Electric Vehicle Battery Charging Utilizing High
Gain Luo Converter with Power Factor Correction
Ashutosh Gupta
Electrical Engineering Department
Delhi Technological University
ashutoshgupta.nitm@gmail.com
Dheeraj Joshi
Senior IEEE Member
Delhi Technological University
joshidheeraj@dce.ac.in
Abstract - This study focuses on enhancing the performance of
electric vehicle (EV) battery chargers by reducing stress on the
devices used in power factor correction (PFC). It achieves this by
incorporating a high step-up gain Luo converter in conjunction
with a flyback converter. The high-gain Luo converter is
advantageous because it operates with a shorter duty cycle
thanks to a switched inductor at the input. This leads to reduced
conduction losses and better overall charger efficiency when
compared to conventional Luo converters. To manage the
battery's current, a flyback converter is employed, operating in
discontinuous conduction mode (DCM) while maintaining a
constant voltage-constant current mode. The design of the highgain converter and the flyback converter involves the use of fewer
sensors and a smaller magnetic volume, resulting in cost and size
savings. The system is also tested to evaluate the improved power
quality (PQ) performance of the charger under both stable and
rapidly fluctuating line voltages. In comparison to other voltagelift-type Luo converters found in the literature, this converter
showcases reduced device stress, fewer components, and
improved efficiency. Moreover, a closed-loop control system
using cascaded proportional-integral (PI) controllers in the
flyback converter ensures more precise and controlled output.
These findings are validated through MATLAB SIMULINK
simulations of the converter's performance.
efficiency, as well as the speed of charging. The availability of
better-charging infrastructure and the ease of electrifying
homes are contributing factors to the steady growth of the EV
industry [1].
Nevertheless, traditional chargers with diode bridge rectifiers
(DBR) fed AC-DC converters violate world power quality
standards (PQ) due to poor power factor (PF) performance.
The input PF of a conventional charger with a DBR is less
than unity, and the input current has a higher total harmonic
distortion (THD) of 50-60% at the rated battery load. Thus, a
traditional charger provides a basis for improving line side
indices to meet the limits of the IEC 61000-3-2 standard. A
standard boost PFC converter used at the output of a DBR is a
suitable option to enhance PQ at the front end of EV chargers.
The charger's PQ performance is evaluated using approved PQ
guidelines, such as the IEC 61000-3-2 standards. Compared to
a single-stage architecture, a two-stage charger offers two
advantages, including reduced intermediate DC-link
capacitance and low second-order harmonic current ripples in
the battery current [3]. Various buck-boost-based PFC
converters for EV chargers have been investigated in the
literature, including Cuk, SEPIC, CSC Landsman, and Luo
converters [4]. Among these, a Luo converter is popular for
DC-DC converters due to its excellent voltage regulating
characteristic and high efficiency at low loads. Traditional
non-isolated inverted output Luo converters [5]-[7]
demonstrate adequate power quality performance with
improved voltage level gain and current and voltage ripples.
Fig. 1. (a) illustrates the fundamental principle for a PFC EV
charger based on an elemental negative output Luo converter
[8].
Nonetheless, utilizing the traditional boost or fundamental Luo
converter to provide the highest output voltage requires
operating at excessively high duty-cycle ratios, leading to
increased system stresses, larger size, and reduced charger
efficiency. Previous studies have focused on optimizing switch
Keywords– PFC, High step gain Luo converter, Flyback converter,
EV Battery Charger, DCM.
I. INTRODUCTION
The demand for clean energy production worldwide is
multiplying due to fuel shortages and pollution concerns. This
has led to increased research in the electric vehicle (EV)
domain, as it offers practical and sustainable solutions to
address these issues, particularly concerning global warming.
