SiC-based Inverter Improves Efficiency in EVs, Renewables

Article By : Maurizio Di Paolo Emilio

SiC-based inverters are increasing efficiency in power converters, especially in EVs and renewable applications.

Power converters are essential parts used in all electrical applications, including inductors, systems for converting renewable energy sources, and energy storage. The need to electrify systems formerly run on the burning of fossil fuels is more urgent as worldwide efforts to minimize greenhouse gas emissions pick up speed. The effectiveness and performance of converters, the primary component of these systems, are becoming increasingly crucial as the world moves toward the usage of electric systems powered by renewable energy.

By taking a market position in energy transition, Hillcrest Energy Technologies intends to lead by example to decarbonize the energy sector. Its technology is focused on creating solutions that unlock efficiencies in electrification and maximize the performance of integrated electric systems. “While we have a focus on traction inverter technology for electric vehicles (EVs), the technologies we are developing can also be applied to a wide range of industries to make electrical systems more efficient,” said Chief Technology Officer Ari Berger at Hillcrest.

According to Berger, Hillcrest technology eliminates traditional design tradeoffs faced across the power industry—deploying higher switching frequencies has historically meant a greater increase in losses: lower system efficiency and higher heat. “Through a combination of hardware and control software expertise, Hillcrest technology materially eliminates switching losses, enabling power applications to leverage higher switching frequencies, realize improved power system performance and reliability, and operate at higher power levels without compromising efficiency,” said Berger.

SiC-based inverter technology

By reducing inverter losses, Hillcrest’s inverter technology reduces the amount of thermal management needed across the whole powertrain system. By reducing the complexity of the system and relieving some of the pressure on the power components, this might result in considerable cost savings.

Technologies based on silicon, such as MOSFETs, have been created to alleviate conduction losses. Utilizing the wide-bandgap semiconductors silicon carbide and gallium nitride, low-conducting–resistance switches have also been developed recently. These also reduce switching losses by reducing the transition time, but this in turn has created issues with EMI and high dV/dt, which has a negative effect on reliability.

The other way to reduce or eliminate switching losses is by eliminating the current and voltage transition overlap. The well-known approach is based on zero-voltage switching (ZVS).

ZVS enables “soft switching,” which prevents switching losses that often occur with traditional PWM and synchronization.

When a MOSFET is soft-switched, any voltage or current overlap is removed, reducing losses. The voltage drops to zero (not just to idle) before the MOSFET is turned on or off. (The method may also be used in order to flip the MOSFET when the current, rather than the voltage, approaches zero. Zero-current switching, or ZCS, is the name of this technique.) The fact that soft-switching waveforms reduce EMI is another benefit.

The most accurate definition of soft switching (ZVS) is the conversion of standard PWM power while the MOSFET is in operation but with “resonant” switching transitions. The method may be compared to a PWM power supply that uses a constant control to vary the conversion frequency or to maintain output voltage regulation in service time. This technique is equivalent to fixed-frequency conversion with adjustable duty cycle for a given unit of time.

Two additional benefits of ZVS are its ability to operate at higher frequencies, which results in reduced noise, simpler filtering, and the usage of fewer filter components, as well as its ability to minimize the harmonic spectrum of any EMI (by focusing it on the switching frequency).

The disadvantage is that there is no assurance that the MOSFET will have used all of its energy before switching off, particularly at high frequencies. This “stored” energy over time may lead to component failure, particularly in a fast-switching voltage regulator. In order to ensure that all energy is evacuated from the transistor, power module manufacturers solve this issue by connecting a fast body diode in parallel with the switch.

The existing ZVS methods are not suitable for traction applications, as they suffer from issues such as temperature and load-dependent performance and narrow operating range. Hillcrest’s inverter technology implements the ZVS method controlled by novel control software algorithms using microcontrollers. This method enables soft switching and materially eliminates switching losses. Therefore, with eliminated switching losses, switching frequency of the converter can be increased and the system can enjoy the benefits of high switching frequency, such as better performance and smaller size.

