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Design and Control of a Bi-directional ResonantDC-DC Converter for Automotive Engine/Battery

Hybrid Power Generators

Junsung Park, Minho Kwon and Sewan Choi, IEEE Senior Member Department of Electrical and Information EngineeringSeoul National University of Science and Tech nology

E-mail: [email protected]

Abstract In this paper a bidirectional DC-DC converter isproposed for automotive engine/battery hybrid powergenerators. The two-stage bidirectional converter employing a

fixed-frequency series loaded resonant converter(SRC) isdesigned to be capable of operating under zero-current-switching(ZCS) turn on and turn off regardless of voltage andload variation, and hence its magnetic components and EMIfilters can be optimized. Also, a new autonomous and seamlessbidirectional voltage control method that combines twoindividual controllers for low voltage side control and highvoltage side control by introducing a variable current limiter isproposed to provide uninterrupted power to critical AC loadsand reduce the size of the DC bus capacitor. Experimental resultsfrom a 5 kW prototype are provided to validate the proposedconcept.

Keywords hybrid power generator, bidirectional DC-DCconverter, series loaded resonant converter, zero cur-rent switching,seamless transition

I. I NTRODUCTION Standby or emergency generators are often used as backup

power supplies for buildings, industrial facilities, and power plants in the event of a loss of utility power [1]. In addition,remote power generation for military, industrial, and personaluse requires a reliable, compact, and lightweight powergeneration system. The diesel generation system has been usedas backup power supplies or remote power generators[2]. Sincethe engine generator may not be able to respond to sudden loadchanges, energy storage devices should be used along with theengine generator to level out the erratic changes in power

balance between the generation and load consumption[3][4].Energy storage device is used along with a bidirectional DC-DC converter(BDC) in order to match the voltage level and/orachieve efficient charging and discharging operation[2].

Fig. 1 shows an automotive engine/battery hybrid powergeneration system. The BDC is located between the highvoltage DC bus and the low voltage battery which is alsoconnected to DC loads such as anti-lock brakes, electric powersteering, heated seats, electronic ignition and HVAC in thevehicle. The DC-AC inverter converts the DC power to AC

power to supply the critical AC load in the vehicle such as

broadcasting equipment of outside broadcast van andcommunications equipment of tactical vehicle. The AC-DCconverter converts the AC power from the engine generator to

the DC power, regulating the high voltage DC bus[5]. If theengine generator is capable of supplying total demanded powerof AC and DC loads, the AC-DC converter will be able toregulate the high voltage DC bus, and the BDC will deliver the

power from the engine generator to the low voltage side. If theengine generator is shut down or total demanded power of theAC and DC loads is greater than the maximum power of theengine generator, high side bus voltage will drop off to avoltage depending on the capacitances of the DC bus capacitor.Then, the BDC is required to take over the regulation duty ofthe high voltage DC bus by changing over from V L control(battery charging) to V H control (battery discharging) so that itshould be able to deliver power from the battery to the ACload. Therefore, in order to provide uninterrupted power to thecritical AC loads and reduce the size and cost of the DC buscapacitor, the transition from V L control to V H control of theBDC should be seamless and as short as possible. This is acrucial performance of the BDC, especially, in the automotiveapplication where electrolytic capacitors cannot be used due tolimited lifespan and bulky nature[6]-[9]. So far, bidirectionalvoltage control methods with seamless mode transition of theBDC have not been discussed.

The BDC should provide galvanic isolation and high stepup/down voltage conversion ratio in the application where thelow voltage battery is used. Typical topology candidates with

Fig. 1. Automotive engine/battery hybrid power generation system

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these requirements include half-bridge, full-bridge and push- pull PWM converters [10][11], dual active bridge(DAB)converters [12][13], and resonant converters [14]-[17]. Thehalf-bridge, full-bridge and push-pull PWM converters usuallynecessitate passive or active clamping on the low voltage sidewith inductors to clamp the surge voltage generated by theleakage inductance of the transformer. The active clampingtechnique makes the converter not only clamp the surgevoltage, but achieve zero-voltage-switching (ZVS) turn on ofall switches. A drawback of the active clamped PWMconverter is high switch turn off losses[18]. The DAB has amodular and symmetric structure and can achieve ZVS turn onwithout auxiliary components. However, the DAB has limited

ZVS range and high circulating currents for applicationsrequiring wide voltage variation. The ripple current of the DABconverters is high and especially problematic in the low voltageapplication[19]. The bidirectional resonant DC-DC convertergenerally sees different resonant tanks in forward and reversemodes, respectively, resulting in different voltage gains. Thisoften makes it difficult to satisfy the required voltage gain for

both modes of operation[14][15]. The dual bridge SRCconverter has large current in the resonant tank compared to theDAB converter[16]. A minimum current operation for the dual

bridge SRC converter has been proposed with a complicatedswitching method[17]. The bidirectional CLLC resonantconverter in [14] requires four resonant components to bedesigned and therefore has a challenging issue for high volumemanufacturing associated with resonant component tolerances.

