Abstract:
A current sharing control system of a power supply, wherein a plurality of switching power supplies are connected in parallel to feed DC power to an external load. Each switching power supply comprises an output current sensing circuit for obtaining a signal corresponding to the output current of the power supply; an ideal diode circuit comprising an anode coupled to receive a signal corresponding to the output current and a cathode coupled to receive a signal corresponding to the maximum output current of the plurality of parallel connected switching power supplies; an error amplifier for outputting an error signal representing difference between the anode and cathode potentials of the ideal diode circuit; and an output voltage regulator for adjusting the output voltage thereof to cancel the error signal. Advantageously, the system enables the output currents of the plurality of parallel connected power supplies to be maintained at the same level, thereby preventing variations in output voltage when any one or more of the switching power supplies fail, or are connected or disconnected while the power line is active. Also, advantageously, the system enables control of variations of current to be within a certain range when short circuit or open circuit of a parallel operation control signal of the system occurs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a current sharing control system of a power supply, wherein a plurality of switching power supplies are connected in parallel to feed DC power to an external load. 
     2. Description of the Prior Art 
     A conventional current sharing control system of a power supply using a plurality of switching power supplies to supply DC power to an external load is shown in FIG. 1, wherein a convention switching power supply  10  is used as the power supply system and comprises an output current sensing circuit  1  for outputting an output current sensing signal, corresponding to the circuit&#39;s output current. The sensing circuit  1  is connected to the negative input terminal of an error amplifier U 2  through a resistor R 12 . Also, an output voltage sensing circuit  2 , which outputs an output voltage sensing signal, corresponding to the circuit output voltage, is connected to the negative input terminal of an error amplifier U 1 . 
     The positive input terminal of the error amplifier U 1  is connected to a voltage reference Vrl and the amplifier output terminal is connected to the anode of diode D. The cathode of diode D is connected to both the negative input terminal of error amplifier U 2 , through a resistor R 11 , and a parallel operation control signal terminal CT. The positive input terminal of error amplifier U 2  is connected to a reference voltage Vr 2  and the output terminal thereof is connected to a switching regulator  3 . 
     The switching regulator  3  is a DC power supply circuit, whose output current is controlled by the output of error amplifier U 2 . The switching regulator  3  comprises a pulse width modulation circuit  3   a , that modulates the output of error amplifier U 2  by pulse width, and a switching converter  3   b , that inputs the output of pulse width modulation circuit  3   a  to a switching device. The output of switching converter  3   b  is connected to output terminals OUT+ and OUT−. The output terminals CT, OUT+ and OUT− of the plurality of switching power supplies  10 , 10   a , and  10   b , as described above, are connected in parallel as depicted in FIG. 2, in order to supply power to a load  50 . The sum of the output currents of the switching power supplies  10 ,  10   a  and  10   b  is thus supplied to load  50 . Although FIG. 2 shows the case of three switching power supplies  10 , 10   a , 10   b  connected in parallel, it is possible to increase or decrease the number of power supplies according to the current capacity required by load  50 . 
     In each switching power supply, connected as shown in FIG. 2, a voltage signal, which corresponds to the power supply output current, detected by the output current sensing circuit  1  of the power supply (called output current sensing signal Vc) is compared with the voltage of the voltage reference Vr 1  by means of the error amplifier U 1  (see FIG.  1 ). The resulting comparison signal is outputted to parallel operation control signal terminal CT through diode D. 
     Since the parallel operation control signal terminals CT of the plurality of switching power supplies  10 , 10   a ,  10   b  are connected in parallel, the highest level of signals outputted from the terminals CT of the parallel connected switching power supplies (called parallel operation control signals Vp) is supplied to the resistor R 11  of each switching power supply by the operation of diode D. The switching power supply that outputs the highest level parallel operation control signal Vp serves as a master switching power supply for controlling the other switching power supplies. 
     For each parallel connected switching power supply, a parallel operation control signal Vp outputted by the master switching power supply is added to the output current sensing signal Vc of the parallel connected switching power supply by means of resistors R 11  and R 12 , and error amplifier U 2 . The resulting addition signal is compared with the reference voltage Vr 2 , and the resulting comparison output signal is fed to switching regulator  3  through error amplifier U 2 . 
     The switching regulator  3  of each switching power supply controls the power supply output current according to the comparison output signal supplied by error amplifier U 2 . Thus, in the current sharing control system of the power supply,described above, it is possible to control the output currents of the parallel connected plurality of switching power supplies , to substantially the same level by setting the power supplies reference voltage Vr 2  to the same value. 