The electrification of vehicles is also becoming a pressing
demand due to the modernization of energy and globalization,
prompting governments to encourage the widespread adoption
of electric vehicles and boost the EV sector. However, the key
challenges in EV development are the battery and charging
DBR
Filter
-ve Output PFC Luo Converter
Flyback Converter
Df
DBR
High Step-up Gain Luo Converter
Flyback Converter
Df
Ib
Ib
Lf
Lf
Do
S1
Cf
Lo
is
Io
Vs
Vin
L1
C1
To S1
Control unit-1
Lo
Io
Vin
C1
Ib*
Sawtooth
generator
Ib
Vo
Vb
Voltage
controller
Voref
Sf
Co
Ib
Current
controller
V b*
PWM
generator
Sawtooth
generator
Vb
Cb
Do
L2
Sf
PWM
generator
Voltage
controller
Cf
S2
S1
Co
Vo
PWM
generator
Sawtooth
generator
L1
Vb
Cb
is
Vs
Filter
To S1
To S2
PWM
generator
Voltage
controller
Control unit-1
(a)
(b)
Fig.1 Charger configuration based on (a) conventional luo PFC converter (b) High gain Luo converter
Ib*
Vb
Voltage
controller
Voref
Control unit-2
Current
controller
Sawtooth
generator
V b*
Control unit-2
voltage and current for AC-DC PFC converters. These
approaches modify the control mechanism by utilizing a
linked inductor with coupling coefficient changes or an extra
converter that consumes the difference in incoming and
outgoing power, thus minimizing stress on the output voltage.
However, there has been no topology transition to minimize
system stresses in power factor correction converters. This
could be achieved by combining the high step-up gain
principle used in various buck-boost converters, including the
SEPIC, Cuk, Zeta, and Luo converters[9]-[11]. While several
strategies have been proposed in the literature to increase the
gain of the positive output Luo converter, such as voltage lift,
re-lift, super-lift, and ultra-lift techniques, it is essential to note
that these methods require many systems and equipment that
are not well-suited for high-power density EV chargers.
As a result, this work presents a new high stepping-up gain
Luo converter with a switching inductor design at the input
end for PFC at the front end of an electric vehicle battery
charger, as illustrated in Fig. 1. (b). The following are the
crucial benefits of this innovative Luo converter topology:
1) The voltage gain is equal to twice the of a traditional
Luo converter.
2) Compared to other Luo converters described in the
literature, the voltage lift is achieved with fewer circuit
elements.
3) To maintain the same DC-link voltage, the converter
runs at a lower duty ratio, significantly reducing circuit
conduction loss.
4) When compared to a traditional Luo converter, a
reduced switch voltage stress is obtained, resulting in a smaller
and less expensive charger.
5) The DCM design also reduces the size and cost of the
product because the sensing requirements are lower.
In a traditional Luo converter, the switch's voltage stress is
equivalent to the supply voltage, Vin, which is smaller
in this converter. This means this converter must run at a
reduced AC voltage at the input to provide stepping-up
operation for a particular DC-link voltage and power capacity.
As a result, this paper presents a design for a 750W power
220V AC to 400V DC converter. A flyback converter is
executed at the next level to manage the battery current in
constant voltage-constant current mode. At the steady state
and suddenly varying line voltages of 200V-240V, the
converter performance is tested for better PQ-based charges
and reduced system stresses.
II. CONFIGURATION AND OPERATION OF HIGH GAIN LUO
CONVERTER
Figures 1(a) and 1(b) show this EV charger's frameworks
having high step-up gain Luo converter and standard Luo
converter. This converter is made by dividing the input
inductor Li into two halves and adding one more switch S 2 to a
standard Luo converter. As a result, the switched inductor
structure of this converter consists of two inductors L1,2 with
switches S1,2 at the input. With diode Do, the intermediate
capacitors C1 and an output inductor Lo, operate in standard
manner. At the PFC converter stage, the peak switch voltage
across switches S1,2 is decreased to input voltage Vin, which is
lower in this high gain Luo converter than in a standard Luo
converter, to provide a rated output voltage, Vo of 400V. At
the next stage, a flyback converter is employed to regulate the
battery current in constant voltage - constant current mode. In
DCM, the design of two converters is preferred. The flyback
converter works like a standard converter, as shown in [7]. On
the other hand, the input inductances L1 and L2 of the provided
Luo converter have been designed to work in DCM. Figures
1(a)-1(d) show the corresponding switching waveforms and
operating modes over one switching interval, which are
explained as follows.
Mode 1: This mode is activated when both switches S1, and
S2 are turned off simultaneously. As demonstrated in Figs.
2(a) and 2(b), the inductances L1, L2 begin to store energy
from the source. The capacitor C1 begins to discharge,
transferring energy to the output through the inductance L o.
During this time, the output diode Do is in reverse bias.
