A Novel 10kW Hybrid Converter

for the Electric Vehicle Charge Applications

Dai-Duong Tran, Hai-Nam Vu, Hyun-Hwa Lee, Van-Long Pham and Woojin Choi Electrical Engineering Department

Soongsil University Seoul, Korea

trandaiduong188@gmail.com, namvh.hust@gmail.com, cwj777@ssu.ac.kr

Abstract— A hybrid converter for the on-board charger consisting of a soft switching full bridge (SSFB) and a half bridge (HB) LLC resonant converter is proposed in this paper. The proposed topology adopts an additional switch and a diode at the secondary side of SSFB converter to guarantee the wide ZVS range of primary side switches and to eliminate the circulating current. The output voltage is regulated by controlling the duty cycle of secondary side switch. The effectiveness of the proposed converter was experimentally verified using a 10 kW prototype circuit. The experimental results show 96.8% peak efficiency.

Keywords— High efficiency, hybrid resonant and PWM power conversions, hybrid-switching circuit, minimized circulating current, Zero-Voltage-Switching (ZVS).

I. INTRODUCTION

The most common on-board charger (OBC) system architecture includes ac active-front-end ac-dc converter, and an isolated dc-dc converter [1]. First, an ac-dc converter converts ac power to dc power to maintain the intermediate dc bus from 360 to 400 V; second, the isolated dc-dc converter will recharge the EV battery pack within the output voltage range from 250 to 450 V. This paper will focus on the isolated dc-dc converter with 10-kW power.

Phase-shifted full bridge (PSFB) converter has been widely used in high power conversion applications due to its high power handling capability and ZVS realization of all switches with no auxiliary circuit [2]. However, it has two major drawbacks when used for battery charge application. Firstly, ZVS range of the lagging-leg is limited due to the wide range of output power variation. When the OBC circuit works at constant output voltage (CV) mode, the charge current decreases continuously. Since the ZVS of the lagging-leg switches is achieved with the energy stored in the leakage inductor, the lagging-switches tend to lose ZVS at this light load condition. The second main drawback of PSFB is the circulating current which causes conduction loss and efficiency degradation. Several approaches have been introduced to improve the performance of PSFB converter [3-7] by adding auxiliary circuit.

The other topology is an LLC resonant converter which exhibits very high efficiency at its resonant frequency [8]. The best application for resonant converter is the bus converter which can be designed at the optimal operation point with minimal circulating loss to get high efficiency. When the wide

variation of the input/output voltage is required, the variation of the switching frequency is very large, which makes the resonant operation impractical.

Recently, some hybrid topologies of phase-shift full-bridge and LLC converter are introduced in [9-10]. However their control switching scheme is still complicated and circulating current is not totally removed.

In this paper, a new hybrid topology which combines a soft-switching full-bridge (SSFB) converter and a half-bridge LLC (HBLLC) converter. In the SSFB part, an additional switch and a freewheeling diode are employed in the secondary side to control the output voltage. The proposed converter possesses following advantages: high DC conversion ratio with negligible duty loss, ZVZCS of all primary switches, wide ZVS range, high efficiency with no circulating current, especially at the light load, small size of the passive components in the secondary and a simple switching scheme.

II. OPERATING PRINCIPLE OF THE PROPOSED CONVERTER

The circuit diagram of the proposed converter is shown in . 1. In this converter, diagonally opposite switches ( and ) and ( and ) from each leg are switched as pairs and the

secondary side switch is synchronized with the primary switch operation with a small delay at both ends. During the delay between the primary switches and the secondary switch, primary current is reset zero, which makes it possible for the primary switches to turn on with ZVS and turn off with ZCS. There are six switching modes for the proposed converter in a half switching cycle: ([ ~ ]): At , and turn OFF. The primary current of SSFB transformer charges and