     As discussed, in the conventional current sharing control system of a power supply, it is possible to control the output currents of a plurality of parallel connected switching power supplies to the same level with accuracy. However, the conventional systems have a problem, namely, that if a switching power supply, serving as the master switching power supply, fails or is connected or disconnected while the power line is active, the output voltage variation becomes unacceptably large for a short period of time during which another switching power supply takes the place of the master switching power supply. 
     Furthermore, disadvantageously, in the conventional system, the output voltage of the switching power supply, other than the master switching power supply, is not directly controlled. Instead, the concerned power supply is controlled in such a manner that the sum of the parallel operation control signal and the power supply output current is kept constant. This results in a problem, namely, that the output voltage variation becomes unacceptably large when an output voltage sensing circuit, such as a current transformer, which involves a certain delay of detection, is used. 
     SUMMARY OF THE INVENTION. 
     Accordingly, an object of the invention is to overcome the aforementioned and other disadvantages and deficiencies of the prior art. 
     Another object is to provide a current sharing control system of a power supply that enables the output currents of a plurality of parallel connected switching power supplies to be maintained at the same level, thereby preventing output voltage variations in the power supplies from occurring in the event any one or more switching power supplies fail, or are connected, or disconnected while the power line is still active. 
     A further object is to provide such system which enables control of the output voltage variations to be within a certain range when short circuiting or open circuiting occurs in a parallel operation control signal used in such system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram depicting a conventional switching power supply. 
     FIG. 2 is a block diagram depicting a conventional current sharing control system of a power supply. 
     FIG. 3 is a block diagram depicting an illustrative embodiment of a switching power supply of the invention. 
     FIG. 4 is a schematic diagram depicting characteristics of an ideal diode circuit of the invention. 
     FIGS. 5A and 5B are schematic diagrams depicting an ideal diode circuit of the invention. 
     FIG. 6 is a block diagram depicting a Zener diode of the invention. 
     FIG. 7 is a block diagram depicting a low pass filter of the invention. 
     FIG. 8 is a block diagram depicting an output voltage sensing circuit of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows an illustrative embodiment of a switching power supply used for a current sharing control system of a power supply, wherein components that are the same as those in FIG. 1 are similarly labelled and are not discussed here at for sake of clarity. In FIG. 3, an output terminal of the output current sensing circuit  1 , of power supply  20  is connected to the anode of an ideal diode circuit D 1 . The cathode of diode D 1  is connected to a parallel operation control signal terminal CT. The anode of diode D 1  is connected to the negative input terminal of an error amplifier U 2  through a resistor p 27 . The cathode of diode circuit D 1  is connected to voltage dividing resistors R 25  and R 26 . The common junction between the resistors R 25  and R 26  is connected to the positive terminal of error amplifier U 2 . 
     The ideal diode circuit refers to a diode that has the characteristics of a standard ordinary diode even when the range of applied forward voltage is as narrow as from 0 to 0.5 volts. On the other hand, the standard ordinary diode does is not turned ON, that is pass current , unless a voltage of approximately 0.6 volts is applied in the forward direction FIG. 4 shows characteristics of an ideal diode circuit D 1 . As is evident from the characteristic curve  200 , the ideal diode circuit is characteristically equivalent to a switching circuit that is turned ON when voltage is applied in the forward direction and is turned OFF when voltage is applied in the reverse direction. 
     An ideal diode circuit D 1  can be realized, for example, by the circuit shown in FIG. 5A, wherein an input terminal  41  is connected to the positive input terminal of error amplifier U 4 . The output terminal of error amplifier U 4  is connected to an output terminal  43  through diode  42 . The common junction between the cathode of diode  42  and the output terminal  43  is connected to the negative input terminal of error amplifier U 4 . In such an ideal diode circuit, the input terminal  41  serves as an anode and the output terminal  43  serves as a cathode. 
     In FIG. 3, the output terminal of error amplifier U 2  is connected to both low pass filter  4  and the negative input terminal of error amplifier U 2  through feed back resistor R 28 . The low pass filter  4  can be realized by feeding back the output signal from error amplifier U 2  through a capacitor C 1 , for example, as shown in FIG.  5 B. Alternatively, the low pass filter  4  can be realized, for example, by connecting the positive input terminal of error amplifier U 1  to a common potential through a capacitor C 3 , such as shown in FIG.  7 . 
