Converter controller

Provided is a converter controller capable of preventing destruction of an element such as an auxiliary switch by preventing operation interference between auxiliary circuits of respective phases in a multiphase soft switching converter. A duty threshold input unit receives, as an input, an obtained acceptable duty deviation value. A duty deviation computation unit judges whether or not the duty deviation between the phases does not exceed an acceptable duty deviation value. When the duty deviation between the phases exceeds the acceptable duty deviation value, the duty deviation computation unit corrects an adjusted U-phase duty ratio, adjusted V-phase duty ratio and adjusted W-phase duty ratio under the PID control rule, and outputs the resultant duty ratios to an FC converter control circuit. On the other hand, when the duty deviation between the phases does not exceed the acceptable duty deviation value, the duty deviation computation unit does not correct the adjusted U-phase duty ratio, adjusted V-phase duty ratio and adjusted W-phase duty ratio and outputs them to the FC converter control circuit.

This is a 371 national phase application of PCT/JP2009/060714 filed 11 Jun. 2009, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a converter controller that controls an output voltage of a fuel cell.

BACKGROUND ART

Regarding fuel cell systems mounted on automobiles, etc., various hybrid fuel cell systems each having, as a power source, a fuel cell and a battery, have been proposed in order to deal with a rapid change in a load that exceeds the power generation capability of the fuel cell.

In a hybrid fuel cell system, an output voltage of a fuel cell and an output voltage of a battery are controlled by a DC/DC converter. As a DC/DC converter for carrying out such a control, a DC/DC converter having a configuration in which a switching element such as a power transistor, IGBT or FET is made to perform a PWM operation for voltage conversion has been used widely. Together with reduced power consumption, reduced size and increased performance of electronic equipment, a reduced loss, increased efficiency and reduced noise of the DC/DC converter have further been demanded. In particular, reductions of a switching loss and a switching surge that are associated with the PWM operation have been demanded.

One of the techniques for reducing the above switching loss and switching surge is a soft switching technique. Here, soft switching refers to a switching method for realizing ZVS (Zero Voltage Switching) or ZCS (Zero Current Switching), the soft switching involving a low switching loss of a power semiconductor device and a low stress given to the semiconductor device. Meanwhile, a switching method for directly turning on/off a voltage/current with a switching function of a power semiconductor device is referred to as hard switching. In the descriptions below, a method in which both or one of ZVS and ZCS is realized is referred to as soft switching, whereas the other methods are referred to as hard switching.

Soft switching is realized by a general pressure increase/decrease DC/DC converter provided with, for example, an inductor, a switching element and a diode, the DC/DC converter being additionally provided with an auxiliary circuit for reducing a switching loss (the so-called soft switching converter) (see, for example, Patent Document 1).

Meanwhile, in order to realize enhanced speed, increased capacity and reduced ripple, a multiphase DC/DC converter (multiphase converter) in which a plurality of DC/DC converters are connected in parallel has been used in the related art.

Regarding such a multiphase converter, when employing a soft switching converter as a converter of each phase, this can attain the enhanced speed and increased capacity, but might lead to upsizing of the converter. In light of this problem, it can be considered that a component of an auxiliary circuit included in the soft switching converter of each phase, e.g. an auxiliary coil, can be made to serve as a common coil shared by the converters of the phases. Accordingly, downsizing of the multiphase soft switching converter can be attained.

PRIOR ART REFERENCE

Patent Document

Patent Document 1: JP2005-102438 A

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

However, in a multiphase soft switching converter, when a current for two or more phases flows to an auxiliary coil due to operation interference between the auxiliary circuits of the respective phases, the inductance characteristic of the auxiliary coil deteriorates.

The reason for the above is described below. An auxiliary coil is generally designed such that the maximum acceptable current Imax is set assuming the flow of a current for one phase (seeFIG. 16); however, when a current Iu equal to or larger than the maximum acceptable current Imax (i.e., a current for two or more phases) flows to the auxiliary coil due to the operation interference between the auxiliary circuits of the phases, the inductance characteristic of the auxiliary coil deteriorates. This has caused the problem of the flow of a current equal to or larger than a rated current to a circuit element (e.g., switching element) other than the auxiliary coil included in the auxiliary circuit, leading to, in the worst case scenario, destruction of an element.

The present invention has been made in light of the above circumstances, and an object of the invention is to provide a converter controller capable of preventing destruction of an element such as an auxiliary switch by preventing operation interference between auxiliary circuits of the respective phases in a multiphase soft switching converter.

Means for Solving the Problem

In order to solve the problem described above, the present invention provides a converter controller for a multiphase soft switching converter including auxiliary circuits of respective phases, the converter controller controlling an output voltage of a fuel cell, including: a calculation unit that calculates a duty ratio of an auxiliary switch included in each of the auxiliary circuits of the phases; a deviation derivation unit that derives a duty deviation between the auxiliary switches of the phases; and a control unit that controls the duty ratio of the auxiliary switch of each phase so that the derived duty deviation does not exceed a preset threshold value, in which the auxiliary circuits of the phases include an auxiliary coil, the auxiliary coil being a common coil shared by the auxiliary circuits of all the phases.

