Multiphase interleaved boost DC/DC converter for fuel cell/battery powered electric vehicles

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1 ISSN , England, UK World Journal of Modelling and Simulation Vol. 13 (2017) No. 3, pp Multiphase interleaved boost DC/DC converter for fuel cell/battery powered electric vehicles Ahmed Medhat M. Youssef Aircraft Electrical Equipment Department, MTC, Egypt (Received May , Accepted April ) Abstract. Fossil-fuel pollutants being pumped from internal combustion engine vehicles have a major threat to our climate and our health. Electric vehicles have been proposed as alternatives to conventional vehicles and are considered as one of the pillar of eco-friendly solutions to overcome the problem of global pollution and radiations due to greenhouse gases. Generally fuel cells are low-voltage high-current sources and their output voltages are sensitive to changes in the loads, therefore integrating them with the high voltage of the DC-link of electric vehicles traction drives represents a major challenge for vehicle designers. Boost DC/DC converters are commonly used to interface the elements in the electric power-train, hence the objective of this paper is to design an appropriate boost DC/DC converter structure to manage the energy transfer from the fuel cell stack to the DC-bus based on high efficiency range. A multi-phase interleaved boost DC/DC converter structure is proposed, which offers reduction of: input current ripples, voltage stresses on switches, output voltage ripples, and passive component sizes. Improving transient response and reliability are among the many advantages of using such converters. Keywords: Brushless DC motor, interleaved boost converter, fuel cell, electric vehicle 1 Introduction Air pollution and greenhouse gases emissions are of serious concern in densely populated urban areas. Transport sector represents the main source of carbon dioxide emissions due to fossil fuel consumption worldwide and consequently is estimated to be responsible for around 1.3 million deaths every year. Due to environmental awareness, rapid consumption of natural resources, and increasing of fuel price, there is a great demand to search for green transportation alternatives which are independent on fossil fuel. Electric vehicles have been proposed as alternatives to internal combustion engine (ICE) vehicles and are considered as one of the pillar of eco-friendly solutions to overcome the problem of global pollution and radiations due to greenhouse gases. In general, an electric vehicle propulsion system consists of an electric energy supply system and a traction system; interconnected to each other through a high voltage DC-link. Traction system constitutes the electric motor and its drive system. Among various electric motors, Brushless Direct Current (BLDC) motor appears to be the most suitable motor for electric vehicles due to its higher reliability, higher power density, higher efficiency, lower maintenance requirements, lower cost and lower weight [3]. Supply system was conventionally consisted of a pack of batteries as energy source due to their advantages, such as: high power density, fast dynamic response time to fluctuations in a load, energy recovery during braking operation, and nearly zero emission. Unfortunately, batteries have some constraints that limit the pure battery driven vehicles, such as: low energy density, long charging time, high cost, maintenance needs and limited lifetime. Nowadays, researches are focused on renewable energy sources because they have higher power and Corresponding author. address: ammyk.khater@mtc.edu.eg. Published by World Academic Press, World Academic Union

