Phase control device for DC/DC converter

A phase control device for use with a voltage-boosting DC/DC converter includes a current sensor for detecting a current flowing through at least one of the primary and secondary windings of a transformer of the DC/DC converter, a ripple determining section for, on the basis of a current signal detected by the current sensor, determining an amount of current ripple occurring when a first switching element or second switching element switches to an ON state, and a phase regulating section for, on the basis of the amount of current ripple determined by the ripple determining section, regulating a phase related to an ON time of each of the first and second switching elements such that the amount of current ripple is reduced to zero.

FIELD OF THE INVENTION

The present invention relates to a phase control device for a DC/DC (digital-to-digital) converter particularly suitable for use in a power supply of an electric vehicle, and also to a program embodied in a computer readable medium for use in phase control of a DC/DC converter.

BACKGROUND OF THE INVENTION

Various types of voltage-boosting DC/DC converters are known as disclosed, for example, in Japanese Patent Laid-Open Publications (JP-A) Nos. 2003-111390, 2003-216255 and 2006-149054.

As shown inFIG. 13hereof, the voltage-boosting DC/DC converter disclosed in JP 2006-149054 A generally comprises an input-side smoothing capacitor C1, an inductor L0, a primary winding L1, a secondary winding L2, four switching elements SW1, SW2, SW3and SW4, and an output-side smoothing capacitor C2.

The input-side smoothing capacitor C1is connected between a common reference terminal11and an input terminal12, while the output-side smoothing capacitor C2is connected between the common reference terminal11and an output terminal13.

The primary winding L1and the secondary winding L2form an essential part of a transformer T1. The transformer T1includes a single core (ferrite core, iron core or the like) F1on which the primary winding L1and the secondary winding L2are wound with opposite winding directions and connected together in an oppositely-wound configuration. The winding ratio between the primary winding L1and the secondary winding L2is preferably 1:1.

The switching elements SW1to SW4are each in the form of, for example, an IGBT (Insulated Gate Bipolar Transistor) capable of conducting a high current and withstanding a high voltage. Each of the switching elements SW1to SW4has a collector, emitter and gate. Further, a diode D3is connected in parallel between the collector and emitter of each of the switching elements SW1-SW4in a forward direction from the emitter toward the collector.

The inductor L0is connected at one end to the input terminal12, which forms an upper terminal of the input-side smoothing capacitor C1. The other end of the inductor L0is connected to a common terminal “c” of the primary and secondary windings L1and L2of the transformer T1. Two T-match circuits are connected in parallel between the other end of the inductor L0and the output terminal13. The parallel T-match circuits comprise a first T-match circuit including the primary winding L1of the transformer T1and switching elements SW1and SW3, and a second T-match circuit including the secondary winding L2of the transformer T1and switching elements SW2and SW4.

In the first T-match circuit, a point between the collector and emitter of the switching element SW1is connected between a terminal “a” of the primary winding L1and the common reference terminal11, and a point between the collector and emitter of the switching element SW3is connected between the terminal “a” and the output terminal13. Further, in the second T-match circuit, a point between the collector and emitter of the switching element SW2is connected between a terminal “b” of the secondary winding L2and the common reference terminal11, and a point between the collector and emitter of the switching element SW4is connected between the terminal “b” and the output terminal13. Gate signals SG1and SG2for controlling ON/OFF action of the two switching elements SW1and SW2are supplied from a control device or controller (not shown) to the respective gates G1and G2of the switching elements SW1and SW2. Similarly, gate signals for controlling ON/OFF action of the remaining switching elements SW3and SW4are also supplied from the non-illustrated controller to the respective gates of the switching elements SW3and SW4. In the circuit configuration shown inFIG. 13, however, the switching elements SW3and SW4are kept in an OFF state. In this instance, when current flows from the terminal “a” or the terminal “b” toward the output terminal13, the diode D3of the corresponding switching element SW3or SW4allows the current to flow therethrough to the output terminal13.

