Method of driving DC/DC converter, and DC/DC converter

When the three U-, V-, W-phase arms of a DC/DC converter are turned on, they are alternately turned on by gate drive signals. When the U-, V-, W-phase arms are alternately turned on, an upper arm switching device of the U-phase arm, for example, is turned on, and thereafter a lower arm switching device of the U-phase arm is turned on. Thereafter, an upper arm switching device of the V-phase arm which is next to the U-phase arm is turned on, and thereafter a lower arm switching device of the V-phase arm is turned on. The upper and lower arm switching devices are thus turned on in a rotation switching process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of driving a chopper DC/DC converter, and a DC/DC converter.

2. Description of the Related Art

There have heretofore been widely used DC/DC converter apparatus as switching power supplies having switching devices such as MOSFETs, IGBTs, or the like.

For example, there has been proposed a vehicle (hereinafter referred to as “electric vehicle”) incorporating a DC/DC converter apparatus for increasing and reducing a DC voltage, which is connected between an electricity storage device and a motor that is energized by an inverter. On the electric vehicle, when the motor is energized, the voltage across the electricity storage device is increased by the DC/DC converter apparatus and applied to the inverter, and when the motor regenerates electric power, the regenerated voltage from the inverter is lowered by the DC/DC converter apparatus and applied to charge the electricity storage device.

There has also been proposed a vehicle (hereinafter also referred to as “fuel cell vehicle”) which also uses a motor as a propulsive source. The fuel cell vehicle includes a fuel cell directly connected to the motor which is energized by an inverter. A DC/DC converter apparatus for increasing and reducing a DC voltage is connected between an electricity storage device and the junction between the fuel cell and the motor. The fuel cell is used as a main power supply, and the electricity storage device as an auxiliary power supply for assisting the main power supply.

On the fuel cell vehicle, when the motor is energized, the voltage across the fuel cell and the voltage across the electricity storage device, which has been increased by the DC/DC converter apparatus, are added together, and the sum voltage is applied to the inverter. When the motor regenerates electric power, the regenerated voltage from the inverter is lowered by the DC/DC converter apparatus and applied to charge the electricity storage device. If the electric power generated by the fuel cell contains an excessive amount of electric power, then it is lowered in voltage and applied to charge the electricity storage device.

FIG. 16of the accompanying drawings shows a DC/DC converter apparatus16disclosed in Japanese Laid-Open Patent Publication No. 2004-357388 that is applied to an electric vehicle. As shown inFIG. 16, the DC/DC converter apparatus16basically comprises a DC/DC converter6including reactors2A,2B,2C and a switching device comprising three-phase arms made up of upper and lower arm switching devices including transistors3A,3B,3C,4A,4B,4C that are connected inversely across respective diodes7A,7B,7C,8A,8B,8C, and a control means5for controlling the DC/DC converter6.

The DC/DC converter apparatus16has a function to convert the voltage of a DC power supply1at a low-voltage terminal TL into a voltage that is m times higher and apply the converted voltage to a load11through a high-voltage terminal TH (voltage increasing mode), and also a function to convert the voltage at the high-voltage terminal TH into a voltage that is 1/m times lower and apply the converted voltage to the DC power supply1through the low-voltage terminal TL (voltage reducing mode).

As shown inFIG. 17of the accompanying drawings, while the DC/DC converter apparatus16is in the voltage increasing mode, when it is driven at a duty ratio of 92 {≈(11/12)×100}[%] in a switching period 2π, the transistors4A,4B,4C of the lower arm switching devices of the three-phase arms are turned on at timings that are 2π/3 out of phase by gate drive signals ULA, ULB, ULC from the control means5.

While the transistors4A,4B,4C are being energized, since the terminals of the reactors2A,2B,2C which are connected to the respective transistors4A,4B,4C are grounded, the current from the DC power supply1flows through the reactors2A,2B,2C to ground. At this time, the reactors2A,2B,2C store an amount of energy which is proportional to the product of the square of the current flowing therethrough and the inductance of the reactors2A,2B,2C.

When the transistors4A,4B,4C are then turned off, a current depending on the energy stored in the reactors2A,2B,2C flows through the diodes7A,7B,7C to the high-voltage terminal TH. The voltage at the high-voltage terminal TH is monitored by a voltage detecting circuit6a.

While the DC/DC converter apparatus16is in the voltage reducing mode, the transistors3A,3B,3C of the upper arm switching devices are turned on at timings that are 2π/3 out of phase by gate drive signals UHA, UHB, UHC from the control means5. When the transistors3A,3B,3C are energized, a current flows from the high-voltage terminal TH through the transistors3A,3B,3C and the reactors2A,2B,2C to the DC power supply1through the low-voltage terminal TL, storing energy in the reactors2A,2B,2C.

When the transistors3A,3B,3C are then successively turned off, the diodes8A,8B,8C are successively turned on correspondingly, causing a current to flow from ground through the diodes8A,8B,8C and the reactors2A,2B,2C to the DC power supply1. The DC/DC converter apparatus16thus operates as a voltage reducing circuit.

If voltage increasing/reducing DC/DC converter apparatus need to produce an output current higher than the rated current of switching devices such as MOSFETs or IGBTs, then the DC/DC converter apparatus are required to have multiphase arms, rather than a single phase arm, as is the case with the DC/DC converter apparatus16. Since the output current is distributed to the phase arms, the DC/DC converter apparatus require multiphase reactors, e.g., the three reactors2A,2B,2C for the three phases in the DC/DC converter apparatus16shown inFIG. 16.

