Performance Comparison of Si IGBT and SiC MOSFET Power Module Driving IPMSM or IM under WLTC

Abstract

The cumulative inverter losses and power consumption of a silicon insulated gate bipolar transistor (Si IGBT) and three types of silicon carbide metal-oxide-semiconductor field-effect transistors (SiC MOSFETs) were evaluated on an electric motor test bench under a worldwide harmonized light vehicles test cycle (WLTC). SiC MOSFETs showed higher performance than Si IGBT regardless of the motor type and test vehicles. In the case of driving an interior permanent magnet synchronous motor (IPMSM), the latest 4th generation SiC MOSFET (SiC-4G) in ROHM has the lowest inverter loss and energy consumption compared with the other generations. In the case of driving an induction motor (IM), on the other hand, the 2nd generation SiC MOSFET (SiC-2G) in ROHM has the best energy consumption despite the fact that the inverter losses of SiC-2G are slightly larger than the loss of SiC-4G. The latest or later generation power device does not necessarily contribute to better performance in a total system by simply replacing early power devices.

1. Introduction

Fossil fuel-powered vehicles cannot stop emitting green-house-gas, particle matter, etc. wherever they go. Vehicle electrification is one of the worldwide trends to reduce air pollution, energy consumption, and the environmental load. Automotive electrification can reduce fossil fuel dependence and the exhaust emission of passenger cars and road freight vehicles [1,2,3].

Electric vehicles (EVs) can convert energy into driving force more efficiently than a combustion engine. However, EVs can only run a much shorter distance than petrol cars because the energy density of a battery is smaller than fuel. In order to extend driving distance, it is necessary to improve electricity conversion efficiency from a battery to a motor. xEV, referring to all kinds of EVs, has several electric power components, e.g., a motor, a traction inverter, a dc–dc converter, and a battery charger. Semiconductors used in power conversion circuits are silicon metal oxide semiconductor field effect transistors (Si MOSFETs), silicon insulated gate bipolar transistors (Si IGBTs), and silicon carbide metal oxide semiconductor field effect transistors (SiC MOSFETs). Si IGBTs are mainly used for xEV’s traction inverters because of their high withstand voltage, low loss in the high current range, and their continuing performance evolution. Meanwhile, SiC MOSFETs are steadily catching up to Si IGBTs in terms of performance and are increasingly adopted in commercial traction inverters. The SiC MOSFET has excellent characteristics: low transient loss, low conduction loss, high-speed operation, and high-temperature operation. These features are effective in terms of energy saving and achieving a reduction in size and weight.

Technical innovation and improvement often occur on an individual technology basis. In the power device field, new material is selected based on the performance of physical properties for a next generation power device [4,5,6,7]. Device structures are devised to unlock the material potential for closing or exceeding the ideal performance [4,8,9,10,11]. However, there are not too many reports of actual use in a power converter or a system except for simple circuits to evaluate the tolerance and reliability of materials and device structures [12,13,14,15]. In the power conversion field, there reports of, e.g., increasing the power density and downsizing the total volume of a power converter [16,17,18], improving circuit configurations or control methods [19,20], analyzing losses of each electronic components in a circuit [21,22], etc. The evaluations focus on only a power converter, although there have been reports comparing performance among several power devices [23,24,25,26,27]. In the xEV system field close to electricity, the main contents are the loss distribution and junction temperature of power devices not including mechanical parts [28,29,30,31]. On the other hand, in the field close to the system or machines, the main topic is far from electricity, such as the difference between the energy consumption of standardized drive cycle and real use [32], comparison among commercial cars [33], and motor optimization and analysis [34]. Few reports evaluating the transmission of electricity to machines compare performances with changing drive cycles or motors [35,36] but not power devices.

ROHM released a commercial planar, trench, and improved-trench SiC MOSFET [37,38,39,40]. The structure arrangement certainly improves the static characteristics of conduction resistance, switching loss, short-circuit withstand time, etc. It is, however, not certain whether a system that combines individually optimized devices will perform as intended, because complex systems cannot be built up without mutual interactions. We are interested in how the difference in the types of power devices influences the performance of a power converter or a total system.

In this paper, a type of Si IGBTs and three types of ROHM SiC MOSFETs are applied to a three-phase, two-level inverter assuming application to EVs. An interior permanent magnet synchronous motor (IPMSM) and an induction motor (IM) are employed as a traction motor in order to verify whether there is a difference depending on the motor type. An electric motor test bench gives a traction motor road load according to vehicle specifications. The evaluation method mostly follows the worldwide harmonized light duty driving test procedure (WLTP). Performance indexes are integrated inverter loss, energy consumption, and energy ration over a driving test cycle.

