Wide bandgap semiconductors (WBG) have recently drawn a lot of interest as main switches for power conversion processes. Owing to their inherent properties, materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) offer some advantages over silicon, therefore representing a solution to the quest for increased power density, safer thermal operation, better efficiency and reduced system size. Among such two breakthrough technologies, Silicon Carbide is the most mature wide-bandgap semiconductor material currently available in the market. STMicroelectronics is among the first manufacturers having introduced and produced high-voltage silicon carbide (SiC) Power MOSFETs, and the first one achieving the industry’s highest temperature rating of 200°C in a molded package.
Figure 1 shows a comparison mapping of SiC and GaN in terms of breakdown voltage and power of systems. Very little of an overlap exists between them; this is the reason why STMicroelectronics made the decision to embark in the development for two such breakthrough technologies.
As can be seen from the chart, GaN lends itself to be used over a voltage range from hundreds of volts up to 1000V maximum as opposed to the SiC that fits into applications requiring voltages in excess of 1200V and even higher; depending upon the voltage bus chosen, both technologies may find a proper place in the automotive ecosystem for advancing plans aimed to increase the amount of on-board electrification.
SILICON CARBIDE POWER TECHNOLOGIES 1200V SiC MOSFET
At voltages in excess of 1200V, the SiC MOSFET benchmark is the IGBT for the reasons highlighted earlier; in fact, even a super-junction product would exhibit too high a Ron for it to be a possible choice. Table 1 quantifies and summarizes the above statement.
|Von @ 25C, 20A (V)||Von @ 25 °C, 20A (V)||Eon @ 25 °C /175 °C (J)||Eoff @ 25 °C /175 °C (J)||Chip size (rel.)|
Table 1: 1200V SiC MOSFET vs. IGBT
The data shown in table 1 was obtained comparing a 100 mΩ SiC MOSFET with the state-of-the-art IGBT with the same voltage rating. What immediately emerges from a glance is that the SiC MOSFET exhibits much lower switching losses even at higher temperatures. This translates into far higher switching frequencies that can be reached, which, in turn, imply smaller magnetic components and therefore reduced form factors.
Another feature of ST’s SiC MOSFET to underline is the less pronounced variation of Eon and Eoff as temperature increases: another tangible benefit of WBG (wide band-gap) products. This improvement holds true also for the Ron parameter as shown in fig. 2 respectively, for standard silicon MOSFET as well as SiC MOSFET from our competitors.
SIC MOSFET VALUE PROPOSITION
Experimentally, it has been verified, therefore, that the SiC MOSFET ensures the same efficiency as that of silicon IGBT but at a frequency which is four times bigger, as shown in fig. 3.
This translates into the possibility of reducting the impact of passive components and obtaining smaller, lighter, less costly systems, as clearly shown in fig. 4 with an analysis conducted by an electromagnetic company on the above described 5kW Boost Converter (report available).
SIC MOSFET PRODUCT LINE ENLARGMENT
STMicroelectronics’ SiC MOSFETs SCT30N120 (1200V, 100mΩ) and SCT20N120 (200V, 290 mΩ) were released in mass production in 2014. Target applications include solar inverters, high-frequency power supplies, power factor correctors (PFCs) as well as fully electric and hybrid electric vehicles (EV and HEV).
The success of this product is due to the “normally-off” structure and to outstanding intrinsic parameters such as
- Almost-flat RDS(on) derating vs temperature,
- Ultra-low Qg and capacitances,
- Very fast intrinsic and robust body-diode
ST’s SiC Power MOSFETs represent the “mandatory next step” vs traditional silicon metal–oxide–semiconductor field-effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs), for all the applications in power conversion running at a frequency ≥ 100 kHz. Additionally, the fully-guaranteed 200°C TJ (max) opens new frontiers in terms of thermal management, reliability, and robustness.
STMicroelectronics is currently working to broaden the product line-up by introducing new RDS(on) specs, new innovative packages and higher breakdown voltage devices (i.e. 1700V).