Silicon Carbide Inverter for EV/HEV Application featuring a Low Thermal Resistance Module and a Noise Reduction Structure.
The downsizing and improving efficiency are required of inverters to convert battery direct current (DC) voltage to alternating current (AC) voltage for driving traction motors. Si power semiconductors are used in inverters. And the characteristics of inverters are advanced by the improvement of component parts (e.g. power semiconductor, power module and capacitor)  . In addition, inverters that use SiC power semiconductor are being researched and developed for the further downsizing and improving efficiency  .
Improvement of fuel efficiency has been reported in driving tests of a hybrid electric vehicle (HEV) equipped with an inverter built with a SiC chip in place of a Si module . A reduction of surge voltage during switching and improvement of SiC inverter efficiency at a maximum current of 130 Arms have been reported by using an air-cooled power module to reduce stray inductance .
This paper focuses on the application of water cooling to downsize and improve the efficiency of a SiC inverter with large current output. The objective of this work was to develop inverter technologies for eliciting the advantages of SiC MOSFETs and compensating their drawbacks. Attention was focused specifically on technologies in two areas. Firstly, power module technologies are described for improving cooling characteristics to facilitate downsizing, while simultaneously increasing the output current of the inverter by applying a small die size of semiconductor like a SiC chip. Making the switching speed faster improves inverter efficiency, but it worsens inverter noise. The technologies of a noise reduction structure for addressing this issue are then described.
2. SiC INVERTER PROTOTYPE
2.1. Outline of SiC Inverter Prototype
The SiC inverter prototype developed in this work is shown in Figure 1. A schematic diagram of the prototype SiC inverter is shown in Figure 2. It is a three-phase water-cooled AC inverter consisting of SiC MOSFETs, a gate driving circuit, capacitors and a cooling structure. External diodes are eliminated because the built-in diodes of the MOSFETs are utilized. The specifications of the SiC inverter prototype are shown in Table 1. It is 2.9 L in size and can drive an 80-kW motor.
2.2. Motor Test Bench Evaluation and Estimation of Driving Range Improvement
The fabricated SiC inverter prototype was evaluated on a motor test bench. Turn-on and turn-off waveforms are shown in Figures 3 and 4, respectively, at the rated voltage of 360 V and inverter output current of 550 A as one example of the motor test bench results. Ids characteristics show a switching time within 100 nsec at turn-on and turn-off. The SiC inverter prototype achieved average efficiency of 98.5% in the WLTC driving mode by power module cooling technologies and noise reduction technologies.
3. DIRECT-COOLED POWER MODULE FOR LOW THERMAL RESISTANCE
3.1. Outline of Direct-Cooled Power Module Prototype
Firstly, the power module cooling technologies for facilitating inverter downsizing are described. The direct-cooled power module prototype fabricated in this work is shown in Figure 5. Six plastic mold packages (1-in-1 type) and the water jacket are soldered via insulated substrates. One plastic mold package consists of one arm of the inverter.
The plastic mold package features a thick Cu heat spreader located under the chip for lower thermal resistance. Thermal resistance can be reduced by using a thick Cu heat spreader because the heat generated by the chip spreads not only directly under the chip but also laterally. However, stress is increased due to differences in the coefficient of thermal expansion of the soldered parts. To address this issue, a thin closed aluminum (Al) water jacket was adopted to suppress stress. Thermal resistance and stress simulation results and evaluation results of the fabricated prototype are presented below.
3.2. Simulation Results: Thermal Resistance
A conventional direct-cooled module (e.g. the module having pin-fin type water jacket  ) and the direct-cooled module prototype with a thick Cu heat spreader are shown in Figure 6.
The proposed system has a 5-mm-thick Cu heat spreader located under the semiconductor chip to spread the heat generated by the chip for lower thermal resistance.
Thermal resistance simulation results are presented in Figure 7, and Table 2 shows the materials and properties of the power module parts. They were modelled using STAR-CD software made by CD-adapco. The boundary thermal exchange condition between the parts and air was negligible, so the condition was adiabatic. A turbulence model was applied for the boundary condition between the coolant and the water jacket wall surface.
