Power supply system for a vehicle, and method

The invention relates to a power supply system for a vehicle having a DC/DC converter arrangement for connection of a fuel cell apparatus to an energy storage apparatus, and to a corresponding method for operation of the power supply system.

Power supply systems which produce power at least temporarily and autonomously normally have a power generator and an energy storage apparatus. The power generator is used to generate power during autonomous operation, and the energy storage apparatus is used to compensate for reduced power supplies at times. In this case, it is necessary for the power generator to charge the energy storage apparatus when excess power is available during operation, and for the energy storage apparatus to emit power when the power supply is insufficient.

One known technical problem in this case is that the operating voltage of the power generator and of the energy storage apparatus fluctuates in particular as a function of the power being consumed or of the fuels being supplied. It is known for so-called buck converters and boost converters to be connected between the power generator and the energy storage apparatus in order to adapt the various operating voltages which result from this. In the case of a buck

converter, the output voltage is always less than the input voltage. By way of example, this is used when the operating voltage of the power generator is higher than that of the energy storage apparatus. In the opposite situation, in which the operating voltage of the power generator is less than the operating voltage of the energy storage apparatus, the boost converter is used to set the output voltage to be higher than the input voltage.

In one entirely different technical specialist field, specifically, the voltage supply for a digital camera, the document JP 2006042461-A proposes a buck-boost voltage supply in which the inrush current surge can be suppressed by clocked driving of a plurality of switches.

The document EP 1 146 629 A2 describes a buck-boost switched- mode regulator which allows an output voltage of the switched-mode regulator to be set to a higher, lower or identical voltage value in comparison to its input voltage. The input voltage is made available by a voltage source, and the output voltage is applied to a load.

The document DE 198 53 626 Al describes a switched-mode regulator and a method for operation of the switched-mode regulator, as is used, for example, in mobile radio telephones. In this case as well, a circuit arrangement is proposed for supplying power, which increases or decreases a DC voltage without any transformers, and produces a regulated voltage on the output side.

The document EP 1 388 927 A2 also discloses a switched-mode regulator, with this document describing the matching of the input voltage from a voltage source to an output voltage for a load.

Furthermore, DC/DC converter arrangements are known for connecting a fuel cell apparatus to an energy storage apparatus, such as those which are already in use nowadays.

The document JP 63277470 AA, which actually forms the closest prior art, describes a power generating system having a fuel battery, which is connected to a load via a matching circuit, in order to supply the load with a stable predetermined electrical power. The matching circuit has diode-based switching elements which allow the current to flow in only one direction.

The invention is based on the object of proposing a power supply system which is distinguished by simple design and good usage characteristics.

This object is achieved by a power supply system having the features of Claim 1 with a DC/DC converter arrangement or a high-voltage intermediate circuit - referred to jointly in the following text as a DC/DC converter arrangement - and a method for operation of the power supply system having the features of Claim 11. Preferred and/or advantageous embodiments are described in the dependent claims and in the following description, and/or the attached figures.

The power supply system according to the invention has a DC/DC converter arrangement which is suitable and/or designed to connect a fuel cell apparatus to an energy storage apparatus. As will also be explained further below, the fuel cell apparatus is preferably a fuel cell system for a vehicle and, in particular, uses PEM technology. The energy storage apparatus may be in the form of a rechargeable battery or a capacitor, in particular in the form of a so-called supercap.

In principle, the DC/DC converter arrangement can be used wherever the output voltage range and input voltage range overlap, for example in a DC voltage intermediate circuit or as a high-voltage intermediate circuit.

The DC/DC converter arrangement has a voltage input and a voltage output, with the expressions voltage input and voltage output being chosen for better representation. The DC/DC converter arrangement can be used bidirectionally so that the voltage input and the voltage output can interchange their functions with one another. The voltage input is preferably designed and/or connected as the interface for connection of the fuel cell apparatus, and the voltage output is preferably designed and/or connected as the interface for connection of the energy storage apparatus.

