Industry Article

What Do Higher Voltages Mean for SiC Devices?

May 19, 2024 by Didier Balocco, onsemi

In this article, onsemi looks at higher voltages and what they mean for SiC devices.

With its higher electron mobility, lower losses, and ability to work at higher temperatures, the case for silicon carbide (SiC) devices in challenging power applications is widely accepted. While the benefits over silicon are significant in key applications, further advancement is required, especially to reduce size and cost while improving efficiency. 

In the automotive and renewable sectors, there is a trend toward increasing voltages to meet these goals. However, to achieve this, SiC devices capable of operating at these voltages must be freely available.

 

Image used courtesy of Adobe Stock

 

Ohm’s Law

The move to higher voltages in a bid to increase efficiency has its foundations in the most fundamental law in physics, Ohm’s Law. This law tells us that losses increase with the square of current, so reducing current will benefit efficiency. Ohm’s Law also tells us that, for the same power, if we want to halve current, we have to double the voltage.

By halving the current, the static losses due to conduction losses in semiconductors and cabling are reduced by a factor of four. This is why electricity grids operate at extremely high voltage levels.

While it might then seem obvious that industrial and automotive applications move to extremely high voltages, this is not practical due to the availability of components supporting such voltages.

 

Power Device Switching

In a switching power converter, a power device has three roles: blocking, conducting, and switching between the two previous roles. As voltage increases, the challenge is with the blocking aspect, as high voltages will appear across the device. Without the appropriate construction and materials, a catastrophic failure could occur.

 

Figure 1. During switching, MOSFETs are required to block large voltages between their drain and source. Image used courtesy of onsemi

 

Therefore, to access the efficiency benefits of higher voltage operation, SiC devices with higher blocking voltages across their drain and source pins (VDS) will be required. Currently, many devices have 650 V capability, and 1200 V-rated devices are becoming more common. However, higher voltage-rated devices will be needed to address these higher voltages, with an appropriate design margin to ensure reliability.

 

Applications Moving to Higher Voltage Operation

Greater efficiency will benefit applications operating at higher power levels. Wind and solar power generation are moving to higher voltages. With solar photovoltaic (PV) systems, the DC bus voltage from the PV panels has increased from 600 V to 1500 V to enhance efficiency.

Greater efficiency simply means that more natural solar or wind energy is available to be used immediately or stored for later use. While both forms of energy may appear to have infinite capacity, they are both subject to the vagaries of weather, which can limit output.

Also, if the system is more efficient, it will be smaller and lighter, which is a significant benefit for roof-mounted installations.

 

Figure 2. Renewable energy and automotive powertrains are two applications moving to higher voltages to enhance efficiency. Image used courtesy of onsemi

 

One of the reasons often cited for slower-than-expected growth in EVs is the time taken to charge and the limited battery range. Automakers are replacing 400 V battery strings with 800 V versions to address this. Charging speed is determined by the charger’s output power, which is limited by the system voltage and output current. While increasing the current reduces charging times, it also increases the heat generated and system energy loss, reducing the charger's efficiency and elevating the cooling demands. Alternatively, the EV charger's power output is significantly increased by increasing the voltage and maintaining a similar current level. This lowers the vehicle’s charging time without substantially raising the thermal considerations or reducing system efficiency.

The reduced current and enhanced efficiency will reduce the size, cost, and weight of the onboard charger (OBC), as thinner cables can be used and less heatsinking is required. As the OBC remains on the vehicle, any reduction in weight will result in an increase in the vehicle's range.

Looking to the future, as electric propulsion moves into commercial vehicles, the batteries will be significantly larger, and more power will need to be transferred efficiently to charge in a reasonable time. As a guide, the Megawatt Charging System (MCS) is rated for a charge rate of 3.75 MW - 3,000 A @ 1,250 VDC.

 

SiC Breakdown Voltages

One of the challenges in developing semiconductor power devices in vertical structures with higher breakdown voltages is that an increase in RDS(ON) causes a corresponding increase in conduction losses. The drift layer is generally thicker in devices with higher withstand voltages, which leads to an increase in conduction losses.

So, while increasing operating voltage has efficiency benefits, the corresponding increase in conduction losses within the MOSFET will negate some or all of these benefits.

However, SiC-based power devices can offer higher breakdown voltages with a substantially thinner drift layer than the equivalent silicon device, resulting in lower forward voltage drop and reduced conduction losses.

 

A Modern High-Breakdown SiC MOSFET

With the need for MOSFET devices with breakdown voltages in excess of the operating voltage, onsemi has developed a range of new SiC MOSFETs specifically for such applications.

Onsemi’s NTBG028N170M1 is an N-channel planar EliteSiC MOSFET for high-voltage fast switching applications with a VDSS of 1700 V and extended VGS of -15/+25 V. The device supports drain currents (ID) up to 71 A continuously and 195 A when pulsed.

 

 

Figure 3. NTBG028N170M1. Image used courtesy of onsemi

 

The robust device mitigates conduction losses with an excellent RDS(ON) value of just 28 mΩ (typ.), while an ultra-low gate charge (QG(tot)) of just 222 nC ensures low losses during high-frequency operation. The device is housed in a surface mountable D2PAK–7L package, which reduces parasitic effects during operation.

Supporting the MOSFETs, onsemi offers a range of 1700 V-rated SiC Schottky diodes that enhance the voltage margin between VRRM and the peak repetitive reverse voltage of the diode. In challenging applications, the 1700 V diodes will deliver lower VFM, maximum breakdown voltage, and excellent reverse leakage current even at high temperatures, equipping design engineers to achieve stable high voltage operation at elevated temperatures.

 

Efficiency is the Ultimate Goal

In every area of power electronics, efficiency is a primary goal. Not only does it reduce operating costs, it also allows for smaller, lighter, lower-cost designs. With highly efficient semiconductors already commonplace, designers are looking to other areas of the system to deliver more efficiency.

Increasing the system voltage reduces the current and significantly reduces losses. However, this has proved challenging until now, as increasing the breakdown voltage of MOSFETs has also increased their conduction losses.

The latest MOSFET devices from onsemi are based on SiC, where the breakdown voltage can be increased without a pronounced effect on the conduction losses. As a result, devices such as the NTBG028N170M1 offer 1,700 V VDSS capability while maintaining low loss performance.