A DSP-based interleaved boost DC–DC converter for fuel cell applications

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A DSP-based interleaved boost DCeDC converter for fuel cell applications Sakda Somkun*, Chatchai Sirisamphanwong, Sukruedee Sukchai School of Renewable Energy Technology, Naresuan University, Phitsanulok, Thailand

article info

abstract

Article history:

Multiphase interleaved boost DCeDC converters are typically used as a fuel cell power

Received 6 February 2015

conditioner due to their low input ripple current. This paper presents analysis and design

Received in revised form

of a 2-phase interleaved boost DCeDC converter controlled by a single chip DSP to enhance

12 March 2015

reliability and flexibility. The PI regulator for the output voltage control was tuned by the

Accepted 16 March 2015

extended symmetrical optimum method to optimally operate in the fuel cell ohmic region

Available online xxx

with a guaranteed phase margin of 45
for the entire fuel cell voltage range. The set point weighting technique was used for soft starting, which was effective. The effects of

Keywords:

switching and sampling time delays were analysed and compensated. Experimental results

Fuel cell

of the 1-kW, 120-V prototype converter connected with a PEM fuel cell stack closely agreed

DCeDC converter

with simulation results in MATLAB/Simulink. The DSP-based fuel cell converter has the

Hydrogen energy

potential for further development as a compact fuel cell generator or fuel cell electric

Renewable energy

vehicle with embedded online diagnosis and fault-tolerant operation.

Digital control

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Climate change and global warming are of concern worldwide. Reducing fossil fuel usage e the main cause of global warming, by the adoption of renewal energy sources, is now an imperative [1]. However, important renewable energy sources, such as wind and solar energy, are intermittent in nature [2]. Production and storage of hydrogen from water electrolysis using excess supply of renewable energy is a measure to overcome such intermittency [2e4]. Fuel cells are commonly used to convert hydrogen energy back to electricity, which can be supplied over an AC network of distributed generation or for use in electric vehicles. The characteristics of these fuel cells are high efficiency, low maintenance, low audible noise, and zero emissions [5].

Polymer electrolyte membrane (PEM) fuel cells, alternatively called proton exchange membrane fuel cells, have gained attention among several fuel cell types, due to compact construction, high efficiency, and low working temperature, which can be started at room temperature. A stack of PEM fuel cells provides an unregulated low DC voltage, approximately 1 V/cell at the no load condition. A step-up or boost DCeDC converter is commonly used as the fuel cell power conditioner to raise the fuel cell voltage high enough for the load and prevent the stack from overloading [6e9]. Low and high frequency components in the fuel cell current created by power converters have been studied for many years [10e15] as it de-rates the stack output power [10e13], lowers durability of the membranes [14], and increases fuel consumption [15]. Parallel connection and interleaved switching of power converters reduce

* Corresponding author. Naresuan University, School of Renewable Energy Technology, Muang, Phitsanulok 65000, Thailand. Tel.: þ66 55963192; fax: þ66 55963182. E-mail addresses: [email protected] (S. Somkun), [email protected] (C. Sirisamphanwong), [email protected] (S. Sukchai). http://dx.doi.org/10.1016/j.ijhydene.2015.03.069 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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the fuel cell ripple current and increase power output [16e21]. Control of a multiphase interleaved boost DCeDC converter for fuel cell applications generally consists of an inner fast current control loop and a pulse width modulator (PWM) for each parallel circuit and another outer slow voltage control loop. This requires a large number of analogue components [16e18], which lowers the system's reliability. In recent years, digital control has been applied to DCeDC power converters, due to the decreasing scale in CMOS technologies [22e24]. A lesser component count in the digital control systems enhances the reliability of the power converters. Modern digital signal processors (DSPs) designed for power electronic applications possess high performance computing machines, built-in analogue to digital converters (ADCs) and PWMs, which makes it possible to develop a complex power converter control scheme on a single chip [23]. In addition, flexibility and adaptability are other important benefits of the digital control [24]. This paper elaborates the analysis and design of a DSPbased interleaved DCeDC converter to combine the benefit of digital control and low fuel cell ripple current. The DCeDC converter was constructed as a power conditioner for a 1-kW, 43.2 V PEM fuel cell stack [25] to produce an output voltage of 120 V. The paper begins with a description of the system, followed by modelling of the fuel cell stack and the power converter. Generalised expressions of the input ripple current and output ripple voltage of a multiphase interleaved boost DCeDC converter were derived. Inductor current and output voltage controllers were designed by the pole placement and extended symmetrical optimum methods. Practical issues of implementation in the DSP, performance improvement and further applications of fuel cells are discussed.

System configuration Fig. 1 exhibits a 72-cell PEM fuel cell stack, connected with an N-phase interleaved boost DCeDC converter. The fuel cell stack, model H-1000 from Horizon, was specified to have the output voltage ranging from 43.2 to 67.8 V with a maximum electrical power of 1015.2 W at 43.2 V, 23.5 A [25]. The stack is an open-cathode architecture, in which an air stream is forced through the cathode channels by the blowers to ensure adequate oxygen (O2) for electrochemical reaction and also for cooling by heat convection [26]. Ultra high purity (99.999%) dry hydrogen was regulated at 0.5 ± 0.05 bar (gauged pressure) and fed to the anode channels in the dead-end mode, where the supply valve was continuously opened and the purging valve was periodically opened every 10 s so as to remove inert gases accumulated at the anode side. Parallel connection of the boost DCeDC converters allows the total inductor core volume to decrease by a factor of approximately 1/N2 and the interleaved switching technique helps to reduce the DIFC as well as the size of the inductors and output capacitor [21]. The design goals of the DCeDC converter in this work were intended to have the nominal output voltage, Vo of 120 V and the converter operated in the continuous conduction mode (CCM) so as to draw DIFC less than 0.94 A, 4% of its rated current [10]. Typical loads of this system can be an inverter for small power generation or a fuel cell electric vehicle.

Fig. 1 e A PEM fuel cell connected with an N-phase boost DCeDC converter.

System modelling PEM fuel cell modelling Accurate modelling of a fuel cell system deals with multiphysical domains [27], covering electrochemistry for electrochemical reactions at the electrodes, fluid mechanics for reactant mass transportation, and thermodynamics for stack temperature cooling. These all affect the terminal voltage and performance of the stack. However, in this work, only the electrical transient characteristic of the stack is of interest for observation of the interaction between the fuel cell stack voltage and the DCeDC converter. Due to the simplicity of the PEM fuel cell system, only the electrical dynamic in a single cell of the stack was simulated based on the equivalent electric circuit shown in Fig. 2. This model was based on the following assumptions [28]: 1) 2) 3) 4)

One-dimensional modelling. Constant stack temperature. Neglected temperature variation in the individual cells. Event distribution of gases in the channels.

