Conference-201310-Kerim Colak-Dual Closed Loop Control of LLC Resonant Converter for EV Battery...

8/9/2019 Conference-201310-Kerim Colak-Dual Closed Loop Control of LLC Resonant Converter for EV Battery Charger

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Dual Closed Loop Control of LLC Resonant

Converter for EV Battery Charger

Kerim Colak, Erdem AsaElectrical and Computer Engineering,

Polytechnic Institute of New York University

New York, USA

[email protected], [email protected]

Dariusz CzarkowskiElectrical and Computer Engineering,

Polytechnic Institute of New York University

New York, USA

[email protected]

AbstractIn this paper, LLC resonant converter simulation with

dual closed loop control is demonstrated for Electric Vehicles

(EVs) battery charger. To improve efficiency and fast charging

conditions, switching losses must be diminished to have a

maximum energy transfer among the variable converter

components. The presented dual closed loop control technique is

implemented under soft-switching conditions with constantcurrent and constant voltage control methods. The designed

DC/DC resonant converter, fed by Power Factor Corrector

(PFC) outputs 200-300 V, provides 60 V / 20 A at 1.2 kW. The

simulation results show that differences in soft switching

conditions are obtained by realizing input current and input

voltage in phase under different output load conditions. At the

same time, a battery is charged using Constant Current (CC) and

Constant Voltage (CV) control techniques.

Keywords-constant current (CC), constant voltage (CV), phase

locked loop (PLL), resonant LLC, converter

I. INTRODUCTION

The importance of EV chargers is significantly increasingfor fast and efficient charging of lithium-ion batteries. Withinthe rechargeable batteries, lithium-ion battery has higherenergy density, wider temperature range, low self-discharge,smaller volume, lighter weight, and longer lifecycle [1]-[2].However, the chemical structure of these batteries is moresensitive than the other battery types. Even very small failure inthe battery chemistry can cause very severe reduction in the

battery life and performance [3]. Therefore, the battery chargevoltage and current should be adjusted in the charging process.

Figure 1. General EV charger circuit

EV battery chargers are classified as on-board and off-board, according to the Society of Automotive Engineers(SAE) [4]. Depending on a battery capacity and depth ofdischarging, on-board systems charge the battery in 6-8 hours,while off-board quick charging systems supply 80% charge tothe battery in 15-30 minutes [5]-[6]. Figure 1 shows the mostcommon EV charger system. It consists of a rectified AC

source followed by a Power Factor Corrector (PFC) and anisolated DC-DC converter supplying power into the battery [7]-[8]. PFC is utilized to control the input voltage of LLCresonant converter for constant current and constant voltageoperation in [9]-[11].

Resonant topologies are used in the DC-DC converter stageso that switching losses of the components are reduced [12]-[14]. One of the resonant converter topologies, LLC resonantconverter, decreases current and voltage stresses on theswitching devices in a widely operating frequency range [15].This converter also gives high efficiency at the resonantfrequency under different load conditions [16]-[20]. However,LLC resonant converter is vulnerable to variation of circuit

parameters [9]. This variation changes resonant frequency anddecreases the converter efficiency. To eliminate this problem,the converter should track resonant frequency regardless of theoutput load conditions.

In this paper, dual closed loop control is applied for EV

chargers. The constant current-constant voltage (CC-CV)control is achieved by the PFC providing effective and fastcharging. In order to transfer maximum power, PLL controltechnique is performed for the LLC resonant converter. Arelated circuit and control system is set up inMATLAB/Simulink program.

II. SYSTEM CONFIGURATION

Figure 2. Power factor corrector

While the AC-DC stage improves the power factor of input

with PFC, the DC-DC stage makes an arrangement of

frequency control of the half bridge LLC resonant converter

with galvanic isolation. Considering cost, reliability and

978-1-4799-1464-7/13/$31.00 2013 IEEE

International Conference on Renewable Energy Research and Applications Madrid, Spain, 20-23 October 2013

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efficiency, LLC resonant converter is the proper topology for

EV batteries [21].

PFC must comply with Standard IEC 6100-3-2 to satisfy

PFC limitations for EV battery chargers. In Figure 2, a PFC

circuit that has a front-end rectifier and a boost PWM DC-DC

converter [22] are displayed. After the input voltage is

rectified, current harmonic distortion of the system is reduced

in this stage. Additionally, constant current and voltagecontrol methods are carried out by the boost converter.

The applied battery charging method is first constant

current and then voltage limit control until the battery is fully

charged. The voltage and current characteristics of a lithium-

ion battery is shown in Figure 3. The battery is charged at

constant current until voltage reaches rated value in stage 1.

