ROHM Semiconductor – Advantages of Silicon Carbide Diodes, MOSFETs and Modules

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By: ROHM Semiconductor

ROHM Semiconductor is a pioneer in silicon carbide (SiC) development and one of only a few companies manufacturing SiC diodes, FETs, and modules for industrial and power-related applications. SiC provides a number of key advantages over silicon, including significantly higher efficiency in high voltage applications.

Improving performance and reducing device cost while shrinking packaging size is fundamental to virtually all semiconductor products. For power devices, improved performance is measured by increased efficiency and power density, higher power handling capability, and wider operating temperature range. Such improvements depend largely on the characteristics of the power components used, such as low switching and conduction losses, high switching frequency, stable electrical characteristics over a wide temperature range, high operating temperature, and high blocking voltage.

However, as silicon power components approach their theoretical limits, compound semiconductor materials such as silicon carbide are emerging as viable alternatives due to their superior characteristics – delivering a level of performance not possible with their silicon counterparts.

This is particularly true for the most basic components in power electronics: diodes and transistors. SiC Schottky barrier diodes (SBDs) and transistors have been available for more than a decade but were not commercially viable until only recently. As a pioneer in SiC technology, ROHM Semiconductor expects that volume production will lead to SiC’s acceptance in more and more applications.

Advantages of SiC
SiC is a compound semiconductor comprised of silicon (Si) and carbon (C). SiC provides a number of advantages over Si, including:

  • 10x the Dielectric Breakdown Field Strength (0.3 MV/cm vs 3 MV/cm)
  • 3x the Energy Bandgap (1.12eV vs 3.26eV)
  • 3x the Thermal Conductivity (1.5 W/cm°C vs 4.9 W/cm°C)

As a result, SiC offers lower resistivity, higher breakdown voltages (600V+), and can operate at higher temperatures (up to +175°C), making it ideal for power devices in PFC circuits and secondary bridge rectifiers in switch mode power supplies. Other applications include air conditioners, solar power conditioners, EV chargers and industrial equipment.

SiC Diodes
SiC-SBDs allow system designers to improve efficiency and increase switching frequency to lower the cost and size of passives and heat sinks in high voltage (600V+) applications that far exceed the upper limit of Si-SBDs.

While Si-SBDs have the advantage of low forward and negligible switching losses, the narrow band-gap of silicon limits their use to a maximum voltage of ~200V. For breakdown voltages above 200V, silicon fast, super-fast and ultra-fast recovery diodes (FRDs) are used. However, compared to silicon FRDs, SiC-SBDs feature significantly lower reverse recovery current and recovery time, which dramatically reduces recovery loss and noise emission. And unlike silicon FRDs these characteristics do not change significantly over current and operating temperature ranges.

Silicon FRDs also have high transient current at the moment the junction voltage switches from the forward to the reverse direction, resulting in significant switching loss. This is due to minority carriers stored in the drift layer during conduction phase when forward voltage is applied. The higher the forward current (or temperature), the longer the recovery time and the larger the recovery current.

In contrast, since SiC-SBDs are majority carrier (unipolar) devices that use no minority carriers for electrical conduction, they do not store minority carriers. The reverse recovery current in SiC-SBDs is present only to discharge junction capacitance. Therefore, the low reverse recovery current in SiC-SBDs leads to substantially lower switching losses compared to Si-FRDs. Figure 1 shows a comparison of recovery time and current for an SiC-SBD and Si-FRD. Note the ~2/3 reduction in switching loss compared to silicon FRDs.

Figure 1: Switching waveforms of SiC-SBD and Si-FRD

Figure 1: Switching waveforms of SiC-SBD and Si-FRD

Another benefit of SiC-SBDs is that transient current is nearly independent of temperature and forward current. Figure 2 shows the reverse recovery time (trr) over temperature of SiC-SBDs and silicon FRDs. Unlike silicon FRDs whose trr increases with temperature, the reverse recovery time of SiC-SBDs remains stable, thereby achieving lower switching loss and stable fast recovery in high temperature environments.

Figure 2: Reverse recovery time vs temperature for SiC-SBD and Si-FRD

Figure 2: Reverse recovery time vs temperature for SiC-SBD and Si-FRD

SiC FETs
IGBTs (Insulated Gate Bipolar Transistors) are the most common silicon power transistors for high-voltage (>600V), high-current applications. However, the IGBT’s advantage of low resistance at high breakdown voltages is achieved at the expense of switching performance.