Fig. 2 e Equivalent circuit of a PEM fuel cell.

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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5) Constant pressure in the gas flow channels. 6) Equilibrium water content in the polymer membranes. 7) Stack parameters lumped together from individual cells. The voltage of each cell at the steady state condition, Vcell is governed by the reversible voltage (ENerst), also called Nerst voltage, and associated losses including activation loss Dvact, ohmic loss Dvohm, and mass transportation loss Dvtrans [5], given below Vcell ¼ ENerst  Dvact  Dvohm  Dvtrans :

(1)

The Nerst voltage is used to approximate the reversible voltage at a given temperature and pressure. If the operating temperature is below 100
C and the reactant product is water in the liquid phase, the Nerst voltage at the reference temperature of 25
C is then simplified as
 ENerst ¼ 1:482  0:00485T  0:0000431T$ln PH2 PO2

(2)

where T is the cell temperature, PH2 and PO2 are the hydrogen and oxygen partial pressures respectively [28]. The ohmic loss is due to the electrical resistance of the electrode and mainly the resistance of ionic flow in the electrolyte, which is strongly affected by the water content [5,28]. In equilibrium of membrane humidity, the ohmic resistance, rohm can be assumed to be constant and the ohmic voltage is linear with the fuel cell current, as given by Dvohm ¼ rohm $iFC

(3)

Decrease in the partial pressures, PH2 and PO2 due to the large consumption of the reactant gases at a high current results in an additional voltage drop due to mass transportation loss or concentration loss. This phenomenon can be described by an empirical model with parameters m and n given below [5]. Dvtrans ¼ m$eniFC

(4)

The activation loss describes the slowness in the electrochemical reactions at the electrode, which has a great variation in reaction depending on the type of fuel cells and the electrode materials. For PEM fuel cells, the reaction at the anode is comparatively fast and can be negligible [5]. Thus, only the over voltage at the cathode can be approximated by the Tafel equation given by DVact ¼

  RT iFC ln 2aF i0

(5)

where R is the universal gas constant, F is the Faraday constant, a is the charge transfer coefficient, i0 is the exchange current describing the activeness of the electrodes. In fact, there is a small amount of electrons flowing through the polymer electrolyte, called internal current and some fuel migrates from the anodes to the cathodes without producing useful current, called fuel cross over. These two effects can be represented by parameter, in. If the fuel cell temperature remains constant and includes the effects of internal current and fuel cross over, the Tafel equation can be re-written as   iFC þ in DVact ¼ A ln i0 where A is the Tafel slope.

(6)

Double layer capacitance, Cdl is due to the charges stored at the boundaries between the electrolyte and electrodes. Whenever the fuel cell current changes, the activation over voltage slowly increases/decreases toward the steady state value given by the Tafel equation in (6). Thus, the instantaneous activation over voltage is determined from the equivalent circuit in Fig. 2, which is given by Ref. [29].   dDvact 1 DVact ¼ iFC  Cdl dt ract ðiFC Þ

(7)

where the equivalent activation resistance, ract was calculated from ract ¼

DVact iFC þ in

(8)

Finally, the fuel cell stack voltage, vFC was obtained as vFC ¼ Ncell  vcell

(9)

where Ncell is the number of cells in the stack. The fuel cell model was simulated in MATLAB/Simulink using the parameters in Table 1. Fig. 3 compares the specified polarisation curve with the modelling, and the measured and modelled actual polarisation curves. The stack delivered the maximum power of 600 W at 43.2 V, which is believed to be due to the degradation in the stack [30]. The degraded characteristic did not enter the mass transportation loss regime because the stack voltage was too low for the controller's working range. Hence, the mass transportation loss was removed from the model. Fig. 4 compares the measured and simulated transient characteristics, where the double layer capacitance was determined by the current interruption technique. The difference between the measured and simulated fuel cell voltages when the fuel cell current was suddenly removed is due to the fact that Cdl also varies with the fuel cell current [29]. However, this situation will not be noticeable when connected to the boost DCeDC converter, where the change in the fuel cell current is restricted by the converter control system.

Interleaved boost DCeDC converter modelling Fig. 5 illustrates the basic operation of a single module boost DCeDC converter operating in CCM [31]. When the power switch is closed (qN ¼ 1) during ton, the inductor stores energy from the fuel cell in its magnetic core, while the output

Table 1 e Parameters for H-1000 PEM fuel cell model. Parameters Cell number, Ncell Stack temperature, T H2 pressure, PH2 O2 pressure, PO2 Tafel slope, A Exchange current, i0 Internal current, in Double layer cap.,Cdl Ohmic resistance, rohm Mass trans. parameter, m Mass trans. parameter, n

Specification

Degraded

72 cells 338.15 K 1.494 atm 0.21 atm 0.055 V 5  103 A 0.47 A e 0.001 U 1.7  104 A 0.27 A1

72 cells 338.15 K 1.494 atm 0.21 atm 0.055 V 1.5  103 A 0.13 A 0.3 F 0.006 U e e

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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Fig. 3 e Static polarisation curves of the H-1000 PEM fuel cell stack.

capacitor is discharged through the load. Once the power switch is opened (qN ¼ 0) during toff, the stored energy in the inductor, and also energy from the fuel cell, charges the output capacitor through the diode, DN. Thus, the inductor currents for an N-phase boost DCeDC converter can be expressed as follows 9
 diL1 ðtÞ > > þ RL1 iL1 ðtÞ ¼ vFC ðtÞ  1  q1 ðtÞ vo ðtÞ > L1 > > dt > > > > >
 diL2 ðtÞ = þ RL2 iL2 ðtÞ ¼ vFC ðtÞ  1  q2 ðtÞ vo ðtÞ L2 dt > > > « > > > > >
 > diLN ðtÞ ; þ RLN iLN ðtÞ ¼ vFC ðtÞ  1  qN ðtÞ vo ðtÞ > LN dt

(10)

where L1,L2,/,LN and RL1,RL2,/,RLN are the input inductors and their corresponding winding resistances, and q1(t),q2(t),/,qN(t) 2 {0,1} are the switching functions of the power switches. Neglecting the equivalent series resistance of the output capacitor, the output voltage is given by Co

N N X
 dvo ðtÞ X ¼ 1  qN ðtÞ iLN ðtÞ  io ðtÞ iDN ðtÞ  io ðtÞ ¼ dt i¼1 i¼1

(11)

where iDN(t) is the diode currents. This model is called the switched circuit model [32], which was implemented in MATLAB/Simulink, and connected with the fuel cell dynamic model described in the previous section.

