converters. In the case where Ts2=0, the bidirectional switch S2and the inductor winding to which it connects may be removed from the circuit for analysis, which becomes a polarity-inverting Buck-boost converter that provides a negative output. In the case where Ts1=0, bidirectional switch S1may be removed from the circuit for analysis, which then becomes a non-polarity inverting Buck-boost converter where inductor winding3–4is the input or primary winding and winding1–2becomes the output or secondary winding. The output is positive due to the phase relationship between the windings.

Between these two limit cases, both Ts1and Ts2are nonzero. In this mode there is no analog to known converters. From the discussion above, it is clear that the two inductor windings with their associated switches store energy in the inductor in opposite directions. The closure of S1causes the current flowing into a reference-phase (dotted) inductor terminal to increase, while the closure of S2causes the current flowing into a reference-phase (dotted) inductor terminal to decrease. The action of the switches S1and S2oppose each other and the net volt-second product across an inductor winding terminals1–2or3–4results from the difference in the switch on-times. Where S1is closed for a longer time than S2, the net volt-second product across inductor terminals1–2is positive; where S2is closed for a longer time than S1, the net volt-second product across inductor terminals1–2is negative. It is this net volt-second product that determines the polarity of the output voltage.

It will be noted that, if switch S3and capacitor24are removed, the circuit ofFIG. 1is similar to the primary circuit of a push-pull Buck or forward converter with a center-tapped transformer primary that consists of two windings with an equal number of turns. The present invention differs from the push-pull Buck converter in that it controls and varies the magnitude and sign of the winding volt-second product and explicitly stores energy in an inductor which is transferred to an output. In the case of the Buck converter with a push-pull primary, the volt-second product is either not controlled, or if controlled, it is done such that the net volt-second product across either winding is minimized.

This minimization is necessary to prevent the saturation of the Buck-converter transformer core which is not designed to store a significant amount of energy. Further, in such a push-pull Buck converter, any energy stored in the transformer is not deliberately varied as a means to vary the output of the converter and delivered to the converter output as in the present invention.

Given that the switches used in the present invention are bidirectional, meaning that they can control current flow in both directions, power may flow from the input voltage source to the output, or into the output and to the input voltage source (Vin ofFIG. 1). Thus, the labels “input” and “output” are arbitrary. An example of an application for the “reverse” power flow would be an AC-to-DC converter, such as a battery charger or power-factor correction circuit.

If the Output-voltage Control Signal input to the Pulse Modulator is AC, the output voltage will also be AC and the converter will function as an amplifier.

Referring now toFIG. 2, a block diagram shows the elements of an other embodiment of a power converter50similar to power converter10ofFIG. 1. Power converter50and power converter10can share many of the same elements including pulse modulator12, voltage source14, bidirectional switches16,18and20as well as capacitor24. In contrast though, power converter50includes an inductor52having a third inductor winding5,6in addition to the first (1,2) and second (3,4) inductor windings. Power converter50has an output that is electrically isolated from the input. This isolation is achieved by means of the third inductor winding5,6that connects only to the output. The pulse modulator12inFIG. 2can be identical to the pulse modulator12ofFIG. 1. Where the number of turns in this third winding is equal to the number of turns in windings1–2and3–4, the relationship between input voltage, switch times, and output voltage is identical to power converter10ofFIG. 1. Where the number of turns in output winding5–6is not equal to that of1–2and3–4the relationship between input voltage, switch times, and output voltage is changed as follows:
Vout=Vin×N×[(Ts2−Ts1)/Ts3]
where N is the ratio of the number of turns in winding5–6to the number of turns in both windings1–2and3–4.

In the power converter10ofFIG. 1, current is always flowing in either winding1–2or winding3–4. In the power converter50ofFIG. 2, when bidirectional switch20(S3) is closed, current ceases to flow in windings1–2and3–4and transfers to flow in winding5–6. This behavior is much like that of a conventional isolated buckboost or Flyback converter.

