RECTIFIER CIRCUIT FOR PROVIDING LIMITED OUTPUT VOLTAGE RANGE

Abstract: A rectifier circuit connected with a variable voltage AC source for providing a range of DC output voltages narrower than the range of AC voltage input by controllably switching between half-wave and full wave rectification.

Description: The present invention relates in general to apparatus for rectifying the output of an AC source and more specifically to a rectifier circuit controllably operating to limit the output voltage range of a variable voltage AC source. Background of the Invention Rectifier circuits are well known in the art and are frequently used within a power supply circuit which generates a regulated DC output for use by electronic circuit components. The rectifier circuit is typically coupled between the AC source, from which the power supply circuit derives unregulated power, and the voltage regulator portion of the supply. The function of the voltage regulator portion is, among other things, to provide a substantially constant output voltage irrespective of variations in the AC source. The rectifier circuit, however, normally provides a rectified output that follows variations in the AC source to which it is coupled. Thus, the voltage regulator must be capable of performing its function for an anticipated wide range of rectifier output voltages. However, regulators are normally rated for operation with an input voltage that varies over only a limited range. For example, a typical input voltage range for a regulator may be 10 to 50 volts, i.e. a voltage variation of one to five. Thus, if the AC source voltage, and hence the rectifier output voltage, is expected to vary over a range greater than that for which the regulator to be used is rated, application of a single such regulator may not be suitable. One known solution to this problem is to apply multiple regulators to the rectifier output either in cascaded or parallel connection. The multiple regulators represent additional expense, take up additional space and have an adverse effect on overall power supply reliability. One example of a variable voltage AC source which has an exceptionally wide output voltage range is a 3-phase permanent magnet alternator. Such an alternator includes a mechanically driven shaft carrying a plurality of permanent magnets. One use of such alternators is in aircraft where they are driven by the aircraft engine. Since the alternator generates an AC voltage whose magnitude is in direct proportion to the rotational speed of the mechanical driver, in aircraft applications, no-load voltages can vary over a range exceeding twenty to one. The electrical output of such alternators in aircraft applications is typically coupled to a switched mode type of power supply, a supply type well known in the art. One characteristic of a switched mode power supply is that it draws substantially constant power from the source to which it is coupled irrespective of the source voltage. As a result, when the alternator output voltage is low, a proportionately higher current will be drawn by the power supply in order to maintain a constant power input. However, due to the large series inductive reactance often found in such alternators, the alternator output voltage will be further depressed by the higher current. Thus, the overall output voltage range of such an alternator when coupled to this type of power supply can exceed the no-load voltage range. One solution to the problem of accommodating a large alternator output voltage range known in the art is to provide the alternator armature with winding taps. The taps are utilized to shunt portions of the winding as a function of output voltage in order to reduce the overall output voltage range. This solution requires the fabrication of a special alternator with winding taps, as well as control circuitry and a tap changing mechanism for effecting the appropriate shunt connections between taps. When the alternator output voltage range exceeds the rating of the regulator to be applied thereto, and multiple regulators, as previously described, are applied to a high impedance alternator capable of producing a wide output voltage range, an additional problem results. That is, the components of the regulators must be capable of withstanding both the high currents experienced during low voltage conditions and the additional voltage stresses imposed during high voltage conditions. US 3205426 describes a rectifier circuit which comprises a first full wave centre-tap rectifier connected with an A.C. source to provide a D.C. output at one power level, a second full wave bridge rectifier connected with the A.C. source to provide a D.C. output at a higher power level, and switching means including a diode selectively operable to render said rectifiers alternately effective for supplying the D.C. output power to the output terminals of the rectifier circuit. Objects of the Invention It is an object of the present invention to provide a rectifier circuit which is automatically operable to compress the range of voltages developed by a variable AC voltage source. Another object of the present invention is to provide a rectifier circuit which obviates the need for multiple voltage regulators in the power supply as in the past. An additional object of the present invention is to provide a rectifier circuit which eliminates the need for tap changing mechanisms on alternator armatures. A further object of the present invention is to provide a rectifier circuit which enables use of a voltage regulator, the components of which may be rated for limited ranges of voltage and current. Summary of the Invention These and other objects are accomplished by the present invention which is directed to a rectifier circuit for accepting a wide range of voltages from a variable voltage AC source and delivering a relatively narrow range of rectified output voltages. "The rectifier circuit comprises first and second rectifier means for providing output voltages of opposite polarity, a switching means selectively operable to render one of the