ISA Transactions 114 (2021) 485–498
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ISA Transactions
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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,∗
aTechnical Sciences Vocational School, Gazi University, Ankara, Turkey
bElectrical 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
abstract
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 light-
ing. 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 re-
duce 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).
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 com-
plicated. Besides, SI-MIMO converters have limited voltage gain
because they usually cannot provide the symmetrical output volt-
age. 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 measure-
ment 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
https://doi.org/10.1016/j.isatra.2020.12.023
0019-0578/©2020 ISA. Published by Elsevier Ltd. All rights reserved.
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 re-
duced 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 buck-
boost 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 re-
quirement, 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 cur-
rents 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 discontinu-
ous 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 devel-
oped 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 tech-
niques complicates the implementation of the control method.
In this paper, the average current control of the proposed mode-
changeable converter is implemented without neither calculation
method nor observer. CCM operation is provided with the selec-
tion of converter parameters in accordance with the methodology
reported in [30] and then model predictive current control struc-
ture is applied. Both the simulation and the experimental results
show that the proposed converter and its control algorithm suc-
cessfully 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 sev-
eral 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 algo-
rithm 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 algo-
rithm automatically integrates the active sources to the system
by changing the operational mode between MIMO and SIMOs.
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N. Güler and E. Irmak ISA Transactions 114 (2021) 485–498
Fig. 2. Flowchart of the mode detection algorithm.
Table 1
Switch positions for operational modes.
Mode IGBTs Switches of the Aux
relays for power routing
modenS1S2s3s4s5s6s7s8
SIMOV1 1 Gate1Gate21 0 1 0 0 1
SIMOV2 2 Gate1Gate21 0 0 0 1 1
MIMO 3 Gate1Gate20 1 0 1 1 0
2.1. Analysis of MIMO operation
According to Fig. 1 and Table 1, if the contacts of auxiliary
relays s4,s6and s7are ON while the others are OFF, the converter
starts to operate in MIMO mode. In this case, V1and V2sources
are connected to separate boost converters as seen from Fig. 1.
Duty cycles of the IGBTs (S1and 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, S1and S2are ON. Once their
modes are changed to ON, the inductances L1and L2start to
being charged through the input sources. At the same time, the
capacitors C1and C2are discharged through the loads.
Switching State 2: In this case, S1and S2are OFF. Since the
inductance L1is in discharging mode, the diode D1is forward bi-
ased. Therefore, the energy is transferred from the L1inductor and
the V1input source to the C1capacitor. The first layer operates as
a typical boost converter in this mode. Similarly, the energy flows
from the L2inductance and the V2input source to the C2capacitor
on the second layer.
For MIMO operation, the relation between the output voltages
(VOUT 1and VOUT 2) and the inputs is calculated by using the typical
boost converter equations as given in (1) and (2), where d1and
d2indicates the duty cycle ratios of the switches S1and S2,
respectively.
VOUT 1=V1
1
(1 −d1)(1)
VOUT 2=V2
1
(1 −d2)(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 s7is OFF in SIMOV1
mode. Therefore, the source V2is completely separated from the
system. Similarly, the source V1is separated from the system
through the auxiliary switches (s5and s6) during the SIMOV2
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 (SIMOV1and SIMOV2), while the first layer operates in
buck-boost mode, the second one operates in boost mode. Thus,
SIMOV1and SIMOV2operations can be analyzed according to the
ON and OFF positions of the IGBTs as following.
Switching State 1: In this case, both the S1and the S2are 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 S1and the S2.
Switching State 2: In this case, both the S1and the S2are OFF
and the diode D1is forward biased because the inductance L1is
in discharge mode. Thus, the energy flows to the loads and to the
capacitor C1via the inductance L1. In the second layer, the energy
also flows to the loads and to the capacitor C2via the inductance
L1and 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 d1and d2indicate the duty cycle ratios of the S1and the S2,
respectively.
VOUT 1(1−d1)+VOUT 2(1−d2)=V1,2(1 +d1) (3)
2.3. Mathematical analysis
Mathematical models are separately created for each oper-
ating 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, 1and 2indicate the first and second layers, respec-
tively. Also, V1and V2indicate the voltages of first and second
input sources, respectively. If both the S1and the S2are 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
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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 =1
L1,2V1,2−RL1,2iL1,2(4)
diL1,2
dt =1
L1,2V1,2−RL1,2iL1,2−VOUT1,2(5)
diL1,2
dt =1
L1,2V1,2−RL1,2iL1,2−VOUT1,21−S1,2 (6)
where;
S1,2=0for OFF position of the switch 1or 2
1for ON position of the switch 1or 2
The differential equations of the output voltage can be de-
scribed with Eqs. (7) and (8) for ON and OFF positions of the
switch, respectively.
dVOUT1,2
dt = − 1
C1,2R1,2VOUT1,2(7)
dVOUT1,2
dt =iL1,2
C1,2
−1
C1,2R1,2VOUT1,2(8)
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
dt =1
L1V1,2−RL1iL1(9)
diL1
dt =1
L1−RL1iL1−VOUT1(10)
diL1
dt =1
L1V1,2(S1)−RL1iL1−VOUT1(1 −S1)(11)
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 dif-
ferential equations are structured based on the activated source.
Since V1or V2can 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 (V1or V2) with V1,2in 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).
Vin1=
V1S1for mode1(represents the SIMOV1mode)
V2S1for mode2(represents the SIMOV2mode)
V1for mode3(represents the MIMO mode)
0for mode4(both input sources are disabled)
(12)
Vin2=
V1for mode1(represents the SIMOV1mode)
V2for mode2(represents the SIMOV2mode)
V2for mode3(represents the MIMO mode)
0for mode4(both input sources are disabled)
(13)
where; S1=0for OFF position
1for 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).
diL1,2
dt =1
L1,2Vin1,2moden−RL1,2iL1,2−VOUT1,2(1 −S1,2)(14)
where;
n= {1,2,3,4mode number used in Eqs.(12) and (13)}
As mentioned earlier, operational mode of the converter is
determined by a mode detection algorithm and it is configured to
generate the nvalue 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.
2.4. Continuous-time model
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).
˙
x=Ax +Bu (15)
y=Cx +Du (16)
The state vector of the proposed converter can be defined as in
Eq. (17).
x=iL1VOUT1iL2VOUT2T(17)
Since the inherent of switched power converters, averaged mod-
els 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.
Aavg=dA1+(1−d)A2(18)
where, ddenotes the duty ratio of switching signal and A1is the
state matrix for S=ON, while A2represents the state for S=OFF.
Eqs. (19) and (20) describes both layers of the proposed model in
matrix form.
A1=
−RL1
L1000
0−1
C1R10 0
0 0 −RL2
L20
000−1
C2R2
(19)
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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.
A2=
−RL1
L1−1
L10 0
1
C1−1
C1R10 0
0 0 −RL2
L2−1
L2
0 0 1
C2−1
C2R2
(20)
B=[1/L10 1/L20]T(21)
C=[0 1 0 1](22)
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
load resistance, and the results are presented in Fig. 3(b), (c), and
(d), respectively [31]. Clearly, the converter is stable despite all
variations.
3. Model predictive control of the proposed system
The proposed mode-changeable converter is based on a two-
layer 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, vice-
versa. It is clear that the control method should exhibit a good
match with the changes in topology. A model-based reconfig-
urable 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. Consider-
ing 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
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