To meet the challenging requirements of high efficiency and high power density, power electronics applications are increasingly moving toward advanced materials, such as wide-bandgap semiconductors. These materials, including silicon carbide, offer superior performance over existing semiconductor technologies like silicon-based MOSFETs and IGBTs.
SiC power devices guarantee lower losses, higher switching frequencies, higher operating temperatures and higher breakdown voltages. SiC power devices can operate at much higher voltages, frequencies, and temperatures than conventional silicon, achieving lower switching and conduction losses. SiC has about 10× better conducting and switching properties than silicon, and the die area of SiC MOSFETs is nearly half that of IGBTs.
The main properties of SiC power devices can be summarized as follows:
- High breakdown voltage
- Low on-state resistance, also known as RDS(on)
- High switching frequency
- Low switching and commutation losses
- Excellent thermal management due to higher junction temperature than silicon
The critical breakdown strength of SiC is approximately 10× higher than that of silicon. Moreover, its drift layer (the main cause of electrical resistance) is one-tenth of the thickness (see Figure 1). This allows a large reduction in electrical resistance and, in turn, in power losses.
Mitsubishi FMF400DY-24B power module
Mitsubishi Electric has recently released a new 400-A, 1,200-V dual SiC power module that includes an anti-parallel, low-Vf, zero-recovery–loss SiC Schottky barrier diode. The module, provided in a current industry-standard footprint (62 × 108 mm) is suitable for medical power supplies and general industrial applications.
“Our new silicon carbide dual module is focused on medical power-supply–type applications and has been developed in conjunction with some of our industry partners,” said Adam Falcsik, senior product manager of Mitsubishi Electric US’s Power Device Group. “Historically, they have used high-speed silicon IGBT devices but are now looking to expand and improve their products by moving to silicon carbide.”
The device implements Mitsubishi Electric’s second-generation SiC MOSFETs, with voltages ranging from 1,200 V up to 3.3 kV and currents of several hundred amperes.
“From our first to our second commercial SiC generation, we have seen a further decrease in loss,” said Falcsik. “Additionally, we have also seen an increase in productivity because we moved to a larger wafer size in the manufacturing of those devices.”
Because higher cost is considered one of the key factors limiting the wide adoption of SiC, this downward trend of SiC pricing achieved by Mitsubishi Electric can be considered as a major result. According to Falcsik, while the cost of a first-generation SiC device was 7× to 10× the cost of a similar-rated silicon device, they are now seeing a cost 4× to 7× higher than an equivalently rated Si device.
Requiring a gate-source voltage of 15 V in the on state, the module is compatible with standard IGBT gate drivers and can be smoothly installed on already-existing mechanical layouts for an easy replacement of conventional Si-based devices.
An important aspect to underline is that the design of a SiC MOSFET gate driver is not much different from the design of a standard IGBT or regular silicon MOSFET driver. SiC devices typically require a negative voltage in the off state and a positive voltage (15 V, in this case) in the on state. While a conventional IGBT would require a zero or a single-sided power supply, a negative-bias gate drive voltage is normally required to maintain a SiC device in the off state. That solution provides dV/dt immunity, avoiding undesired commutation due to ripple or noise.
“The idea behind our new power module was to maintain mechanical compatibility with previous generations of power modules,” said Falcsik. “That provides customers with high flexibility, since by changing the gate drive, they are able to take advantage of silicon carbide.”
The new FMF400DY-24B SiC-based power module has the same package as the IGBT type but is far more efficient. And although it has a higher cost, this is paid for by lower power consumption. Even though its package is not technically optimized, it is required by the market for legacy support and compatibility. Additionally, the package features an extreme low inductance, thus allowing the device to switch at almost 100 kHZ without affecting its efficiency. The new power module (Figure 2), based on Mitsubishi Electric second-generation SiC MOSFET chip technology, adds to the company’s growing family of SiC products, providing best-in-class performance and flexibility. Capable of operating at higher switching frequencies, the new SiC module reduces power loss by approximately 70% compared with an equivalently rated Si IGBT.
Even though its second-generation SiC devices are based on a robust planar structure, Mitsubishi is planning to move to a trench structure for its third generation of SiC MOSFETs.
“That will allow us to shrink the chip size and increase the power density,” said Falcsik. “Our second generation still uses a planar structure because we were able to outperform some of the other trench structures available on the market. We think we have hit the limit of what we can do with planar structure, so we’re moving toward a trench-type silicon carbide MOSFET in the third generation.”
The Semiconductor & Device Division of Mitsubishi Electric US offers a wide portfolio of semiconductor and electronic devices, including next-generation optical devices and high-frequency gallium nitride, gallium arsenide, and silicon RF devices used in applications like data centers, satellite base stations, and two-way radios to support today’s rapidly evolving telecommunications networks. Moreover, the division offers highly efficient power modules for both traditional and renewable energy sources that distribute power effectively and reliably.
“In the Power Device Group, we are focused on silicon carbide MOSFETs.,” said Falcsik. “In the way that we’re structured, we start with our lowest device ratings, 600 V, and we go on from there up to 1.2-, 1.7-, and 3.3-kV–rated devices. In this space, we think silicon carbide is the way to go.”
For the production of SiC devices, Mitsubishi Electric procures the raw SiC wafers from a variety of foundries, and then from there, all of the fabrication steps required by the chip (epi, ion implantation, and diffusion) are made in-house.