A reactor has impedance that is greater in proportion to the frequency of a current flowing therethrough. If the output current is constant, then the reactor needs to have a greater inductance as the frequency is lower. Though a reactor with a smaller Q is preferable for a lower resistance loss, a thicker conductive wire is necessary to make a reactor with a smaller Q.

It is desirable that DC/DC converter apparatus be as small and light as possible. However, the need for as many reactors as the number of multiphase arms presents one of obstacles to efforts to reduce the size and weight of the DC/DC converter apparatus.

The DC/DC converter apparatus16shown inFIG. 16employs high-power transistors as switching devices. With the three-phase arms, as shown in plan inFIG. 18Aof the accompanying drawings, the six transistors3A,3B,3C,4A,4B,4C are fixedly mounted on a metal heat radiating plate (heat spreader)12.

As shown inFIG. 17, during the switching period 2π, three transistors are simultaneously energized during a time which is 9/12 of the switching period2π, and two transistors are simultaneously energized during a remaining time which is 3/12 of the switching period 2π.

FIG. 18Bof the accompanying drawings shows a pattern, shown hatched, in which the heat from the transistors is transferred through the heat spreader12. As shown inFIG. 18B, while the three transistors are simultaneously energized, the heat transferred therefrom concentrates in regions, shown cross-hatched, of the heat spreader12, resulting in a poor heat radiation efficiency. In order to meet thermal conditions such as maximum allowable temperatures for the transistors, it is necessary to increase the volume and surface area of the heat spreader12, and to increase the rate of a coolant supplied to cool the heat spreader12. As a result, the DC/DC converter apparatus16tends to be large in size.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of driving a DC/DC converter having an excellent heat radiating capability and enabling reduction in size and weight, and a DC/DC converter which is small in size and light in weight.

A method of driving a DC/DC converter according to the present invention comprises the steps of: connecting a plurality of phase arms parallel to each other between a first electric power device and a second electric power device, each of the phase arms comprising a series-connected circuit of an upper arm switching device and a lower arm switching device and diodes connected inversely across the upper arm switching device and lower arm switching device; alternately turning on the phase arms; and when one of the phase arms is turned on, turning on either the upper arm switching devices or the lower arm switching device, or alternately turning on the upper arm switching device and the lower arm switching device.

With the method of driving a DC/DC converter, the upper arm switching devices and the lower arm switching devices are not simultaneously turned on, and the different phase arms are not simultaneously turned on. Rather, at most one switching device is turned on at all times. Therefore, the DC/DC converter is of an excellent heat radiating capability, i.e., can easily be designed for heat radiation. As a result, the DC/DC converter apparatus can be reduced in size and weight.

With the above method, the upper arm switching devices and the lower arm switching devices are not simultaneously turned on, and the different phase arms are not simultaneously turned on. Rather, at most one switching device is turned on at all times. Therefore, the DC/DC converter is of an excellent heat radiating capability, i.e., can easily be designed for heat radiation. As a result, the DC/DC converter apparatus can be reduced in size and weight.

A DC/DC converter according to the present invention comprises a plurality of phase arms connected parallel to each other between a first electric power device and a second electric power device and comprising a series-connected circuit of upper arm switching devices and lower arm switching devices and diodes connected inversely across the upper arm switching devices and lower arm switching devices, the phase arms having respective midpoints connected to each other, and a reactor inserted between the connected midpoints and the first electric power device or the second electric power device.

The DC/DC converter for increasing or reducing the voltage with the multiphase arms includes the single reactor, and hence is reduced in size and weight. Inasmuch as the DC/DC converter needs only one reactor, the DC/DC converter may be smaller in size and weight than the DC/DC converter having the multiphase arms according to the related art, as the number of the phases of the multiphase arms increases.

With the DC/DC converter having the multiphase arms (three-phase arms for an easier understanding of the invention) according to the related art, each of the reactors of the three-phase arms is energized once in one switching period. According to the present invention, the single reactor is energized once in one switching period 2π by the three-phase upper arm switching devices or the three-phase lower arm switching devices during a voltage increasing mode or a voltage reducing mode. The present invention also covers a process wherein the single reactor is energized once alternately by the three-phase upper arm switching devices and the three-phase lower arm switching devices in one switching period 2π.

According to the present invention, since the operating frequency of the single reactor is three times higher, the inductance value thereof may be one-third of the inductance values of the reactors according to the related art. Accordingly, the reactor may be reduced in size.

Although the DC/DC converter may become slightly difficult to design for heat radiation, under the condition that the switching devices are used in a range of their rated currents (allowable device temperatures), the upper arm switching devices of a plurality of phases may be simultaneously turned on and then, after a dead time, the lower arm switching devices of a plurality of phases may be simultaneously turned on. Such a process is also covered by the present invention. Also, a plurality of lower arm switching devices may be simultaneously turned on and then, after a dead time, a plurality of upper arm switching devices may be simultaneously turned on. Furthermore, a plurality of upper arm switching devices may be simultaneously turned on and then, after a dead time, a plurality of upper arm switching devices may be simultaneously turned on. Alternatively, a plurality of lower arm switching devices may be simultaneously turned on and then, after a dead time, a plurality of lower arm switching devices may be simultaneously turned on. These processes are also covered by the present invention. Since the single reactor is shared by the phases in these processes, the DC/DC converter is small in size and weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Vehicles incorporating a method of driving a DC/DC converter and a DC/DC converter according to the present invention will be described below with reference to the drawings.