Then constant voltage control is performed in stage 2. When

the charging current approaches a low level the charging cycle

is completed [23].

V, I

Charge Current

Charge Voltage

t

stage 1 stage 2

Figure 3. Current and voltage characteristic of a lithium-ion battery

Another crucial issue for the life of an EV battery ischarging the battery with low ripple current and voltage. In

addition, the EV charger should be small to satisfy higherenergy transfer efficiency at the high power. These

requirements are met by LLC resonant converter operating

soft-switching techniques at the resonant frequency [24]. The

LLC converter components have some tolerances influenced

by temperature and aging factors.

In the LLC resonant converter, maximum power energytransfer occurs at the resonant frequency in which the resonantconverter reads the load as a resistive. If the operatingfrequency is higher or lower than the resonant frequency,converter experiences high voltage and current stresses duringthe turn on and turn off processes. The diode reverse recoverystress is large when the diode turns off. These situations resultin high switching losses and stresses across the switches andmay destroy the transistors [25]. Moreover, conduction lossesand stresses raise the junction temperature of the switches. Thecomponent electrical parameters change over time because ofaging factors. Charge and discharge cycle of capacitors alsochange with time. Furthermore, their electrical characteristicsdeteriorate with high temperature [26]. Transformers havesome tolerances in the design procedure. These tolerancevalues influence the converter modeling [27].

The factors mentioned above affect resonant componentvalues and change the resonant frequency of the converter, sothat power transfer efficiency decreases. Thus, the controlalgorithms compensate for the impact of component values andalso protect battery charging characteristics.

III. LLCRESONANT CONVERTER DESIGN

Figure 4. Resonant LLC converter

In Figure 4, half bridge LLC resonant converter is shown. Itconsists of a half bridge LLC resonant inverter and a center

tapped rectifier. The resonant tank is formed with resonantcapacitor Cr, resonant inductance Lr, and magnetizinginductance Lm. The following parameters describe thecharacteristic of circuit:

The corner frequency (or the undamped natural frequency)is calculated at open circuit conditions. The magnetizing andresonant inductances are connected in series with the capacitor.The value is

( )

1

2o

m rr

f

L L C

=

+

(1)

Short circuit conditions which eliminates the magnetizinginductance determine the critical frequency in the circuit. Thecritical resonant frequency is given by:

1

2rs

r r

fL C

= (2)

The normalized equations are given in Table I.

TABLE I. NORMALIZED EQUATIONS

Equivalent inputresistance

2 2

8

n RLRi

=

Characteristic

impedance( )L Lr mZo

Cr

+=

Quality factor2 2

8

R n Ri LQZ Zo o

= =

Ratio between the two

inductancesLrALm

=

Normalized frequency fswfnfo

=


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Normalized parameters help to find the transfer function ofthe system using design parameter values. These equations inTable I are explained by the following statements.

The primary equivalent resistance iR is calculated from the

load resistance LR , assuming that the primary power is equal

to the output power. The quality factor is defined by proportionof the total average stored energy and dissipated energy. Theratio between resonant and magnetizing inductances is A. Asshown in Table I, normalized frequency depends on switchingfrequency and corner frequency.

The voltage transfer function of the system is

( )

4

1 1 12 1 12 1

VM

An A j f n

Q A ff nn

= + + +

(3)

The voltage gain function of the LLC resonant converterversus quality factor and normalized frequency is plotted inFigure 5. The voltage gain can be arranged at a desired value

by controlling the switching frequency, and it can be controlledagainst output load and input voltage variations [28].

Figure 5. The voltage transfer function of LLC resonant converter

The following equations present the converter designprocedure:

The ratio of the transformer determines output voltage andoutput current on the load side. A high turns ratio gives lowoutput voltage and causes high currents on the components.The calculation of the ratio is given in (4).

2 2 in

out

Vn

V= (4)

At high voltage gain conditions, the output voltage can beregulated in a narrow operating frequency range. The voltagegain must be chosen properly to control the system in optimalfrequency range. The equations of maximum and minimumvoltage gain functions are shown in (5).

max min

,min , max

,out out V Vin in

V VM M

V V

= = (5)

The resonant capacitor value is a proportion of qualityfactor to corner frequency and input resistance (6).

2r

o i

QC

f R= (6)

The sum of Lr and Lm can be calculated by substitutingresonant capacitor value into the corner frequency equation in(1). The ratio A is used to calculate resonant and magnetizinginductance values in (7) and (8) respectively.