FIG. 14collectively shows the ON/OFF action of the switching elements SW1and SW2occurring in response to the gate signals SG1and SG2applied respectively thereto, waveforms of currents I1and I2flowing through the primary and secondary windings L1and L2, respectively, according to the ON/OFF action of the switching elements SW1and SW2, and the waveform of an ideal current I3flowing through the primary and secondary windings L1and L2.

In the voltage-boosting DC/DC converter10shown inFIG. 13, when the switching element SW1is turned on, an exciting current I1flows through the primary winding L1of the transformer T1. As the exciting current I1flows through the primary winding L1, an excited current (induced current) I2is produced in the secondary winding L2on the basis of the mutual induction action. Alternatively, when the switching element SW2is turned on, an exciting current I2flows through the secondary winding L2of the transformer T1. As the exciting current I2flows through the secondary winding L2, an excited current (induced current) is produced in the primary winding L1on the basis of the mutual induction.

The two switching elements SW1and SW2are designed to perform switching operation such that, as shown inFIG. 14, the timing of switching action of one switching element occurring in response to one of the two gate signals of different phases is the same as the timing of switching action of another switching element. The switching actions of the switching elements SW1and SW2have the same time period A, B corresponding to unit waveforms of the currents I1and I2flowing through the primary and secondary windings L1and L2. The switching elements SW1and SW2have the same ON time C, D.

In the conventional DC/DC converter10, the current flowing through the primary winding L1of the transformer T1and the current flowing through the secondary winding L2ideally have a waveform (ideal current waveform) I3, which is continuous in regions15occurring repeatedly at switching of mutual energization of the primary and secondary windings L1and L2. In practice, however, due to a difference in inductance of the primary and secondary windings L1and L2, or a difference in ON/OFF characteristic of the switching elements SW1and SW2, switching of mutual energization of the primary and secondary windings L1and L2produces a current difference, which creates an abrupt change (or stepped portion) in each of regions16of the waveforms of the currents I1and I2. The region15in the ideal current waveform I3and the region16of the waveforms of actual currents I1and I2are shown on enlarged scale inFIGS. 15A and 15B, respectively.

The abrupt change (stepped portion)16occurring in the regions16of the waveforms of the currents I1and I2increases current ripple in the transformer T1, which may sometimes be 5 or more times as large as the ideal current waveform I3. With this increase in the current ripple, iron loss of the transformer T1increases, resulting in undue temperature rise and efficiency reduction of the transformer T1. In some cases, the transformer T1undergoes magnetic saturation. At the stepped portion16a(FIG. 15B) of the waveforms of the currents I1and I2, a harmonic component is involved, which causes the transformer T1to generate unpleasant vibration noises. Further, an increased current ripple gives a negative influence on currents flowing through the switching elements SW1and SW2so that the diodes D3associated with the switching elements SW1to SW4are subjected to an increased peak current. This may require use of switching elements of higher capacity capable of providing an increased current rating (withstanding current).

With the foregoing drawbacks in view, the present invention seeks to provide a phase control device and a phase control program, which are capable of suppressing creation of an unnecessary abrupt change (stepped portion) in waveforms of currents flowing in the primary and secondary windings of a transformer of a DC/DC converter.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a phase control device for use with a voltage-boosting DC/DC converter including a low-voltage-side port and a high-voltage-side port, the DC/DC converter further including a transformer of a magnetic-field cancellation type having a primary winding and secondary winding interconnected in an oppositely-wound configuration with a common terminal of the primary winding and secondary winding being connected to a positive-pole terminal of the low-voltage-side port, first switching means for controlling an energizing current of the primary winding which flows to a common reference terminal, and second switching means for controlling an energizing current of the secondary winding which flows to the common reference terminal, the phase control device comprising: current detecting means for detecting a current flowing through at least one of the primary and secondary windings; determining means for, on the basis of a current signal detected by the current detecting means, determining an amount of current ripple occurring when the first switching means or the second switching means switches to an ON state; and phase regulating means for, on the basis of the amount of current ripple determined by the determining means, regulating a phase related to an ON time of each of the first and second switching means such that the amount of current ripple is reduced to zero.