2. Materials and Methods

2.1. System Configuration Outline

Figure 1 shows the overall experimental system configuration for evaluating power modules. The device under test (DUT) includes components of a traction inverter and 4 types of power module, namely Infineon Si IGBT (FF450R12ME4), Rohm 2nd generation planar SiC MOSFET (SiC-2G: BSM400D12P2G003), Rohm 3rd generation trench SiC MOSFET (SiC-3G: BSM400D12P2G002), and Rohm 4th generation improved trench SiC MOSFET (SiC-4G: the same module packaging as the others). The power modules are selected under the condition of the same module packaging, voltage rating, and current capacity. The specifications of the traction motors of IPMSM and IM are the same as the dynamometer of the motor test bench: 12,000 rpm/300 Nm/100 kW. The dynamometer imitates the preset road load. The controller (PE-Expert4 of Myway Plus) generates a gate drive signal following the worldwide harmonized light vehicles test cycle (WLTC). There are two-line liquid cooling systems for heat dissipation of the motor and the power modules. The coolant is an ethylene glycol-based water solution.

Input and output electric power of the inverter are measured and integrated by the power meter (PW6001 of Hioki) with the current sensors (CT6876 of Hioki) and motor power is calculated with number of rotations and torque measured by the torque meter (T40B of HBM).

Table 1. Equipment detail.

	Maker	Model	Specifications
Power Supply	SINFONIA TECHNOLOGY	-	850 V/500 A/100 kW
DUT	INFINEON	Si IGBT FF450R12ME4 [41]	1200 V/450 A
	ROHM	SiC-2G BSM400D12P2G [42]	1200 V/400 A
		SiC-3G BSM400D12P3G [43]
		SiC-4G prototype
Torque Meter	HBM	T40B	20,000 rpm/500 Nm
Traction Motor	MOTION SYSTEM TECH	IPMSM	850 V/500 A/100kW 12,000 rpm/300 Nm/100 kW
		IM
Dynamometer	SINFONIA TECHNOLOGY	-	12,000 rpm/300 Nm/100 kW
Controller	MYWAY PLUS	PE-EXPERT4	-
Power Meter	HIOKI	PW6001	w/CT6876

presents the equipment details.

2.2. Experimental Conditions

The evaluation cycle shown in Figure 2 illustrates the pre-set reference vehicle speed to control motor speed by the controller. This is one dynamic segment of the WLTC Class 3b Shortened Type 1 test that excludes the extra high-speed phase in Japan. The original Shortened Type 1 test has two dynamic segments and two continuous speed segments [44]. We did not conduct a reproduction of the battery profile, and battery voltage drop depended on current in this study for simplifying the following analyses.

The motor dynamo determines the road load as follows:

$$T_{R/L}=J·\dot{\omega }+\left(0.5C_{d}A\rho v^2+\mu mg\right)·r/G$$

(1)

where $T_{R/L}$ is the road load torque; $J$ is the vehicle inertia automatically calculated with set vehicle parameters in the motor dynamo system; $\dot{\omega }$ is the angular acceleration; $C_d$ is the aerodynamic drag coefficient; $A$ is the frontal area; $\rho$ is the air density; $v$ is the vehicle speed; $\mu$ is the rolling resistance coefficient; $m$ is the vehicle mass; $g$ is the gravity constant; $r$ is the tire radius; $G$ is the gear ratio.

We choose two vehicles for comparative evaluation. One is the 2017 Nissan Leaf G which is one of the major EVs in Japan. The other is the 2022 BMW i4 eDrive 40 whose required current is the maximum in other vehicles for which we set available parameters.

Table 2. Parameters of test vehicles.

Parameter	Symbol	Unit	2017 Nissan Leaf G	2022 BMW i4 eDrive 40
Tire Radius	r	m	0.323	0.356
Gear Ratio	G	-	8.193	8.774
Aerodynamic Drag Coefficient	Cd	-	0.28	0.24
Frontal Area	A	m2	2.48	2.41
Vehicle Mass	m	kg	1646	2251
Air Density	ρ	kg/m3	1.189
Rolling Resistance Coefficient	μ	-	0.011
Gravity Constant	g	m/s2	9.8

presents vehicle parameters for calculating road load [45].