Regarding the multiphase soft switching converter, with such a configuration, the duty deviation between the auxiliary switches of the phases is derived, and the duty ratio of the auxiliary switch of each phase is controlled so that the derived duty deviation does not exceed the preset threshold value. Accordingly, the operation interference between the auxiliary circuits of the phases is prevented, whereby the occurrence of a circuit failure (element destruction, etc.) can be prevented.

In the configuration above, it is preferable that a converter of each of the phases includes a main voltage increasing circuit and the auxiliary circuit, that the main voltage increasing circuit includes: a main coil in which one end of the ends is connected to a high-potential-side terminal of the fuel cell; a main switch that performs switching and in which: one end of the ends is connected to the other end of the main coil; and the other end is connected to a low-potential-side terminal of the fuel cell; a first diode in which a cathode is connected to the other end of the main coil; and a smoothing capacitor provided between an anode of the first diode and the other end of the main switch, and that the auxiliary circuit includes: a first series connection including a second diode and a snubber capacitor, the first series connection being connected in parallel with the main switch and being connected to the other end of the main coil and the low-potential-side terminal of the fuel cell; and a second series connection including a third diode, the auxiliary coil and the common auxiliary switch, the second series connection being connected between a connecting part between the second diode and the snubber capacitor and the one end of the main coil.

Further, in the configuration above, it is preferable that: the converter of each phase includes a free-wheel diode for keeping, when the auxiliary switch is turned off while the auxiliary coil is being energized, a current flowing in the same direction as that during the energization; and the free-wheel diode includes an anode terminal connected to the low-potential-side terminal of the fuel cell and a cathode terminal connected to a connecting part between the auxiliary coil and the auxiliary switch.

Further, in the configuration above, it is preferable that the preset threshold value is represented by expression (10) below:

where Dth represents the preset threshold value, f represents a drive frequency of the auxiliary switch, n represents the number of drive phases, and Tso represents an energization time period of the auxiliary coil.

Further, in the configuration above, it is preferable that the energization time period Tso of the auxiliary coil is represented by expression (11) below.

The present invention provides another converter controller, for controlling an output voltage of a fuel cell, for a multiphase soft switching controller including auxiliary circuits of respective phases, in which: the auxiliary circuits of the phases include an auxiliary coil, the auxiliary coil being a common coil shared by the auxiliary circuits of all the phases; and a lower-limit energization capacitance of the auxiliary coil is set to be larger than the total current value of currents that flow for the phases when an auxiliary switch of each of the phases is turned on.

Effect of the Invention

According to the present invention, in a multiphase soft switching converter, operation interference between auxiliary circuits of the respective phases is prevented, thereby preventing destruction of an element such as an auxiliary switch.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment according to the present invention will be described with reference to the drawings.FIG. 1shows the configuration of an FCHV system mounted on a vehicle according to the embodiment. Although the following description assumes a fuel cell hybrid vehicle (FCHV) as an example of vehicles, the FCHV system may also be applied to electric vehicles, etc. In addition, the FCHV system may be applied not only to the vehicles but also to various mobile objects (e.g., ships, airplanes and robots), stationary power supplies and mobile fuel cell systems.

A-1. Overall Configuration of System

In an FCHV system100, an FC converter2500is provided between a fuel cell110and an inverter140, and a DC/DC converter (hereinafter referred to as a battery converter)180is provided between a battery120and the inverter140.

The fuel cell110is a solid polymer electrolyte cell stack in which a plurality of unit cells are stacked in series. The fuel cell110is provided with a voltage sensor V0for detecting an output voltage Vfcmes of the fuel cell110and a current sensor I0for detecting an output current Ifcmes. In the fuel cell110, the oxidization reaction represented by formula (1) occurs at an anode, and the reduction reaction represented by formula (2) occurs at a cathode. The electromotive reaction represented by formula (3) occurs in the entire fuel cell110.
H2→2H++2e−(1)
(1/2)O2+2H++2e−→H2O  (2)
H2+(1/2)O2→H2O  (3)

A unit cell has a configuration in which: a polymer electrolyte membrane, etc., is interposed between two electrodes, a fuel electrode and an air electrode, to form an MEA; and the MEA is interposed between separators for supplying fuel gas and oxidant gas. In the anode, an anode catalyst layer is provided on a porous support layer, and in the cathode, a cathode catalyst layer is provided on a porous support layer.

The fuel cell110has a system for supplying the fuel gas to the anode, a system for supplying the oxidant gas to the cathode and a system for providing a coolant, which are not shown in the figure, and is able to generate desired power by controlling the fuel gas supply and the oxidant gas supply according to control signals from a controller160.

The FC converter2500has a role of controlling the output voltage Vfcmes of the fuel cell110, and is a bidirectional voltage converter that converts (increases or decreases) the output voltage Vfcmes input to a primary side (input side, fuel cell110side) to have a voltage value different from the primary side and outputs it to a secondary side (output side, inverter140side), and conversely converts a voltage input to the secondary side to have a voltage different from the secondary side and outputs it to the primary side. The FC converter2500controls the output voltage Vfcmes of the fuel cell110to be a voltage in accordance with a target output.

