Abstract:
In accordance with the present invention, the active rectifier is a circuit which directly takes the place of a passive rectifier by using a switching module (or simply a device in cases where a single device is used) controlled by a sensing circuit. Where passive devices have a single knee value determined by the physical properties of the semi-conductive material being used, the active circuit can be designed to a range of knee voltages and other performance criterion. Additional flexibility is available to the designer through the active rectifiers ability to allow for manipulation of the curve of response from the circuit in the knee region. Flexibility both in production, in designs, and in characteristics make the active rectifier highly valuable for engineering firms designing larger electronic circuits.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/869,350, filed Dec. 11, 2006, entitled “Active Rectifier” incorporated herein by reference in its entirety. 
    
    
     FEDERALLY FUNDED RESEARCH 
     Not Applicable 
     SEQUENCE LISTING OR PROGRAM 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to the field of electrical engineering/electrical circuits. Specifically, this electrical circuit is intended to be used in larger circuits. Performance issues, cost issues, part availability issues, as well as other factors make this circuit expressly desirable as a replacement for passive rectifier alternatives in certain applications. 
     2. Prior Art 
     The first device used for rectification of current was the vacuum tube rectifier. This included a heated cathode, a heating wire, and an anode. The electrons on the cathode would gain sufficient energy from the heater that they would break free of the cathode and would head toward the anode. Electrons that collected at the anode, though, would not have sufficient energy to leave, and hence would be stuck at the anode. In this way, electrical current would flow in only one direction. 
     After the hollow state rectifier, the solid state p-n junction rectifier was used for controlling the direction of electricity in electrical circuits.  FIG. 3  shows a graph of an actual p-n junction showing resistance vs. forward biased voltage on the junction. As the voltage increases, the resistance on the junction decreases. The two main features that are important on this diagram are the existence of a voltage where the device begins to turn on, called the knee voltage, and also the average slope of the tangent line to characteristic line within the knee region. The knee voltage in a semiconductor is intrinsic to the semi-conductive material being used while the shape of the knee region varies. Little adjustment is possible even at the silicon foundry. This is not a problem for power rectification of AC waves to DC voltage, for power regulation using non-precision voltage references like zener diodes, or for comparison of voltages where the signal is significant. When higher precision is necessary, having control of additional variables can make design of larger electrical circuits easier. Additionally, in instances where cheap, high current, low resistance active devices are available, it may be cheaper to use an active high power device rather than a high power passive rectifier. The flexibility to substitute a power circuit utilizing an active rectifier in place of a high power solid state rectifier allows additional flexibility by designers in what is becoming a highly competitive field. In the area of power rectification, the power component is by far the most expensive part, and its substitution has the greatest capability of decreased cost and increased profit. Even in low power applications, the additional flexibility allowed by changing the characteristic curves make this circuit valuable. 
     3. Objects and Advantages 
     This circuit takes the place of a passive rectifier in applications where:
         a) A different knee voltage is desired from the knee voltages available from the different passive diodes, or   b) A different slope on the knee is desired than available from commonly available diodes, or   c) There is a need to dissipate lower total wattage than with a conventional passive rectifier.   For example, a FETs (Field Effect Transistor, a common switching device) typically generate substantially less heat and are more power efficient than their passive junction cousins. It is actually possible to use parts which cause this active rectifier to dissipate lower wattage overall than any diode commonly available.   d) Accomplishes rectification like a solid state rectifier utilizing a different power component allowing for additional flexibility in manufacturing power circuits based on economic conditions.   e) Additional variables including speed of switch, slope, thermal dissipation, etc. are required that would either be impossible to obtain using stock passive parts, or would cost more.       

     Of noticeable importance is that this invention is capable of having lower knee voltages than any power diode presently in production or having, conversely, higher knee voltages than any diode presently in production. Beyond these benefits, the active rectifier circuit can also dissipate lower wattage than the passive cousins which gives many reasons to spend the extra effort to build this active circuit with substantially more parts. Also note, however, that these are not the only advantages of this invention. This invention has at least two degrees of flexibility over the passive alternative. Hence, for any application where the passive solution doesn&#39;t quite meet the requirements, the active rectifier may. 
     Additionally, as mentioned above, being able to accomplish an old task in an alternative way opens new avenues depending on economics and part availability. In a condition where there is high demand for high power transistors, the price for parts may be sufficiently lower than traditional power rectifiers so that utilization of the active rectifier as a replacement may in the future prove useful to reduce price. If that is not the case, then other issues including availability, temperature ranges, integration, etc. could also prove an obstacle where a manufacturer may prefer to use this complex circuit over the conventional solid state high power PN junction. 
