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Boost-Cascaded-by-Buck Power Factor Correction Converter for Universal On-board Battery Charger in Electric Transportation A.V.J.S.Praneeth, Student Member, IEEE, Lalit Patnaik, Member, IEEE and Sheldon S Williamson, Senior Member, IEEE Advanced Storage Systems and Electric Transportation (ASSET) Laboratory Smart Transportation Electriﬁcation and Energy Research (STEER) Group UOIT-Automotive Center of Excellence (UOIT-ACE) Department of Electrical, Computer, and Software Engineering University of Ontario Institute of Technology,Canada E-mail ID: Jaya.Av@uoit.net, Sheldon.Williamson@uoit.ca Abstract—A two-stage battery charger in battery operated electric vehicles (BEVs) and plug-in-hybrid electric vehicles (PHEVs) with wide output voltage range of 100–500 V is most suitable for all vehicle architectures. Existing battery chargers have a different limited range of output voltages of 36–48 V, 72–150 V and 200–450 V is achieved by varying the output voltage of DC/DC converters keeping a ﬁxed voltage at DC link. A universal charger, which can address this wide range of battery pack voltages is suitable for all vehicle architectures. The most feasible way to meet this requirement of wide output voltage range is to vary the voltage at DC link with a ﬁxed conversion voltage ratio at DC/DC converter. In this paper, we propose the use of cascaded converter in power factor correction (PFC) converters to achieve the wide DC link voltages for battery chargers. The primary focus of the paper is on the analysis and operation of boost-cascaded by buck (BoCBB) converter. The control implementation presented in the paper achieves a high input power quality, wide DC link voltages with universal input voltage ranges of 85–265 V. It also provides the degree of control freedom to operate even if the V/Vm (output voltage to the peak of Input) < 0.5. Simulations of the proposed converter with 1 kW power rating are carried out in PSIM 11.0 software and the results with wide DC link voltage of 150–400 V are presented in the paper. Index Terms—AC–DC power converters, Power Factor correction (PFC), Plug-in Hybrid Electric Vehicle (PHEV)/ Electric Vehicle (EV) I. I NTRODUCTION To reduce the carbon emissions in the environment and encourage green energy electric vehicles (EVs) and plug-inhybrid vehicles (PHEVs) have gained a lot of interest and attention worldwide. Battery as a source of power provides an efﬁcient and smooth drive compared with internal combustion engines. Efﬁcient on-board or off-board charging of these battery packs is the most crucial and challenging [1]. Due to the limitations in the fast charging infrastructure, most of the electric vehicles are equipped with an on-board battery charger allowing them to charge from ubiquitous utility socket [2]. These chargers when connected to grid should adhere harmonic regulations and standards, such as IEC 61000- l-))) Fig. 1: Two stage layout of on-board battery charger 3-2 and IEEE 519 for high power quality [3]. The twostage converter for EV battery charging consists of AC/DC power factor correction (PFC) stage followed with an isolated DC/DC converter. The most commonly used PFC converter in all applications is with boost topology, which maintains the high power quality for a universal input voltage of 85265 V. At high power levels, it requires a bulky inductor and output capacitor for operation with universal input voltages and provides efﬁcient operation only when the required DC output voltage is greater than peak of the input voltage [4].Moreover,various control methods emboided using PI, PR, nonlinear controllers in on-board battery chargers have been studied [5]–[7]. It is an attractive solution to provide a step-up or step-down voltage of DC output voltage for a universal input voltage applied to the converter. However, the conventional singleswitch topologies with buck-boost conﬁgurations like conventional buck-boost, single-ended primary inductance converter (SEPIC), ﬂyback and Cuk converters can fulﬁll the operation [8]–[9] but have high component sizes and stress on devices compared