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Multi-Loop Structure Based Sliding Mode Controller
for Positive Output Split Inductor Boost Converter G.Saritha1*,D.Kirubakaran2
1. Research scholar, Sathyabama Institute of Science and Technilogy, Chennai.
2 .Professor, St.Joseph’s Institute of Technology, Chennai.
* Assistant Professor, Sri Sai Ram Institute of Technology, Chennai..
Abstract
This article investigates the Multi-Loop Structure Based Sliding Mode
Controller (MLSBSMC) for Positive Output Split Inductor Boost Converter
(POSIBC) operated in Continuous Conduction Mode (CCM). It converts the
positive DC input voltage into the positive DC output voltage. On account
of the on-off switching with time varying of POSIBC and its dynamic
analysis becomes non-linear. The traditional Proportional-Integral (PI)
controller is non-capable to regulate the output voltage and inductor (coil)
current of POSIBC. As a result to improve the output voltage and inductor
current along with dynamic analysis, MLSBSMC is designed. The sliding
mode controller co-efficients are computed from the state space averaging
model of the POSIBC. The performance of the designed model is validated
at various operating conditions by making the Matrix Laboratory
(MATLAB)/Simulation Link (Simulink) software platform. The results are
presented to show the proficient of the designed MLSBSMC.
Keywords: DC-DC power conversion, positive output split inductor boost
converter, sliding mode controller, state space averaging method.
1. Introduction
In current scenario, Luo-Converters (LC) are DC chopper and it plays a
main role in power source for various applications such as solar energy, fuel
cell, DC/AC micro grid and medical equipments etc.,. LC has good voltage
transfer gain, reduced ripples of capacitor voltage/inductor current and
proficient power density. In this article, Positive Output Split Inductor Boost
Converter (POSIBC) is chosen for study which is one of the topology of
LC. The controller design is a difficult one for POSIBC due to their
complex structure [1-2].
The Sliding Mode Controller (SMC) is one of the variable structure based
non-linear controller and it is more apt for variable structure system like
POSIBC. The main design of SMC has sliding surface and control law
based on this to satisfy its conditions. The classical proportional integral
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(PI) controller has not satisfied the large line and the load variation for the
system. The main benefits of SMC have proficient output voltage regulation
during the line and load disturbances, stable output voltage even though the
circuit parameters variation, flexible selection of sliding surface designs and
simple implementation. Novel SMC for buck-boost converter has been
simulated using MATLAB/Simulink [3]. However, the output voltage
regulation during line and load variation has produced more peak overshoot
using designed SMC. New controller for PV application is well presented
[4]. But, results of output voltage have generated large overshot and more
oscillation in line and load disturbance regions. Integral based SMC for DC
chopper is reported [5]. But, in this article addressed only for mathematical
analysis of the converter. Compound controller design for step-down
chopper has been reported [6]. Still, this controller has produced small
overshoot in disturbance operating conditions. New fixed frequency SMC
approach for power converters was reported [7]. But, this design for this
converter has generated less overshoot and rapid settling time. The detailed
study of SMC for buck converter is reported [8]. From this article, the
detailed analysis of SMC and its operating conditions were presented. The
fixed frequency based SMC for positive output triple-lift split inductor-type
boost converter is well presented [9]. But, the responses of this converter
with this controller has generated more maximum overshoots and settling
time in line and load variation in addition the transient region. The SMC for
Luo-Converters with fixed frequency operation has been executed [10].
From these articles, it is well visibly marked that the output voltage and
inductor current of the converters has created high overshoots and taking
long settling time with SMC. The current distribution control for shunt
connected various dc-dc converters using SMC is well presented [11]. Still,
these articles discussed about the regulation of the output current and
voltage for the SMC, which reported the additional number of sensors unit,
is essential, computation is complexity, and huge overshoots in dynamic
conditions. A PWM based double-integral type of SMCs for switched mode
power converter has been addressed [12-13]. Even if the results for used
control technique for the converter has produced more start-up overshoot,
more peak at line and load disturbances conditions, more steady state error
and settling time. Reduced order based fixed frequency SMC for Luo-
converter is well reported [14]. Still, the converter using this control method
has generated huge overshoots in line and load disturbances regions. The
fixed frequency based SMC for complex dc-dc converters has been reported
[15]. However, the troubles of this control method have more calculations,
implementation obscurity and needs of more sensors. The above problems
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are solved by Multi-Loop Structure Based Sliding Mode Controller
(MLSBSMC).
