Study and Implementation of Four Modules in Parallel 10kWBoost DC-to-DC Converter for Fuel Cells
Shih-Jen Cheng,Shu-WeiKuo, Yu-Kang Lo, and Huang-Jen Chiu
Department of Electronic Engineering, NTUST, Taiwan, ROC
Corresponding author: Yu-Kang Lo
Email:
Add.: No.43, Sec.4, Keelung Road, Taipei, Taiwan, ROC
Keywords: Fuel cells, current-fed, interleaved, voltage-double, constant current.
Abstract
In this thesis, this choice of architecture is the current-fed full-bridge converter and use of voltage-double in the secondary circuit, the turns ratio can be reduced by half, thereby reducing losses, improving overall efficiency and practicality. And use interleaved PWM control mode to parallel with four modules,in order to reduce the current ripple on primary side for fuel cells. To ensure that the battery life and reliability in working condition, so design a feedback of constant voltage (CV) /constant current (CC) feedback circuit in terminal. Finally, implement a four modules in parallel 10kW DC-DC boost converter, and has a soft start time of 30 seconds to protect the fuel cells to a steady state. In the high-voltage input full load output, the maximum efficiency can reach 91%.
1Introduction
Since the 18th century industrial revolution, men have been using the fossils for fuels (petroleum, natural gas, coal) to improve technologies and create a practical living environment. Recently, technologies are still improving, thus making living quality better and better, but this energy consuming will eventually cause the natural energy to become less and less, create pollutions, produce harms towards earth and impact the whole ecosystems, such as global warming, sour rain, damage of atmosphere, etc. Our earth has existed for about 4.6 billion year, with the natural energy from the fossils having been consumed by us merely in 200 years, meaning that they can only enough for the upcoming tens to hundreds of years. If human beings are still considering to exist, we should make more progress on energy saving and carbon reducing. Thus, the development of other energy resources[1-3] technologies have become focus of human beings. Some of these technologies include solar energy, wind energy, hydraulic energy, burn energy, nuclear energy, and fuel cells. Each of them focuses solely on clean and pollution-free.
Till now, there are many ways to acquire energy resources, and there exist various disadvantages from each electrical generation way. Wind and hydraulic energy are somewhat dependant on geometric shape, solar energy still need much more improvement in energy transformation and application efficiency. These energies are generating electrical energy in collective sites, and will result in high power loss during the transmission process. When they are malfunctioning, the electrical energy consuming system will be disturbed massively. These renewable energies naturally have variations on voltage level, power density, and response speed, thus making power electronics technologies become significant to improve the transient and steady-state characteristics, including also the fuel cells technology [4-5]. Consequently, the power converter should be designed with so many considerations.Fuel cell is a device that converts chemical fuels into electrical energy, with high power density, clean power generation, high efficiency, and diverse energy applications as main advantages [6]. But, the fuel cell is not similar to conventional batteries, because it does not have capability to store energy and only supply for low voltage output with poor output voltage regulation range. In order to stabilize this as voltage source, a DC-DC converter should be inserted between fuel cells and loads.
In due to its broad regulation range low output voltage and slow dynamic response [7], a high-voltage battery is shunt at the fuel cells output, thus converting the voltage level into 365V through the mentioned DC-DC stage and supplying the loads and batteries. From fuel cells characteristics, the hydrogen density inside the fuel cells is higher during very light load and results in higher output voltage, and will decrease to more stable lower output voltage during heavy load. This is the reason a constant-voltage (CV) and constant-current (CC) feedback control and auxiliary supply are added at the output points to improve the slow dynamic response and output voltage stability. With simple topology and high efficiency, the circuit application and implementation is optimized. Finally, a 4-parallel-module of 10kW laboratory prototype with current-fed full-bridgetopology [8], including the CV/CC control, auxiliary supply, is implemented.