Fig. 5 e Basic operation of a single module boost DCeDC converter in CCM [31].

If ripple components are ignored, the switching functions, qN(t) in (10) and (11) are substituted by their local averages within a switching period, Ts defined below 1 Ts

tþTs Z

qN dt ¼ dN ðtÞ ¼ DN þ d~N ðtÞ

(12)

t

where dN(t) is the continuous duty ratio, DN is the steady state duty ratio, and d~N ðtÞ is the small AC component of DN around an operating point. This yields an averaged circuit model, used for designing the control system and steady state analysis. Therefore, at a steady state condition, the voltage conversion ratio is given by Vo 1  RL1 IL1 =VFC 1  RL2 IL2 =VFC 1  RLN ILN =VFC ¼ ¼ ¼/¼ VFC 1  D1 1  D2 1  DN

(13)

Equation (13) indicates that the input inductors should have identical construction for equal current balancing among the circuit modules. If the inductor winding resistances are negligible, and D1,D2,/,DN z D, the voltage conversion ratio becomes Vo 1 ¼ VFC 1  D

(14)

which is similar to the conventional boost DCeDC converter [31].

Analysis of the ripple components Fig. 6 explicates key waveforms of a 2-phase boost DCeDC converter operating in the CCM. It is assumed that the inductors, L1 ¼ L2 ¼ L and their winding resistances are negligible. The switching function, q2(t) delays the q1(t) by Ts/N ¼ Ts/ 2 or 180
. The fuel cell current, iFC(t) equals the summation of the inductor currents. Hence, the fuel cell ripple current, DIFC is determined from Refs. [15,20].

Fig. 4 e Modelled and measured dynamic characteristic of the H-1000 PEM fuel cell stack when the fuel cell current changed from 0 to 5 A, and vice versa.

N diFC ðtÞ X diLN ðtÞ ¼ dt dt i¼1

(15)

Equation (15) is used to derive DIFC in the time interval, Dt in Fig. 6 for 0  D  0.5, and 0.5 < D < 1.0. This yields DIFC as a

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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Fig. 6 e Steady state waveforms of a 2-phase boost DCeDC converter operating in the CCM: (a) D ≤ 0.5, (b) D > 0.5.

piecewise function of the duty cycle D, with 2 subfunctions. At the boundary between adjacent subfunctions (D ¼ 0.5), DIFC ¼ 0 because q1(t) is complementary to q2(t). Applying a similar analysis to the N-phase boost converters, the general form of DIFC can be expressed by DIFC ¼

  Vo k1 k1 k ðk  N$DÞ for D D N N N Lfs

(16)

where k ¼ 1, 2,/, N, defining the subfunction of DIFC. Thus, DIFC ¼ 0 when D ¼ k/N and the maxima of DIFC can be given by DIFC;max ¼

Vo 4NLfs

at D ¼

2k  1 2N

(17)

The output voltage ripple, DVo can be directly analysed in the time interval, Dt in Fig. 6 using the definition in (11). In Fig. 6(a) for 0  D  0.5, there are always the incoming currents from the diodes charging through the output capacitor and the load. Thus, DVo in this regime is comparatively low. On the other hand, for 0  D  0.5 in Fig. 6(b), Co is discharged by the load current without the incoming currents from the diodes when all MOSFETs are switched on. Consequently, large ripple voltage arises in this duty cycle range. So, a generalised expression for DVo of a multiphase boost DCeDC converter consists of low ripple and high ripple regimes and is given by Ref. [15].    j1 j D D Po j 1 N1 N N for  D< ; and N  2 ðD1Þ N N DVo ¼ Vo Co fs >   > > > Po N1 N1 > : for D1 D N N Vo Co fs 8 > > > > > <

(18)

where j¼1,2,/,(N  1), defining the subfunction of DVo in the low ripple regime. At D ¼ j/N, DVo ¼ 0 similar to DIFC, but DVo,max occurs at D ¼ 1. Fig. 7 visualises the normalised DIFC and DVo as functions of the duty ratio, which shows that DIFC,max and DVo,max decrease with the number of parallel circuits. Equation (17), in other word, indicates that the inductance can be reduced with the factor of 1/N at a given value of DIFC. This substantially reduces the inductor core volume VCore, which derives from the maximum stored energy in the core as follows [33]. 2 b 2 VCore 1 2 m LbI B 0VCore ¼ c 2LN Wm ¼ LbI LN ¼ 2 2mc b B

(19)

b and mc is the peak flux density and permeability of the where B core. Each inductor carries the average current of IFC/N. The total core volume, VCore,tot can be approximated from VCore;tot zN 


  mc ðLðDIFC Þ=NÞ I2FC N2 m LðDIFC ÞI2FC z c 2 b2 b N2 B B

(20)

Greater converter efficiency is another benefit of the multiphase boost converters as the smaller average inductor current reduces the I2R losses in the inductors and power MOSFETs [16]. In addition, a greater number in the phase legs also raises the reliability in the case of power switch faults, where the converter can still operate at a lower output power [15]. However, the control system of an N-phase converter requires components proportionally, i.e. current sensors, ADCs, PWMs, gate drivers. According to the fuel cell polarisation curve in Fig. 3, the duty cycle should be in the range of 0.5e0.64. It can be observed in

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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Fig. 7 e Normalised ripple input current and ripple output voltage of a multiphase boost DCeDC converter as functions of the duty ratio.

Fig. 7 that the nomalised DIFC of the 3-phase and 4-phase converters is not significantly lower than the 2-phase converter for such duty cycle range. Also, the average fuel cell current increases with the duty cycle, which is quite similar to DIFC of the 2-phase converter. This makes the ratio of DIFC/IFC is approximately constant throughout the duty cycle range. With these reasons, the 2-phase topology was chosen in this work [21].

