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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

SiC MOSFETS in Automotive Motor Drive Applications and Integrated Driver Circuit

R. Vrtovec, J. Trontelj

Abstract – Silicon carbide (SiC) MOSFETs are about to replace conventional silicon-based power switches. Due to their blocking, thermal, conducting and switching characteristics they represent a better solution for high power and high voltage applications, such as automotive motor drives in hybrid electric vehicle. Operating conditions and advantages of using SiC MOSFET in a typical automotive motor drive application are presented. Furthermore, SiC MOSFET driving requirements are described and a driving concept based on a SiC integrated circuit(IC) is presented. The SiC IC is comprised of componentsavailable in recently presented target technology – SiC depletion and enhancement mode MOSFETs, polysilicon and diffusion resistors.

I. INTRODUCTION

In the last two decades, a scientific breakthrough inthe silicon carbide (SiC) semiconductor development was accomplished, resulting in commercially available power SiC n-MOSFETs. So far, non-mature technology and consequently high costs are constraining its use to a smallset of applications. However, their outstanding characteristics, as later presented in this article, make SiC MOSFET a better engineering solution and potential leader in high voltage and high power range.

A typical application representative, where SiC is about to play an important role, is the power switch of a boost DC-DC and DC-AC converter in Hybrid Electric Vehicle (HEV) drive. From this standpoint, this paper first observes operating conditions and advantages of using SiC MOSFETs over its silicon competitors. In addition, driving requirements are discussed and driving concept based on SiC integrated circuit (IC) technology is presented.

II. SIC MOSFET ADVANTAGES INAUTOMOTIVE MOTOR DRIVE APPLICATIONS

Advantages of SiC MOSFETs over its silicon (Si)counterparts (IGBTs and MOSFETs) originate from superior SiC material properties. Key parameterscomparing 4H-SiC, which is the most prominent SiC polytype, and Si are listed in Table 1 and further discussedregarding the operation conditions in the HEV motor drive applications.

TABLE IKEY MATERIAL PROPERTIES COMPARISON [1]

Property 4H-SiC SiEc - Critical electric field [V/cm] 0.3 3Eg - Energy Bandgap [eV] 3.26 1.12λ - Thermal Conductivity [W/(cm K)] 4.9 1.5µe - Electron mobility [cm2/(Vs)] 900 1400

Contemporary motor drive applications in the HEVs range from 300 V to 750 V with increasing power and voltage tendency [2] [3]. This requires further research and development of highly efficient and reliable high voltage power switches.

To achieve high blocking voltages of common Si MOSFETs, its n-drift region has to be lightly doped andwide, to ensure proper depletion width distribution. This is done at the expense of increased on-resistance whichresults in large conducting losses and generally limits use of Si MOSFETs in power applications to voltages below 300 V [4] [5].

Technological advance to power switch high voltageoperation was enabled by IGBT development. By electroninjection into lightly doped n-drift region, forward voltage drop is decreased, enabling operation at voltages exceeding 1200 V with reasonable conducting losses [5]. Today, IGBTs are the optimal choice for high voltage and highpower applications, with some drawbacks which limit its use.

As an IGBT is a minority carrier device, the accumulated charge in the drift region has to be removed during reverse recovery. This manifests in the tail current and significantly increases switching losses and turn-off time. Therefore, IGBTs are limited to low frequency (mainly up to 50 kHz) applications [6]. Furthermore, some difficulties arise from the positive temperature coefficient of certain IGBT technologies, making it harder to use in parallel circuit [6].

SiC MOSFETs overcome many of described Si MOSFET and IGBT drawbacks and bring significant technological improvement. Approximately 10 times higher breakdown electric field (Ec) compared to Si allows thinner and heavily doped drift region for the same blocking voltage capability. Consequently, specific on-resistance is about 300 times lower [7] (despite the lower electron mobility (µe) of SiC) enabling high voltage MOSFET operation [4]. Typically for MOSFETs, there is no tail current at turn-off, which, together with lower

J. Trontelj and R. Vrtovec are members of Laboratory for Microelectronics at Faculty of electrical engineering, University of Ljubljana, Tržaška 25, SI-1000 Ljubljana e-mail: rok.vrtovec@fe.uni-lj.si

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parasitic capacitances, enables their usage at higher frequencies (unlike IGBT). Different reports show animprovement in switching and conducting losses [8] [1] [9][7]. Due to the negative temperature coefficient there is also no concern for the temperature runaway.