2 World Journal of Modelling and Simulation, Vol. 13 (2017) No. 3, pp energy densities, lower cost, longer lifetime, non-carbon emission, lower acoustic and heat emissions, more efficient power, and are maintenance free compared to the batteries. Among these renewable electric energy sources is the fuel cell. Fuel cells have some disadvantages, such as: sensitive to sudden changes in the loads, slow dynamic response time and relatively long warming up time before full power output is available [14]. To exploit the advantages of both fuel cells and batteries and undermines their disadvantages, this paper proposed a hybrid power supply system based on a combination of two energy sources. Fuel cells are low-voltage current sources, therefore for automotive high-power applications, there is a need for DC/DC step-up or boost converters to be connected between the fuel cells to the motor drive DC-link. Using conventional boost converters for such high-power applications leads not only to high input current ripples which reduce the fuel cell stack lifetime, but also to electrical/thermal stresses on the switching devices which decrease the overall converter s reliability. A solution to this problem is considered in this paper; by designing a multiphase interleaved boost converter for fuel cell/battery powered electric vehicle (FCBEV) applications. The paper is organized in six sections: Section 2 describes briefly the components of the FCBEV. Section 3 explains the principle of operation of the traction system components. The operational characteristics and the mathematical modeling of the fuel cell are detailed in Section 4. Section 5 explains the design of the multiphase interleaved boost converter and its simulation results. Finally, the paper concludes with a brief summary in Section 6. 2 System description The basic configuration of the FCBEV is shown in Fig. 1. Its traction system constitutes a BLDC motor, and its associated voltage source inverter and drive system. The power supply system is composed of a fuel cell stack as a main source, and a battery pack as an auxiliary source to power the propulsion motor during starting the vehicle and to assist the propulsion of the vehicle during transients. The fuel cell stack is connected to the DC-bus through a boost converter, whereas the battery pack is connected to the DC-bus via a bidirectional DC/DC converter. Original 12 V accessories, such as: lights, horn, and so on are powered by tapping the full battery pack voltage and cuts it down to a regulated 12 V output using a buck DC/DC converter. Fig. 1: Fuel cell/battery powered electric vehicle configuration It is obvious that the performance of the electric vehicle supply system is limited considerably by the performances of the converters. This paper is one of a series of papers by the author on the modeling and simulation of the overall FCBEV configuration components. It carries out the design and simulation of an interleaved boost DC/DC converter that interfaces the fuel cell with the power-train of the electric vehicle. WJMS for subscription: info@wjms.org.uk

3 202 A. M. Youssef: Multiphase Interleaved Boost DC/DC Converter 3 Traction system components The electric motor is the main component of an electric vehicle traction system. Selecting a proper type of motor with suitable rating is very important. Possible motor candidates for powering electric vehicles are: the induction motor, the switch reluctance motor, and the BLDC motor. BLDC motors are characterized by their capability of offering higher torque and power density together with higher reliability and efficiency, compactness, longer operating life, higher dynamic response, better speed versus torque characteristics, and noiseless operation compared to the motors of the same size and other types. Therefore BLDC motors are preferred for electric vehicle applications. Complete electric vehicle traction system is composed of BLDC motor, inverter bridge, rotor position sensor, controller and driver circuit. A BLDC motor is a synchronous motor with permanent magnets on the rotor and armature windings on the stator. It is powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor, i.e. the motor accomplishes commutation electronically. The commutation instants are determined by the rotor position. Detecting the rotor position in BLDC motors is performed either by position sensors like Hall sensor, position encoder and resolver etc. or by sensorless techniques. A three-phase BLDC motor has three stator windings, which are oriented 120 o apart. When the motor rotates, each winding generates a voltage called back-emf, which has an opposite polarity to the energized voltage. There are two types of stator windings: trapezoidal and sinusoidal, which refers to the shape of the back-emf signal. Trapezoidal motor is a more attractive alternative for most applications due to its simplicity, lower price and higher efficiency [4]. A three-phase trapezoidal stator windings BLDC motor requires three Hall sensors to detect its rotor position, each hall sensor is typically mounted 120 o apart and produces 1 whenever it faces the North pole of the rotor, i.e. every 60 o rotation. Therefore, it takes six steps to complete an electrical cycle. The Hall sensor signals are fed to the control circuit which controls the direction and speed of the motor by producing pulsewidth modulation (PWM) signals for triggering the electronic switches; MOSFET or IGBT, of the six-step inverter bridge via an interface driver. Fig. 2 shows the Simulink diagram of the BLDC motor. The inverter bridge structure is shown in Fig. 3. The switching sequence, the current direction and the position signals are shown in Tab. 1.The back-emf e is a function of rotor position θ and has the amplitude E = K e ω. ; where K e is the back-emf constant and ω is the rotor mechanical speed. Fig. 4 shows the Hall sensor signals with respect to back-emf and the phase current. For this paper, the selected motor is a 5kW, 72V, three phase BLDC motor made by Golden Motor, shown in Fig. 5, and its parameters are shown in Tab. 2 [11]. Fig. 2: BLDC Simulink diagram Fig. 3: Inverter bridge structure According to the Dynamic Test data provided by the manufacturer for the HP M5000 B BLDC motor [2], the motor torque constant can be estimated by taking the slope of the line fitted to the data points as shown in Fig. 6. The calculated torque constant is [N.m/A]. Usually BLDC motor drive system controllers include both a speed controller and a current controller to control the motor performances. Simulink model of the traction system is shown in Fig. 7, where a cascaded WJMS for contribution: submit@wjms.org.uk