Control of the FC converter Fig. 8 shows the cascade control configuration used for the interleaved boost DCeDC converter. The fast inner loops were used to regulate the average inductor current, iLN using proportional-integral (PI) controllers. The continuous duty ratios, dN from the PI controllers were then sent to the PWM

modules for generation of the switching functions, qN. A phase shift of Ts/N was added between adjacent PWMs for interleaved switching. The output voltage was controlled by the slower outer loop with a PI controller. The reference fuel cell current, iFC,ref from the voltage controller was divided by N as the references of the current control loops. The fuel cell current slope, diFC/dt was not limited as the fuel starvation [17] was not found during the experiments. Small signal analysis was applied to (10) and (11) to determine the converter transfer functions for design of the PI controllers [31,32]. By substituting every variable, x(t) in (10) and (11) with xðtÞ ¼ X þ x~ðtÞ, where X is the average value of x(t) at a given operating point and x~ðtÞ small changes around X, the small signal models of the inductor currents and output voltage are given as follows d~iL1 ðtÞ ~FC ðtÞ  ð1  D1 Þv ~o ðtÞ þ Vo d~1 ðtÞ þ RL1~iL1 ðtÞ ¼ v dt d~iL2 ðtÞ ~FC ðtÞ  ð1  D2 Þv ~o ðtÞ þ Vo d~2 ðtÞ þ RL2~iL2 ðtÞ ¼ v L2 dt « L1

LN

Co

d~iLN ðtÞ ~FC ðtÞ  ð1  DN Þv~o ðtÞ þ Vo d~N ðtÞ þ RLN~iLN ðtÞ ¼ v dt

9 > > > > > > > > > > > = > > > > > > > > > > > ;

(21)

N N X ~o ðtÞ X dv ~  ~io ðtÞ ¼ ð1  DN Þ~iLN ðtÞ  ILN dðtÞ dt i¼1 i¼1

¼

N N X VFC X ~iLN ðtÞ  ~  ~io ðtÞ ILN dðtÞ Vo i¼1 i¼1

(22)

It should be noted that the higher order terms ðx~1 ðtÞ$x~2 ðtÞÞ are neglected and this analysis is valid for the bandwidth of the control scheme well below the switching frequency [31,32]. Fig. 9 depicts the converter transfer functions, ~iLN ðsÞ=d~N ðsÞ and v ~o ðsÞ=~iLN ðsÞ derived from (21) and (22), and their control block diagrams. The open loop transfer function of the current control can be written as

Fig. 8 e Cascade control block diagram of a multiphase boost DCeDC converter.

ðTni s þ 1Þ 1 Kci $ $ GOL;i ðsÞ ¼ Kpi Tni s ðTd s þ 1Þ ðTl s þ 1Þ |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflffl{zfflfflfflfflfflffl} |fflfflfflfflfflffl{zfflfflfflfflfflffl} PI controller

PWM delay time

(23)

Converter

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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In this paper, the extended symmetrical optimum method proposed by Preitl and Precup [35] was selected for tuning the voltage controller. Fig. 10 illustrates that the phase angle, q of the open loop transfer function of such process in (27) remains unchanged even if the process gain deviates. This is achieved by selecting the open loop gain at the median, Knorm to be symmetry with a slope of 20 dB/decade around the cross over frequency, uc. The maximum phase margin, 4max is defined by parameter, b as follows  . pffiffiffi

4max ¼ tan1 ðb  1Þ 2 b

The recommended values of b range from 4 to 16, corresponding to the phase margin of 36
e60
[35]. The higher value of b leads to a lesser output voltage deviation and a longer settling time during when the load changes. From the polarisation curve in Fig. 3, VFC,min ¼ 43 V and VFC,max ¼ 68 V resulting in the median fuel cell voltage, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi VFC;norm ¼ VFC;min VFC;max ¼ 54:1 V coincidently lying in the ohmic region, where the fuel cell stack operates optimally [5,28]. The parameter, b ¼ 6, at which the settling time is close to the minimum with an acceptable overshoot [35], was chosen. This yielded 4max ¼ 45.6
. The controller parameters, Kpv and Tnv were calculated from

Fig. 9 e (a) Inductor current control block diagram, (b) Output voltage control block diagram.

where Td is the delay time due to PWM generation and current sensing, Kci ¼ Vo/RL and Tl ¼ L/RL are the converter gain and time constants. If Td is comparatively small and negligible, the closed loop transfer function can be written as T sþ1 ni 

GCL;i ðsÞ ¼ Tni Tl 2 s Kci Kpi

þ

1þKci Kpi Kci Kpi

(28)

(24) Tni s þ 1

The current controllers were tuned by using the pole placement method [34], because the loop bandwidth, uc,i was easily selected from the controller parameters as follows Kpi ¼

2zi uc;i Tl  1 Kci

(25)

Tni ¼

2zi uc;i Tl  1 u2c;i Tl

(26)

Co Vo Kpv ¼ pffiffiffi $ bTd;i VFC;norm

(29)

Tnv ¼ bTd;i

(30)

Table 2 lists the parameters of a 2-phase boost DCeDC converter used in this study. The inductance and capacitance values were chosen to create DIFC less than 0.94 A and DVo at VFC,min less than 1 V at the maximum output power of 1000 W, which was explained in detail in our previous work [21]. The phase margin was iteratively checked in the entire fuel cell voltage range, which guaranteed the minimum phase margin of 45.1
as summarised in Table 3. The DCeDC converter connected to the fuel cell stack was simulated in MATLAB/simulink using the switched circuit model described in section system modelling and parameters in

For the design of the voltage control loop, the current loop was approximated as a first order transfer function with a   1þK K time constant of Tdi ¼ K Kci pi . The open loop transfer funcci pi

tion of voltage control is given by ðTnv s þ 1Þ 1 VFC 1 GOL;v ðsÞ ¼ Kpv $ $ Tnv s ððTdi þ TFv Þs þ 1Þ Vo sCo |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflffl{zfflfflfflfflffl} PI controller

Current loop&filter

(27)

Converter

where Tdi includes the time constant of the voltage sensor and filter. Equation (27) shows that the voltage control loop of the boost DCeDC converter is a variable gain process. In some previous works [6,7], the voltage controller was designed at the fuel cell rated output with a given phase margin, 4. However, the phase margin is lower when the fuel cell voltage increases at light loads. This could destabilise the converter operation if a small phase margin is selected.

Fig. 10 e Frequency response diagram of the open loop transfer function with the PI controller tuned by the extended symmetrical optimum [35].