The battery120is connected in parallel with the fuel cell110with respect to a load130, and functions as a storage source of surplus power, a storage source of regenerated energy during regeneration braking, and an energy buffer during a load variation as a result of acceleration or deceleration of a fuel cell vehicle. The battery120is constituted by a secondary battery such as a nickel/cadmium battery, a nickel/hydrogen battery or a lithium secondary battery.

The battery converter180has a role of controlling an input voltage of the inverter140, and has the same circuit configuration as that of, e.g., the FC converter2500. A voltage increase converter may be employed as the battery converter180, and a voltage increase/decrease converter that can perform a voltage increasing operation and a voltage decreasing operation may also be employed. A converter having any configuration that can control the input voltage of the inverter140may be employed.

The inverter140is, for example, a PWM inverter driven by a pulse width modulation method. In accordance with a control command provided by the controller160, the inverter140converts direct-current power output from the fuel cell110or the battery120to three-phase alternating current power, thereby controlling a rotation torque of a traction motor131.

The traction motor131serves as the main motive power of the vehicle in this embodiment, and it also generates regenerative power during deceleration. A differential132is a decelerator, decelerating a high-speed rotation of the traction motor131to a predetermined rotation frequency and rotating a shaft to which tires133are provided. The shaft has a wheel speed sensor (not shown), etc., thereby detecting the vehicle speed, etc., of the vehicle. In this embodiment, all the equipment (including the traction motor131and the differential132) operable upon receiving power supplied from the fuel cell110are collectively referred to as the load130.

The controller160is a computer system for controlling the FCHV system100, and has a CPU, RAM, ROM, etc. The controller160receives, as inputs, various signals (e.g., a signal representing an acceleration opening degree, a signal representing a vehicle speed, and a signal representing an output current or output terminal voltage of the fuel cell110) supplied from a sensor group170, and obtains the power required from the load130(i.e., the power required for the entire system).

The power required from the load130corresponds to the total value of, for example, vehicle driving power and auxiliary-apparatus power. The auxiliary-apparatus power includes, e.g., power consumed by vehicle-mounted auxiliary apparatuses (humidifier, air compressor, hydrogen pump, cooing water circulation pump, etc.), power consumed by devices necessary for vehicle driving (change gear, wheel control device, steering device, suspension device, etc.), and power consumed by devices arranged in an occupant space (air-conditioning device, illumination device, audio equipment, etc.).

The controller (converter controller)160determines an output power distribution ratio of the fuel cell110and the battery120and computes a power generation command value. The controller160calculates the power required for the fuel cell110and the battery120, and then controls the operations of the FC converter2500and the battery converter180in order to obtain the above required power.

A-2. Configuration of FC Converter

As shown inFIG. 1, the FC converter2500has a circuit configuration of a three-phase resonance converter which includes a U-phase, a V-phase and a W-phase. The circuit configuration of the three-phase resonance converter has a combination of a circuit portion similar to an inverter, which temporarily converts an input direct-current voltage to an alternating current, and a portion that rectifies the alternating current again and converts it to a different direct-current voltage. In this embodiment, a multiphase soft switching converter having a free-wheel circuit (described below in detail) (hereinafter referred to as a multiphase FC soft switching converter) is employed as the FC converter2500.

A-2-1. Description of Multiphase FC Soft Switching Converter

FIG. 2is a diagram illustrating a circuit configuration of the multiphase FC soft switching converter2500mounted on the FCHV system100.FIG. 3is a diagram illustrating a circuit configuration for one phase of the multiphase FC soft switching converter2500.

In the description below, the respective FC soft switching converters of the U-phase, V-phase and W-phase, which constitute the multiphase FC soft switching converter2500, are referred to as FC soft switching converters250a,25band250c, and when there is no particular need to distinguish these converters, they each are simply referred to as an FC soft switching converter250. Also, a voltage before voltage increase which is input to the FC soft switching converter250is referred to as an input voltage Vin, and a voltage after voltage increase which is output from the FC soft switching converter250is referred to as an output voltage Vout.

As shown inFIG. 3, each of the FC soft switching converters250is provided with a main voltage-increasing circuit22afor performing a voltage increasing operation, an auxiliary circuit22bfor performing a soft switching operation and a free-wheel circuit22c.

In the main voltage-increasing circuit22a, with a switching operation of a switching circuit constituted by a first switching element S1constituted of, e.g., an IGBT (Insulated Gate Bipolar Transistor) and a diode D4, energy stored in a coil L1is released to the load130via a diode D5, thereby increasing the output voltage of the fuel cell110.