FIG. 1shows a fuel cell vehicle20according to an embodiment of the present invention. As shown inFIG. 1, the fuel cell vehicle20basically comprises a hybrid power supply system including a fuel cell (FC)22and an electricity storage device (referred to as “battery”)24which is an energy storage, a travel motor26for being supplied with a current (electric power) from the hybrid power supply system through an inverter34, and a DC/DC converter apparatus {also referred to as “VCU (Voltage Control Unit)”}23for converting voltages between a primary end1S connected to the battery24and a secondary end2S connected to the fuel cell22and the motor26(the inverter34).

The VCU23comprises a DC/DC converter36and a converter controller54for controlling the DC/DC converter36.

The fuel cell22is of a stacked structure made up of cells each comprising an anode, a cathode, and a solid polymer electrolytic membrane sandwiched between the anode and the cathode. The fuel cell22is connected to a hydrogen tank28and an air compressor30by pipes. The fuel cell22generates a current If due to electrochemical reaction between hydrogen (fuel gas) and air (oxidizing gas) as reactant gas. The generated current If is supplied through a current sensor32and a diode (also referred to as “disconnecting diode”)33to the inverter34and/or the DC/DC converter36.

The inverter34conducts conversion from direct current into alternating current, and supplies a motor current Im to the motor26. The inverter34also conducts conversion from alternating current into direct current in a regenerative mode, and supplies the motor current Im from the secondary end2S to the primary end1S through the DC/DC converter36.

A secondary voltage V2, which may be the regenerated voltage or the generated voltage Vf across the fuel cell22, is converted into a low primary voltage V1by the DC/DC converter36. The low primary voltage V1is further converted into a lower voltage by a downverter42. The downverter42supplies, under the lower voltage, an accessory current Iau to accessories44such as lamps, etc. and also supplies any excess current as a battery current Ibat to charge the battery24.

The battery24, which is connected to the primary end1S, may comprise a lithium ion secondary battery or a capacitor. In the present embodiment, the battery24comprises a lithium ion secondary battery.

The battery24supplies the accessory current Iau to the accessories44through the downverter42, and also supplies the motor current Im through the DC/DC converter36to the inverter34.

Smoothing capacitors38,39are connected respectively to the primary and secondary ends1S,2S. A resistor40is connected across the smoothing capacitor39, i.e., across the fuel cell22.

The fuel cell22, the hydrogen tank28, and the air compressor30make up a system controlled by an FC controller50. The inverter34and the motor26make up a system controlled by a motor controller52which includes an inverter driver. The DC/DC converter36makes up a system controlled by the converter controller54which includes a converter driver.

The FC controller50, the motor controller52, and the converter controller54are controlled by a general controller56which serves as a higher-level controller for determining a total load Lt on the fuel cell22, etc.

Each of the general controller56, the FC controller50, the motor controller52, and the converter controller54comprises a CPU, a ROM, a RAM, a timer, input and output interfaces including an A/D converter, a D/A converter, and, if necessary, a DSP (Digital Signal Processor), etc.

The general controller56, the FC controller50, the motor controller52, and the converter controller54are connected to each other by communication lines70such as of a CAN (Controller Area Network) as an intravehicular LAN, and perform various functions by sharing input and output information from various switches and various sensors and executing programs stored in the ROMs under the CPUs based on the input and output information from the various switches and various sensors.

The switches and the sensors for detecting vehicle states include, in addition to the current sensor32for detecting the generated current If, a voltage sensor61for detecting a primary voltage V1which is equal to a battery voltage Vbat, a current sensor62for detecting a primary current I1, a voltage sensor63for detecting a secondary voltage V2which is substantially equal to the generated voltage Vf across the fuel cell22when the disconnecting diode33is rendered conductive, a current sensor64for detecting a secondary current I2, an ignition switch (IGSW)65, an accelerator sensor66, a brake sensor67, a vehicle speed sensor68which are connected to the communication lines70, and temperature sensors69connected to the converter controller54.

The general controller56determines a total demand load Lt on the fuel cell vehicle20based on the state of the fuel cell22, the state of the battery24, the state of the motor26, the state of the accessories44, and the input signals from the switches and the sensors (load demands), determines the shares of a fuel cell allocated load (demand output) Lf to be allocated to the fuel cell22, a battery allocated load (demand output) Lb to be allocated to the battery24, and a regenerative power supply allocated load Lr to be allocated to the regenerative power supply, through an arbitration process, based on the total demand load Lt, and sends commands indicative of the determined shares to the FC controller50, the motor controller52, and the converter controller54.

The DC/DC converter36comprises three phase arms connected parallel to each other between a first power device in the form of the battery24and a second power device in the form of the fuel cell22or the regenerative power supply (the inverter34and the motor26). The three phase arms include a U-phase arm UA (81u,82u), a V-phase arm VA (81v,82v), and a W-phase arm WA (81w,82w) which are made up of upper arm switching devices81(81u,81v,81w) and lower arm switching devices82(82u,82v,82w) such as IGBTs or the like.

Diodes83u,83v,83w,84u,84v,84ware connected inversely across the respective arm switching devices81u,81v,81w,82u,82v,82w.

A single reactor90for discharging and storing energy at the time the DC/DC converter36converts voltage between the primary voltage V1and the secondary voltage V2is inserted between the battery24and the commonly connected midpoints of the U-phase arm UA, the V-phase arm VA, and the W-phase arm WA.