The DC/DC converter arrangement has at least one induction component which allows an inductance to be converted and, in particular, is in the form of a coil and/or an inductor. The induction component is functionally connected such that, during operation of the DC/DC converter arrangement, it contributes to a voltage reduction in an output voltage produced at the voltage output in a buck mode, and contributes to a voltage increase in the output voltage in a boost mode. This function is already known from the known buck converters and boost converters .

The induction component is arranged and/or connected in a simple and at the same time advantageous manner centrally between a buck section and a boost section of the DC/DC converter arrangement. Preferably, the induction component is even provided only at the position between the buck section and the boost section. The induction component can

nevertheless be formed from a plurality of individual induction units.

The expressions buck section and boost section should also be understood in the descriptive form since - as already stated - the DC/DC converter arrangement can be used bidirectionally, so that the functions of the buck section and boost section can be interchanged, and/or are interchanged during operation.

The central arrangement of the single induction component in the DC/DC converter arrangement on the one hand saves components since there is no need to distribute a plurality of induction components in a decentralized form in a DC/DC converter arrangement. Secondly, the central arrangement of the induction component greatly simplifies the design of the DC/DC converter arrangement, thus simplifying, for example, the choice of the individual components in the circuit. Furthermore, the proposed DC/DC converter arrangement has high efficiency which may be up to more than 96%.

In one preferred embodiment, the buck section has a first switching element which is connected in series with the induction component and the voltage input and/or output, and a second switching element which is connected to earth.

The boost section preferably likewise has a third switching element which is connected to earth, and a fourth switching element which is connected in series with the induction component and the voltage output and/or input .

In one alternative, simplified embodiment, the boost section has one switching or the third switching element, which is connected to earth, and a diode which is connected in series

to the induction component and to the voltage output and/or input and is used instead of one or of the fourth switching element. The switching elements are, for example, MOSFET or IGBT modules and/or switching elements which can be clocked and, in particular, can be clocked quickly via control electronics. Provision is preferably made for a freewheeling diode to be connected in parallel with each of the switching elements or at least one switching element.

It is particularly preferable for the DC/DC converter arrangement to be designed to be topologically symmetrical, with the induction component forming the axis or point of symmetry. This particular embodiment indicates that - as already mentioned - the expressions voltage input, voltage output, boost section and buck section are each intended only for descriptive purposes, and their functions can in each case be interchanged. Specifically, this is because the DC/DC converter arrangement can be operated bidirectionally so that a power current can flow either from the so-called voltage input to the so-called voltage output, or in the opposite direction.

The switching elements which can be clocked are clocked by a control device, preferably using a clock frequency of more than 5 kHz, in particular of more than 50 kHz, for example in the region of 100 kHz. The switching elements are preferably driven with square-wave signals or, alternatively, with sawtooth or triangular waveform signals. The duty cycle indicates the ratio of the length of the switched-on state (pulse duration) to the period duration of the square-wave signal. A matched definition can be chosen for signals with different waveforms.

The DC/DC converter arrangement can preferably be used to achieve any desired choice of the following operating modes:

Charging buck operating mode

In this case, power flows from the voltage input to the voltage output, that is to say, in the connected state, from the fuel cell apparatus to the energy storage apparatus. The first and second switching elements in the buck section are switched alternately. In the case of square-wave signals, this can be done for example by means of an inverter. The third switching element is switched on or quasi-on by appropriate clocking (in particular 100% duty cycle) , and the fourth switching element is switched off or quasi-off by clocking (in particular 0% duty cycle) . The bucking level, that is to say the voltage reduction in the input voltage applied to the voltage input with respect to the output voltage produced at the voltage output, is set by the ratio of the duty cycle of the first switching element to the duty cycle of the second switching element . The greater the voltage difference which is intended to be set between the input voltage and the output voltage, the higher is the duty cycle of the second switching element .

Charging boost operating mode

Once again, this is based on power flowing from the voltage input to the voltage output, with the input voltage which is applied to the voltage input being less than the output voltage which is produced at the voltage output . In this operating mode, the third and fourth switching elements are switched alternately, and the first switching element is switched on or quasi-on by clocking (in particular 100% duty

cycle) , and the second switching element is switched off or quasi-off by clocking (0% duty cycle) .