More specifically, one end of the coil L1is connected to a high-potential side terminal of the fuel cell110, one of the electrodes of the first switching element S1is connected to the other end of the coil L1, and the other electrode of the first switching element S1is connected to a low-potential side terminal of the fuel cell110. Also, a cathode terminal of the diode D5is connected to the other end of the coil L1, and a capacitor Cd that functions as a smoothing capacitor is connected between an anode terminal of the diode D5and the other end of the first switching element S1. The main voltage-increasing circuit22ais provided with a smoothing capacitor C1on the fuel cell110side, thereby reducing a ripple of an output current of the fuel cell110.

Here, a voltage VH applied to the capacitor C3serves as the converter output voltage Vout of the FC soft switching converter150, and a voltage VL applied to the smoothing capacitor C1serves as an output voltage of the fuel cell110, and also serves as the converter input voltage Vin of the FC soft switching converter150.

The auxiliary circuit22bincludes a first series connection having: a diode D3connected in parallel with the first switching element S1; and a snubber capacitor C2serially connected to the diode D3. In the first series connection, a cathode terminal of the diode D3is connected to the other end of the coil L1, and an anode terminal of the diode D3is connected to one of the ends of the snubber capacitor C2. The other end of the snubber capacitor C2is connected to the low-potential side terminal of the fuel cell110.

Further, the auxiliary circuit22bincludes a second series connection constituted by a diode D2, a second switching element S2, a diode D1and an auxiliary coil L2that is common to the phases.

In the second series connection, an anode terminal of the diode D2is connected to a connecting part between the diode D3and the snubber capacitor C2in the first series connection. Also, a cathode terminal of the diode D2is connected to one of the electrodes of the second switching element (auxiliary switch) S2. The other electrode of the second switching element S2is connected to a connecting part between the auxiliary coil L2and the free-wheel circuit22c. An anode terminal of a free-wheel diode D6is connected to the low-potential side terminal of the fuel cell110, while a cathode terminal of the free-wheel diode D6is connected to the auxiliary coil L2. The free-wheel circuit22cincludes the free-wheel diode D6shared by the phases, and is a circuit for realizing a fail-safe function which is provided to prevent the occurrence of a surge voltage that destructs the second switching element S2even when an open fault of the second switching element S2occurs while the auxiliary coil L2is energized. Note that the present invention may be applied also to a configuration not including the free-wheel circuit22c.

In the FC soft switching converter25configured as described above, the controller160adjusts a switching duty ratio of the first switching element S1of each phase, thereby controlling a ratio of the voltage increased by the FC soft switching converter25, i.e., the ratio of the converter output voltage Vout to the converter input voltage Vin. Also, the switching operation of the first switching element S1is combined with the switching operation of the second switching element S2in the auxiliary circuit12b, thereby realizing soft switching.

Next, a soft switching operation by the FC soft switching converter25will be described with reference toFIGS. 4 to 8.FIG. 4is a flowchart showing a cycle of processing of the FC soft switching converter25via a soft switching operation (hereinafter referred to as soft switching processing), and the controller160carries out steps S101to S106inFIG. 4sequentially so as to form one cycle. In the description below, the modes that represent the current/voltage states of the FC soft switching converter25are denoted by mode1to mode6, the states being illustrated inFIGS. 5 to 8. InFIGS. 5 to 8, the currents flowing through circuits are indicated by arrows.

Soft Switching Operation

First, the initial state of the soft switching processing shown inFIG. 4is the state where power required by the fuel cell110for the load130is being supplied, i.e., the state where both the first switching element S1and the second switching element S2are turned off so that a current is supplied to the load130through the coil L1and the diode D5.

In step S101, while the turn-off of the first switching element S1is held, the second switching element S2is turned on. By performing such a switching operation, the current flowing on the load130side gradually moves to the auxiliary circuit12bside through the coil L1, the diode D3, the second switching element S2and the auxiliary coil L2due to the potential difference between the output voltage VH and the input voltage VL of the FC soft switching converter150. Note that, inFIG. 5, the state of the movement of the current from the load130side to the auxiliary circuit12bside is indicated by an outline arrow.

The second switching element S2is turned on, whereby a current circulation in the direction of arrow Dm11shown inFIG. 5is generated. Here, the rate of current change of the second switching element S2increases in accordance with the voltage across the auxiliary coil L2(VH−VL) and the inductance of the auxiliary coil L2; however, the current flowing to the second switching element S2is reduced by the auxiliary coil L2. As a result, a soft turn-off of the current flowing to the load130side through the diode D5(see arrow Dm12inFIG. 5) is realized.

Here, a shift completion time tmode1for a shift from mode1to mode2is represented by expression (4) below.

t⁢⁢mod⁢⁢e⁢⁢1=max⁡(Ip-Δ⁢⁢I2,0)×L⁢⁢2⁢id(VH-VL)(4)
Ip: phase current
L2id: inductance of the auxiliary coil L2

After the above shift completion time passes and the processing proceeds to step S102, the current flowing through the diode D5becomes zero, and the current flows to the auxiliary circuit12bside via the coil L1and the diode D5(see arrow Dm21inFIG. 6). Meanwhile, the charge stored in the snubber capacitor C2flows to the auxiliary circuit12bside due to the potential difference between the voltage VH of the snubber capacitor C2and the voltage VL of the fuel cell110(see arrow Dm22inFIG. 6). The voltage applied to the first switching element S1is determined in accordance with the capacitance of the snubber capacitor C2.