with boost PFC converters[10]–[14]. This conventional buck-boost and Cuk converters have inverting output voltages and provide zero direct energy transfer from the input source to load. The minimum direct energy transfer will increase the size of components for energy storage, which increases the stress on the devices. The ability of direct energy transfer path leads to lower component stress, high efﬁciency of the converter and reduces the role of storage elements for a given voltage conversion ratio [15]. The single switch buck-boost topologies are therefore unsuitable for high voltage applications. To mitigate the issues on stress, component Authorized licensed use limited to: UNIV OF ENGINEERING AND TECHNOLOGY LAHORE. Downloaded on October 03,2021 at 19:20:03 UTC from IEEE Xplore. Restrictions apply. Fig. 2: Topology of boost-cascaded by buck (BoCBB) converter size independent controlled two-switch buck-boost converter topologies have been proposed in literature [16]–[18]. These converters have a non-inverting output voltage and can operate in buck as well as boost converter based on the input voltage applied and the desired output voltage. The challenge in these converters is to provide a smooth transition in the change of mode. The researchers have investigated many ways of smooth transition implementation by providing an additional mode between buck and boost, providing a pseudo-continuous conduction mode state and adding a compensated duty ratio in between the modes (pre and post buck-boost) of operations [19]–[21]. The drawback of the topologies is complicated control structures and less efﬁcient due to minimum direct energy paths. The review of topologies involved in PFC applications are analysed with drawbacks before proposing the control strategies[22]. This paper proposes the use of a two-switch boostcascaded-by-buck as a PFC converter is been analyzed and the parameters are derived. The two operating modes of the converter are explained in detail. Moreover, the design and implementation of a control loop for the converter are also pictured which is able to vary the PFC output voltage to a value above and below the maximum value of the input voltage. This variable DC-link voltage provides a smooth transition between the buck and boost mode of operation and improves the power quality of the converter. A 1 kW system is simulated in PSIM and the results obtained with wide PFC voltages are presented and discussed. II. OPERATION OF THE PFC STAGE In single switch buck-boost and Cuk converter topologies, most of the power transfer from source to load happens through the passive components which results in high voltage and current stresses, low efﬁciencies. The circuit in Fig. 1 shows a two-switch boost cascaded by buck (BoCBB) converter which consists of two switches, S1 and S2 .The total number of switching states possible with the two switches is four. In BoCBB converter, the converter operation is comprised Fig. 3: Operationg modes of the BoCBB converter a) Boost mode of operation b) Buck mode of operation of S1 and D1 switches in boost mode of operation and the buck mode of converter comprises of S2 and D2 . If the dc output voltage (VDC ) is higher than the peak of the input voltage (Vmax ), then the circuit will operate in only boost mode and the switch S2 is turned-on continuously. If the dc output voltage is less than the peak of the input voltage (Vmax ), then the mode of operation of the converter will depend on the switching states shown in Table.1 and Fig. 2. To perform the analysis of this BoCBB converter, the following simplifying assumptions are made: 1 All the components are ideal; ESR of the components and forward on-state voltage drops are neglected. 2 The rectiﬁed sinewave is considered ideally same as input sinewave, i.e.,V1 = Vm |sinωt| where Vm is the peak of Authorized licensed use limited to: UNIV OF ENGINEERING AND TECHNOLOGY LAHORE. Downloaded on October 03,2021 at 19:20:03 UTC from IEEE Xplore. Restrictions apply. the input voltage and ω is the angular frequency 3 The selection of the inductors (L1 and L2 ) are such that the converter always operates in continuous conduction mode (CCM). 