Regulated bi-directional DC-to-DC voltage converter which maintains a continuous input current during step-up conversion
BACKGROUND OF THE INVENTION 
     The present invention provides a regulated bi-directional DC-to-DC voltage converter. More particularly, a bi-directional DC-to-DC voltage converter having a regulated output, and capable of converting between a high-potential direct current voltage and a low-potential via a pulse-width modulation (&#34;PWM&#34;) technique. Additionally, the disclosed invention accomplishes the voltage conversions with a minimal number of components, and therefore provides for a reliable converter which offers advantages in size and weight over prior art designs. 
     Regulated bi-directional DC-to-DC voltage converters are employed extensively in aerospace applications to provide a means of coupling batteries (typically a 28 volt arrangement of cells) and the high-voltage (typically 270 V) DC bus which provides power to the various flight systems. When in a &#34;step-up&#34; mode the converters function to increase the battery voltage level to the required high-voltage bus level, and when in an &#34;step-down&#34; mode, the converters reduce the high voltage available on the bus to a level suitable for battery charging. 
     As with most components intended for aerospace applications, the minimization of the size and weight of bi-directional voltage converters is of great concern. The primary contributors to both the size and weight of such a voltage converter are its transformer and inductors (the so-called &#34;magnetic components&#34;). As typical regulated bi-directional DC-to-DC voltage converters require at least two inductors (one on each side of the transformer) to be employed to decouple the rectifying and switching components of the converter from its DC capacitors, the impact of the magnetic components on the converter&#39;s size and weight is increased even further. Also of concern in an aerospace system is the reduction of electromagnetic interference (&#34;EMI&#34;) generated by any component. Typically, power converters create EMI as a result of current switching performed during conversion. 
     Accordingly, it is the object of the present invention to provide a regulated bi-directional DC-to-DC voltage converter which employs a single transformer and a single inductor (thus reducing the size and weight of the converter), and which provides a stable output voltage level over a wide range of input voltage variation in either the step-up or step-down mode. In addition, the invention insures the minimization of generated EMI as it maintains a continuous input current during step-up operation. 
     SUMMARY OF THE INVENTION 
     This invention provides a bi-directional DC-to-DC voltage converter having a regulated output, and capable of converting between a high-potential direct current voltage and a low-potential direct current voltage using a PWM technique. Within the disclosed converter, the magnetic components, as well as several of the semiconductor rectifiers, perform dual functions (one in the step-up mode, and another in the step-down mode). This dual functionality serves to minimize the total component count, allowing the converter to be both compact and light weight; two critical attributes for aerospace applications. Additionally, the invention allows input current to remain continuous during voltage step-up conversion, thus reducing the EMI which is typically generated by voltage converters current switching during step-up conversion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of this invention will be apparent on consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1A is a schematic diagram of a preferred embodiment of the invention; 
     FIG. 1B is a schematic diagram illustrating a circuit equivalent to the preferred embodiment of FIG. 1A in the &#34;down on  &#34; state when the first negative and second positive bridge transistors are biased on; 
     FIG. 1C is a schematic diagram illustrating a circuit equivalent to the preferred embodiment of FIG. 1A in the &#34;down on  &#34; state when the second negative and first positive bridge transistors are biased on; 
     FIG. 1D is a schematic diagram illustrating a circuit equivalent to the preferred embodiment of FIG. 1A in the &#34;down off  &#34; state; 
     FIG. 1E is a schematic diagram illustrating a circuit equivalent to the preferred embodiment of FIG. 1A in the &#34;up on  &#34; state; 
     FIG. 1F is a schematic diagram illustrating a circuit equivalent to the preferred embodiment of FIG. 1A in the &#34;up off  &#34; state when the positive boost transistor is biased on; 
     FIG. 1G is a schematic diagram illustrating a circuit equivalent to the preferred embodiment of FIG. 1A in the &#34;up off  &#34; state when the negative boost transistor is biased on. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in FIG. 1, a preferred embodiment the invention consists of two sub-circuits: a boost converter 1 and a bridge converter 2. These sub-circuits share a common negative line and are also interconnected by a transformer 3. The transformer&#39;s primary winding has a first terminal and a second terminal. The secondary winding of the transformer has a first terminal, a second terminal, and a center-tap terminal. The turn ratio between the primary and secondary windings is N:1. 
     The boost converter sub-circuit has a positive input/output terminal 4 and a negative input/output terminal 5, with a filter capacitor 6 connected between them. In operation, the input/output lines of this sub-circuit are connected to the low-voltage supply. The positive input/output terminal is also connected to one terminal of an inductor 7. The other terminal of the inductor is coupled to the center-tap terminal of the transformer&#39;s secondary winding. The negative input/output terminal of the boost sub-circuit is coupled the negative input/output terminal of the bridge converter sub-circuit, and to the emitters of two NPN transistors 8, 9. One of the transistors 8, designated the positive boost transistor, has its collector lead coupled to the first terminal of the transformer&#39;s secondary winding. The emitter and collector of this transistor are bridged by a diode 10, connected so that the anode of the diode is coupled to the transistor&#39;s emitter, and the cathode of the diode is coupled to the transistor&#39;s collector. The other transistor 9, designated the negative boost transistor, has its collector lead coupled to the second terminal of the transformer&#39;s secondary winding. The emitter and collector of this transistor are bridged by a second diode 11, connected so that the anode of the diode is coupled to the transistor&#39;s emitter, and the cathode of the diode is coupled to the transistor&#39;s collector. 