     In FIG. 3, the output terminal of the output voltage sensing circuit  2  is connected to the negative input terminal of error amplifier U 1  through resistor R 21 . The output terminal of low pass filter  4  is connected to the positive input terminal of error amplifier U 1  through resistor R 23 . The common junction “b” between the negative input terminal of error amplifier U 1  and resistor R 21  is connected to the anode of the ideal diode circuit D 1  , at the point indicated by “a” in FIG. 3, through resistor R 22  and Zener diode Dz which are connected in series. The common junction between the positive input terminal of the error amplifier U 1  and the resistor R 23  is connected to the common potential through a resistor  24  and a reference voltage Vr 1  connected in series. In this specification, reference is often made to a connection to a signal. It is to be understood that this means that a connection is made to a means which carries such signal. The output terminal of error amplifier U 1  is connected to the switching regulator  3  in the same way as a conventional system. The output terminal,CT, OUT− and OUT+ of the plurality of switching power supplies  20  are connected in parallel, as shown in FIG. 2, in order to feed DC power to a load  50 . Hence, the sum of output currents provided by the switching power supplies  10 ,  10   a  and  10   b  is applied to the load  50 . If a plurality of switching power supplies  20  are connected parallely, as shown in FIG. 2, an output current sensing signal Vc obtained by an output current sensing circuit  1  is supplied to a parallel operation control signal terminal CT through the ideal diode circuit D 1  as a parallel operation control signal Vp(see FIG.  3 ). Since the parallel operation control signal terminals CT of all of the plurality of switching power supplies are connected in parallel,the highest level of parallel operation control signals Vp provided by the plurality of parallel connected switching power supplies is applied to resistor R 25 . The particular switching power supply that outputs the highest level parallel operation control signal Vp serves as the master switching power supply for controlling the output signal of the other switching power supplies. 
     Each of the plurality of parallel connected switching power supplies compares a parallel operation control signal Vp outputted by the master switching power supply with its own output current signal Vc by means of error amplifier U 2 . The switching power supply then feeds the resulting comparison output signal to low pass filter  4 . The output of low pass filter  4  is added to the reference voltage Vr 1  borough resistors R 23  and R 24 . Then, the resulting addition signal is supplied to the positive input terminal of error amplifier U 1 . Since an output voltage sensing signal Vs, corresponding to the output voltage of error amplifier U 1 , is supplied to the negative input terminal of error amplifier U 1 , the switching power supply controls the switching regulator  3  so as to increase its own output current when the output current is smaller than that of the master switching power supply. Conversely, the switching power supply controls the switching regulator  3  so as to decrease its own output current when the output current is larger than that of the master switching power supply. 
     A The output currents of the switching power supplies are controlled so that they are kept equal over a certain range defined by the degree of amplification of error amplifier U 2  and resistors R 23 , and R 24 . The time required for the output current of each switching power supply to reach the same level, that is the settling time, is dependent on the time constant of the low pass filter  4 . If the time constant is set to a value far greater than the time constant for controlling the output voltage, which is usually determined by connecting a CR circuit across the input and output terminals of the error amplifier U 1  for phase compensation, by a factor of 100 or more, for example, the tracking of a sudden change in the load is started only by the system of controlling the output voltage of each switching power supply. This method makes it possible to minimize output voltage variations. Although it is possible to minimize the output voltage variation by the tracking of the sudden change in the load, this tracking is not limited to the foregoing method alone. The ratio of output current shared by each switching power supply depends solely on the dynamic impedance thereof. Hence, if the dynamic impedance is not equal for the plurality of switching power supplies, such as for reasons of variations during manufacture, a switching power supply having the lowest dynamic impedance will provide a current corresponding to the sudden change in load. This will result in a remarkable increase in the electrical stress of that switching power supply. This problem can be prevented by using a Zener diode Dz and resistors R 21 ,R 22  that form an addition resistor network. 
     An output current sensing signal Vc corresponding to the output current is supplied to the junction “a” of the cathode of the Zener diode Dz. When the voltage level of the output current sensing signal Vc exceeds the Zener voltage of the Zener diode Dz, the extra voltage level is divided in a ratio determined by the resistors R 21  and R 22 . Then, the resulting voltage level is supplied to the negative input terminal of error amplifier U 1 . Consequently, the switching regulator  3  is operated so as to decrease its own output current. More specifically, when the output current exceeds a certain current level defined by the Zener voltage of Zener diode Dz, the switching power supply output current is decreased by means of Zener diode Dz and resistors R 21 , R 22 , forming an addition resistor network, according to a ratio determined by the resistors R 21  and R 22 , against a change in excess of that current level. This makes it possible to prevent any excess current from being supplied to the switching power supply. 