rectifier means non-conducting, and an output voltage range compression circuit means for limiting the ratio of the output DC voltage range to a lesser value than the ratio of the input voltage range including control means as set forth in claim 1 which generates switching voltages for controlling the switching means to switch rectifier circuit between half wave and full wave rectification modes dependent on the level of the rectified output voltage. The invention, together with the objects, features and advantages thereof are more fully considered in the following detailed description when read together with the accompanying drawings in which applicable reference designations have been carried forward. Brief Description of the Drawings Figure 1 is a circuit schematic diagram of a preferred embodiment of a rectifier circuit constructed in accordance with the present invention; Figure 2 is a graphical representation of an exemplary 3-phase AC voltage waveform; Figure 3 is a graphical representation of an exemplary 3-phase full wave rectified voltage waveform; Figure 4 is a graphical representation of an exemplary 3-phase half-wave rectified positive voltage waveform; and Figure 5 is a graphical representation of the operation of the rectifier circuit of the present invention. Description of the Invention Referring to Figure 1, a rectifier circuit, generally indicated at 10, includes a first half-wave rectifier network 12 and a second half-wave rectifier network 20. The inputs of rectifier networks 12 and 20 are connected to a three-phase voltage source 28. For illustrative purposes, the three phases of the source have been respectively designated A, B and C. Source 28 is wye-connected and includes a neutral point 30, designated N and accessible for electrical connection. In the illustrated embodiment of the present invention, rectifier network 12 comprises three diodes 14, 16 and 18, the cathodes of which are connected electrically in common at a junction 12a, while rectifier network 20 comprises three diodes 22, 24 and 26 whose anodes are connected to a common junction 20a. Each diode in each rectifier circuit is connected to a different phase of the voltage source. As will be seen, rectifier networks 12 and 20 are oppositely poled so as to rectify voltages of opposite polarities. Thus, rectifier 12 is configured to conduct upon application thereto of a positive polarity source voltage while rectifier 20 conducts when a negative polarity source voltage is applied to it. The output junction 12a of rectifier network 12 is directly connected to a first output terminal 32 of circuit 10. The output junction 20a of rectifier network 20 is connectable through a switch to a second output terminal 36 of the circuit. In the illustrated embodiment of the present invention, this switch comprises an NPN bipolar transistor switch 34. A resistor 35 is connected between the base and emitter terminals of transistor switch 34 to serve as a leakage path for junction current when the transistor is nonconducting. A diode 38 is coupled between terminal 36 and source neutral point 30. Additionally, terminal 36 is connected to a common voltage reference point 37 for circuit 10. Circuit 10 further comprises a control circuit 50 which, in the illustrated embodiment, includes voltage scaling means for supplying a signal voltage proportional to that appearing on terminal 32. In the preferred embodiment of the present invention such scaling means is provided as a voltage divider 40 consisting of a pair of resistors 42 and 44 connected in series between terminal 32 and common voltage reference point 37. An output 46 of the divider is connected to a node 48 which joins resistors 42 and 44. Control circuit 50 further includes a voltage comparator 52 of which a first input 54 is coupled to an external reference voltage source, designated V ref in Figure 1, through a resistor 58. A second input 56 of the comparator is coupled to output 46 of divider 40. A resistor 60 is coupled between comparator output 62 and input 56. Output 62 is also coupled to means 64 for conditioning transistor switch 34 to close and open. In the illustrated embodiment of the present invention, means 64 comprises a typical transistor driver circuit for operating transistor switch 34. This driver circuit includes a PNP bipolar transistor 66 the base of which is coupled to comparator output 62 through a resistor 67. The emitter of transistor 66 is coupled to an external positive voltage source designated +V cc . The collector of transistor 66 is coupled to the base of an NPN bipolar transistor 68 through a resistor 69. A resistor 70 is coupled between the base and emitter terminals of transistor 66 to serve as a leakage path for junction current when the transistor is nonconducting. The collector of transistor 68 is coupled to external voltage source +V cc through a resistor 72. The emitter of transistor 68 is coupled to the base of transistor switch 34. A diode 74 is coupled between the base of transistor 68 and output 36 of circuit 10. In operation, rectifier circuit 10 operates as a 3-phase full wave rectifier with transistor switch 34 closed and as a 3-phase half-wave rectifier with the switch open. Since, in general and for a given source AC voltage level, the output voltage of a half-wave rectifier is significantly less than that of a full wave rectifier, circuit 10 is operated with transistor switch 34 in an open circuit condition above a first predetermined value of rectifier circuit output voltage. Below a second predetermined circuit output voltage value, less than the first, circuit 10 is operated with the switch in a closed circuit condition. This achieves the desired result of limiting the rectified output voltage range as compared to the source voltage range. Figure 2 illustrates a balanced 3-phase AC output voltage waveform generated by source 28 and applied to rectifier networks 12 and 20. The phase-to-neutral voltages are respectively designated V AN , V BN and V CN . The rms value of the phase-to-neutral voltages is designated V s . With transistor switch 