Mode 2: Both switches S1, and S2 are currently turned off. The
output diode Do enter the conducting condition, as shown in
Figs. 2(a) and (c). The stored energy is released via the
inductances L1, L2, and the transfer capacitor C1 starts
charging reversely. During this period, the diode Do supply the
load current via inductance Lo and L1, L2.
Mode 3: Because none of the switches are conducting, this
mode is termed DCM or freewheeling time. The stored energy
in the inductances L1 and L2 is entirely drained, as seen in
Figures 2(a) and 2(c). via the inductance Lo, and DC link
Capacitor Co gives sufficient energy to the load via the transfer
capacitor C1.
During mode1 and mode2,
(1)
(2)
The transfer voltage is given as:
(3)
As a result, as shown in Fig. 2(a), the step-up voltage gain of
this converter is twice that of a standard Luo converter. The
maximum voltage stress between switches and diodes can be
calculated using the formula:
(4)
L1
L1 charging
L2 charging
Co discharging
S2
Do
Lo
L1
C1
Lo
Io
Vin
Flyback
Converter
Co
S2
Do
Io
Vin
S1
L1 Discharging
L2 Discharging &
C1 charging in
opposite direction
S1
L2
C1
Co
Flyback
Converter
L2
(a)
(b)
C1discharging
S2
L1
Do
Lo
Io
Vin
S1
C1
Co
Flyback
Converter
L2
By applying KVL in the loop,
across the diode is given as,
(c)
Fig. 2(a)-(c) Operating Principle of high gain Luo converter over one cycle
TABLE I: SPECIFICATION OF THE CHARGER OF HIGH GAIN LUO CONVERTER
Do-L2-L1-C1, voltage stress
Specification
(5)
The peak value of the switch and diode voltage stress of this
converter is relatively smaller than that of a traditional Luo
converter (VS1=Vo, VDo=Vin+Vo) because the supply voltage to
achieve the required duty cycle is lower than in a standard
DCM converter due to the lower input voltage applied for the
same output voltage.
III. DESIGN CONSIDERATIONS OF EV CHARGER
To achieve inherent power factor correction at the front end of
the Electric vehicle charger, a 600W (Pi) high gain Luo
converter coupled with a flyback converter is developed in
DCM. In discontinuous conduction mode, the flyback
converter is designed traditionally using the techniques
described in [7]. The output voltage of this charger with a high
step-up gain Luo converter is maintained at 300V. Using
expression (3) to consider the instantaneous supply voltage
Vin, the equation for the duty cycle D(t) is as follows:
(6)
According to equation (6), this converter should perform with
reduced AC voltage at the input to give a step-up operation for
specific DC-link voltage and power rating, equivalent to the
standard Luo converter. As a result, the new Luo converter is
built for voltage conversion from 220V AC to 400V DC, with
a range of 200V (Vsmin)-250V. (Vsmax). This high gain
converter's switching frequency fs is set to 20kHz. Table-I
contains the charger characteristics.
Input voltage
Filter inductance
Filter capacitance
Input inductance
Series capacitor
DC link capacitor
Battery rating
Output capacitor
Output inductance
Switching frequency
Output power
Output voltage/current
Permissible Current Ripple
in the output inductor
Permissible Voltage Ripple
in series capacitor
Permissible Voltage Ripple
in DC-link Capacitor
Permissible Voltage Ripple
in Output Capacitor
High gain Luo
converter
220V single-phase
AC
4mH
1µF
70µH
550nF
500µF
48V/100Ah
2000µF
2.08mH
20kHz
600W
320V/2.739A
Flyback
320V DC
50kHz
52V/12.03A
20%
10%
3%
0.1%
A. Design of High Gain Luo Converter:
High gain Luo converter is supplied by DC voltage generated
at the output of DBR and EMI filter, i.e., the average output is
indicated as Vin and is given as follows:
(7)
Where Vs is the input voltage, at the input, the converter has
two inductors, L1 and L2, designed to work in Discontinuous
Conduction mode. As a result, the critical value of L1,2 is
determined as follows:
(8)
The selected value of L1,2 is 70µH.
The critical value of the series capacitor is given by:
(15)
The selected value of Cb is 2000µF.
IV. CONTROL ALGORITHM
(9)
The selected value of the critical series capacitor is 550nF.
The critical value of the output inductor of the converter is
given as:
(10)
The selected value of Lo is 2mH.
