SiC Entering Low-power Applications

Article By : fast SiC semiconductor

SiC MOSFETs' thermal, EMI, space, and noise immunity characteristics help improve the performance of highly integrated power stages, and also open up opportunities for these solutions to reach higher power levels.

Silicon carbide (SiC) has been gaining momentum since Tesla started adopting transistors made of this wide bandgap material in its best-seller electric vehicle (EV) Model 3.

SiC MOSFETs and SiC Schottky Barrier Diodes (SBDs) rated from 650V to 1700V are now penetrating increasingly medium- to high-power applications, which include SMPS for servers in datacenters, UPSes, on-board chargers (OBCs) and DC/DC converters for EVs and PHEVs, traction inverters for EVs and fuel-cell vehicles (FCVs), inverters for high-speed trains (such as Japan’s Shinkansen N700S, made by Hitachi) and commuter train (such as Taiwan’s EMU3000, also made by Hitachi), and PV inverters, wind turbine inverters, and solid state relays for energy storage systems (ESS).

The power rating of these applications ranges from several kW to multiple MW. The obvious advantages of SiC MOSFETs and SiC SBDs in performance enable them to replace their silicon counterparts Si-IGBT and Si-FRD in a way whenever the system or BOM cost can be justified despite the higher component cost.

Eliminating price gap between SiC and Si is a matter of time

It is true there is still an obvious gap between the price of SiC and Si. The high cost of SiC could be be attributed to its own material characteristics. SiC is a tough material with high thermal, mechanical, and chemical stability. Processing SiC usually requires a much higher temperature compared to Si. In particular, the slow, extremely high temperature sublimation growth (up to 2,600℃) of SiC crystal makes SiC substrates expensive, and account for more than 40% of final device cost. But with the aggressive expansion plans initiated by leading SiC substrate providers Wolfspeed, II-VI, and Sicrystal and their effort to shift the mainstream wafer size from 6in to 8in, plus the capacities of processing fabs gradually reaching economies of scale, the real cost of SiC devices will drop rapidly in the following years.

Combining the lowering material and processing cost and the advances in technology, the price-parity between SiC MOSFETs and Si super-junction (SJ) MOSFETs in the 600V/650V class will happen sooner or later. The specific on-resistance of current 650V SiC MOSFETs is only 1/5 of the most advanced Si SJ-MOSFETs and less than 1/10 of many legacy parts.

The chip size of SiC MOSFET is also 1/2 to 1/4 of GaN transistors, depending on the supplier. The smaller die size of SiC MOSFET make itself already competitive in price to many GaN transistors despite the higher cost of processed wafer. In addition to that, because the Rdson of SiC MOSFET only increases slightly with increasing temperature (for example, the Rdson of SiC MOSFET at 150°C is only 1.2x to 1.3x of Rdson at 25°C, while the Rdson of Si SJ-MOSFET and GaN transistor at 150°C is 2x to 3x of Rdson at 25°C. This feature allows engineers to use SiC MOSFET with higher Rdson to replace Si SJ-MOSFET or GaN transistor with lower Rdson, to further reduce the price gap. The concern that a smaller die size may result in a worse thermal performance can be addressed by the nearly 3x higher thermal conductivity of SiC compared to Si and GaN.

Benefits of SiC MOSFETs in low power applications: Thermal and EMI

A simple drop-in-replacement test shows how SiC MOSFET can improve thermal and EMI even with single-ended topologies such as QR Flyback converters. Figure 1 illustrates the thermal images and EMI spectra of a commercially available 65W adaptor. It can be observed that the MOSFET temperature improves by around 10°C and EMI improves by 10dB between 10MHz and 30MHz when the original 168mΩ Si SJ-MOSFET was replaced by a 370mΩ SiC MOSFET.

Figure 1 Thermal images and EMI spectra of a commercially available 65W adaptor.

In another test, a 320mΩ SiC MOSFET was utilized in a 65W charger with the ACF controller of Silanna Semiconductor. The highest efficiency achieved is around 94%, which is on par with GaN transistors, but the thermals are significantly better than GaN especially at low line. The EMI spectra also show that >6dB margin for radiated emission can be readily achieved with SiC MOSFET.

Figure 2 Thermals, efficiencies and EMI spectra of a 65W charger with SiC MOSFET and Silanna’s ACF controller.

In most cases, the initial evaluations of SiC MOSFETs can be performed by simple drop-in-replacements or even by flying-wires if the pin out is not compatible. SiC MOSFETs exhibit good immunity to noise and is strong enough to endure transient voltage spikes because their ability to withstand high avalanche energy. SiC MOSFETs’ ease-of-use and robustness, as well as thermal and EMI benefits, can help engineers address the challenges in the quest for higher power density when the new USB PD 3.1 standard raises the maximum power level to 240W.

SiC MOSFETs enable higher level of integration

The concept of “everything that can be integrated should be integrated” has prevailed in many fields of integrated circuits, but in power electronics, the integration of power stages has been limited to a relatively low power level because of stringent challenges encountered in dealing with thermals, EMI, and limited space. Brushless DC (BLDC) and permanent-magnet synchronous (PMSM) motor drive for applications including medical equipment, home appliance, building controls, and industrial automation is one such example. The trend of BLDC/PMSM motor drive is moving towards increasing levels of integration to save space, reduce external component counts, shrink system size, and decrease maintenance.

Figure 3 illustrates how the PCB size and the number of external components for a motor drive can be significantly reduced with a highly integrated SiC sIPM developed by Richtek Technology. This SiC sIPM achieves 99% efficiency at 100W of output power, with improved EMI, and with sensed temperature only 14.4°C higher than the ambient.

Figure 3 The driving board of the all-in-one system integrated full SiC intelligent power module (abbreviated as full SiC sIPM) developed by Richtek Technology, mounted on a motor for the air cleaner. This full SiC sIPM integrates six SiC MOSFETs provided by FastSiC with an ARM-based 32-bit Cortex-M0 CPU with 60MHz of frequency, 16kB of memory, 4kB of SRAM, internal ROM with embedded motor control library, seven channels of 10-bit ADC, one channel of 8-bit DAC, high/low-side gate drivers with UVLO and deadtime interlock algorithm, and propagation delay matching strategy.

Figure 4 shows that the six SiC MOSFETs used in the power stage only occupy a small footprint of the package. This suggests that the size of the package can be further reduced, or the size of package can be kept but lower Rdson SiC MOSFETs can be used to increase the output power level. At 600V/650V level, SiC MOSFETs can readily achieve Rdson which is only 1/2 to 1/4 of GaN transistors and 1/5 to 1/50 of Si MOSFETs when the die sizes are the same. The benefits provided by SiC MOSFETs in thermal, EMI, space, and immunity to noise help improve the performance of these highly integrated power stages, and also open up opportunities for these solutions to reach higher power levels for applications such as industrial servo motor drive for robotics, where proposals to help get rid of bulky and expensive cables will be welcomed.

Figure 4 The 23-Lead SOP plastic package body of the full SiC sIPM, where six SiC MOSFETs were used for a 3-phase-bridge power stage. The dimension of this full SiC sIPM is 29-by-12-by-3.3mm, and is expected to deliver up to 250W of output power.

 

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