ISA Transactions 114 (2021) 485–498 Contents lists available at ScienceDirect ISA Transactions journal homepage: www.elsevier.com/locate/isatrans Practice article Design, implementation and model predictive based control of a mode-changeable DC/DC converter for hybrid renewable energy systems Naki Güler a , Erdal Irmak b , a b ∗ Technical Sciences Vocational School, Gazi University, Ankara, Turkey Electrical and Electronics Eng. Department, Fac. of Technology, Gazi University, Ankara, Turkey article info Article history: Received 1 February 2020 Received in revised form 7 November 2020 Accepted 10 December 2020 Available online 13 December 2020 Keywords: Hybrid renewable energy system Mode-changeable converter Model predictive control Multi-input multi-output system a b s t r a c t Since renewable energy sources such as PV and wind provide intermittent energy generation, this paper presents an advanced DC/DC converter that is able to set its operational mode automatically to either multi-input multi-output (MIMO) or single-input multi-output (SIMO) depending on the input source conditions. Power flow is controlled through the auxiliary relays added to a double-layer boost converter. Considering the transient events require fast dynamic response, model predictive control (MPC) is used to achieve the current control processes for both layers. Furthermore, the MPC is modified to adapt itself to changes in the topology. The proposed system is verified by simulations and experimentally. Results show that the proposed mode-changeable converter successfully determines the optimum power route after deciding the best operational mode in accordance with the input source conditions. Furthermore, the control method achieves a powerful and effective control process in both MIMO and SIMO modes. © 2020 ISA. Published by Elsevier Ltd. All rights reserved. 1. Introduction Over the past decades, DC–DC power converters have been adopted for a wide variety of applications ranging from power supplies, renewable energy systems, motor drives and LED lighting. Although the main role of DC–DC conversion is specified as control the voltage gain, it has gained another importance with the multi-port operation ability in the last decade [1–4]. The integration of multiple sources and feeding of multiple loads can be highlighted as the major advantage of the multi-port power converters. Multi-input multi-output (MIMO) DC/DC topologies are used to control the power flow from multiple sources to multiple loads. Thanks to the power control capability, MIMO converters have widespread usage in such applications as battery charging systems [5], energy transferring operations between multiple sources [6,7], electrical vehicles [8] and the power systems those have loads requiring different voltage levels [9]. In order to reduce the number of circuit components, single-inductor MIMO (SI-MIMO) types are especially preferred, where the inductor is charged by switching the input sources sequentially while the output power is controlled by using different switches. In these ∗ Corresponding author. E-mail addresses: [email protected] (N. Güler), [email protected] (E. Irmak). https://doi.org/10.1016/j.isatra.2020.12.023 0019-0578/© 2020 ISA. Published by Elsevier Ltd. All rights reserved. structures, each layer requires two switches at least [1,9–11], which makes the control harder. Due to the inductor current is the sum of all layer currents, the cross-regulation problem is inevitable, and the control of inductor current is very complicated. Besides, SI-MIMO converters have limited voltage gain because they usually cannot provide the symmetrical output voltage. Although the SI-MIMO topology presents a simple multi-port structure, its complicated control strategy and cross-regulation problem are the main challenges for implementation. Single-input multi-output (SIMO) converters are another type of multi-port converters. Similar to other types, these structures have independent power layers too. In general, SIMO models include additional control switches attached to the output of a main converter [12]. Thus, each switch controls the power and voltage level of its own layer. Since the unsymmetrical currents of the layers, the control of input current is complicated. Besides, the voltage gain band is limited especially in buck type SIMOs [12– 14]. Similar to SI-MIMO, the cross-regulation problem is the key challenge in control of single inductor SIMO models. The power control of converters is as important as their circuit topology and a great deal of studies in recent literature deal with this subject [8,15]. In this context, increased speed and measurement capabilities of microcontrollers have made it possible to use of model-based control systems widespread. As compared with the conventional techniques [16–18], model predictive control (MPC) method stands out with its some features such as fast N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Fig. 1. Block diagram of the proposed mode-changeable converter and MPC scheme. dynamic response and easy implementation [19–22]. Therefore, MPC is widely used for controlling the input and the output power in multi-port converters. Usage of MPC method offers better solutions to provide the power sharing between the input sources in MIMO converters [9] and to achieve the control of output voltages both in MIMO and SIMO converters [12]. Considering such issues summarized above, this paper presents an advanced converter that can automatically switches its topology to either MIMO or SIMO depending on the voltage level of the input sources. The idea behind this topology is to maintain the power flow by changing the operational mode against the interruptions in the sources. Besides the fundamental difference between the existing topologies, the total cost is reduced by using simple relays instead of power diodes to turn on or off the sources. As mentioned earlier, the voltage gain is limited in SI-MIMO structures because of their non-symmetrical outputs. In order to overcome this problem, the proposed converter uses a double layer boost converter