2Operating Principles
As shown in Fig.1 is the implementation of a 10kW current-fed full-bridge [6-8] block diagram. First, a DC-DC flyback converts the fuelcells voltage from 37~80V into stable 5~15V as the auxiliary supply for digital signal processor (TMS320F2808) and current-fed full-bridge. Then, the 37~80V voltage is boosted to 365V and connected in parallel to the high voltage batteries load. When rated output power of 10kW is reached, the current is limited with a CC circuit to avoid over-current through the high-voltage batteries and functions deterioration, thus optimizing the life and operational reliability of the high-voltage batteries.
Fig.1 Overall converter block diagram
Conventional full-bridge converter should use 4 power switches, which is double the number or switches in half-bridge and push-pull, thus making it more complicated. The switches of half-bridge need more on the lower voltage stress rating but higher current stress rating, meanwhile the ones of push-pull with the lower current rating but higher voltage rating. Full-bridge topology adopts the advantages of both topologies and is very suitable for high-power applications.
As seen from Fig.2, there is an inductor at the front end of the current-fed full-bridge converter, of which acting the same as boost inductor, making the primary-side switches should choose the twice input voltage rating. A voltage-doubler is added at the secondary side to minimize the rectifier diode voltage rating as the same with output voltage only, thus reducing transformer turns ratio and minimizing the leakage inductance probably resulting from the winding process.
Fig.2 Current-fed full-bridge converter
As current-fed mode, the switches duty cycle should be larger than 50%, with some important waveforms shown in Fig.3. In the two-pair interleaving cycles, there exists overlap time for the input inductor to store energy. During this overlap time, transformer T1 can be seen as shorted and disabling the input inductor to supply energy to load. At this moment, the load energy is supplied from output capacitor CO1 and CO2. The operation is divided into 4 states, with each operational principle and mode discussed below.
Fig.3 Waveforms of current-fed full-bridge converter
At t = t0, shown in Fig.4, all the power switches are ON and all the output diodes DO1, DO2 are OFF. This is the overlap time and the input inductor Lin is storing energy. Transformer T1 is shorted in due to conducting state of all switches, resulting in zero voltage across both primary and secondary windings. There is no energy transfer in the transformer, and output capacitors CO1, CO2 are supply energy to load.
At t=t1, shown in Fig.5, QA and QD are ON, QBand QC are OFF. Current flows through input inductor, QA, primary winding, QD, and back to input port. This current is added with the stored current in Lin during previous state, thus making this almost similar to boost converter. Simultaneously, the primary and secondary windings voltage are built now with dot point as positive polarity, forcing the output diode DO1 to conduct, and let the current from NS dot polarity and CO2 flow to output load. At this time CO1 is storing energy and CO2 is releasing energy. In this duration, Vin and Lin are supplying energy to load and CO1.
At t = t2, shown in Fig.6, all the power switches are ON and all the output diodes DO1, DO2are OFF. This state is exactly similar mode 1.
At t=t3, shown in Fig.7, QB and QC are ON, QA and QD are OFF. Current flows through input inductor, QC, primary winding, QB, and back to input port. This current is added with the stored current in Lin during previous state, thus making this almost similar to boost converter. Simultaneously, the primary and secondary windings voltage are built now with non-dot point as positive polarity, forcing the output diode DO2 to conduct, and let the current from NS non-dot polarity and CO1 flow to output load. At this time CO2 is storing energy and CO1 is releasing energy. In this duration, Vin and Lin are supplying energy to load and CO2.
Fig.4 Conducting current path during t0＜t＜t1
Fig.5Conducting current path during t1＜t＜t2
Fig.6Conducting current path during t2＜t＜t3
Fig.7Conducting current path during t3＜t＜t4
3Constant current circuit analysis and design
The output of this converter is shunt to high voltage batteries, such that an over-voltage or over-current will influence the batteries life and operational reliability, this is why a constant current (CC) feedback circuit is designed.
As shown in Fig.8, LM358 OP will act as the feedback part, a current sense resistor (Rsense) is connected in series in the output current loop, transferring the output current signal into Vsense and is being fed as non-inverting input at LM358. Then, ongoing addition function with reference of 2.5V, LM358 provides the sample points VFB to the primary side, thus keeping the current controlled at a fixed value.
