Abstract:
A multiple-output DC-DC converter has an electronically controllable reactor (64,66) connected in series with each output circuit. In response to variations in voltage of the output circuits (80,82), control signals are fed back to vary the permeance of the electronically controllable reactor (64,66) in each output circuit. The voltage in any output circuit is thereby made independent of input voltage variations and load fluctuations in any other output circuit.

Description:
TECHNICAL FIELD 
     This invention relates to multiple-output DC-DC converters and, in particular, to the regulation of multiple-output DC-DC converters. 
     BACKGROUND OF THE INVENTION 
     In a DC-DC converter having a plurality of voltage outputs, one of the voltage outputs may be directly sensed and fed back to a controller for varying the duty cycle of a switching transistor connected to the converter input. The regulation of the remaining outputs is achieved by using the magnetic coupling between the secondary output circuits. Such a method of controlling the output voltages is referred to as cross regulation. Because the magnetic coupling between secondary windings is never perfect, however, as the load connected to one of the output circuits fluctuates, the voltages of the other output circuits may also fluctuate. It is desirable to be able to regulate independently each of the voltage outputs regardless of load variations. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrative embodiment of the present invention, both method and apparatus are disclosed for regulating the voltage in each output circuit of a multiple-output DC-DC converter independently of variations in the load connected to each other output circuit. This voltage regulation is obtained by varying the inductance in each output circuit in response to load fluctuations. 
     More particularly, a electronically controllable reactor is connected in series with each output circuit, namely, the secondary-winding circuit. According to one embodiment, the load voltage or a proportion thereof in one of the output circuits is compared with a reference voltage to develop an error signal. In response to the error signal, a control current in each output circuit is adjusted and fed back to control the permeance of the electronically controllable reactor. 
     According to another embodiment, the output current in one of the output circuits is fed back to control the permeance of the electronically controllable reactor connected thereto. In each of the remaining output circuits, the output voltage is compared with a reference voltage to produce an error signal. In response to the error signal, a control current is adjusted and fed back to control the permeance of the electronically controllable reactor in that output circuit. 
     In each of the aforesaid embodiments, input voltage variations and load fluctuations across any secondary circuit are prevented from affecting the load voltage across any of the remaining secondary circuits of a multiple-output DC-DC converter. 
     One advantage of the present invention is the highly efficient method of regulating multiple-output DC-DC converters by using low loss components, namely, electronically controllable reactors, thereby minimizing the loss of energy. 
     Another advantage is the method of generating an arbitrarily precise output voltage in any output of a multiple-output DC-DC converter regardless of load fluctuations in any of the remaining outputs thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the prior art method of cross regulation in a multiple-output DC-DC converter; 
     FIG. 2 shows one embodiment of the present invention for regulation of a multiple-output DC-DC converter; 
     FIG. 3 shows details of an embodiment of the electronically controllable reactor shown in FIG. 2; 
     FIG. 4 shows a characteristic φ vs. ΣNi curve for the electronically controllable reactor in FIG. 3; 
     FIG. 5 shows another embodiment of the electronically controllable reactor shown in FIG. 3; and 
     FIG. 6 shows another embodiment of the present invention for regulation of a multiple-output DC-DC converter. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown a prior art energy-storage transformer 10 comprising a primary winding 12 and a plurality of secondary windings 14,16. The pulsating current in secondary winding 14 is rectified to direct current (d.c.) by diode 18 and capacitor 20. The rectified current is delivered to a load 22, shown connected across the capacitor 20. Likewise, pulsating current in secondary winding 16 is rectified by diode 24 and capacitor 26 and d.c. is delivered to load 28. In response to the sensed d.c. output voltage V 01  delivered to load 22, controller 30 varies the duty cycle of switching transistor 32, shown connected in series with the primary winding 12. 
     Transformer 10 is designed to deliver energy to loads 22 and 28 within a specified range. As the current drawn by load 22 varies within its specified range, the current delivered to load 28 varies appreciably. It is the purpose of this invention to deliver load current to load 28 within a specified range regardless of load variations in load 22. 
     Referring to FIG. 2, there is shown a transformer 40 having a primary winding 42 and a plurality of secondary windings 44,46. Diode 48 and capacitor 50 provide d.c. to a load 52 shown connected across the capacitor 50. Likewise, diode 54 and capacitor 56 provide d.c. to a load 58. The output voltage, V 03 , across load 52 is sensed by controller 60. Controller 60 adjusts the duty cycle of the switching transistor 62, connected in series with the primary winding 42. 