Here,FIG. 12is a diagram illustrating voltage/current behavior in the process of a shift from mode2to mode3, where: the voltage of the fuel cell110is indicated by a thick solid line; the voltage of the snubber capacitor C2is indicated by a thin solid line; and the current of the snubber capacitor C2is indicated by a dashed line.

After energization of the path of Dm21inFIG. 6is started (see (A) inFIG. 12), energization of the path of Dm22inFIG. 6, i.e., energization to the auxiliary coil L2is started due to the potential difference between the voltage VH of the snubber capacitor C2and the voltage VL of the fuel cell110(see (B) inFIG. 12). Here, as shown inFIG. 12, the current of the snubber capacitor C2continues to increase until the voltage of the snubber capacitor C2reaches the voltage VL of the fuel cell110. More specifically, the charge stored in the snubber capacitor C2starts to be regenerated on the power supply side due to the potential difference between the voltage VH of the snubber capacitor C2and the voltage VL of the fuel cell110(arrow Dm22inFIG. 6). However, the original potential difference is (VH−VL), and therefore, the flow of the electric charge (discharge) stored in the snubber capacitor C2stops when the voltage VH reaches the power supply voltage (i.e., the voltage VL of the fuel cell110) (see point in time T1inFIG. 12); meanwhile, the characteristic of the auxiliary coil L2(i.e., the characteristic that keeps a current flowing continuously) keeps the electric charge flowing even when the voltage of the snubber capacitor C2is equal to or lower than the voltage VL (see (C) inFIG. 12). At this point, if expression (4)′ below holds, the entire electric charge of the snubber capacitor C2flows (discharges).

12⁢L*I2>12⁢C*V2(4)′
Left side: energy stored in the auxiliary coil L2
Right side: energy remaining in the snubber capacitor C2

When the electric charge stored in the snubber capacitor C2is emptied out of the snubber capacitor C2, a free-wheel operation is performed using the path of Dm23inFIG. 6to continue energization (see (D) inFIG. 12). As a result, all the energy stored in the auxiliary coil L2is discharged. Note that the LC resonance is half-wave resonant because the anode of the diode D2is connected to the relevant end of the auxiliary coil L2. Therefore, the snubber capacitor C2holds 0 V after discharge.

Here, a shift completion time tmode2for a shift from mode2to mode3is represented by expression (5) below.

C2d: capacitance of the capacitor C2

When the operation in which a current flows in the Dm22path inFIG. 6, and the electric charge is emptied out of the snubber capacitor C2or is at the minimum voltage (MIN voltage), the first switching element S1is turned on, and the processing proceeds to step S103. With the voltage of the snubber capacitor C2being zero, the voltage applied to the first switching element S1is also zero, and thus ZVS (Zero Voltage Switching) is attained. In such a state, a current II1flowing through the coil L1corresponds to the sum of a current Idm31flowing on the auxiliary circuit12bside which is indicated by arrow Dm31and a current Idm32flowing via the first switching element S1which is indicated by arrow Dm32(see expression (6) below).
Il1=Idm31+Idm32  (6)

The current Idm31flowing through the first switching element S1is determined in accordance with the rate of reduction of the current Idm31flowing to the auxiliary circuit12bside. The rate of current change of the current Idm31flowing to the auxiliary circuit12bside is represented by expression (7) below. More specifically, the current Idm31flowing to the auxiliary circuit12bside is reduced at the rate of change of expression (7) below, and therefore, even with the first switching element S1being turned on, the current flowing through the first switching element S1does not rise rapidly. As a result, ZCS (Zero Current Switching) is attained.

In step S104, the amount of current flowing to the coil L1is increased due to the continuation of the state of step S103, thereby gradually increasing energy stored in the coil L1(see arrow Dm42inFIG. 8). Here, the diode D2exists in the auxiliary circuit12b, and therefore, an inverse current does not flow to the auxiliary coil L2, so that the snubber capacitor C2is not charged via the second switching element S2. Also, the first switching element S1has been turned on at this point, and therefore, the snubber capacitor C2is not charged via the diode D3. Accordingly, the current for the coil L1equals the current for the first switching element S1, and the energy stored in the coil L1is increased gradually. Here, a turn-on time Ts1of the first switching element S1is approximately represented by expression (8) below.
Ts1=(1−VL/VH)*Tcon(8)

Tcon: control period

Note that the control period refers to a time period during which soft switching processing is performed with the procedure of from steps S101to step S106being assumed as one period (one cycle).