“We are currently using 1,200-V SiC semiconductors for our developments, because from today’s point of view, they are the most suitable for traction applications,” said Harald Hengstenberger, Managing Director and Founder of Systematec GmbH, a strategic partner of Hillcrest with expertise in power electronics and electromechanical component design. He commented that the switching frequencies of today’s traction systems are about 10 kHz. “We see considerable advantages in using our high-efficiency inverter technology to variably adjust this switching frequency to the optimum value for overall system efficiency, which is up to approximately 100 kHz. GaN switches are suitable for much higher switching frequencies, and we see their use in chargers or DC/DC converters.”

According to Hilllcrest, all markets that have high operating times or high-efficiency requirements could find this type of inverter beneficial. “In addition to electric vehicles and the mobility industry, the Hillcrest inverter technology platform can improve efficiency and reliability across a variety of end uses and applications in which inverters play a key role, including grid-tied renewables, charging and storage systems, and high-voltage/high-power applications like utility-scale grid, rail, and container ships,” said CTO Berger.

Because Hillcrest’s inverter technology materially eliminates switching losses, it makes higher switching frequencies feasible, which results in better output power quality and lower total harmonic distortion and smaller DC-link capacitor size. It also reduces the dV/dt of the main power switches without adversely affecting losses, which helps to protect motor windings and cabling from insulation breakdown and decreases problems traditionally caused by EMI.

According to Hillcrest, these benefits offer valuable system-level advantages, such as reduced motor size and cooling requirements, lower torque ripple, and increased lifetime of mechanical parts in traction applications.

“In the development of semiconductors, a compromise is usually made between the lowest possible forward losses and good switching properties,” said Berger. “With our high-efficiency inverter technology, we have basically eliminated the switching losses of power semiconductors, and therefore, future generations of semiconductors that are optimized for the lowest possible on-state losses could lead to further increases in efficiency and range. The special switching method used in Hillcrest’s high-efficiency inverter technology also reduces the ripple current in the DC circuit, which can lead to an increase in battery life. In our view, the type of semiconductor is not decisive in this respect.”

Graph of H=hard-switching turn-off under 5-A inward load current for SiC-based inverter
Figure 1: Hard-switching turn-off under 5-A inward load current (Source: Hillcrest Energy Technologies)
Graph of soft-switching turn-off under 5-A inward load current for SiC-based inverter
Figure 2: Soft-switching turn-off under 5-A inward load current (Source: Hillcrest Energy Technologies)

Figures 1 and 2 result for turn-off transition under 5-A current toward the converter (negative direction). In the hard-switching scheme, approximately 65μJ is dissipated in the switch with peak power of 1.5 kW. In this scheme, the dV/dt is about 13 kV/µs, which is quite high and can damage cables and motors. On the other hand, the losses of the soft switching during transition is amazingly low, at approximately 2 µJ. The dV/dt value in this scheme is 1.1 kV/µs, which results in longer life of the motor and cables, lower EMI, and lower shielding requirements.

Challenges

The main drivers for inverter development are always size, weight, and cost. The ultimate efficiency of a device is a consequence of the interplay/tradeoffs of these three requirements in relation to each other.

“By eliminating the switching losses, the already-high efficiency of an inverter of 97.5% to 98% is increased by another 1.5%,” said Berger. “While increased inverter efficiency is valuable, the decisive benefit can be found in the higher switching frequencies of the inverter. By increasing the switching frequency of the inverter, the quality of the supply current for the electric machine is increased considerably and the system efficiency in the partial load range is increased by approximately 14%, which then has a much more substantial effect on range.”

According to Hillcrest, its inverter technology can help reduce packaging challenges by making some components smaller. “For product design, our partner Systematec has many years of automotive experience in hardware design, and we also plan to partner/collaborate with technology companies that specialize in this area, including global Tier 1 automotive suppliers and OEMs,” said Berger.

 

This article was originally published on EE Times.

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.

 

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