Therefore, in this article, it is developed to a design of a MLSBSMC for
POSIBC operated in Continuous Conduction Mode (CCM). The modeling
of POSIBC is derived with help of the famous state space averaging method
at first and then SMC parameters are derived.
2. Operation and Modeling of POSIBC
2.1 Operation and modeling of POSIBC
The power circuit of the POSIBC is illustrating in Fig. 1 (a). The POSIBC
consists of double inductors (L1, L2), output capacitor Co, two power
switches (S1, S2), output diode D, Vo is the output voltage and load
resistance R. The switches are controlled concomitantly using control
signal. POSIBC is assumed that all the elements are idyllic and also, the
POSIBC operates in CCM. The operation of the POSIBC will be divided in
to two modes through the switches-ON and the switches-OFF. Figure 1 (b)
and Figure 1(c) indicates the modes of operation of POSIBC [16].
(a) (b)
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(c)
Fig. 1 Power circuit of POSIBC, (a) topology, (b) equivalent circuit during
mode 1 operation, and (c) equivalent circuit during mode 2 operation.
Fig. 1(b) indicate the equivalent circuit of POSIBC in mode 1 operation. In
mode 1 operation, when (S1, S2) are ON and the diode D is not conducting.
The energy stored in Co liberates to the R. The Vin is connected to the
inductors in shunt arrangement; therefore, both the L1, L2 inductors are
energized. So, the VL1, VL2 can be expressed as equation (1)
1 2L L inV V V (1)
The state space equation of the POSIBC in mode one operation can be
written as equation (2)
11
0 00
Lin
diL V
dt
dV VC
dt R
(2)
In the mode two equivalent circuit of this converter as shown in Fig. 1(c). In
this mode, S1, S2 are in OFF mode, Co, D, L1, L2, and Vin are linked in series
arrangement. As per this construction, the net energy from the input source,
energy stored in the storage elements is transferred to the R. After that, the
voltage across the L1, L2 can be expresses as equation (3)
1 22
in oL L
V VV V
(3)
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The state space equation of this converter during the mode 2 working can be
inscribed as equation (4)
011
0 00 1
2
inL
L
V VdiL
dt
dV VC i
dt R
(4)
In POSIBC circuit, there are two inductors that are identical values. So, the
current flows through both in (iL1=iL2). Consequently, removing the any one
inductor (like trade-off selection) in the POSIBC. The state space variables
of the POSIBC are chosen such as the iL1, and Vo respectively x1, and x2.
Using (1) and (2), the modeling of the POSIBC can be written as equation
(5)
0
1 1 11 1
1
010 0
0 0 0 0
10 1
2 2 22
1 1 20
in
L L
in
L
V V
L L Li iLd V
ViV V
C RC C RC
(4 a)
.
X AX Bd C
Where, d is the switches status, x and .
X are the vectors of the state
variables (iL1, Vo) and their derivatives respectively,
1
0
S ONd
S OFF
(5)
2.2 Design of Multi-Structure Based Sliding Mode Controller
Step 1: Select the sliding surface
1 1 1 2 2,L oSliding Surface S i V M M (6)
Where, sliding controller coefficients M1 and M2 good positive gain values,
ε1 is the feedback inductor current error, and ε2 is the feedback output
capacitor voltage error,
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1 1 1L L refi i (7)
2 .o orefV V (8)
By inserting equations (7) and (8) in equation (6) to found (9)
1 1 1 1 2, ( ) ( )L o L L ref o orefS i V M i i M V V (9)
Equation (9), is applied to hysteresis modulator generates the PWM gate
pulses to power switches. The resulted MLSBSMC is shown in Figure 2.
Status of the switch (d) is regulated by hysteresis block H, that objectives to
reduce the error of variables iL1, and Vo. The stability of this converter is
entirely depends on the good selection of controller coefficients M1 and M2
and its circuit components.
Fig. 2. Multi-Loop Structure Based Sliding Mode Controller for POSIBC.
Step 2: Select the SMC parameters
Assume the POSIBC has ideal power switches, power supply free of dc
ripple and operating at high-switching frequency.
Based on the controller parameters, the POSIBC circuit components are
selected L1, L2 is designed from specified input and output current ripples,
Co is selected so as to limit the output voltage ripple in the case of fast and
more value of load resistance (R) changes, and maximum operation
frequency is based on POSIBC capacity and selection of the power switch.