IGBTs provide lower on-resistance than MOSFETs by injecting minority carriers into the drift region, a phenomenon called conductivity modulation. However, when the IGBT is turned off, it takes time for these minority carriers to recombine and
“dissipate”, thus generating a tail current and increasing switching time and power compared with MOSFETs (which in principle do not generate tail current).

In contrast, SiC devices do not need conductivity modulation to achieve low on-resistance since they have much lower drift-layer resistance than Si devices. As a result, SiC-MOSFETs feature much lower switching loss than IGBTs.

For example, ROHM’s SCH2080KE SiC-MOSFET delivers 90% lower switching loss during turnoff, while its integrated SiC-SBD further improves switching loss by shortening the diode recovery time during turn-on. Figure 3 shows the turn-off characteristics of an SiC-MOSFET vs Si-IGBT, while Figure 4 shows a comparison of total loss between SiC-MOSFETs and Si-IGBTs.

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Figure 4: Loss comparison of SCH2080KE SiC-MOSFET+SBD and Si-IGBT

Figure 4: Loss comparison of SCH2080KE SiC-MOSFET+SBD and Si-IGBT

The high switching loss in IGBTs increases the chip’s junction temperature, frequently limiting the switching frequency to 20kHz or less. SiCMOSFETs, on the other hand, provide significantly lower turn-off losses and allow for higher switching frequencies above 50kHz, simplifying thermal design and enabling smaller passives to be used. Figure 5 shows a comparison of LC filter component sizes at 20kHz vs 50kHz.

Figure 5: LC filter size for 20kHz and 50kHz switching frequencies

Figure 5: LC filter size for 20kHz and 50kHz switching frequencies

SiC Modules
IGBT modules that combine Si-IGBTs and Si-FRDs are commonly used as power modules to handle high currents and high blocking voltage. However, to minimize switching loss associated with IGBT tail current and FRD recovery current, ROHM has developed the first commercial power modules with integrated SiC-MOSFETs and SiC-SBDs. Benefits include:

  • Improved conversion efficiency due to lower switching losses
  • Simpler thermal management (e.g. smaller, less expensive heat sink/cooling system, natural cooling vs water/forced air)
  • Support for smaller passive components (i.e. inductors, capacitors) by allowing for higher switching frequencies

SiC power modules are being increasingly adopted in power supplies for industrial equipment, PV power conditioners, and more.

Summary
Compound semiconductors such as silicon carbide are quickly emerging as the ideal successors to silicon in order to meet the need for higher efficiencies and power handling capability demanded in today’s power applications.

Silicon carbide features a number of inherent advantages over silicon, including 10x higher dielectric breakdown field strength, 3x higher bandgap energy, and 3x higher thermal conductivity, making it possible to operate at higher voltages and temperatures and providing a level of performance not possible with silicon devices.

ROHM offers a wide range of power SiC SBDs, MOSFETs, and modules optimized for voltages up to 1,200V, and is currently developing 1700V products.

Part Number Product Type Voltage (V) Current (A) Package
SCS220KG SiC Diode VR = 1200 IF = 20 TO-220AC
SCS215KGSiC Diode VR = 1200 IF = 15TO-220AC
SCS210KGSiC Diode VR = 1200 IF = 10TO-220AC
SCS205KGSiC Diode VR = 1200 IF = 5TO-220AC
SCS220AGSiC Diode VR = 650IF = 20TO-220AC
SCS210AGSiC Diode VR = 650IF = 10TO-220AC
SCT2080KESiC N-MOSFETVDSS = 1200 ID = 40TO-247
SCT2160KESiC N-MOSFETVDSS = 1200 ID = 22TO-247
SCT2280KESiC N-MOSFETVDSS = 1200 ID = 14TO-247
SCT2450KESiC N-MOSFETVDSS = 1200 ID = 10TO-247
SCT2120AFSiC N-MOSFETVDSS = 650 ID = 29TO-220AB
BSM180D12P2C101SiC Half Bridge ModuleVDSS = 1200 ID = 180see datasheet
BSM120D12P2C005SiC Half Bridge ModuleVDSS = 1200 ID = 120see datasheet

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