     There is shown connected in series with the secondary winding 44, an electronically controllable reactor 64. Likewise, secondary winding 46 is connected in series with electronically controllable reactor 66. The output voltage, V 04 , across load 58, is sensed through resistors 68 and 70. A potential drop, proportional to the output voltage, is developed across the voltage divider comprising resistors 68 and 70. The sensed voltage is compared with a reference voltage from reference 72 by comparator 74 to generate an error signal. This error signal is used for controlling the current sources 76 and 78. Current sources 76 and 78, in turn, adjust the amount of the control current flowing through leads 77 and 79 to electronically controllable reactors 66 and 64, respectively. 
     When the load current to load 58 drops, the comparator 74 operates to change the settings of the current sources 76 and 78, thereby regulating the amount of control current flowing through leads 77 and 79 to the electronically controllable reactors 66 and 64, respectively. The control current to the electronically controllable reactors 66 and 64 changes the permeance thereof. That is, the inductance of the load windings of each electronically controllable reactor of each output circuit 80 and 82 corresponding to the secondary windings 44 and 46, respectively, will be adjusted so that the load voltages will be regulated to specified values. 
     Referring more particularly to FIG. 3, there is shown one embodiment of electronically controllable reactor 64 or 66. The leads in FIG. 3 bear the same indicia corresponding to those of FIG. 2. Each of the electronically controllable reactors 64,66 comprise two toroidal, saturable cores 84 and 86, a control winding 85 and load windings 87 and 89. The control winding 85 bears the d.c. feedback or control current from current source 76 or 78 in output circuit 82 or 80. When the number of turns of winding 85 is made large, the amount of control current necessary to saturate the cores 84 and 86 will be decreased. Thus, the power required to control the load voltages will be quite small. Consequently, the efficiency of the system will be increased. 
     The secondary windings 87 and 89 each have equal number of turns. Furthermore, windings 87 and 89 are connected in such a way that the a.c. flowing therethrough induce magnetic flux in cores 84 and 86, respectively, flowing in opposite directions, shown by broken directional lines. Because of these two conditions, no net a.c. flux will link the control winding 85 and no net a.c. voltage will be induced across winding 85. 
     The magnetic flux φ (shown by solid directional lines) is induced in cores 84 and 86 by the control current in winding 85. 
     Referring to FIG. 4 there is shown by the solid curve a typical characteristic φ vs. ΣNi curve (i.e., flux versus ampere-turns) for each core of the electronically controllable reactors 64,66. The slope of the φ vs. ΣNi curve at any point thereon is a measure of the permeance of the cores 84,86. As stated hereinabove, the d.c. level of magnetic flux φ induced in core 84 is related to the d.c. control current flowing in winding 85. This relationship is shown by the φ vs. ΣNi curve of FIG. 4. For small a.c. excursions about this d.c. operating point, the control current determines the permeance. This phenomenon can be better understood by referring to point 90 on the φ vs. ΣNi curve. Small increases and decreases of the aforesaid magnetic flux occur simultaneously in small excursions about point 90 along the tangent to the φ vs. ΣNi curve thereat. It is well-known that this tangent is the slope of the curve at point 90, thereby indicating the permeance. 
     Referring to FIG. 5, there is shown an alternative embodiment to the electronically controllable reactor shown earlier in FIG. 3. The windings on the reactor in FIG. 5 bear the same indicia as those in FIG. 3. Furthermore, the embodiment shown in FIG. 5 operates in substantially the same manner as that shown in FIG. 3. 
     Referring to FIG. 6, there is shown another embodiment of the present invention. Transformer 100 has primary winding 102 and secondary windings 104,106. Output circuit 110 has connected to secondary winding 104, a diode 108, electronically controllable reactor 112, capacitors 114,116 and a load 118. The output voltage of output circuit 110 is sensed by lead 119 and conveyed to controller 120 for varying the duty cycle of switching transistor 122. The load current is fed back to control winding 124 to vary the permeance of electronically controllable reactor 112. Capacitor 116 insures that any alternating currents induced in control winding 124 will be filtered. 
     Likewise, output circuit 130 comprises diode 126, electronically controllable reactor 128, capacitor 132 and load 134. The output voltage is sensed by voltage divider 136,138 and compared with a reference voltage at comparator 140 to produce an error signal. This error signal is used to adjust the setting of current source 142, thereby regulating the amount of control current fed back to control winding 144. 
     When there is a fluctuation in load 118, because of the magnetic coupling between secondary windings 104 and 106, there will be a change in the load current delivered to load 134. This change will be detected by comparator 140. In response thereto, the current source 142 will be adjusted to regulate the feedback current to control winding 144 for changing the permeance of electronically controllable reactor 128. In turn, the load current for load 134 flowing through the load windings of electronically controllable reactor 128 will be adjusted thereby compensating for fluctuations in load 118.