As per the variable structure system (VSS) theory, the POSIBC equations
can be written in as equation (10)
.
x Ax Bd D (10)
Where, x denotes the vector of state-variables errors.
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.
*x x X (11)
Where, X* = [iL1ref, Voref] T is the transverse vector of references. By
inserting equation (11) in equation (4) to found
*D AV C (12)
1 1
1
0 0
10
22
1 10
inL ref
oref
VL i
LDV
C RC
(13a)
1
2 2
orefin
L ref oref
o o
VV
L LD
i V
C RC
(13b)
Inserting equation (11) in equation (9), the sliding surface equation will be
expressed as
1 1 2 2( ) TS x M x M x M x (14)
Where, MT = [M1 , M2] and x = [x1 ,x2] T
The existence condition of the MLSBSMC needs that all phase trajectories
near the surface can be directed toward the sliding line. MLSBSMC can
enforce the converter state to remain near the sliding plane by suitable
operation of the POSIBC switches.
To build this converter state travel toward the sliding surface, the condition
of equation (15) is necessary and sufficient and it can be engraved as
.
.
( ) 0, ( ) 0
( ) 0, ( ) 0
S x if S x
S x if S x
(15)
MLSBSMC is found by feedback control strategy that relates to the position
of the switch with the value of )(xS is expresses as
0, ( ) 0
1, ( ) 0
for S x
for S x
(16)
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The existence condition of equation (15) will be written as
.
( ) 0, ( ) 0T TS x M Ax M D S x (17)
.
( ) 0, ( ) 0T T TS x M Ax M B M D S x (18)
From a simulation study point of view, assuming that error variables xi are
suitably smaller than references X*, equation (17) and equation (18) will be
expressed as
0, ( ) 0TM D S x (19)
0, ( ) 0T TM B M D S x (20)
By using matrices B and D in equation (19) and equation (20), one
determines
1 21
1
0in oref L ref oref
o
M MV V Ri V
L C R (21)
1 2
1
0in oref
o
M MV V
L C R (22)
The existence condition is fulfilled if the disparities equation (21) and
equation (22) are true.
Step 3: Switching frequency calculation
In sliding mode at non-finite switching frequency, state trajectories are
moved towards the sliding surface and stir exactly along it. A real time
system could not operate switch at non-finite frequency. As a result, a
typical control circuits a practical relay, as indicated in Fig. 3.
Fig. 3. Switching function γ.
A practical relay always exhibits hysteresis modeled by
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.
.
0,
0( )
1,
0
when S or
when S and Sd s
whenS or
when S and S
(23)
Fig. 4. The waveform of S(x).
Where, δ is an arbitrarily small positive quantity and 2δ is the amount of
hysteresis in S(x). The hysteresis characteristic makes it impossible to
switch the control on the surface S (x) = 0. As a consequence, switching
occurs on the lines S =± δ, with a frequency depending on the slopes of iL1.
This hysteresis causes phase plane trajectory oscillations of width 2δ,
around the surface S (x) = 0 as shown in Fig. 4. Note that Fig. 4 simply
confirms that in t1, the function S(x) must increase from (–δ to δ) (.
S 0),
while in t2, it must decrease from +δ to δ (.
S 0). The switching frequency
equation is obtained from Fig. 4 by considering that the state trajectory is
invariable, near to the sliding surface S(x) = 0 and is given by
1 2
1s
ft t
(24)
Where, t1 is conduction time of the switch S and t2 is the off time of the
switch S. The conduction time t1 is derived from equation (22) and it is given by
11 2
1
2
in oref
o
tM M
V VL C R
(25)
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The off time, t2 is derived from equation (21), and it is given by
21 2
1
1
2
in oref L ref oref
o
tM M
V V Ri VL C R
(26)
The maximum value of switching frequency is obtained substituting
equation (25) and equation (26) in equation (24) with the assumption that
the converter is operating in no load (iL1ref = 0 and 1/R=0) and the output
voltage reference is crossing its maximum value (Voref(max) ). The optimal
value of switching frequency is found by equation (27)
1(max)
1 max
12
in ins
oref
M V Vf
L V
(27)
Step 4: Duty Cycle
The duty cycle d(t) is defined by the ratio between the conduction time of
the switch S and the switch period time, as represented by
1
1 2
( )t
d tt t
(28)
Considering the SMC, an instantaneous control, the ratio between the output
and the input voltages must satisfy the fundamental relation at any working
condition.
