Description of the Related Art Electrochemical fuel cells convert fuel and oxygen to electricity.

Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled to conduct electrons between the electrodes through an external circuit. Typically, a number of MEAs are electrically coupled in series to form a fuel cell stack having a desired power output.

In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have flow passages to direct fuel and oxygen to the electrodes, namely the anode and the cathode, respectively. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxygen, and provide channels for the removal of reaction products, such as water formed during the fuel cell operation. The fuel cell system may use the reaction products in maintaining the reaction. For example, reaction

water may be used for hydrating the ion exchange membrane and/or maintaining the temperature of the fuel cell stack.

The stack's capability to produce current flow is a direct function of the amount of available reactant. Increased reactant flow increases reactant availability. Stack voltage varies inversely with respect to the stack current in a non-linear mathematical relationship. The relationship between stack voltage and stack current at a given flow of reactant is typically represented as a polarization curve for the fuel cell stack. A set or family of polarization curves can represent the stack voltage-current relationship at a variety of reactant flow rates. Fuel cell stacks are generally more efficient under high loads. In typical applications, the desired output voltage is the controlling parameter, and the reactant flow is adjusted accordingly. This results in the fuel cell stack operating less efficiently (i. e., along a less than optimal polarization curve) than desired.

In most practical applications, it is desirable to maintain an approximately constant voltage output from the fuel cell stack. One approach is to employ a battery electrically coupled in parallel with the fuel cell system to provide additional current when the demand of the load exceeds the output of the fuel cell stack and to store current when the output of the fuel cell stack exceeds the demand of the load.

The many different practical applications for fuel cell based power supplies require a large variety of different power/voltage delivery capabilities.

Typically this requires using a fuel cell stack with a higher rating than actually required, or alternatively, specially designing the fuel cell stack for the particular application. In most instances, it is prohibitively costly and operationally inefficient to employ a power supply capable of providing more power than required by the application. It is also costly and inefficient to design, manufacture, validate, and maintain inventories of different power supplies capable of meeting the demand of each potential application (e. g. , 1 kW, 2 kW, 5 kW, 10 kW, etc. in power, 24V, 48V, etc. in voltage). Further, it is desirable to increase the reliability of the power supply without significantly increasing the

cost. Thus, a less costly, less complex, more flexible, and/or more efficient approach to fuel cell based power supplies is desirable.

BRIEF SUMMARY OF THE INVENTION In one aspect, a circuit to selectively provide power between a power source and a load comprises a main power converter having a primary side and a secondary side, the primary side of the main power converter electrically coupled directly to the power source without at least one of a switch and a diode therebetween, and the secondary side of the main power converter electrically couplable to the load ; and at least one controller coupled to control the main power converter. A power storage device may be electrically coupled in parallel across the secondary side of the main power converter to buffer power. Further, an auxiliary isolated power supply may be electrically coupled between the power storage device and at least one controller to provide power to the main converter controller and at least one controller to provide power to the main converter power stage and driver and to at least one first auxiliary load of the power supply, for example a fan such as such as a cooling fan of a fuel cell system. One or more switches may be selectively operable to electrically couple the auxiliary isolated power supply to the first auxiliary load in a first state and to alternatively electrically couple the power source to the first auxiliary load in a second state.

In another aspect a power supply that selectively provides power to a load comprises a fuel cell stack; a main isolated DC/DC converter comprising a transformer, a primary side and a secondary side, the primary side of the main isolated DC/DC converter electrically connected directly to the fuel cell stack; a power storage device electrically coupled to the secondary side of the main isolated DC/DC converter to receive power therefrom; and at least a first auxiliary fuel cell system component load alternatively electrically coupable to the fuel cell stack to receive power therefrom and the power storage device to receive power therefrom.

In yet another aspect, a power supply that selectively provides power to a load via voltage bus, comprises a fuel cell stack; a power bus to electrically couple at least one external load to the fuel cell stack, the power bus comprising a main isolated DC/DC converter wherein the main isolated DC/DC converter is the only on/off switching device between the fuel cell stack and the load ; and at least one controller coupled to control the main isolated DC/DC converter.

In a further aspect a method of selectively providing power to a load from a fuel cell stack comprises: electrically directly connecting a fuel cell stack to a main isolated DC/DC converter; selectively operating the main isolated DC/DC converter to supply power to the load a first time; and selectively stopping operation of the main isolated DC/DC converter to stop supplying power to the load at a second time.