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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Fig. 11 e Simulated waveforms of a 2-phase boost DCeDC converter connected with an H-1000 PEM fuel cell stack when load changed from 192 W to 480 W, and vice versa.

Table 3. Fig. 10 shows a simulation result when the load changed from 192 W to 480 W, and vice versa. The steady state fuel cell voltage, VFC obeys the polarisation curve in Fig. 3. It can be observed that the output voltage variation when the load is decreasing is greater than for an increasing load despite the same absolute amount of load variation. This is explained by the bode diagram in Fig. 10. While the fuel cell supplied the 480 W load, the fuel cell voltage was close to VFC,min resulting lower phase margin than at 192 W. Nevertheless, the minimum phase margin of 45.1
also assured the stability of the voltage control.

DSP-based implementation Prototype construction [21] A prototype converter with the same parameters as listed in Table 2 was constructed and assembled in a single printed

Table 2 e Simulation parameters of a 2-phase interleaved boost DCeDC converter. Parameters

Values

Output voltage, Vo Inductance, L Inductor resistance, RL Output capacitance, Co Maximum output power, Po,max Switching frequency, fs PWM delay time, Td Current sensor&filter delay time, TFi Bandwidth of current loops, uc,i Damping ratio of current loops, zi Proportional gain of current loops, Kpi Integral time of current loops, Tni Current loop equivalent delay time, Tdi Voltage sensor&filter delay time, TFv Voltage loop parameter, b Proportional gain of voltage loop, Kpv Integral time of voltage loop, Tnv Simulation time resolution

120 V 0.68 mH 0.1 U 330 mF 1000 W 25 kHz 40 ms 0 700 Hz 0.707 0.0341 0.31 ms 0.48 ms 0.16 ms 6 0.6251 2.9 ms 2 ns

circuit board (PCB) as shown in Fig. 12. The converter was electrically isolated from the DSP board. Table 4 summarises the devices of the converter. In fact, the prototype converter was designed for the fuel cell's specified polarisation curve, delivering the maximum output power of 1000 W. The fuel cell voltage and output current were also measured for a protection scheme.

DSP-based control of the DCeDC converter A TMS320F28035 32-bit 60-MHz microcontroller from Texas Instruments [36] was chosen for control of the DCeDC converter. It possesses three 32-bit timers, 14 PWM channels and 16 channels of 12-bit dual sampling and hold ADCs, which was suitable for this application. The processor was set to execute the control algorithm every 40 ms, the same rate as the switching frequency as illustrated in Fig. 13(a). Timer 1 and timer 2 were set to operate in the up-down continuous mode with the maximum value of 1200. The switching and sampling frequencies were 25 kHz, determined from 60 MHz/(2  1200). Timer 2 was configured to delay timer 1 by 180
for interleaved switching. The interrupt request signal was evoked when timer 1 had reached zero, which subsequently commanded the ADCs to simultaneously sample iL1(k) and iL2(k), then vo(k) and io(k), and finally vFC(k) at the time instant, k. With this timing diagram, the inductor currents, iL1 and iL2 were measured at the centres of their rising and falling edges, and the average inductor currents were controlled. There was, in fact, a switching time delay in the gate drivers and power MOSFETs, which are exhibited as the dashed lines in the iL1, iL2

Table 3 e Phase margins of the output voltage control loop at different fuel cell voltages. VFC VFC,min ¼ 43 V VFC,norm ¼ 54 V VFC,max ¼ 68 V

4

uc


45.1 45.6
45.1

114 Hz 136 Hz 162 Hz

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difference approximation [34]. Fig. 14 shows the discrete time control block diagram of the output voltage. The sampling frequency of the voltage control loop was 2.5 kHz, executed every 10 current control loops. This combination of the sampling frequencies gave satisfactory steady state results. The discrete proportional, Kpvd and integral, Kivd constants were determined from the continuous time PI controller as follows, VB IB

(31)

Kpv Th2 Tnv

(32)

Kpvd ¼ Kpv

Kivd ¼

Fig. 12 e Photograph of the prototype 2-phase boost converter.

and vo signals. This created an imbalance between the inductor currents at the steady state condition. This problem was corrected by swapping the sampling sequence. The vFC(k) was read first with a sampling window of 1 ms, then iL1(k), iL2(k), and vo(k), io(k). Once all the analogue signals had been acquired, they were divided by the base values of current and voltage (IB and VB) for per unit system conversion. The voltage and current control algorithms were performed to calculate the duty ratios, d1(k) and d2(k), which were scaled and updated to the corresponding compare registers. Then, the program control returned to the waiting loop for another interrupt request. At the time instant, k þ 1, d1(k) and d2(k) of the previous sampling loaded in the compare registers were used to generate the switching functions, q1(k) and q2(k) as shown in the figure. The programing flowchart of the control algorithm is illustrated in Fig. 13(b). After resetting the processor, the relevant peripherals and the interrupt were configured. At the beginning of each interrupt service routine (ISR), the program read the status of an external switch to enable of the converter operation. At the end of each ISR, the interrupt flags were cleared to be ready for the next interrupt request. The output voltage and inductor current controllers designed in the previous section were digitised using backward

where Th2 ¼ 10Ts is the sampling period. An integral anti wind-up was used to reset the integrator through the tracking constant, Ktv. The reasonable value of Ktv should be greater than the integral constant [34]. The set point weighting parameter, b, whose value ranges from 0 to 1, was used to limit a sudden change in the fuel cell current during start-up periods. This set point weighting technique does not deteriorate the performance of load disturbance rejection [34]. High frequency components in the output voltage were attenuated by a first-order low pass filter, of which a constant, a is given by a¼

Th1 Th1 þ TFv

(33)

The proportional and integral constants of the digital current controllers (Kpid and Kiid) were determined from Kpid ¼ Kpi IB Kiid ¼

Kpi Th1 Tni

(34)

(35)

The sampling period in (33) and (35) equalled the switching period (Th1 ¼ Ts). Table 5 lists the parameters of the digital PI controllers translated from the continuous time domain. With this parameter set, oscillation and overshoot in the output voltage during load disturbances were slightly greater than the simulation. This is the nature of the discrete time system due to additional phase lag in the sampling time [37]. This was compensated by increasing the proportional gain, Kpv and integral time, Tnv, denoted in the brackets. After this adjustment, the load disturbance rejection was very close to the simulation in the continuous time domain.

