Analysis and implementation of LLC-T series parallel resonant converter using fuzzy logic controller

Abstract

A modified LLC-T (inductor inductor capacitor) series parallel resonant converter (SPRC) has been simulated and the performance is analyzed. The limitations of two element resonant topologies have been removed by adding a third reactive element. The output voltage is regulated using fuzzy logic controller. The simulation study indicates the superiority of fuzzy control over the conventional control methods. The proposed approach is expected to provide better voltage regulation for dynamic load conditions. A prototype 300 W, 100 kHz converter is designed and built to experimentally demonstrate, dynamic and steady state performance for the LLC-T series parallel resonant converter. A comparative study is performed between experimental results and the simulation studies. The analysis shows that the output of converter is free from the ripples, and has constant current, regulated output voltage.

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International Journal of Engineering, Science and Technology
Vol. 2, No. 10, 2010, pp. 35-43

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Analysis and implementation of LLC-T series parallel resonant converter
using fuzzy logic controller

C. Nagarajan1*, M. Madheswaran2

1Bharath University, Chennai. Tamilnadu, INDIA
2Centre for Advanced Research, Muthayammal Engineering College, Rasipuram, Tamilnadu, INDIA.
*Corresponding Author: e-mail: nagaraj2k1@gmail.com, Tel +91-9894620123,


Abstract

A modified LLC-T (inductor inductor capacitor) series parallel resonant converter (SPRC) has been simulated and the
performance is analyzed. The limitations of two element resonant topologies have been removed by adding a third reactive
element. The output voltage is regulated using fuzzy logic controller. The simulation study indicates the superiority of fuzzy
control over the conventional control methods. The proposed approach is expected to provide better voltage regulation for dynamic
load conditions. A prototype 300 W, 100 kHz converter is designed and built to experimentally demonstrate, dynamic and steady
state performance for the LLC-T series parallel resonant converter. A comparative study is performed between experimental
results and the simulation studies. The analysis shows that the output of converter is free from the ripples, and has constant current,
regulated output voltage.

Keywords: Resonant Converter, Fuzzy Logic, Control System, Power Electronics, MATLAB.

1. Introduction

The design and development of various DC-DC resonant converters (RC) have been focused for telecommunication and
aerospace applications in the recent past year, it has been found that these converters experience high switching losses, reduced
reliability, electromagnetic interference (EMI) and acoustic noise at high frequencies. The LCL tank circuit based DC-DC
converter have been experimentally demonstrated and reported in Raju and Doradla (1995) and Bhat (1994a, 1997). It has been
found from the literature that the LCL tank circuit has been connected in series with the load and operated in above resonant
frequency. It is also observed that the series parallel resonant converter improves the load efficiency (Bhat, 1993, 1994b). Later,
Borage et al. (2005) have demonstrated an LCL-T half bridge resonant converter with clamp diodes, in which the output voltage of
constant current power supply increases linearly. Therefore a constant-voltage limit must be incorporated in the converter. In order
to get these operations the clamp diode is required. Borage et al. (2009) have demonstrated the characteristics of LCL-T Resonant
converter using Asymmetrical duty cycle (ADC).The converter operated at fixed resonant frequency and its analyzed using state
space approach. Belaguli and Bhat (2000) have experimentally demonstrated with independent load when operated at resonant
frequency, making it attractive for application as a constant voltage (CV) power supply. It has been found from the literature that
the LCL tank circuit connected in series-parallel with the load and operated in above resonant frequency improves the load
efficiency and independent operation. China and Sng (2009) have demonstrated LCC series parallel resonant converter using
robust control method, the converter gain was derived by using the state plan analysis. The voltage and current type load was
presented with variable load condition. The closed loop operation was presented using PI controller with load independent
operation as obtained.
Later, Gu et al. (2005) investigated LLC topology in detail using fundamental approximation, but simplified the AC equivalent
circuit by ignoring the leakage inductance in the secondary side. Lu et al. (2006) have demonstrated LLC resonant converter, the
performance of controller has been evaluated and found that the load independent operation may not be possible. Nagarajan and
Madheswaran (2010) have simulated an LCL-T series parallel resonant converter using fuzzy logic controller and PID controller.
The performance of the controller has been presented. The fuzzy logic controller was found to have better performance compared