The coolant input temperature was 25 deg. C and the flow rate is 6 L/min. The semiconductor chip temperature increase was simulated from 25 deg. C when power of 100 W was applied to the chip surface, and the thermal resistance was calculated. With the conventional direct cooling system, heat generated by the semiconductor chip only reaches the cooling fins just under the chip and does not spread because of the thin copper substrate and insulated substrate with high thermal resistance positioned directly below the chip. The Cu heat spreader of the proposed system serves to spread the heat generated by the semiconductor chip efficiently to the cooling fins.
Figure 8 shows the thermal resistance simulation results for various die sizes of the semiconductor chip shown in Figure 6. The results indicate that the proposed structure reduces thermal resistance regardless of the die size. The rate of reduction in thermal resistance increases with a smaller die size. For a die size of 25 [mm.sup.2], thermal resistance is reduced by 34% compared with the conventional direct cooling system.
3.3. Simulation Results: Stress
The direct-cooled power module with the Cu heat spreader has metal junctions formed of dissimilar Cu and Al metals on both sides of the insulated substrate. This causes stress-strain due to the difference in the coefficient of thermal expansion between Cu and Al. The large stress causes cracks after soldering and problems during long term reliability test. So it is desired that the stress of proposed structure is not larger than that of conventional one. To address this issue, we simulated a thin closed Al water jacket that was shown to suppress stress more than a thick Al plate. And cooling fins connected to back side also contributed to suppress stress. The modelling and simulation software was the same as that used in the thermal resistance simulation. Stress was simulated in a linear analysis using a finite volume method. The differential temperature in the simulation was 220 deg. C. This temperature was assumed when a power module is cooled from a high temperature to room temperature at the time of soldering. A method of reducing stress was simulated and verified by conducting a prototype temperature cycle test. The simulation was focused on obtaining the stress tendencies.
The simulation results revealed that insulated substrate stress equal to that of a conventional power module structure can be achieved by designing the upper-side plate of the water jacket with a thickness of 1 mm even though the Cu heat spreader has a thickness of 5 mm, as shown in Figures 9 and 10. The simulation results in Figure 10 are shown relative to the conventional module result as 1.
Experimental Results: Thermal Resistance
Measured and simulated thermal resistance results for the module in Figure 5 are shown in Figure 11. Thermal resistance was obtained experimentally by measuring the SiC chip temperature difference after applying 100 W to the chip. The difference between the measured and simulated results was below 6%. This evaluation result was in line with the design.
3.4. Experimental Results: Stress
A schematic diagram and the cross-sectional structure of the power module prototype are shown in Table 3 along with the temperature cycle test results. The prototype consisted of the Cu heat spreader and a multi-port tube water jacket having the same configuration and dimensions as that used in the thermal resistance evaluation. The prototype was subjected to a temperature cycle test in a temperature range of -40 deg. C to 130 deg. C. Stress was generated in the solder in proportion to that of the insulated substrate, so the insulated substrate and solder crack conditions were checked during the test. The solder condition was checked by scanning acoustic tomography (SAT).The test results confirmed that the multi-port tube prototype did not display any insulated substrate and solder cracks until 1000 cycles. These results indicate that stress can be suppressed even with a thick Cu heat spreader.
4. NOISE REDUCTION STRUCTURE
4.1. Outline of the Noise Reduction Structure
Next, the technologies for reducing the noise generated by the switching of the power semiconductors are described. dV/dt and dI/dt must be increased to reduce the switching losses of power semiconductors. However, the normal mode noise and common mode noise that cause radiation noise increase in proportion to a faster switching speed.
A cross-sectional view of the structure concept for reducing normal mode noise and common mode noise is shown in Figure 12. The two key features of this noise reduction structure are described here.
1. P and N bus bars are located between the plastic mold package and capacitors. The configurations of P and N bus bars are designed to eliminate the resonance.
2. The P and N bus bars are separated from the housing, making the parasitic capacitance smaller. The bus bars are designed with the same area, resulting in the same parasitic capacitance at the part of the bus bars closest to the housing. (The bottom side of the P and N bus bars is designed to have the parasitic capacitance [C.sub.P1]=[C.sub.N1] in Figure 12.)