Vo1

Vo2

S1 S3

S2 S4

TR11 : n1

TR21 : n2

Llk1

Lo

Co1

Co2

S5

CinVdc

Iin

Ipri1

Llk2

Ipri2

Lm2

ILm2

b

a

cCr

D1 D2

D3 D4

D5 D6

D7 D8

Rb

Cb

Ba

ttery m

od

el

D9

Vds1

Coss1

Vds3

Coss3

Vds2

Coss2

Vds4

Coss4

Ids4Ids2

Isec1

Isec2

ID9

ILo

Lm

1ILm1

. 1. Proposed hybrid dc-dc converter for the on-board charger

discharges , respectively. During this time, and

simultaneously flow out from to charge and discharge . After the voltage and increase to

while the voltage and decay to zero, the body diodes of switch and conduct, creating ZVS condition for

and . In the LLC circuit, since is equal to the magnetizing current at , no power is transferred through the LLC circuit. ([ ~ ]): At , and turn on with ZVS. Because is still turned off, only LLC part transfer current to output port by the resonance between and . ([ ~ ]): Secondary side switch turns ON at

and the current commutates from diode to the switch . Therefore, current flowing through decreases to zero. At primary side, is charged by , thus this period represents the duty cycle loss. ([ ~ ]):In this mode, the SSFB transformer delivers the power to the load through the switch , and . The primary current of the transformer 1 reaches the reflected output inductor current and increases with the slope as

shown in = ( − ) ( / ) Since the output capacitors, and , are large enough, the LLC primary current, , increases by the resonance between the leakage inductor and and it is given by

( ) = − 2 − ( ) sin[ ( − )]− ( ) (1)

where, = / and = ; ( ) is the

voltage across the resonant capacitor at . The slope of the magnetizing current can be represented as ( ) = .

The LLC converter also participates in transferring the energy to the secondary side at the resonant frequency where the voltage gain is unity and independent of the load and the inductance ratio / . Thus, the can be designed relatively large, which leads to a small magnetizing current of LLC transformer, . ([ ~ ]): When is turned OFF at , the primary current resets to almost zero current. Hence the circulating current can be minimized in this topology. The output inductor current of the SSFB freewheels through diode . ([ ~ ]): At , and are turned OFF under ZCS condition. The same principle is applied to the other half of the switching period.

III. ANALYSIS OF THE PROPOSED CONVERTER

A. Voltage conversion ratio M

By using the voltage-second balance of the output inductor shown in . 3and the secondary rectifier voltage waveforms shown in . 4. ( ), we can obtain = = + 2 (2)

Where, is the duty cycle of the secondary side switch . . 4. ( ) and ( ) shows the rectifier output voltage waveform of the conventional PSFB converter and the

. 2. Key waveforms of the proposed converter

Lo

Co1

S5

Vo

D9

n2Vdc/2

n1Vdc

vLo

Lo

Co1

S5

Vo

D9

n2Vdc/2

n1Vdc

vLo

. 3. ( ) Equivalent circuit when is on, ( ) Equivalent circuit when is off.

. 4. Voltage waveforms of rectifier. (a) Conventional full-bridge converter. (b) Proposed converter.

proposed converter. Comparing the two figures, in the freewheeling time, of PSFB is maintained at zero. This mode does not exist in the proposed converter because the input power is continuously transferred to the output port without any circulating current in the primary side.

B. Transformer turns ratio

The output voltage of LLC converter is = , while

the output voltage of FB converter is = . Since duty cycle variation of the secondary side switch is =0.2~0.9, we can derive it as , . ,. .

Hence the optimal value of can be determined as = , −0.9 = , −0.2

Therefore, the turn ration of the LLC transformer can be

calculated as = .

C. Power sharing between SSFB and LLC ciruit

Since the outputs of each converter in the hybrid SSFB-LLC converter are connected in series, the LLC resonant converter transfers energy to the output during the whole switching period. The output voltage of the hybrid converter is adjusted by the duty cycle of secondary side switch of the SSFB converter. Hence the power distribution ratio of each converter, β1andβ2, can be given as = ≅ + 2 (3)

= ≅ 2 + 2 (4)

D. ZVS condition for the primary switches

In this proposed converter, both primary currents of two transformers 1 and 2 can contribute to charge and discharge the parasitic capacitors of four primary switches.

The peak magnetizing current of the SSFB transformer 1 can be represented as , = 4

The peak magnetizing current of the LLC transformer 2can be represented as , = /2 4 .