     As with the boost converter sub-circuit, the bridge converter sub-circuit also has a positive input/output terminal 12 and a negative input/output terminal 13 which are bridged by a filter capacitor 14. In operation, the input/output lines of this sub-circuit are connected to the high-voltage bus. The bridge converter sub-circuit also contains a network of four NPN transistors 15, 16, 17, 18, and four diodes 19, 20, 21, 22. 
     The collector of one of the transistors 15, designated the first negative bridge transistor, is coupled to the second terminal of the transformer&#39;s primary winding, and its emitter is coupled to the negative input/output terminal of the bridge converter sub-circuit. The emitter and collector of this transistor are bridged by a diode 19, connected so that the anode of the diode is coupled to the transistor&#39;s emitter, and the cathode of the diode is coupled to the transistor&#39;s collector. 
     The emitter of the second transistor 16, designated the first positive bridge transistor, is coupled to the second terminal of the transformer&#39;s primary winding, while its collector is coupled to the positive input/output lead of the bridge converter sub-circuit. The emitter and collector of this second transistor are also bridged by a diode 20, connected so that the anode of the diode is coupled to the transistor&#39;s emitter, and the cathode of the diode is coupled to the transistor&#39;s collector. 
     The collector of the third transistor 17, designated the second negative bridge transistor, is coupled to the first terminal of the transformer&#39;s primary winding, and its emitter is coupled to the negative input/output terminal of the bridge converter sub-circuit. The emitter and collector of this transistor are bridged by a diode 21, connected so that the anode of the diode is coupled to the transistor&#39;s emitter, and the cathode of the diode is coupled to the transistor&#39;s collector. 
     The emitter of the fourth transistor 18, designated the second positive bridge transistor, is coupled to the first terminal of the transformer&#39;s primary winding, while its collector is coupled to the positive input/output lead of the bridge converter sub-circuit. The emitter and collector of this second transistor are also bridged by a diode 22, connected so that the anode of the diode is coupled to the transistor&#39;s emitter, and the cathode of the diode is coupled to the transistor&#39;s collector. 
     As stated earlier, the invention performs both step-up and step-down conversions. Operation of the above described embodiment in both of those modes is discussed below: 
     For step-down conversion the converter is repeatedly switched between two states, &#34;down on  &#34; and &#34;down off  &#34;, to achieve the proper output voltage regulation. In the down on  state the four transistors within the bridge converter sub-circuit are switched between conductive (&#34;on&#34;) and non-conductive (&#34;off&#34;) states as pairs. One pair, consisting of the first negative bridge transistor and the second positive bridge transistor are first biased on, while the other pair, consisting of the second negative bridge transistor and the first positive bridge transistor are biased off (see the equivalent circuit illustrated in FIG. 1B). The biasing is then changed to reverse the conductive states of the transistor pairs (see the equivalent circuit illustrated in FIG. 1C). This bias switching, the frequency of which is determined as a function of the converter&#39;s application, effectively delivers high-voltage DC pulses of alternating polarities to the primary winding of the transformer. While the conductive states of these transistor pairs are being alternately switched, the two transistors contained in the boost sub-circuit are biased to off throughout step-down operation, allowing their companion diodes to act as a single phase push-pull rectifier, and deliver a series of low-voltage pulses having the same polarity to the inductor, the boost sub-circuit filter capacitor, and any load coupled to the boost sub-circuit input/output terminals. In the down off  state, the four transistors within the bridge converter sub-circuit and the two transistors in the boost sub-circuit are biased off (see the equivalent circuit illustrated in FIG. 1D). This allows energy stored in the inductor to freewheel through the secondary winding of the transformer, the boost sub-circuit diodes, and any load connected to the boost sub-circuit input/output terminals. 
     For step-up conversion the converter is also repeatedly switched between two states, &#34;up on  &#34; and &#34;up off  &#34;, to achieve the proper output voltage regulation. In the up on  state the four transistors within the bridge converter sub-circuit are biased off, and the two transistors contained in the boost sub-circuit are biased on (see the equivalent circuit illustrated in FIG. 1E). This effectively prohibits any DC power entering on the boost sub-circuit input/output terminals from being transferred through the transformer to the bridge sub-circuit, while allowing the energy previously stored in the inductor and the bridge converter filter capacitor to supply the high-voltage load (connected to the input/output terminals of the bridge sub-circuit) with power. In the up off  state the four transistors within the bridge converter sub-circuit remain biased in an off state, but the two transistors contained in the boost sub-circuit are alternately biased on and off--One being &#34;off&#34; while the other is &#34;on&#34; (see the equivalent circuits illustrated in FIGS. 1F and 1G). The frequency at which the biasing is switched being a function of the specific application in which the converter is employed. This switching permits the induced voltage across the inductor (due to the energy stored in the up on  state), as well as the voltage applied to the boost sub-circuit input/output terminals (which is delivered as low-voltage DC pulses of alternating polarities to the secondary winding of the transformer) to be stepped up through the transformer and delivered to a load connected across the input/output terminals of the bridge converter sub-circuit. 
     The transistors referred to in the above preferred embodiment description may be standard bipolar type, insulated gate bipolar transistors, or field effect transistors. In addition, transistors are by no means the only type of switching device which could be employed in the invention. For example, thyristors may be substituted for all or some of the aforementioned transistors. 
     It will be understood that the particular embodiments described above are only illustrative of the principles of the present invention, and that various modifications could be made by those skilled in the art without departing from the scope and spirit of the present invention, which is limited only by the claims that follow.