High Power Buck-Boost DC/DC Converter for Automotive Powertrain Applications

Transcription

1 High Power Buck-Boost / Converter for Automotive Powertrain Applications B. Eckardt*, M. März*, A. Hofmann*, M. Gräf +, J. Ungethüm + * Fraunhofer Institute of Integrated Systems and Device Technology, Schottkystrasse 10, Erlangen - Germany, + German Aerospace Center (DR) e.v. Institute of Vehicle Concepts, Pfaffenwaldring 38-40, Stuttgart Abstract A high power buck-boost / converter for use in the powertrains of hybrid cars is presented. A special digital control strategy is implemented that allows a smooth change between both energy transfer directions. Equipped with this feature the converter can realize the energy management in the electric powertrain. The advantages of the hybrid system are shown by a hybrid fuel cell research car as an example. Application oriented data and test results of the used / converters are given. Two prototypes of highly efficient 4kW and 70kW bidirectional / converters were developed and evaluated. The digital control provides full protection against overvoltage, overcurrent and overtemperature. By integrated liquid cooling up to C - and very low losses, a high power density up to 5W/cm³ is achieved. Characterization data of the converter and measurements in the target application - a hybrid fuel cell car - are presented. Keywords: / converter, bidirectional, digital control, fuel cell, hybrid car 1. Introduction The development of ultra low emission vehicles (UEV) is a great challenge for power electronics in automotive applications [1]. A key component, whether talking about hybrid or fuel cell cars, is a powerful and highly efficient / converter. Measurements of the DR institute of vehicle concepts of a battery car capable of recuperating braking energy, clearly show power savings of up to 4% for inner city driving. Statistic City and Motorway Distance / km Time / s Average speed / km/h Recuperation ratio Motor->Batt / Batt->Motor 4.1% 6.7% 4.9% Table 1: Evaluation of recuperation energy Because of the low energy density of batteries in comparison to fuel the cruising range was very poor. To get a useful car with both, very low emissions and high efficiency, a hybrid fuel cell propulsion system was developed and tested. The hybridisation of a fuel cell car with a battery not only improves the efficiency because of the recuperation, it also provides a much better acceleration.. Hybrid Powertrain Fuel Cell 0kW Electric Motor AC 130V -60V 10kW - 4kW 1V Battery 1V oad Battery NiMH SuperCap Figure 1: Powertrain of a hybrid fuel cell car with high voltage link, a storage battery and conventional 1V powernet The test car has an electric motor with 1kW continuous and 39kW peak power. As primary energy source a fuel cell with a maximum of 0kW energy output by a bus voltage of 140V to 40V is used. To be able to recuperate braking energy and to improve acceleration, the fuel cell system is actually equipped with a 48V Pb battery. It is planned

2 to replace it by a SuperCap of about V. This energy storage is coupled to the high voltage bus by the described 4kW / converter with a maximum power transfer capability of up to 11.5kW in this configuration. The complete powertrain is controlled via CAN-Bus interface and allows a maximum cruising speed of 10km/h. needed chip area to carry the maximum current of up to A per phase. In the voltage range up to 450V there are only MOSFETs with a relatively high R DSon. IGBTs can carry more current per square than high end MOSFETs in this voltage range. Because of this fast NPT-IGBTs were used in parallel with fast Si-pn diodes. The integrated half bridges are SKM1GB063DN modules. For the prototypes only standard components of electrolyte and foil capacitors were used. The ferrite main inductors were specially designed for high saturation current and with RF litz wires for low winding losses. 4. Current Mode Control Figure : Hyite Fuel cell test car on a winter test run in Stuttgart I m 3 set current compensation ramp 3. High Power / Converter m 1 m inductor current A non isolating topology was chosen for the / converter for efficiency reasons. Since the voltage difference between battery and fuel cell side is quite small and a highly efficient energy transfer in both directions is necessary, a buck boost topology was employed. The converter was realized with three phases which are switched by a 10 degree phase shift (see Fig. 3) to get low current ripple on the capacitor banks. Fuel Cell High Side I C 1a C 1b C 40 Figure 3: Multi phase buck / boost converter Battery ow Side That allows a cheap and compact design. The semiconductors were chosen according to the t Figure 4: Slope compensation for peak current mode control The phases are current balanced by peak current mode control with an adaptive slope compensation for duty cycles greater than 50% as described in [3]. The slope compensation is calculated by the known inductance and measured voltages on the battery V V and fuel cell side V HV under the assumption that the voltages are constant during a switching period and: di V =, di V m, m = dt dt (1) For buck mode operation the slope is calculated as: Vl1 m1 Buck = with Vl1 = VHV VV () Vl m Buck = with Vl = V V (3) For control loop stability the slope m 3 of the compensation ramp has to be at least one half of the slope m. VV m3buck (4)