     SUMMARY 
     In accordance with the present invention, the active rectifier is a circuit which directly takes the place of a passive rectifier by using a switching module (or simply a device in cases where a single device is used) controlled by a sensing circuit. Where passive devices have a single knee value determined by the physical properties of the semi-conductive material being used, the active circuit can be designed to a range of knee voltages and other performance criterion. Additional flexibility is available to the designer through the active rectifiers ability to allow for manipulation of the curve of response from the circuit in the knee region. Flexibility both in production, in designs, and in characteristics make the active rectifier highly valuable for engineering firms designing larger electronic circuits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Active Rectifier Schematic—This drawing uses standard electronics notation to depict a working embodiment of the invention. This schematic shows that there is an embodiment that is producible. Additional information on its operation can be found in OPERATION— FIG. 1 . 
         FIG. 2 : Alternate embodiment—This schematic adds diode D 5 . This diode causes the system to trigger differently and supports current suppositions in the claims. Additional information on its operation is found in operation of alternate embodiment. 
         FIG. 3 : Prior Art, Concept of Perfect Diode—This drawing is of a graph of a typical passive diode curve with resistance as the y-axis and current as the x-axis. This is to help clarify the issue as there is a great deal of difference between the dotted line and the actual passive diode curve line. 
         FIG. 4 : Functional Block Diagram of Active Rectifier—This Diagram breaks the invention into logical blocks so that the abstract idea can be clearly captured 
         FIG. 5 : Comparison of passive and active rectifiers—This diagram shows the standard symbol for a passive rectifier next to the block diagram of an active rectifier. It is to accentuate the differences in complexity and flexibility of the two options for rectification. 
     
    
    
     DETAILED DESCRIPTION 
     FIG.  1 —Preferred Embodiment 
     A preferred embodiment of the present invention is illustrated in  FIG. 1 . 
     In the preferred embodiment, transistor Q 1  is a field effect transistor with intrinsic reverse body diode, the intrinsic body diode is oriented in the same direction as the desired direction of rectification. Q 2  and Q 3  are NPN bipolar transistors, with bases connected together to form a differential pair. Although widely differing transistors can be used for Q 2  and Q 3 , the preferred embodiment uses two transistors substantially the same. The diode D 1  connects the emitter of Q 3  to offset diode D 1 &#39;s anode. The cathode of D 1  is attached to the drain side of FET, Q 1 . The emitter of Q 2  is attached directly to the source of Q 1 . Thus the differential pair Q 2 ,Q 3  senses the difference in voltage across the FET Q 1  with an additional voltage offset provided by D 1 . 
     In the preferred embodiment, An optional diode, D 2 , is connected from emitter to base of Q 1 , with the cathode of D 2  attached to the base of Q 3 . Another optional diode, D 3 , is attached to Q 3 &#39;s base and collector with D 3 &#39;s anode connected to the Base, and D 3 &#39;s cathode attached to the collector. 
     Zener Z 1 &#39;s anode is connected to Q 3 &#39;s collector, and Z 1 &#39;s cathode is connected to the base of Q 4 . 
     The collector of Q 4  is connected to the collector of Q 2  and to the gate of transistor Q 1 . 
     The anode of diode D 4  is connected to an external supply, +supp, of voltage with respect to at least one of Port 1  in, and Port 1  out. The cathode of D 4  is connected to one terminal each of three resistors, R 1 , R 2 , and R 3 . 
     The remaining terminal of R 1  is connected to the base of Q 3 . The remaining terminal of R 3  is connected to the base of Q 4 . And the remaining terminal of R 2  is connected to the emitter of Q 4 . The differential pair (Q 2  and Q 3 ), diode D 1 , zener diode Z 1 , transistor Q 4  and resistors R 2  and R 3  form a difference amplifier. The operation of the difference amplifier is discussed further below. 
     Operation— FIG. 1   
     D 4  serves to protect the diode circuit from application of reverse bias. The cathode side of protection diode D 4  serves as the positive power supply of the preferred embodiment. Resistor R 1  is a convenient source of current to bias at least one of Q 2  and Q 3  into forward active or saturated mode of operation. 
     Although differing transistors can be used to a similar effect, in the preferred embodiment an explicit diode, D 1 , is used insure that when Q 1  has a voltage drop from source to drain substantially smaller than the voltage drop across diode D 1 , that Q 3  will be biased into cutoff. Hence for current to flow through the collector of Q 3 , a necessary condition is that a small voltage drop exist from the source to the drain of Q 1 . This voltage would also naturally forward bias the internal body diode of Q 1 . Furthermore, since the emitters of a matched differential pair tend to be at zero potential with respect to each other—the voltage drop across D 1  will necessarily be nearly the same as the voltage drop across from source to drain of Q 1 . This similarity will cause a similar/proportional current to flow through D 1  as is flowing through the intrinsic body diode of Q 1 . 