By thus regulating the phase related to the ON time of each switching means, it is possible to suppress generation of waveform discontinuity of the currents flowing through the primary and secondary windings of the transformer. Since the currents flowing through the primary and secondary windings are substantially free from ripple, the transformer can operate silently without generating unpleasant vibration noise. The ripple-free currents can obviate the need for switching elements of higher capacities.

According to a second aspect of the present invention, there is provided a program embodied in a computer readable medium for use in phase control of a voltage-boosting DC/DC converter including a low-voltage-side port and a high-voltage-side port, the DC/DC converter further including a transformer of a magnetic-field cancellation type having a primary winding and secondary winding interconnected in an oppositely-wound configuration with a common terminal of the primary winding and secondary winding being connected to a positive-pole terminal of the low-voltage-side port, first switching means for controlling an energizing current of the primary winding which flows to a common reference terminal, second switching means for controlling an energizing current of the secondary winding which flows to the common reference terminal, the program comprising, and current detecting means for detecting a current flowing through at least one of the primary and secondary windings, the program comprising: a determining function to, on the basis of a current signal detected by the current detecting means, determine an amount of current ripple occurring when the first switching means or the second switching means switches to an ON state; and a phase regulating function to, on the basis of the amount of current ripple determined by the determining function, regulating a phase related to an ON time of each of the first and second switching means such that the amount of current ripple is reduced to zero.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIGS. 1 to 4, a description will be made as to an embodiment of a phase control device for a DC/DC converter according to the present invention. The DC/DC converter10shown inFIG. 1can be used as a voltage-boosting DC/DC converter (electric power converter) embodied in an electric system of an electric vehicle (not shown). The phase control device or controller30shown inFIG. 1is designed to control switching elements SW1and SW2in an appropriate manner so as to suppress ripple current in the circuitry. In terms of fundamental configuration, the DC/DC converter10shown inFIG. 1is substantially the same as the conventional DC/DC converter shown inFIG. 13.

InFIG. 1, the DC/DC converter10is shown as dual-port circuitry (four-terminal circuitry). In the case where the DC/DC converter10should operate as a voltage-boosting DC/DC converter, the left-side port functions as a low-voltage-side input port, while the right-side port functions as a high-voltage-side output port.

Here, the general configuration of the DC/DC converter10will be described again. The DC/DC converter10comprises an input-side smoothing capacitor C1, an inductor L0, a transformer T1, switching elements SW1and SW2, and an output-side smoothing capacitor C2. The inductor L0may be omitted.

The input-side smoothing capacitor C1is connected between a common reference terminal (earth or ground terminal)11and an input terminal12, while the output-side smoothing capacitor C2is connected between the common reference terminal11and an output terminal13. The transformer T1is formed by a core F1, a primary winding L1and a secondary winding L2. The primary winding L1and the secondary winding L2are wound with opposite winding directions and connected together in an oppositely-wound configuration. The winding ratio between the primary winding L1and the secondary winding L2is preferably 1:1.

In the DC/DC converter10, the primary and secondary windings L1and L2that are wound on the core F1of the transformer T1are so arranged as to form a magnetic-field cancellation structure in which magnetic fluxes produced by the primary and secondary windings L1and L2cancel each other. With this magnetic-field cancellation structure, the core F1of the transformer T1can be prevented from magnetically saturating easily and does not have to provide a gap. This allows use of a small-sized core, which will contribute to reduction in size and weight of the transformer T1. Furthermore, the inductor L0is provided to ensure boosting of the input voltage to a desired level within a range of one to two times the input voltage and to realize a function of continuous variability rather than a high voltage-boosting function. Accordingly, as compared to a circuit arrangement so configured as to perform a voltage-boosting operation only by means of an inductor L0without using a transformer T1, the DC/DC converter10of the present invention allows for a small-sized inductor used as the inductor L0.

Each of the switching elements SW1and SW2has a collector, emitter and gate. Further, a diode D3is connected in parallel between the collector and emitter of each of the switching elements SW1and SW2in a forward direction from the emitter toward the collector.