4 World Journal of Modelling and Simulation, Vol. 13 (2017) No. 3, pp Table 1: Switching sequence of BLDC motor Switching interval Sequence no. Position sensors Switch Phase current Hall a Hall b Hall c closed a b c I S5 S4 off II S1 S4 + - off III S1 S6 + off IV S3 S6 off V S3 S2 - + off VI S5 S2 - off + Fig. 4: Back-EMF, Hall position sensors and phase current Fig. 5: HPM5000B BLDC motor Table 2: BLDC motor technical specifications (Model: HPM5000B) Voltage 72 V Number of poles 8 Rated power 5 KW Phase resistance 12 mω Efficiency 90.6% Phase inductance 154 µh Fig. 6: Calculation of motor torque constant speed control scheme consisting of an inner current control loop and an outer speed control loop is employed. The whole design and analysis of BLDC motor drive system controllers are carried out in a variety of references (see for example [6]). WJMS for subscription: info@wjms.org.uk

5 204 A. M. Youssef: Multiphase Interleaved Boost DC/DC Converter Fig. 7: Simulink model of the traction system 4 Supply system components The supply system is composed of a fuel cell stack as the main power source and batteries as the auxiliary power source to power the propulsion motor during starting the vehicle and to assist the propulsion of the vehicle during transients. Since this paper is concerned with the designing and modeling of the boost converter which interfaces the fuel cell stack to the DC bus, therefore only the fuel cell operational characteristics and mathematical modeling are presented. A fuel cell is an electro-chemical device that converts chemical energy into electrical energy and heat as a byproduct. The fuel cell will continue delivering electric power as long as fuel is supplied. The most commonly used fuels are hydrogen and oxygen. Among fuel cells available today are: proton exchange membrane fuel cells, alkaline fuel cells, phosphoric acid fuel cells, solid oxide fuel cells, and molten carbonate fuel cells [8]. In this paper, Polymer Electrolyte Membrane or Proton Exchange Membrane fuel cells (PEMFCs) are used because of their comparatively high efficiency, high energy density, low working temperature ( o C), compactness, easy and safe operational modes [9]. A PEMFC consists of a solid polymer electrolyte membrane sandwiched between two electrodes; anode and cathode, as shown in Fig.8. When the hydrogen is injected at the anode and enters the electrolyte, it ionizes: Anode: H 2 2H + + 2e. (1) Only protons are allowed to pass through the electrolyte, therefore the protons move across the electrolyte to the cathode to rejoin with oxygen, while the freed electrons from the hydrogen atoms travel through an external circuit to recombine with oxygen at the cathode. Cathode: Therefore, the overall chemical reaction becomes: O 2 + 4H + + 4e 2H 2 O. (2) 2H 2 + O 2 2H 2 O + Electricity + Heat (3) i.e., hydrogen gas is recombined with oxygen gas producing electricity with water vapor as emission. A closed loop system could be operated whereby the water from of the PEMFC can be electrolyzed into oxygen and hydrogen for later re-use. Oxygen is generally obtained from the surrounding air. WJMS for contribution: submit@wjms.org.uk

