Interleaved 3 Phase DC/DC Converter for Automotive Applications

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1 010, 1th International Conference on Optimization of Electrical and Electronic Equipment, OTIM 010 Interleaved 3 hase DC/DC Converter for Automotive Applications O. Cornea, N. Muntean, M. Gavris olitehnica University of Timisoara/Department of Electrical Engeerg, Timisoara, Romania octavian.cornea@et.upt.ro; nic_muntean@gmail.com; mihaela.gavris@et.upt.ro Abstract- The paper presents an terleaved 3-phase step-down converter that can be used to charge the 1 battery and to feed the correspondg loads from the 4 bus, automotive dual voltage system. The converter has a maximum 3 kw put power and efficiency above 93% at full load. The simulation results show good responses under various put and put conditions. I. INTRODUCTION In 1994 the automotive dustry realized that the voltage level of electrical systems at that time (1-14) would not be sufficient for the future vehicles and a new voltage level (4) was soon proposed [1,11]. Now the 4 power bus is ternationally accepted as the automotive electrical system of the future. At this moment, however, the electrical system must be dual voltage architecture because the standardized (cheap) components such as small power electronics and motors, lamps etc. are 14 electrical devices. Thus a dual voltage method is used the transition period between the existent and the future electrical system. The 4 bus supplies high-power loads and the 14 bus supplies lowpower devices. The dual voltage architecture can have one or two batteries. The structure shown Fig. 1 has one 4 battery and one 14 battery. Fig.. A dual voltage electrical system that has only one 36 battery. converter is operatg step-down mode, the contuous put power required is 600W and the peak put power is 1 kw. The efficiency of the converter should be 96% at 100% put power and 93% at 30% put power. Obtag this very high efficiency is a real challenge for the design engeer. The required peak put power when the converter is operatg step-up mode is only 40% of the value stepdown mode and the efficiency is not very important because this mode of operation is activated only emergencies and works only for limited periods of time Due to the tendency of the automotive systems electrification, it was foreseen that the peak put power of the buck converter will rise to -3kW or even 5kW. Fig. 1. A dual voltage electrical system that has a 36 battery and a 1 battery. The dc/dc converter is used to charge the 14 battery. The configuration presented Fig. has only one 4 battery and the low-power devices are feeded through the 4/14 dc/dc converter. The dc/dc converter, unidirectional or bidirectional, that terfaces the two voltage buses, is the most important element of the dual-voltage electrical system. Reference [1] presents a table with the required ma features of a bidirectional dc/dc converter for a dual-battery, dual-voltage electrical system of a C segment car. When the II. EXISTING SOLUTIONS Both ductors and capacitors have been used as energy storage elements the terface between 4 and 14 buses. The most simple and first used topology was the classical buck converter, which can be used for low power levels. However, when the power creases the buck converter presents several disadvantages which have been discussed the literature. For high power levels a multiphase terleaved buck converter can be used [1]. Each converter has its ductor and the put capacitive filter is common. To improve the efficiency the diodes which are normally used to recirculate the current through the ductors can be replaced by power MOSFETs. This is called synchronous rectification. The advantage of the synchronous rectification is the lower voltage drop on the transistors when they are conduction. In the terleaved buck converters the put current is the sum of the phase currents. One of the advantages of usg a multiphase configuration is the reduction of the total current /10/$6.00 '010 IEEE 589