34 closed, i.e. the transistor in a conducting mode, full wave rectification results. Further, diode 38 is nonconducting due to the reverse biasing voltage across it. During this mode, the voltage on terminal 32 with respect to terminal 36, designated V out in Figure 1, has a waveform as illustrated in Figure 3. This waveform is simply that of the output of a 3-phase full wave rectifier and, as is well known in the art, has a peak value equal to √3 x √2 x V s or approximately 2.45 x V s , where V s is the source rms phase-to-neutral voltage. The average value of the V out waveform is equal to √3 x √2 x (3/π ) x V s or approximately 2.33 x V s . With transistor switch 34 open, i.e. the transistor in a nonconducting mode, half-wave rectification results and V out has a waveform as illustrated in Figure 4. During half-wave rectification, rectifier circuit 20 is nonconducting and diode 38 is conducting due to the forward biasing voltage applied to it, thus providing a return path to source 28. The latter waveform is simply that of a 3-phase positive half-wave rectifier and, as is well known in the art, has a peak value of √2 x V s or approximately 1.41 x V s . The average value of the V out waveform during half-wave rectification is equal to 3 x √3 x V s /( √2 x π ) or approximately 1.17 x V s . Thus, half-wave rectification reduces the peak value of V out achieved during full wave rectification by a factor of 2.33/1.17 or 1.73 and the average value of V out by a factor of 2.45/1.41 or 1.99. Control circuit 50 operates to open and close transistor switch 34 respectively at first and second predetermined values of V out . Such values are selected in accordance with the expected output voltage range of source 28 as well as the voltage range capability of the voltage regulator circuit to be used in the power supply. The reference voltage V ref is selected in accordance with these predetermined values of V out . A signal voltage appearing at node 48 of divider 40 is proportional to V out since resistors 42 and 44 are coupled between terminal 32 and rectifier circuit common voltage reference point 37. This signal voltage is applied to input 56 of comparator 52 by way of divider output 46. When the voltage at input 56 is less than that applied at input 54; i.e. V ref , the output of the comparator is in a low state. In the low state, output 62 is connected by the circuitry of the comparator to circuit common voltage reference point 37. As a result, the base of transistor 66 is also connected to the circuit common reference. Due to the application of +V cc to the emitter of transistor 66, this transistor is forward biased and hence in a conducting state. With transistor 66 conducting, a positive polarity voltage is applied to the base of transistor 68. As a result, transistor 68 is forward biased and conducting. This in turn results in a positive polarity voltage being applied to the base of transistor switch 34. Transistor switch 34 is thereby forward biased and conducting. As a result, rectifier networks 12 and 20 are both conducting, diode 38 is nonconducting and circuit 10 operates in a full wave rectification mode. In the illustrated embodiment of the present invention, diode 74 is connected to operate as a Baker clamp in order to increase the operating speed and control the base drive current of transistor 68. This circuit design technique is well known in the art. When V out exceeds the first predetermined voltage value, the signal voltage at divider node 48 exceeds V ref . Thus, the voltage at comparator input 56 exceeds that at input 54 and the output of the comparator swings to a high state such that a positive voltage appears on the comparator output. As a result, transistor 66 is no longer forward biased and hence turns off, i.e. becomes nonconducting. With transistor 66 nonconducting, transistors 68 and 34 are no longer forward biased and respectively turn off. With transistor switch 34 turned off, rectifier network 12 and diode 38 are conducting, rectifier network 20 is nonconducting and circuit 10 operates in a half-wave rectification mode. The operation of control circuit 50 relies on a hysteresis effect such that the first predetermined voltage value of V out at which transistor switch 34 opens, as V out increases, is greater than the second predetermined value of V out at which the switch closes as V out decreases. With comparator output 62 in the low state, resistor 60 is connected to the circuit voltage reference point 37. As a result, resistor 60 is electrically in parallel with resistor 44. Since the parallel combination of resistors 44 and 60 provides a lower effective resistance than resistor 44 alone, a lesser portion of V out applied to divider 40 appears across resistor 44 than would appear if resistor 60 were not in parallel with it. With comparator output 62 in the high state, resistor 60 is no longer in parallel with resistor 44 and a greater portion of V out appears across resistor 44 than would appear if resistor 60 were in parallel with resistor 44. Thus, first and second proportionality relationships exist between the signal voltage at node 48 and V out such that for a particular value of V out , the signal voltage will, respectively, be disproportionately high or low with respect to V out . This in turn depends on whether the comparator output is respectively in the high or low state. Thus, during full wave rectification, i.e. with transistor switch 34 closed, the comparator output is in the low state and the signal voltage at node 48 is disproportionately low. Because of this, a disproportionately higher value of V out is required in order for the signal voltage to exceed V ref . This higher value of V out is the first predetermined value at which switching from full wave to half-wave rectification takes place during a rising output voltage V out at terminal 32. During half-wave rectification, i.e. with transistor switch 34 open, the comparator output is in the high state and the signal voltage at node 48 is disproportionately high. Because of this, a disproportionately lower value