2018.12.06 SiC Power Device
This final section ends by summarizing the key points of the physical properties of SiC, SiC-SBDs (Schottky barrier diodes), SiC-MOSFETs and full-SiC power modules.
・SiC power devices are next-generation, low-loss elements that are excellent for reducing power loss and for operating in high-temperature environments.
・Although these are new semiconductor devices, they have already been widely used even in automotive markets that require high quality and reliability.
＜What is SiC (silicon carbide)?＞
・The physical properties of SiC are well-suited to power devices.
・Compared with Si semiconductors, losses are low, and dynamic characteristics in high-temperature environments are excellent.
・SiC has been developed as one solution to energy-related problems.
・In addition to reducing losses, SiC offers the major advantage of miniaturization.
＜SiC Schottky Barrier Diode (SiC-SBD)＞
・Features of SiC-SBDs are excellent high-speed operation combined with a high voltage.
・Compared with Si PN diodes having high voltages, SiC devices afford excellent reverse recovery times and other high-speed characteristics, and so make possible lower losses and more compact equipment.
・An SiC-SBD has a faster trr and much smaller reverse recovery current compared with a Si-PND (FRD), and so losses are small.
・The reverse recovery characteristic (trr and reverse recovery current) of an SiC-SBD exhibits almost no temperature dependence.
・The VF of an SiC-SBD rises with temperature, whereas the VF of an Si-PND (FRD) falls.
・The increase in the VF of an SiC-SBD at high temperatures causes the IFSM to decline, but there is no thermal runaway, such as occurs in Si-PNDs (FRDs), the VF of which declines.
・In second-generation SiC-SBDs, the VF falls, and at present they can be called the power diodes that are best able to help reduce losses.
・ROHM SiC-SBDs have already evolved to the third generation.
・Third-generation products offer improved TFMS and reduced leakage currents, and further reduce the low VF values achieved in the second generation.
・The trr is fast, so that recovery losses can be dramatically reduced, for higher efficiency
・For a similar reason, the reverse current is small so that noise is low, and the number of noise/surge suppression components can be reduced, enabling enhanced miniaturization
・High frequency operation enables miniaturization of inductors and other peripheral components
・At ROHM, tests conforming to industry standards for semiconductor devices are conducted to evaluate the reliability of SiC-SBDs.
・SiC-MOSFETs can contribute to reduced losses and smaller application size relative to Si-MOSFETs and IGBTs.
・The features of power transistors differ depending on the materials and structures.
・There are various advantages and disadvantages where characteristics are concerned, but SiC-MOSFETs exhibit excellent characteristics overall.
・In order to obtain a low on-resistance for a SiC-MOSFET, the Vgs must be set higher than that for a Si-MOSFET, to around 18 V or so.
・The internal gate resistance of a SiC-MOSFET is higher than that of a Si-MOSFET, and so the external resistance Rg is set low; but surge protection should also be considered.
・The on-resistance characteristic of SiC-MOSFET Vd-Id characteristics changes linearly, and SiC-MOSFETs have an advantage over IGBTs at low currents.
・Switching losses of SiC-MOSFETs can be greatly reduced compared with IGBTs.
・The forward characteristic Vf of the body diode of a SiC-MOSFET is high compared with that of an Si-MOSFET.
・The trr of a SiC-MOSFET body diode is fast, and the recovery loss can be reduced relative to that of an Si-MOSFET.
・ROHM has achieved mass production of SiC-MOSFETs that adopt an original double-trench structure.
・Trench-structure SiC-MOSFETs have an ON-resistance lower by about 50%, and an input capacitance lower by about 35%, compared with DMOS-structure products.
・The effectiveness of SiC-MOSFETs should be considered carefully, taking hints from case studies that utilize SiC-MOSFETs.
・The reliability of ROHM SiC-MOSFETs is equivalent to that of Si-MOSFETs currently in use.
＜Full-SiC Power Modules＞
・Full-SiC power modules are configured using SiC-MOSFETs and SiC-SBDs developed and manufactured by ROHM.
・Faster switching and greatly reduced losses can be achieved compared with Si-IGBT power modules.
・Full-SiC power modules continue to evolve, adopting the most advanced third-generation SiC-MOSFETs.
・Full-SiC power modules are capable of dramatic cuts in switching losses compared with IGBT modules.
・The difference is particularly stark at higher switching frequencies.
・SiC power modules can perform rapid switching even while greatly reducing losses.
・"False gate turn-on" is one issue requiring consideration in relation to gate driving in a full-SiC power module.
・False gate turn-on arises due to the fast dV/dt during high-side switch-on and the low-side parasitic gate capacitance and gate impedance.
・Methods for suppressing false gate turn-on include ① setting Vgs to a negative voltage when turned off, ② adding an external capacitor CGS, and ③ adding a mirror clamp MOSFET.
・By optimizing the gate driving of a full-SiC power module, clean operation with still lower losses is possible.
・In order to exploit high-speed switching performance, parasitic inductances in electric wiring must be suppressed insofar as possible.
・Capacitors are connected near power terminals to reduce wiring inductance.
・Surges and ringing can be dramatically suppressed by using a specialized gate driver and a snubber module.
・Where losses are concerned, there is an increase in Eon and a decrease in Eoff. When comparing overall losses (Eon+Eoff), losses are reduced.
・A full-SiC module loss simulator and other support tools are available.
・The support tools are useful for selection and initial studies of full-SiC modules.