What Does “99% Power Efficiency” Really Mean in Silicon Carbide MOSFETs?

There's a good reason that analysts expect silicon carbide (SiC) MOSFETs to boom in sales over the next few years. Specifically, Allied Market Research anticipates this sector to surge to $1.1 billion dollars by 2025 with a CAGR of 18.1% from 2018 to 2025. 

SiC MOSFETs have major advantages over competitive technologies. They can operate in temperatures of 200°C, reduce switching loss, and increase power conversion efficiency with high thermal conductivity.

Factors that affect SiC adoption. Image used courtesy of Allied Market Research

These characteristics make SiC MOSFETs an apt candidate for a broad number of applications, including:

• Electric vehicle (EV) charging
• Data centers
• Energy storage for solar and smart grid systems
• Uninterruptible power supplies (UPS)
• Portable power supplies
• Industrial motor control and drives

What Do We Mean by "Power Efficiency?"

Whenever we discuss power semiconductors, the feature that often takes center stage is power efficiency. But what exactly do we mean by that term?

When power conversion takes place within a power supply unit (PSU)—whether it is a DC-DC conversion or a DC-AC inversion—100% efficiency can never be achieved. There is always a loss in the power conversion process in the form of heat. Therefore, a 95% efficient power supply will have a 5% heat generation. A 1% efficiency increase translates to a 10% reduction in heat, which can be significant. 

A more efficient power supply will need a smaller heat sink and smaller components. This will reduce the total billing-of-material (BOM) cost of the unit. 

Over the years, the power industry has developed a de facto standard called "80 Plus" to measure the efficiency of power conversion. The most stringent requirement is the 80 Plus Titanium standard which requires up to 94% efficiency for 115 V applications and 96% for 230 V applications.

To help device manufacturers achieve this standard, many semiconductor companies started introducing a new version of SiC MOSFETs.

Wolfspeed Touts 99% Power Efficiency

Wolfspeed, a Cree company, is among the first, if not the first, to introduce third-generation SiC MOSFETs in the 650 V range. Packaged in a through-hole, industry-standard TO-247, the product family supports a maximum current rating of 120 A.

Packaging of C3M0015065K. Image used courtesy of Wolfspeed

For example, the C3M0015065K supports the TO-247-4 (4-pin), 120 A configuration while the C3M0015065D supports the TO-247-3 (3-pin), 37 A configuration. Surface-mount (TO-263-7) packaging is also available. The product operates over a wide temperature range from -40°C to +175°C.

Low Drain-to-Source Resistance

To reduce switching losses and increase overall system power efficiency, the SiC MOSFETs should have low drain-to-source resistance commonly referred to as on-state resistance (R_DS(on) at 25°C). A high value will mean high conduction loss with heat generation.

Graph of how C3M0060065K compares to a competitor in a boost converter. Image used courtesy of Wolfspeed

Wolfspeed is able to achieve 15 mΩ and 60 mΩ (R_DS(on) at 25°C) at 650 V depending on the product type. To achieve the 99% power efficiency, the products are able to maintain R_DS(on) at 79 mΩ at 175°C with 60 mΩ at a lower temperature range.

Low Reverse-Recovery Charge

Another important parameter of the devices is a low reverse-recovery charge (Q_rr), which reduces switching losses and can reduce the size of PSU components such as transformers, inductors, and capacitors. The Q_rr of the Wolfspeed 60-mΩ MOSFET is 62 nC.

Output Capacitance

The third parameter is output capacitance, C_oss. A low value will reduce parasitic capacitances and switching losses as the switching frequency increases. Wolfspeed’s SiC MOSFETs have 80 pF (60-mΩ models) and 289 pF (15-mΩ models) accordingly.

The SiC MOSFET Market Heats Up

Other manufacturers like Microsemi/Microchip, ST Microelectronics, and Infineon have recently offered similar products. This is good news for customers since they will now have many choices.

As depicted below, most products offer similar specifications without publishing the power efficiency percentage—except for Infineon.

Comparison of power SiC MOSFET semiconductors. Note that several companies do not publish power efficiency in product specifications.

They specify a 98% power efficiency with higher on-state resistance R_DS(on) (at 25°C). They also feature lower reverse-recovery charge (Q_rr) and output capacitance (C_oss) values compared to Wolfspeed.

It is important to note that as the overall power efficiency is approaching 100%, pushing power efficiency becomes a major product design challenge.

When testing the power SiC MOSFET semiconductors, make sure they are tested in a controlled environment against the specification provided by the vendors because each manufacturer may have tested their products in a different way.

Featured image used courtesy of Wolfspeed.

What's your hands-on experience with SiC MOSFETs? What are their strengths and limitations? Share your thoughts in the comments below.