Conference-201304-Chun-Liang Liu-Design and Implementation of a Digitally-controlled LLC Resonant...

8/9/2019 Conference-201304-Chun-Liang Liu-Design and Implementation of a Digitally-controlled LLC Resonant Converter for

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Design and Implementation of a digitally-controlled

LLC resonant converter for battery charging

applicationsChun-Liang LiuDepartment of Electrical

Engineering, NTUSTTaipei, Taiwan, R.O.C

[email protected]

Yi-Hsun ChiuDepartment of Electrical


[email protected]

Yi-Feng LoGreen Electronics Design &Application Dept., Divisionfor Biomedical & IndustrialIC Technology, ICL, ITRIHsinchu, Taiwan, R.O.C

[email protected]

Shun-Chung WangLunghwa University of Science

and Technology, TaoyuanCounty, Taiwan, R.O.C

[email protected]

Yi-Hua LiuDepartment of Electrical


[email protected]

AbstractIn this paper, a digitally-controlled LLC resonant

converter is developed for LEV battery charging applications.

LLC resonant converter boasts the advantages of high efficiency

and wide input voltage range; therefore is a suitable candidate

for medium power battery charger. To enhance the

performance of the developed battery charger, five-step constant

current (CC) charging method is implemented in the proposeddigital controller. Experimental results show that the proposed

charger can successfully realize the five-step CC charging

algorithm, and the measured conversion efficiencies of the

designed LLC resonant converter are all higher than 88% under

all output voltage and load conditions

I. INTRODUCTION

Due to the continuous growth of the global energy demandand the increasing concern about environmental issues,interests in using and developing zero emission light electricalvehicle (LEV) are growing. For LEV, secondary batteries

play a significant part in energy storage solutions. The

performance and longevity of secondary batteries depend onthe quality of their chargers. Therefore, designing a goodcharger for secondary batteries is essential. The objectives ofa high-quality charger include high efficiency, long cycle lifeand short charging time [1-3]. The commonly adoptedcharging method for secondary batteries is the constantcurrent- constant voltage (CC-CV) method. For CC-CVmethod, large constant current is applied at the beginning ofthe charging cycle. When the battery voltage increases to amaximum allowable value, the charger switches to constantvoltage charging mode and continues in that mode until thetermination criterion is satisfied. However, constant voltagecharging part seriously extends the charging time and alsoreduces the cycle life of the battery.

In this paper, a digitally-controlled LLC resonant converteris developed for LEV battery charging applications. The LLCresonant topology allows for zero voltage switching (ZVS) ofthe main switches, therefore dramatically lowering switchinglosses and boosting efficiency [4-6]. To enhance the

performance of the developed battery charger, five-stepconstant current (CC) charging method is utilized in this paper.The five-step CC charging algorithm is proven to have the

advantages of longer cycle life, higher charge/dischargeenergy efficiency, and shorter charging time [7, 8]. In addition,the dsPIC33FJ16GS502 from Microchip corp. is used as thedigital variable frequency controller of the LLC seriesresonant converter [9]. The advantages of the digitalcontroller include components cost reduction and more design

flexibility. Experimental results show that the proposedcharger can successfully realize the five-step CC chargingalgorithm, and the measured conversion efficiencies of thedesigned LLC resonant converter are all higher than 88%under all output voltage and load conditions.

II. SYSTEM CONFIGURATION

Fig. 1 shows the block diagram of the proposed chargersystem. In Fig. 1, the input power source of the proposed Li-ion battery charger is a 400 V DC voltage from the powerfactor corrector (PFC) stage, and the battery pack used is a 48V, 22 Ah lead-acid battery module for light electric vehicles.Fig. 2 shows the hardware configuration of the proposed

charger. In Fig. 2, the dsPIC33FJ16GS502 digital signalcontroller (DSC) from Microchip Corp. is used to implementthe charging algorithm, provide the required gating signals forthe power switches in the power converter and then gather andanalyze data from the data acquisition circuit. PWMmodulation strategies and interfacing IC driving signals arealso realized using the DSC to achieve better performance.From Fig. 2, the whole system can be divided into three major

parts: input/output interfacing unit, digital controller unit andpower converter unit. Detailed descriptions about each unitwill be given in the following sections:

a. Input/output interfacing unit: I/O interfacing unitincludes feedback circuit which is used to measure the voltage

and current information from the battery side and signalconditioning circuits which performs amplification and rangeadaptation on feedback signals. It should be noted that forconventional LLC resonant converter, only one feedbacksignal (output voltage or output current) is required. However,for battery charging applications, both battery voltage and

battery current information is essential.

