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

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Powertrain Applications. B. Eckardt*, M. März*, A. Hofmann*, M. Gräf+, J. Ungethüm+. * Fraunhofer Institute of Integrated Systems and Device Technology,.

High Power Buck-Boost DC/DC 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, 91058 Erlangen - Germany, www.iisb.fraunhofer.de +

German Aerospace Center (DLR) e.V. Institute of Vehicle Concepts, Pfaffenwaldring 38-40, 70569 Stuttgart

Abstract A high power buck-boost DC/DC 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 DC/DC converters are given. Two prototypes of highly efficient 24kW and 70kW bidirectional DC/DC converters were developed and evaluated. The digital control provides full protection against overvoltage, overcurrent and overtemperature. By integrated liquid cooling – up to 85°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: DC/DC converter, bidirectional, digital control, fuel cell, hybrid car

1. Introduction The development of ultra low emission vehicles (ULEV) 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 DC/DC converter. Measurements of the DLR institute of vehicle concepts of a battery car capable of recuperating braking energy, clearly show power savings of up to 24% for inner city driving. Statistic City Land Motorway Distance / km 14.7 18.1 24.6 Time / s 2774 1084 1105 Average speed / km/h 19.1 60.1 80.2 Recuperation ratio Motor->Batt / 24.1% 6.7% 4.9% Batt->Motor 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.

2. Hybrid Powertrain Fuel Cell 20kW

12V Battery

DC

12V Load

DC

10kW - 24kW

DC

AC DC Electric Motor

130V -260V

DC

Battery NiMH SuperCap

Figure 1: Powertrain of a hybrid fuel cell car with high voltage DC link, a storage battery and conventional 12V powernet

The test car has an electric motor with 12kW continuous and 39kW peak power. As primary energy source a fuel cell with a maximum of 20kW energy output by a DC bus voltage of 140V to 240V 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

to replace it by a SuperCap of about 100V. This energy storage is coupled to the high voltage DC bus by the described 24kW DC/DC 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 120km/h.

needed chip area to carry the maximum current of up to 95A per phase. In the voltage range up to 450V there are only MOSFETs with a relatively high RDSon. 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 SKM195GB063DN 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 I

set current m3

Figure 2: HyLite Fuel cell test car on a winter test run in Stuttgart m1

Fuel Cell High Side



Battery Low Side

I1 120° C1a

C1b

C2

240°

Figure 3: Multi phase buck / boost converter

That allows a cheap and compact design. The semiconductors were chosen according to the

inductor current

m2

3. High Power DC/DC Converter A non isolating topology was chosen for the DC/DC 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 120 degree phase shift (see Fig. 3) to get low current ripple on the capacitor banks.

compensation ramp

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 L and measured voltages on the battery VLV and fuel cell side VHV under the assumption that the voltages are constant during a switching period and:

V dI dI , ≡m , m= L dt dt L

VL = L

For buck mode calculated as:

operation

the

Vl1 with Vl1 = V HV − V LV L V = l 2 with Vl 2 = −VLV L

(1) slope

m1Buck =

(2)

m2 Buck

(3)

is

For control loop stability the slope m3 of the compensation ramp has to be at least one half of the slope m2 .

⇒ m3 Buck ≤ −

V LV 2L

(4)

The same approach for the boost mode slope compensation:

Vl1 with Vl1 = V LV (5) L V m2 Boost = l 2 with Vl 2 = VLV − V HV (6) L V − V HV (7) ⇒ m3 Boost ≤ LV 2L m1Boost =

5. Buck/Boost Mode Transition

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 12bit 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 VLV IHV

CPU FPGA

D/A Converters

Ipeak

(setvalue)

slope

+ _

+

¾ CAN-Bus communication breakdown is followed by a shut down of the converter after 100ms time out

ILV

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 DC/DC converter tries to keep the voltage on the fuel cell DC bus constant. An appropriate voltage level can be set via CANBus 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.

ILV CH1: Phase Current

VHV VLV Figure 5: Digital control loop of the converter

CH4: Iset

The microcontroller calculates the maximum phase current Ipeak 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. A sophisticated safety system is implemented in hard- and software. It protects the converter against the following events: ¾ Overvoltage on the high and low voltage side by load dump under high power operation - by hardware ¾ Overcurrent because of short circuit on the low side - prevented by the digital control loop ¾ Overcurrent on the high side in case of short circuit - prevented by a fast high voltage fuse ¾ Overtemperature - leads to a derating of the maximum possible current

CH2: Slope Compensation

Figure 6: Boost to Buck Mode Transition CH1: Current of one phase CH2: Slope compensation CH4: Value for peak current Iset

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 shows the set value Iset for the peak current control and Ch 1 the current pulses of one phase. After the transition Iset does not reach a constant value because of the increasing low side voltage due to the charging process of the battery. Figure 7: Prototype of the 24kW DC/DC converter (left) and the 70kW Converter (right)