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Nagarajan and Madheswaran / International Journal of Engineering, Science and Technology,
Vol. 2, No. 10, 2010, pp. 35-43
36
to the PID controller. The harmonic spectrum and dynamic analysis fed RLE load was presented. Prabhakar et al. (2009) have
simulated an LCC resonant converter. The voltage gain expressions are derived for different resonant topologies. Here the load
variation and load independent operation was not presented and there was no static and dynamic analysis.
It is clear from the literature studies that the output voltage regulation of the converter against load and supply voltage
fluctuations have important role in designing high-density power supplies. In the proposed LLC-T series parallel resonant
converter is operating in constant output current and constant output voltage is presented. The series capacitor limits the circulating
current. Therefore the clamping diodes are not necessary which reduces the circuit complexity. It reduces the component stress,
there by it is easily paralleling to the other converter with current sharing. The closed loop module and AC analysis has been
derived and simulate using MATLAB/Simulink. A prototype 300W, 100 kHz the LLC-T series parallel resonant converter is
implemented and the experiment results are compared with the simulation results. The simulation results agree with the
experimental results.

2. Proposed LLC-T Series Parallel Resonant Converter

The circuit diagram of the LLC-T series parallel resonant converter with capacitive and inductive output filter is shown in Figure
1. The resonant tank consisting of three reactive energy storage elements (LLC) has overcome the conventional resonant converter
that has only two elements. The first stage converts a dc voltage to a high frequency ac voltage. The second stage of the converter
is to convert the ac power to dc power by suitable high frequency rectifier and filter circuit. Power from the resonant circuit is
taken either through a transformer in series with the resonant circuit (or) series in the capacitor comprising the resonant circuit as
shown in Figure 1. In both the cases the high frequency feature of the link allows the use of a high frequency transformer to
provide voltage transformation and ohmic isolation between the dc source and the load.

Figure 1. LLC-T series parallel resonant converter circuit

In LLC-T series parallel resonant converter the load voltage can be controlled by varying the switching frequency or by varying
the phase difference between the inverter. The phase domain control scheme is suitable for wide variation of load condition
because the output voltage is independent of load. The dc current is absent in the primary side of the transformer, there is no
possibility of current balancing. Another advantage of this circuit is that the device currents are proportional to the load current.
This increases the efficiency of the converter at light loads to some extent because the device losses also decrease with the load
current. If the load gets short circuited at this condition, very large current would flow through the circuit. This may damage the
switching devices. To make the circuit short circuit proof the operating frequency should be changed.

3. AC Analysis of the Proposed LLC-T Series Parallel Resonant Converter

In the ac analysis, the output rectifier and filter are replaced by the equivalent ac resistance and the square-wave input voltage
source is replaced by its fundamental sinusoidal equivalent. The power transfer from input to output is assumed to be only via the
fundamental component and the contribution of all the harmonics is neglected. Without losing generality, the turn’s ratio (N1/N2)
of the isolation transformer n x1 is assumed to be unity. The equivalent circuit for LLC-T series parallel resonant converter is
shown in Figure 2.The output power is associated to the output load resistance Ro. Rac is the ratio of the instantaneous voltage and
current.

V
R
R
8
π
Iorms
2 Rac= Idc
2 Ro.,
o
o
dc
ac
ac
oac
2
R
R
V
2
22
=


o ac
RR
2
=
(1)

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Nagarajan and Madheswaran / International Journal of Engineering, Science and Technology,
Vol. 2, No. 10, 2010, pp. 35-43
37

Figure 2. Equivalent circuit for LLC-T series parallel resonant converter
Transfer Function:
The ac resonant tank can be defined by the following transfer function H(s).
1
(
1
)(
sLRn
sC
Voltage gain:
V
out
2
From the above equation the voltage and current gain of LLC-T series parallel resonant converter can be derived as

()⎟⎟
⎠
⎝
8
ω
δ
n

()⎟⎟
⎠⎝
8
ωδ
n
Q
The voltage conversion ratio is equal to one half the module of resonant tank. The resonant frequency and normalized switching
frequency are defined as
1
,
rr
f
LC LC
ω
π
2
outoin
Vn
R
L
=
δ