11 2

ir

o

RL

f QA

=

+

(7)

( )1 2i

m

o

RL

A f Q=

+ (8)

With the equations (1) - (8), half bridge LLC resonantconverter design parameters are calculated and summarized inTable II.

TABLE II. DESIGN PARAMETER VALUES

Parameters Value

PFC Output DC Voltage 200-300 V

Output DC Voltage 60 V

Max. Output Current 20 A

Max Output Power 1.2 kW

Output Voltage Ripple


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fast and safe charging without disturbing the DC-DC stageLLC converter control area. The PLL control technique in theLLC converter traces the resonant frequency regardless ofoutput battery charging conditions. This lowers switching andconduction losses and increases the efficiency of the system

power transfer.

+-

+

Figure 6. Simulation of the EV charger

The waveforms in Figure 7 show the switch voltage 2SV

and resonant tank current iI of the transient and steady state.

The voltage and current are not in phase during the transientstate. PLL control detects the phase difference between voltageand current, and then it catches the resonant frequency byincreasing the switching frequency. In the steady state, thevoltage and current are in phase. Hence, zero voltage and zerocurrent switching is achieved by the PLL control.

0 0.5 1 1.5 2 2.5

x 10-5

-20

-10

0

10

20

Time [s]

Current[A]

Resonant Tank Voltage and Current Before Catching Resonant Frequency

0 0.5 1 1.5 2 2.5

x 10-5

-100

0

100

200

300

Voltage[V]

0 0.5 1 1.5 2 2.5 3

x 10-5

-20

-10

0

10

20

Time [s]

Current[A]

The Voltage and Current in the Resonant Tank

0 0.5 1 1.5 2 2.5 3

x 10-5

-100

0

100

200

300

Voltage[V]

Current

Voltage

Current

Voltage

Figure 7. Resonant tank voltage and current waveforms before and catching

resonant frequency

The performance of the Constant Current control with stepload change is illustrated in Figure 8 when the load resistance

changes from 2.5 ohm to 3 ohm. The control technique keeps

the current value constant until the voltage reaches the

programmed value. The current stays at 20 A in steady state,

while the output voltage increases with load changes until 60V. It is worth mentioning that the steady state condition is

obtained in less than 0.15 ms.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-3

0

100

200

300

Time [s]

Voltage[V]

The Peak Voltage and Current with Load Changes

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-3

-20

0

20

40

Current[A]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-3

51

54

57

60

63

Time [s]

Voltage[V]

The Output Voltage and Current

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-3

14

20

26

Current[A]

Voltage

Current

Voltage

Current

Figure 8. The current and voltage vaweforms with load changes

Figure 9 shows the constant voltage control performanceunder step load changes. The controller is tested by changingthe load resistance from 3 ohm to 3.5 ohm. The output voltageis stabilized at 60 V in less than 1.5 ms.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 10-3

0

50

100

150

200

250

Time [s]

Voltage[V]

The Peak Voltage and Current with Load Changes

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 10-3

-20

0

20

40

Current[A]

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 10-3

60

62

64

66

Voltage[V]

The Output Voltage and Current

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 10-3

16

18

20

22

Time [s]

Current[A]

Voltage

Current

Voltage

Current

Figure 9. Constant voltage control with input voltage changes

In order to see the performance of the dual closed loopcontrol, a charging cycle of the battery is simulated. The

battery is modeled as a variable resistor which is increasedfrom 1.5 ohm to 7 ohm linearly in 100 ms. Related simulationresults are presented in Figure 10. During the transition

between CC and CV controls, a smooth transition is achievedwith less than 1.2 V overshot.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

20

40

60

80

Time [s]

Voltage[V]

Simulated Battery Charging Cycle - Conctant Current and Voltage Control

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

10

15

20

25

Current[A]

Current

Voltage

Constant Voltage

Stage 1 Stage 2

Smooth Transient

Constant Current

Figure 10. Simulated battery charging cycle CC and CV


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V. CONCLUSIONS

In this study, the dual closed loop control technique ispresented for EV battery charger systems. Design equation ofthe system is explained by calculating parameters, and thesimulation results are displayed as a waveform. The simulationis performed with 200 V-300 V outputs of PFC DC voltage and1.2 kW output power. It is shown that the proposed controlmethod provides both soft switching and battery charge

management. Soft switching conditions are obtained by tracingresonant frequency. Maximum power transfer is achieved byreducing switching losses. Battery fast charging methods CCand CV are provided by PFC. The battery is charged underreliable charging conditions with decreasing output voltageripple.

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