The prototypes are especially designed for automotive applications with liquid cooling. Water glycol mixtures (50% / 50%) with coolant temperatures up to 85°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 24kW min.: 40V max.: 120V min.: 130V max.: 260V max.: 240A max.: 110A 65°C

35kW Peak 70kW min.: 200V max.: 290V min.: 300V max.: 450V max.: 280A max.: 240A 85°C

Low Side Voltage Range High Side Voltage Range Low Side Current High Side Current Max. Cooling Temperature Switching 27kHz 17kHz frequency Main Inductor 35µH 90µH Value Low Side 8.5mF 6mF Capacitance C2 High Side 5.3mF 3mF Capacitance C1a High Side Pulse 44µF 45µF Capacitance C1b Physical 360 x 260 x 360 x 260 x Dimensions 130 mm³ 150 mm³ Power Density 1.9 W/cm³ 5.0W/cm³ Efficiency 90% 92% 10% to 90% to to Power Output 94% 98% Table 2: Specifications of the converter prototypes

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 24kW prototype with different input voltages. In Fig. 9 the efficiency of the same converter is plotted for buck mode operation. 100

95 Efficiency [%]

6. Prototypes

90

Vout=160V

85

Vin=120V Vin=80V Vin=40V

80 0

5000

10000

15000

20000

25000

Pout [W]

Figure 8: Efficiency for the 24kW 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 VCE,sat of 2.4V 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 240A low side current a maximum of up to 10kW can be transferred. The efficiency and the transfer capability improves significantly at higher input voltages.

100

95

95 Efficiency [%]

Efficiency [%]

100

90

Vout=100V

85

90

Vout=250V

85

Vin=240V

Vin=300V

Vin=160V

Vin=400V

80

80 0

5000

10000

15000

20000

25000

Pout [W]

0

20000

40000

60000

Pout [W]

Figure 9: Efficiency of the 24kW converter in Buck mode

Figure 11: Efficiency of the 70kW 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 EMI of the DC/DC Converter converter is reasonable because of the very low switching ripple on the high and low side. The measured voltage ripple is smaller than 100mV 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.

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. 100

Efficiency [%]

95

90

Vout=350V

85

Vin=300V Vin=200V

80 0

20000

40000

60000

Pout [W]

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 24kW prototype.

8. Measurements of the powertrain Measurements of the automotive powertrain comprising a fuel cell stack, a DC/DC 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. 12 a load step from 10A to 130A (from approx. 1.4kW to 18kW) high side current is plotted. It shows that 95A of the demanded current step of 120A is provided by the DC/DC-Converter out of the storage battery. Only a minor current of 25A, 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 240A. That is the reason it only delivers 95A to the high side instead of the possible 105A. The difference of 25A is provided by the fuel cell because of a voltage drop to 120V of the DC bus voltage in second 22 to 25 in Fig. 12. Afterwards the output power of the fuel cell stack ramps up until it reaches the demanded current of 130A. In second 78 in Fig. 12 the demanded current falls back from 130A to 10A. The fuel cell needs about three seconds

to adjust its power output to the new demands. In this three seconds the DC/DC converter transfers power to the storage battery. I-Fuel Cell

150

500

100

400

0 300 -50

I-DC/DC-Converter

Setpoint current [A]

Current [A]

50

200 -100

100

Current Step

-150

-200 0

10

20

30

40

50

60

70

80

0 100

90

Time[s]

Figure 12: Load step from 10A to 130A with electronic load on test bed

U-Fuel Cell

225 200

Voltage / Current [V] / [A]

175 150

I-Motor

125 100

energy to the motor. At the end of the acceleration phase the current rises up to 156A and the power demand reaches 24.9kW. This can not be handled by the fuel cell alone. The converter transfers the missing 16A to the electric motor.

9. Conclusion The presented DC/DC 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.

I-Fuel Cell

75 50

10. Acknowlegment

25 0 -25

I-DC/DC-Converter

-50 -75 0

10

20

30

40

50

60

70

80

90

100

110

This work was partially supported by the „HighTech Initiative Bayern“ in the framework of the mechatronics program (BKM).

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 24.5kW. There for it is supported by the DC/DC converter. The second acceleration phase is much longer, it lasts 25 seconds. In the beginning the converter transfers nearly no

11. References [1]

[2]

[3]

Dr. Peter Treffinger et al.: “Light Weight Electric Vehicles – Vehicles of the future?” German Aerospace Center (DLR), Stuttgart R. W. Erickson: “Fundamentals of Power Electronics”, fourth printing 1999, Kluwer Academic Publishers, Massachusets C. K. Tse an Y. M. Lai.: “Control of Bifurcation in Current-Programmed DC/DC Converters: A Reexamination of Slope Compensation”, ISCAS 2000, Geneva, Switzerland

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