2 ripple which is filtered by the put capacitors; the value of the put filter capacitance is smaller. There is also possible a shift the technology of the put capacitors. Ceramic capacitors can be used for filterg stead of bulky electrolytic capacitors. This fact has the consequence of reducg the power loss because the ceramic capacitors have much lower values of the equivalent series resistance. In the terleaved buck converters the put current ripple is reduced and the efficiency is always better compared with classic converter. If the synchronous rectification is used the same converter can be utilized step-down mode or step-up mode, only by changg the control signals. The terleaved converter can be implemented usg offthe-shelf ductors but there are other possibilities to build them. One possibility is to use coupled ductors, where the current slope of one ductor is affected by the voltage across the other ductors [, 7]. The ma advantage usg coupled ductors is that the current ripple steady-state is reduced comparison with the case of the uncoupled ductors. If the current ripple is lower, the commutation frequency of the MOSFETs control can be reduced thus lowerg the switchg losses power transistors. Reference [3] presents a 4 phase terleaved buck converter design, which not all the ductors are coupled. A coupled ductor prototype is compared with an uncoupled ductor prototype. Regardg the coupled design, only the ductors of two phases, that are driven by 180 shifted signals, are coupled together. The wdgs are tegrated the prted circuit board. Because of the higher value of steady-state ductance the current ripple is reduced and the power losses of the switches and the copper wdgs are also reduced for the prototype with coupled ductors. The power losses beg smaller the temperatures at different measurement pots are several degrees lower for the coupled ductor prototype. The dynamic responses of the two prototypes are the same due to the fact that the transient ductance is unchanged. Two terleaved buck converters design with high number of phases are presented [4]. Both have the nomal put power of 1 kw; one design has 16 and the other has 3 terleaved phases, a modular construction. The ductors are tegrated and the magnetic cores are added at the moment when the other components are placed on the CB. The maximum, full load (70A), efficiency of the prototype with 16 phases, is 95%. For the other converter the maximum efficiency is ab 93%. The high number of phases brgs problems the control part of the system. The authors used a FGA custom design to implement the control signals for the 16 and 3 phases. A converter with nomal put power of 1kW with magnetic components was presented [5,6]. The converter efficiency can be higher than 98% but is dependent on the switchg frequency and state of charge of the batteries. The structure of the converter, the driver circuits and the control signals tend to be quite complicated but its efficiency cannot be reached by the converters with magnetic components. The converter can be operated at a frequency between 1-10 khz but the efficiency for 1 khz is below 90%. It is most efficient at 100% battery voltage and a frequency of 10 khz. The put voltage of this type of converter can t be controlled as the case of the converter with magnetic components. This can be a disadvantage because the ratio between the voltages of the two batteries is constant and if one battery is overcharged or empty the voltage on the other side of the converter is different than the expected value. The voltage control can be implemented by changg the switchg frequency, boostg the complexity of the converter. The efficiency improvement for this type of converters is obtaed maly by reducg the switchg looses because the switchg frequency is the range of 1 10 khz comparison to the converters with magnetic components where it is general khz. This paper is focused on a 3kW unidirectional dc/dc buck converter that is used to charge the 14 battery from the 4 bus. III. THE ROOSED SOLUTION The ma specifications of the proposed terleaved dc-dc converters are the followg: Input oltage: ; Output oltage: 14 ; Nomal ower (1Q): 3 kw; No of hases: 3. The block diagram of the converter is presented Fig. 3. There are three buck converters parallel. Each of them has its own synchronization voltage that is shifted from the other two and an put ower Good, which dicates if the converter gives the right voltage at the put. The three ower Good puts are combed to give a global dicator of the put voltage which is send to the hierarchical superior system (driven by a microcontroller or a DS). There is also a global shutdown. The buck converters are implemented usg LTC3810 [8]. It is a current mode synchronous switchg regulator controller IC from Lear Technology that can be used for step-down converters. LTC3810 allows an put voltage up to 100 which makes this IC well suited for automotive applications. The ma features of this IC, that are important regardg the overall system efficiency, are summarized below: 590

3 Fig. 3. Block diagram of the proposed converter. - Freewheelg of the ductor current is made through a power MOSFET (the so called synchronous rectifier or active freewhilg ); - There is no need of a sense resistor. The dra-source voltage of the ferior MOSFET is measured to estimate the ductor current; - The MOSFET drivers cluded LT3810 have very low put impedance for fast switchg of the power MOSFET to reduce the switchg losses; - The IC supply is derived from the put through an ternal low drop regulator. The schematic of one branch of the terleaved converter used for simulation is presented Fig. 4. There are three identical branches the terleaved converter, as Figure 3 shows. The schematic presented Fig. 4 contas also the components necessary for simulation of a step load. The voltage controlled switch SW3 is used to connect or disconnect a resistive load R load Ω. I. CONERTER EFFICIENCY The converter efficiency is determed usg the basic equation (1). In the followg is presented the efficiency calculation at full load for put voltage 57 and put voltage 14. (1) η + losses The put power is:. I () If the converter is runng at full power, the put current is 14A. This current is divided between the three branches of the converter; each phase has an average current of 7A at full load. The ma losses of the converter are given below: 1. High side transistor looses (S1) The conduction losses the high-side transistor are given by equation (3). I,max cond, loss, S1 ρ T RDS( on) (3) W. In equation (3) the dra to source resistance is the typical value R DS(on) 3.3mΩ, the R DS(on) temperature coefficient is ρ T, which is the maximum value the transistor datasheet and the put current is I,max 14A. Fig. 4. The schematic of one buck converter. 591

