Design and Experimental Realization of a DC/DC Converter for

Electric Vehicle

Joao Pedro Gomes da Silva Beneditojoao.benedito@tecnico.ulisboa.pt

Instituto Superior Tecnico, Lisboa, Portugal

October 2017

Abstract

This thesis contains the design and experimental realization of a dc/dc converter for a electricalvehicle. The developed dc/dc converter is designed for a Formula Student car (FST07e). The designedconverter is a Full-Bridge with Phase-Shift and Zero-Voltage-Switch (FB-PS-ZVS) dc/dc converterwith a reactive LCC branch, working with an input voltage range of 475V DC to 600V DC, a frequencyof 200kHz and providing a nominal output power of 600W. In the power converter design simulationswere performed, (using the operation conditions of the converter in the car) in order to identify themost efficient devices to use in the power circuit. The design of a control unit based on a self-developeddigital control using a microcontroller, sensors and circuits for sensors signal conditioning is presented.The system used to supply the control unit was also self-developed and designed using a monolithicstep-down regulator. The converter is designed to have smaller dimensions, in order to achieve thisgoal, PCB prototypes were developed and some of the systems were tested.Keywords: FB-PS-ZVS, Zero voltage switching, Formula student, Electric vehicle, Phase-shift

1. IntroductionThe use of DC/DC converters in electric vehicles isquite usual. Switching power converters are usedfor processing the electrical energy that flows intothe energy storage system in a efficient way, mak-ing them essential for the development of more effi-cient and reliable electric vehicles. In electric vehi-cles the power management converters are normallyconnected between the high voltage channel and thelow voltage circuit [5]. The challenge in power con-verters design for electric vehicles is the ability of ef-ficiently convert energy at high operating frequencyat different loads.

Figure 1: Electric formula student car FST06e.

Usually Formula student cars (Figure 1) have twobatteries. One battery is the high voltage batteryused to supply the inverters and the three-phasemotors, the other battery is used to supply the lowvoltage circuit (24V DC). Using two batteries in-

creases costs and the electric system complexity.

Figure 2: DC/DC converter connections betweenthe low voltage system and the accumulator con-tainer.

The main objective of this thesis is to substi-tute the low voltage system supply by a new self-developed dc/dc converter, as presented in Figure 2.With the design and construction of this purposeddc/dc converter, the assembly of the low voltagesystem power supply will be more simple. In fact,the batteries from the LiPo cell stack will not beused and therefore the system will remain more safe.In addition, the costs tend to decrease.

Due to the power converter specifications theFull-Bridge with Phase-Shift and Zero-Voltage-Switch (FB-PS-ZVS) dc/dc converter with a reac-tive LCC branch is the chosen topology to applyin the dc/dc converter to be developed. Using theFB-PS-ZVS dc/dc converter with a reactive LCCbranch is possible to achieve high efficiency underhigh load cases and it is possible to operate with afix operating frequency with the use of phase-shift

1

control method. Due to the fact that the chosen de-vices to use in the inverter circuit are MOSFETs,the soft-switching method used is the zero-voltage-switching which is the most efficient method whenMOSFETs are used.

In Table 1 is presented the power converter char-acteristics.

Table 1: Target values defined for the dc/dc con-verter.

Input Voltage range: 475V DC to 600V DCOutput Voltage: 24V DCOutput Power: 600WEfficiency: High efficiency (> 80%)Protections: Over voltage protection

Under voltage protectionOver current protection

Compactness

2. Background

The section describes an overview of the dc/dcconverter topology FB-PS-ZVS with reactive LCCbranch [1] [3] [4] as well as the description of themost common control methods used in dc/dc con-verters [6] [2].

2.1. FB-PS-ZVS dc/dc converter with reactive LCCbranch

The FB-PS-ZVS with reactive LCC branch(Figure 3) is composed by: Four MOSFETs(Q1, Q2, Q3andQ4) and to each MOSFET a reso-nant component which is a capacitor is placed inparallel with the MOSFET. An inductance LP (alsoa resonant component) is connected in series withthe power transformer primary winding when, theprimary winding leakage inductance is lower thenthe necessary inductance to achieve ZVS conditions.A power transformer is used to reduce the voltageamplitude and to provide galvanic isolation betweenthe high voltage side and the low voltage side. Atthe power transformer secondary winding is con-nected a rectifier bridge and the low pass filter com-posed by LF and CF .