Table 4 e Two-phase interleaved boost DCeDC converter specification. Devices Power MOSFETs: q1 and q2 Diodes: D1 and D2 Inductors: L1 and L2

Output capacitance, Co Gate drivers Current sensors: iL1, iL2 and io Voltage sensors: vo and vFC

Specification 2  IRFP4232PbF (250 V, 42 A, 30 mU) 2  MUR3020WT (200 V, 30 A) 0.68 mH, 0.1 U 2  Double ferrite ETD-59 cores (EPCOS N-97, 58 turns of 3  60/36AWG Litz wires, 3-mm air gap) 1  330 mF 400-V electrolytic 2  HCPL-3120 3  LEM CASR-6NP (Closed-loop flux gate, 6 A, 200 kHz) 2  (ISO124 and voltage dividers)

Experimental results Fig. 15 shows the experimental system. The fuel cell stack was tested with an electronic load for parameter identification of the modelling. The output of the boost DCeDC converter supplied a resistive load adjusted for the desired operating points. The converter input was connected to a 50-V switching power supply for preliminary validation, and then with the fuel cell stack for full system tests. The current signals were measured by Agilent 1146B and Chauvin Arnoux E3N current probes, whose bandwidths are specified at 100 kHz (3 dB). According to their frequency response curves, the indicted waveforms of inductor and fuel cell currents were estimated

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Fig. 13 e (a) Timing diagram of PWM generation and signal sampling, (b) Programing flowchart of the control algorithm.

to be lower than the actual values by 6% and 11% respectively. The voltage waveforms were measured by Tektronix TPP0101100-MHz voltage probes. A Tektronix TBS1064 60-MHz digital oscilloscope was used to display and recorded the measured signals.

Preliminary tests with a 50-V power supply

was forced to the current loops. This resulted in the output voltage of 100 V and 120 V respectively. The effect of the switching time delay on the inductor current sharing is explained in section 4.2 and can be observed in Figs. 16 and 17. The current loops were tuned at various frequency bandwidths and a satisfactory steady performance was found at 700 Hz, where measurement noise was less susceptible, as depicted in Fig. 18, which also shows good agreement of the

A resistor network of 37.5 U was connected to the converter output, and the fuel cell current set point of 5.33 A and 7.68 A Table 5 e Parameters for discrete time voltage and current controllers. Parameters Base voltage, VB Base current, IB Current loop sampling period, Th1 Current loop proportional constant, Kpid Current loop integral constant, Kiid Current loop tracking constant, Ktid Output voltage low pass filter constant, a Voltage loop sampling period, Th2 Voltage loop proportional constant, Kpvd Voltage loop integral constant, Kivd Voltage loop tracking constant, Ktvd

Fig. 14 e Discrete time voltage control block diagram.

Values 120 V 8.33 A 40 us 0.2838 0.0365 0.0730 0.2000 400 us 9.9778 (23) 1.5452 (0.94) 3.0508 (1.88)

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Fig. 15 e Photograph of the experimental system.

converter input current between the experimental result and the simulation. The slight difference in the overshoot and settling time is due to additional phase lag from the sampling time added into the experimental system [37]. Another possibility is due to the uncertainty in the converter parameters including the values of inductance, resistance and additional resistance from the power switches. The voltage loop was implemented with the controller's parameters in Table 5. The converter was tested under the load power of 96e576 W, and found that the efficiency lied between 92.2 % and 94.7 %.

Tests with an H-1000 fuel cell stack The ripple components of vFC, iFC, iL1 and iL2 are portrayed in Fig. 19. Substituting k ¼ 2 and D ¼ 0.63 in (16), the calculated value DIFC of 0.68 A is close to the measurement, considering the limited bandwidth of the current probes. The measurement shows that the ripple frequency of iFC doubles the switching frequency as illustrated in Fig. 6. The AC components in vFC is due to the interaction of iFC with the fuel cell internal impedance [7]. At such high frequency, the

Fig. 17 e Steady state waveforms of vo (Ch. 1), iFC (Ch. 2), iL1 (Ch. 3) and iL2 (Ch. 4) at iFC,ref ¼ 7.68 A, the input voltage of 50 V without switching time delay correction.

membrane resistance mainly contributes to the stack ~FC should have theoretiimpedance [12]. The phase angle of v cally been opposite to that of ~iFC . The phase delay in ~iFC was caused by the low bandwidth of the current probes. The set point weighting technique helped the converter to start smoothly as depicted in Fig. 20. Figs. 21 and 22 display the dynamics of vo, vFC, iFC and iL1 when the output load changed from 192 W to 480 W, and vice versa. It can be observed that the experiment closely agrees with the simulation in Fig. 11. This indicates that the fuel cell model is sufficient for this application. The high switching noise in of vo at the output power of 480 W is due to the equivalent series resistance of the output capacitor, which was not included in the simulation. However, this did not affect the dynamic response significantly.

Discussion Performance of the DSP-based control scheme The pole placement method was useful for the design of the current loops as the loop bandwidth could be easily shaped to

Fig. 16 e Steady state waveforms of vo (Ch. 1), iFC (Ch. 2), iL1 (Ch. 3) and iL2 (Ch. 4) at iFC,ref ¼ 7.68 A, the input voltage of 50 V with switching time delay correction.

Fig. 18 e Step responses of measured and simulated input current of the DCeDC converter (vo ≈ 100 V).

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Fig. 19 e AC components of vo (Ch. 1), iFC (Ch. 2), iL1 (Ch. 3) and iL2 (Ch. 4) at the steady state condition with the load power of 480 W.

obtain the satisfactory dynamic response and measurement noise rejection. The extended symmetrical optimum method is well suited to the voltage control loop because a desired phase margin was assured in the entire fuel cell voltage range. However, the sampling delay time caused additional overshoot and oscillation [37]. The additional phase lag decreased in the phase margin at high frequencies as illustrated in Fig. 23. The open loop gain may be not noticeable, but the phase is more pronounced [37]. This can be minimised by increasing the sampling frequency, but it was found that measurement noise was picked up into the voltage control loop. The increased proportional gain, Kpv and integral time, Tnv (dashed line) extended the 20 dB/decade slope of the open loop transfer function, which subsequently raised the phase margin. The overshoot and settling time of the output voltage during load changes were observed while adjusting the controller parameters. After the parameter refinement, the experimental dynamic responses depicted in Figs. 21 and 22 were very close to the simulation results in Fig. 11. Set point weighting was found to be very effective for limiting the instant change in the fuel cell current when starting the converter. In Fig. 20, the proportional controller acted immediately when the command changed, and the fuel

Fig. 20 e Dynamic responses of vo, iFC and vFC when started up with a 75-U resistive load (192 W) with/without set point weighting.