4 The output capacitance of the boost converter is very high, to which it is considered and placed across the load to have small twice line frequency ripple and provide a stiff output voltage Vdc . The BoCBB converter mainly has two modes: buck mode and the boost mode of operation. The controlling of switchS1 and diode D1 corresponds to the boost mode as shown in Fig.3(a) and the switch S2 and D2 to buck mode as shown in Fig.3(b).Let us deﬁne the duty ratio of the switch in boost mode is d1 and that of in buck mode is d2 . The BoCBB converter is a cascaded combination of two converters (boost,buck) the overall gain (M1 ) of the converter is given in (1) d2 (1) M1 = 1 − d1 which is the product of individual gains in buck and boost mode. For analysis the forward voltage drop of the diodes is neglected and the rectiﬁed voltage is same as the input voltage which is given as V1 (t) = Vm |sinωt| (2) where Vm is √ the maximum value of input voltage and is given as Vm = 2Vac,rms .For a ideal PFC rectiﬁer (neglecting losses) the power at input and output is assumed to be constant. For a particular value of the output voltage Vdc the input current i1 (t) is proportional to the input voltage V1 (t) √ ⎧ ⎪ ⎪ i1 (t) = 2Vac,rms |sinωt| ⎨ Ri (3) ⎪ (t) V ⎪ ⎩ i1 (t) = 1 Ri where Ri is the emulated resistance related to active power P demanded by the load. As shown in Fig. 4, the reference voltage is applied to BoCBB converter with an input voltage. For analysis one half cycle of line period with half wave symmetry is considered. In the boost converter, the reference voltage selected should always be higher than the peak of the input voltage and for the buck converter, the reference voltage should be less than the input voltage. In the interval [0, t1 ] the converter operates in boost mode as the reference voltage is higher than the input voltage. In [t1 , T4s ] the converter operates in buck mode which has the input voltage is higher than reference voltage and it repeats for [ T4s -t2 , t2 - T2s ] in buck and boost modes. The switchover from boost to buck mode and vice versa is taken care by the controller to provide smooth transitions in every quarter of a cycle. Fig. 3(a) shows the operation of BoCBB converter in boost mode. The boost switch cell S1 ,D1 are conducting and the buck switch S2 always on, d2 =1 and the converter is operated in continuous conduction mode. It has the continuous load current with low ripple current. The dc voltage gain of boost mode is determined by the inductance volt-sec balance relation to L1 , Fig. 4: Waveform of the rectiﬁed input voltage and reference dc output L2 during on and off of the boost switch S1 over a cycle is given as, V1 d1 Ts + (V1 + Vc1 )(1 − d1 )Ts = 0 (4) V1 + Vc1 (1 − d1 ) = 0 (5) On the similar way, the volt-sec balance on ``L2 ´´gives (V1 − Vc1 − Vdc )d1 Ts + (V1 − Vdc )(1 − d1 )Ts = 0 V1 − Vdc = Vc1 d1 (6) (7) The voltage gain with boost mode of operation obtained by solving the equations (5) and (7) which is given as 1 Vdc = (8) M1 = V1 1 − d1 In the steady state operation the duty ratios are deﬁned in terms of the varying nature of input voltage. ⎧ ⎨ d = 1 − V1 (t) 1 Vdc (9) ⎩ d2 = 1 Fig. 3(b) shows the operation of BoCBB converter in buck mode. The buck switch cell S2 , D2 are conducting and the boost switch S1 always off, d1 =0 and the converter is operated in continuous conduction mode.It also provides continuous load current with low ripple current. The LC ﬁlter is formed with circuit parameters L2 and C2 .The dc voltage gain of buck mode is determined by the inductance volt-sec balance relation L1 , L2 during on and off of the buck switch S2 over a cycle which is given as (V1 − Vc1 )d2 Ts + (V1 − Vc1 )(1 − d2 )Ts = 0 (10) V1 = Vc1 (11) (V1 − Vdc )d2 Ts + (−V1 − Vdc + Vc1 )(1 − d2 )Ts = 0 (12) V1 d2 = Vdc (13) The voltage gain with boost mode of operation obtained by solving the equations (11) and (13) which is given as Vdc = d2 (14) M2 = V1 Authorized licensed use limited to: UNIV OF ENGINEERING AND TECHNOLOGY LAHORE. Downloaded on October 03,2021 at 19:20:03 UTC from IEEE Xplore. Restrictions