In this paper, we set the inverter input voltage as 800 V for high voltage use evaluation although the battery voltage of the original vehicles is around 400 V. The gate resistance values of each type of power module are decided such that the peak voltage of the switching surge is approximately 1100 V when the test current is approximately 800 A in the double pulse test. The on-/off- gate resistances of Si IGBT, SiC-2G, SiC-3G, and SiC-4G are 0.0/6.8 Ω, 1.2/2.2 Ω, 1.2/2.2 Ω, and 2.7/6.8 Ω. The gate driver circuit (BSMGD2G17D24-EVK001) can output the recommended gate voltage for the inverter use of SiC power modules, while negative voltage for Si IGBT is limited to −4 V. The on-/off- gate voltages of Si IGBT, SiC-2G, SiC-3G, and SiC-4G are 15/−4 V, 18/−4 V, 18/−2 V, and 18/−2 V. The switching frequency and the dead time of the inverter are set 10 kHz and 2 us. The list of parameters is shown in

Table 3. Parameters of driven DUT.

Parameter	Unit	IGBT	SiC-2G	SiC-3G	SiC-4G
Inverter Input Voltage	V	800
Inverter Input Capacitor	μF	960 (320 μF × 3: 947C321K122CDMS)
		150 (25 μF × 6: B32778G1256K000)
Snubber Capacitor	nF	4050 (1350 nF × 3: EVSM1D72J2-142H16)
Discharge Resistance	MΩ	0.73 (2.2 MΩ × 3-parallel )
Gate Resistance (ON/OFF)	Ω	0.0/6.8	1.2/2.2	1.2/2.2	2.7/6.8
Gate Voltage (ON/OFF)	V	15/–4	18/–4	18/–2	18/–2
Switching Frequency	kHz	10
Dead Time	us	2

.

2.3. Evaluation Index

The original energy consumption requires complex calculation procedures [41]. However, in this paper, we employed a simplified calculation as follows:

$$EC=E{C}_{\mathrm{DC}}/D$$

(2)

where $EC$ is the energy consumption per 100 km; $E{C}_{DC}$ is the total integrated inverter input energy; $D$ is the total driving distance standardized by 100 km.

$$E{C}_{\mathrm{DC}}=\sum P_{\mathrm{DC}}·∆{\tau }_1$$

(3)

where $P_{\mathrm{DC}}$ is the inverter input power; $\Delta {\tau }_1$ is 50 ms (refresh rate is 20 Hz) and is set by the accumulation function of the power meter, PW6001.

$$D=2\pi r·N_{\mathrm{total}}/\left(4096·G\right)$$

(4)

where $N_{\mathrm{total}}$ is the total number of the a/b pulses converted from a resolver of the traction motor; $G$ is the gear ratio.

The integrated inverter loss energy $E_{\mathrm{loss}\_\mathrm{inv}}$ is calculated as follows:

$$E_{\mathrm{loss}\_\mathrm{inv}}=\sum \left(P_{\mathrm{DC}}-P_{\mathrm{AC}}\right)·∆{\tau }_1$$

(5)

where $P_{\mathrm{AC}}$ is inverter output power.

The integrated motor loss energy $E_{\mathrm{loss}\_\mathrm{mtr}}$ is calculated as follows:

$$E_{\mathrm{loss}\_\mathrm{mtr}}=\sum P_{\mathrm{AC}}·∆{\tau }_1-E_{\mathrm{trc}}$$

(6)

where $E_{\mathrm{trc}}$ is the traction energy.

The traction energy $E_{\mathrm{trc}}$ is calculated as follows:

$$E_{\mathrm{trc}}=\sum P_{\mathrm{MC}}·∆{\tau }_2$$

(7)

where $P_{\mathrm{MC}}$ is the motor output; $∆{\tau }_2$ is approximately 200 milliseconds (refresh rate is 5 Hz) and is decided by the data accumulation speed between a PC and other measurement equipment.

The motor output $P_{\mathrm{MC}}$ is calculated as follows:

$$P_{\mathrm{MC}}=2\pi ·T·N/60$$

(8)

where $T$ is the torque and $N$ is the number of rotations. Both parameters are measured using the torque meter.

3. Experimental Results and Discussion

In Figure 3, total energy consumption overall the evaluation cycle is compared in a combination of three categories, a type of power modules, traction motors and target vehicles. Each stacked bar of EC is color-coded by three categories, traction energy, motor loss energy, and inverter loss energy. Each data label on the bars means traction energy consumption (EC_trc) calculated with only traction energy, mechanical energy consumption (EC_mc) with traction energy and motor loss energy, and total energy consumption (EC_ttl) with energy of all 3 categories. EC_ttl and EC_mc are important values because the accuracy is guaranteed by the power meter. On the other hand, EC_trc is just a reference value because the torque meter does not guarantee the accuracy of transient response and the refresh rate of communication speed is slow. The ratio of the traction energy and the motor loss energy is not so accurate.