1
1 ( )
o
in
V
V d t
(29)
Step 5: Inductor Current
The high-frequency or maximum inductor current ripple is obtained
from Fig.2 and given by
11
1
inL t
Vi
L (30)
Step 6: Voltage Capacitor
The controller operates over the status of the switch to make the voltage
Vc(t) to follow the reference. As a consequence, on the capacitor Vc(t), a high
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frequency voltage ripple (which is a characteristics function switching
frequency) is imposed. The capacitor voltage ripple is given by
1
( )( )
2oo t
V t tV
RC (31)
It is interesting to note that the switching frequency, inductor current ripple,
and capacitor voltage ripple depend the control parameters and circuit
parameters, viz. reference voltage, output capacitor voltage Vo(t), inductor
current iL1(t) etc. It is important to determine the circuit parameters and
coefficients M1 and M2 to have agreement with desirable values of
maximum inductor current ripple, maximum capacitor voltage ripple,
maximum switching frequency, stability, and fast response for any operating
condition.
3. Simulation Study and Discussion
The main purpose of this section is to use the earlier inferred equations
to compute MLSBSMC elements value, controller parameters and perform
simulation studies. The validation of the MLSBSMC performance is done
for three regions viz. line variation, load variation and elements
modifications. Simulations have been performed on MLSBSMC circuit with
specifications are listed in Table I.
A. Calculation of Vc
From (29) and a simulation point of view, the output voltage is chosen
to produce a variation of the duty cycle close to 0.677. The selected value of
of the output voltage is 48 V which is in Table I, and a variation of the duty
cycle between dmin = 0.3 and dmax = 0.0.6 is expected. Finally Vcmax = 48V.
B. Determination of Ratio K1 /L
Inserting Vin, Vcref (max) = Vcmax and δ=0.3 in (27), the value of K1/L is
computed as K1 / L =7553.
C. Determination of Ratio K2 /C
From (21) and (22) and taking iLref = iL(max) = 4.111A, one founds 1308 <
K2 / C < 247433. There are some degrees of freedom in choosing the ratio
K2/C. In this controller, the ratio K2/Co is a tuning parameter. It is
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recommendable to select the ratio K2/C to agree with essential levels of
stability and response speed. The ratio K2/Co is chosen by iterative
procedure (i.e. the ratio is modified until the transient response is
acceptable), and it is validated by simulation. The final acceptable value is,
K2/Co=7666.
D. Calculation of L
The maximum inductor current ripple is chosen to be equal to 15%
maximum inductor current and the inductor value is which is obtained from
(30) as L 1 & L 2=100μH.
E. Calculation of C
The maximum capacitor ripple voltage Vcomax is chosen to be equal to
1% maximum capacitor voltage and Co is determined using (31) as 300μF.
F. Values of the coefficients K1 and K2
Having decided on the values of the ratio K1/ L1 and inductor, the value
of K1 is unswervingly obtained (K1=1). Similarly the K2 (K2 = 5) is
computed using the ratio K2/Co and the Co.
TABLE I
Parameters of POSIBC using MLSBSMC
Parameters name Symbol Value
Input Voltage Vin 12 V
Output Voltage Vo 48 V
Inductor L1, L2 100 µH
Capacitor Co 300 µF
Nominal switching
frequency
Fs 100 kHz
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Load resistance R 50
Output power Po 46.08W
Input power Pin 49.404W
Input current Iin 4.111A
Efficiency 93.62%
Duty cycle d 0.6
Output Current Io 0.96 A
Fig. 5. MATLAB/Simulink model of MLSBSMC for POSIBC.
The static and dynamic performance of MLSBSMC for POSIBC operated in
CCM is evaluated using MATLAB/Simulink software platform. The
MATLAB/Simulink simulation model is depicted in Fig. 5. Figs. 2 and 5
shows that ε1 (feedback current) and ε2 (feedback current) is obtained by the
respective differences of feedback reference inductor current/capacitor
voltage and feedback inductor current/capacitor voltage, which gives
feedback current error and feedback voltage error. Afterwards both error
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signals are multiplied by the coefficients K1 and K2 to obtain the sliding
surface signal S (iL1, Vo) by simulation model, which guided by (9) and S
(iL1, Vo) applied to the hysteresis modulator to generate the gating of switch.