     When Q 3  is in cutoff, and therefore no substantial current is flowing through D 1 , no substantial current will flow through zener Z 1 . Q 4 &#39;s base-emitter voltage and current will therefore be determined solely by resistors R 3  and R 2 . Thus, the absence of current in Z 1  will cause the voltage drops across resistors R 3  and R 2  to be zero, and thus transistor Q 4  will also be in cutoff. 
     When Q 3  is in cutoff mode, only Q 2  can be forward biased and therefore the voltage at the collector of Q 2  with respect to its emitter will drop until Q 2  saturates and the voltage from the gate to source of Q 1  is drawn down to a voltage much lower than the threshold of Q 1 , and thus Q 1  will be turned off. 
     However, when sufficient voltage is applied from source to drain of Q 1 , diode D 1  will become forward biased. Under this condition, and provided that there is a voltage at the +supply sufficiently large to overcome the zener drop of Z 1 , current will flow through resistor R 3 . The voltage across R 3  caused by this zener current will tend to forward bias the emitter base junction of Q 4 , and will also induce a voltage drop across resistor R 2 . The voltage across resistor R 2  will cause a positive current to flow into the emitter of Q 4 . The majority of the current flowing into Q 4 &#39;s emitter will arrive at the collector of Q 4 . 
     Thus R 3  and R 2  and Q 4  form a current mirror with gain determined by the values of these components. In the current mirror, the current through R 3  is mirrored by the current through R 2  and Q 4 . 
     And the positive current flowing out of Q 4 &#39;s collector will be in proportion to the current flowing in diode D 1 . Since the diode D 1  tracks the intrinsic body diode of the FET, Q 1 , a forward bias across the source-drain terminals of Q 1  will cause current to flow in Q 4 . 
     Since the voltage on the anode of D 1  tracks the voltage at the source of FET Q 1 , when Q 3  saturates in attempt to maintain the voltage at the anode of D 1  because of a forward bias across the source drain terminals of Q 1 , the Zener diode&#39;s cathode will have a voltage approximately equal to the voltage rating of the diode above the voltage at the source. Since Q 4  will saturate with a voltage on its collector that is a very small diode drop above the voltage at the zener&#39;s cathode—the voltage across the gate-source terminals of FET Q 1  will be nearly the voltage of the Zener diode. 
     Thus in normal operation, the bias across the source and drain terminals of FET Q 1  will cause the difference amplifier to produce a voltage across Q 1 &#39;s Gate-source terminals proportional to the said bias. (Q 1  being the switch block of FIG.  4 ,) 
     This action amplifies the small bias across Q 1 &#39;s source and drain such that a small forward bias across the intrinsic body diode of Q 1  will cause Q 1  to be turned very solidly on. This action will cause the effective forward bias resistance of the body diode of Q 1  to be reduced—and hence less power will be dissipated in the body diode. However, when the body diode of Q 1  is reverse biased, the same amplification will cause Q 1  to be turned off. The overall effect is to make Q 1  act like a diode whose forward bias voltage drop is much less than that of it&#39;s intrinsic body diode—and whose characteristics can be controlled by selection of zener Z 1  voltage, diode D 1  voltage, and current mirror Q 4 -R 2 -R 3 &#39;s values. 
     Optional Diode D 2  serves merely to prevent the leakage current of D 1  when in reverse bias from inducing hot carriers in Q 3 —thus extending its lifespan. Optional diode D 3  serves to reduce the saturation charge in Q 3  allowing it to be reduced to cutoff faster, and hence to improve the reverse recovery of the FET Q 1  when becoming reverse biased. 
     Operation of Additional Embodiments ( FIG. 2 ) 
     The alternate embodiment of  FIG. 2  has the same characteristics as that of  FIG. 1 , except that differential voltage across the source drain of transistor Q 1  required to effectively produce current in D 1  is reduced. Hence the circuit becomes more sensitive to the bias across the source drain terminals of Q 1 . 
     CONCLUSION, RAMIFICATIONS, AND SCOPE 
     This circuit would seem at first to not be something that is needed. However, having found applications which require reaction to voltage differentials a fraction of the knee voltage of silicon, there are presently no other alternatives. This circuit allows designers the opportunity to take more control of the response curves of their rectifier allowing them to utilize advantages of FET technology as well as other technologies. 
     The ramifications of this circuit are not limited to, but include sensing circuits, voltage references, as well as a host of other precision applications. The number of applications indeed are as varied as the applications for the original p-n junction. ( FIG. 5  submitted for contrast) 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, the switching device can be a bipolar transistor driven by a set of Darlington transistors thus splitting the switching functions into two parts, or perhaps a driving circuit to drive a relay breaking the circuit by mechanical means, etc. In any case, the salient points are contained within the claims, and the claims and their legal equivalents ought to be used to determine the breadth of this filing as opposed to the preferred examples.