Figure 3: FB-PS-ZVS with reactive LCC branchdc/dc converter.

In order to ensure the enough energy to achieveZVS a LCC reactive branch is used, as presentedin Figure 3, increasing the primary winding current(IP ) in the passive-to-active transition.

The FB-PS-ZVS dc/dc converter with reactiveLCC branch waveforms are presented in Figure 4.

Figure 4: FB-PS-ZVS with reactive LCC branchdc/dc converter waveforms [3].

As previously mentioned, to achieve ZVS energyin the resonant components must be ensured. Toguarantee ZVS existence, the auxiliary inductor en-ergy must be greater or equal to the energy tocharge the capacitors, WL >WC , in which:

WL =Io

2

2.n2.LTrf +

T 2.Vin2

128.Ls(1)

WC =1

2.(C2 + C4).Vin

2 (2)

2.2. Control methodsIn order to achieve the converter proper operation,an efficient control method must be implementedin order to ensure the desired output voltage at thedesired output load and also monitoring the con-verter devices to avoid damaging the converter. Us-

2

ing an efficient control method, the converter effi-ciency may be increased. Based on previous studieson FB-PS-ZVS dc/dc converters control methods,mentioned in [6], the following control methods aredescribed:

• Alternating Control Method;

• Random Control Method;

• Temperature Control Method.

Alternating Control Method

This method consists in changing the devices inthe leading (A-P) and lagging (P-A) arm using atimer with a pre-defined preset time.

Random Control Method

The random control method is based in a PRBSgenerator (Figure 5), which is a linear feedbackshift-register composed by three XOR gates.

Figure 5: Random control method diagram.

When the output is 0 the LE is composed byQ1 and Q2 and the LA is composed by Q3 and Q4

. When the output is 1, the transistor of the LE

changes for the LA and the transistor of the LA

changes for the LE .

Temperature Control Method

This control method monitors the temperature ofboth inverter circuit arms (LA and LE) in order todefine which inverter arms is phase-shifted. In Fig-ure 6 is presented the temperature control methodgraphical representation.

Figure 6: temperature control method graphicalrepresentation [6].

To implement this method a DSP, a ADC mod-ule, temperature sensors and the sensors signal con-ditioning circuit are used.

3. Implementation

This section describes the implementation processused to identify the best devices and circuits topol-ogy to use in the converter by means of simulationsto each converter sub-systems. As presented in Fig-ure 7, the converter to be implemented is dividedin two systems. The Power system is the convertermain circuit and is composed by power semicon-ductor devices, the power transformer, the recti-fier/filter circuit and the output low-pass filter. Thelow voltage system is composed by the controllerunit, the supply circuit used to adapt the voltageto the low voltage devices and the gate-driver cir-cuit.

Figure 7: DC/DC converter to be developed sub-systems.

As presented in Figure 7 the power transformerhas a second secondary winding to supply the lowvoltage supply circuit used to adapt the voltagefrom the second secondary winding to a proper volt-age used by the low voltage system devices.

3.1. Inverter circuit devices

To select the device that best fit to the converteroperating conditions, two devices topologies werechosen: The Cascode GaN FET HEMT and theSiC MOSFET.

It is important to mention that all the performedsimulations to the inverter circuit in this Thesiswere performed using the PSIM.

Cascode GAN FET

The cascode GAN FET devices are composed bya high electron mobility transistor (HEMT) made ofvarious GaN layers grown on a substrate. To switchON a GaN FET transistor a positive bias voltagebetween the gate and the source must be applied,

3

these positive bias voltage will create a bidirectionalchannel between the drain and the source.

An advantage of using cascode GaN FET is thefact of the transistor behavior be similar to a lowvoltage Silicon fet, with its operating voltage ex-tended by the GaN HEMT. The GaN HEMT isconnected to the silicon fets drain and extends therange to 600V.Cascode GAN FET SimulationA simulation of the converter using the Cascode

GAN FET transistor real realized was performed, tosettle the FET transistor efficiency under the con-verter operating conditions. In this simulation threeload cases were used (300W; 450W and 600W).

Figure 8: Results obtained in the simulations withthe TPH3208PS at full load.

The simulation presented in Figure 8, is the con-verter simulation using the TPH3208PS real param-eters with a 600W load.