apply. Fig. 5: Control loop implementation of BoCBB Converter Fig. 6: Two loop Control implementation of converter Fig. 8: Output voltage waveform for 400 V Fig. 7: Input voltage and current waveforms at 400 V In the steady state operation the duty ratios are deﬁned in terms of the varying nature of input voltage. ⎧ ⎨ d1 = 0 (15) V ⎩ d2 = dc V1 (t) The mode of operation repeats for [ T4s , T2s ]. The controller is to be designed to provide the smooth transitions between the modes and to have a wide DC output voltages with reduced ripple. Fig. 9: Input voltage and current waveforms at 250 V Authorized licensed use limited to: UNIV OF ENGINEERING AND TECHNOLOGY LAHORE. Downloaded on October 03,2021 at 19:20:03 UTC from IEEE Xplore. Restrictions apply. TABLE I: Parameters of the BoCBB converter Item Buck Mode Boost Mode Input Voltage Vin (rms) Output Voltage Vdc Power Rating Switching frequency fs Inductor L1 Inductor L2 Capacitor C1 CapacitorC2 Duty cycle D Output Load Resistance R 85-265 V 250 V 1 kW 20 kHz 6 mH 2.52 mH 8 μF 950 μF 0.266 62.5Ω 85-265 V 400 V 1 kW 20 kHz 6 mH 2.52 mH 8 μF 950 μF 0.7175 160Ω III. C ONTROL I MPLEMENTATION Fig. 10: Output voltage waveform for 250 V reference The layout for control implementation with the plant transfer functions is shown in Fig.6. G2 (s) is the transfer function of current to duty ratio and G1 (s) is the transfer function of voltage to duty ratio. The BoCBB PFC converter uses two loop control for maintaining the wide output voltage of converter and high input power quality. The control loop needs to operate in boost and buck modes of operation. The detail control structure is shown in Fig.5. Two PI control loops for the voltage and control loops are used in boost and buck mode to generate the duty cycle d1 andd2 . GV Bu (s) represents the voltage controller in buck mode and GIBu (s) represents the current controller in buck mode. Similarly,GV Bo (s) represents the voltage controller in buck mode and GIBo (s) represents the current controller in boost mode. IV. R ESULTS AND D ISCUSSION Fig. 11: Input voltage and current waveforms at 150 V Fig. 12: Output voltage waveform for 150 V A 1 kW BoCBB Converter model with the proposed control has been simulated in PSIM (11.0) software with parameters from Table I. The simulation results in Fig. 7 shows the input voltage of 230 V(rms) applied to converter whose reference value is greater than the peak of the input voltage. The top waveform contains the input voltage and the reference voltage need to be followed by the PFC converter and bottom is the current wave which is in-phase with the input voltage giving high power factor of 0.996. Fig. 8 shows the waveform of output voltage of the PFC converter which is same as reference 400 V. similarly, Fig. 9 shows the waveforms for input voltage and current of PFC converter with the reference voltage of 250 V. As we see the top ﬁgure contains the reference voltage is slightly less than the peak of input voltage making the converter to toggle between in buck and boost modes of operation. The input current during this mode also maintains high power quality as seen in the bottom waveform. Fig.10 shows the waveform of output voltage of the PFC converter which is same as reference 250 V. Fig.11, shows the waveforms input voltage and current of PFC converter in a buck-boost mode with a reference voltage of 150 V and Fig. 12 shows the output voltage obtained at PFC converter which is same as a reference voltage. This fulﬁlls the requirements of wide output voltage control of the PFC converter maintaining high power quality at the input side. Authorized licensed use limited to: UNIV OF ENGINEERING AND TECHNOLOGY LAHORE. Downloaded on October 03,2021 at 19:20:03 UTC from IEEE Xplore. Restrictions apply. V. C ONCLUSION A non-inverting boost-cascaded-by-buck (BoCBB) PFC converter for the universal battery charger is been simulated to attain wide output voltages. The output voltage of PFC converter is varied for a value greater than the peak of the input voltage and lesser than the peak of the input voltage. 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