In this paper, a two-stage BDC is proposed for automotiveengine/battery hybrid power generators. The proposed two-stage BDC consists of a non-isolated converter and a fixedfrequency SRC. The SRC is designed be capable of operatingunder ZCS turn on and turn off regardless of voltage and loadvariation in both forward and reverse operation. A method ofadjusting dead time of the SRC will be presented to minimizethe switch turn on losses associated with energy stored inMOSFETs output capacitances during the ZCS turn on

process. Also, a new autonomous and seamless bidirectionalvoltage control strategy is proposed to provide uninterrupted

power to the critical AC loads and reduce the size of the DC bus capacitor.

II. PROPOSED B IDIRECTIONAL DC-DC CONVERTER The proposed BDC consists of two power conversion

stages: a non-isolated converter and a fixed frequency SRC, asshown in Fig. 2. Since the SRC is operated at fixed frequencyand fixed duty all components can be designed with minimumvoltage and current rating. The non-isolated converter isoperated to regulate either high side voltage V H or low sidevoltage V L according to demanded load power and availabilityof the engine generator. Figs. 3 and 4 show key waveforms andoperation states of the proposed SRC, respectively. Mode I

begins with Lr -C r resonance when switches S H1 and S L2 areturned on at t 0. The angular resonant frequency of the resonantcircuit can be expressed as,

12r r

r r f

L C = = (1)

where resonant inductance and resonant capacitance can bedetermined respectively by,

2

2m ks

r kpm ks

L n L L L

L n L

= +

+

(2)

1 2 3r r r C C C = + . (3)

It is seen from Figs. 3 and 4 that low side current i L (= iSL2)at Mode I (t 0-t 1) becomes purely sinusoidal if the on-time dutycycle is selected such that DT s = 0.5 f r . Then it can be expressedas,

,( ) sin2 L DC

L r

I i t t

= (4)

Fig. 2. Proposed two-stage bidirectional DC-DC converter

S H1 , S L2

i Lr

v Lr

vCr2

i SH1

v SH1

i SL2

v SL2

t 0 t 1 t 2

i Lm

0.5T s0

V SH,on

I Lm,pk

D d T s

T s

i SH2

v SH2

S H2 , S L1

v SL1

i SL1

t 3 t 4

V SL,off

Near ZCS turn-off Near ZCS turn-on

ZCS turn-off ZCS turn-on

Near ZCS turn-off ZCS turn-on

ZCS turn-off ZCS turn-on

vCr1

V SL,on

I L,pk

DT s= 2f r 1

2V i

V i

V SL,ON

V SH,on I SH,off = I Lm,pk

Fig. 3. Key waveforms of the proposed SRC

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However, the turn on losses of the switches may beconsiderable in the high voltage application. The turn on loss

P SL,loss(on) of the low side switch is negligible since V SL,on issmall in this low voltage application, and the turn-on loss

P SH,loss(on) of the high side switch is also small due to the two-stage configuration. In the proposed SRC, P SH,loss(on) can further

be reduced by increasing Dd T s and, in turn, decreasing V SL,on, asshown in Fig. 3. However, increasing Dd T s may causeincreased current ratings and undesired resonance, and hence itshould properly be chosen. Therefore, it is noted that both turnoff and turn on switching losses of the proposed SRC is madenegligible in this application.

In the conventional frequency-controlled SRC, in general,resonant inductance Lr should be made large to reduce theswitching frequency range. In the proposed SRC, on thecontrary, Lr is chosen to be small since the SRC is not used forregulation, resulting in very small gain variation according toload variation in both charging and discharging modes, as

shown in Fig. 5. Furthermore, small Lr leads to less sensitive tothe resonant component tolerances, eliminating the voltageregulation issues and saturation problem of magnetic devicesthat was introduced in the conventional frequency-controlledSRC[22][23]. This also allows Lr to be easily embedded in thetransformer. Also, the proposed SRC is able to achieve ZCSturn on and turn off of the switch without regard to voltage orload variation, as shown in Fig.6, by choosing the resonantfrequency f r as follows,

1 1

1 12 1 2r s sd

f f f D D

= = (15)

III. PROPOSED CONTROL STRATEGY The high side DC bus is regulated to either 400V by the

AC-DC converter or 380V by the BDC, respectively, accordingto condition of V H . The conventional control of the BDC is ingeneral realized with two individual controllers of V L controlfor battery charging and V H control for battery discharging, andtherefore may not be able to avoid large transient during thetransition from V L control to V H control of the BDC.