The obtained waveforms from the GAN FET sim-ulation were very similar to the obtained waveformsin the converter simulation with ideal devices.

The obtained converter output voltage and the

secondary winding voltage have the expected wave-form. The output current ripple (last waveform) islower as expected.

Using the TPH3208PS in the inverter circuit, itis possible to achieve efficiencies higher then 90%under the converter operating conditions. It is pos-sible to conclude that the converter is more efficientwith V in = 600V DC at full load (Figure 9). Forall the simulated load cases, ZVS was achieved.

Figure 9: Obtained simulation results in the sim-ulations with the TPH3208PS for three load cases(300W; 450W and 600W).

SiC MOSFETComparing to other Silicon MOSFETs SiC (Sili-

con Carbide) MOSFETs have a lower drift layer re-sistance, allowing high withstand voltage with lowresistance when used as high speed devices.

As the GAN FET transistor, to switch OFF theSiC MOSFET it also needs a negative bias voltagebetween the gate and the source.

SiC MOSFET SimulationThe performed simulations to the SiC MOSFET

were realized under the same conditions used in theGAN FET transistor simulation. But in this simu-lation the SiC MOSFET real parameters were used.The simulation presented in Figure 10, is the con-verter simulation using the C3M0065090J real pa-rameters with a 600W load.

The obtained waveforms from the SiC MOSFETsimulation were very similar to the obtained wave-forms in the converter simulation with ideal devices.

In this simulation ZVS was achieved for all sim-ulated load cases.

As presented in Figure 11, it is possible to achievehigher efficiencies then 90% under the converteroperating conditions using the C3M0065090J. Theconverter is more efficient with a 450W load forboth input voltages.

The results obtained with the C3M0065090J arebetter then the results obtained with the idealMOSFTEs, due to the new values of the LCC

4

Figure 10: Results obtained in the simulations withthe C3M0065090J at full load.

Figure 11: Obtained simulation results in the simu-lations with the C3M0065090J for three load cases(300W; 450W and 600W).

branch obtained in dimensioning. With the valuesfor the LCC branch it is possible to achieve higherefficiency at all load cases, due to the LCC branchlow equivalent impedance.

Conclusions

Based on the presented results the switching de-vice with the best efficiency under the dc/dc con-verter operating conditions is the TPH3208PS Cas-code GaN FET. But the GaN FET HEMT are moreefficient when used as synchronous rectifiers, so touse the GAN FET transistor the rectifier diodesused in the full-bridge rectifier circuit should be re-placed by GAN FET, this implementation processcost is higher and is more complex to implementwhen compared with the full-bridge rectifier com-posed by rectifier diodes. The SiC MOSFET arethe preferred devices to use.

3.2. Power transformer

The power transformer is one of the fundamentalcomponents in the dc/dc converter, because thiscomponent provides a galvanic isolation betweenthe high and low voltage and also due to the factthat, the converter efficiency mainly depends in thepower transformer construction process.

In this thesis six different design configurationsfor the power transformer were simulated, in or-der to identify the best design for the power trans-former, the following parameters were taken intoaccount the voltage per turn, turn ratio and powertransformer dimensions.

The best configuration to be adopted for thepower transformer is the ETD39-N97 due to thelow voltage per turn, turn ratio and dimensions.The ETD39-N97 power transformer specificationsare presented in Table 2.

The power transformer was built and tested.

3.3. Power supply unit

As previously mentioned, the power supply unit willadapt the voltage provided by the second powertransformer secondary winding, to the necessaryvoltage to supply all the devices connected to thelow voltage system (Table 3). This system was de-signed using specific design tools provided by themain component (LT8616) used in the power sup-ply unit manufacturer.

The LT8616 is a dual 42V synchronous mono-lithic step-down regulator with low current con-sumption.

The achieve the +5V DC and the +15V DC, theschematic presented in Figure 12 was used.

5

Table 2: ETD39-N97 power transformer specifica-tions.

Data ETD39-N97VN [V] 537.50DN [%] 0.58Vn [V] 15

VSec [V] 56.02NPri 35NSec 4m 8.75

ISecEF [A] 18.54IPriEF [A] 2.12Rp [Ω] 0.09Rs [Ω] 0.0012

lPPrikm [m] 2.42lPSeckm [m] 0.28

RWirePri [Ω/km] 36.92RWireSec [Ω/km] 4.22

WindingPriLength [mm] 45.85WindingSecLength [mm] 5.24

ku.WaPri [mm] 152.83ku.WaSec [mm] 17.47

Table 3: Table with the supply voltages of the con-trol system devices and their peripherals.