978-1-4673-1792-4/13/$31.00 2013 IEEE 804


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b. Digital controller unit: digital controller unit controlsthe charging current command according to the measured data.The digital controller gathers and analyzes battery status data(voltage, current) and then determines the necessary chargingcurrent. The digital filter and digital PID-control algorithmare also implemented in the digital controller. The digital

controller computes the gating signals of the power converteraccording to the charging current command and currentfeedback information. Detailed descriptions about thesoftware flowchart of the proposed digital controller will be

provided in section IV.

c. Power converter unit: power converter unit is used tosupply the electric power to the battery pack. As shown in Fig.1, a LLC resonant converter is used as the charging unit. Byadequately controlling the PWM gating signal, the powerconverter unit can transfer the required energy to the lead-acid

battery pack. LLC resonant converter is adopted as a batterycharger due to its low EMI noise, high power integration andhigh efficiency. Detailed descriptions about the operating

modes of the proposed LLC resonant converter will beprovided in section III.

Fig. 1: The block diagram of the proposed charger system

Fig. 2: The hardware configuration of the proposed charger

III. LLCRESONANT CONVERTER

From Fig. 2, the primary side of the LLC resonantconverter is a half-bridge configuration. The secondary side isa center-tapped rectifier followed by a capacitive filter.Switches S1and S2are both driven by 50 % duty cycle gatingsignals, with a small amount of dead time introduced between

the consecutive transitions. The circuit has three passivecomponents, Lr, Cr and Lm, where Lm is the magnetizinginductance that acts as a shunt inductor, L r is the seriesresonant inductor, and Cris the resonant capacitor.

Fig. 3 shows the typical waveforms of the presented LLCresonant converter. From Fig. 3, the operation of half ofswitching cycle can be divided into four modes.

(1) Mode 1 (t0< t < t1)at t=t0, S1turned on. During this

mode, output rectifier diode D1 conduct. The transformervoltage is clamped at Vo. Lm is linearly charged with outputvoltage, so it doesnt participate resonant during this period

and p outV n V= . The current Lri and Lmi increases. The

energy flows through the resonant tank and transformer and to

the load. This mode ends when Lri current is the same as Lmi

current. Output current reach zero.

(2) Mode 2 (t1< t < t2)at t=t1, , the two inductor current

Lri and

Lmi are equal. Output current reaches zero. Both

output rectifier diodes D1 and D2 is reverse biased.Transformer secondary voltage is lower than output voltage.Output is separated from transformer. During this period,since output is separated from primary, Lm is freed to

participate resonant. This mode ends when S1is turned off.

(3) Mode 3 (t2< t < t3)at t=t2, S1 is turned off. During

this mode, S1 and S2 are both off. The resonant current Lri

charges (discharges) the parasitic capacitance1oss

C (2oss

C ) of

the power switches. When the voltage across1oss

C equals Vin,

the body diode of S2is turned on.

(4) Mode 4 (t3< t < t4)The body diode of S2is turned on

in previous mode, which creates a ZVS condition for S2. Gatesignal of S2should be applied during this mode. When S2 is

turned on, Lri decreases and this will force secondary diode

D2conduct and ioutbegin to increase. Also, from this moment,transformer sees output voltage on the secondary side. Lm is

clamped with constant voltage p outV n V= , so it doesnt

participate resonant during this period.

For next half cycle, the operation is same as analyzedabove and is omitted here.

Fig. 3 Typical waveforms of LLC Resonant Converter

IV. SOFTWARE CONFIGURATION OF THE PROPOSED

CHARGER

In this paper, the dsPIC33FJ16GS502 DSC from

Microchip corp. is used as the digital controller. The output

voltage and current are sensed through the built-in analog-to-

digital (ADC) converter and the measured output current will

be fed into a PID controller to determine the controller output.

This controller output is then converted into PWM signals

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using the built-in PWM module and utilized to drive the

power switches of the half-bridge LLC resonant converter.

Fig. 4 shows the software flowchart of the proposed system.