     Assume that the output voltage Vd of the low pass filter  4 , or error amplifier U 2 , is within the range of from 0 to 1 volt. Then, the input voltage, which is the reference voltage level of the output voltage, of the error amplifier U 1 , when the output voltage is 0 volt, is: 
     
       
         R 23 /(R 23 +R 24 )×Vr 1   (1) 
       
     
     When, the output voltage Vd is 1 volt, the input voltage of the error amplifier U 1 , is: 
     
       
         R 23 /(R 23 +R 24 )×Vr 1 +R 24 /(R 23 +R 24 )×Vr 1   (2) 
       
     
     This means that it is possible to freely determine the variable range of the output voltage of each switching power supply by suitably selecting the values of resistors R 22  and R 24 . 
     Hence, the output voltage of each switching power supply never exceeds the variable output voltage range determined by the resistors R 23  and R 24 , whether the parallel operation control signal Vp is open circuited, or short circuited. Also, it is possible to prevent excess current from being supplied to any particular switching power supply by the effect of the Zener diode Dz and the addition resistor network R 23 , 24 . This further makes it possible to minimize the output voltage variation in the power supply system. 
     In the current sharing control system of a power supply described above, if, for example, any concurrently running switching power supply fails due, for example, to short circuiting, an electric current flows into that particular switching power supply from the other normal switching power supplies, thereby causing a drop in the output voltage of the power supply system. Furthermore, for example, if one of the concurrently switching power supplies is in an “OFF” state, the output voltage of each of the other active switching power supplies is applied across the output terminals OUT+ and OUT− of the inactive switching power supply. Hence, the output voltage sensing circuit detects the output voltage, and an output voltage sensing signal Vs corresponding to the output voltage is at all times applied to the negative input terminal of error amplifier U 1 . Accordingly, the output voltage of the inactive switching power supply may overshoot or undershoot, when the power supply is turned “ON”, for example. The foregoing phenomenon can be avoided, for example, by adding a protection circuit to the output voltage sensing circuit of each of the plurality of parallel connected switching power supply. 
     FIG. 8 shows an example of an output voltage sensing circuit that makes it possible to avoid the foregoing phenomenon and can be used for the switching power supplies of the current sharing control system of a power supply. FIG. 8 is a partial view showing the output block of the switching power supply shown in FIG.  3 . Hence, components identical to that shown in FIG. 3 are provided the same reference symbols and will be omitted from discussion here at for sake of clarity. 
     In FIG. 8, the positive output terminal of a switching converter  3   b  is connected to an output terminal OUT+ through a diode D 31 . The negative output terminal of switching converter  3   b  is connected to an output terminal OUT−. The anode of diode D 31  and the output terminal OUT− are connected via resistor R 34 . The cathode of diode D 31  and the output terminal OUT− are connected through voltage dividing resistors R 31 ,R 32 , and R 33 . The common junction between the voltage dividing resistors R 31  and R 32  and the anode of diode D 31  are connected through diode D 32 . The potential at the common junction between the voltage dividing resistors R 32  and R 33  is outputted as an output voltage sensing signal Vs. 
     In the output voltage sensing circuit, the diode D 31  blocks a reverse current flowing from output terminal OUT+, when, for example, the switching power supply fails due to short circuiting. Furthermore, diode D 32  and resistor R 34  operate so as to lower the level of the output voltage sensing signal Vs when, for example, the switching power supply is in an OFF state. Hence, it is possible to avoid the above described phenomenon. 
     In the description above, only specific preferred embodiments are provided for the purpose of describing the invention and showing examples of carrying out the invention. The above embodiments are thus to be considered as illustrative and not restrictive. The invention may be embodied in other ways without departing from the spirit and essential character thereof. Accordingly it is to be understood that all modifications and extensions falling within the spirit and scope of the invention are covered by the claims appended hereto. 
     For example, the Zener diode Dz may be realized by using a circuit enclosed by a dotted line and indicated by numeral  60  in FIG.  6 . In circuit  60 , the positive input terminal of error amplifier U 3  is connected to a connection point “a” through a resistor R 10 . The negative input terminal of error amplifier U 3  is connected to a reference voltage Vr 3 . The output terminal of error amplifier U 3  is connected to both resistor R 22  and to the positive input terminal thereof through a feedback resistor R 9  and capacitor C 2 . By using circuit  60  in place of Zener diode Dz, it is possible to precisely set a voltage level corresponding to a certain current level determined by the Zener voltage of Zener diode Dz. In circuit  60 , such a voltage level appropriate to the Zener voltage is set in reference voltage Vr 3 . 
     As discussed, the invention enjoys the following and other advantages: The current sharing control system of a power supply of the invention enables the output voltage variation to be minimized against sudden changes in the load. Hence, it is possible to prevent the output voltage variations that may occur when any single switching power supply fails or is connected or disconnected, while the power line is active. It is also possible with the invention to control the output voltage variation to within a certain range against the open circuiting or short circuiting of the system parallel operation control signal which is a common signal.