How SiC and GaN technology is affecting passive component development

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The power conversion and motor drive industries have recently benefited from several new semiconductor process technology innovations. These include the development of wide band-gap materials (WBG) silicon carbide (SiC) and gallium nitride (GaN). Compared to conventional silicon-based semiconductors, a wide band-gap semiconductor exhibits up to four times the energy required, measured in electron volts (eV) for an electron to cross the 'gap' between the non-conductive valence and conduction bands.

There are several key benefits of using wide band-gap materials in semiconductors, the most important of which is that they can operate at much higher voltages than their silicon counterparts. Also, they have better thermal characteristics that allow WBG devices to operate at higher temperatures. Another critical aspect of WBG semiconductors is that they are capable of much higher switching frequencies, a crucial factor in the design of many switched-mode power converters, and motor control gate drives.

Both SiC and GaN devices are now widely available in the market and are responsible for advancing the next generation of power conversion and motor drive applications. Although sharing many similar benefits, including much smaller physical package sizes, there are a few differences in the type of use cases they suit.

SiC WBG transistors have a higher voltage capability, typically 1200 V and above, and are fast replacing IGBTs in industrial motor drives, DC/DC converters, and photovoltaic inverters. Their switching losses are lower than silicon devices and are capable of much higher switching frequencies, although not as high as GaN. Since switching losses are smaller, SiC devices aid a more energy-efficient conversion process, generating less waste heat.

By comparison to SiC, GaN has taken a little while longer to gain market traction. Still, it is making good progress not only for power conversion and motor drives but in other high-power sectors such as long-range radar and RF broadcast systems. They too are challenging MOSFETs and IGBTs in high voltage applications up to 650 V, although not as high as SiC devices. Capable of switching up to x1000 faster than silicon discretes, they also significantly lower switching and conduction losses, providing a high energy-efficient conversion or drive function.

Figure 1 - WBG semiconductors - the big 3 for WBG - (Source KEMET)
 

WBG devices challenge passive component capabilities

To keep up with the semiconductor process innovations, SiC and GaN present some unique challenges to the essential passive components used in power conversion and motor drive circuits. A key benefit of the higher switching frequency capability is that the value, and hence the corresponding size of inductors and capacitors can be considerably smaller. Together with the smaller footprint SiC and GaN devices require, smaller passive components mean that the overall size of a converter or drive is significantly smaller. Low conduction (Rds-on) losses result in less heat dissipation requiring thermal management, make the solution more energy efficient and further reduce the overall system size.

However, the nature of many WBG devices’ other benefits means that many capacitors and inductors previously used in power circuits will no longer be suitable.

Capacitors will need higher working voltages and to be capable of handling high dv/dt transients, even though their size is smaller due to the higher switching frequencies. Another critical aspect of the higher switching frequency is that the equivalent series resistance (ESR) and the resulting internal self-heating characteristic has a notable impact on performance. Also, lead lengths, particularly for film capacitors, introduce an equivalent series inductance (ESL) to complicate circuit design further.

Ceramic capacitors are proving to be an excellent capacitor for use in WBG-based designs. An example is the CeraLink series from TDK which is designed specifically for use as DC-link and snubber capacitors and is constructed of a unique ceramic material using a multi-layer (MLCC) approach that provides a high capacitance density. DC-link capacitors used in power conversion and inverter drive applications typically have a high capacitance value and are physically larger, often occupying a significant area of board space.

Figure 2 - TDK CeraLink ceramic capacitors for use in DC-link and snubber circuits 

By contrast, TDK's CeraLink capacitors are smaller, and they offer an excellent thermal management capability that allows them to operate up to + 150 degrees C, making them ideal for placing next to high-temperature WBG semiconductors.

Another product example is the KC-Link series of automotive-qualified multi-layer ceramic capacitors from KEMET that feature a high-frequency operation up to 10 MHz, and extremely low ESR and ESL characteristics.

Inductors, transformers, and other magnetic components will also reduce in size thanks to the higher switching frequencies. Losses within any inductive part increase proportionally with the frequency, so new inductor material innovations will also be necessary for circuit designs to take full advantage of the benefits of WBG semiconductors. An example is the compact and shielded Vishay IHLP-0H series of low-profile inductors. Capable of handling up to 45 A, the series offers low inductance values from 0.10 µH to 1.0 µH that exhibit extremely low loss and DC resistance characteristics, suiting high frequency conversion applications up to 10 MHz.

SiC and GaN technologies are advancing power conversion and motor drive applications considerably in terms of power efficiency and PCB footprint. Passive components are now coming to market with the specifications necessary to fully exploit these wide band-gap semiconductor innovations. To learn more about how to optimise your Wide Bandgap applications, register for our webinar, where KEMET FAE Pranjal Srivastava will discuss the design challenges in more detail.