structure with separate inductors. During the SIMO operation, the positive layer operates in buckboost mode while the negative one operates in boost mode. Thus, the voltage gain band is expanded to be use with different type and size of loads. Since each layer of the proposed converter is structured with independent inductors, the cross-regulation problem is eliminated and the control strategy is considerably simplified. Moreover, comprising fewer components than some similar types in [9–11] is another simplification in terms of the model-based control strategy. On the other hand, a fast dynamic response requirement is essential due to the mode transition feature of the proposed topology. Besides the fundamental requirement, the control method should have a good match with the changes from boost to buck-boost in SIMO modes. Therefore, the control algorithm should be able to adapt itself to changes in the topology. MPC method is preferred to control the input currents of the proposed mode-changeable converter, by considering control requirements [9,23]. Thanks to model based structure of the MPC method, the operational changes in the topology are defined into the control algorithm. The current control operation of converters in discontinuous conduction mode (DCM) needs mean value calculation of input currents [24] and this decreases the dynamic capability of the control process. Therefore, some control methods have been proposed to perform the control process in DCM and CCM separately [25,26]. Similarly, the current control operation developed in this study is achieved in only CCM. MPC based average current control of DC/DC converters was presented by using average calculation [24,27], Kalman filter [28], observer [29], and RMS calculation [24]. However, the design process of these techniques complicates the implementation of the control method. In this paper, the average current control of the proposed modechangeable converter is implemented without neither calculation method nor observer. CCM operation is provided with the selection of converter parameters in accordance with the methodology reported in [30] and then model predictive current control structure is applied. Both the simulation and the experimental results show that the proposed converter and its control algorithm successfully provides fast and stable operation not only in the steady state conditions but also during the transitional states. Especially the results of SIMO mode show that the MPC method adapts itself to change in topology. The proposed mode-changeable topology can be used in several applications, some of which are as follows: • Multiple PV strings can be connected to multi inputs of the converter for hybrid operation. • Different types of energy sources like renewables and the grid can be combined on the same bus [25]. • Multiple loads such as industrial loads and batteries can be fed by the system thanks to its multi output feature [11]. 2. Proposed mode-changeable DC/DC converter model In order to explain and analyze the proposed system, a sample two-input two-output converter model is designed consisting of two boost converters as shown in Fig. 1 [31]. Unlike typical double-layer boost converters, the proposed system includes four auxiliary relays to route the power from multiple sources to multiple loads, in different configurations by altering the switch positions of the relays. Thus, the converter is able to set its mode to either SIMO or MIMO automatically. Switches of the newly added relays are numbered from S3 to S8 and their connection diagrams are illustrated with dashed lines in Fig. 1. Table 1 shows the relevant switch positions for each mode. The mode SIMOV1 indicates that only the first input source (V1 ) is active and the system has two outputs. Similarly, the mode SIMOV2 indicates the case where only the second source (V2 ) is active and the system has still two outputs. All inputs and all outputs are active in the last mode called MIMO. A mode detection algorithm is used to select an appropriate operational mode automatically. Fig. 2 shows the flowchart of the mode detection algorithm which determines the positions of auxiliary relays depending on the source voltages [31]. Clearly, the algorithm compares the source voltages with a predefined threshold voltage and it determines the positions of the auxiliary relays according to Table 1. Consequently, the mode detection algorithm automatically integrates the active sources to the system by changing the operational mode between MIMO and SIMOs. 486 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 being charged through the input sources. At the same time, the capacitors C1 and C2 are discharged through the loads. Switching State 2: In this case, S1 and S2 are OFF. Since the inductance L1 is in discharging mode, the diode D1 is forward biased. Therefore, the energy is transferred from the L1 inductor and the V1 input source to the C1 capacitor. The first layer operates as a typical boost converter in this mode. Similarly, the energy flows from the L2 inductance and the V2 input source to the C2 capacitor on the second layer. For MIMO operation, the relation between the output voltages (VOUT 1 and VOUT 2 ) and the inputs is calculated by using the typical boost converter equations as given in (1) and (2), where d1 and d2 indicates the duty cycle ratios of the switches S1 and S2 , respectively. VOUT 1 = V1 VOUT 2 = V2 1 (1 − d1 ) 1 (1 − d2 ) (1) (2) 2.2. Analysis of SIMO operation In order to obtain the SIMO structure, the switches of the auxiliary relays are set to the relevant positions according to Table 1. As seen from the table, the switch s7 is OFF in SIMOV 1 mode. Therefore, the source V2 is completely separated from the system. Similarly, the source V1 is separated from the system through the auxiliary switches (s5 and s6 ) during the SIMOV 2 operation. In the model, the first layer associated with the first IGBT (S1 ) controls the first output voltage (VOUT 1 ) while the second layer controlled by the second IGBT (S2 ) manages the second output voltage (VOUT 2 ). Thus, both outputs are