When changing from the charging buck operating mode to the charging boost operating mode, that is to say for example when the operating voltage of the fuel cell apparatus falls to the operating voltage of the energy storage apparatus, or below it, the duty cycle component of the first switching element is first of all increased in steps to 100%, and the duty cycle of the second switching element is reduced in the opposite direction to 0%. When the input voltage once again falls, the component of the duty cycle of the third switching element is increased in steps starting from 0% and the component of the duty cycle of the fourth switching element is reduced from 100% in the opposite direction. This results in a smooth transition from the charging buck operating mode to the charging boost operating mode.

The invention relates in particular to the power supply system for a vehicle, which has a fuel cell system and an energy storage apparatus, characterized in that the fuel cell system and the energy storage apparatus as well as the DC/DC converter arrangement are designed according to one of the preceding claims, or as just described.

In this embodiment, in particular, it is possible to provide for the fourth switching element and/or the second switching element to be replaced by a single diode.

A further object of the invention relates to a method for operation of the DC/DC converter arrangement and/or of the power supply system, with the control device driving one of the described operating modes alternately or successively, as required.

In principle, the described DC/DC converter arrangement can be used for any desired control strategy.

One particularly preferred control strategy, in particular for the use of the power supply system, is for the control to be in the form of a control process, with the switching elements being driven on the basis of a measured input voltage and a variable output voltage.

Another control strategy relates to regulation, in particular cascaded regulation which is designed in particular as average-current regulation. It may be found to be particularly advantageous for a plurality of the arrangements to be operated with offset clocking and the same input and output capacitance C.

Further advantages, features and effects of the invention will become evident from the following description of one preferred exemplary embodiment and/or on the basis of the attached figure, in which:

Figure 1 shows a block diagram of a first exemplary embodiment of the invention in the form of a power supply system for a vehicle with a DC/DC converter arrangement .

Figure 1 shows a schematic block diagram of a power supply system 1 for a vehicle. The power supply system 1 has a fuel cell system 2 which, for example, is in the form of a PEM fuel cell system, as well as a battery 3 which, for example, is in the form of a supercap. Other embodiments of the fuel cell system 2 and of the battery 3 are also possible.

The fuel cell system 2 and the battery 3 are connected to one another by a DC/DC converter 4. The DC/DC converter 4 has a voltage input 5 and a voltage output 6.

Starting from the fuel cell system 2, which has a first output 7 which is connected to earth or ground and has a second output 8 which, in comparison to the output 7 is at, for example, an operating voltage of 20 V to 40 V, one connection leads to the voltage input 5 of the DC/DC converter 4.

Within the DC/DC converter 4, a connection leads from the voltage input 5 to a first junction point at which, for example, the input voltage U-IN can be tapped off by measurement. A first branch from the first junction point is connected to earth via a capacitor Cl. Starting from the first junction, the line leads on to a first switching unit Tl . The switching unit Tl has a switching element and a freewheeling diode connected in parallel with it. The switching element can be driven clocked by means of a driver gate drive 1.

Starting from the switching unit Tl, the line then leads on to a second junction point. A first branch from the second junction point leads via a switching unit T2 , which is designed analogously to the switching unit Tl and is driven by a driver gate drive 2, to earth or ground.

A second branch from the second junction point leads to an induction component Ll which, for example, is in the form of a coil and/or inductor. In one possible configuration of the coil or inductor, the inductance is in the region above 12 μH, at 80 amperes.

Starting from the induction component Ll, the line then leads to a third junction point. A first branch of the third junction point leads via a switching unit T3 , which is designed analogously to the switching units Tl and T2 , and is driven via a driver gate drive 3, to earth or ground. A second branch of the third junction point leads to a fourth switching element T4 , which is designed analogously to the switching elements Tl, T2 and T3 , and is driven via a driver gate drive 4.