The inductor L0is connected at one end to the input terminal12, which forms an upper terminal of the input-side smoothing capacitor C1. The other end of the inductor L0is connected to a common terminal “c” of the primary and secondary windings L1and L2of the transformer T1. Two T-match circuits are connected in parallel between the other end of the inductor L0and the output terminal13. The parallel T-match circuits comprise a first T-match circuit including the primary winding L1of the transformer T1, the switching element SW1and a diode D1, and a second T-match circuit including the secondary winding L2of the transformer T1, the switching element SW2and a diode D2.

In the first T-match circuit, a point between the collector and emitter of the switching element SW1is connected between a terminal “a” of the primary winding L1and the common reference terminal11, and the diode D1is connected between the terminal “a” and the output terminal13. Further, in the second T-match circuit, a point between the collector and emitter of the switching element SW2is connected between a terminal “b” of the secondary winding L2and the common reference terminal11, and the diode D2is connected between the terminal “b” and the output terminal13. Gate signals SG1and SG2for controlling ON/OFF action of the switching elements SW1and SW2are supplied from the phase control device or controller30to the respective gates G1and G2of the switching elements SW1and SW2.

A power supply21is connected to the common reference terminal11and the input terminal12, and a load22is connected to the common reference terminal11and the output terminal13.

The phase control device or controller30is provided to perform phase control of an output current of the DC/DC converter10. To this end, the phase controller30supplies gate signals SG1and SG″ to the switching elements SW1and SW2. In response to the gate signals SG1, SG2, the phase control is performed on the output current from the transformer T1. When the present invention is embodied in an electric system of an electric vehicle, the phase controller30shown inFIG. 1may be incorporated in an electronic control unit (ECU). The phase controller30receives state-detection signals from various component parts contained in the electric system and supplies control signals to the same component parts. For instance, the phase controller30receives from a main battery signals related to battery information (voltage, current, temperature of the main battery) and supplies control signal (gate signals SG1and SG2) to the DC/DC converter.

As shown inFIG. 1, two current sensors (current detecting means)31and32are associated with the primary winding L1and the secondary winding L2, respectively, of the transformer T1. The current sensor31is disposed between the primary winding L1and the diode D1, while the current sensor32is disposed between the second winding L2and the diode D2. Detection signals SG3, SG4from the current sensors31,32are input into the phase controller30. The detection signals SG3, SG4are also supplied to a current waveform display33. The current waveform display33is configured to receive and display the gate signals SG1, SG2.

FIG. 2shows a general configuration of the phase controller30. The phase controller30is in the form of a computer and includes functional sections that can be realized by way of software processing using a computer program embodied in a computer readable medium for use in phase control of the DC/DC converter10. The functional sections of the phase controller30may be partly or entirely formed by hardware. The phase controller30includes an input section41, which receives detection signals (analog signals) SG3, SG4from the respective current sensors31,32. At the input section41, the detection signals SG3, SG4are current-regulated, then translated into digital detection signals, and finally input into a ripple calculating section42.

The ripple calculating section42calculates an amount of change in current value (i.e., current ripple) occurring at the stepped portions of the waveform of each of the detection signals SG3, SG4that have been detected by the current sensors31,32. A signal output from the ripple calculating section42as representing a calculated amount of current ripple is delivered into the ripple determining section43.

The ripple determining section43judges, through ranking, for example, the level of current ripple occurring when the switching element SW1or the switching element SW1switches to an ON state. A signal output from the ripple determining section43as representing judgment results is input into a phase regulating section44.

Based on the judgment results, the phase regulating section44regulates the phase of currents I11, I21flowing respectively through the primary winding L1and the secondary winding L2, by controlling the respective ON times of the switching elements SW1and SW2in such a manner that the amount of current ripple output from the ripple calculating section42is reduced to zero. The regulation of ON/OFF actions of the switching elements SW1and SW2, which is performed by the phase regulating section44, is carried out by way of proper adjustment of the signal waveforms of the gate signals SG1and SG2supplied to the respective gates G1and G2of the switching elements SW1and SW2(i.e., ON/OFF timing of the switching elements SW1, SW2). The phase adjustment or regulation will be described in greater detail with reference to a typical example.