of V out is required in order for the signal voltage to fall below V ref . This lower value of V out is the second predetermined value at which switching from half-wave to full wave rectification occurs during a falling output voltage V out at terminal 32. Thus, it is seen that the second predetermined value of V out is less than the first. In summary, switching from full wave to half-wave rectification takes place during rising output voltage V out by opening transistor switch 34 at the first predetermined value of V out . Switching from half-wave to full wave rectification takes place during falling V out by closing transistor switch 34 at the second predetermined value of V out . The embodiment of the invention illustrated in Figure 1 may be constructed with the following information: With the invention so constructed, the first and second predetermined values of V out may be calculated as follows: Initially, during the full wave rectification mode with comparator output 62 in the low state, the magnitude of the voltage V₅₆ at comparator input 56 is where R₄₂ = resistance of resistor 42 and R₆₀ = resistance of resistor 60 For the resistance values listed above, V₅₆ = 0.01334 V out . With an increasing V out and V ref = 2.5 volts, the comparator output will swing to the high state when V₅₆ exceeds 2.5 volts. The value of V out at which this swing occurs is the first predetermined voltage and is In the half-wave rectification mode, comparator output 62 is in the high state and with a decreasing V out the comparator output will swing to the low state when V₅₆ falls below V ref (2.5V). The value of V out for which this swing occurs is the second predetermined value of V out . This value of V out may be described in terms of a summation of currents at input 56: where R₄₄ = resistance of resistor 44 and R₆₇ = resistance of resistor 67 V cc , which varies with V out in the illustrated embodiment, is approximately 10.05V at the point of interest. Using this value of V cc and the resistance values listed above, the second predetermined value of V out = 72.6V. The operation of circuit 10 can be graphically illustrated as in Figure 5 where V out is plotted against V s , the source rms phase-to-neutral voltage. The voltage values indicated below are those for the illustrated embodiment and the specific component values listed above. The minimum value of V s expected during normal operation is V₁ = 8.2V. The corresponding value of V out due to the full wave rectification mode is v₅ = 2.33 x V₁ = 19.1V. As the source voltage increases, V out increases to the first predetermined value at V₉ = 187V, as calculated above, which corresponds to a source voltage of V₃ = V₉/2.33 = 80V. At this point, circuit 10 switches to the half-wave rectification mode and the value V out in the half-mode wave is V₇ = V₃ x 1.17 = 94V. In the half-wave mode the circuit may operate up to the maximum source voltage V₄ = 171V which corresponds to a circuit output voltage V₁₀ = V₄ x 1.17 = 200V. If only full wave rectification were available, the circuit output voltage would be V₁₁ = 2.33 x V₄ = 398V. As the source voltage decreases during the half-wave rectification mode, V out falls to the second predetermined voltage V₆ = 72.6V, as calculated above, at which circuit 10 switches to the full wave rectification mode. The corresponding source voltage at this point is V₂ = V₆/1.17 = 62V. As a result of the switch to full wave rectification, V out increases to V₈ = V₂ x 2.33 = 144V. Thus, the source voltage V s varies over a range from V₁ = 8.2V to V₄ = 171V while V out varies from V₅ = 19.1V to V₁₀ = 200V. In the absence of the output voltage range limitation provided by circuit 10, V out would range from V₅ = 19.1V to V₁₁ = 398V. In the illustrated embodiment, the range of V s from V₁ to V₄ or the range of V out from V₅ to V₁₁ would correspond to a voltage variation of approximately 1 to 20.8 while the range from V₅ to V₁₀, resulting from the operation of circuit 10, corresponds to a voltage variation of only approximately 1 to 10.5. While the illustrated embodiment herein is directed to a rectifier circuit adapted for connection to a 3-phase voltage source, the invention is not so limited. It will be readily apparent to those skilled in the art that switching between half-wave and full wave rectification in order to limit the rectified output voltage range is applicable to a single phase source of voltage. Such a source may comprise a center tapped transformer secondary winding wherein the tap point is, for circuit connection purposes, analogous to the neutral point of the 3-phase source. Additionally, each of the two half-wave rectifier networks may comprise two parallel connected diodes rather than three as in the 3-phase circuit illustrated herein. Thus, with the transistor switch closed, the single phase rectifier circuit would be configured as a full wave bridge rectifier. In the illustrated embodiment herein, voltage scaling means is coupled to the rectifier circuit output in order to develop a proportional signal voltage. It will be appreciated by those skilled in the art that the proportional signal voltage may instead be derived from the AC voltage source by coupling thereto through appropriate scaling means. While the transistor driver circuit of the present invention is configured as disclosed above, this is not a requirement of the present invention. Numerous transistor driver circuits are well known in the art and may be adapted for use herein. The preferred embodiment of the present is shown herein as including a bipolar transistor for switch 34. However, the invention is not so limited. Other switching devices such as electromechanical switches or other semiconducting devices may be used. Corresponding changes to the control circuit would be required. In the embodiment illustrated above, diodes are used in each half-wave rectifier network. The invention need not be so limited. Numerous unidirectionally conducting devices are known in the art and may be substituted for the diodes to achieve equally effective rectifier networks.