After the desired energy is stored in the coil L1in step S104, the first switching element S12is turned off, and then a current flows through the path indicated by arrow Dm51inFIG. 9. Here,FIG. 11is a diagram illustrating the relationship in mode5between the voltage of the snubber capacitor C2in mode5(hereinafter referred to as a snubber capacitor voltage) Vc, the voltage applied to the first switching element S1(hereinafter referred to as element voltage) Ve, and the current flowing through the first switching element S1(hereinafter referred to as element current) Ie. When the above switching operation is performed, the snubber capacitor C2, which has been emptied of its electric charge and brought into a low-voltage state in mode2, is charged, whereby the snubber capacitor voltage Vc rises toward the converter output voltage VH of the FC soft switching converter150. At this point, the rate of increase of the element voltage Ve is suppressed by the charging of the snubber capacitor C2(i.e., the rising edge of the element voltage is slowed), and as a result, a ZVS operation for reducing the switching loss in an area of the element current Ve where a tail current exists (see a inFIG. 11) becomes possible to be performed.

After the snubber capacitor C2is charged to reach the voltage VH, the energy stored in the coil L1is released to the load130side (see arrow Dm61inFIG. 9). A turn-off time period Ts2of the first switching element S1is approximately represented by expression (9) below.
Ts2=(VL/VH)*Tcon(9)

By performing the soft switching processing described above, while the switching loss of the FC soft switching converter150is reduced as much as possible, the output voltage of the fuel cell110can be increased to a desired voltage so that the relevant voltage is supplied to the load130.

FIG. 13is a diagram illustrating energization patterns in the respective modes of the FC soft switching converter25, where: the current flowing through the coil L1is indicated by a thick solid line; and the current flowing though the auxiliary coil L2is indicated by a dashed line.

As shown inFIG. 13, when the second switching element is turned on, the auxiliary circuit12boperates, so that a current flows through the auxiliary coil L2(see mode1and mode2inFIG. 13). When time periods during which a current flows in the auxiliary coil L2(hereinafter referred to as auxiliary circuit operating time periods) Tso overlap between the FC soft switching converters25of the respective phases, the operations of the auxiliary circuits of the respective phases interfere with one another, and as a result, a current Iu equal to or larger than the maximum acceptable current Imax (i.e., a current of two or more phases) flows through the auxiliary coil L2, leading to deterioration of the inductance characteristic of the auxiliary coil L2(see the section of the problem to be solved by the invention andFIG. 16).

In order to solve the above problem, in this embodiment, the deviation between the duty ratios set for the second switching elements S2of the respective phases is controlled not to exceed an acceptable duty deviation value Dth indicated by expression (10) below.

f: drive frequency of the switching element S2

n: number of drive phases

Here, the auxiliary circuit operating time period Tso is represented by expression (11) below.

In this embodiment, a control is carried out such that the duty deviation between the phases does not exceed the acceptable duty deviation value Dth obtained using expression (10). More specifically, the control is carried out such that a U-phase duty ratio D(u), a V-phase duty ratio D(v) and a W-phase duty ratio D(s) satisfy expressions (12) to (14) below.
D(v)−D(u)<Dth(12)
D(w)−D(v)<Dth(13)
D(u)−D(w)<Dth(14)

Here, the acceptable duty deviation time period Tth of each phase will be described with reference toFIG. 13, taking, as an example, the case of a duty ratio of each phase being 50%.FIG. 14is a diagram illustrating waveforms of duty ratio control pulses in the FC soft switching converters250of the three phases with a phase shift in the order of the U-phase, through the V-phase, to the W-phase.

The duty ratio control pulses of the phases inFIG. 14are generated by a pulse generator (not shown) that generates triangular waves each of which is phase-shifted by 120 degrees. These duty ratio control pulses control the duty ratio of the second switching element S2that constitutes the auxiliary circuit22bof the U-phase, V-phase or W-phase.

As shown inFIG. 14, the acceptable duty deviation time period Tth of each phase for the case of a duty ratio of 50% is represented by expression (10)′.
Tth=Tso−Tsc/n(10)′

As described above, in this embodiment, the DC/DC converter20is controlled such that the duty deviation between the phases does not exceed the acceptable duty deviation value Dth represented by expression (10) (in other words, the duty deviation time period between the phases does not exceed the acceptable duty deviation time period Tth represented by expression (10)′). This prevents operation interference between the auxiliary circuits22cof the phases, thereby solving a problem in the related art, i.e., preventing the occurrence of a circuit failure (e.g., element destruction). Hereinafter, a control of the duty ratio of the second switching element S2for preventing the operation interference between the auxiliary circuits22cof the phases (hereinafter referred to as interference prevention duty control) will be described in detail with reference to the functional blocks inFIG. 15.

Interference Prevention Duty Control

FIG. 15is a functional block diagram explaining an interference prevention duty control function realized by the controller160, etc. As described above, this embodiment assumes the case where the output of the fuel cell110is controlled using the three-phase resonance type FC soft switching converter2500that includes the U-phase, V-phase and W-phase.

An FC-request power input unit210derives a request power command value with respect to the fuel cell110(hereinafter referred to as an FC-request power command value) Preq based on the power required from the load130, and outputs the derived value to a command current computation unit240.

An FC voltage input unit220receives, as an input, the output voltage Vfcmes of the fuel cell110which is detected by the voltage sensor V0, and outputs the output voltage Vfcmes to the command current computation unit240and a deviation computation unit250.