Fig.8 Constant current circuit block diagram
After the fuelcells have started up, the output voltage will be raised to 365V by the current-fed full-bridge converter, then with a controlled CV control from voltage divider resistors, the over-voltage signal will be fed to PWM controller IC and order the duty-cycle to be larger, regulating the output voltage. At this regulated output voltage, the CC circuit detects the current feedback signal from the current sense resistor. The over-current signal will be fed to PWM controller IC and order the duty-cycle to be smaller, regulating the output current.
4Digital control system analysis and design
As shown in Fig.9 is the circuit structure of the auxiliary supply, and Fig.10 is the circuit structure offour-paralleled module of current-fed full-bridge converter. The negative feedback control is implemented from proportional ratio of output voltage and a voltage reference, feeding a control voltage to TMS320F2808. During output voltage variations, TMS320F2808 will provide CPL3120 with switch driver signal regulation to modulate the duty cycle, thus stabilizing the output voltage.
Fig.9 Auxiliary supply block structure
Fig.10 Block structure of current-fed full-bridge converter
The paralleled modules are adopting interleaved PWM control method to minimize the current ripple on fuelcells stage and optimize the 10kW rated output power, as interpreted in Fig.11. In due to this four-paralleled module system, each control signal is designed to be 90-degree phase-different, as mentioned in (1), with n the number of modules.
(1)
Fig.11 Interleaved control diagram
Here, a DSP (TMS320F2808) is used to produce four phase of synchronous driving signal for switches. Each ePWM module has synchronous input (EPWMxSYNCI) and output (EPWMxSYNCO), and by using the second module ePWM2 during “0” value of counter to create synchronous signal, producing EPWM2SYNCO output to other ePWM modules. When ePWM2 counter is 0, EPWM2SYNCI creates 20-30ns pulse signal, other ePWM modules are according to the setting of TBPHS and PHSDIR register to decide the insertion of counter value and phase when receiving the synchronous signal. For our example, when the switch frequency is 30kHz, register of each module TBPRD is 1668, as shown in Table 1.
TBPRD / TBPHS / PHSDIR / PhaseePWM2 / 1668 / 0 / 1 / 0°
ePWM3 / 1668 / 417 / 0 / 90°
ePWM4 / 1668 / 1668 / 0 / 180°
ePWM5 / 1668 / 417 / 1 / 270°
Table1: Register value setting for each module.
The adoption of digital control as microprocessor should, beforehand, set the programming environment in order to complete its functions. In Fig.12, it is shown that for DSP initiation flow-chart, its setting parts include system control register, input/output port,PIE vector diagram, and the most importantly, the PWM modules and ADC channels. After interfacing with environment, the procedure program is the operational processes control required by hardware users. The whole integration diagrams are shown in Fig.13.
Fig.12Environmental initiation system flow-chart
Fig.13Operational system flow-chart
6Experimental Results
This paper outlines the implementation of a high-efficiency converter for fuel cells application, which improved the dynamic response speed and output voltage stability. The output voltage will raise during very light load and fall during rated load, thus resulting in a need to design for broad voltage range. To verify this converter effectiveness, a 10kW current-fed full-bridge prototype that contains auxiliary supply and CC/CV circuit has been built. The circuit operational specifications are as listed in Table 2.
Input voltage / 37V~80VOutput voltage / 365V
Maximum output power / 10kW
Maximum output current / 27.4A
Switching frequency / 30kHz
Efficiency / 90%
Table2: Parameters of the current-fed full-bridge.
Fig.14 and 15 are the waveforms of Switch driver, transformer primary voltage, and input inductor current atVin=37V~57V、PO= 2.5kW for each module.
Fig.14Each module of Vin=37V、PO= 2.5kW
Fig.15Each module of Vin= 57V、PO = 2.5kW
While operating in very light load, output voltage will raise. To ensure normal operating system, the broad input voltage range is used, as in Fig.16, where the waveforms of output power at 1kW for80V input voltag.