The value of filter inductance and filter capacitance is given
by:
In Constant current and Constant voltage modes, the Luo
converter requires a voltage regulator technique for DC-link
voltage control, and a flyback converter is regulated for small
ripple-based charging batteries. The following are the controls
for two converters.
A. High gain Luo converter control
A PI (Proportional-Integral) controller in voltage regulator
mode shapes the supply current precisely as the mains voltage
by sensors to detect any variations in the DC link voltage of
the high gain Luo converter in ratio to the supply voltage
fluctuations emerging on the side of the main to control the
output of the PFC high gain Luo converter flexibly for
considerable differences in supply voltage.
Vo
(11)
PWM
generator
(12)
The selected value of Lf and Cf is 2mH and 1µF, respectively.
DC link capacitor, i.e. output capacitor of high gain Luo
converter, is given as:
(13)
The selected value of Co is 500µF.
B. Design of Flyback Converter:
It is designed by a transformer that provides isolation, a diode
Df and a capacitor at the output. Selecting the correct
magnetizing inductance Lm in the operation of high-frequency
transformers and flyback converters Lm is very significant. It is
assigned a significantly lower value than the calculated critical
value. Lm is given by:
(14)
Lm is selected as 130µH.
An output capacitor of flyback converter Cb keeps the output
current ripple to a minimum value. It can be calculated as
follows:
Sawtooth
generator
Voltage
controller
To S1
To S2
Voref
Fig. 3 Control unit of High gain Luo converter
As shown in Figure 3, the DC link voltage Vo developed at the
output end of the High gain Luo converter, which is compared
to the reference DC voltage, i.e., Voref regulating and given to
the voltage PI controller.
B. Flyback converter control
As shown in Figure 4, the controlling of a flyback
converter requires a dual loop control of Proportional Integral
in which the battery voltage Vb is measured and compared to a
constant voltage taken as reference, i.e., Vb*, and the resulted
value is given to a voltage PI controller and connected in
series with a current Proportional Integral controller which
helps to generate controlled pulses for the flyback converter
switch.
Ib
PWM
generator
Current
controller
Ib*
Vb
Voltage
controller
Sawtooth
generator
Fig. 4 Control unit of Flyback converter
V b*
V. RESULTS
The device shows the values of different variables of a
48V, 79.97Ah Li-ion battery that is being charged by the
developed framework. Supply voltage, supply current, Battery
voltage, State of charge (SOC)%, and battery current are
represented in Fig.5. (a)-(e). Both the supply voltage and
current are in phase in Fig.5. (a) and Fig.5. (b), indicating that
the circuit has power factor correction. The battery charges
gradually as the SOC% increases in Fig.5. (d). Figure 6 depicts
the fluctuation in THD in the system, which is lower than
5.2%. With a steady current of roughly 15A, the battery is
charged to a voltage of 52V. A negative current value
indicates that the battery is being charged.
Fig.6. Input Current THD
THD fluctuation among different topologies connected to Luo
can be noticed in Table II. It shows how a high-gain Luo
converter reduces the Total harmonic distortion of supply
current, making the device more fuel efficient. As shown in
Table II, the power factor varies with load change and various
converter topologies. It can be observed that with a high gain
Luo converter, approximately unity power factor is obtained,
and the significance of the power factor correction unit is also
clearly obtained.
Table II: Comparision between chargers for THD and power factor
(a)
Charger Type
THD
Power Factor
Conventional EV Charger (No PFC)
Conventional Luo PFC-Based Charger
EV Charger with High Gain Luo Charger
56.1%
12.6%
5.16%
0.79
0.82
0.92
VI. CONCLUSION
(b)
(c)
This work presents a high step-up gain Luo converter with
decreased system stresses for power factor correction at the
front end of an Electric vehicle battery charger. In the
fundamental Luo converter, this decrement in switch voltage is
obtained by introducing one switch and dividing the input
inductance. The benefit of this converter over the standard
boost and Luo PFC converters in producing the same DC-link
voltage is accomplished in terms of fewer device stresses, as
this Luo converter operated at a lesser duty cycle due to the
switching inductor design at the input. This Luo converter is
suitable for high-power EV chargers because the switch
voltage and current stresses are significantly decreased.
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Fig.5. (a) Input Voltage (V), (b) Input Current (A), (c) Battery Voltage (V),
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