An FC measurement power input unit230receives, as an input, an actual output power measurement value of the fuel cell110(hereinafter referred to as an FC output power measurement value) Pfcmes, and outputs the actual output power measurement value to the deviation computation unit250. Here, the FC output power measurement value Pfcmes may be obtained from the output voltage Vfcmes of the fuel cell110, which is detected by the voltage sensor V0, and the output current Ifcmes of the fuel cell110, which is detected by the current sensor10. Also, the FC output power measurement value Pfcmes may be directly obtained using, e.g., a power meter (second measurement unit).

The command current computation unit240derives a request current command value with respect to the fuel cell110(hereinafter referred to as an FC-request current command value) Iref by, for example, dividing the FC-request power command value Preq provided by the FC-request power unit210by the output voltage Vfcmes of the fuel cell110provided by the FC voltage input unit220. The command current computation unit240then outputs the derived FC-request current command value Iref to a command current correction unit260.

The deviation computation unit250obtains a power deviation (difference) between the FC-request power command value Preq and the FC output power measurement value Pfcmes, and outputs the obtained power deviation to a PID correction computation unit270.

The PID correction computation unit270computes a correction Icrt of the request current command value with respect to the fuel cell110based on the power deviation output from the deviation computation unit250under a PID control rule, and outputs the calculated correction Icrt to the command current correction unit260.

The command current correction unit260adds the correction output from the PID correction computation unit270(PID correction factor) Icrt to the FC-request current command value Iref output from the command current computation unit240, thereby producing an adjusted FC current command value Iamref. The command current correction unit260then outputs the produced adjusted FC current command value Iamref to a phase current distribution unit280.

The phase current distribution unit280derives a target current command value of each phase by dividing the adjusted FC current command value Iamref by the number of drive phases that maximizes the conversion efficiency of the FC converter150. Here, the number of drive phases that maximizes the conversion efficiency of the FC converter150depends on the power required for the fuel cell110, operating environment, etc. (hereinafter collectively referred to as “operation status”). Therefore, the correspondence relationship between the operation status and the number of drive phases that maximizes the conversion efficiency of the FC converter150is obtained in advance through experiments, etc., and is formed into a map. The obtained map is held as a number-of-drive-phases determination map. Upon receiving the adjusted FC current command value Iamref from the command current correction unit260, the phase current distribution unit280recognizes the operation status of the fuel cell110; determines the number of drive phases that maximizes the conversion efficiency of the FC converter150under the current operation status by referring to the number-of-drive-phases determination map; and divides the adjusted FC current command value Iamref by the determined number of drive phases, thereby deriving the target current command value of each phase, more specifically, a target U-phase current value Iref(u), a target V-phase current d value Iref(v) and a target W-phase current value Iref(w).

A U-phase measurement current input unit290areceives, as an input, a U-phase reactor current measurement value Ilmes(u) detected by a current sensor11, and outputs the U-phase reactor current measurement value Ilmes(u) to a U-phase deviation computation unit300a. The U-phase deviation computation unit300aobtains a U-phase current deviation by subtracting the U-phase reactor current measurement value Ilmes(u) from the target U-phase current value Iref(u).

A U-phase PID correction computation unit310acomputes a U-phase duty ratio correction Dcrt(u) based on the U-phase current deviation output from the U-phase deviation computation unit300aunder the PID control rule, and outputs the computed U-phase duty ratio correction Dcrt(u) to a U-phase duty ratio correction unit330a.

A U-phase basic duty ratio input unit320areceives, as an input, a U-phase basic duty ratio Ds, and outputs the U-phase basic duty ratio Ds to the U-phase duty ratio correction unit330a. Here, the U-phase basic duty ratio Ds is derived using expression (15) below. Note that the basic duty ratio Ds is constant regardless of phase (i.e., common to the U-phase, V-phase and W-phase), and thus is hereinafter simply referred to as the basic duty ratio Ds if not otherwise specified.
Ds=(VH−VL)/VH(15)
VH: inverter input voltage (high-potential side voltage)
VL: FC voltage (low-potential side voltage)

The first U-phase duty ratio correction unit (calculation unit)330aadds the U-phase duty ratio correction Dcrt(u) output from the U-phase PID correction computation unit310ato the U-phase basic duty ratio Ds output from the U-phase duty ratio input unit320a, thereby producing an adjusted U-phase duty ratio Dam(u). The first U-phase duty ratio correction unit330athen outputs the produced adjusted U-phase duty ratio Dam(u) to an interference prevention duty control circuit340.