Fig.16Each module ofVin= 80V、PO = 1kW
The four modules are paralleled, and the waveforms during 37V to 57V input voltage, output power 6kW to 10kW are shown in Fig.17、18 and 19.
Fig.17Output current waveforms for each module during Vin=37V、PO = 6kW
Fig.18Output current waveforms for each module during Vin= 47V、PO= 8kW
Fig.19Output current waveforms for each module during Vin= 57V、PO = 10kW
Because converter input is connected to fuel cells, the low start-up condition of fuel-cells should be considered in design, the output voltage needs about 30 seconds to reach its stable point. Fig.20 shows the soft-start waveforms during 44V input voltage and 500W output power.
Fig.20Soft-start waveform ofVin = 44V and PO = 500W.
Fig.21 shows the output power and efficiency graphs of 10kW four-paralleled module, from Vin = 37V to 57V and PO = 1kW to 10kW.
Fig.21 Converter output power and efficiency graphs
7Conclusion
This paper focuses on the design of a DC-DC converter for fuelcells power generating system, with improvements on fuelcells slow dynamic response and stability. The overall topology uses current-fed full-bridge as main stage, voltage-doubling circuit on secondary stage to half-reduce the transformer turns ratio, thus increasing overall efficiency. By using interleaved PWM control method on the four paralleled modules, the current ripple on fuelcells stage is minimized. And with an auxiliary supply to implement soft-start, the slow output voltage rising of fuelcells is solved. At the output of the current-fed full-bridge converter, a CC/CV feedback control is added to avoid over-voltage or over-current at the back-stage high voltage batteries, thus optimizing the life and operational reliability of the high-voltage batteries. Finally, a prototype of four-paralleled module of 10kW output power with current-fed full-bridge topology is built, having measured several related waveforms and converter efficiency. At higher input voltage of 57V and full-load output, the measured efficiency reaches more than 91%.
References
[1]R. Y. Chen, R. L. Lin, T. J. Liang and J. F. Chen, K. C. Tseng,“Current-fed Full-bridge Boost Converter with Zero Current Switching for High Voltage Applications,”IAS 2005. IEEE, vol. 3, pp. 2000-2006, Oct. 2005.
[2]M. Schenck, K. Stanton, and J. S. Lai, “Fuel cell and power conditioning system interactions,” in Proc. IEEE Applied Power ElectronicsConf., Austin, TX, pp. 114–120, Mar. 2005.
[3]X. Kong, Student Member, “Analysis and Implementation of a High Efficiency, Interleaved Current-Fed Full Bridge Converter for Fuel Cell System,” IEEE TRANSACTIONS ON POWER ELECTRONICS, vol. 22, no. 2, pp. 543-550, Mar. 2007.
[4]X. Yu, M.R. Starke, L.M. Tolbert and B. Ozpineci, ”Fuel cell power conditioning for electric power applications a summary,” IET Electr. Power Appl., vol. 1, no. 5, pp. 643-656, Sep. 2007.
[5]T. F. Wu, Y. S. Lai, J. C. Hung, and Y. M. Chen, “Boost converter with coupled inductors and buck–boost type of active clamp,” IEEE Trans. Ind. Electron., vol. 55, no. 1, pp. 154–162, Jan. 2008.
[6]G. J. Su and L. Tang, “A multiphase, modular, bidirectional, triple-voltage DC–DC converter for hybrid and fuel cell vehicle power systems,” IEEE Trans. Power Electron., vol. 23, no. 6, pp. 3035–3046, Nov. 2008.
[7]A. Averberg, K.R. Meyer and A. Mertens, “Current-fedFullBridge Converter for Fuel Cell Systems,” PESC 2008. IEEE, pp. 866-872, Jun. 2008.
[8]K. Jin, M. Yang, X. Ruan, and M. Xu, “Three-level bidirectional converterfor fuel-cell/battery hybrid power system,” IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 1976–1986, Jun. 2010.