Status of the switch, γ is controlled by hysteresis block H, and thus
maintains the variable error of iL1 and Vo near zero. The system response is
determined by the circuit parameters and coefficients K1 and K2. With a
proper selection of these coefficients in any operating condition, high
control robustness, stability, and fast response can be achieved. Fig. 5
shows the dynamic behavior in terms of the output voltage and inductor
current start-up of the POSIBC for input voltage 12V using the MLSBSM
and PI controller. It can be seen that output voltage of the POSIBC has a
negligible overshoot and settling time of 0.065s (MLSBSMC), whereas the
same converter for PI controller has maximum overshoots of 12V and
settling time of 0.064s (PI controller) for Vin = 12V.
(a)
0 0.02 0.04 0.06 0.08 0.1-10
0
10
20
30
40
50
Time (s)
Ou
tput
Vol
tag
e (V
), I
nduc
tor
Cu
rren
t (A
) an
d In
put
Vol
tag
e (V
)
Vo
iL1
Vin
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(b)
Fig. 6. Simulated startup response of output voltage of the POSIBC using (a) MLSBSMC,
(b) PI controller.
(a)
0 0.02 0.04 0.06 0.08 0.1-10
0
10
20
30
40
50
60
70
Time (s)
Ou
tput
Vol
tag
e (V
), I
nduc
tor
Cu
rren
t (A
) an
d In
put
Vol
tag
e (V
)
Vo
iL1
Vin
0 0.05 0.1 0.15 0.20
10
20
30
40
50
Time (s)
Ou
tput
Vol
tag
e (V
) an
d In
put
Vol
tage
(V
)
Vo
Vin
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(b)
Fig. 7. Simulation responses of the output voltage of the POSIBC using designed controller,
(a) for input step change from 12V to 10V at time of 0.01s with R = 50, and (b) for input
step change from 12 to 14V at time of 0.1s with R = 50.
0 0.05 0.1 0.15 0.20
10
20
30
40
50
60
Time (s)
Inpu
t V
oltage (
V)
an
d O
utp
ut
Voltage
(V
)
Vo
Vin
0 0.05 0.1 0.15 0.20
10
20
30
40
50
60
70
Time(s)
Ou
tput
Voltag
e (
V)
SMC
PI controller
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Fig. 8. Simulation responses of output voltage of POSIBC using designed controller, when
load value takes a step changes from 50 to 80 at time 0.1s with Vin=12V.
Fig. 9. Simulation responses of output voltage and output current of POSIBC using
designed controller.
Figs. 7 (a) and (b) show the simulation response of output voltage of the
POSIBC using MLSBSMC for input voltage step change from 12V to 10V
and 12V to 14V at time of 0.1s. From these figures, it is clearly found that
the POSIBC using designed controller small overshoots and settling time at
line variation
Fig. 8 show the simulation response of output voltage of the POSIBC using
a MLSBSMC and PI controller for load step change 50 to 80 at time =
0.1s. It could be seen that the simulation results of output voltage of the
POSIBC using a designed controller negligible overshoot and settling time
in comparison with PI controller.
0 0.05 0.1 0.15 0.20
10
20
30
40
50
Time (s)
Ou
tput
Voltag
e (
V)
and O
utp
ut
Curr
ent
(A)
Vo
Io
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Fig. 10. Simulation responses of output power, input power and efficiency of POSIBC
using designed controller
Fig. 9 and 10 show the average output current and average output voltage
respectively. It is showed that the average output current is 0.72A which is
closer to theoretical value in Table I. Using simulation analysis computes
that the input and output power values are 46.25 W and 49.62W
respectively, which is closer to the calculated theoretical value listed in
Table I.
4 Conclusion
In this article, design and implementation of MLSBSMC POSIBC has been
successfully demonstrated through the computer simulation with help of
MATLAB/Simulink software platform. The many simulation results are
presented to show the proficient of the designed controller via. Transient
region, steady state region, line and load variations. It is applied for power
sources in various low and medium power applications. The designed
0 0.02 0.04 0.06 0.08 0.10
20
40
60
80
100
Time (s)
Inpu
t P
ow
er
(W),
Outp
ut
Pow
er
(W)
an
d E
ffic
iency
Efficiency: 93.12
Po: 49.62 W
Pin: 46.25 W
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controller with POSIBC has produced efficiency of 93.62%. The ripples of
this converter have produced minimal ripple voltage as well as current.
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ISSN: 0008-6452
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