In an even further aspect, a method of operating a fuel cell system comprising a fuel cell stack, a fan, a main isolated power converter, and a power converter controller comprises electrically coupling a power storage device in parallel across a secondary or output side of the main isolated DC/DC converter; supplying power to the power converter controller via the auxiliary power supply ; supplying power to the fan via the auxiliary power supply at a first time; and supplying power to the fan directly from a fuel cell stack without the use of the auxiliary power supply at a second time.

In even a further aspect a method of operating a power supply comprising a fuel stack, a fan and at least one power storage device comprises: in a startup state, supplying power to the fan from the power storage device via an auxiliary power supply ; and in a boost state, supplying power to the fan from a fuel cell stack, and enabling a main power converter to supply power to a load from the fuel cell stack via the main power converter. The method may further comprise in an idle state, supplying power to the fan from a fuel cell stack, and disabling the main power converter to prevent the supplying of power to the load from a fuel cell stack. The method may further include : in a failure state, disabling the main power converter to prevent the supplying of power to the

load from a fuel cell stack, and supplying power to the fan from the power storage device via the auxiliary power supply. The method may even further include in a standby state, operating the auxiliary power supply, stopping a reactant flow to the fuel stack to stop the fuel cell stack operation, and disabling the main power converter to prevent the supplying of power to the load from a fuel cell stack.

In an even further aspect, multiple power supplies may be electrically coupled in series and/or parallel, preferably in parallel, in a modular fashion to provide power at a different power rating and at a desired voltage.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements and angles are arbitrarily enlarged and positioned to improve drawing legibility.

Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.

Figure 1 is a schematic diagram of a fuel cell system powering an external load, the fuel cell system comprising a fuel cell stack, fan, main isolated power converter, isolated auxiliary power converter, power storage device, fuel cell controller, DC/DC controller and a pair of switches, according to one illustrated embodiment.

Figure 2 is a schematic diagram of the fuel cell system including a single throw, double pole switch, according to one alternative embodiment.

Figure 3A is a state transition diagram for operating the fuel cell system according to one illustrated embodiment.

Figures 3B and 3C are a state transition table for operating the fuel cell system according to the state transition diagram of Figure 3A.

Figure 4 is a schematic diagram of a number of fuel cell systems electrically coupled in series to supply a desired power a load at a desired voltage.

Figure 5 is a schematic diagram of a number of fuel cell systems electrically coupled in parallel to supply power a load at a desired voltage.

DETAILED DESCRIPTION OF THE INVENTION In the following description, certain specific details are set forth in order to provide a thorough understanding of the various embodiments of the present power converter architectures and methods. However, one skilled in the art will understand that the present power converter architectures and methods may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, fuel cell systems, reactant delivery systems, power storage devices such as batteries and"super"or"ultra" capacitors, temperature control systems, controllers, and power converters such as DC/DC converters, have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the present power converter architectures and methods.

Unless the context requires otherwise, throughout the specification and claims which follow, the word"comprises"and variations thereof, such as"comprises"and"comprising"are to be construed in an open, inclusive sense, that is as"including, but not limited to." Figure 1 shows a power supply 6 providing power to an external load 8 according to one illustrated embodiment of the present power converter architectures and methods. The external load 8 typically constitutes the device to be powered by the power supply 6, such as a vehicle, appliance, computer and/or associated peripherals, lighting, and/or communications equipment. The power supply 6 may also provide power to one or more internal loads, for example control electronics, as discussed below.

The power supply 6 comprises a fuel cell system 10, a main power converter 12, and a voltage bus 14.

Fuel cell system 10 comprises a fuel cell stack 16 composed of a number of individual fuel cells electrically coupled in series. The fuel cell stack 16 receives reactants, such as hydrogen and air via reactant supply systems (not shown) which may include one or more reactant supply reservoirs or sources, a reformer, and/or one or more control elements such as compressors, pumps and/or valves. Operation of the fuel cell stack 16 produces reactant product, typically including water. The fuel cell system 10 may reuse some or all of the reactant products. For example returning some of the water to the fuel cell stack 16 to humidify the hydrogen and air at the correct temperature, to hydrate the ion exchange membranes, and/or to control the temperature of the fuel cell stack 16. Operation of the fuel cell stack 16 produces a voltage VFC across rails 14a, 14b of the voltage bus 14. In some embodiments, the voltage bus 14 electrically couples the fuel cell stack directly to a primary side of the main power converter 12 without the use of any intervening switches or diodes. This takes advantage of galvanic isolation between the fuel cell stack 16 and load 8, discussed in detail below.