The ZVS condition can be achieved from true zero-load to full-load because the peak values of and are

independent of load. Since is typically limited to less than 5% of the PWM period, the magnetizing inductance and

must satisfy the following condition to guarantee ZVS under no-load condition. /[4(3 ) ] (5) /[8( ) ] (6)

E. ZCS condition for primary switches

The ZCS condition for the primary switches can be achieved if the primary switch turns off after the secondary switch turns off after a certain period of time as shown in . 6.

F. Duty cycle loss

From . 6 and . 2( 3([ ~ ]) ), the duty cycle loss can be derived as ∆ = ( )(2 )

where, is the output current of SSFB. It is clear that if is very small, the duty cycle loss is also very small. In the proposed topology, the leakage inductance of the TR1, , can be designed as small as possible because the ZVS range is independent of the value of .

IV. EXPERIMENTAL RESULTS AND DISCUSSIONS

A 10 kW prototype circuit has been built and tested to verify the operation principles and the performance of the proposed dc-dc converter. A digital control board with a Texas Instrument’s DSP 28F335 is employed as the controller.

. 7. Prototype of the proposed dc-dc converter for 10 kW On-board charger

. 5. The equivalent circuit during 1 . 6. Waveforms of rectified voltage , primary current of transformer 1

The specification of the proposed converter can be found in the . The key parameters resulting from the design example are given in the , where actual measured values are also shown. The circuit components used in the prototype converter are provided in the .

. 8 illustrates the voltage and current waveforms of the primary side switch of the proposed converter. This switch can achieve both ZVS turn-on and ZCS turn-off thank to the delay time between the primary side switches and the secondary side switch.

. 9 demonstrates the drain-source voltage and current of switch . It shows a perfect ZVS turn-on and ZCS turn-off characteristics of the lagging leg switch S4. The current flowing through switch is the combination of each current flowing through the full-bridge converter and the LLC converter.

. 10 shows the primary currents of full-bridge transformer (TR1) and LLC transformer (TR2) . After switch is turned off, is reset to the magnetizing current . Therefore, the circulating current in full-bridge converter during the freewheeling time is minimized, which

TABLE I DESIGN SPECIFICATION OF THE PROPOSED CONVERTER

Parameter Designator Value Input Voltage Range , ~ , 380 – 400 [V]Input Voltage Nominal , 390 [V] Output Voltage Range , ~ , 330 – 430 [V]Output Voltage Nominal , 400 [V] Maximum Output Power , 10 [kW] Switching frequency 38.4 [kHz]

TABLE II PARAMETER DESIGN RESULTS

Parameter (Designator) Calculated

Value Measured

Value Turn ratio of the SSFB transformer ( : )

11: 6 11: 6

Leakage inductance of the SSFB transformer ( )

10.22 [µH]

Magnetizing inductance of the SSFB transformer ( ) 1.5[mH]

1.4 [mH]

Turn ratio of the LLC transformer ( : )

14: 16 14: 16

Leakage inductance of the LLC transformer ( )

25 [µH] 25.96 [µH]

Magnetizing inductance of the LLC transformer ( )

[mH] 628 [µH]

Resonant capacitor [μF] 0.65 [μF] Output inductor of SSFB converter ( )

540 [µH] 500 [µH]

TABLE III COMPONENTS USED IN THE PROTOTYPE CONVERTER

Component Manufacturer Part # MOSFET IXYZ IXFB110N60P3 Diode Rectifiers IR 60APU06 Resonant Film Capacitors

EPCOS B32912B3334M

Magnetic Ferrite Core of Transformer

Changsung PQ7252

. 8. Voltage and current waveforms of the switch

. 9. Voltage and current waveforms of the switch

. 10. Primary current waveforms of the Full-bridge transformer and the LLC transformer

. 11. Measured efficiency of the proposed converter.

leads to a reduction of the conduction losses. Since switching frequency is equal to the resonant frequency, waveform is sinusoidal and LLC converter works at optimal condition with unity gain.

The measured efficiency with different load and various output voltage under 400 V nominal input voltage condition is given in . 11. It can be seen the peak efficiency can achieve 96.6% at 5 kW output power with VO = 420V.