Although in the illustrated embodiment just described above the phase controller30is arranged to receive detection signals from the two current sensors31,32, it is possible according to the invention to employ another embodiment in which either one of the primary and secondary windings L1and L2of the transformer T1is provided with a current sensor, and the phase controller30receives a detection signal from the single current sensor.

Operation of the DC/DC converter10will be described with reference toFIGS. 3 to 7. In the illustrated embodiment, the DC/DC converter10serves as a voltage-boosting power converter.

As shown inFIG. 3, the above-mentioned gate signals SG1and SG2are given to the respective gates of the switching elements SW1and SW32to turn on/off the switching elements SW1and SW2. In the voltage-boosting DC/DC converter10, the DC voltage V1is applied as an input voltage as illustrated inFIG. 4. In the voltage-boosting operation, the DC voltage V1input to the left-side input terminal12is converted so that the DC voltage V2of a level equal to or greater than the input DC voltage V1is output from the right-side output terminal13. In the DC/DC converter10, the voltage-boosting operation is performed in a forward direction from the left, low-voltage side toward the right, high-voltage side.

Signal waveforms of the gate signals SG1and SG2are shown inFIG. 3. The gate signals SG1and SG2are of pulse waveforms having the same period t1and same duty cycle t2and hence the same ON time, but these gate signals SG1and SG2are phase-shifted from each other so that the two switching elements SW1and SW2are not turned on simultaneously. The switching elements SW1and SW2alternately repeat ON/OFF action in response to such gate signals SG1and SG2. The duty cycle t2, determining the ON time of the switching elements SW1and SW2, is variable as necessary within a range not exceeding 50% so as to avoid the switching elements SW1and SW2from being turned on simultaneously. In this manner, the output voltage V2can be increased or boosted from the level of the input voltage V1within a range of one to two times the input voltage V1.

The voltage-boosting operation of the DC/DC converter10is described in greater detail with reference toFIGS. 4 to 7.FIG. 5shows current flows in the individual circuit components of the DC/DC converter10when only the switching element SW1is turned on to energize the primary winding L1of the transformer T1.FIG. 6shows current flows in the individual circuit components of the DC/DC converter10when only the switching element SW2is turned on to energize the secondary winding L2of the transformer T1.

In the DC/DC converter10shown inFIG. 4, the gate signal SG1is supplied to the gate of the switching element SW1to turn on/off the switching element SW1. As shown inFIG. 5, when the gate signal SG1is in the ON state, the switching element SW1is turned on. Because the DC voltage V1has been input to the input terminal12, an exciting current I1flows through the primary winding L1of the transformer T1once the switching element SW1is turned on. This exciting current I1flows through a route of the input terminal12, inductor L0, primary winding L1and switching element SW1. While the gate signal SG1is ON, the exiting current I1gradually increases in level. Once the gate signal SG1turns into the OFF state, the exciting current I1decreases in level and ultimately reaches a zero level. Broken-line portions I1−1 of the exciting current I1shown inFIG. 5represent current portions that flow as a result of discharge of energy accumulated in the inductor L0when the gate signal SG1is turned off. The energizing current represented by the broken-line portion I1−1 decreases in level more slowly (i.e., taking a longer time) as the inductance of the inductor L0is greater. This exciting current I1−1 flows, through the primary winding L1and diode D1, to the output terminal13.

As the exciting current I1flows through the primary winding L1of the transformer T1as set forth above, an induced current (excited current)12is produced in the secondary winding L2on the basis of the mutual induction action. The induced current I2flows through the diode D2to the output terminal13. As shown inFIG. 5, the induced current I2thus produced in the secondary winding L2has variation characteristics substantially identical in shape to the exciting current I1and also has similar level values to the exciting current I1on the basis of the winding ratio (1:1). The smoothing capacitor C2is charged with the induced current I2, as a result of which the DC voltage V2is output to the output terminal13on the basis of the induced current I2.