Eliminating unnecessary switches and diodes provides a number of benefits such as reducing the parts counts, reducing costs associated with high current rated devices such as high current rated power relays and high current rated diodes, and reducing the significant losses associated with such devices.

The fuel cell system 10 may include one or more controllers, such as fuel cell controller 18. The fuel cell controller 18 can take a variety of forms, for example, a microprocessor, application specific integrated circuit (ASIC), or other programmed or programmable integrated circuit and the like. The fuel cell controller 18 receives input from one or more customer interfaces 20 such as an ON/OFF switch, voltage adjusting switch, etc. The fuel cell controller 18 also receives operational data 22 for the fuel cell stack 16, for example, readings or measurements of temperature, reactant flows, and valve and/or switch conditions. The fuel cell controller 18 provides commands or stack control signals 24 to various actuators for controlling the operation of the fuel cell stack 16. For example, stack control signals 24 may actuate actuators

such as solenoids for opening and closing valves to start, stop or adjust reactant flows.

The fuel cell system 10 includes one or more fans, such as a cooling fan 26 that is selectively operable to provide an air flow 28 for maintaining the temperature of the fuel cell stack 16 within acceptable bounds or reactant supply fan for supplying fuel or oxidant (e. g., air or oxygen) to the fuel cell stack 16. The fuel cell controller 18 may control the cooling fan 26 via fan speed commands 30.

The main power converter 12 may take a variety of forms such as a full-bridge DC/DC converter, a half-bridge DC/DC converter, a forward DC/DC converter, or their derivatives. For example, the main power converter 12 may take the form of an isolated, full-bridge DC/DC converter power stage and driver electrically coupled on the voltage bus 14 between the fuel cell stack 16 and the load 8, as illustrated in Figures 1 and 2. In the illustrated embodiment, the main power converter 12 is operable to convert the DC voltage VFc produced by the fuel cell stack 16 to a desired DC output voltage VOUT suitable for the load 8.

A variety of DC/DC converter topologies may be suitable, which typically employ semiconductor switching devices in a circuit that uses an inductor, a transformer or a capacitor as an energy storage and filter element to transfer energy from the input to the output in discrete packets or pulses. For example, the DC/DC converter may employ a full-bridge DC/DC converter topology, a push-pull DC/DC converter topology, a half-bridge DC/DC converter topology, or a forward DC/DC converter topology. In particular, the main power converter 12 may employ a high switching frequency (e. g., 100kHz) approach, in order to reduce size, cost and weight. The details of these and other suitable converter topologies will be apparent to those of skill in the art. The main power converter 12 may rely on the galvanic isolation inherent in the transformer in the main power converter 12 to provide isolation between a primary side and a secondary side of the main power converter 12.

The main power converter 12 is operable under a variety of control techniques, such as frequency modulation, pulse-width modulation (i. e., PWM), average-current control, and peak-current control, as will be apparent to those of skill in the art.

The power supply 6 may include one or more power converter controllers to control the main power converter 12 via appropriate drivers, for example, a DC/DC controller 32. The DC/DC controller 32 may operate in conjunction with the fuel cell controller 18, communicating data and/or commands therebetween. For example, the fuel cell controller 18 may provide to the DC/DC controller 32: a voltage reference signal 34 representing the value of a desired output voltage VOUT, a fan enable signal 36 identifying a state <BR> <BR> (e. g. , ON/OFF; High, Medium, Low) of the cooling fan 26, a DC/DC enable<BR> signal 38 identifying a desired state (e. g. , ON/OFF) of the main power converter<BR> 12, and/or a wakeup signal 40 identifying a state (e. g. , ON/OFF) of main power converter 12. The DC/DC controller 32 may provide a status signal 42 to the fuel cell controller 18 identifying an operational status of the DC/DC controller 32 and/or main power converter 12. The DC/DC controller 32 may also receive feedback signals 44 from the main power converter 12. The DC/DC controller 32 produces control signals, such as pulse width modulated signals 46, to control the operation of the main power converter 12 via appropriate drivers.