In this paper a new autonomous and seamless bidirectionalvoltage control strategy, as shown in Fig. 7, is proposed to

provide uninterrupted power to the critical AC loads andreduce the size of the DC bus capacitor. The two outer loop

voltage controllers for V L control and V H control are combined by VCL whose output I * LB is automatically selected to be either I LB,H , the output of the high side voltage controller, or I LB+ , the positive limit of VCL which varies with the output of the lowside voltage controller. This makes it possible to share innerloop current controller, resulting in autonomous and seamlesstransition from V L control(charging mode) to V H control(discharging mode), and vice versa. The peak values ofthe positive and negative limit, I LB-,pk and I LB+,pk , of the VCL aredetermined by,

, ,i

LB pk LB pk i

P I I

V +

= = (16)

According to C-rate of the battery used, I LB+,pk may bechosen smaller than eqn. (16). I LB+ varies with magnitude of V L while I LB- is always fixed at I LB-,pk . The anti-windup is used to

Anti-windup

I LB

G L(s)

Controller

G H (s)

Controller

G I (s)

Controller

PWM Generator

Variable currentlimiter

Anti-windup

Anti-windup

Fig. 7. Control block diagram of the proposed battery charger

Mode III Mode I Mode II

10K

5K

0K

400

395

390

385

380

375

29

28

27

26

25

24

40

20

0

-20

P o w e r

[ W ]

V o

l t a g e

[ V ]

V o l

t a g e

[ V ]

C u r r e n

t [ A ]

Fig. 8. Simulation waveform of the proposed bidirectional voltagecontrol for seamless transition from V L control to V H control

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prevent the saturation of the controllers. For the sake ofsimplicity it is assumed that the DC-load is constant and all the

power losses of the AC-DC converter, the DC-AC inverter andthe BDC in Fig. 1are neglected.

A. Transition from V L control to V H controlFigs. 8 shows PSIM simulation waveforms for illustration

of the operating principle of the proposed bidirectional controlstrategy for transition from V L control to V H control.

Mode I : Assume that the battery has already been fullycharged. The engine generator is supplying the AC and DCloads during this mode. V H is regulated to 400V by the AC-DCconverter, and the reference voltage V * H of the BDC is set at380V. Since the high side voltage controller G H (s) is saturatedthe reference current I * LB of the BDC is determined by I LB+ which is the same as I L,DC .

Mode II : This begins when the AC load increases and thesum of the AC and DC loads is greater than P G,max , themaximum power that can be produced by the generator. Then,the AC-DC converter is not able to regulate the DC bus, and V H drops off from 400V, which makes I * LB be changed to I LB,H , asshown in Fig. 8. I * LB decreases, changes its sign andcontinuously increases. This means that the BDC starts todischarge the battery and regulate V H to 380V. As the batteryvoltage decreases, I LB+ which is the output of the low sidevoltage controller G L(s) increases up to I LB+,pk . This is the endof the mode.

Mode III : I * LB is fixed at constant value since the AC load isconstant. The BDC keeps discharging the battery andregulating V H to 380V. I LB+ is kept at I LB+,pk .

B. Transition from V H control to V L controlFigs. 9 shows PSIM simulation waveforms for illustration

of the operating principle of the proposed bidirectional controlstrategy for transition from V H control to V L control.

Mode I : This mode is identical to Mode III of Section III-A.The BDC is discharging the battery and regulating V H to 380V.

I * LB is determined by I LB,H , and I LB+ is the same as I LB+,pk .

Mode II : This mode begins when the AC load decreasesand the sum of the AC and DC loads becomes smaller than

P G,max . This makes the AC-DC converter be capable ofregulating V H , recovering it back to 400V. Therefore, I

* LB(= I LB,H )

decreases and changes its sign, meaning that the BDC is able toregulate V L to 28V, and continuously increases until it reachesto I LB+,pk . Now, I

* LB is determined by I LB+ (= I LB+,pk ) since the high

side voltage controller G H (s) is saturated. Then, the BDC startsto charge the battery with constant current of I B,CC which is

determined by,

, , , B CC LB pk Load DC I I I += (17)

Mode III : When the battery voltage V L gets close to V * L the

reference current I * LB which is determined by I LB+ starts todecrease. During this mode the BDC charges the battery withconstant voltage of V * L .

