Abstract:
A voltage supplying apparatus using fuel cell to be a voltage source is provided. The voltage supplying apparatus comprises a fuel cell, a DC-DC voltage converter and a control circuit. The DC-DC voltage converter is used to receive the voltage outputted from the fuel cell and then output another voltage. Then, the voltages outputted from the fuel cell and the DC-DC voltage converter are combined to be the output voltage of the voltage supplying apparatus. The control circuit is used to control the operation of the DC-DC voltage converter according to magnitude of the output voltage of the voltage supplying apparatus.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
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   BACKGROUND OF THE INVENTION 
   The invention relates to a type of voltage supplying apparatus specifically designed to use fuel cell as the voltage supplying source for the apparatus. 
   With the diminishing of global energy source and the rising of environmental protection, conventional energy sources such as gasoline and combustion are becoming more inadequate. Therefore, many researches have been focusing on the development of new energy sources, fuel cell being the main focus. 
   Emphasis has been put on fuel cell because of its unique features that are in tune with the modern trend for energy: efficiency, cleanness, and quietness. The efficiency of fuel cell system is extremely high, exceeding over 40% efficiency. Combine with cogeneration technique to recycle the heat produced from the reaction and fuel cell can achieve 80% efficiency. In term of cleanness, fuel cell does not produce any kind pollutants during the energy producing process, including voice pollution. Fuel cell can be applied to different fields including electricity, industry, transportation, space industry, and military. Application can be found in power plant, back-up battery, portable power supply, forklift, robot, electric cars, small submarine and even powers for space shuttles. 
   But fuel cell itself still has some problems, one of them being polarized dependency loss. Due to the inner chemical characteristic of fuel cells, when fuel cells are connected to a load, the voltage of the connected side is easy to alter with the load current. Also, the ratio of alteration can reach 50%. When the load current increases, the amount of change of the fuel cell also increases. Therefore, usually the direct output voltage of the fuel cell is not used. Instead, techniques of power electronics will be used first to stabilize output voltage from the fuel cell. Then, the stabilized voltage will be output for further use. For example, the technique of high frequency switching can be applied, using a DC-DC voltage converter to stabilize the voltage output by the fuel cell. 
     FIG. 1  demonstrates including a DC voltage supplying device  100 , a DC-AC voltage converter, and an Auxiliary Battery  108 . DC-AC voltage converter  206  can accept DC voltage supplied by DC voltage supplying device  100  or Auxiliary Battery  108 . Then, it can convert the accepted DC voltage to AC output voltage  110 . This AC output voltage  110  can be used on many applications, such as e-vehicles or household electricity supplies, etc. 
   Special attention should be paid that the voltage source output from DC voltage supplying device  100  is produced by a fuel cell  102 . However, the voltage output from fuel cell  102  is not directly used as the output voltage for DC voltage supplying device. Intead, it has to be stabilized by a DC-DC voltage converter  104  before it can be used as the output voltage for DC voltage supplying device  100 . Fuel cell  102  and DC-DC voltage converter  104  are connected in series. In other words, the output voltage from the fuel cell must be completely processed by the DC/DC converter before it can be output. Nevertheless, there are usually some energy losses when DC-DC voltage converter  104  is put to work. Therefore, when DC-DC voltage converter is stabilizing the output voltage from fuel cell  102 , some part of the voltage energy is wasted, causing a decrease in efficiency. 
   Thus, in voltage supplying devices those use fuel cells as voltage sources, a circuit structure that can lower the energy losses is needed, in order to raise the efficiency of electrical energy used. 
   BRIEF SUMMARY OF THE INVENTION 
   The main purpose of this invention is to give a circuit structure for the voltage supplying device. 
   Another purpose of our invention is to give a voltage supplying circuit structure that increases output efficiency, in which its voltage source is fuel cells. 
   One more purpose is to give a voltage supplying circuit structure that is used to stabilize output voltage, in which its voltage source is fuel cells. 
   The fourth purpose of this invention is to give a voltage supplying circuit structure that gives temporary large power output, in which its voltage source is fuel cells. 