With reference toFIG. 3, a block diagram illustrates yet another embodiment of a power converter70that can have many of the same elements as power converters10and50including pulse modulator12, voltage source14, and capacitor24. Pulse modulator12of power converter70can be identical to the pulse modulator in converter10ofFIG. 1. In contrast to the multi-winding inductors and (single) bidirectional switches used in power converters10and50, power converter70uses an inductor82having a single winding (1–2) and a series of bidirectional switch pairs (76–81). Where power converter10ofFIG. 1uses two windings which are connected in opposing phase to the input voltage,FIG. 3uses bidirectional switch pairs76&78(S1/S3) and77&79(S2/S4) to connect a single inductor winding (in inductor82) to the input voltage in opposite polarities. Bidirectional switch pair80&81(S5/S6) connects the inductor winding1–2to the output. Switches76and78are opened and closed together, switches77and79are opened and closed together, and switches80and81are opened and closed together. The switch pair76/78(S1/S3) is analogous to switch16(S1) ofFIG. 1, switch pair77/79(S2/S4) is analogous to18(S2) ofFIG. 1, and switch pair80/81(S5/S6) is analogous to20(S3) ofFIG. 1. One terminal of the bidirectional switch80(S5) connects to terminal1of the inductor82. Its other terminal connects to the positive output86. One terminal of bidirectional switch81(S6) connects to terminal2of inductor82while its other terminal connects to the negative output28.

The closure of switches76(S1) and78(S3) causes the current flowing into the reference phase (dotted) inductor terminal to increase, while the closure of switches77(S2) and79(S4) causes the current flowing into the reference (dotted) inductor terminal to decrease. The actions of switch pairs76/78(S1/S3) and77/79(S2/S4) oppose each other and the net volt-second product across the inductor winding1–2results from the difference in these switch-pair on-times. Where S1and S3are closed for a longer time than S2and S4, the net volt-second product across the winding1–2is positive. Where S2and S4are closed for a longer time than S1and S3, the net volt-second product across the winding1–2is negative. Where S1and S3are closed for the same duration as S2and S4, the net volt-second product across the winding1–2is zero. As with power converter10ofFIG. 1, it is this net volt-second product that determines the polarity of the output voltage. The relationship between input voltage, switch times, and output voltage of the power converter70ofFIG. 3is identical to that of the power converter70ofFIG. 1.

Unlike power converter10, the output of power converter70is inherently isolated electrically from the input. However, either output terminal may be connected to either input terminal with no change in operation of the converter.

While the inductor82ofFIG. 3is simpler than inductor22ofFIG. 1or inductor52ofFIG. 2due its single winding, this improvement is offset by the added complexity of three additional bidirectional switches. Further, unlike power converter50ofFIG. 2, where electrical isolation of the input and output is provided by separate inductor windings, the isolation of power inductor70is provided solely by the bidirectional switches (76–81).

Referring toFIG. 4, a flow chart illustrating a method100of power conversion comprises the step102of selectively converting an input signal to an output signal operating in any one of four voltage-current quadrants and the step106of selectively coupling at least one output terminal with an input terminal, where an output current is independent of an output voltage and the output voltage is unconstrained by an input voltage of the input signal. The method100can optionally include the step104of switching a plurality of bidirectional switches that operate in a cyclical sequence controlled by a pulse modulator and the step108of switching a bidirectional switch among the plurality of switches such that an on-time of the bidirectional switch is a constant amount of time during a period of the cyclical sequence.

In light of the foregoing description of the invention, it should be recognized that the present invention can be realized in hardware, software, or a combination of hardware and software. A method and system for providing power conversion according to the present invention can be realized in a centralized fashion in one computer system or processor, or in a distributed fashion where different elements are spread across several interconnected computer systems or processors (such as a microprocessor and a DSP). Any kind of computer system, or other apparatus adapted for carrying out the methods described herein, is suited. A typical combination of hardware and software could be a general purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the methods described herein.

Additionally, the description above is intended by way of example only and is not intended to limit the present invention in any way, except as set forth in the following claims.