Silicon Carbide power semiconductors are rapidly emerging in the commercial market. These devices offer several benefits over conventional Silicon-based power semiconductors. SiC MOSFETs can improve overall system efficiency by more than 10%, and the higher switching capability can reduce the overall system size and cost.
Avago Technologies gate drive optocouplers are used extensively to drive Silicon-based semiconductors like IGBT and Power MOSFETs. Optocouplers are used to provide reinforced galvanic insulation between the control circuits from the high voltages and the power semiconductors. The ability to reject high common mode noise (CMR) will prevent erroneous driving of the power semiconductors during high frequency switching. We will discuss how the next generation of gate drive optocouplers can be used to protect and drive SiC MOSFETs.
Silicon Carbide is a wide bandgap (3.2 eV) compound. Wide-bandgap SiC, besides being able to operate at high voltage, frequency and temperature, exhibits on-resistance and gate charge by an order of magnitude lower than silicon material. In an evaluation conducted by CREE to compare the second generation 1200 V/20 A SiC MOSFET with a silicon high speed 1200 V/40 A H3 IGBT using a 10 KW hard-switching interleaved boost dc-to-dc converter, the results showed that even with five times the switching frequency, the SiC solution was able to achieve a maximum efficiency of 99.3% at 100 KHz, reducing losses by 18% from the best efficiency of the IGBT solution at 20 KHz.
CREE’s recent release of the C2M family of MOSFETs give engineers a wide range of competitively priced 1200 V and 1700 V SiC MOSFETs for a wide range of applications. Cree has been able to bring the MOSFET’s cost down significantly, while providing improved switching performance and lower Rds (on). Increasing the switching frequency can significantly reduce the size of the inductor. The lower conduction and switching losses allow engineers to reduce the size of the heat sink, or potentially remove fans and move to passive cooling solutions. Although a SiC semiconductor costs more than Si, the overall system BOM costs can be lower than Si technology by 20%.
SiC technology is now widely recognized as a reliable alternative to silicon. More than 30 companies worldwide have established SiC technology manufacturing capability with related commercial and promotional activities. Many power module and power inverter manufacturers have included SiC in their roadmaps for future products.
Solar inverter manufacturers and Server power supplies are the earliest adopters of SiC semiconductors as efficiency is critical in their technological ranking. In 2013, Europe’s top solar inverter makers, REFU, SMA and Delta announced a new model with SiC inside. Photon magazine evaluated SMA SiC inverters and efficiency increased from 98% to 99%, while the inverters’ physical weight was reduced by 30% as compared to equivalent IGBT inverter. The ramp-up of SiC semiconductor production will increase adoption in applications like motor drives, railway traction industrial uninterruptible power supplies, and hybrid vehicles.
Avago Technologies’ gate drive optocouplers have been used extensively to drive Silicon-based semiconductors like IGBTs. The company has been working closely with SiC market leader CREE Inc, to determine suitable gate drive optocouplers to fit SiC MOSFET operations. We have evaluated gate drive optocouplers ACPL-W346 and ACPL-339J with CREE C2M SiC MOSFET using an 8 A SEPIC dc-to-dc C cconverter at 100 kHz.
To match the low switching loss of CREE’s SiC MOSFET, the gate driver must be able to deliver high output current and voltage with fast slew rate to overcome the gate capacitance of the SiC MOSFET. The scope capture in Fig. 2 shows ACPL-W346 with a 20 V, fast rise and fall times signal profile at the gate of the SiC MOSFET which is necessary to switch the SiC MOSFET efficiently.
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