Starting from the fourth switching unit T4 , the line leads to a fourth junction point. A measurement point for the output voltage U-Out is preferably provided at this junction point. A first branch of the fourth junction point leads via a second capacitor C2 through to earth or ground. A second branch of the fourth junction point leads to the voltage output 6, which has already been mentioned above and is also connected to the battery 3.

A control unit 10 is used to drive the driver gate drives 1 to 4 and drives them with a pulse-width-modulated square-wave signal at a frequency in the region of 100 kHz, as will be described in the following text, as well.

From the functional point of view, the described DC/DC converter 4 is able to adapt the voltage applied to the voltage input 5 and the voltage produced at the voltage output 6 such that the DC/DC converter provides not only a buck converter but also a boost converter, that is to say a buck-boost converter, overall. The described symmetrical topology, which is designed to be symmetrical with respect to the induction component Ll, allows voltage adaptation not only when power is flowing from the fuel cell system 2 to the battery 3, but also in the opposite direction.

Two operating modes will be explained in the following text, with the power flowing from the fuel cell 2 to the battery 3. However, because of the symmetrical structure, corresponding operating modes are possible when the power is flowing in the opposite direction.

In a buck operating mode the voltage of the fuel cell system 2 is higher than the voltage of the battery 3.

In the buck operating mode, the duty cycle for the drive for the fourth switching unit T4 is 100% so that the switching unit is switched on or quasi-on. The third switching unit T3 , in contrast, is driven with a duty cycle of 0%, so that it is switched or clocked to be off or quasi-off . The first switching unit Tl and the second switching unit T2 are clocked in the same way as in a conventional buck converter, with the switching units Tl and T2 being activated in synchronism with the opposite senses, which means that, when the switching unit Tl is opened, the switching unit T2 is closed without delay, and vice versa. This drive reduces the input voltage U-IN to the output voltage U-Out in a known manner.

In the situation in which more power is being drawn from the fuel cell system 2 or the supply of fuels to the fuel cell system 2 is reduced, the voltage of the fuel cell system 2 falls. In order to match the falling input voltage to the desired output voltage, the duty cycle of the first switching unit Tl is increased to an ever greater extent and, in consequence, the duty cycle of the second switching unit T2 is reduced.

As soon as the voltage of the fuel cell system 2 and of the battery 3 are virtually the same, to be more precise when the voltage of the battery 3 is higher than the voltage of the fuel cell system 2 by the voltage losses in the converter arrangement, the duty cycle of the switching unit Tl reaches 100%, and the duty cycle of the switching unit T2 reaches 0%.

If the voltage of the fuel cell system 2 falls further, the duty cycle of the switching unit T3 is increased from 0% initially while, in contrast, the duty cycle of the switching unit T4 is reduced in a corresponding manner. This results in a smooth transition from the buck operating mode to the boost operating mode.

In the boost operating mode, the voltage of the fuel cell system 2 is lower than the voltage of the battery 3. In this operating mode, the duty cycle of the switching unit Tl is 100%, while, in contrast, the duty cycle of the switching unit T2 is reduced to 0%. The switching units T3 and T4 operate in synchronism but in opposite senses, as in a boost converter that is known from the prior art, with the switching unit T3 in each case being closed when the switching unit T4 is open analogously to the buck operating mode .

Therefore, as illustrated, the output voltage is matched to the input voltage by distribution of the components of the duty cycle between the switching units Tl and T2 in the buck operating mode, and between the switching units T3 and T4 in the boost operating mode.

This driving of the switching units Tl, T2 , T3 and T4 takes place via a control device 9 which produces the appropriate square-wave signals. In principle, any type of closed-loop

and/or open-loop control can be used for control of the control device 9.

In a first embodiment, a control process is used in which a control signal is calculated by means of a characteristic on the basis of the input voltage U-In and the desired output voltage U-Out.

A second embodiment comprises the regulation of the output voltage, based on an "Average-Current-Mode Control" concept which is known in specialist circles and can be optimized in terms of speed and stability by optional pilot control.