controlled separately. In both modes (SIMOV 1 and SIMOV 2 ), while the first layer operates in buck-boost mode, the second one operates in boost mode. Thus, SIMOV 1 and SIMOV 2 operations can be analyzed according to the ON and OFF positions of the IGBTs as following. Switching State 1: In this case, both the S1 and the S2 are ON. Each inductor in the circuit is charged from the input source connected to its own layer. The output energy is supplied through the capacitors and the power to be transferred can be changed by controlling the duty cycles of the S1 and the S2 . Switching State 2: In this case, both the S1 and the S2 are OFF and the diode D1 is forward biased because the inductance L1 is in discharge mode. Thus, the energy flows to the loads and to the capacitor C1 via the inductance L1 . In the second layer, the energy also flows to the loads and to the capacitor C2 via the inductance L1 and the active input source. Eq. (3) gives the continuous mode voltage gain of the SIMO mode, where Vin specifies the voltage of the active input source, and d1 and d2 indicate the duty cycle ratios of the S1 and the S2 , respectively. Fig. 2. Flowchart of the mode detection algorithm. Table 1 Switch positions for operational modes. Mode SIMOV1 SIMOV2 MIMO IGBTs Switches of the Aux relays for power routing moden S1 S2 s3 s4 s5 s6 s7 s8 1 2 3 Gate1 Gate1 Gate1 Gate2 Gate2 Gate2 1 1 0 0 0 1 1 0 0 0 0 1 0 1 1 1 1 0 2.1. Analysis of MIMO operation VOUT 1 (1 − d1 ) + VOUT 2 (1 − d2 ) = V1,2 (1 + d1 ) According to Fig. 1 and Table 1, if the contacts of auxiliary relays s4 , s6 and s7 are ON while the others are OFF, the converter starts to operate in MIMO mode. In this case, V1 and V2 sources are connected to separate boost converters as seen from Fig. 1. Duty cycles of the IGBTs (S1 and S2 ) are separately controlled so that the output voltages can be obtained independently. Thus, it is possible to feed the loads requiring different voltage levels. To better analyze the dynamic response of the proposed model in MIMO mode, ON and OFF states of the IGBTs are studied separately as following. Switching State 1: In this state, S1 and S2 are ON. Once their modes are changed to ON, the inductances L1 and L2 start to 2.3. Mathematical analysis (3) Mathematical models are separately created for each operating mode to better analyze the control process of the input currents in CCM. During the MIMO operation, both layers are in the boost mode as mentioned previously. Similar to the previous notations, 1 and 2 indicate the first and second layers, respectively. Also, V1 and V2 indicate the voltages of first and second input sources, respectively. If both the S1 and the S2 are ON, the current of both inductors (iL1 , iL2 ) can be calculated using the same equation as given in Eq. (4). If they are OFF, the total output power is supplied from input sources and also from the inductor 487 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 that passes to discharging mode as seen from Eq. (5). Instead of using separate equations for ON and OFF positions, Eqs. (4) and (5) can be combined in only one statement as given in Eq. (6). diL1,2 ( = dt diL1,2 1 = diL1,2 = ) 1 ) (4) ( ) ( ( V1,2 − RL1,2 iL1,2 − VOUT1,2 L1,2 ( dt ) 1 ( V1,2 − RL1,2 iL1,2 L1,2 ( dt ) S1,2 = )) (6) diL1,2 1 for ON position of the sw itch 1 or 2 dt dt dVOUT1,2 dt C1,2 R1,2 iL1,2 = VOUT1,2 C1,2 ( − 1 dt diL1 dt diL1 dt ( = ) = 1 ) L1 ( 1 ( V1,2 − RL1 iL1 L1 ( = 1 (8) ) L1 ) ( ) V1,2 (S1 ) − RL1 iL1 − VOUT1 (1 − S1 ) Vin1 = for mode1 (represents the SIMOV 1 mode) ⎪ V1 ⎪ ⎪ ⎪ ⎩ for mode3 (represents the MIMO mode) 0 for mode2 (represents the SIMOV 2 mode) Vin1,2 ] moden − RL1,2 iL1,2 − VOUT1,2 (1 − S1,2 ) ) The inductor current and the output voltage of the converter can be described as independent states for each layer. The exact representation of state–space model can be written as in Eqs. (15) and (16). ẋ = Ax + Bu (15) y = Cx + Du (16) (10) The state vector of the proposed converter can be defined as in Eq. (17). (11) x = iL1 VOUT1 iL2 VOUT2 [ ]T (17) Since the inherent of switched power converters, averaged models are used to analyze the converter in one switching period. As seen from Eq. (18), the average state equation is created by combining the state equations of both ON and OFF positions. Clearly, the source voltage in the differential equations may change depending on the activated source. Therefore, the SIMO operation is separated as SIMOV1 and SIMOV2 modes and the differential equations are structured based on the activated source. Since V1 or V2 can be used as the source in SIMO mode, source selectivity is essential to define the active source in Eq. (11). Source selectivity means that substituting the voltage of activated source (V1 or V2 ) with V1,2 in Eq. (11). In order to define the active source in a common current equation for all operational modes, the input voltage arrays depending on the input sources are created as given in Eqs. (12) and (13). Moreover, the difference between differential equations of boost and buck-boost is defined by using switch position (S1 ) in Eq. (12). ⎧ V1 S1 ⎪ ⎪ ⎪ ⎪ ⎨V2 S1 L1,2 [ 2.4. Continuous-time model (9) ) ( −RL1 iL1 − VOUT1 )( As mentioned earlier, operational mode of the converter is determined by a mode detection algorithm and it is configured to generate the n value which is used in Eqs. (12) and (13). Thus, the changes in the operational mode are defined in the system model. As seen from Eq. (14), the input voltages are determined from the arrays according to the operational mode of the converter. Consequently, the input currents can be calculated by using the same equation even the input sources are changed. For the SIMO operation, the first layer is in buck-boost mode while the second one is in boost mode. Since the type of the second layer is not changed, Eq. (6) given above can also be used for the second layer in SIMO modes. It is worth noting that the input source in Eq. (6) may change depending on the active source in SIMO mode. The differential equations of the first layer can be expressed with Eqs. (9) and (10). As clearly seen from