The upper arm switching devices81(81u,81v,81w) are turned on by gate drive signals (drive voltages) UH, VH, WH output from the converter controller54when the gate drive signals UH, VH, WH are high in level. The lower arm switching devices82(82u,82v,82w) are turned on by gate drive signals (drive voltages) UL, VL, WL output from the converter controller54when the gate drive signals UL, VL, WL are high in level.

The primary voltage V1, typically the open circuit voltage OCV across the battery24at the time no load is connected to the battery24, is set to a voltage higher than a minimum voltage Vfmin of the generated voltage Vf of the fuel cell22as indicated by a fuel cell output characteristic curve (current-voltage characteristics)91shown inFIG. 2. InFIG. 2, OCV≈V1.

The secondary voltage V2is equal to the generated voltage Vf of the fuel cell22while the fuel cell22is generating electric power.

When the generated voltage Vf of the fuel cell22becomes equal to the voltage Vbat (=V1) of the battery24, the fuel cell22and the battery24are in a directly coupled state as indicated by the thick dot-and-dash line inFIG. 2. In the directly coupled state, the duty ratios of the gate drive signals UH, VH, WH supplied to the upper arm switching devices81(81u,81v,81w) are, for example, 100 [%], and the duty ratios of the gate drive signals UL, VL, WL supplied to the lower arm switching devices82(82u,82v,82w) are, for example, 0 [%]. In the directly coupled state, when a current is to flow from the secondary end2S to the primary end1S in a charging direction (regenerating direction), the current flows through the upper arm switching devices81(81u,81v,81w). When a current is to flow from the primary end1S to the secondary end2S in a propulsive direction, the current flows through the diodes83u,83v,83w.

In a directly coupled state (referred to as “directly coupled state for high output power” or “first directly coupled state”) for supplying or sourcing the secondary current I2from the secondary end2S of the DC/DC converter36to the inverter34for producing high output power, the secondary voltage V2is represented by V2=V1−Vd (Vd is a forward voltage drop across the diodes83u,83v,83w).

The directly coupled state is not limited to the time when high output power is to be produced, but may be employed if necessary for control. For example, the directly coupled state may be utilized when the fuel cell vehicle20is stopped. When the fuel cell vehicle20is stopped at a traffic signal or the like, the air compressor30is inactivated and the hydrogen tank28does not supply the fuel gas for better fuel economy. At this time, when the remaining fuel gas in the fuel cell22is used up, the generated voltage Vf (generated current If) of the fuel cell22falls to zero as it is discharged by the resistor40and supplied to the accessories44including an air conditioner. However, the accessory current Iau is continuously supplied from the battery24to the accessories44.

When the fuel cell22is to generate electric power by releasing the brake pedal and pressing the accelerator pedal while the fuel cell vehicle20is being stopped in an idling state, the voltage at the secondary end2S of the DC/DC converter36is kept at a level in a directly coupled state in order for the VCU23to resume its output control on the fuel cell22smoothly. In this directly coupled state (referred to as “idling directly coupled state” or “second directly coupled state”), the resistor40serves as the load, and the secondary voltage V2at the secondary end2S of the DC/DC converter36is held at the level V2=V1−Vd.

The output control performed on the fuel cell22by the VCU23will be described below.

When the fuel cell22generates electric power while it is being supplied with the fuel gas from the hydrogen tank28and the compressed air from the air compressor30, the generated current If of the fuel cell22is determined by the converter controller54setting the secondary voltage V2, i.e., the generated voltage Vf, through the DC/DC converter36on the characteristic curve91, also referred to as “function F(Vf)”, shown inFIG. 2. The generated current If is determined as a function F(Vf) value of the generated voltage Vf. Since If=F(Vf), if the generated voltage Vf is set as Vf=Vfa=V2, a generated current Ifa is determined as a function value of the generated voltage Vfa(V2) according to Ifa=F(Vfa)=F(V2).

Inasmuch as the generated current If of the fuel cell22is determined when the secondary voltage V2(the generated voltage Vf) is determined, the secondary voltage V2(the generated voltage Vf) is set as a target voltage (target value) when the fuel cell vehicle20is controlled for its propulsion. In special cases when the battery24(the first power device) is regarded as suffering a failure such as when the battery24is opened due to a wire disconnection between the downverter42and the battery24, the primary voltage V1is used as the target voltage.

In a system including the fuel cell22, such as the fuel cell vehicle20, the VCU23is controlled to set the secondary voltage V2at the secondary end2S of the DC/DC converter36as the target voltage, and the output (the generated current If) of the fuel cell22is controlled by the VCU23. The output control performed on the fuel cell22by the VCU23has been described above.

A basic operation of the DC/DC converter36that is controlled by the converter controller54will be described below with reference to the flowchart ofFIG. 3.

As described above, the general controller56determines a total demand load Lt on the fuel cell vehicle20based on the state of the fuel cell22, the state of the battery24, the state of the motor26, the state of the accessories44, and the input signals from the switches and the sensors (load demands), determines the shares of a fuel cell allocated load (demand output) Lf to be allocated to the fuel cell22, a battery allocated load (demand output) to be allocated to the battery24, and a regenerative power supply allocated load Lr to be allocated to the regenerative power supply, through an arbitration process, based on the total demand load Lt, and sends commands indicative of the determined shares to the FC controller50, the motor controller52, and the converter controller54.