From Fig. 4, the main tasks of the dsPIC controller include:

Performing digital filter and digital PID controller

Provide the gating signals of LLC resonant converter

Performing five-step constant current charging

The digital filter and digital controller are performed

every 10s. The switching frequency of the proposed power

stage is between 50 kHz and 150kHz. The digital filter used

in the proposed system is a 16-order finite impulse response

(FIR) filter for both voltage and current feedback signal [10,

11]. The equation describing a FIR filter can be expressed as

in (1).1

0

[ ] [ ]T

k

k

y n a x n k

=

= (1)

where x is the filter input, y is the filter output and akis

the corresponding coefficient of the designed FIR filter.

The digital PID controller is used to calculate the

required PWM command. A simple incremental PID

controller is utilized in this paper and the utilized PID control

algorithm can be designed as in (2).

I( ) ( ( ) ( 1)) ( )

( ( ) 2 ( 1) ( 2))

P

D

u n K e n e n K e n

K e n e n e n

= +

+ + (2)

where e(n) is the error signal and u(n) is the output of

the PID controller.

According to the literatures, multistage constant-current

charging algorithm has the advantages such as longer cycle

life, higher charge/discharge efficiency and shorter charging

time. Therefore, five-step CC charging algorithm isimplemented in the proposed battery charger system. Fig. 5

illustrates the concept of the five step CC charging pattern

used in this paper. From Fig. 5, the total charging period is

divided into five stages. In each stage, the charging current is

set to a pre-determined value. During charging, the voltage

of battery will increase. When the voltage exceeds the preset

limit voltage VTR, the stage number will increase and a new

charging current set value will be applied accordingly. The

same procedure will continue until stage number reaches 5.

In summary, the gating signals of power switch are

determined by the difference of feedback current and current

command using the PID controller and the stage number

which is determined by the feedback battery voltage.

It should be noted that the LLC resonant converter

works with variable frequency control. That is, LLC resonant

converter regulates their output voltage by changing the

frequency of the gating signals so that the impedance of the

resonant circuit changes. Therefore, the switching frequency

instead of the duty cycle is chosen as the control variable for

the proposed LLC resonant converter. This concept can be

illustrated as follows. Fig. 6 shows the PWM module built in

dsPIC33FJ16GS502. For conventional PWM controller, the

output of PID controller should be fed into the PWM Duty

Cycle (PDC) register while the Period register in Fig. 6

should be fixed as a constant value. However, for the

presented digital controller, the output of PID controller

should be fed into thePeriod register while thePDCregister

should be set as half the value of that in Period register to

obtain 50 % duty cycle.

Fig. 4 Software flowchart of the proposed charger

Fig. 5 Software flowchart of the five-step CC chargingmethod

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Fig. 6 PWM module in dsPIC33FJ16GS502 [9]

V.

EXPERIMENTAL RESULTS

In order to verify the correctness of the proposed system,

some experiments are carried out. The specification of the

presented prototype system isInput voltage: 380~420 Vdc

Output voltage: 44~52 Vdc

Maximum output current: 10 A

Fig. 7 shows the turn on transient of S1. Observing Fig.

7, the proposed LLC resonant converter can achieve ZVS.

Fig. 8 and Fig. 9 show the measured key waveforms of the

proposed LLC resonant converter. In Fig. 8, the input voltage

is fixed as 380 Vdcand the load varies from light load to full

load. In Fig. 9, the load current is fixed as 5 A (half-load)

and the input voltage varies from 380 V to 420 V. From Fig.

8 and Fig. 9, the proposed system operates in LLC resonantmode correctly. Fig. 10 and Fig. 11 show the measured

efficiency of the proposed LLC resonant converter under

minimum and maximum output voltage. From Fig. 10 and 11,

the efficiency of the proposed LLC resonant converter is all

higher than 88 %. Fig. 12 shows the recorded voltage/current

profile for the proposed five-step CC charging algorithm.

From Fig. 12, the proposed charger can accurately follow the

charging command. Fig. 13 shows the photo of the proposed

system.