Eq. (9), the inductor is charging by the activated power source (V1 or V2 ) for ON position of the S1 . For OFF state of the switch, the inductor current can be expressed with Eq. (10), where it is seen that the output power is only supplied via the inductor. According to the switch position, Eq. (11) can be derived by combining Eqs. (9) and (10). diL1 1 n = {1, 2, 3, 4 mode number used in Eqs. (12) and (13)} (7) VOUT1,2 ( = where; ) C1,2 R1,2 for OFF position (14) ) 1 for mode4 (both input sources are disabled) 1 for ON position Finally, Eq. (14) is obtained after the input voltage arrays in Eqs. (12) and (13) are combined with the current equations in Eqs. (6), (11). for OFF position of the sw itch 1 or 2 ( (13) where; S1 = 0 =− for mode3 (represents the MIMO mode) 0 (5) The differential equations of the output voltage can be described with Eqs. (7) and (8) for ON and OFF positions of the switch, respectively. dVOUT1,2 ⎪ V2 ⎪ ⎪ ⎪ ⎩ for mode2 (represents the SIMOV 2 mode) 0 where; { for mode1 (represents the SIMOV 1 mode) { V1,2 − RL1,2 iL1,2 − VOUT1,2 1 − S1,2 L1,2 Vin2 = ⎧ V1 ⎪ ⎪ ⎪ ⎪ ⎨ V2 Aavg = dA1 + (1 − d ) A2 (18) where, d denotes the duty ratio of switching signal and A1 is the state matrix for S = ON, while A2 represents the state for S = OFF. Eqs. (19) and (20) describes both layers of the proposed model in matrix form. ⎡ ( ) RL1 ⎢− L1 ⎢ ⎢ ⎢ 0 ⎢ A1 = ⎢ ⎢ ⎢ 0 ⎢ ⎢ ⎣ (12) 0 for mode4 (both input sources are disabled) 488 ⎤ 0 ( − 1 0 0 C1 R1 0 0 0 ) ( − RL2 ) L2 0 − ⎥ ⎥ ⎥ ⎥ 0 ⎥ ⎥ ⎥ ⎥ 0 ⎥ ( )⎥ ⎦ 1 C2 R 2 (19) N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Fig. 3. Magnitude and phase responses under a: steady-state, b: various input voltage, c: various duty ratio, d: various load conditions. ⎡ ( ) RL1 − ⎢ ⎢ ( L1) ⎢ 1 ⎢ ⎢ C ⎢ 1 A2 = ⎢ ⎢ ⎢ 0 ⎢ ⎢ ⎣ ( ) 1 − L ( 1 ) 1 − 0 0 C1 R 1 ( 0 0 0 0 RL2 ) − L ( 2) 1 C2 ⎤ ⎥ ⎥ ⎥ ⎥ 0 ⎥ ⎥ ( ) ⎥ ⎥ 1 ⎥ − ⎥ L ( 2 )⎥ ⎦ 1 − load resistance, and the results are presented in Fig. 3(b), (c), and (d), respectively [31]. Clearly, the converter is stable despite all variations. The proposed mode-changeable converter is based on a twolayer DC/DC boost converter and each layer is a second-order system. Although the layers are derived using the conventional topologies, the mode-changeable structure of the proposed model requires some specifications in terms of designing the controller. The requirements in the control strategy can be described as the fast dynamic response and a good match with the changes in topology. In general, the fast dynamic response is needed to achieve a good reference tracking under variations in system inputs and outputs. In addition to this, the dynamic response is important for mode transitions times in the proposed topology. Furthermore, the type of the first layer is changing between boost and buck-boost in the transition from MIMO to SIMO, viceversa. It is clear that the control method should exhibit a good match with the changes in topology. A model-based reconfigurable controller type is essential and special for the proposed mode-changeable converter. On the other hand, MPC exhibits an excellent dynamic performance for power converters. Considering the requirements, model predictive control is used to control the proposed converter. The average input currents are controlled with the model predictive control (MPC) technique, block diagram of which is C2 R 2 B = [1/L1 0 1/L2 0] (21) C = [0 1 0 1] (22) T 3. Model predictive control of the proposed system (20) The averaged state–space model can be obtained by substituting Eqs. (18), and (21) into Eq. (15). Since the design of the converter plays a crucial role in the performance of closed-loop controller, the open-loop bode diagram is presented to show the stability of the converter. Fig. 3 shows the bode plots which are obtained using the system parameters and state–space model for the first layer. As mentioned before, the proposed model consists of a two boost converter. Hence, the results of each layer are the same and the dynamics are investigated for only the first layer. The bode result of the steady-state condition is shown in Fig. 3(a). The phase angle and magnitude are negative at the crossover frequency. Clearly, the selected design parameters are suitable for steady-state conditions. However, large variations in the system input and outputs may cause instabilities. Therefore, the bode analysis is evaluated by variations in input voltage, duty ratio, and 489 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Table 2 Model parameters. Switching frequency Inductors (L1 and L2 ) Output filters (C1 and C2 ) IGBTs (S1 and S2 ) TS 5–10 kHz 1 mH 1000 µF 2MBI100U4A – 120 – 50 10 µs The key point of the MPC algorithm is the cost function, which is created by using the estimation results and the reference values (i∗L and i∗L ). Eq. (25) presents the created cost function for each 1 2 layer of the converter. [ gidc1,2 (k + 1) = i∗L1,2 − iL1,2 (k + 1) ]2 (25) By utilizing the cost function, the algorithm decides the ON or OFF positions of the IGBTs. Since this digital control method does not use a comparator, the switching frequency is variable. In the study, the frequency is reduced by benefiting the advantages of MPC method, as it is able to control multiple system parameters together. In order to reduce the switching frequency (fsw ), the cost function given in Eq. (26) is derived [32], where λ indicates the weighting factor that has been determined as 0.01 by using the cost function classification technique. A detailed information about the use of this technique can be found in [19]. gsw1,2 (k + 1) = λ ∗ ⏐S1,2 − S1,2 (k − 1)⏐ ⏐ ⏐ (26) After defining an ideal weighting factor, it is used for reducing the maximum switching frequency. Eq. (27) gives the complete cost function obtained by combining the Eqs. (25) and (26). g1,2 (k + 1) = gidc1,2 (k + 1) + gsw1,2 (k + 1) Fig. 4 illustrates the flowchart of the MPC algorithm that starts to operation by measuring the input parameters. In addition, n parameter which is generated by the mode detection algorithm is integrated with the MPC to define the operational mode changes in the