Another great and distinctive advantage of the SiCMOSFET is its ability to operate at high junctiontemperatures (200 °C as reported and more) [10]. Almost three times wider energy bandgap (Eg) reflects innegligible intrinsic carrier concentration (ni) over a wide temperature range and allows SiC devices to keep their native doping properties [4] [11]. This results in general device parameter stability [8] [10]. Moreover, higher SiCthermal conductivity (λ) facilitates dissipation and increase current rating for same junction temperature, compared to Si. However, many constrains remain, arising from the oxide instability, surface passivation, metallization and packaging, that limits the operating temperature of the SiC devices to max 135 [12] or 150 °C [13] [10].

In many automotive and other similar applications, the silicon temperature boundary of 150 or 175 °C represents a serious challenge. For example, combustion engine and traction motor in HEV elevate ambient temperature up to 150 °C and require water cooling system to dissipate energy from the converter power unit [14]. The SiC advanced thermal capabilities are therefore of great potential to reduce cooling costs and simplify overall construction. Among the other described benefits, SiC MOSFET presents potential leading switching device in high power and high voltage segment.

III. SIC MOSFET DRIVING REQUIREMENTS

Distinctive SiC material properties result in specific SiC MOSFET driving requirements. Conclusions in this chapter are based on the state of the art SiC DMOSFET devices from Cree Inc. and Rhom Co., Ltd. [12] [13].

Lower electron mobility in conducting channel and short-channel effects result in modest transconductance (gm) and wide triode region, as seen in the typical output characteristic in Fig. 1 [4] [15]. To reach the adequatechannel conductivity for the fully “ON” state, the applied gate-source voltage (Vgs) should be higher as in the case ofsilicon MOSFETs, approx. 18 to 20 V. Furthermore, wide triode region, where a MOSFET acts more like voltage controlled resistor, requires fast transitions of the gate signal to minimize switching losses [8].

The gate signal fidelity is also an important factor. Voltage spikes in Vgs, as the consequence of high switching rates in e.g. totem-pole configuration, may cause unintentional turn-on or partial turn-off of the transistor due to the relatively low threshold voltage of the device [8]. Experts advise to apply a negative bias of 2 V to max 5 V to the gate, when the SiC MOSFET is in the “OFF” state, to ensure sufficient immunity, or to connect gate and source terminal with a resistor [8].

Similarly, for fully “ON” state, Vgs noise-related decrease of few volts changes operating point and results in additional losses and device overloading. Any excessive ringing in driving circuit should be therefore avoided; this is especially important during turn-on and turn-off transitions [8].

Fig. 1. Typical output characteristic of SiC DMOSFET based on [12] and [13]

To conclude, the driver should fulfill following requirements: low impedance output, capability of high current peaks and providing adequate voltage swing (~24V) [8] [1]. For effective driving it is also crucial to minimize connection length and the corresponding loop between driver power stage and SiC MOSFET gateterminal to reduce the inductance and to achieve fast transients and immunity [16]. Thus, the best solution is to place the driver circuit right next to the SiC power MOSFET in the converter power stage. Concerning the latter and regarding the high operating temperature capability of the SiC devices, which is one of the key advantages, the conventional Si drivers are inadequate, due to their temperature limits [14].

A way to overcome temperature limit and to improve inductance and space related issues in the power stage of the converter is a full integration of the driving circuit together with the SiC power MOSFET.

IV. DRIVING CIRCUIT WITH INTEGRATED MOSFET DRIVER

Many analogue and digital 4H-SiC integrated circuits(ICs) have been presented in different technologies likebipolar, JFETs or MOSFETs [17]. Although it is not expected for SiC ICs to be available for LSI or VLSI, uncomplicated circuits, such as power MOSFET gate buffer for example, will play an important role in high temperature and high reliability electronics.

In this section, a concept of use of a monolithicintegrated SiC power MOSFET and driving circuit will be

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presented. Target technology, which allows the integration of a driving circuit and a power SiC MOSFET was recently reported [18]. It comprises N-channel enhancement and depletion mode MOSFETs and polysilicon and diffusion resistors, fabricated in a 2 µm 4H-SiC process.

Fig. 2 shows fabricated and evaluated driver circuit (gate buffer) [18] with an addition of SiC power MOSFET.Such circuit could be used for any switching application ineither a low or high side configuration.