Since some embodiments directly couple the fuel cell stack 16 to the main power converter 12 without any intervening switches and/or diodes, the operation of the main power converter 12 serves as the ON/OFF control between the fuel cell stack 16 and main power converter 12 and/or load 8.

Thus, power from the fuel cell stack 16 can be turned ON and OFF by enabling and disabling the main power converter 12.

The power supply 6 may also include an power storage device 48, such as a"super"or"ultra"capacitor and/or a battery, electrically coupled in parallel across the load 8, at the output side of the main power converter 12.

The open circuit voltage of the power storage device 48 is selected to be similar to the desired maximum output voltage of the power supply 6. An internal

resistance of the power storage device 48 is selected to be much lower than an internal resistance of the main power converter 12, thus the power storage device 48 acts as a buffer, absorbing excess current when the fuel cell stack 16 produces more current than the load 8 requires, and providing current to the load 8 when the fuel cell stack 16 produces less current than the load 8 requires. The coupling of the power storage device 48 across the load 8 reduces the maximum power rating requirement of the fuel cell stack 16. The power storage device 48 also supplies energy to the internal loads of the power supply 6 when the fuel cell stack 16 is, for example, in a startup state, failure state and/or standby state, as more fully discussed below.

The power supply 6 includes an auxiliary power converter 50 to provide power to the various internal loads of the fuel cell system 10. For example, the auxiliary power converter 50 may provide power to the main power converter 12, the DC/DC controller 32 and/or the fuel cell controller 18.

A single auxiliary power converter 50 may also supply power to other internal loads of the fuel cell system for example the cooling fan 26. Thus, the architecture of the power supply 6 takes advantage of the existing auxiliary <BR> <BR> power converter used to power the control circuitry (e. g. , DC/DC controller 32, fuel cell controller 18) to eliminate a dedicated cooling fan power supply typically found in fuel cell systems. The auxiliary power converter 50 may take the form of a widely-used flyback converter. The auxiliary power converter 50 may be isolated, for example, relying on the galvanic isolation associated with the flyback transformer in the auxiliary power converter 50, to provide protection between the remainder of the power supply 6 and/or the load 8.

The power supply 6 may employ one or more switches selectively operable to supply power to the cooling fan 26 directly from the fuel cell stack 16, or alternatively, supply power to the cooling fan 26 via the auxiliary power converter 50. For example, a first switch SW1 may electrically couple the cooling fan 26 to the voltage bus 14 in a closed state, and electrically uncouple the cooling fan 26 from the voltage bus 14 in an open state. A second switch SW2 may electrically couple the cooling fan 26 to the auxiliary power converter

50 in a closed state, and electrically uncouple the cooling fan 26 from the auxiliary power converter 50 in an open state. The DC/DC controller 32 may control the state (e. g., ON/OFF) of the switches SW1, SW2 in response to the fuel cell controller 18. The power supply 6 may further include a pair of diodes D1, D to protect against reverse current flow.

Figure 2 shows an alternative embodiment of the power supply 6.

This alternative embodiment, and those alternative embodiments and other alternatives described herein, are substantially similar to previously described embodiments, and common acts and structures are identified by the same reference numbers. Only significant differences in the operation and structure are described below.

In particular, the power supply 6 of Figure 2 employs a single switch SW3 in place of the first and second switches SW1, SW2, and a single diode D3. The switch SW3 is selectively operable to alternatively electrically couple the cooling fan 26 directly to the fuel cell stack 16 or to the power storage device 48 via the auxiliary power converter 50. This alternative embodiment may be simpler to operate and less costly than the embodiment of Figure 1, but may not be capable of functioning under several of the operating states discussed below.

Figure 3A is a state transition diagram and Figures 3B and 3C are a state transition table illustrating a state machine 100 for operating the power supply 6.

The state machine 100 involves a variety of states or operating modes, some of which are activated by a user selecting an appropriate control on the customer interface 20, and others which are automatically entered via the fuel cell controller 18 and/or DC/DC controller 32 in response to certain operating conditions.

Figure 3A shows the valid transitions for the state machine 100.

