This paper proposes a high voltage ratio and low ripple interleaved boost DC-DC converter, which can be used to reduce the output voltage ripple. This converter transfers the low DC voltage of fuel cell to high DC voltage in DC link. The structure of the converter is parallel with two voltage-doubler boost converters by interleaving their output voltages to reduce the voltage ripple ratio. Besides, it can lower the current stress for the switches and inductors in the system. First, the PSIM software was used to establish a proton exchange membrane fuel cell and a converter circuit model. The simulated and measured results of the fuel cell output characteristic curve are made to verify the correctness of the established simulation model. In addition, some experimental results are made to validate the effectiveness in improving output voltage ripple of the proposed high voltage ratio interleaved boost DC-DC converters.
Owing to worldwide energy crisis and awareness of environmental protection in recent years, to seek for substitute energy has become an important issue. Among many substitute energies, solar energy, wind energy, hydroelectric power, biomass energy, and fuel cells are green energies with potential development. As for fuel cells, there tend to have been more and more researches and applications recently. The fuel cell is a clean energy without pollution. Its energy, derived from reversed reaction of electrolyzed water, produces dynamic power. Only water is produced after the reaction; hence, there is hardly any environmental pollution. Fuel cells as a source of power are usually applied to electric hybrid automobiles, distributed electric generation system, and portable and stationary power. Among them proton exchange membrane fuel cells (PEMFCs) are the most commonly used because of the following merits: (1) lower temperature during operation, accordingly leading to rapid turning on and off and rapid reaction to the load change; (2) lower operation pressure, thus with higher safety; (3) easily set in mode system; and (4) lower emission ratio and higher conversion ratio [
Although the proton exchange membrane fuel cell has the advantages mentioned above, due to its own activation loss, ohmic loss, and concentration loss, the output voltage is lowered as a result of load increase. Namely, the fuel cell lowers the output voltage but raises the output current gradually as the output power rises under the added load. Thus, it is a low-voltage high-current output equipment. If we can transfer the low voltage produced by the fuel cell to high voltage, sending it to DC link, there will be a wider range of application [
Based on this, presented in this paper is a high voltage ratio interleaved DC-DC converter parallelly connected and further interleaved by means of two sets of voltage-doubler boost converters. So besides the advantages of high voltage ratio converter, also because of the effect of parallel connection, the current is dispersed into four routes, thus lowering the current stress of the switch and inductance. In this way it can withstand the high current output while there is a high load. Through the parallel connection of two sets of converters and controlling their interleaved voltage, it is possible to lower the output voltage ripple ratio. Figure
The system of the presented dual interleaved voltage doubler of high voltage ratio converter.
There is a great variety of fuel cells; also there are different ways to classify them. The common approach is to classify them according to the various qualities of the electrolyte. Thus, they can be divided into the following six kinds: proton exchange membrane fuel cell, PEMFC, alkaline fuel cell, AFC, phosphoric acid fuel cell, PAFC, molten carbonate fuel cell, MCFC, solid oxide fuel cell, SOFC, direct methanol fuel cell, DMFC.
Among them, the proton exchange membrane fuel cell is the best choice when we choose fuel cells for the source of the applied power because of the following reasons: (1) lower operation temperature, thus it can be rapidly turned on and off; (2) lower operation pressure, hence greater safety; (3) it can be easily set into mode system; (4) lower emission ratio and higher conversion ratio.
As for fuel cells, this paper adopts the NEXA proton exchange membrane fuel cell produced by Ballard Company. The specifications of this proton exchange fuel cell are shown in Table
Specifications of the Ballard NEXA proton exchange membrane fuel cell [
Power | Rated power | 1200 W |
Operating voltage range | 22–50 |
|
Voltage at rated power | 26 V | |
Current at rated power | 46 A | |
Startup time | 2 minutes | |
| ||
Emissions | Noise | 72 dBA |
Water | 870 mL/hr | |
| ||
Physical | Dimensions |
|
Mass | 13 kg | |
| ||
Fuel | Purity | 99.99% H2 (vol) |
Pressure | 0.7–17.2 bar | |
Consumption | <18.5 SLPM |
In building up the proton exchange membrane fuel cell math model, currently there are many simple precise model parameters and calculation formulae being presented and developed [
The math model of the proton exchange membrane fuel cell is shown in
And the thermodynamic output voltage of every piece of fuel cell can be shown as follows.
As for activation loss voltage, it can be shown this way:
And the respective coefficients of the activation loss are
As for ohmic loss voltage, it can be shown as follows:
The resistance coefficient of the membrane therein is
The resistance coefficient of the membrane can be shown to be
Concentration loss formula is shown to be
Therein, the current density of the cell is
The equivalent circuit of the fuel cell.