3 The same approach for the boost mode slope compensation: Vl1 m1 Boost = with V l1 = VV (5) Vl m Boost = with Vl = VV VHV (6) VV VHV m (7) 3Boost These equations can easily be solved in real time by the used Infineon XC164 16bit microcontroller. The result for the slope of the compensation ramp is transmitted to an integrator by a 1bit digital to analog converter. The ramp at the output of the integrator is added to the measured current ramp. When the sum of both is equal to the maximum set current, the phase is switched off until a new switching period starts. VHV VV CPU FPGA D/A Converters Ipeak (setvalue) slope + I V + _ CAN-Bus communication breakdown is followed by a shut down of the converter after ms time out 5. Buck/Boost Mode Transition The application of fuel cells requires a voltage control mode with an automatic direction change for the energy transfer from or to the battery. The / converter tries to keep the voltage on the fuel cell bus constant. An appropriate voltage level can be set via CAN- Bus control to enable the fuel cell to deliver the needed current. In case of a voltage increase on the fuel cell side over the selected value the converter transfers the surplus energy into the storage battery. This can happen because of recuperation or an actual higher power output of the fuel cell than used by the electric motor. If the driver wants to accelerate the car there suddenly is a higher power demand than the fuel cell can deliver. The converter transfers the missing energy until the fuel cell current output is ramped up. To get a fast reduction of the fuel cell current the dc voltage is raised. I HV VHV I V VV CH1: Phase Current Figure 5: Digital control loop of the converter The microcontroller calculates the maximum phase current I peak needed to receive the set output values selected via CAN-Bus. It is possible to use the converter as a current or voltage source either on the low voltage side or on the high voltage side. The user can set individual maximum values as limitations for voltages and currents. CH: Slope Compensation CH4: I set A sophisticated safety system is implemented in hard- and software. It protects the converter against the following events: Overvoltage on the high and low voltage side by load dump under high power operation - by hardware Overcurrent because of short circuit on the low side - prevented by the digital control loop Overcurrent on the high side in case of short circuit - prevented by a fast high voltage fuse Overtemperature - leads to a derating of the maximum possible current Figure 6: Boost to Buck Mode Transition CH1: Current of one phase CH: Slope compensation CH4: Value for peak current I set For bidirectional power transfer the converter is switched between buck and boost mode. A very smooth transition is achieved by a small dead zone of switching pulses at the time of transition. This was realized by the short pulse suppression of the used driver circuit. Fig. 6 shows the moment of transition from boost to buck mode operation due to recuperation. Ch

4 4 shows the set value I set for the peak current control and Ch 1 the current pulses of one phase. After the transition I set does not reach a constant value because of the increasing low side voltage due to the charging process of the battery. 6. Prototypes The prototypes are especially designed for automotive applications with liquid cooling. Water glycol mixtures (50% / 50%) with coolant temperatures up to C can be used. The converter housing is designed in accordance to IP54 and the electric components are evaluated, placed and secured to withstand automotive vibration tests. Two different prototypes have been realized with the following technical data: Converter Rating 15 kw Peak 4kW 35kW Peak 70kW ow Side Voltage Range min.: 40V max.: 10V min.: 00V max.: V High Side Voltage Range min.: 130V max.: 60V min.: 300V max.: 450V ow Side Current max.: 40A max.: A High Side Current max.: 110A max.: 40A Max. Cooling 65 C C Temperature Switching 7kHz 17kHz frequency Main Inductor 35µH µh Value ow Side 8.5mF 6mF Capacitance C High Side 5.3mF 3mF Capacitance C1a High Side Pulse 44µF 45µF Capacitance C1b Physical Dimensions 360 x 60 x 130 mm³ 360 x 60 x 150 mm³ Power Density 1.9 W/cm³ 5.0W/cm³ Efficiency 10% to % Power Output % to 94% 9% to 98% Table : Specifications of the converter prototypes Figure 7: Prototype of the 4kW / converter (left) and the 70kW Converter (right) 7. Measurements of the converters The converters were tested under several load and temperature conditions and the efficiency of the prototypes was measured in buck and boost mode operation. Fig. 8 shows the boost mode efficiency of the 4kW prototype with different input voltages. In Fig. 9 the efficiency of the same converter is plotted for buck mode operation Vout=160V Vin=10V Vin=V Vin=40V Figure 8: Efficiency for the 4kW converter in Boost mode with different input voltages The plotted efficiency in Fig. 8 for boost mode clearly shows that the efficiency of the converter depends on the input voltage. The nearly constant IGBT saturation voltage V CE,sat of.4v leads to a less efficient energy transfer at low input voltages. Higher input voltages yield a better performance of the converter due to the fact that the relation between input and saturation voltage is more convenient. With only 40V input voltage and a max. of 40A low side current a maximum of up to 10kW can be transferred. The efficiency and the transfer capability improves significantly at higher input voltages.