Essential part of each gate driving circuit is its output stage which charges and discharges gate capacitance of the power device; a common totem-pole circuit is used. Due to the absence of P-channel device, it is composed of two N-channel enhancement-mode pull-up and pull-downtransistors [18].

Fig. 2. The SiC gate buffer IC as proposed in [24] with power MOSFET integration

Complementary drive signals for the output stageMOSFETs (Mpu and Mpd) are provided by two inverters, the design of which is challenging due to limited set of devices. Authors propose a single stage design with N-channel pull-down and a passive or active pull-up; aresistor or a depletion mode MOSFET, respectively. Toimprove consumption characteristics, the source follower could be added as a second stage of an inverter, but on theexpense of the increased voltage drop [18].

However, only single stage inverter variant with resistor pull-up was fabricated and tested [18]. Vdd2 wasset to 18 V and gate buffer was loaded with 1.65 nF capacitor, which simulated power MOSFET input capacitance [18]. The circuit performance was measured as a function of Vdd1, which was found to have a significant influence on output stage voltage amplitude (Vg), rise and fall times, delays and overall consumption [18]. Beside thelatter, higher Vdd1 means better performance. It isconcluded, that Vdd1 should be set at least 8 V above Vdd2to ensure 18 V at the gate of power MOSFET (Vg) and to reach better dynamic characteristic. This is a consequence of the low transconductance and the Body effect of Mputransistor [18].

According to [12] and [13], there is a maximum allowed gate-source voltage of 22 or 25 V respectively. Regarding safety margin, this voltage must not exceed 20V. Therefore, unifying Vdd1 and Vdd2 is not possible. To use this circuit as an integrated driver and SiC power MOSFET, different voltage levels Vdd1 and Vdd2 present an inconvenient requirement. Authors provided some further investigation and possible improvements to decrease inverter supply voltage [18].

We propose a concept of use of described circuit that helps to improve Vdd1 issue. To ensure proper gate immunity to unintentional turn-on, negative gate voltage bias should be provided to the SiC power MOSFET gate in“OFF” state. Aside from that, negative bias could also serve to increase absolute supply voltage of inverter circuits.

The circuit is shown in Fig. 3. The inverters and the output stage of the driver circuit are both supplied with the same voltage level (Vdd = 24 V), which is the sum of two serial sources, U1 and U2. Regarding the data from the paper [18], 24 V inverter supply is sufficient for the functional operation and the adequate turn-on of the power MOSFET.

To open power MOSFET, input signal should be set to “1” – Mpu transistor then opens and charges power MOSFET gate capacitance. Because source terminal of power MOSFET is not tied to Vss, but raised for U1, the gate-source voltage does not exceed critical gate voltage(Vg – Vs ≈ 20 V). In the opposite situation, where power MOSFET is turned “OFF”, Mpd pulls its gate to Vss, i.e. 4 V (U1) lower than Vs. Therefore, Vg – Vs ≈ - 4 V and power MOSFET immunity against unintentional turn-on is significantly raised.

Fig. 3. The concept of use of the driver circuit to improve double power supply issue and unify voltage supplies

The described concept of use of the IC driver (gate buffer) effectively increases supply voltage for inverters, unifies the supply for the driving circuit and also increases the immunity against the accidental turn-on.

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The functionality of the concept was proven by LT Spice simulation; the corresponding circuit is shown in Fig. 4. M1 and M2 are common silicon devices, used only to evaluate the concept functionality. Inverters were omitted as the complementary drive signals for M1 and M2 are provided by voltage sources V2 and V5. For the SiC power MOSFET Spice model CMF20120 from Cree was used.

Fig. 4. The simulation circuit from LT Spice

The transient simulation output (Fig. 5) confirms concept functionality. From the middle waveform it is evident, that the gate-source (Vgs = Vg – Vs) voltage reaches demanded negative bias in the power MOSFET “OFF” state and 20 V level in the “ON” state. Top and bottom traces indicate driving signal (only V5 is plotted) and power MOSFET drain current, respectively.

Fig. 5. LT Spice transient simulation output waveforms

V. CONCLUSION

The overview of the current solutions with SiC technology for HEV is presented in the paper. A driver concept based on the SiC integrated circuit technologyusing dual supply is proposed. Simulation was performed to prove the concept.

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