Although the above description has been made with the U-phase operation control serving as an example, the same controls are also carried out regarding the V-phase and the W-phase. These controls are briefly described below. A V-phase PID correction computation unit310bcomputes a V-phase duty ratio correction Dcrt(v) based on the V-phase current deviation output from the V-phase deviation computation unit300bunder the PID control rule, and then outputs the computed V-phase duty ratio correction Dcrt(v) to a first V-phase duty ratio correction unit330b. The first V-phase duty ratio correction unit (calculation unit)330badds the V-phase duty ratio correction Dcrt(v) output from the V-phase PID correction computation unit310bto the V-phase basic duty ratio Ds output from the W-phase duty ratio input unit320b, thereby generating an adjusted V-phase duty ratio Dam(v). The first V-phase duty ratio correction unit330boutputs the produced adjusted V-phase duty ratio Dam(v) to the interference prevention duty control circuit340.

Similarly, a W-phase PID correction computation unit310ccomputes a W-phase duty ratio correction Dcrt(w) based on the W-phase current deviation output from the W-phase deviation computation unit300cunder the PID control rule, and then outputs the computed W-phase duty ratio correction Dcrt(w) to a first W-phase duty ratio correction unit330c. The first W-phase duty ratio correction unit (calculation unit)330cadds the W-phase duty ratio correction Dcrt(w) output from the W-phase PID correction computation unit310cto the W-phase basic duty ratio Ds output from the W-phase duty ratio input unit320c, thereby generating an adjusted W-phase duty ratio Dam(w). The first W-phase duty ratio correction unit330coutputs the produced adjusted W-phase duty ratio Dam(w) to the interference prevention duty control circuit340.

Interference Prevention Duty Control Circuit340

The interference prevention duty control circuit340includes a duty deviation computation unit341and a duty threshold input unit342.

The duty threshold input unit342receives, as an input, the acceptable duty deviation value obtained using expression (10) above. Meanwhile, the duty deviation computation unit (deviation derivation unit)341judges whether or not the duty deviation between the phases does not exceed the acceptable duty deviation value Dth by substituting the input adjusted U-phase duty ratio Dam(u), adjusted V-phase duty ratio Dam(v) and adjusted W-phase duty ratio Dam(w) in expressions (12) to (14) above (see expressions (12)′ to (14)′ below).
Dam(v)−Dam(u)<Dth(12)′
Dam(w)−Dam(v)<Dth(13)′
Dam(u)−Dam(w)<Dth(14)′

When the computation results do not satisfy expressions (12)′ to (14)′, the duty deviation computation unit341performs a correction such that the adjusted U-phase duty ratio Dam(u), the adjusted V-phase duty ratio Dam(v) and the adjusted W-phase duty ratio Dam(w) are corrected under the PID control rule so as to satisfy expressions (12)′ to (14)′, and outputs the resultant duty ratios as a U-phase interference prevention duty ratio Du, a V-phase interference prevention duty ratio Dv and a W-phase interference prevention duty ratio Dw to an FC converter control circuit350. On the other hand, when the computation results satisfy expressions (12)′ to (14)′, the duty deviation computation unit341does not correct the adjusted U-phase duty ratio Dam(u), the adjusted V-phase duty ratio Dam(v) and the adjusted W-phase duty ratio Dam(w), and outputs them as the U-phase interference prevention duty ratio Du, the V-phase interference prevention duty ratio Dv and the W-phase interference prevention duty ratio Dw to the FC converter control circuit350. Note that, if expressions (12)′ to (14)′ are satisfied, the adjusted U-phase duty ratio Dam(u), the adjusted V-phase duty ratio Dam(v) and the adjusted W-phase duty ratio Dam(w) may be corrected under the PID control rule.

The FC converter control circuit (control unit)350controls the operations of the auxiliary circuits22bby setting the U-phase interference prevention duty ratio Du, the V-phase interference prevention duty ratio Dv and the W-phase interference prevention duty ratio Dw output from the interference prevention duty control circuit340as the duty ratios of the second switching elements S2of the respective phases. The interference prevention duty control described above can prevent the operation interference between the auxiliary circuits22cof the phases, thereby preventing the occurrence of a circuit failure (element destruction, etc.).

Although the occurrence of a circuit failure is prevented by preventing the operation interference between the auxiliary circuits22cof the phases in this embodiment, the occurrence of a circuit failure may be prevented by, for example, setting the maximum acceptable current Imax (see the section of the problem to be solved) of the auxiliary coil L2constituting the auxiliary circuit22cto have a value that allows a current for a number of phases to flow.

For example, when the three-phase resonance type FC soft switching converter250that includes the U-phase, V-phase and W-phase as inFIG. 2is employed, the maximum acceptable current Imax of the auxiliary coil L2(seeFIG. 16; lower-limit energization capacitance) is set to be larger than a current for the three phases. As a result, even if the operation interference between the auxiliary circuits22bis caused for some reason, resulting in the current for the three phases (total current value) flowing to the auxiliary coil L2, this does not lead to the deterioration of the inductance characteristic of the auxiliary coil L2because the maximum acceptable current Imax of the auxiliary coil L2has been set to be larger than the current for the three phases. Accordingly, such a configuration also can prevent a problem in that a current equal to or larger than a rated current flows to another circuit element (e.g., the switching element) that constitutes the auxiliary circuit, leading to, in the worst case scenario, destruction of an element.

DESCRIPTION OF SYMBOLS