Device Supply voltageCurrent Sensor +15V DC and -15V DC

Gate driver +15V DC;-4V DC;+5V DCMicrocontroller +2.6V DC to +5V DC

SPI to CAN +5V DC

Figure 12: LT8616 typical connections.

To supply negative voltages to the low voltagesystem a charge pump circuit was implemented us-ing a 555 timer and the IC TPSA..., as presentedin Figure 13.

Figure 13: Negative voltage supply circuit.

3.4. Controller unit

The controller unit is responsible for sensors datamanagement and provide signal to the gate drivercircuit using voltage, current and temperature con-trol methods.

The implemented controller unit is based in a dig-ital control method composed by: microcontrollerwith a DSP, a resistive voltage divider to acquirethe converter output voltage and manage the sig-nals to the gate driver, a hall effect current sensorto measure the current at inductor LF and by fourNTC temperature sensors. In Figure 14 is presenteda controller block diagram with all the devices pre-sented in the controller unit.

Figure 14: Controller unit devices.

The implemented control unit schematics andPCB layouts are self-developed.

3.5. SiC MOSFET gate driver

The circuit to be adopted for the gate driver circuitis a common gate driver circuit used for switchingSiC MOSFETS. The implemented circuit is basedin a commercial solution provided by CREE.

The SiC MOSFET gate driver uses an integratedcircuit to increase the current in the MOSFET gate,to ensure enough power to switch the MOSFET.

To minimize the switching losses, a resistor wasplaced in series with the MOSFET gate and to min-

6

imize, the output current ringing a Kelvin sourceconnection is used.

4. ResultsThis section presents the obtained results for thefollowing components / systems:

• Implemented control methods: Voltage con-trol; Current control and Temperature control;

• Power transformer;

• Self-developed air core inductors.

Due to the delays in the budget availability, itwas not possible to built and test the converter pro-totype on time. In order to predict the converterbehavior under real conditions, a simulation usingall components real parameters was performed forboth input voltages and for the following load cases:60W; 120W; 180W; 240W; 300W; 450W and 600W

4.1. Control UnitVoltage and current measurement

and output power calculation

Table 4: Obtained results for output voltage (Vout)and current (Iout) measurement and output powercalculation.

Parameters Theoretical ExperimentalOutput voltage [V] 21.50 21.41Output current [A] 896 891Output power [kW] 19.26 19.08

The obtained results for the converter output volt-age, current and power were the expected ones. Theresults have a small error compared to the theoreti-cal results, this error can be reduced by calibratingthe sensors.

Offset angle calculation

Table 5: Obtained results for the offset angle calcu-lation.

Vout [V] φ Theoretical [o] φ Experimental [o]13,2 90,00 90,0021,41 16,36 16,0021,41 161,66 161,0021,41 -1,21 -1,1921,41 181,35 181,00

The obtained results for the offset angle calculationswere the expected ones.

Temperature measurement

Table 6: Obtained results in temperature measure-ment.

Temperature [oC] Measured temperature [oC]23,5 23,5323,5 23,5323,5 23,5223,5 23,5323,5 23,51

Phase angle control based on theinverter circuit devices temperature

Figure 15: Obtained results in the phase angle dis-tribution based on the inverter circuit devices tem-perature.

As presented in Figure 15 the phase value presentedin the LE is changed to the LA arm due to theLA average temperature was higher then the LE

average temperature.

4.2. Power transformer

Figure 16: Obtained waveforms in the power trans-former tests [Blue: Voltage 5V/div]; [purple:current500mA/div] at 60kHz.

As presented in Figure 16 visible two peaks in thesecondary voltage waveform as well as in the sec-ondary current waveform, because the used inverter

7

circuit does not have snubber circuits. It is also im-portant to refer that, the transformer did not satu-rate for V in = 60V and F = 20kHz, as expected. InFigure 17 the developed power transformer pictureis presented.

Figure 17: Self-developed power transformer.

4.3. Air core inductors

As presented in Figures 18 and 19, air core induc-tors were developed instead of inductors using reg-ular core materials.

Figure 18: Output filter inductor measurement.

Figure 19: Output filter inductor measurement.

