Current sharing control system of power supply and output voltage sensing circuit

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.

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's output current. The sensing circuit 1 is connected to the
 negative input terminal of an error amplifier U2 through a resistor R12.
 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 U1.
 The positive input terminal of the error amplifier U1 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 U2, through a resistor R11, and
 a parallel operation control signal terminal CT. The positive input
 terminal of error amplifier U2 is connected to a reference voltage Vr2 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 U2. The switching
 regulator 3 comprises a pulse width modulation circuit 3a, that modulates
 the output of error amplifier U2 by pulse width, and a switching converter
 3b, that inputs the output of pulse width modulation circuit 3a to a
 switching device. The output of switching converter 3b is connected to
 output terminals OUT+ and OUT-. The output terminals CT, OUT+ and OUT- of
 the plurality of switching power supplies 10,10a, and 10b, 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, 10a and 10b is thus supplied to load 50. Although FIG. 2
 shows the case of three switching power supplies 10,10a,10b 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 Vr1 by means of the error amplifier U1 (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 10,10a, 10b 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 R11 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 R11 and R12, and error
 amplifier U2. The resulting addition signal is compared with the reference
 voltage Vr2, and the resulting comparison output signal is fed to
 switching regulator 3 through error amplifier U2.
 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 U2. 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 Vr2 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.

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 D1. The cathode of diode D1 is
 connected to a parallel operation control signal terminal CT. The anode of
 diode D1 is connected to the negative input terminal of an error amplifier
 U2 through a resistor p27. The cathode of diode circuit D1 is connected to
 voltage dividing resistors R25 and R26. The common junction between the
 resistors R25 and R26 is connected to the positive terminal of error
 amplifier U2.
 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 D1. 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 D1 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 U4. The output terminal of
 error amplifier U4 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
 U4. 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 U2 is connected to both
 low pass filter 4 and the negative input terminal of error amplifier U2
 through feed back resistor R28. The low pass filter 4 can be realized by
 feeding back the output signal from error amplifier U2 through a capacitor
 C1, for example, as shown in FIG. 5B. Alternatively, the low pass filter 4
 can be realized, for example, by connecting the positive input terminal of
 error amplifier U1 to a common potential through a capacitor C3, 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 U1 through
 resistor R21. The output terminal of low pass filter 4 is connected to the
 positive input terminal of error amplifier U1 through resistor R23. The
 common junction "b" between the negative input terminal of error amplifier
 U1 and resistor R21 is connected to the anode of the ideal diode circuit
 D1 , at the point indicated by "a" in FIG. 3, through resistor R22 and
 Zener diode Dz which are connected in series. The common junction between
 the positive input terminal of the error amplifier U1 and the resistor R23
 is connected to the common potential through a resistor 24 and a reference
 voltage Vr1 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 U1 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, 10a and 10b 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 D1 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 R25. 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 U2. 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 Vr1 borough resistors R23 and
 R24. Then, the resulting addition signal is supplied to the positive input
 terminal of error amplifier U1. Since an output voltage sensing signal Vs,
 corresponding to the output voltage of error amplifier U1, is supplied to
 the negative input terminal of error amplifier U1, 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 U2 and resistors R23, and R24. 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 U1 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 R21,R22 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 R21 and R22. Then, the resulting
 voltage level is supplied to the negative input terminal of error
 amplifier U1. 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 R21, R22, forming an addition
 resistor network, according to a ratio determined by the resistors R21 and
 R22, 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 U2, 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 U1, when the output voltage is 0 volt, is:
EQU R23/(R23+R24).times.Vr1 (1)
 When, the output voltage Vd is 1 volt, the input voltage of the error
 amplifier U1, is:
EQU R23/(R23+R24).times.Vr1+R24/(R23+R24).times.Vr1 (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 R22 and R24.
 Hence, the output voltage of each switching power supply never exceeds the
 variable output voltage range determined by the resistors R23 and R24,
 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 R23,24. 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 U1.
 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 3b is
 connected to an output terminal OUT+ through a diode D31. The negative
 output terminal of switching converter 3b is connected to an output
 terminal OUT-. The anode of diode D31 and the output terminal OUT- are
 connected via resistor R34. The cathode of diode D31 and the output
 terminal OUT- are connected through voltage dividing resistors R31,R32,
 and R33. The common junction between the voltage dividing resistors R31
 and R32 and the anode of diode D31 are connected through diode D32. The
 potential at the common junction between the voltage dividing resistors
 R32 and R33 is outputted as an output voltage sensing signal Vs.
 In the output voltage sensing circuit, the diode D31 blocks a reverse
 current flowing from output terminal OUT+, when, for example, the
 switching power supply fails due to short circuiting. Furthermore, diode
 D32 and resistor R34 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 U3 is connected to a connection
 point "a" through a resistor R10. The negative input terminal of error
 amplifier U3 is connected to a reference voltage Vr3. The output terminal
 of error amplifier U3 is connected to both resistor R22 and to the
 positive input terminal thereof through a feedback resistor R9 and
 capacitor C2. 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 Vr3.
 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.