For example, the power supply 6 may transition from an off state 102 to a standby state 104. The power supply 6 may transition from the standby state 104 to the off state 102 or to a startup state 106. The power supply 6 may

transition from the startup state 106 to the standby state 104, to a fault state 108, or to an idle state 110. The power supply 6 may transition from the fault state 108 to the standby state 104. The power supply 6 may transition from the idle state 110 to the fault state 108 or to a boost state 112. The power supply 6 may transition from the boost state 112 to the fault state 108 or the idle state 110.

The above transitions are represented by arrows on the state transition diagram (Figure 3A), each of the arrows having a reference number that identifies the transitions in the state transition table (Figures 3B and 3C).

The off state 102 is the beginning state for the power supply 6. In the off state 102 the various subsystems such as the fuel cell stack 16, main power converter 12, fuel cell controller 18, cooling fan 26, DC/DC controller 32 and/or auxiliary power converter 50 are not operating.

The standby state 104 maintains the controllers in an operational state after receiving the wake-up command, while the housekeeping power supply for controllers is activated, and controllers in power supply 6 are awake and ready to communicate with customer interface 20. The standby state 104 may be activated by an appropriate user input via the customer interface 20.

To enter the standby state 104, the fuel cell controller 18 causes the DC/DC controller 32 to open the first switch SW1, if not already open, to electrically uncouple the cooling fan 26 from the fuel cell stack 16. The fuel cell controller 18 also causes the DC/DC controller 32 to open the second switch SW2, if not already open, to electrically uncouple the cooling fan 26 from the auxiliary power converter 50. The fuel cell controller 18 further disables the fuel cell stack 16, for example, by stopping reactant flow to the fuel cell stack 16. The fuel cell controller 18 further causes the DC/DC controller 32 to disable the main power converter 12.

The startup state 106 may be entered in response to the user selecting an appropriate ON/OFF switch, or the automatic sensing of a loss of power from an independent power source such as a public or private electrical grid. The startup state 106 may allow the various subsystems of the power

supply 6 to come up to operational levels, for example, allowing the fuel cell stack 16 to come up to its open circuit voltage VFC. To enter the startup state 106, the fuel cell controller 18 causes the DC/DC controller 32 to open the first switch SWr, if not already open, in step 106 to electrically uncouple the cooling fan 26 from the voltage bus 14. The fuel cell controller 18 also causes the DC/DC controller 32 to close the second switch SW2, if not already closed, to electrically couple the cooling fan 26 to the power storage device 48 to receive power via the auxiliary power converter 50.

The fault state 108 may be entered when one or more operating values goes out of bounds or some other erroneous condition occurs, the failure state protecting the various subsystems of the power supply 6, as well as the load 8. To enter the fault state 108, the fuel cell controller 18 causes the DC/DC controller 32 to open the first switch SW1, if not already open, to electrically uncouple the cooling fan 26 from the voltage bus 14. The fuel cell controller 18 also causes the DC/DC controller 32 to disable the main power converter 12. The fuel cell controller 18 further causes the DC/DC controller 32 to close the second switch SW2, if not already closed, to electrically couple the cooling fan 26 to the power storage device 48 via the auxiliary power converter 50.

The idle state 110 may be entered to maintain the power supply 6 in an operational state, while the load 8 does not require power. The idle state 110 may be activated by an appropriate user input via the customer interface 20, or by automatically sensing of the loss of load 8. To enter the idle state 110, the fuel cell controller 18 causes the DC/DC controller 32 to open the second switch SWz, if not already open, to electrically uncouple the cooling fan 26 from the auxiliary power converter 50. The fuel cell controller 18 also causes the DC/DC controller 32 to close the first switch SW1, if not already closed, to electrically couple the cooling fan 26 directly to the fuel cell stack 16 via the voltage bus 14. The fuel cell controller 18 further causes the DC/DC controller 32 to disable the main power converter 12.

The boost state 112 may be entered once the power supply 6 is fully operational, to supply power to the load 8. The boost state 112 may be activated by an appropriate user input via the customer interface 20, or by automatically sensing of the load 8. To enter the boost state 112, the fuel cell controller 18 causes the DC/DC controller 32 to open the second switch SW2, if not already open, to electrically uncouple the cooling fan 26 from the auxiliary power converter 50. The fuel cell controller 18 also causes the DC/DC controller 32 to close the first switch SW1, if not already closed, to electrically couple the cooling fan 26 directly to the fuel cell stack 16 via the voltage bus 14.