Fig. 7 The turn on transient of S1

(VGS10 V/div, VDS200 V/div, Time4 s/div)

(a) light load (1 A)

(VGS: 10 V/div, VDS: 200 V/div, Vp: 200 V/div, ir: 5 A/div, Time: 10 s/div)

(b) full load (10 A)(VGS: 10 V/div, VDS: 200 V/div, Vp: 200 V/div, ir: 5 A/div, Time: 10 s/div)Fig. 8 Measure key waveforms of the proposed LLC resonant

converter when Vinis fixed (Vin= 380 V)

(a) Vin=380 V(VGS: 10 V/div, VDS: 200 V/div, Vp: 500 V/div, ir: 5 A/div, Time: 4 s/div)

(b) Vin=420 V(VGS: 10 V/div, VDS: 200 V/div, Vp: 500 V/div, ir: 5 A/div, Time: 4 s/div)

Fig. 9 Measure key waveforms of the proposed LLC resonantconverter when load is fixed (load current = 5 A)

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90

91

92

93

94

95

96

1 2 3 4 5 6 7 8 9 10

(%)

Output Current(A)

380V 400V 420V

Fig. 10 Measured efficiency of the proposed LLC resonant

converter (Vo= 44 V)

Fig. 11 Measured efficiency of the proposed LLC resonantconverter (Vo= 52 V)

Fig. 12 Recorded voltage/current profile for the proposed

charging algorithm

Fig. 13 Photo of the proposed system

VI. CONCLUSION

In this paper, a digitally-controlled LLC resonant

converter is developed for LEV battery charging applications.

The LLC resonant topology allows for zero voltage switching

of the main switches, thereby dramatically lowering

switching losses and boosting efficiency. The proposed

digitally-controlled LLC resonant converter can operate in

both constant output voltage mode and constant output

current mode. Five-step constant current charging pattern is

also realized in this paper. According to the experimental

results, the conversion efficiencies of the proposed LLC

resonant converter are all higher than 88% under all load

conditions, and the proposed charger can accurately follow

the charging command.

ACKNOWLEDGMENT

This work was sponsored by the R&D Piloting CooperationProjects between Industries and Academia at the Hsinchu

Science Park, project number: 101A21

REFERENCES

[1] L. R. Chen, "A Design of an Optimal Battery Pulse Charge System byFrequency-Varied Technique," IEEE Transactions on IndustrialElectronics,Vol. 54, No. 1, February 2007, pp.398- 405.

[2] L. R. Chen, R. C. Hsu, and C. S. Liu, A design of a grey-predicted Li-

ion battery charge system, IEEE Trans. Ind. Electron., vol. 55, no. 10,pp. 36923701, Oct. 2008.

[3] L. R. Chen, Design of duty-varied voltage pulse charger for improvingLi-ion battery-charging response, IEEE Trans. Ind. Electron., vol. 56,no. 2, pp. 480487, Feb. 2009.

[4] J. Feng, Y. Hu, W. Chen, and C. C. Wen, "ZVS analysis ofasymmetrical half-bridge converter," IEEE Proc. Power ElectronicsSpecialist Conference, vol. 1, pp. 243-247, 2001.

[5] R. Liu and C. Q. Lee, "The LLC-type series resonant converter variableswitching frequency control," Proc. Midwest Symposium Circuits andSystems, vol. 1, pp. 509-512, Aug. 1989.

[6] B.Yang, F. C. Lee, A. J. Zhang, and G. Huang, "LLC resonantconverter for front end DC/DC conversion," IEEE Proc. Applied PowerElectronics Conference and Exposition, vol. 2, pp. 1108-1112, Mar.2002.

[7]

Y. H. Liu, Y. F. Lo, Search for an Optimal Rapid Charging Pattern forLi-Ion Batteries Using the Taguchi Approach, IEEE Transactionson Industrial Electronics, Vol. 57, No. 12, Dec. 2010, pp. 3963-3971.

[8] Y. H. Liu, C. H. Hsieh, Y. F. Lo, Search for an Optimal Five StepCharging Pattern for Li-ion Batteries Using Consecutive OrthogonalArrays, IEEE Transactions on Energy Conversion, Vol. 26, No. 2,June 2011, pp. 654-661.

[9] Microchip Technology Inc.," dsPIC33FJ06GS101/X02 anddsPIC33FJ16GSX02/X04,"Available: http://www.microchip.com.

[10] "Implementing FIR and IIR Digital Filters Using PIC18Microcontrollers," Application Note AN852.

[11] Filter Design for dsPIC DSC Digital Filter Design and AnalysisSystem, Momentum Data Systems, Inc., 2008.

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