   To achieve the purposes of this invention mentioned above, a fuel cell, a DC-DC voltage converter and a control circuit are included in a preferred embodiment of voltage supplying device that suits this invention. The DC-DC voltage converter is used to accept voltage output from the fuel cell. Then, the converter converts the accepted voltage into its own output voltage. Finally, the total output voltage from the voltage supplying device is obtained by combining the output voltages from the fuel cell and from the DC-DC voltage converter. The control circuit controls the conversion of DC-DC voltage converter in accordance to the magnitude of the total output voltage. This allows the output voltage of the DC-DC voltage converter to be adjusted when the output voltage of the fuel cell undergoes fluctuation. If required by the actual application, the DC-DC voltage converter can be substituted with common flyback, forward, half bridge, full bridge, or push-pull isolated converter. 
   In a preferred embodiment of voltage supplying device that suits the present invention, a battery backup unit can also be included. This battery backup unit can be common rechargeable batteries, such as lead acid batteries. It is also connected in parallel with the output jacks of the DC-DC voltage converter. This way, this battery backup unit can store electrical energy output from the DC-DC voltage converter. When, the voltage supplying device needs large power output, the electrical energy in the battery backup unit will be released, allowing the voltage supplying device to temporarily give large power output. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The aspect of the invention will become apparent upon reading the following detailed description in conjunction with the accompany drawings, in which: 
       FIG. 1  illustrates a block diagram of a known voltage supplying circuit with fuel cell as the voltage source. 
       FIG. 2  illustrates a block diagram of a preferred embodiment of the present voltage supplying apparatus invention. 
       FIG. 3  illustrates a topology of a flyback isolated converter. 
       FIG. 4  illustrates a block diagram of another preferred embodiment of the present voltage supplying apparatus invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Although a known voltage supplying apparatus uses a DC-DC Voltage Converter and a fuel cell in a serial circuit to solve the huge electric energy loss associated with the voltage outputted from fuel cell, this method will decrease the working efficiency of the voltage supplying apparatus. Thus, the basic aspect of the present invention is to combine the output voltage of both the fuel cell and the DC-DC Voltage Converter and making the total output voltage of the voltage supplying apparatus to be the sum of the fuel cell and the DC-DC Voltage Converter not just of the DC-DC Voltage Converter. With this method, the huge electric energy loss associated with DC-DC Voltage Converter can be averted. 
     FIG. 2  illustrates a schematic circuit diagram of a voltage supplying apparatus  200  of a preferred embodiment of the present invention. The voltage supplying apparatus  200  comprises of a fuel cell  202 , a DC-DC Voltage Converter  204 , and a control circuit  206 . The voltage source for the voltage supplying apparatus  200  is produced by fuel cell  202 . The output voltage supplied by fuel cell  202  is directly transmitted to DC-DC Voltage Converter  206 . In another word, the positive output terminal and the negative output terminal of fuel cell  202  are connected to the positive input terminal and the negative input terminal of DC-DC Voltage Converter  206 . 
   Because of the chemical feature of fuel cell, the output voltage of fuel cell  202  is slightly unstable and creates huge fluctuation as a result. DC-DC Voltage Converter  204 , on the other hand, takes advantage of high-frequency switching and stabilizes the unstable direct current voltage. Finally, the output voltage of both fuel cell  202  and DC-DC Voltage Converter  204  are combined to get the total output voltage  210  for the voltage supplying apparatus  200 . In this preferred embodiment, the positive output terminal of DC-DC Voltage Converter  204  is connected to the negative output terminal of fuel cell  202 . The potential difference between the positive output terminal of fuel cell  202  and the negative output terminal of DC-DC Voltage Converter  204  is total output voltage  210 . The actual circuit connection is not restricted to this preferred embodiment. As long as the output voltage of fuel cell  202  and DC-DC Voltage Converter  204  are outputted together, the idea will still work. 