. 12 shows the loss analysis when the proposed converter works at 10 kW, 5 kW and 1 kW, respectively. In three cases, thank to ZVS turn-on and ZCS turn-off of the primary switches, the loss at the switches is 14% in max. and that at secondary rectifier diodes is less than 24%. Due to the hard switching of additional MOSFETs and diode, the more the load increases, the more the switching loss increase. In contrast to this, the core loss of two transformers dominate over other losses at light load and it becomes relatively small at heavy load.

V. CONCLUSION

In this research a novel soft-switching hybrid converter combining an soft-switching full bridge and a half bridge LLC resonant converter is proposed for the EV On-board charger. Experimental results with a 10 kW prototype have been presented to verify the validity of the proposed converter. The proposed converter has a wide range of the ZVZCS characteristics and shows high efficiency characteristics, especially at the light load condition.

References [1] Yilmaz, M.; Krein, P.T., "Review of Battery Charger

Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles," in Power Electronics, IEEE Transactions on , vol.28, no.5, pp.2151-2169, May 2013

[2] Tae-Hoon Kim; Seung-Jun Lee; Woojin Choi, "Design and control of the phase shift full bridge converter for the on-board battery charger of the electric forklift," in Power

Electronics and ECCE Asia (ICPE & ECCE), 2011 IEEE 8th International Conference on , vol., no., pp.2709-2716, May 30 2011-June 3 2011

[3] Eun-Soo Kim and Yoon-Ho Kim, "A ZVZCS PWM FB DC/DC converter using a modified energy-recovery snubber," in IEEE Transactions on Industrial Electronics, vol. 49, no. 5, pp. 1120-1127, Oct 2002.

[4] H. K. Yoon, S. K. Han, E. S. Choi, G. W. Moon and M. J. Youn, "Zero-Voltage Switching and Soft-Commutating Two-Transformer Full-Bridge PWM Converter Using the Voltage-Ripple," in IEEE Transactions on Industrial Electronics, vol. 55, no. 3, pp. 1478-1488, March 2008.

[5] T. Mishima, K. Akamatsu and M. Nakaoka, "A High Frequency-Link Secondary-Side Phase-Shifted Full-Range Soft-Switching PWM DC–DC Converter With ZCS Active Rectifier for EV Battery Chargers," in IEEE Transactions on Power Electronics, vol. 28, no. 12, pp. 5758-5773, Dec. 2013.

[6] I. O. Lee and G. W. Moon, "Phase-Shifted PWM Converter With a Wide ZVS Range and Reduced Circulating Current," in IEEE Transactions on Power Electronics, vol. 28, no. 2, pp. 908-919, Feb. 2013.

[7] K. B. Park, C. E. Kim, G. W. Moon and M. J. Youn, "Voltage Oscillation Reduction Technique for Phase-Shift Full-Bridge Converter," in IEEE Transactions on Industrial Electronics, vol. 54, no. 5, pp. 2779-2790, Oct. 2007.

[8] Haoyu Wang; Dusmez, S.; Khaligh, A., "Design and Analysis of a Full-Bridge LLC-Based PEV Charger Optimized for Wide Battery Voltage Range," in Vehicular Technology, IEEE Transactions on , vol.63, no.4, pp.1603-1613, May 2014

[9] Bin Gu; Chien-Yu Lin; Baifeng Chen; Dominic, J.; Jih-Sheng Lai, "Zero-Voltage-Switching PWM Resonant Full-Bridge Converter With Minimized Circulating Losses and Minimal Voltage Stresses of Bridge Rectifiers for Electric Vehicle Battery Chargers," in Power Electronics, IEEE Transactions on , vol.28, no.10, pp.4657-4667, Oct. 2013

[10] Jih-Sheng Lai; Miwa, H.; Wei-Han Lai; Nan-Hsiung Tseng; Chi-Seng Lee; Chin-Hone Lin; Ya-Wen Shih, "A high-efficiency on-board charger utilitzing a hybrid LLC and phase-shift DC-DC converter," in Intelligent Green Building and Smart Grid (IGBSG), 2014 International Conference on , vol., no., pp.1-8, 23-25 April 201

(a)

(b)

(c) . 12. Loss analysis at (a) 10 kW, (b) 5 kW, (c) 1 kW