In step S1shown inFIG. 3, the general controller56determines (calculates) a total demand load Lt from the power demand of the motor26, the power demand of the accessories44, and the power demand of the air compressor30, which all represent load demands. In step S2, the general controller56determines the shares of a fuel cell allocated load Lf, a battery allocated load Lb, and a regenerative power supply allocated load Lr for outputting the determined total demand load Lt. When the general controller56determines the fuel cell allocated load Lf, the general controller56takes the efficiency η of the fuel cell22into account.

Then, in step S3, the converter controller54determines a generated voltage Vf of the fuel cell22, i.e., the secondary voltage V2in this embodiment, depending on the fuel cell allocated load Lf.

After the secondary voltage V2is determined, the converter controller54controls the DC/DC converter36to achieve the determined secondary voltage V2in step S4.

Specifically, the converter controller54energizes the DC/DC converter36in a voltage increasing mode, a voltage reducing mode, or a directly coupled mode, depending on the determined secondary voltage V2.

In the voltage increasing mode for sourcing the secondary current I2from the secondary end2S of the DC/DC converter36to the inverter34in step S4, the converter controller54controls the DC/DC converter36in a rotation switching process by turning on the lower arm switching device82u(storing energy in the reactor90with the primary current I1produced by subtracting the accessory current Iau from the battery current Ibat and at the same time sourcing the secondary current I2from the capacitor39to the inverter34). Then, the converter controller54controls the DC/DC converter36by rendering the diodes83u,83v,83wconductive (discharging the energy from the reactor90, storing the energy in the capacitor39, and sourcing the secondary current I2to the inverter34). Thereafter, the converter controller54controls the DC/DC converter36by turning on the lower arm switching device82vin the same manner as above, then rendering the diodes83u,83v,83wconductive in the same manner as above, then turning on the lower arm switching device82win the same manner as above, then rendering the diodes83u,83v,83wconductive in the same manner as above, thereafter turning on the lower arm switching device82u, and so on.

The upper arm switching devices81u,81v,81wand the lower arm switching devices82u,82v,82whave their ON duty ratios determined to keep the output voltage V2at the command voltage from the general controller56.

In the directly coupled mode for high output power wherein the secondary current I2is sourced from the secondary end2S of the DC/DC converter36to the inverter34in step S4, the diodes83u,83v,83ware rendered conductive, and the secondary voltage V2is represented by V2=V1−Vd.

In the voltage reducing mode for supplying or sinking the secondary current I2from the secondary end2S of the DC/DC converter36to the accessories44and the battery24at the primary end1S in step S4, the converter controller54controls the DC/DC converter36in a rotation switching process by turning on the upper arm switching device81uso as to store energy in the reactor90with the secondary current I2output from the capacitor39and at the same time supply the primary current I1from the capacitor38to the accessories44and, if necessary, the battery24. Then, the converter controller54controls the DC/DC converter36by rendering the diodes84u,84v,84wconductive. In this case, the diodes84u,84v,84ware rendered conductive as flywheel diodes discharging the energy from the reactor90, storing the energy in the capacitor39, and supplying the primary current I1to the accessories44and, if necessary, the battery24. Thereafter, the converter controller54controls the DC/DC converter36by turning on the upper arm switching device81vin the same manner as above, then rendering the diodes84u,84v,84wconductive in the same manner as above, then turning on the upper arm switching device81win the same manner as above, then rendering the diodes84u,84v,84wconductive in the same manner as above, thereafter turning on the upper arm switching device81u, and so on.

If a regenerated voltage exists, then the regenerative power supply allocated load Lr is added to the secondary current that is sunk in the voltage reducing mode. In the voltage reducing mode, the upper arm switching devices81u,81v,81wand the lower arm switching devices82u,82v,82walso have their ON duty ratios controlled depending on the determined output voltage V2.

The secondary voltage V2and the primary voltage V1are controlled by the converter controller54while it is controlling the DC/DC converter36in the PID operation based on a combination of a feed-forward control process and a feedback control process.

The basic operation of the DC/DC converter36controlled by the converter controller54has been described above.

As shown inFIGS. 4A and 4B, the arm switching devices81u,81v,81w,82u,82v,82ware assembled as a so-called 6-in-1 module13fixedly mounted on a metal heat radiating plate (heat spreader)12. The arm switching devices81u,81v,81w,82u,82v,82ware associated with the respective temperature sensors69. The temperature sensors69and the gate terminals of the arm switching devices81u,81v,81w,82u,82v,82ware connected to the converter controller54. The diodes83u,83v,83w,84u,84v,84wthat are paired with arm switching devices81u,81v,81w,82u,82v,82ware omitted from illustration inFIGS. 4A and 4B.

The inverter34for driving the motor26is also in the form of a 6-in-1 module as is the case with the DC/DC converter36. Therefore, the fuel cell vehicle20is of a reduced cost.

For driving the motor26with the inverter34, however, the midpoints of the three phase arms of the inverter34are not connected in common, but connected to the U-, V-, and W-phase coils of the motor26in a full bridge configuration.

The fuel cell vehicle20according to the present embodiment is basically constructed and operates as described above. The rotating switching process controlled by the VCU23including the DC/DC converter36will be described in detail below.

FIG. 5is a timing chart of a voltage reducing mode (for sinking the secondary current I2) of the VCU23, andFIG. 6is a timing chart of a voltage increasing mode (for sourcing the secondary current I2) of the VCU23.