   An unstable voltage is generated by fuel cell  202 , so total output voltage  210  is also an unstable voltage. However, as the circuit diagram in  FIG. 2  illustrates, total output voltage  210  can be compensated to a stable voltage by using DC-DC Voltage Converter  204 . The control circuit  206 , in voltage supplying apparatus  200 , is used in combination with DC-DC Voltage Converter  204  to control the conversion carried out by DC-DC Voltage Converter  204 , so it can regulate the magnitude of DC-DC Voltage Converter output voltage. Thus, in the preferred embodiment, the control circuit  206  monitors the magnitude of the total output voltage  210 . When total output voltage  210  becomes unstable, stabilization can be achieved through controlling the magnitude for the output voltage of DC-DC Voltage Converter  204 . In another word, when output voltage of fuel cell  202  decreases, the output voltage of DC-DC Voltage Converter  204  will increase upon control. When output voltage of fuel cell  202  increases, the output voltage of DC-DC Voltage Converter  204  will decrease and maintains a set value for the total output voltage  210 . 
   Furthermore, the stability of the total output voltage  210  can be enhanced by having DC Bulk Capacitors  208  in a parallel circuit with the two terminals of the total output voltage  210 . 
   Isolated converter such as flyback, forward, half bridge, full bridge, or push-pull can be used as the DC-DC Voltage Converter  206  for the above mentioned voltage supplying apparatus  200 . 
     FIG. 3  illustrates a flyback isolated converter circuit that is used to achieve the same function as the DC-DC Voltage Converter  204  illustrated in  FIG. 2 . The circuit comprises a transformer  302 , a Switching Component  204 , a diode  306 , and a DC Bulk Capacitor  308 . In this preferred embodiment, the metal-oxide-semiconductor field-effect transistor (MOSFET) is used as the Switching Component  304 . The gate terminal of switching component  304  is controlled by the same control circuit  206  shown in  FIG. 2 . When control circuit operates Switching Component  304 , electric current will immediately flow through the first side of transformer  302  and induced voltage will be produced from the second side of transformer  302 . The first side and the second side of transformer  302  have opposite polarity, so diode  306  will sever the transmission of induced voltage and allows the energy transmitted by the first side to be stored in transformer  302  until component switch  304  is cut-off. The DC Bulk Capacitor  308  is also used to stabilize the output voltage on the second side. 
   The above mentioned example is of the flyback isolated converter circuit, but because the principles are the same with other types of isolated converter circuit, they can also be applied in the present invention. Different types of isolated converter circuit have different features. For example, flyback and forward isolated converter have lower cost, so they are perfect for low power application. On the other hand, half bridge, full bridge, and push-pull isolated converter have effective transformer utilization rate, they are more suited for high power application. 
   Fuel cells are often applied as the power source for advance vehicles. The vehicle itself has variable load that, beside the standard output range, will require high power output (when climbing a hill) as well. When high power output is required, just as the schematic circuit of the voltage supplying apparatus  400  in  FIG. 4 , a battery backup unit  402  in parallel circuit with the output terminal of DC-DC Voltage Converter  204  will help with the supply of high power output voltage  404 . Beside the addition of battery backup unit  402  and having control circuit  206  monitors the terminal voltage of the battery backup unit  402 , other parts of the voltage supplying apparatus are the same as the voltage supplying apparatus  200  shown in  FIG. 2 . Battery backup unit  402  can be any common rechargeable batter such as a lead acid battery. This way, the electric energy outputted by DC-DC Voltage Converter  204  can first be stored in battery backup unit  402  until high power output is required by voltage supplying apparatus  400 . Battery backup unit  402  helps in supplying part of the output power. Thus, in this preferred embodiment, control circuit  206  monitors the terminal voltage of battery backup unit  402 . When the energy stored in battery backup unit  402  is low, DC-DC Voltage Converter  204  is immediately activated to recharge battery backup unit  402  and maintain the availability of battery backup unit  402 . As can be seen in the schematic circuit of the preferred embodiment of the present invention, the goal of high power output can easily be achieved. 
   Although the present invention is based on a preferred embodiment as shown above, its application does not restrict to the present invention. Anyone who is knowledgeable in this field is able to make any modification within the concept and perimeter of the present invention. Therefore in order to seek protection, the following claims are stated.