2
2
2
2
1
2
2
)
1
s
()
1
(
)
sLRn sL
sLRn
sC
n
sH
ac ac
ac
++++
+
=
(2)
M
nV
dc
1
=
(3)
⎞
⎜⎜
⎛
+−+
⎟⎠
⎞
⎜⎝
⎛ +
1
==
1
1
1
2
δω
πδ
n
in
o
Qj
V
V
M
(4)
⎞
⎜⎜
⎛
+−+
⎟⎠
⎞
⎜⎝
⎛ +
1
=
1
11
1
2
δω
πδ
n
j
H

.,,
2
1
s
n
s
nrr
f
ff
====
ω
ωω

Quality factor:
22
8
out
ac
PZZ
Q
π
==

Induction ratio:
2
1
L
The characteristic impedance:
Cf
Lf
C
L
Z
r
rro
π
2
π
2
1
1
===


4. Results and Discussion
4.1. Design parameters of the converter
For the design of LLC-T series parallel resonant converter (Borage et al., 2009), the design specifications are minimum and
maximum value of dc voltage, maximum output current (Io), corresponding to the full-load condition and switching frequency (fs).
In the analysis of previous sections, the transformer turns ratio (N1/N2 ) was assumed to be unity. It is desired to design the
converter with the following specifications:
The values used for all the elements are presented. These values were obtained using a design procedure (Borage et al., 2009) to
assure resonance for almost different power ranges (load-independent design) and also to limit the current and voltage peak values.
The fuzzy controller used is robust to parameter variations as well as for a wide load operating conditions. The simulation and
implementation are carried out for 300 W load power. The load used on the tests is composed of a series connection of a resistor
and a small inductor.

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Nagarajan and Madheswaran / International Journal of Engineering, Science and Technology,
Vol. 2, No. 10, 2010, pp. 35-43
38
Table.1.Design Parameters
Parameter
Power output
Minimum input voltage
Minimum output voltage
Maximum load current
Transformer Turns ratio
Switching frequency (fs)
Series Inductance L1, L2
Parallel Capacitance (C)
Load Inductance (Lo)
Load Capacitance (Co)
S.No
1
2
3
4
5
6
7
8
9
10
Value
300W
100V
100V
3A
1
100KHz
39.18 μH
66 nF
1mH
650μF

4.2. Fuzzy Logic Control (FLC)

Fuzzy control involves three stages: fuzzification, inference or rule evaluation and defuzzification. Series parallel resonant
converter is modeled using MATLAB software. Fuzzy control is developed using the fuzzy toolbox. Five triangular membership
functions are chosen for simplicity. Table 2 shows the fuzzy rule base created in the present work based on intuitive reasoning and
experience. Fuzzy memberships NB, NS, Z, PS, PB are defined as negative big, negative small, zero, positive small, and positive
big.
Table.2.Fuzzy Rules










Figure 3. Flowchart for the FLC
Error (e)

NB
NS
Z
PS
PB
NB
NB
NB
NB
NM
Z
NS
NB
NM
NS
Z
PM
Z
NB
NS
Z
PS
PB
PS
NM
Z
PS
PM
PB
PB
Z
PM
PB
PB
PB
Change in error
(ce)

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Nagarajan and Madheswaran / International Journal of Engineering, Science and Technology,
Vol. 2, No. 10, 2010, pp. 35-43
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Where Vr is the reference or the desired output voltage, VL is the actual output voltage and pe is previous error. If the output
voltage continues to increase gradually while the current is low during the charging process the fuzzy controller will maintain
increase in voltage to reach the set point. A drop in the output voltage level triggers the fuzzy controller to increase the output
voltage of the converter by modifying the Modulation Index (MI) of the converter. The resolution of fuzzy logic control system
(Figure 3) relies on the fuzziness of the control variables while the fuzziness of the control variables depends on the fuzziness of
their membership functions.

4.3. Simulation Results
The closed loop simulation using FLC is carried out using MATLAB/Simulink software. Depending on error and the change in
error, the value of change of switching frequency is calculated. The fuzzy set parameters instruction and function blocks available
in MATLAB are used to update the new switching frequency of the pulse generators. The entire system is simulated with a
switching frequency of 100 KHz. The converter resonant voltage, resonant current and output load voltage are shown in Figure.4a-
4e.

(a) (b)



(c)

(d)

(e)
Figure 4. (a) Inverter Voltage (VAB), (b) Inverter Current (Transformer Primary side), (c) Voltage across Capacitor, (d) Current
through Inductance L1 , (e) Output voltage fed RLE load for Vr=100V.