Semiconductor devices that exhibit wide bandgap characteristics, such as silicon carbide (SiC) and gallium nitride (GaN), are far more freely available and are becoming popular in high voltage, high power applications – such as the traction motors and inverters found in the automotive sector.
These devices allow higher voltages to be used (600V to 1700V) as well as the ability to operate at higher switching rates, both of which improve efficiency and reduce size and weight (which also improves vehicle efficiency). Efficiency is hugely important, especially in electric vehicles (EV) and hybrid electric vehicles (HEV) as it extends range and provides designers with options, such as the ability to use smaller battery packs in more compact vehicles.
However, using the new wide bandgap devices has required new standard CMOS drivers, which has necessitated a re-design of some elements of the power system. An additional challenge has been the need for new packaging types which has presented some difficulties with automotive qualification.
Benefits of SiC include a higher critical electric field, improved thermal conductivity and better dielectric constant – these are detailed in Table 1. As a result of these parametric enhancements, SiC devices have low on-resistance, reduced leakage current at all temperature levels, and generally perform better than silicon devices at elevated temperatures. Operation at faster switching speeds is possible with GaN devices, delivering higher efficiency due to their reduced electron saturation velocity.
Transitioning to SiC
Compared to silicon, the 3eV bandgap of SiC has a breakdown field that is five times stronger and its thermal conductivity is over three times higher. The excellent performance / properties was the major reason that SiC became the first wide bandgap material to make its way into cars and trucks.
SiC devices have well-controlled switching times that do not vary appreciably with changes in temperature. As a result, less design margin is required to allow for temperature variation, allowing designers to achieve better performance from these devices. The ability to push the devices closer to their limit makes them ideally suited to applications such as the main inverters in EV and HEV.
Reliability is key in all automotive applications and, for that reason, diodes and MOSFETs are often qualified to AEC-Q101 to demonstrate their suitability. One such device is ROHM Semiconductor’s SCS220KGHR 1200V epitaxial planar Schottky diode. The device is available in TO-220 packaging and has a total capacitance (Qc) of just 65nC, allowing for high-speed operation with significantly reduced switching losses. Devices with lower voltage ratings, such as the 650V SCS215, can reduce system size due to the TO-263AB (also referred to as SOT-23) packaging.
Ford’s latest electric vehicle will be using 900V-rated SiC MOSFET devices from the Cree C3M0120090D series in a high power density, low-cost, on-board inverter. The motor systems in this vehicle are 88kW and the target running cost is said to be $8/kW for when mass production of the vehicle starts around 2020. In part, costs are being managed by reducing the overall weight, aiming to deliver 1.4kW/kg.
SiC devices were selected to deliver the very best on-resistance across the entire operating temperature range, as well as for their improved avalanche energy handling that is expected to increase reliability by a factor of 10x. The Cree MOSFETs have 10mΩ on-resistance and delivers better light-load efficiency than silicon IGBTs at low operating frequencies. As a result, the inverter losses are predicted to be 67% lower than an equivalent silicon solution. This dramatic reduction allows for vastly reduced cooling that will significantly reduce the size and weight of the inverter as well as the cooling costs.
GaN gains traction
GaN devices can achieve efficiencies as high as 97%, due in part to their higher bandgap (3.4eV) and higher electron mobility than SiC devices. Initially, GaN was adopted in RF applications due to their ability to perform well at elevated frequencies. More recently, GaN is now making inroads to power applications – especially automotive power.
Comparing GaN devices with silicon shows that they have comparable conduction losses per unit area to superjunction MOSFETs, but their switching characteristics are superior. Automotive-qualified GaN devices are now in production using enhancement mode device structures, giving these devices the ability to achieve efficiencies as high as 98%.