Claims: A rectifier circuit (10) for providing a variable DC output voltage from a varying input AC source (28) to which the rectifier circuit is coupled, wherein the variable DC output range is less than the varying AC input range, said rectifier circuit comprising: (a) first multiphase rectifier means (12) for connection to an AC source to provide an output voltage of one polarity at a first output terminal (32) of said rectifier circuit; (b) second multiphase rectifier means (20) for connection to an AC source (28) to provide output voltages of an opposite polarity at a second output terminal (32) of said rectifier circuit; and switching means (34) selectively operable to render one of said rectifier means nonconducting; characterized by, (c) output voltage range compression circuit means (50,64,40,52) for limiting the ratio of the output DC voltage range to a lesser value than the ratio of the AC input voltage range, including: (1) control means (50) responsive to the output DC voltage for generating switching voltages in response to the rectified output voltage, said control means generating a first switching voltage when said rectified output voltage is at a predetermined upper level and generating a second switching voltage when said rectified output voltage is at a predetermined lower level, and (2) means coupling (64) said switching voltages to said switching means for selectively actuating said switching means to operate said rectifying means in a half wave rectification mode above said upper level and in a full wave rectification mode below said lower level and in both half wave and full wave rectification modes between said levels, said switching means operating to maintain full wave rectification between said levels when said output voltage is increasing and half wave rectification when said output voltage is decreasing. The rectifier circuit of claim 1 characterized in that said second rectifier means is rendered nonconducting when said switching means is actuated to operate said rectifying means in the half wave mode. The rectifier circuit of claim 2 characterized by a unidirectional conducting means (38) coupled between the variable AC source and the second terminal, wherein said switching means is connected in series between the source and said second terminal. The rectifier of claim 3 characterized by said control means further including: switch actuating means (66) coupled to said switching means: a comparator (52) having a first input for coupling to a reference voltage source, a second input and an output coupled to said switch actuating means; and voltage scaling means (40) connected to said first output terminal of said rectifier circuit for supplying a signal voltage to said comparator second input proportional to said DC output voltage; said comparator controlling said switch actuating means to open said switching means when said signal voltage proportional to said first predetermined voltage exceeds said reference voltage and to close said switching means when said signal voltage proportional to said second predetermined voltage is less than said reference voltage. The rectifier circuit of claim 6 characterized by means (60) controlled by said comparator for conditioning said voltage scaling means to establish first and second different proportionality relationships between said signal voltage and said DC output voltage depending upon the condition of said switching means. The rectifier circuit of claim 5 characterized in that said first and second proportionality relationships occur when said switching means is respectively open and closed; said signal voltage being disproportionately high and low for said first and second proportionality relationships respectively. The rectifier circuit of claim 6 characterized in that said voltage scaling means comprises a voltage divider including first (42) and second (44) series connected resistors, one end of said first resistor being coupled to said first output terminal of said rectifier circuit and one end of said second resistor being coupled to a circuit voltage reference point, the series connection of said resistors connected to said comparator second input: and a third resistor (60) coupled between said output and said second input of said comparator; and said comparator operating to switch said third resistor into and out of parallel circuit relation with said second resistor whereby to establish said second and first proportionality relationships respectively. The rectifier circuit of claim 1 characterized in that said source is a 3-phase source (28), and each of said first and second rectifier means is configured as a 3-phase half-wave rectifier (14,16,18; 22,24,26).

Inventors: PETERSON, WILLIAM ANDERS
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1986
Main classification: H02M7
IPC codes: H02M7