By Nagarajan Sridhar
A couple of months ago, I got a chance to test-drive a new autonomous vehicle. My general observation: it was a computer on wheels! Because of the traction motor (a key characteristic of electric vehicles [EVs]), it was totally silent, with no whirring sound from the integrated circuit-based engine. Also memorable was experiencing autopilot mode, which is the predecessor to the autonomous driverless vehicular concept.
Will drivers have little to no control over their cars in the near future? Many articles predict that EV numbers will rise significantly starting in 2020, and at some point will even outnumber gasoline-powered vehicles. And with the increase of wideband gap materials such as silicon carbide (SiC), the vision of EVs becoming lighter, cooler and more efficient is becoming a reality.
EVs have a completely electric drive train comprising high-voltage DC/DC power-conversion units from 400V down to 12V, along with a high-voltage traction inverter to drive the motors to move the vehicle. The charging infrastructure, which can be grid-infrastructure-to-vehicle or solar-to-vehicle, entails wirelessly charging the batteries from renewable sources.
Both the drive train and charging infrastructure need high-voltage gate drivers operating under switched-mode power conversion at high frequencies (in the order of 20 to hundreds of kilohertz) in order to operate under high efficiencies. From a power-electronics perspective, you can see why it is important to have less power to the driver (gate driver) as well.
To achieve high efficiency, which in turn affects the miles per charge in a vehicle, the automotive industry is looking toward adopting SiC instead of silicon for the power switches. SiC has the following advantages over silicon:
- High power density – 10x more than silicon.
- High breakdown voltage.
- Drives higher current in a reduced footprint.
- High thermal conductivity.
- High mobility/ability to switch at high frequencies.
High power density enables the power-train system to be smaller in size for a given power level requirement. This in turn helps reduce vehicle weight. High thermal conductivity enables efficient dissipation of heat from the system. Finally, the ability to switch at higher frequencies enables a size reduction of passives such as capacitors and magnetics, which in turn enables weight reduction.
Let’s expand on two of these features of SiC, starting with high drive current. A power switch turns “on” or “off,” whether it be Si or SiC. An ‘on’ state indicates current conduction. At this stage, there is a conduction loss associated with it, which is indicated by its “on-state resistance.” The smaller the resistance, the smaller the power loss. SiC has a five to 10 fold lower “on-state resistance.” This enables SiC to run high power applications (few to hundreds of kilowatt) at high current with high efficiencies.
The other important feature of SiC is the ability to operate at a high temperature, close to 300°C. This is twice that of silicon. The capability to function at a higher temperature allows the SiC modules (that has a bank of SiC FETs tied together) to operate with high efficiency and reliability for high temperature applications, such as the engine hood in HEVs, that operate in excess of 200°C.
If you are looking for solutions for electric drive train and charging infrastructures, check out TI’s automotive-qualified gate drivers.
Author: Nagarajan Sridhar is a marketing manager for TI’s High-Voltage Power Products.