If we take the dynamic response of the fuel cell into consideration, when two different substances come into contact or the load current flows from one end to the other, accumulation of charge is produced on the contact area. In the fuel cell, the layer of change between the electrode and electrolyte (or compact contact face) will accumulate electric charge and energy, whose action is similar to capacitance. So when the load current changes, there will be charge and discharge phenomena happening on the charge layer. Meanwhile, activation loss voltage and concentration loss voltage will be under the influence of transient response, causing delay. But ohmic loss voltage will not be influenced or delayed. We can take this into consideration to let first-order lag exist in activation loss voltage and concentration loss voltage. Thus, its dynamic response equation can be shown to be [
The analysis shown above can be used to build up the mathematical model of the proton exchange membrane fuel cell so as to carry on the simulation analysis of the system.
In this paper PSIM simulation software is used to build up the simulated model of the proton exchange membrane fuel cell. Its composition module is shown in Figure
The fuel cell model built up by means of PSIM software.
The simulated circuit of capacitance equivalent dynamic action built up by means of PSIM software.
The DLL in Figure
After building up fuel cell model, we have its load current operated within fixed rate and value. The hydrogen and oxygen pressures are, respectively, set up at 1 bar. The characteristic curve of the simulated fuel cell output voltage and power rate is shown in Figure
The curve of the fuel cell output by means of PSIM software simulation.
The curve of the actual measuring output of Ballard Co. NEXA fuel cell [
Shown in Figure
Circuit structure of voltage-doubler boost converter.
The four switch modes of voltage-doubler boost converter in the duty cycle: (a) model 1, (b) model 2, (c) model 3, and (d) model 4.
The equivalent circuits of mode 1 and mode 3 are exhibited in Figures
Figure
The equivalent circuit of mode 4 is exhibited in Figure
Through the analysis of the four modes mentioned above, only
Voltage waveform of diode
After getting the clamp capacitor voltage, we work out (
From (
The output and input power can be shown, respectively, in
The waveform of inductance currents is exhibited in Figure
The waveform of the change of inductance current.
The condition on which the converter can be operated in continuous current mode is that
Because the maximum and the minimum induction currents of inductance
From the mathematic function
The load impedances of so-called light load and heavy load in this paper, are respectively, 2,020 Ω and 450 Ω. So at switching frequency 15 kHz, heavy load duty cycle about 0.85 when it is substituted into (
The change of output capacitor current is shown in the
The switch signal, inductance, and capacity waveforms under each operation mode.
So the result is
Therefore in the converter, we can decide the size of the capacitor according to the amount of voltage ripple ratio. From (
By means of the above-described operation mode of the converter, the switch control signal in the circuit, inductance and capacity current waveform can be exhibited in Figure
From (
The circuit structure of the dual interleaved voltage doubler of high voltage ratio converter presented in this paper is shown in Figure
The circuit structure of dual interleaved voltage doubler of high voltage ratio converter.
Figure
The ripple waveforms of switch control signal, inductance current, and output voltage under each operation mode.
In order to prove the feasibility of the dual interleaved voltage doubler of high voltage ratio converter set forth in this paper, a test will be carried on under two different loads. The fuel cell produces output voltage about 26 to 43 V, to be upgraded to 300 V, and the electronic load is, respectively, adjusted at 2,020 Ω (about output power 43 W) and 450 Ω (about output power 200 W) under test.
Figure
The switch signal waveforms of dual interleaved voltage doubler of high voltage ratio converter.
The switch signal and input/output voltage waveforms under output power 43 W.
The switch signal and input/output voltage waveforms under output power 200 W.
Figures
The switch signal,
The switch signal,
Figures
The output voltage ripple waveform of single voltage-doubler boost converter under output power 43 W.
The output voltage ripple waveform of the presented dual interleaved voltage-doubler of high voltage ratio converter under output power 43 W.
Figures
The output voltage ripple waveform of single voltage-doubler boost converter under output power 200 W.
The output voltage ripple waveform of the presented dual interleaved voltage doubler of high voltage ratio converter under output power 200 W.
This paper sets forth an ameliorated dual interleaved voltage doubler of high voltage ratio converter to improve the problem of output ripple voltage of single set voltage-doubler boost converter. With two parallelly connected voltage-doubler boost converters to interleave the output voltage ripple, we further lower the output voltage ripple. Not only does it maintain the advantages of voltage-doubler boost converter, but also, owing to the interleaved single set converter with two separate current routes and the two sets of switches of the double voltage booster once again in parallel connection leading to four separate current routes, it is thus possible to further lower the current stress of the switch and inductance. Through test and experiment, this paper proves and confirms the feasibility of the presented dual interleaved converter.
This work was supported by the National Science Council, Taiwan, under the Grant no. NSC99-2623-E-167-001-ET.