InFIGS. 5 and 6, the primary current I1flowing through the reactor90has a positive (+) sign when it flows from the primary end1S to the secondary end2S in the voltage increasing mode (a source current flowing from the secondary end2S of the DC/DC converter36to the inverter34), and a negative sign (−) when it flows from the secondary end2S to the primary end1S in the voltage reducing mode (a sink current flowing from the fuel cell22or the inverter34to the secondary end2S of the DC/DC converter36).

Of the waveforms of the gate drive signals UH, UL, VH, VL, WH, WL output from the converter controller54, periods that are shown hatched represent periods in which the arm switching devices which are supplied with the gate drive signals UH, UL, VH, VL, WH, WL (e.g., the upper arm switching device81uis supplied with the gate drive signal UH) are actually turned on, i.e., currents are flowing through the arm switching devices. It is to be noted that even when the arm switching devices are supplied with the gate drive signals UH, UL, VH, VL, WH, WL, currents do not flow through the arm switching devices unless the corresponding parallel diodes83u,83v,83w,84u,84v,84ware turned off.

As shown inFIGS. 5 and 6, in either of the voltage reducing mode and the voltage increasing mode of the DC/DC converter36, as can be understood from the waveforms of the gate drive signals UH, UL, VH, VL, WH, WL output from the converter controller54, the U-, V-, W-phase arms UA, VA, WA are alternately turned on by the gate drive signals UH, UL, VH, VL, WH, WL in the rotation switching process per one switching period 2π. When the U-, V-, W-phase arms UA, VA, WA are turned on, the upper arm switching devices81u,81v,81wof the U-, V-, W-phase arms UA, VA, WA are turned on by the gate drive signals UH, VH, WH (seeFIG. 5), or the lower arm switching devices82u,82v,82wof the U-, V-, W-phase arms UA, VA, WA are turned on by the gate drive signals UL, VL, WL (seeFIG. 6).

As shown inFIGS. 5,6, and7, in order to prevent the upper and lower arm switching devices81,82from being turned on simultaneously and hence to prevent the secondary voltage V2from being short-circuited, dead times dt are inserted between the gate drive signals UH, UL, the gate drive signals VH, VL, and the gate drive signals WH, WL for alternately turning on the upper arm switching devices81u,81v,81wor the lower arm switching devices82u,82v,82w. When the U-, V-, W-phase arms UA, VA, WA are turned on, dead times dt are inserted between the gate drive signals UL, VH, the gate drive signals VL, WH, and the gate drive signals WL, UH. In other words, so-called synchronous switching is performed with the dead times dt inserted between the ON times.

In the voltage reducing mode shown inFIG. 5, while the upper arm switching device81uis being turned on by the gate drive signal UH in a period between time t1and time t2, energy is stored in the reactor90through the upper arm switching device81uby the secondary current I2from the fuel cell22and/or the regenerative power supply. In a period from time t2to time t5which includes a dead time d5, an ON time of the gate drive signal UL (with no current flowing through the lower arm switching device82u), and another dead time dt, the energy stored in the reactor90is discharged as the primary current I1to the primary end1S through the diodes84u,84v,84wthat function as flywheel diodes and are rendered conductive. From time t5, the upper arm switching devices81v,81w,81u, . . . are successively turned on repeatedly.

In the voltage increasing mode shown inFIG. 6, while the lower arm switching device82uis being turned on by the gate drive signal UL in a period between time t13and time t14, energy is stored in the reactor90by the primary current I1from the battery24. In a period from time t14to time t17which includes a dead time dt, an ON time of the gate drive signal VH (with no current flowing through the upper arm switching device81v), and another dead time dt, the energy stored in the reactor90is discharged to the secondary end2S through the diodes83u,83v,83wthat function as rectifying diodes and are rendered conductive. From time t17, the lower arm switching devices82v,82w,82u, . . . are successively turned on repeatedly.

FIG. 7shows showing transitions between the voltage increasing mode and the voltage reducing mode. InFIG. 7, in a period (shown hatched) between time t20and time t21during which the upper arm switching device81uis turned on by the gate drive signal UH, energy is stored in the reactor90through the upper arm switching device81uby the secondary current I2from the fuel cell22and/or the regenerative power supply.

In a period from time t21to time t22when the direction of the current is inverted (the sign of the current changes from negative to positive), the energy stored in the reactor90is discharged to the primary end1S through the diodes84u,84v,84wthat function as flywheel diodes and are rendered conductive.

In a period between time t22and time t23during which the lower arm switching device82uis turned on by the gate drive signal UL, energy is stored in the reactor90by the primary current I1from the battery24. In a period from time t23to time t24at which time the direction of the current is inverted (the sign of the current changes from positive to negative), the energy stored in the reactor90is discharged to the secondary end2S through the diodes83u,83v,83wrendered conductive. The same operation as describe above will subsequently be repeated. In the three-phase rotation switching process according to the present embodiment, as described above, smooth switching is made between the voltage increasing mode and the voltage reducing mode.

According to the embodiment described above, the VCU23has the DC/DC converter36comprising the three phase arms, i.e., the parallel-connected phase arms UA, VA, WA, having the upper arm switching devices81and the lower arm switching devices82, connected between the secondary end2S as the junction between the fuel cell22and the inverter34and the primary end1S connected to the battery24, and the converter controller54for controlling the DC/DC converter36.