The evaluation results for the normal mode and common mode of the SiC inverter prototype that are shown in Figure 1 are described here.
4.2. Improvement of Normal Mode Characteristics
Firstly, the normal mode characteristics were evaluated by measuring the impedance as shown in Figure 13. An impedance analyzer was connected to the P and N terminals as the input of the inverter. The evaluation results presented in Figure 14 show that the impedance between the P and N bus bars does not have any resonance from 1 MHz to several hundred MHz. This confirmed that normal mode noise does not cause any effects, especially around 100 MHz in the FM radio frequency band.
4.3. Improvement of Common Mode Characteristics
Next, the reduction of common mode noise is described. The evaluation condition of the common mode current measurement is shown in Figure 15. A differential signal was input to the P and N bus bars, and the common mode current that flowed to the ground (GND) via Csw (the parasitic capacitance between the power semiconductor and the water jacket) and Cwh (the parasitic capacitance between the water jacket and the housing) were measured.
The evaluation results are presented in Figure 16. The SiC inverter prototype reduced common mode current noise by as much as 8 dB in the FM radio frequency band of 70-100 MHz compared with the conventional inverter. The characteristics obtained in this evaluation showed no resonance in the FM radio frequency band.
An 8-dB reduction means that the SiC inverter prototype reduced the common mode current by 60% compared with that of the conventional inverter according to the equations below. Reducing the common mode current increases the switching speed, thus reducing the switching loss. This contributes to improving inverter efficiency.
-8dB = 20[log.sub.10][I.sub.SiCprototype]/[I.sub.conventional] (1)
[I.sub.SiCprototype]: common mode current of SiC inverter prototype
[I.sub.conventional] : common mode current of conventional inverter
A SiC inverter prototype of 2.9 L in size for driving an 80-kW motor was fabricated and evaluated on a motor test bench. The SiC inverter prototype achieved average efficiency of 98.5% in the WLTC driving mode. Power module technologies for obtaining lower thermal resistance and a noise reduction structure were applied to facilitate inverter downsizing and efficiency improvement. It was confirmed by simulation that a new direct-cooled power module with a thick copper (Cu) heat spreader located under the semiconductors reduced thermal resistance by 34% compared with a conventional direct-cooled power module. Evaluation results for the inverter prototype were confirmed to be in line with the simulation. In addition, the results of a temperature cycle test performed on the prototype confirmed that it did not show any fluctuation in reliability during the short-term test. Regarding the noise reduction structure, it was confirmed that the P and N bus bar configuration located between the plastic mold packages and capacitors eliminated resonance and reduced the common mode current by 60% compared with that of a conventional inverter. Reduction of the common mode current increases the switching speed to reduce the switching loss, which contributes to improving inverter efficiency.
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Keiichiro Numakura, Kenta Emori, Akinori Okubo, Taku Shimomura, and Tetsuya Hayashi
Nissan Motor Co., Ltd.
Table 1. Specifications of SiC inverter prototype Parameter Specification Unit Typical DC input 360 V voltage Max. motor output 80 kW Size 2.9 L Cooling method Water cooling - Table 2. Parts of the power module Part Material Thermal Coefficient of conductivity thermal expansion (W/mK) (ppm/K) Semiconductor SiC 450 3 Solder SnPb (a) 39 29 SnSb (b) 48 23 Cu heat spreader Cu 402 17 Cu Cu 402 17 DCB substrate Insulated [Si.sub.3][N.sub.4] 90 2.6 substrate Al water jacket Al 236 24 Coolant [H.sub.2]O 0.62 - (a) Solder below a semiconductor (b) Solder on lower side of DCB (a) conventional direct cooling (b) Solder on upper and lower sides of DCB (b) proposed direct cooling
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|Title Annotation:||/hybrid electric vehicles|
|Author:||Numakura, Keiichiro; Emori, Kenta; Okubo, Akinori; Shimomura, Taku; Hayashi, Tetsuya|
|Publication:||SAE International Journal of Passenger Cars - Electronic and Electrical Systems|
|Date:||May 1, 2017|
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