Silicon Carbide MOSFETs Challenge IGBTs

Sep 1, 2008 12:00 PM
By Michael O'Neill, Applications Engineering Manager, CREE, Durham, N.C.

Three-Phase High-Power-Factor Rectifier

A three-phase six-switch rectifier followed by an isolated dc-dc converter is typically used in three-phase applications that require high power factor (HPF) and galvanic isolation between input and output. The rectifier shown in Fig. 3 (a 3-kW zero current switching resonant topology) can achieve the same goal with only two switches. When switches S1 and S2 are turned on, the stored energy in C1 to C3 is quickly moved to the secondary-side resonant capacitor (C_D) through the transformer (T_R) and resonant inductor (L_R). The discharging time is designed to be approximately equal to one-half of the resonant period (T_O). For an optimal design, T_O should be relatively shorter than the switching period (T_S) to achieve a low total harmonic distortion.

The system was run initially at full load with a pair of standard 1200-V, 25-A-rated IGBTs. These devices were then replaced by a pair of standard 1200-V, 40-A IGBTs, and the system was re-run at full load. The efficiency versus output power curves were recorded.

The IGBTs were then replaced by a pair of 1200-V, 20-A-rated SiC MOSFETs and the exercise repeated. As seen in Fig. 4, the SiC devices resulted in a 2.2% increase in efficiency at a 3-kW output and substantial efficiency improvement throughout the entire load curve. Also of note was a 25°C case-temperature reduction with the MOSFETs versus the 40-A-rated IGBTs and a 36°C difference when compared with the 25-A-rated devices.^[5]

10-kV, 10-A SiC MOSFET in a Boost Converter

SiC technology shows significant improvements in the 1200-V MOSFET arena, as revealed in the two previous examples. The performance improvement becomes even greater when compared to Si power switches rated at 6.5 kV and above.

Recently developed was a 10-kV, 10-A SiC MOSFET. The 10-kV device exhibits a drain-source forward voltage drop of only 4.1 V, while conducting full-rated 10-A drain current with a 20-V gate-source voltage. This is equivalent to a specific on-resistance characteristic of only 127 mV/cm². The drain-source leakage current measured 124 nA at a 10-kV blocking voltage. In a direct comparison with a standard 6.5-kV Si IGBT in a clamped inductive switching test fixture, a SiC MOSFET exhibited 1/200th of the total switching energy of the IGBT. This unipolar SiC MOSFET's turn-on delay time was only 94 ns compared with 1.4 ìs for the IGBT and the turn-off time was only 50 ns instead of the IGBT's 540 ns.

To analyze in-circuit performance, a 10-kV SiC MOSFET was combined with a 10-kV SiC Schottky diode and an air-core inductor in a standard boost-circuit topology (Fig. 5). The circuit was designed to boost a 500-Vdc input to a 5-kVdc output at a switching frequency of 20 kHz. The system ran at 91% efficiency throughout the power band up to 600 W. Considering that this same circuit with standard Si MOSFET switches would only be capable of running at a few hundred Hertz switching frequency, there is an even greater performance advantage with SiC material at these voltage levels. The waveform shown in Fig. 6 exhibits the exceptionally fast turn-off transient of this system.^[6]

With the most recent advancements in SiC materials processing and device design, it will soon be possible to produce reliable MOSFET switches for the commercial marketplace. Considering the recent surge in interest in alternative energy systems, SiC technology is now ready to further improve their benefits. The reduction in power losses that this technology will provide in a PV system's power conversion section will enable more efficient usage of PV panel energy, in turn providing more power to the grid and allowing a reduction in future fossil-fuel generation.

References

1. 4H refers to the SiC crystalline structure used in power semiconductors.

2. Hull, Brett, et al. “Status of 1200 V 4H-SiC Power DMOSFETs,” International Semiconductor Device Research Symposium, December 2007.

3. Burger, B.; Kranzer, D.; Stalter, O.; and Lehrmann, S. “Silicon Carbide (SiC) D-MOS for Grid-Feeding Solar-Inverters,” Fraunhofer Institute, EPE 2007, September 2007.

4. Burger, B.; Kranzer, D.; and Stalter, O. “Cost Reduction of PV Inverters with SiC DMOSFETs,” Fraunhofer Institute, 5^th International Conference on Integrated Power Electronics Systems, March 2008.

5. Yang, Yungtaek; Dillman, David L.; and Jovanovic, Milan M. “Performance Evaluation of Silicon Carbide MOSFET in Three-Phase High-Power-Factor Rectifier,” Power Electronics Laboratory, Delta Products, www.deltartp.com.

6. Das, Mrinal K., et al. “State-of-the-Art 10-kV NMOS Transistors,” 20^th Annual International Symposium on Power Semiconductor Devices and ICs, May 2008.

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