Referring now toFIG. 6, the gate signal SG2is supplied to the gate of the switching element SW2to turn on/off the switching element SW2. The switching element SW2is kept in the ON state while the gate signal SG2is ON as illustrated inFIG. 7. The DC voltage V1has been input to the input terminal12, and thus, an exciting current I3flows through the secondary winding L2of the transformer T1once the switching element SW2is turned on. This exciting current I3flows through a route of the input terminal12, inductor L0, secondary winding L2and switching element SW2. While the gate signal SG2is ON, the exciting current I3gradually increases in level. Once the gate signal SG2turns into the OFF state, the exciting current I3decreases in level and ultimately reaches the zero level. Broken-line portions I3−1 of the exciting current I3shown inFIG. 7represent current portions that flow as a result of discharge of energy accumulated in the inductor L0. The exciting current represented by the broken-line portion I3−1 decreases in level more slowly (i.e., taking a longer time) as the inductance of the inductor L0is greater. This exciting current flows, through the secondary winding L2and diode D2, to the output terminal13.

As the exciting current I3flows through the secondary winding L2of the transformer T1as set forth above, an induced current (excited current) I4is produced in the primary winding L1on the basis of the mutual induction action. As shown inFIG. 7, the induced current I4thus produced in the primary winding L1has variation characteristics substantially identical in shape to the exciting current I3and also has similar level values to the exciting current I3on the basis of the winding ratio (1:1). The smoothing capacitor C2is charged with the induced current I4, as a result of which the DC voltage V2is output to the output terminal13on the basis of the induced current I4.

As set forth above, the voltage-boosting operation of the DC/DC converter10is based on the magnetic-field-cancellation type circuit section (L1, L2and F1). Namely, once the switching element SW1is turned on while the switching element SW2is turned off, an exciting current flows through the primary winding L1, and simultaneously an induced current (excited current) flows through the secondary winding L2in such a direction as to cancel the magnetization of the core F1and is then supplied to the output terminal13. Further, once the switching element SW2is turned on while the switching element SW1is turned off, an exciting current flows through the secondary winding L2, and simultaneously an induced current (excited current) flows through the primary winding L1in such a direction as to cancel the magnetization of the core F1and is then supplied to the output terminal13. Thus, the those currents flow through the primary and secondary windings L1and L2in opposite directions, so that the DC magnetization in the core F1is canceled out and thus the core F1can be prevented from magnetically saturating easily. Thus, even with smaller windings (coils) and core than the conventional counterparts, the embodiment of the DC/DC converter10can appropriately handle greater electric power. Namely, the above-described inventive arrangements can achieve a significant reduction in size (i.e., minitualization) of the DC/DC converter10.

Further, with the inductor L0added between the input terminal12and the common terminal c of the primary and secondary windings L1and L2, the input voltage of the transformer T1is continuously variable with the duty cycle t2of the gate signals SG1and SG2according to the function of the inductor L0. As a consequence, the DC/DC converter10can boost the input voltage V1to a desired level within a range of one to two times the input voltage V1by varying the duty cycle t2of the gate signals SG1and SG2within a range not exceeding 50%.

Next, with reference toFIGS. 8 to 11, a description will be given about various examples of phase adjustment or regulating operations, which are performed by the phase regulating section44(FIG. 2) of the phase controller30.

FIG. 8shows the ON/OFF action of the switching elements SW1and SW2occurring in response to the gate signals SG1and SG2applied respectively thereto, and waveforms of currents I11and I21flowing through the primary and secondary windings L1and L2, respectively, according to the ON/OFF action of the switching elements SW1and SW2. InFIG. 8, the waveforms are shown diagrammatically for the purpose of illustration of an essential part of the present invention.

As for times (time periods) A, B, C, D and E shown inFIG. 8, “C” is an ON time of the switching element SW1, “D” is an ON time of the switching element SW2, “A” is a time period from the rise or leading edge of the ON time of the switching element SW1to the rise or leading edge of the ON time of the switching element SW2, “B” is a time period from the rise or leading edge of the ON time of the switching element SW2and the rise or leading edge of the ON time of the switching element SW1, and “E” is a switching cycle or period. “A” and “B” are alternate with each other and show a nature of two-phase time. When the phase controller30is not operating to offer a phase regulating function, it is usual that A=B, C=D, and A+B=E.