6 World Journal of Modelling and Simulation, Vol. 13 (2017) No. 3, pp Fig. 8: Single PEMFC Generally fuel cells are low-voltage high-current sources. A single cell produces a voltage of approximately 1 V. To produce a higher voltage, multiple cells have to be connected in series to build a fuel cell stack.the relationship between output voltage (V ) and load current (i) is given as: V = N cell ( E n R T z α F ( ) ) i ln 1. i 0 s T d + 1 r. i (4) Fig. 9 shows the used 6 kw, 45V PEMFC stack. Its parameters are given in Tab. 3. The polarization current-voltage (I-V) and current-power (I-P) curves are shown in Fig. 10. Fig. 9: Simulink model of PEMFC stack Fuel cell stacking reduces its reliability and lifetime as a chain of series-connected cells is as strong as the weakest cell. Also fuel cell stack voltage is sensitive to sudden changes in the loads. Therefore, stepping-up the fuel cell stack voltage level is needed to interface the fuel cell stack with the high power train DC-link voltage; 72V. This is achieved by using a boost DC/DC converter. 5 Interleaved boost DC/DC converter A boost DC/DC converter is a stepping-up converter that transfers power from a lower DC voltage bus to a higher DC voltage bus. A DC/DC converter block mainly utilizes power semiconductor switch, a diode, an inductor, and is often accompanied by an output filter capacitor. The most important requirements expected WJMS for subscription: info@wjms.org.uk

7 206 A. M. Youssef: Multiphase Interleaved Boost DC/DC Converter Fig. 10: I-V and I-P curves [1] Table 3: PEMFC parameters Parameter Value Open circuit voltage of one cell E n V Fuel cell resistance r ohms Exchange current i A Gas constant R J/mol Exchange coefficient α Faradays constant F A.s/mol Operating temperature T 338 o K Fuel cellresponsetime T d 10 s Number of moving electrons z 2 Number of cells N cell 65 Nominal power 6000 W Output voltage V Operating current range A Nominal stack efficiency 55% from a boost DC/DC converter are high voltage ratio and low input current ripple, which are associated with volume, weight, reliability, and efficiency constraints [7]. When a conventional boost converter is used for high-power applications; like electric vehicles, the lowinput-voltage causes large input current to be withdrawn. Also with low duty cycle operation, the ripple current through the boost diode and output capacitor becomes very high. These increase the losses enormously and make the conventional boost converter quite inefficient [12].Large duty ratio places a practical limit on the achievable voltage step-up due to the large volume and weight of the required capacitance [13]. Hence conventional DC/DC boost converter only steps-up the voltage, without taking into consideration the input current, output voltage ripple and passive component size. The requirements expected from a boost DC/DC converter are achieved when an interleaved design involving parallel operation of multiple boost converters is used. Fig. 11 shows Simulink model of the designed two-phase interleaved boost DC/DC converter. The gating pulses of the MOSFETS; Q1 and Q2, are shifted by 360 /2 = 180 according to the timing diagram shown in Fig. 12 with four sub-intervals. The converter principle of operation can be summarized as follows: The interleaved-boost approach uses forced current-sharing between the power stages to equalize the power that the stages deliver. Therefore both the inductors and diodes should be identical in both channels of an interleaved design, i.e.; each inductor will have the same inductance value; L1 = L2 = L. It is assumed that the duty cycle of Q1 and Q2; denoted as D1 and D2, are the same, i.e.; D1 = D2 = D. Interval 1 (t 0 t 1 ): Q1 is on and Q2 is off, D2 is conducting and D1 is reverse biased, current ramps up in L1 with a slope d i L1 d t = V in L, storing energy in L1, while L2 discharges through the load with a slope d i L2 d t = V in V o L. WJMS for contribution: submit@wjms.org.uk