In the case of driving IPMSM, the good order of EC_ttl in the types of power modules is SiC-4G < SiC-2G < SiC-3G < Si IGBT regardless of the target vehicles. The order of EC_mc is SiC-4G ≈ SiC-2G < Si IGBT < SiC-3G in Nissan Leaf G and Si IGBT < SiC-4G ≈ SiC-2G < SiC-3G in BMW i4 eDrive 40. SiC MOSFETs much improve the total energy consumption, whereas motor loss can be increased since the traction energy should be uniquely determined by vehicle parameters.

In the case of driving IM, the order of EC_ttl is SiC-2G < SiC-4G < SiC-3G < Si IGBT and the order of EC_mc is SiC-2G < SiC-3G < SiC-4G ≈ Si IGBT regardless of the vehicles. All SiC MOSFETs demonstrated better performance than Si IGBT. SiC-2G of planar structure realizes the best energy saving although SiC-3G and -4G of the trench have better standalone device characteristics.

Pure inverter losses are shown in Figure 4. The lowest order of the loss is SiC-4G < SiC-2G < SiC-3G < Si IGBT regardless of the motor types and the target vehicles. The difference in material properties is very clear. The remarkable point is that SiC-2G of the planar can keep the loss lower than SiC-3G of the trench in power converter level.

Figure 5 shows the relationship of EC and power train loss of E_{loss_mtr} and E_{loss_inv}. The results in Figure 5a are calculated with the data of the overall evaluation cycle of one dynamic segment. Figure 5b–d show the data of the low, middle, and high phases of the cycle. The dashed line shows the Nissan Leaf G and the dot-dashed line the BMW i4 eDrive 40. The two lines are drawn by each simple average of all data. The slop of those lines signifies the impact of the powertrain loss on EC. In the low phase shown in Figure 5b, a short drive distance makes the slope of each line steep, and therefore the loss reduction effect is easily apparent as an improvement in energy consumption. On the other hand, in the high phase shown in Figure 5d, the loss has only a limited effect on energy consumption since the slope of each line is loose. The intercept of each line represents experimentally obtained ideal energy consumption ignoring motor and inverter loss. The intercept is approximately 11.56 kWh/100 km for the Nissan Leaf G, and approximately 14.41 kWh/100 km for the BMW i4 eDrive 40, as shown in Figure 5a. Therefore, the energy increase in traction purely due to the difference between the target vehicles is approximately 25%.

The filled markers in Figure 3 represent motor loss. The motor loss increases in the order of low, middle, and high phase as drive distance increases. The markers are roughly overlapped if grouped by the target vehicles and the motor type except SiC-2G of IM. The open markers mean the total value of motor loss and inverter loss. The distance between the filled and the open marker of the same color and shape shows the amount of inverter loss and the amount of improvable energy consumption. Figure 5 helps to visually understand the ratio and the amount of motor and inverter loss, and the effect of each loss on energy consumption.

Figure 6 shows percentages of traction energy, motor loss energy, and inverter loss energy in each EC_ttl. The inverter loss ratio of SiC-4G is the lowest regardless of the motor types and the target vehicles.

4. Conclusions

In this paper, we investigated the effects of device materials and structures on the power converter and the EV system by just replacing power modules. Two types of vehicle parameter sets did not influence the trend of results except for increasing the required overall energy for the evaluation cycle of one dynamic segment of the Japanese WLTC. However, types of power modules and motors affected inverter loss and mechanical energy converted to traction. The good order in inverter loss and energy consumption was not equal to the individual performance of evaluated power modules, i.e., in the order of Si IGBT, SiC-2G (planar SiC MOSFET), SiC-3G (trench SiC MOSFET), and SiC-4G (improved trench SiC MOSFET). In addition, the type of power module can influence the motor loss.

SiC-2G demonstrated good balance in terms of inverter loss, motor loss, and energy consumption. SiC-4G had the lowest inverter loss and reduced energy consumption to 3% of the entire energy in the case of driving IPMSM. On the other hand, the mechanical energy of SiC-4G is the same as Si IGBT in the case of driving IM. Our future work aims to find what device parameters influence motor loss, mechanical energy, and energy consumption.