The fuel cell controller 18 further causes the DC/DC controller 32 to provide PWM signals 46 to the main power converter 12, enabling the main power converter 12 in order to supply power to the load 8 from the fuel stack 16.

The above teachings may be implemented in a modular approach to providing power supply systems of a large variety of output powers and voltages, as illustrated in Figures 4 and 5.

Figure 4 shows a number of power supplies 61-6n electrically coupled in series on a voltage bus 14 to power a load 8. The ellipses indicate that any number of additional power supplies may be electrically coupled between the first power supply 6 and the nth power supply 6n. This modular approach allows customers to reconfigure a power supply system of a n times output power at n times output voltage while utilizing the same fuel cell stack design and the same power supply 6 module.

Figure 5 shows a number of power supplies 61-6n electrically coupled in parallel on a voltage bus 14 formed by voltage rails 14a, 14b to power a load 8. This modular approach allows a customer to reconfigure a power supply system of a n times output power at the same voltage, while utilizing the same fuel cell stack design and the same power supply 6 module.

The embodiments of Figures 4 and 5 can be combined in various arrangements of series and parallel coupled to provide a modular approach to the manufacture, validation, and distribution of power supply systems.

The disclosed embodiments may provide a number of advantages over existing systems. For example, the above described approaches may reduce the time required to produce a suitable power supply system that meets a customer's specific desired power and voltage requirements. Having a power supply system more closely tailored to the actual load requirements and/or capable of adjusting the output voltage via a power converter saves costs since fewer cells are required in the fuel cell stack 16, and since only a relatively few, or even only one, standard fuel cell stack 16 must be designed, validated, manufactured, inventoried and distributed. Further, having a power supply system more closely tailored to the actual load requirements allows the fuel cell stack 16 to operate more efficiently. Use of the power converter to adjust the voltage, allows the fuel cell stack 16 to operate at maximum load, independent of the desired load voltage, also allowing the fuel cell stack to operate more efficiently along the optimum polarization curve. As noted above, the elimination of costly and lossy high voltage switches and/or diodes also adds to the savings in cost and efficiency. As further discussed above, the elimination of a dedicated power supply for the fan provides significant cost and efficiency savings. The coupling of the power storage device 48 across the load 8 provides significant saving by reducing the maximum power rating of the fuel cell stack 16. Even further, the main power converter 12 may from time-to-time, or as required, generate a current pulse to decontaminate the fuel cell stack 16, thereby improving fuel cell stack performance.

Although specific embodiments of, and examples for, the power supply are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the present power converter architectures and methods, as will be recognized by those skilled in the relevant art. The teachings provided herein can be applied to other fuel cell systems, not necessarily the exemplary fuel cell systems generally described above.

The various embodiments described above can be combined to provide further embodiments. All of the above U. S. patents, patent applications

and publications referred to in this specification, including but not limited to, commonly assigned pending U. S. Patent Applications Serial No. 10/017,480, entitled"Method and Apparatus for Controlling Voltage From a Fuel Cell System" (Attorney Docket No. 130109.436) ; Serial No. 10/017,462, entitled "Method and Apparatus for Multiple Mode Control of Voltage From a Fuel Cell System" (Attorney Docket No. 130109.442) ; and Serial No. 10/017,461, entitled"Fuel Cell System Multiple Stage Voltage Control Method and Apparatus" (Attorney Docket No. 130109.446), all filed December 14,2001 ; Serial No. 60/421,126, entitled"Adjustable Array Of Fuel Cell Systems In Power Supply"filed May 16,2002 (Atty. Docket No. 130109. 449P1) ; and serial No. 60/436,759, entitled"Electric Power Plan With Adjustable Array Of Fuel Cell Systems"filed December 17,2002 (Atty. Docket No. 130109.449P2), are all incorporated herein by reference, in their entirety. Aspects of the present power converter architectures and methods can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments of the present power converter architectures and methods. Suitable methods of operation may include additional steps, eliminate some steps, and/or perform some steps in a different order. For example, the fuel cell controller 18 may employ a different order for determining the operating state, and/or for opening and closing the switches SW1, SW2.

These and other changes can be made to the present power converter architectures and methods in light of the above-detailed description.

In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all fuel cell systems that operate in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.