The current I11flowing through the first winding L1is composed of an exciting current I1flowing during the time period A and an induced current (excited current) I4flowing during the time period B. In correspondence to the alternating arrangement of the time periods A and B, the exciting current I1and the induced current I4flow alternately. Similarly, the current I21flowing through the secondary winding L2is composed of an induced current (excited current)12flowing during the time period A and an exciting current I3flowing during the time period B. In correspondence to the alternating arrangement of the time periods A and B, the induced current I2and the exciting current I3flow alternately.

The waveforms of the currents I11and I21shown inFIG. 8illustrate two different stages or conditions exhibited respectively before and after a phase adjustment or regulating operation is effected. InFIG. 8, broken lines show waveforms of the currents I11and I21before being subjected to the phase adjustment or regulating operation, and solid lines show waveforms of the current I11and I21provided after the phase adjustment or regulating operation. In the example shown inFIG. 8, the currents I11and I21involve discontinuity in waveform (stepped portions), as indicated by circles16, occurring at the moment of turn-on action of the switching element SW2. The waveform discontinuity16is removed by the phase adjustment or regulating operation, which will be discussed later.

In the case where the currents I11and I21include current discontinuity16, i.e., current ripple, the phase controller30performs a phase adjustment or regulating operation by controlling on-off operation of the switching elements SW1and SW2in such a manner as to reduce the current ripple to zero or a level smaller than the current level.

According to a first example of phase regulating operation, the relative ratio of the time C to the time A is increased while the times A, E and D are kept constant. As shown inFIG. 9, the ON time C of the switching element SW1is varied to change the duty cycle of the waveform51(corresponding to the time C) during the time A.

Stated more specifically, as shown inFIG. 9, the ON time of the switching element SW1is increased to C′ where C′>C so that the switching waveform51varies to assume a waveform51′. With this waveform variation, the exciting current I2gradually increases in level until it assumes a waveform indicated by52shown inFIG. 19. The ON time C is adjusted in an appropriate manner while monitoring the current values detected by the current sensors31,32so that the discontinuity in waveform between the current I2and the current I3, which occurs at the leading edge of the ON time D of the switching element SW2, can be removed.

DC (direct current) component of the output current of the DC/DC converter10is constant. Accordingly, as the means value of DC component of the current I21increases, the means value of DC component of the current I11decreases conversely. Thus, waveform portions of the current I11and waveform portions hatching of the current I21, that are indicated by hatching as shown inFIG. 8, have the same area. This means that phase control performed in such a manner as to remove the discontinuity in waveform (stepped portions16) of the current I21will automatically remove the waveform discontinuity (stepped portions16) of the current I11. By thus removing the waveform discontinuities (stepped portions16), the currents I11and I21have waveforms, which are stable as indicated by solid lines shown inFIG. 8.

The foregoing adjustment of the ON time C effected to remove the waveform discontinuity (stepped portions16) of the current I21may be replaced by an adjustment of the ON time C effected in order to remove the waveform discontinuity (stepped portions16) of the current I11. In this instance, the same advantageous effect as discussed above can be attained too.

As thus far described, according to the first example of phase adjustment or regulating operation, the times A, E and D are kept constant and while keeping this condition, the gate signal SG1given to the switching element SW1is adjusted to increase the ON time C of the switching element SW1in such a manner that current ripple contained in the currents I11and I21is reduced to zero or minimized.

Next, other examples of the phase adjustment or regulating operation will be described. In the examples described below, the relative ratio of the time C to the time A is increased without requiring the times A and E to be kept constant. According to a second example of the phase adjustment or regulating operation, the time C is increased while a part (A-C) of the time A excluding the time C is kept unchanged. In a third example of the phase adjustment or regulating operation, a part (A-C) of the time A, which is exclusive of the time C, is decreased. Yet, according to a fourth example of the phase adjustment or regulating operation, the phase adjustment as done in the second example and the phase adjustment as done in the third example are effected in combination. In the second example, the time A becomes longer than the time B. In the third embodiment, the time A becomes relatively short compared to the time B.