system model. For each sampling step, the prediction algorithm is separately run for two different positions (ON and OFF ) of the related switch. Thus, two cost functions are created using the values obtained from the prediction result. Then, the switch positions are determined by minimizing the cost function value and appropriate control signals are generated for the next sampling step. Fig. 4. Flowchart of the control algorithm. illustrated in Fig. 1. As seen, the MPC consists of two steps as the predictive model and the cost function optimization. Current values at the next sampling iteration are estimated by discretetime model. The cost function optimization minimizes the error between the reference value and the estimated value. The control signals applied to the IGBTs are generated according to the minimization result. Since the MPC method operates depending on a specific time step (TS ), it is necessary to discrete the control equation given in Eq. (14) previously. For this purpose, the forward-difference Euler method is used as given in Eq. (23), and the expression obtained after the discretization is given in Eq. (24) that determines the predicted current value and evaluated separately for each layer of the converter. During a sampling step, discrete-time equations are calculated for all possible switch positions. diL1,2 dt ≈ iL1,2 (k + 1) − iL1,2 (k) iL1,2 (k + 1) = [( Ts )( [ ] TS Vin1,2 mode − RL1,2 iL1,2 (k) n L1,2 4. Simulation results In order to test and verify both the proposed converter model and its control algorithm, some simulation studies are performed in MATLAB/Simulink. Table 2 presents the parameters used in the simulation model. In addition to the current control capability of the MPC algorithm, the transient states occurred in mode transitions are tested especially. During the simulation studies, the switching frequency is changed between 5 kHz and 10 kHz. (23) 4.1. Simulation results for MIMO operation When the converter operates in MIMO mode, its capability of tracking a fixed reference is tested under variable input voltages. Each converter layer starts to operate when its input voltage exceeds 10 V that is the predefined threshold value in the control algorithm. Fig. 5 shows the results of the first case where the reference current is set to 2 A. As seen, the input voltage of the first layer is suddenly increased from 20 V to 35 V after 1 s from the start. In order to see the transient response of the system against this situation, the precise time of transition is zoomed and indicated as Z1. As obviously seen, the ] ( )) −VOUT1,2 (k) 1 − S1,2 + iL1,2 (k) (27) (24) where; iL1,2 (k+1) : predicted current values of iL1 or iL2 Ts : Sampling time moden : Corresponding operational mode as given in Eqs. (12) and (13). 490 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 control algorithm immediately makes the related switch OFF for three periods to prevent the current increase when the voltage increases. Furthermore, since the proposed system is able to generate variable switching frequencies, the control algorithm reduces the switching frequency. As the second case, the input voltage is decreased from 35 V to 25 V at 1.4 s. The second zoomed part indicated as Z2 on Fig. 5 presents the system response against this transitional situation. Similar to the previous case, the system successfully tracks the reference by adjusting the duty ratio of the switching signal to an appropriate level for new conditions. The analysis given Fig. 5 is not only achieved for the first (positive) layer, but also for the second (negative) layer. For the first case where the input voltage is suddenly increased from 15 V to 35 V, the control algorithm makes the related switch OFF until the input current decreases to the desired level. This situation can be seen obviously from the zoomed part indicated as Z3 on Fig. 5 After the instant increase on the voltage signal, the system automatically adjusts the PWM ratio to track the reference successfully. As seen from the last zoomed part indicated as Z4, the system successfully continues to track the reference by increasing the duty ratio of the switching signal even the voltage decreases instantly. In addition to the control results of input currents, output voltage of both layers are higher than input voltages. Clearly, both layer operates in boost mode during the MIMO mode. As the second test for the MIMO mode, the system performance under variable references is examined. Fig. 6 presents the simulation results for this case, where the reference currents change instantly while both layers operate at a fixed input voltage. According to the scenario, the reference value of the positive layer increases from 2 A to 4 A at 0.2 s. As shown from the zoomed part indicated as Z1 on Fig. 6, the control algorithm keeps the switch ON for 2 periods in order to increase the input current to the reference. As a further scenario, the reference is decreased to 3 A at 0.25 s. The zoomed part indicated as Z2 shows that the input current is successfully adjusted to desired level in this case too. Ability of the proposed converter for tracking the variable current references is also examined for the second (negative) layer. The zoomed parts indicated as Z3 and Z4 on Fig. 6 present the results for these analyzes in detail and confirm that the proposed control algorithm successfully executes the current control process. As a common result for this section, all simulation results performed to analyze the current control operation in MIMO mode verify that the proposed algorithm serves a fast and effective control for not only in the steady state conditions but also in the transitional events. For both SIMO modes, reference value of the layers are changed several times to analyze the system response against these transient situations. As obviously seen from results in Fig. 7, the proposed control algorithm successfully achieves to track the reference in all cases in similar to MIMO mode operations. Although the input voltages are altered, the average value of the current is not affected from this situation. As mentioned in the theoretical analysis, the first layer is in buck-boost mode while the second one is in boost mode for SIMO modes. It is worth noting that, the output voltage of the first layer is dependent on the reference current, load resistance and input voltage. As clearly seen from Fig. 7, the operational mode of the first layer is changed depending on the reference current and it can be operated in buck and boost modes. Furthermore, the results show that the second layer is operated in boost mode in both SIMO modes. Consequently, the theoretical considerations have been verified by simulation studies for all operational modes of the proposed converter. 