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8/9/2019 Conference-201304-Chun-Liang Liu-Design and Implementation of a Digitally-controlled LLC Resonant Converter for
1/5
Design and Implementation of a digitally-controlled
LLC resonant converter for battery charging
applicationsChun-Liang LiuDepartment of Electrical
Engineering, NTUSTTaipei, Taiwan, R.O.C
Yi-Hsun ChiuDepartment of Electrical
Engineering, NTUSTTaipei, Taiwan, R.O.C
Yi-Feng LoGreen Electronics Design &Application Dept., Divisionfor Biomedical & IndustrialIC Technology, ICL, ITRIHsinchu, Taiwan, R.O.C
Shun-Chung WangLunghwa University of Science
and Technology, TaoyuanCounty, Taiwan, R.O.C
Yi-Hua LiuDepartment of Electrical
Engineering, NTUSTTaipei, Taiwan, R.O.C
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
8/9/2019 Conference-201304-Chun-Liang Liu-Design and Implementation of a Digitally-controlled LLC Resonant Converter for
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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
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[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]
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[9] Microchip Technology Inc.," dsPIC33FJ06GS101/X02 anddsPIC33FJ16GSX02/X04,"Available: http://www.microchip.com.
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