8 World Journal of Modelling and Simulation, Vol. 13 (2017) No. 3, pp Fig. 11: Simulink model of interleaved boost DC/DC converter Fig. 12: Timing diagram Interval 2 (t 1 t 2 ): Q1 and Q2 are off, D1 and D2 are conducting. Currents in L1 and L2 ramp down delivering part of their stored energy to the load and the output capacitor with slopes d i L1 d t = d i L2 d t = V in V o L. Interval 3 (t 2 t 3 ): Q2 is on and Q1 is still off, D1 is conducting and D2 is reverse biased, current ramps up in L2 with a slope d i L2 d t = V in L, storing energy in L2, while L1 continues to discharge with a slope d i L1 d t = V in V o L. Interval 4 (t 3 t 0 ): Q1 and Q2 are off, D1 and D2 are conducting. Hence the circuit behaves the same as in interval 2. The design of interleaved boost converter involves the selection of the inductors, the output capacitor, the power switches and the output diodes. The boost converter can be classified into continuous current mode (CCM) or discontinuous current mode (DCM). Most modern designs use CCM because higher power densities are possible [5], hence this design is based on CCM in which the inductor current remains positive at all times. For details on DCM operation, refer to [10]. Designing the interleaved boost converter in CCM is similar to the design of the conventional single stage boost converter with slight differences. From Tabs. 2 and 3, the design parameters are as follows: Input voltage V in range : 32 : 60 V rms ; Output voltage V o : 72 V; Output power P o Switching frequency F s Current ripple I o Voltage ripple V o : 5 KW; : 20 KHz; : 40% of load current; : 200 mv. WJMS for subscription: info@wjms.org.uk

9 208 A. M. Youssef: Multiphase Interleaved Boost DC/DC Converter The equations below are used for the calculation of the converter parameters: From the maximum and minimum input voltages, and the output voltage, the maximum and minimum duty cycles are calculated. V o V in = 1 1 D D = V o V in V o (6) D max = V o V inmin V o & D min = V o V inmax V o. (7) From the output power and voltage, the load current I o is calculated: I o = P o V o. (8) Output current ripple I o = 40% of load current I o. The converter input current I in ; which is the output current of the fuel cell stack, can be calculated from the load current I o and duty cycle. (5) I in = I o 1 D. (9) The inductor current per phase is one-half the converter input current I in : and consequently the average inductor current (maximum) per phase: I L = 0.5 I in (10) I L (avg) = 0.5 I o 1 D max. (11) The peak-to-peak inductor current ripple per phase; I L, is assumed to be a 20% of the individual phase current. Hence, the peak inductor current per phase is given by: I L = 20 % I L. (12) I L (Peak) = I L (avg) + I L. (13) The inductance value per phase is then calculated using the ripple current, switching frequency, input voltage, and duty cycle information. L = V in min D max I L. F s. (14) At the boundary of CCM, the current in the inductor will just reach zero at the lowest peak of the current waveform once each cycle.therefore to maintain the converter in the CCM, the output current I o should be greater than the boundary current I ob ; I o > I ob (15) V o R > V o D max (1 D max ) 2 2 L. F s. (16) The critical value of the inductance per phase to maintain the converter in CCM is given by: WJMS for contribution: submit@wjms.org.uk

10 World Journal of Modelling and Simulation, Vol. 13 (2017) No. 3, pp where Finally the output capacitor value C can be calculated by: L Critical = R D max (1 D max ) 2 2 F s, (17) R = V o I o. (18) C = I o D max V o. (2F s ). (19) In Eq. (19), F s is doubled because both phases are combined at the output capacitor. Fig. 13: Gating pulses of Mosfets; Q1 and Q2 Fig. 14: Inductor currents According to the above equations, the chosen boost converter parameters are: the duty cycle D is 0.4, the inductance value per phase; L1 = L2, is 400µH, and the capacitor value C is 1200µF. A resistive and inductive load of 1Ω and 100µH are used to simulate motor windings across the power terminals. The simulation waveforms of PWM signals, inductor currents, converter input current, and converter output voltage and current are shown in Figs. 13, 14, 15, 16 and 17. Fig. 13 shows the gating pulses of Mosfets for a duty cycle D of 0.4. Fig. 14 shows that both the inductor currents IL1 and IL2 remain positive at all times hence the converter is in CCM, and that they are phase-shifted by 180 ; that is while one is charging, the other is discharging. The inductor current ripple is 1.85 A. The average value of each of the inductor currents is 60.2 A; that is half of the input current shown in Fig. 15. The ripple of the input current is 0.62 A; which means that it is reduced as compared to the inductor currents, and proves the natural cancellation of the ripple in the interleaved topology. Fig. 16 shows that the output voltage WJMS for subscription: info@wjms.org.uk