5. Experimental study A real prototype of the proposed system, shown in Fig. 8, is designed and implemented to test the converter experimentally in real time conditions. As stated in Table 2, the proposed converter is built by using 2MBI100U4A-120-50 IGBTs. The relays are used to route the power depending on the operation mode of the converter. LV 25-P voltage sensors are used as transducers for measuring the input and output voltages. Input and output currents are sensed using HAS 50-s. The control software is designed using MATLAB/Simulink environments and it is embedded in dSPACE ds1104. Two separate DC voltage sources are used as the main energy sources. To better analyze the proposed modechangeable converter and its control algorithm, experimental results are presented for MIMO and SIMO modes, separately. 5.1. Experimental results for MIMO operation To test the converter in MIMO mode, its response to instant reference changes under fixed input voltages is examined. Fig. 9 shows the dynamic responses of input currents and switching signals for abrupt changes in the references of inductor currents. As seen from Fig. 9(a), when the reference for the first layer changes from 2 A to 4 A, the control algorithm increases the current by keeping the related switch in ON position for two periods (230 µs). After this transient situation, the converter continues to its operation with an average current of 4 A. If the reference decreases from 4 A to 2 A, the MPC algorithm keeps the switch in OFF position until (200 µs) the current decreases to 2 A as seen from Fig. 9(b). The same procedure is also realized for the second layer. Fig. 9(c) verifies that the proposed model successfully provides the average current tracking process when the reference current for the second layer increases from 1.5 A to 3 A. Even though the reference decreases to 1.5 A again, the system continues to track it successfully as seen from Fig. 9(d). Considering the results in Fig. 9, if the reference increases, the switch remains at ON position until the inductor current reaches to the peak value of the desired level. Similarly, if the reference decreases, the switch passes to OFF position until the inductor current reaches to the new peak value. Clearly, the dynamic responses in the switching signal shows that MPC selects an optimal control action independent from switching frequency. As mentioned earlier, the switching signal is generating as the result of a logic comparison in digital control methods. This is the fact that behind the superior dynamic behavior of MPC. Furthermore, the MPC algorithm eliminates the oscillations occurred in the transient state and shortens the response time as well. On the 4.2. Simulation results for SIMO operation SIMO operation means that one of the input sources is deactivated due to operational conditions and thereby the system works with only one input while there are multi outputs at the load side. Accordingly, while SIMOV1 expresses the situation where the second input is turned off and only the first one feeds the system, SIMOV2 describes the opposite of this condition. A critical threshold voltage level as 10 V is determined in the study. Thus, if the voltage level of one of the input sources decreases below this level, the system automatically disables the related input. Fig. 7 presents the simulation results under the variable reference values when the system operates in SIMO mode. As seen, the system starts to operation in SIMOV1 mode as soon as the first input voltage exceeds 10 V. According to the scenario, the first layer is suddenly disabled and the second one is activated simultaneously at 0.5th second. Upon this new condition, the system automatically switches its mode to SIMOV2 . 491 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Fig. 5. Results of tracking a fixed reference under variable input voltages in MIMO mode. Fig. 6. Step change analysis of the currents in MIMO mode. other hand, the results show that the input current of each layer is independent of each other. This feature allows the independent power flow from the input sources. Also, it can be worth noting that the cross-regulation problem has not occurred. Depending on the input voltage and the reference signal, the switching frequency is changed between 5 kHz to 10 kHz during the experimental tests. This is the reason why the oscillation level on the inductor current changes as seen in the current graphs. As reported in [20], even though this issue can be alleviated by increasing the switching frequency, this is not the case for high power applications where a low switching frequency is required. For this reason, controller design for high-power DC–DC converters working at relatively low frequency (<20 kHz) has an increasing practical value. In addition to test the system response against the reference changes, the fixed current control analysis under variable input voltages is also tested experimentally and the maximum error for the average current is measured as 4%. For instance, Fig. 10(a) 492 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Fig. 7. Simulation results under the variable reference currents in SIMO mode. Fig. 8. The real prototype of the model for experimental study. the inductor current in the desired reference against the changes in the input voltage. Consequently, all the experimental results described above separately for each layer verify that the proposed control algorithm