The second example of the phase adjustment or regulating operation will be described in greater detail with reference toFIG. 10. In this example, the phase adjustment of the current I21is effected such that the time C is increased to C′ to thereby increase the times A and E to A′ and E′, respectively. The amount of variation (increase in length) is the same for the times A, C and E. By thus increasing the times A, C and E, the induced current (excited current)12now has a prolonged duration which is long enough to ensure joining of the induced current I2and the next following exciting current I3. The discontinuity in waveform (stepped portions16) of the current I21can thus be removed. The control is performed such that the current ripple (stepped portions16) involved in the current I21is minimized. The same control is performed on the current I11to remove the discontinuity in waveform (stepped portion16) of the current I11.

As thus far described, according to the second example of phase adjustment or regulating operation, the times B and D are kept unchanged and while keeping this condition, the gate signal SG1given to the switching element SW1is adjusted to vary the ON time C of the switching element SW1and the times A and E in such a manner that current ripple contained in the currents I11and I21is reduced to zero or minimized.

In the examples shown inFIGS. 9 and 10, the undesired current discontinuity occurs at a time the switching element SW2is turned on. In the case where the undesired current discontinuity occurs when the switching element SW1is turned on, the ON time D and other time-dependent factors of the switching element SW2will be varied to perform phase adjustment or regulating operation. It will be readily appreciated that the phase adjustment operation effected in connection with the switching element SW2may have four variations (fifth to eighth examples) corresponding to the afore-mentioned first to fourth examples. Furthermore, when the undesired current discontinuity occurs at the moment of turn-on action of each of the two switching elements SW1and SW2, one of the first to fourth examples of phase adjustment operation may be combined with one of the fifth to eighth examples of the phase adjustment operation.

The current waveform display33shown inFIG. 1is designed to concurrently display waveforms of the two currents I11and I21, for example. It is therefore possible to compare a pre-adjustment condition in which the currents I11, I21have not been subjected to phase adjustment operation, and a post-adjustment condition in which the phase adjustment operation has already effected on the currents I11, I21. Further, it is possible for a human operator to perform fine adjustment of the ON/OFF action of the switching elements SW1and SW2while observing waveforms of the currents I11and I21shown on the current waveform display33.

FIG. 11shows waveforms of the gate signals SG1and SG1and currents I11and I12that are actually observed on the current waveform display33before phase control (phase compensation) according to the aforesaid first example is performed by the phase controller30in conjunction with the DC/DC converter10. On the other hand,FIG. 12shows waveforms of the gate signals SG1and SG2and currents I11and I12that are actually observed on the current waveform display33when the phase control (phase compensation) according to the first example is performed by the phase controller30. The switching cycle or period employed in the illustrated example is 55 μs.

In the pre-adjustment condition shown in a time period t1and a time period t2have the same length and, hence, the currents I11and I12involve noticeable waveform discontinuity (stepped portions shown in circles16). By contrast, in the condition shown inFIG. 12, the time period t1is slightly decreased (by 3 μs, for example) while the time period t2is slightly increased (by 3 μs, for example). With this phase adjustment, the currents I11and I12are almost free from waveform discontinuity (i.e., current ripple), which would otherwise occur at portions shown in circles61. By thus shifting the phase of switching operation of the two switching elements SW1and SW2based on the gate signals SG1and SG2, the currents flowing through the primary and secondary windings L1and L2of the transformer T1are substantially freed from waveform discontinuity leading to generation of undue current ripple.

It should be appreciated that the constructions, shapes, positional relationships have been explained above in relation to various examples only to the extent that the present invention can be appropriately understood and carried out, and that the numerical values and materials given above are just illustrative. Namely, the present invention should not be construed as limited to the above-described embodiment and examples and may be modified variously unless it departs from the technical scope indicated by the appended claims.