4 The switchg looses the high side transistor at turn-on are given by equation (4).,max Δ I 3 sw, loss, S 1 ( on ) t on f,max Δ I 3 (4) R dr C miller f 7.58 W In equation (4) the ternal resistance of the MOSFET driver is R DR.5 Ω the miller capacitor is C miller 1 nf, the threshold voltage is th 4.5 and the switchg frequency is f 00kHz. The switchg looses the high side transistor at turn-off are given by equation (5) where CC 10. sw, loss, S 1( off ) W,max 3 Δ I +,max 3 R dr Δ I + C miller t on f th The total losses all high side transistors are: loss, high side cc W th f. Low-side transistor losses (S) The low-side switch doesn t have switchg losses because it is parallel with the Schottky diode D; it is turned-on and turned-off under zero voltage condition. The low-side switch is formed by two MOSFET transistors parallel, so the current through one transistor is half the current of the switch. The losses the low-side switch are given by equation (1). loss, S 1.98W 3 ) The total losses of the low side transistors are: loss, low side (5) (6) ( ρ T ) R DS ( on (7) 3. Inductors The ductor has conduction losses and core losses. The conduction losses are given by equation (9) and the core losses can be taken from the ductor datasheet [8]. We use A low-loss ductors from ulse Engeerg. The typical dc resistance of A is 0.38mΩ, and the maximum value is 0.48mΩ. Two ductors series were used to realize 0.9μH so the losses must be multiplied by. W (8) R cond, loss, ductor 5.8W, max RL (9) 3 The maximum core losses an ductor at a switchg frequency of 00 khz are 0.7 W. The total losses all ductors are given by equation (10) W W W (10) loss, ductor 39 Neglectg the capacitor and the driver looses, the total looses are given by equation (11) (11) tot loss, high side loss, low side loss, ductor The efficiency of the converter related to equation (1) is: 3000 η, 93.3 % (1) loss ductor The converter was simulated LTSpice [10]. The efficiency of the converter was determed from the simulation data, dividg the put power by the average put power of the converter, which was obtaed tegratg the stantaneous put power over a number of switchg periods. The result was 93% at full put power.. SIMULATION WORK AND RESULTS Fig. 5 8 present simulation results obtaed usg the complete simulation model of the terleaved converter. Fig. 5 shows the start-up of the converter and a load step response. The put voltage is settled at 14 less than 0.5ms. In this time the 3 phase currents are limited at ab 150A. The load resistance is modified from R load1 R load to R load, openg the voltage controlled switch SW3 at t 4 ms. The controller responds very fast at this load step and contues to operate steady-state at 14W put power. After 1ms the switch SW3 is closed and the operation is settled aga at 3kW put power. Fig. 6 (a detail of Fig. 5) presents the put voltage, the phase currents and the synchronization voltages at steadystate. After 4 ms the phase currents are completely synchronized with the synchronization voltages and the converter operates steady state. In the first 4 ms the synchronization is not complete and the time tervals between the phase currents are not equal. Fig. 7 presents the waveforms of the phase currents when the put voltage contas an ac component of 7/10kHz superposed on the dc component of 50. The converter is operatg at 5% full power. The amplitude of the phase currents is automatically modified to compensate for the variation the put voltage. 59

5 Fig. 5. Load step response of the terleaved converter. Figure 6 presents the put voltage, the phase currents and Fig. 6. Output voltage, phase currents and synchronization voltages at steady-state. Figure 7. The waveforms of the phase currents when the put voltage has an ac component. 593

6 The converter response to a load step from 100% to 5% put power when the put voltage contas an ac component presented Fig. 8. This simulation was carried to test the response of the terleaved converter when the put condition and the put condition are changed simultaneously. I. CONCLUSIONS The paper presents an terleaved buck converter for the automotive dual-voltage electrical system. The maximum put power of 3 kw is considered to meet the future specification of the automotive dustry. The converter uses synchronous rectification and a commercial specialized IC to boost the efficiency. The calculated efficiency is above 93% at full power, which is close to the value obtaed from the simulation data. The simulation results show that the converter responds well to put voltage and load variations. The prototype is under construction and the experimental results are expected soon. ACKNOWLEDGMENT The authors gratefully acknowledge the support received from the F7 EE-ERT Grant SCS7-GA This work was partially supported by the strategic grant OSDRU 009 project ID of the Mistry of Labor, Family and Social rotection, Romania, co-fanced by the European Social Fond Investg eople. Figure 8. The converter response to the put voltage steps REFERENCES [1] A. Consoli, M. Cacciato, G. Scarcella, A Testa, Compact, Reliable Efficiency, IEEE Industry Applications Magaze, Nov/Dec 004, pp []. Zumel, O. Garcia, J.A. Cobos, J Uceda, Magnetic Integration for Interleaved Converters, Applied ower Electronics Conference and Exposition, Eighteenth Annual IEEE, ol., 9-13 February 003, pp [3] Seung-Yo Lee, A. G. faelzer, J. D. van Wyk, Comparison of Different Designs of a 4-/ 14- DC/DC Converter Regardg Losses and Thermal Aspects, IEEE Transactions on Industry Applications, ol. 43, No, March/April 007, pp [4] O. Garcia, Zumel, A. de Castro, J A. Cobos, Automotive DC-DC Bidirectional Converter Made With Many Interleaved Buck Stages, IEEE Transactions on ower Electronics, ol.1, No. 3, May 006, pp [5] Fang Zheng eng, Fan Zhang, Zaomg Qian, A Magnetic-Less DC- DC Converter for Dual- oltage Automotive Systems, IEEE Transactions on Industry Applications, ol. 39, no March/April 003, pp [6] F. Z. eng, F. Zang, Z. Qian, A Novel Compact DC-DC Converter for 4 Systems, ower Electronics Transportation, ower Electronics Specialist Conference, 003, 34th Annual IEEE, pp [7] J. Czogalla, Jieli Li, C. R. Sullivan, Automotive Application of Multi- hase Coupled Inductor DC-DC Converter, IEEE Industry Applications Society Annual Meetg, Octomber 003, pp [8] LTC3810 Datasheet, Lear Technology Corp., [9] SMT ower Inductors, lanar A1X9XNL Series, ulse Engeerg Corp., [10] LT SICE [11] J.G. Kassakian, Automotive electrical systems The power electronics market of the future,, roc. AEC 000 Conf., 000, vol. 1, pp