Fig. 21 e Dynamic responses of vo (Ch. 1), vFC (Ch. 2), iFC (Ch. 3) and iL1 (Ch. 4) the load increased from 192 W to 480 W.

cell current rapidly rose. This is undesirable for the stack's health [38]. After just about 1.5 ms, the integral controller played an important role to reduce the voltage error. This set point weighting scheme did not affect the load disturbance rejection performance [34]. Although the inductor current control tuned at 700 Hz showed good performance providing clean signals, the loop bandwidth should have operated up to the 1.25 kHz, about 20 times below the switching frequency. This is believed to be due to electromagnetic interference (EMI) mainly from the inductors and power switches. In this prototype, the voltage and current outputs of the power circuit were connected to the DSP board via twisted wires, which were easily susceptible to the radiated EMI. Equal air gaps of the inductors, L1 and L2 were placed in each leg of the ETD ferrite cores to adjust the inductance values and prevent saturation in the cores. The fringe magnetic flux of the outer legs creates large EMIs [39]. This problem can be minimised by using a centred-gap configuration. In addition, the DSP control card should be placed on the power circuit board and careful PCB design will

Fig. 22 e Dynamic responses of vo (Ch. 1), vFC (Ch. 2), iFC (Ch. 3) and iL1 (Ch. 4) the load decreased from 480 W to 192 W.

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bidirectional DCeDC converters for energy storages or DC/AC converters for AC loads/motors to form a hybrid power system or fuel cell electric vehicle. Control area network (CAN) and power line communication (PLC) protocols, commonly used in automotive and smart grid applications, are also available in the DSP to support such applications.

Conclusion

Fig. 23 e Illustration of voltage controller adjustment for sampling time delay (dashed/solid lines for with/without sampling time delay).

increase the sampling frequency of voltage control loop and the bandwidth of the converter control. Sampling of the analogue signals must be carefully taken into account in the discrete time control systems as portrayed in Figs. 16 and 17. This problem will become even more serious when the converter operates close to the rated power. The ferrite cores of the inductor with the greater current will be saturated, creating higher loss and decreasing the output power. The current imbalance will be more prominent when increasing the switching frequency. The delayed sampling technique used in this research solved this problem quite well. However, while it worked for the particular power MOSFETs and gate drivers used in the prototype, this does not necessarily suggest it will work with all other similar components. This problem can be tackled by measuring the inductor currents at twice that of the switching frequency [22]. The average inductor currents can then be calculated from the sampled values on the rising and falling edges, which will be valid regardless of switching delay time.

A DSP-based 2-phase interleaved boost DCeDC converter for PEM fuel cells was developed. A 32-bit TMS320F28035 microcontroller was used to perform control algorithm and interleaving PWM generation. The extended symmetrical optimum method was effective for tuning the PI controller of the output voltage, where the minimum phase margin of 45
was assured for the entire fuel cell voltage range. Switching delay time of the converter caused imbalanced inductor currents, which was corrected by selecting appropriate times for the inductor current measurement. Set point weighting technique was effective to avoid an instant change in the fuel cell current without deteriorating the load disturbance rejection. Faulttolerant operation could be implemented in the DSP-based control scheme by reconfiguration of the interleaved switching in the case of a failure in the power switches. In addition, online diagnosis of the stack can be embedded for healthy operation of the fuel cell system.

Acknowledgement This work was financially supported by Naresuan University, research grant no. R2557C024. S. Somkun would like to thank Dr. Tawat Suriwong for fruitful discussion. We also acknowledge the significant editing of the English in this document, by Mr. Roy Morien of the Naresuan University Language Centre.

references

Further applications for fuel cell systems The TMS320F28035 microcontroller was equipped with a 32-bit central processing unit connected with peripherals for power electronics, automotive and renewable energy applications. Other manufacturers also offer similar products without significant difference in the performance [40]. The other attractive applications of DSPs are condition monitoring and fault diagnosis widely used in electromechanical systems [41]. In recent years, several diagnosis models have been developed for PEM fuel cells [42e46]. Thus, this DSP-based DCeDC converter can be further improved by adding online fault diagnosis capabilities for reliability and durability of the stack. It has been reported that a fault in the power switches of a multiphase DCeDC converter leads to excessive input ripple current [15]. This can be mitigated by adaptation of the interleaved switching, which can be easily implemented in the DSP platforms. There are several channels of PWMs and ADCs, which can be used to control other power converters such as

[1] IEA. World energy outlook 2014: executive summary. International Energy Agency; November 2014. Available online: http://www.iea.org/publications/freepublications/ publication/world-energy-outlook-2014e-executivesummary.html. [2] Winter C-J. Hydrogen energy d abundant, efficient, clean: a debate over the energy-system-of-change. Int J Hydrogen Energy 2009;34(14, Suppl. 1):S1e52. [3] Ozbilen A, Dincer I, Naterer GF, Aydin M. Role of hydrogen storage in renewable energy management for Ontario. Int J Hydrogen Energy 2012;37(9):7343e54. [4] Carapellucci R, Giordano L. Modeling and optimization of an energy generation island based on renewable technologies and hydrogen storage systems. Int J Hydrogen Energy 2012;37(3):2081e93. [5] Larminie J, Dicks A. Fuel cell systems explained. 2nd ed. Wiley; March 2003. [6] Choe SY, Ahn JW, Lee JG, Baek SH. Dynamic simulator for a PEM fuel cell system with a PWM DC/DC converter. IEEE Trans Energy Convers 2008;23(2):669e80.