When the converter controller54turns on the three phase arms UA, VA, WA of the DC/DC converter36, the converter controller54alternately turns on the three U-, V-, W-phase arms UA, VA, WA, i.e., turns on the upper arm switching device81uof the U-phase arm UA (seeFIGS. 5 through 7), thereafter turns on the lower arm switching device82uof the U-phase arm UA (seeFIG. 7), thereafter turns on the upper arm switching device81vof the V-phase arm VA (seeFIGS. 5 through 7), and thereafter turns on the lower arm switching device82vof the V-phase arm VA (seeFIG. 7). In this manner, the converter controller54rotates the switching timings.

FIG. 8shows the manner in which the heat from the arm switching devices81u,81v,81w,82u,82v,82wis radiated when the three phase arms are switched in rotation, i.e., the U-phase arm is turned on, then the V-phase arm is turned on, thereafter the W-phase arm is turned on, then the U-phase arm is turned on, . . . .

As shown inFIG. 8, according to the rotation switching process, only one upper arm switching device81or one lower arm switching device82is turned on at a time. Consequently, as can be understood from heat radiating paths shown hatched inFIG. 8, the heat radiating paths are not superposed simultaneously unlike the superposed regions shown cross-hatched inFIG. 18B. The DC/DC converter36thus has an increased heat radiating capability, and hence the 6-in-1 module13is reduced in size and weight.

The DC/DC converter36shown inFIG. 1includes the three phase arms UA, VA, WA of series-connected circuits of the upper and lower arm switching devices81u,81v,81w,82u,82v,82wand the diodes83u,83v,83w,84u,84v,84wthat are connected inversely across the respective arm switching devices81u,81v,81w,82u,82v,82w, the three phase arms UA, VA, WA being connected parallel to each other between the battery24as the first power device and the fuel cell22and/or the inverter34and the motor26as the second power device and the single reactor90inserted between the commonly connected midpoints of the three phase arms UA, VA, WA and the battery24(or the second power device) as the first power device.

This arrangement allows the single reactor90to be used in the DC/DC converter36for increasing and reducing the voltage with the three phase arms.

Specifically, the converter controller54for controlling the DC/DC converter36alternately turns on the three phase arms UA, VA, WA. When the converter controller54turns on the phase arm UA, for example, it outputs the gate drive signals UH, UL for turning on one of the upper arm switching device81uor the lower arm switching device82uof the phase arm UA (FIGS. 5 and 6) or alternately turning on the upper arm switching device81uand the lower arm switching device82u(FIG. 7). In this manner, the VCU (the DC/DC converter device)23is capable of operating in the voltage increasing mode and the voltage reducing mode with the single reactor90.

The DC/DC converter36and the VCU23are reduced in size and weight because only one reactor90is used in combination therewith.

With the DC/DC converter6having the multiphase arms (three-phase arms for an easier understanding of the invention) according to the related art shown inFIGS. 16 and 17, each of the reactors2A,2B,2C of the three-phase arms is energized once in one switching period 2π. With the DC/DC converter36shown inFIG. 1, the single reactor90shared by the three-phase arms UA, VA, WA is energized once in one switching period 2π during the voltage increasing mode (FIG. 6) or the voltage reducing mode (FIG. 5). In principle, therefore, the operating frequency of the reactor90of the DC/DC converter36shown inFIG. 1is three times higher than the reactors2A,2B,2C of the DC/DC converter6shown inFIGS. 16 and 17.

Since the operating frequency of the reactor90is three times higher, the inductance value thereof may be one-third of the inductance values of reactors2A,2B,2C. Accordingly, the reactor90may be reduced in size. Inasmuch as the DC/DC converter36needs only one reactor90, the DC/DC converter36may be smaller in size and weight than the DC/DC converter having the multiphase arms according to the related art as the number of the phases of the multiphase arms increases.

According to the DC/DC converter36shown inFIG. 1, the upper arm switching devices81and the lower arm switching devices82are not simultaneously turned on, and the different phase arms are not simultaneously turned on. At most one switching device is turned on at all times. Therefore, the DC/DC converter36is of an excellent heat radiating capability, i.e., can easily be designed for heat radiation. As a result, the VCU23can be reduced in size and weight.

When the converter controller54turns on the three U-, V-, W-phase arms UA, VA, WA of the DC/DC converter36, as described above with reference toFIGS. 5 through 7, it alternately turns on the three U-, V-, W-phase arms UA, VA, WA, i.e., turns on the upper arm switching device81uof the U-phase arm UA (FIGS. 5 through 7), for example, thereafter turns on the lower arm switching device82uof the U-phase arm UA (seeFIG. 7) after the dead time dt, thereafter turns on the upper arm switching device81vof the V-phase arm VA (seeFIGS. 5 through 7) after the dead time dt, and thereafter turns on the lower arm switching device82vof the V-phase arm VA (seeFIG. 7) after the dead time d5. In this manner, the converter controller54rotates the switching timings.

As described above, when the converter controller54alternately turns on the upper arm switching devices81and the lower arm switching devices82of the phase arms UA, VA, WA, it alternately turns them on with the dead time dt interposed therebetween, and alternately turns on the phase arms UA, VA, WA with the dead time dt interposed therebetween. In this manner, the upper arm switching devices81u,81v,81wand the lower arm switching devices82u,82v,82ware prevented from being short-circuited and hence the phase arms are prevented from being short-circuited.

FIG. 9is a timing chart of gate drive signals UH, UL, VH, VL, WH, WL in a rotation switching process according to another embodiment.