In both Figures using a impedance analyzer, in-ductors inductance is measured. In Table 7 thecomparison between the desired inductance for bothinductors with the measured inductance is pre-sented.

Figure 20: Converter obtained simulation resultsfor both input voltages.

Table 7: Air core inductors measured inductances.Inductor Theo ExpLF [uH] 240 246.2LS[uH] 1000 1181

4.4. Full converter simulation

Figure 21: Converter performed simulation withVin = 600V DC and 240W load.

8

Due to the high power transformer primary leak-age inductance, the desired output voltage was notreached for higher load cases (75% and 100%).

With Vin = 600V DC the converter is less effi-cient for almost all load cases comparing with thesimulation performed with V in = 475V DC, butin these simulation the input voltage is higher so,it was possible to achieve the desired output volt-age (24V DC) in most of the load cases, comparingwith the simulation at Vin = 475V DC. For 75%and 100% of the nominal load it was not possibleto achieve the desired output voltage due to thereasons presented before.

The waveforms obtained in the simulation per-formed with V in = 600V CC and 240W of load areas expected.

5. ConclusionsDue to the delays in the budget availability, thecomponents were not order on time to test the con-verter.

The study of full-bridge with phase-shift mod-ulation and zero voltage switching converters wasperformed, presenting the full circuit analysis, soft-switching concepts and advantages and disadvan-tages were presented.

Simulations with ideal components were per-formed concluding that, to achieve ZVS underthe dc/dc converter specifications, a reactive LCCbranch must be implemented.

Based on the converter simulation using compo-nent real parameters (presented in section 4) it isnot possible to achieve the desired output voltagefor higher load cases (450W and 600W). At 40% ofthe nominal the converter achieves it’s maximumefficiency for both input voltages. This results isvery important because, when the converter is con-nected in the formula student car, the existing loadat its terminals will be approximately 40% of thenominal load.

In order to perform test to the FB-PS-ZVS dc/dcconverter a bench should be built not only to testthe power circuits but also to test the auxiliary cir-cuits. With this bench it is possible to test thepower converter safely, due to the high voltage usedin the inverter circuit.

Research and development of new control meth-ods techniques using FPGA, because it is even morenecessary the implement of a control method capa-ble to check the output voltages and currents, butalso capable to check the status of the converterdevices at the same time.

AcknowledgmentsThe author would like to thank to his family,friends, to the Institute of Telecommunications forproviding the laboratory for the master thesis ex-perimental realization and prototype contruction

and to all that support the author during the thisthesis realization.

References[1] Joo Pedro Ruivo Beirante. Tcnicas de optimizao

em conversores cc-cc em ponte com comutaono zero da tenso e modulao de fase. Master’sthesis, Instituto Superior de Engenharia de Lis-boa, Avenida Rovisco Pais, n1, 1049-001 Lisboa,Outubro 2001.

[2] IEEE Bo-Yuan Chen, Student Member andIEEE Yen-Shin Lai, Senior Member. Switch-ing control technique of phase-shift-controlledfull-bridge converter to improve efficiency un-der light-load and standby conditions with-out additional auxiliary components. IEEETRANSACTIONS ON POWER ELECTRON-ICS, 25(4):1001–1012, 2010.

[3] Beatriz Vieira Borges and Vitor A Anunciada.Conversores de corrente contnua comutados aalta frequencia. Instituto de Telecomunicaoes,pages 248–258, Janeiro 1995.

[4] Robert W. Erickson and Dragan Maksimovic.Fundamentals of Power Electronics. KluwerAcademic Publishers/Plenum Publishers, Uni-versity of Colorado Boulder, Colorado, 2nd edi-tion, 2001. eBook ISBN: 0-306-48048-4 PrintISBN: 0-7923-7270-0.

[5] Shangzhi Pan Majid Pahlevaninezhad andPraveen Jain. A zvs phase-shift full-bridgedc/dc converter with optimized reactive currentused for electric vehicles. IEEE, 51(2):4546–4551, 2013.

[6] IEEE Zih-Jie Su Student Member IEEE Yen-Shin Lai, Fellow and IEEE Ye-Then Chang,Member. Novel phase-shift control technique forfull-bridge converter to reduce thermal imbal-ance under light-load condition. IEEE TRANS-ACTIONS ON INDUSTRY APPLICATIONS,51(2):1651–1659, MARCH/APRIL 2015.

9