IV. EXPERIMENTAL R ESULTS A 5 kW prototype of the proposed BDC has been built

under the following system parameters.

P o = 5 kW V H 340~440 V V L = 24~32 V N P : N S = 5 : 1

f s1 = 48 kHz f s2 = 20 kHz C f = 110 F C H = 45 F

C i = 100 F C L = 380 F L B = 1 mH L f = 0.42 H

Lr = 5.8 H Dd T s = 600 ns C r1 (=C r2)=0.94 FFigs. 10 and 11 show key experimental waveforms of the

charging and discharging modes at full load, respectively. Aswe can see from Figs. 10(b), (c) and Figs. 11(b), (c) all switchesof the SRC are being turned on and off with ZCS in bothcharging and discharging modes. In fact, all switches of theSRC are always turned on and off with ZCS without regard tovoltage and load variations.

Figs. 12 shows the experimental waveforms of the modetransition. A 24 V/100 Ah lead acid battery was used at the lowvoltage side. Fig. 15 shows that the BDC is regulating V L tocharge the battery, and V H is regulated to 400 V by the AC-DCconverter. When the engine generator is shut down, V H drops

but is recovered to 380 V since the BDC changes over to V H control to discharge the battery.

The efficiency of the proposed BDC including gate driveand control circuit losses is measured by YOKOGAWAWT3000 and shown in Fig. 13. The maximum efficiencies are95.13 % at 1.3 kW in charging mode and 95.08 % at 1.5 kW in

Mode III Mode I Mode II

CC CV

I B,CC

10K

5K

0K

400

395

390

385

380

375

29

28

27

26

25

24

40

20

0

-20

P o w e r

[ W ]

V o

l t a g e

[ V ]

V o l

t a g e

[ V ]

C u r r e n

t [ A ]

Fig. 9. Simulation waveform of the proposed bidirectional voltage controlfor seamless transition from V H control to V L control

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discharging mode, respectively. Fig. 14 shows the photographof the proposed BDC prototype.

V. CONCLUSIONS

This paper proposes a bidirectional DC-DC converter forautomotive engine/battery hybrid power generators. Thefeatures of the proposed BDC are as follows.

The proposed topology preserves the advantages of thetwo-stage DC-DC converter: 1) The switching method issimple in that voltage regulation and mode transition arecarried out only by the non-isolated converter; 2) Allcomponents ratings of the isolated converter areoptimized.

Small Lr can be used since the proposed SRC is notused for regulation, which leads to the following

advantages: 1) The SRC has very small gain variationaccording to load variation, and therefore the proposedBDC can be designed for wider voltage range; 2) TheSRC is less sensitive to the resonant componenttolerances, and therefore suitable for high volume

(a) (b) (c)

Fig. 10. Experimental waveforms of the charging mode (a) inductor current I LB, switch voltages V SB,1 and V SB,2 of the non-isolated converter (b) primary current I pri,high side switch voltages V SH,1 and V SH,2 of the SRC (c) primary current I pri, low side switch voltages V SL,1 and V SL,2 of the SRC

I pri [20A/div]

V SL,1 [50V/div]V SL,2 [50V/div]

[5 s/div]

ZCS turn on& turn off

(a) (b) (c)

Fig. 11. Experimental waveforms of the discharging mode (a) inductor current I LB, switch voltages V SB,1 and V SB,2 of the non-isolated converter (b) primary current I pri ,high side switch voltages V SH,1 and V SH,2 of the SRC (c) primary current I pri, low side switch voltages V SL,1 and V SL,2 of the SRC

Fig. 12. Experimental waveforms of transition from V L control forcharging to V H control for discharging

E f f i c i e n t y

( % )

Fig. 13. Measured efficiencies of the proposed BDC including gate driveand control circuit losses

Fig. 14. Photograph of the proposed BDC prototype

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manufacturing; 3) Small Lr can be easily embedded inthe transformer.

The proposed SRC is capable of achieving ZCS turn onand turn off regardless of voltage and load variation. Amethod of adjusting dead time of the SRC has been presented to minimize the switch turn on lossesassociated with energy stored in MOSFETs output

capacitances during the ZCS turn on process. An autonomous and seamless bidirectional voltage

control method with a variable current limiter has been proposed to provide uninterrupted power to critical ACloads and reduce the size of the DC bus capacitor.

Experimental results from a 5kW prototype were providedto validate the proposed concept. The maximum efficienciesincluding gate drive and control circuit losses are 95.13 % at1.3 kW in charging mode and 95.08 % at 1.5 kW in dischargingmode, respectively.

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