In the rotation switching process according to the other embodiment shown inFIG. 9, not all the gate drive signals UH, UL, VH, VL, WH, WL are rotated, but the gate drive signal VL is inactivated. As shown inFIG. 4A, the arm switching devices81u,81v,81w,82u,82v,82ware associated with the respective temperature sensors69. The lower arm switching device82v, for example, whose temperature as detected by the associated temperature sensor69is higher than a threshold temperature is temporarily disabled, and the rotation switching process is continued with the other arm switching devices81u,81v,81w,82u,82w. When the temperature of the lower arm switching device82vbecomes lower than the threshold temperature and returns to a normal range, the rotation switching process involving all the gate drive signals UH, UL, VH, VL, WH, WL is resumed. A gate drive signal to be inactivated is not limited to one arm. Three gate drive signals UH, VH, VL, for example, may be inactivated.

The intermittent rotation switching process shown inFIG. 9is applicable to a case where when a certain phase arm suffers a fault such as an open circuit, the faulty phase arm is not operated, but the other phase arms only are operated to continue the switching process. The intermittent rotation switching process makes the VCU23and hence the fuel cell vehicle20more reliable.

According to the present invention, as described above, when the converter controller54alternately turns on the three phase arms UA, VA, WA, the converter controller54alternately turns on either one of the upper arm switching devices81(81u,81v,81w) or either one of the lower arm switching devices82(82u,82v,82w) of the phase arms UA, VA, WA.

When the converter controller54alternately turns on the three phase arms UA, VA, WA, the converter controller54may alternately turn on an upper arm switching device and a lower arm switching device of a certain phase at random, and thereafter may alternately turn on a next upper arm switching device and a next lower arm switching device of a next phase at random. Alternatively, when the converter controller54alternately turns on the three phase arms UA, VA, WA, the converter controller54may alternately turn on them one in every 2π-switching period for easy control, or may alternately turn on them one in two switching periods4π or more.

FIG. 10shows a modified rotation switching process in which the converter controller54turns on the upper arm switching devices81(81u,81v,81w) and/or the lower arm switching devices82(82u,82v,82w) a plurality of times in one switching period 2π.

According to the embodiment described above, the upper arm switching devices81and the lower arm switching devices82are not simultaneously turned on, and the different phase arms UA, VA, WA are not simultaneously turned on. Rather, at most one switching device is turned on at all times. Therefore, the DC/DC converter36is of an excellent heat radiating capability.

As shown inFIG. 11, the principles of the present invention are also applicable to a fuel cell vehicle20A incorporating a two-phase DC/DC converter36A, rather than the three-phase DC/DC converter36. The present invention is thus applicable to a DC/DC converter having two or more phases, e.g., four or more phases.

The principles of the present invention are also applicable to a battery-driven vehicle (electric vehicle)21shown inFIG. 12, in addition to the fuel cell vehicles20,20A. The principles of the present invention are further applicable to a parallel or series parallel hybrid vehicle which incorporates an engine, a battery, and a motor.

The motor26is not limited to those for use on vehicles, but may be motors for use with elevators or the like.

As shown inFIG. 13, the principles of the present invention are also applicable to a fuel cell system20B which employs a single-phase load35instead of the inverter34, a load controller53instead of the motor controller52, a power supply switch65ainstead of the ignition switch65, and various sensors66a,67a,68ainstead of the sensors66,67,68. The general controller56controls the VCU23through the converter controller54for thereby controlling a load current IL.

As shown inFIG. 14, the principles of the present invention are also applicable to a fuel cell vehicle20C incorporating a DC/DC converter36B which includes three reactors90u,90v,90wconnected to the respective midpoints of the U-, V-, W-phase arms UA, VA, WA. For example, when the converter controller54is to turn on the multiphase arms of the DC/DC converter36B, it alternately turns on the phase arms UA, VA, WA, and when the converter controller54is to turn on the phase arms UA, VA, WA, it alternately turns on either one of the upper arm switching devices81u,81v,81wor either one of the lower arm switching devices82u,82v,82wof the phase arms UA, VA, WA. Therefore, the upper arm switching devices81u,81v,81wand the lower arm switching devices82u,82v,82ware not simultaneously turned on, and the different phase arms UA, VA, WA are not simultaneously turned on. Rather, at most one switching device is turned on at all times. Therefore, the DC/DC converter36B is of an excellent heat radiating capability, i.e., can easily be designed for heat radiation. As a result, the DC/DC converter apparatus23B can be reduced in size and weight.

The present invention is not limited to the above embodiments, but may be modified in various ways. For example,FIG. 15is a timing chart of a voltage reducing mode of a DC/DC converter apparatus, which corresponds toFIG. 5, according to still another embodiment. As shown inFIG. 15, insofar as the switching devices are used in a range of their rated currents (allowable device temperatures), the upper arm switching devices81u,81v,81wof the three phases are simultaneously turned on by the gate drive signals UH, UV, UW from time t31and then, after a dead time dt, the upper arm switching devices81u,81v,81wof the three phases are simultaneously turned on by the gate drive signals UH, UV, UW. From time t41, two of the three phases are simultaneously turned on and then, after a dead time dt, two of the three phases are simultaneously turned on. In the voltage increasing mode shown inFIG. 6and the alternate voltage increasing and reducing modes shown inFIG. 7, the switching devices may also be simultaneously turned on with dead times dt interposed.