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A DC/DC converter interconnecting a first DC voltage source having a first potential and a second DC voltage source having a second potential and having a stepup/stepdown transformer coupled between the two voltage sources; a controlled switching circuit coupled to the transformer for switching puls

A DC/DC converter interconnecting a first DC voltage source having a first potential and a second DC voltage source having a second potential and having a stepup/stepdown transformer coupled between the two voltage sources; a controlled switching circuit coupled to the transformer for switching pulsed direct current through the transformer between the voltage sources; and a controller controlling a switching operation of the switching circuit to allow the pulsed direct current to flow between the two voltage sources, whereby current flows between the two voltage sources in dependence on load demands imposed on respective ones of the two voltage sources.

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1. A DC-to-DC converter for interconnecting two DC voltage sources comprising a first source having a first potential and a second source having a second potential, the converter allowing current to be drawn from either voltage source by a load connected to either voltage source, the converter compr

1. A DC-to-DC converter for interconnecting two DC voltage sources comprising a first source having a first potential and a second source having a second potential, the converter allowing current to be drawn from either voltage source by a load connected to either voltage source, the converter comprising:a transformer coupling the first and second voltage sources and loads connected to each voltage source;a first controlled switch coupling the transformer and the second voltage source;a second controlled switch coupling the transformer and the second voltage source;a third controlled switch coupling the transformer and the first voltage source;a fourth controlled switch coupling the transformer and the first voltage source;a controller for repetitively turning on and off the first and fourth controlled switches simultaneously and alternately turning on and off the second and third controlled switches simultaneously; whereby current flows from the first voltage source to the second voltage source or from the second voltage source to the first voltage source depending on power demands placed on the first and second voltage sources;wherein the transformer has a plurality of windings including:a first winding coupling the first controlled switch and the third controlled switch;a second winding coupling the second controlled switch and the fourth controlled switch;a third winding coupling the first controlled switch and the first source voltage source; anda fourth winding coupling the fourth controlled switch and the first voltage source. 2. The converter defined in claim 1, wherein the transformer comprises an autotransformer and the first, second, third and fourth windings are connected in series. 3. The converter defined in claim 2, wherein the first, second, third and fourth windings are connected in series in the order first winding, third winding, fourth winding and second winding. 4. The converter defined in claim 1, wherein the controlled switches are selected from the group consisting of MOSFETs, IGBTs and bipolar devices. 5. The converter defined in claim 1, wherein each of the controlled switches is bridged by a respective diode. 6. The converter defined in claim 1, wherein when the first and fourth controlled switches are turned on, current flows between the first and second voltage sources in at least a portion of a first half cycle determined by the controller, and when the second and third switches are turned on, current flows between the first and second voltage sources in at least a portion of a second half cycle determined by the controller. 7. The converter defined in claim 6, wherein the controller includes a pulse width modulator generating first and second control signals for respectively controlling the first and fourth controlled switches simultaneously and the second and third controlled switches simultaneously, to render each pair of the first and fourth controlled switches and the second and third controlled switches conductive during the respective first and seconds half cycle of the controller. 8. The converter defined in claim 7, further comprising a current limiting circuit for preventing excessive current from flowing between the first and second voltage sources. 9. A DC-to-DC converter for interconnecting two DC voltage sources comprising a first source having a first potential and a second source having a second potential, the converter allowing current to be drawn from either voltage source by a load connected to either voltage source, the converter comprising:a transformer coupling the first and second voltage sources and loads connected to each voltage source;a first controlled switch coupling the transformer and the second voltage sourcea second controlled switch coupling the transformer and the second voltage source;a third controlled switch coupling the transformer and the first voltage source;a fourth controlled switch coupling the transformer and the first voltage source;a controller for r epetitively turning on and off the first and fourth controlled switches simultaneously and alternately turning on and off the second and third controlled switches simultaneously; whereby current flows from the first voltage source to the second voltage source or from the second voltage source to the first voltage source depending on vower demands placed on the first and second voltage sources;wherein when the first and fourth controlled switches are turned on. current flows between the first and second voltage sources in at least a portion of a first half cycle determined by the controller, and when the second and third switches are turned on. current flows between the first and second voltage sources in at least a portion of a second half cycle determined by the controller;wherein the controller includes a pulse width modulator generating first and second control signals for respectively controlling the first and fourth controlled switches simultaneously and the second and third controlled switches simultaneously, to render each pair of the first and fourth controlled switches and the second and third controlled switches conductive during the respective first and seconds half cycle of the controller;further comprising a current limiting circuit for preventing excessive current from flowing between the first and second voltage sources; andwherein the current limiting circuit comprises a current detecting circuit for detecting current flowing between the first and second voltage sources and for providing an overcurrent signal to the pulse width modulator to reduce a duty cycle of the first and second control signals. 10. The converter defined in claim 9, wherein the current detecting circuit comprises a comparator connected to a current sensor coupled in a circuit coupling the first and second voltage sources, the comparator providing the overcurrent signal if the current in the current sensor exceeds a predetermined reference. 11. The converter defined in claim 10, wherein the current sensor is connected in series with an inductor coupled with the transformer, so that the current flowing between the first and second voltage sources is smoothed when the pulse width modulator reduces the duty cycle. 12. The converter defined in claim 11, wherein the current limiting circuit further includes a transistor coupled between an output of the comparator and an input of the pulse width modulator for transmitting the overcurrent signal. 13. The converter defined in claim 12, further comprising a plurality of drivers, the drivers being arranged in four pairs each having two series connected drivers so that each pair of the series connected drivers is coupled between an output of the pulse width modulator and a respective controlled switch. 14. The converter defined in claim 1, wherein a selected one of two of the plurality of windings is connected so that it will not pass current between the first and second voltage sources at a specified time in a cycle of the controller. 15. The converter defined in claim 14, further comprising a capacitor coupled between a common ground for the two voltage sources and a point coupled to the two of the plurality of windings and a pair of diodes each coupling the point and a respective one of the two of the plurality of windings for providing a negative voltage supply. 16. The converter defied in claim 1, wherein either of the voltage sources includes an automotive alternator providing a rectified direct current, whereby the alternator can provide current to both of the voltage sources, with current to one of the voltage sources being supplied through the converter. 17. The converter defined in claim 1, wherein each switch comprises a parallel combination of a bidirectional switch and a diode. 18. A DC-to-DC converter comprising:a transformer having a first pair of windings on opposite sides of a central tap and a second pair of windings each connected to an outer terminal of a winding of the first pair; a first pair of alternately operable electronic switches each connected between an outer terminal of the first pair of windings and one pole of a first direct current voltage source, the first direct current voltage source having another pole connected to a ground connection;a second pair of alternately operable electronic switches each connected between an outer terminal of the second pair of windings and one pole of a second direct current voltage source, the second direct current voltage source having another pole connected to the ground connection; andrespective drivers for the electronic switches and a controller for the drivers to turn on one switch of the first pair and turn on one switch of the second pair simultaneously and alternately turn on a second switch of the first pair and turn on a second switch of the second pair simultaneously, thereby allowing pulsed direct current to flow between the two voltage sources in dependence on load demands imposed on the two voltage sources.