successfully achieves the current control process in MIMO mode even the input voltages are changed. presents a sample experimental result where the input voltage of the first layer decreases from 25 V to 15 V. As seen, the average input current is fixed to the reference value with an error of 2.5% while the input voltage is 25 V. After the voltage decreases to 15 V, the control algorithm again sets the input current to the reference with an error of 0.5%. In similar to the first layer, the second one serves satisfactory results for the average current control process. Fig. 10(b) presents an experimental result where the input voltage of the second layer (VIN2 ) increases from 20 V to 30 V. Once the voltage increase is occurred, the control method fixes the average current value to the reference value with an error of 1%. Actually, these error rates given for both layers mainly cause from the measurement accuracy of the sensors and ADC resolution of the controller. The dynamic responses of input and output variables show that the proposed converter and its control method are able to maintain 5.2. Experimental results for mode transitions and SIMO operation As mentioned in the theoretical considerations, the mode transition capability is the main advantage of the proposed converter. Experimental results of mode transitions and SIMO operational modes are investigated in this section. As clearly seen from Fig. 11, the second input source (V2 ) is suddenly disconnected while the system is being operated in MIMO mode. In this case, the control algorithm changes the positions of the auxiliary relays 493 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Fig. 9. Experimental results for tracking the variable references in MIMO mode (a, b) the first layer (c, d) the second layer. Fig. 10. Current control analysis in MIMO mode (a) the first layer (b) the second layer. according to Table 1 thereby the system passes to SIMOV 1 mode. During the transition, the first layer remains at disabled for 5 ms while the relay switches are being repositioned. However, the duration of this temporary time depends on the processing speed of the controller and the specifications of hardware devices used in the system such as relays. As seen from the detailed current graphs shown in Fig. 11, while the average current is not changed after the mode transition from MIMO to SIMO, the oscillation on the current signal increases. This is due to the positive layer operating in the buckboost mode. On the other hand, even if the input source of the second layer is disabled, its inductor current is maintained at 2 A. The result verifies that the proposed converter and its control algorithm successfully route the power flow. As a different experimental scenario, the second input source is again enabled while the system operates in SIMOV1 mode. In this case, the system returns back to MIMO mode as seen from Fig. 11. It is experimentally observed that the average current 494 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Fig. 11. Experimental results of the mode transitions. Fig. 12. System responses in SIMO mode a: Reference change in SIMOV 1 mode (IL1 ), b: Voltage change in SIMOV 2 mode (VIN2 ). values have not changed in this operational condition too. In order to test the system in SIMOV2 mode, the first input source (VIN1 ) is disabled. After that, it has been took 32ms for cutting off the energy of positive layer and changing the converter’s mode from boost to buck-boost. After the mode change process, the positive layer operates as buck-boost while the negative one operates in boost mode. In this case, the MPC algorithm fixes the average current of both layers to 2 A as seen from the detailed current graphs in Fig. 11. Similar to the results of the mode transition from MIMO to SIMOV1 , the results verify that the power flow is continuing in both layers. After analyzing the system response in SIMOV1 and SIMOV2 modes, the first input source is re-activated to switch the converter to MIMO mode as seen from the last part of Fig. 11. Similar to all other cases, the control algorithm successfully continues to achieve the average current tracking process. Thus, all the experimental tests conducted for analyzing the system response during the input source transitions verify that the proposed converter automatically determines its operational mode according to the input conditions. Moreover, the developed MPC algorithm successfully provides the current controlling process under all operational situations. Another experimental test is conducted by changing the reference value of the input current in SIMOV1 mode. Fig. 12(a) Fig. 13. The efficiency analysis for both MIMO and SIMO modes. shows the dynamic responses of input variables for a step change in the reference current from 1 A to 2 A. As clearly seen, the control algorithm keeps the related switch in ON state until the current of positive layer increases to 2 A, and then continues to operate with the new reference. Fig. 12(b) illustrates a sample system response against the input voltage changes in SIMOV2 mode. While the VIN2 input voltage is 15 V in this experimental scenario, the average currents of the first and the second layers are 1 A and 2 A, respectively. As seen from the figure, these 495 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 is to control the inductor current, the robustness of the control method is investigated under variations in inductance. The proposed topology consists of two boost converters with the same component parameters. Since the type and parameters of both layers are the same as each other, the influence of parameter variations is analyzed with the first layer by changing L1 . The inductance value in the control software (L1,control ) is changed in the range ±50%. The variation in the inductance value is calculated by Eq. (31). The current error between the reference and measured current is calculated with Eq. (32). ∆L1 (%) = ∆iL1 (%) = average current values are not changed even the input voltage increases to 25 V. However, the ripple on the current signals increases when the