5 Vout=V Vin=40V Vin=160V Figure 9: Efficiency of the 4kW converter in Buck mode In the buck mode the input voltage value on the high side is not as critical as the input voltage on the low side in boost mode operation (see Fig. 9). But it also shows that with higher voltages a higher efficiency can be achieved. The efficiency of the 70kW converter was also evaluated by several measurements and is shown in Fig. 10 for boost and Fig. 11 for buck mode operation with different input and output voltages Vout=350V Vin=300V Vin=00V Figure 10: Efficiency of the 70kW converter in boost mode Figs. 10 and 11 show that the 70kW converter has a significantly higher efficiency because of the higher voltages. Furthermore the efficiency is not so much depending on the input voltage as it is with the 4kW prototype Vout=50V Vin=300V Vin=400V Figure 11: Efficiency of the 70kW converter in buck mode The EMI of the / Converter converter is reasonable because of the very low switching ripple on the high and low side. The measured voltage ripple is smaller than mv under all load conditions. This was achieved by paralleling three phases and a π-filter on the high voltage side. The CAN-Bus control interface is optocoupled to prevent ground loops. 8. Measurements of the powertrain Measurements of the automotive powertrain comprising a fuel cell stack, a / converter, a storage battery and a drive inverter with the electric motor mounted on a test bed were taken. Additionally an electronic load was attached for fast power transient tests. In Fig. 1 a load step from 10A to 130A (from approx. 1.4kW to 18kW) high side current is plotted. It shows that A of the demanded current step of 10A is provided by the /-Converter out of the storage battery. Only a minor current of 5A, unintentional in the application, comes from the fuel cell. At this test the voltage at the fuel cell dc bus is about 140V. The voltage of the storage battery is about 48V and the converter runs into low side current limitation of 40A. That is the reason it only delivers A to the high side instead of the possible 105A. The difference of 5A is provided by the fuel cell because of a voltage drop to 10V of the bus voltage in second to 5 in Fig. 1. Afterwards the output power of the fuel cell stack ramps up until it reaches the demanded current of 130A. In second 78 in Fig. 1 the demanded current falls back from 130A to 10A. The fuel cell needs about three seconds

6 to adjust its power output to the new demands. In this three seconds the / converter transfers power to the storage battery. 150 I-Fuel Cell 500 energy to the motor. At the end of the acceleration phase the current rises up to 156A and the power demand reaches 4.9kW. This can not be handled by the fuel cell alone. The converter transfers the missing 16A to the electric motor. Current [A] Time[s] Figure 1: oad step from 10A to 130A with electronic load on test bed Voltage / Current [V] / [A] I-/-Converter Current Step I-Motor I-Fuel Cell I-/-Converter U-Fuel Cell Time [s] Figure 13: Current and voltage measurement for two acceleration and braking cycles with the test car The integrated powertrain was tested again under real application conditions. Implemented in the test vehicle the load step is not as critical as under worst case laboratory conditions (see Fig. 13). There is a more slowly and steady increase of power demand when the car accelerates. This can be handled easily by the converter and the fuel cell system. Only in the moment of braking a relatively high current peak is transferred to the battery. The first acceleration takes place in only 15 seconds. During this time the fuel cell can not reach the demanded peak output power of about 4.5kW. There for it is supported by the / converter. The second acceleration phase is much longer, it lasts 5 seconds. In the beginning the converter transfers nearly no Setpoint current [A] 9. Conclusion The presented / converters are well suited for automotive powertrain applications in hybrid fuel cell cars. They feature a very robust and compact design with a very high efficiency. The measurements presented clearly show that higher voltages for the powertrain and storage battery promise higher efficiency due to lower current losses in the used IGBTs. The converters have wide voltage ranges on high and low side for high flexibility in adaption to different fuel cell types and electrical storage technologies like SuperCaps. The converters are tested in application of a hybrid fuel cell test car. They demonstrate the expected performance in the hybrid fuel cell car and allow primary energy savings by recuperation of braking energy as well as higher acceleration. 10. Acknowlegment This work was partially supported by the High- Tech Initiative Bayern in the framework of the mechatronics program (BKM). 11. References [1] Dr. Peter Treffinger et al.: ight Weight Electric Vehicles Vehicles of the future? German Aerospace Center (DR), Stuttgart [] R. W. Erickson: Fundamentals of Power Electronics, fourth printing 1999, Kluwer Academic Publishers, Massachusets [3] C. K. Tse an Y. M. ai.: Control of Bifurcation in Current-Programmed / Converters: A Reexamination of Slope Compensation, ISCAS 000, Geneva, Switzerland