11 210 A. M. Youssef: Multiphase Interleaved Boost DC/DC Converter Fig. 15: Converter input current Fig. 16: Converter output voltage Fig. 17: Converter output current ripple V o is less that 200 mv which meets the design specifications. From Figs. 16 and 17, the waveforms of the converter output voltage and current are the same which indicates that the power factor of the circuit is approximately unity, hence this circuit can be used when power factor correction is needed. From the above design and simulation, it can be deduced that in a typical interleaved converter;where several power stages are connected in parallel and driven with signals shifted by 360 /(number of phases), the effective switching frequency is increased as several times as the number of the interleaved modules without compromising the efficiency, and that interleaving topology offers numerous important benefits such as: High power-density. Reduction of input current and output filter capacitor current due to a ripple cancellation effect. Size reduction of magnetic components by processing power in a number of power conversion stages that are connected in parallel. Reduction of the burden on the output capacitor. Reliability improvement. Higher efficiency by splitting the output current into two paths, substantially reducing I 2 R losses. WJMS for contribution: submit@wjms.org.uk

12 World Journal of Modelling and Simulation, Vol. 13 (2017) No. 3, pp Running the capacitors closer to their optimum due to the increased of effective switching frequency. Longer fuel cell lifetime due to lowering the current ripple. 6 Conclusions Fuel cell electric vehicles are considered as one of the pillar of eco-friendly solutions to overcome the problem of global pollution and radiations due to greenhouse gases. Since fuel cells are low-voltage high-current sources and their output voltages are sensitive to changes in the loads, a high power DC/DC converter is needed to adjust the output voltage, current and power of fuel cells to meet the vehicle requirements. In this paper, a two-phase interleaved boost converter is designed, the selection criterion of its elements is explained, and benefits of using interleaving topology are discussed. Simulation results verified the capability of the designed converter to achieve the requirements on input current, output voltage ripple and passive component size. References [1] N. P. datasheet. Misc title, ps6.pdf. [2] N. P. datasheet. Hpm bldc motor: Dynamic test., [3] A. Emadi. Handbook of automotive power electronics and motor drives. Taylor & Francis, [4] A. Emadi, M. Ehsani, J. M. Miller. Vehicular electric power systems : land, sea, air, and space vehicles. Marcel Dekker, [5] T. Instruments. An-1820 lm5032 interleaved boost converter, Application Report no. SNVA335A. [6] K. N. Mobariz, A. M. Youssef,et al. Electric vehicle propulsion system. International Journal of Advances in Engineering Research, [7] M. Kabalo, B. Blunier, et al. State-of-the-art of dc-dc converters for fuel cell vehicles. in: Vehicle Power and Propulsion Conference, 2010, 1 6. [8] Karunarathne, Lakmal. An intelligent power management system for unmanned aerial vehicle propulsion applications. Cranfield University, [9] J. Meyer, F. D. Plessis, W. Clarke. Design Considerations for Long Endurance Unmanned Aerial Vehicles. InTech, [10] N. Mohan, T. M. Undeland, W. P. Robbins. Power Electronics : Converters, Applications, and Design. Wiley, [11] G. Motor. Hpm bldc motor datasheet., [12] A. S. Samosir, N. Taufiq, A. H. M. Yatim. Simulation and implementation of interleaved boost dc-dc converter for fuel cell application. International Journal of Power Electronics & Drive Systems, 2011, 1(2): [13] J. Scofield, S. Mcneal, et al. Studies of interleaved dc-dc boost converters with coupled inductors [14] R. C. Smith. Design of a control strategy for a fuel cell/battery hybrid power supply. Texas A & M University, WJMS for subscription: info@wjms.org.uk