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Mitsubishi Electric ADVANCE June 2011 3

T ECHNICAL R EPORTS

that the leak current remains sufficiently low when therated voltage is applied.

Figure 2 shows the current–voltage characteristicsof the fabricated prototype SiC-SBD at room tempera-ture. The chip size is about 5 mm square and the effec-tive chip area is about 0.25 cm 2. As shown, a currentbegins to flow at a forward voltage of 0.96 V (V th), andthen the current–voltage characteristic becomes typicalfor the SBD.

Fig. 2 Electric characteristics of SBD

3. Fabrication and Evaluation of Prototype11-kW SiC Inverter

3.1 11-kW SiC Inverter

Using the SiC devices described in the previoussection, we fabricated a prototype high-power inverter.The results are as follows.

Figure 3 shows the arrangement of the chips insidethe module. This module is the building block of theprototype inverter and each module consists of a con-verter, inverter and brake circuits. In the photo, the leftpart is the converter circuit and the right part is theinverter and brake circuits. The converter consists of Sidiodes, and the inverter and brake circuits are con-structed with SiC-MOSFETs and SiC-SBDs. The chipscircled by dashed lines and dotted lines are SBDs andMOSFETs, respectively.

Fig. 3 SiC device layout in the fabricated SiC inverter

The evaluation results of the dynamic characteris-tics (switching waveforms) of the SiC inverter areshown in Fig. 4. The measurement was carried outthrough the upper and lower arms of the module byusing an inductive load. The waveforms shown weremeasured at 125 °C.

Figure 4 shows the current and voltage waveformsobtained when the power is turned on (upper chart) orturned off (lower chart). The dark and light lines indicatethe waveforms of the SiC device and the conventionalSi device, respectively.

Fig. 4 Switching waveform of the inverter

When the power is turned on (upper chart), the SiCdevice exhibits a lower spiking current (recovery cur-rent) than the Si device does. This result is an effect ofthe SBDs, and the oscillation observed in the waveformoccurs because of the parasitic capacitance and induc-tance in the circuit.

When the power is turned off (lower chart), a tailcurrent appears in the waveform of the Si device,

whereas the SiC device promptly shuts off the power.For inverter operation (carrier frequency: 15 kHz,

power factor: 0.8, effective current: 23 A, power output:

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11 kW), the power loss is calculated using the relation-ship between the current value and the switching lossobtained from the switching test above, along with themeasurements of the static characteristics (I–V char-acteristics).

Figure 5 shows the calculated power loss for the Sidevice and SiC device. As shown, overall power loss isgreatly reduced by 70% from that of the conventional Siinverter. Figure 5 also shows the respective conductionand switching (SW) losses, indicating that the SiCdevice exhibits a dramatic reduction in the switchingloss, which measures to 83% lower than that of the Sidevice.

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Fig. 5 Comparison of electrical power loss between aSi-inverter and the SiC-inverter

3.2 Characteristi cs of SiC Inverter A lower inverter loss means that the equipment can

be made smaller. Figure 6 shows the prototype SiCinverter and its current and voltage waveforms obtainedwhen controlling a motor (11 kW output) (4). Because ofthe reduced power loss, a smaller cooling system issufficient, and thus the volume of the SiC inverter isreduced to 1.1 L (1/4 of conventional inverter). As aresult, a power density of 10 W/cm 3 has been achieved,which is about four times greater than that of the Siinverter with the same current capacity.

The lower part of Fig. 6 shows the current andvoltage waveforms for controlling a motor at a poweroutput of 11 kW. As clearly shown, the motor is con-trolled in a stable condition.

We have thus demonstrated that SiC devices usefar less power than conventional inverters do, and thatthe inverter volume can be reduced to one quarter,thanks to the low loss, increasing the power density to10 W/cm 3. We will continue to develop technologies forapplying SiC devices to power conversion equipmenttoward early commercialization.

A part of this research project is the outcome of the“Development of Basic Technologies for Power Elec-tronics Inverters” commissioned by the Ministry ofEconomy, Trade and Industry and NEDO.

Fig. 6 The fabricated inverter and the waveform at PWMoperation

References(1) T. Watanabe, et al.: “Shuttle Activation Annealing

of Implanted Al in 4H-SiC,” Jpn. J. Appl. Phys.47(4) 2841–2844 (2008).

(2) S. Kinouchi, et al.: “High Power Density SiC Con-verter,” Material Science Forum Vols. 600–603,1223–1226 (2009).

(3) S. Nakata, et al.: “Substantial Reduction of PowerLoss in a 14 kVA Inverter Using ParalleledSiC-MOSFETs and SiC-SBDs,” Silicon Carbideand Related Materials 2008, 903–906 (2009).

(4) S. Kinouchi and S. Nakata: “Energy Conservationand Resource Saving by SiC Inverter,” MitsubishiDenki Giho 83, No. 10, 27–30 (2009).

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