Please cite this article in press as: Somkun S, et al., A DSP-based interleaved boost DCeDC converter for fuel cell applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.03.069

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[7] Thounthong P, Rael S, Davat B. Behaviour of a PEMFC supplying a low voltage static converter. J Power Sources 2006;156(1):119e25. [8] Saha SS. Efficient soft-switched boost converter for fuel cell applications. Int J Hydrogen Energy 2011;36(2):1710e9. [9] Giaouris D, et al. Nonlinear stability analysis and a new design methodology for a PEM fuel cell fed DCeDC boost converter. Int J Hydrogen Energy 2012;37(23):18205e15. [10] Gemmen RS. Analysis for the effect of inverter ripple current on fuel cell operating condition. J Fluids Eng 2003;125(3): 576e85. [11] Somaiah B, Agarwal V, Choudhury SR, Duttagupta SP, Govindan K. Analysis and comparative study of pulsating current of fuel cells by inverter load with different power converter topologies. Int J Hydrogen Energy 2011;36(22):15018e28. [12] Fontes G, Turpin C, Astier S, Meynard TA. Interactions between fuel cells and power converters: influence of current harmonics on a fuel cell stack. IEEE Trans Power Electron 2007;22(2):670e8. [13] Kim JH, et al. An experimental analysis of the ripple current applied variable frequency characteristic in a polymer electrolyte membrane fuel cell. J Power Electron 2011;11(1):82e9. [14] Gerard M, Poirot-Crouvezier JP, Hissel D, Ṕra MC. Ripple current effects on PEMFC aging test by experimental and modeling. J Fuel Cell Sci Technol 2011;8(2). [15] Guilbert D, Gaillard A, Mohammadi A, N'Diaye A, Djerdir A. Investigation of the interactions between proton exchange membrane fuel cell and interleaved DC/DC boost converter in case of power switch faults. Int J Hydrogen Energy 2015;40(1):519e37. [16] Hwang JC, Chen LH, Yeh SN. Comprehensive analysis and design of multi-leg fuel cell boost converter. Appl Energy 2007;84(12):1274e88. [17] Thounthong P, Davat B. Study of a multiphase interleaved step-up converter for fuel cell high power applications. Energy Convers Manag 2010;51(4):826e32.  s A, Blanes JM, Liza  n JL. Non-isolated multiphase [18] Garrigo boost converter for a fuel cell with battery backup power system. Int J Hydrogen Energy 2011;36(10):6259e68. [19] Seyezhai R, Mathur BL. Design and implementation of interleaved boost converter for fuel cell systems. Int J Hydrogen Energy 2012;37(4):3897e903. [20] Benyahia N, et al. MPPT controller for an interleaved boost DCeDC converter used in fuel cell electric vehicles. Int J Hydrogen Energy 2014;39(27):15196e205. [21] Somkun S, Sirisamphanwong C, Sukchai S. Design and implementation of an interleaved boost DCeDC converter for PEM fuel cells. Appl Mech Mater 2014;666:87e92. [22] Carrejo CE, Vidal-Idiarte E, Giral R, Martinez-Salamero L. Predictive digital interpolation current control for DCeDC power converters. IET Power Electron 2009;2(5):545e54. [23] Pahlevaninezhad M, Das P, Drobnik J, Jain PK, Bakhshai A. A new control approach based on the differential flatness theory for an AC/DC converter used in electric vehicles. IEEE Trans Power Electron 2012;27(4):2085e103. [24] Pengju S, Luowei Z, Smedley KM. A reconfigurable structure DCeDC converter with wide output range and constant peak power. IEEE Trans Power Electron 2011;26(10):2925e35.

[25] Horizon. H-1000 fuel cell stack user manual v2.0. Horizon Fuel Cell Technologies; 2009. [26] Zhang G, Kandlikar SG. A critical review of cooling techniques in proton exchange membrane fuel cell stacks. Int J Hydrogen Energy 2012;37(3):2412e29. [27] Wang C, Nehrir MH, Shaw SR. Dynamic models and model validation for PEM fuel cells using electrical circuits. IEEE Trans Energy Convers 2005;20(2):442e51. [28] Barbir F. PEM fuel cells: theory and practice. 2nd ed. Academic Press; 2005. [29] Khan MJ, Iqbal MT. Dynamic modelling and simulation of a fuel cell generator. Fuel Cells 2005;5(1):97e104. [30] Strahl S, Gasamans N, Llorca J, Husar A. Experimental analysis of a degraded open-cathode PEM fuel cell stack. Int J Hydrogen Energy 2014;39(10):5378e87. [31] Mohan N, Underland TM, Robbins WP. Power electronics: converters, applications, and design. 3rd ed. Wiley; 2003.  D, Stankovic  AM, Thottuvelil VJ, Verghese GC. [32] Maksimovic Modeling and simulation of power electronic converters. Proc IEEE 2001;89(6):898e912. € lfle WH. Transformers and inductors for [33] Hurley WG, Wo power electronics: theory, design and applications. Wiley; 2013. ˚ stro € m KJ, Ha € gglund T. PID controllers. 2nd ed. ISA; 1995. [34] A [35] Preitl S, Precup R-E. An extension of tuning relations after symmetrical optimum method for PI and PID controllers. Automatica 1999;35(10):1731e6. [36] TI. Piccolo microcontrollers: TMS320F28030, TMS320F28031, TMS320F28032, TMS320F28033, TMS320F28034, TMS320F28035. Texas Instruments; July 2012. Available online: http://www.ti.com/product/tms320f28035. [37] Phillips CL, Harbor RD. Feedback control systems. 4th ed. Prentice Hall; 2000. [38] Pukrushpan JT, Stefanopoulou AG, Huei P. Control of fuel cell breathing. IEEE Control Syst 2004;24(2):30e46. [39] Van den Bossche A, Valchev VC. Inductors and transformers for power electronics. CRC Press; 2005. [40] Kukkonen L. Processor evaluation for low power frequency converter product family [MSc thesis]. Aalto University School of Science and Technology; 2010. [41] Akin B, Seungdeog C, Toliyat HA. DSP applications in electric and hybrid electric vehicles [in the spotlight]. IEEE Signal Process Mag 2012;29(3). 136e133. [42] Zheng Z, et al. A review on non-model based diagnosis methodologies for PEM fuel cell stacks and systems. Int J Hydrogen Energy 2013;38(21):8914e26. [43] Petrone R, et al. A review on model-based diagnosis methodologies for PEMFCs. Int J Hydrogen Energy 2013;38(17):7077e91. €l S, Davat B. [44] Hinaje M, Sadli I, Martin J-P, Thounthong P, Rae Online humidification diagnosis of a PEMFC using a static DCeDC converter. Int J Hydrogen Energy 2009;34(6): 2718e23. [45] Thomas S, Lee SC, Sahu AK, Park S. Online health monitoring of a fuel cell using total harmonic distortion analysis. Int J Hydrogen Energy 2014;39(9):4558e65. [46] Ettihir K, Boulon L, Becherif M, Agbossou K, Ramadan HS. Online identification of semi-empirical model parameters for PEMFCs. Int J Hydrogen Energy 2014;39(36):21165e76.

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