input voltage is increased due to the change of the switching frequency and input voltage. When the input voltage reduced from 25 V to 15 V, the ripples on input currents return back to the initial condition. ηSIMO (%) = POUT × 100 = (VOUT 1 iOUT 1 ) + (VOUT 2 iOUT 2 ) × 100 V1 iL1av g + V2 iL2av g (28) Pin × 100 = (VOUT 1 iOUT 1 ) + (VOUT 2 iOUT 2 ) V1,2 iinav g × 100 (29) ∫ iinav g = d1 t +T1 iL1 (t ) dt + iL2av g iL1av g × 100 (31) (32) The comparison of the proposed mode-changeable topology with four existing multi-port DC/DC topologies is presented in Table 3. The comparison includes topology, control method, number of switching devices, drivers, switching signal generation method, switching frequency, efficiency, maximum power and mode-changeability feature between SIMO and MIMO operational modes. Among the existing topologies, the proposed topology exists the mode-changeability feature which is able to automatically change the operational modes between SIMO and MIMO. Furthermore, the maximum efficiency in the SIMO mode can be specified as a remarkable advantage of the proposed topology. Since the proposed topology is built by two separate inductors, the crosscoupling effect has not occurred. Hence, the efficiency of the proposed topology higher than the single-inductor SIMO converter in [12]. The auxiliary switch requirement can be pointed as a disadvantage of the proposed topology. However, the switches are only contacts of relays and it does not require special driving devices. Therefore, the usage of such switches is a cheap and practical solution for routing the power. On the other hand, while the proposed topology and SIMO-buck [1] can be implemented with two switching devices, the other topologies are needed more switches such as IGBT and MOSFET. In order to analyze the system efficiency, Eqs. (28) and (29) are derived for MIMO and SIMO modes, respectively. The average current values of the inductors are used for calculating the input power. Unlike MIMO operation, cross-coupling effects disturb the total input current in SIMO operation because the first layer operates in buck-boost mode. Therefore, the average value of the total input current can be calculated by using Eq. (30), where T1 indicates the period time of the first switching signal (d1 ). Pin 1 × 100 5.5. Comparison with existing control methods 5.3. Efficiency POUT i∗L L1 − iL1avg Since the cost function has two objectives as control of inductor current and reducing the switching frequency, the influence of the variations is investigated on the current and average switching frequency as given in Fig. 14. Clearly, the average current error increases depending on the parameter variation. It is evident from the figure, the maximum average control error is 2.13% despite −50% variation in the inductance. On the other hand, the influence of the variations is also seen in the average switching frequency. Fig. 14. Current error and average switching frequency results under parameter variations in L1 . ηMIMO (%) = L1,control − L1 (30) t After analyzing the efficiency, a graph is created according to the output power as presented in Fig. 13. While the output power increases, the efficiency decreases in both operational modes. In MIMO mode, the maximum efficiency is about 97% and the minimum efficiency is about 85%. These values for SIMO mode are 87% and 83%, respectively. Efficiency of the similar systems in recent literature is between 80% and 98% depending on the rated power [1,33]. Considering them, the proposed converter offers the similar efficiency. 6. Conclusion In this article, a mode-changeable DC/DC converter with the capability of automatically set its operational mode to either MIMO or SIMO is proposed for hybrid operated energy sources. In case of any interruption in one of the input sources, the modechangeable structure maintains the power flow from the other source. Auxiliary relays are used in the circuit to route power from multiple sources to multiple loads, and their positions are determined by a mode detection algorithm. MPC algorithm is used to control the currents of the input sources and it is modified to adapt the algorithm with the changes in topology. The theoretical considerations are verified by both simulation and experimental studies. The main purpose of this study is to design a mode-changeable converter to be used in intermittent energy sources. The results reveal that the proposed converter is able 5.4. Influence of parameter mismatch The MPC method is a model-based control strategy and it predicts the next value of the control variable depending on the system parameters. Therefore, the accuracy of the controller highly depends on system parameters which are defined in the control software. The parameters of the system components such as inductance and capacitance may vary depending on their tolerances and ages. Since the objective of the MPC in this paper 496 N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498 Table 3 Comparison of four existing multi-port DC/DC topologies with proposed mode-changeable topology. Description [1] [6] [10] [12] Proposed Topology mode-changeability Control approach Number of switching device Number of auxiliary relay Number of driver circuit Switching signal generation method Switching frequency Maximum power Maximum efficiency SIMO-buck Does not exist Robust decentralized 2 Does not exist 2 PWM MIMO-buck-boost Does not exist PI 4 Does not exist 4 PWM SIMO-buck Does not exist MPC 3 Does not exist 3 Digital Multi-mode (SIMO and MIMO) Exists MPC 2 4 2 Digital 1 kHz Not reported Not reported 20 kHz 240W Not reported MIMO Does not exist MPPT based PWM controller 5 Does not exist 5 Requires a special PWM generation strategy Not reported 2.5 mW 87.6% Not reported 100 W 83.1% 5 kHz–10 kHz 195 W 97% (MIMO) 87% (SIMO) to set operational mode according to the voltage level of input sources. Moreover, the modified MPC exhibits excellent performance under all operating conditions. It reveals that the modified MPC is adapt itself to changes in the operational modes. 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