GaN/SiC for Your Next Design - Fight or Flight?

28 Mar 2023

GaN/SiC for Your Next Design – Fight or Flight?

As a technical consultant, I have seen various new technologies implemented in both new and old applications. In the semiconductor industry, wide band gap (WBG) devices such as Silicon carbide (SiC) and Gallium Nitride (GaN) transistors have been gaining attention due to their small size, fast speed, and better thermal performance. The introduction of these new semiconductors into the consumer market came after a series of military and other commercial applications in everything from electric vehicles to radar systems.

GaN devices have enabled a much better form factor for product design than their Silicon counterparts, and as they become cheaper (Moore’s law states that as a device becomes smaller, it also becomes cheaper), we expect to see them widely adopted in power-switching modules worldwide.

But new technology often serves as a double-edged sword. The most significant advantage of a GaN device (superfast switching, say 100V/ns) also brings a challenge for controlling the EMI. As we all know, the faster the switching action (i.e defined by the rise time), the harder it is to contain the EMI, especially above the frequency of 1/πt, where t is the rise/fall time of a switching event.

“We swapped the GaN with a Silicon, and we passed the EMC”

Many engineers have chosen to swap the GaN device with a Silicon MOSFET to pass EMC tests, considering time-to-market is often critical for companies to profit. But this defeats the spirit of making a higher efficiency product. Facing greater EMI challenges, many engineers choose to “flight” rather than “fight” under time and cost pressure.

In other cases, engineers have chosen to use silicon MOSFETs based on trade-off calculations in the design. For instance, if using a GaN device results in requiring an additional filter to pass EMC, it is not a good idea as the filter would add cost and weight.

General EMC Design Rules for WBG Devices

To fully realize the potential of WBG (Wide Band Gap) devices, such as GaN (Gallium Nitride), it is essential to apply state-of-the-art EMC (Electromagnetic Compatibility) design right from the design stage. As explained, any attempt to optimize the filter will inevitably prove to be bulky and costly.

Technically speaking, GaN semiconductors are high-electron-mobility transistors (HEMTs), meaning they do not have the doped region in a PN junction like MOSFETs, this enables faster electron flow, hence higher switching speed. Because HEMTs do not have the PN structure, they also do not have a body diode. In applications such as motor drives, we can switch on the HEMT for free-wheeling rather than relying on the body diode. Traditionally, the EMI issues associated with the reverse recovery charge of a body diode during the deadtime can be a problem. To fix the issues, engineers often place a Schottky diode in parallel with the MOSFET as Schottky diodes switche faster and do not have reverse recovery charge. Now that the switching speed of a GaN is faster than a Schottky diode, and it does not have reverse recovery charge either. The HEMT has a “quasi diode” mode in the deadtime region, and we need to control the deadtime well.

On the layout, depending on the topology, minimizing the loop size and reducing the switch node should be the design engineers’ top priority. Loop areas are associated with near-field magnetic energy, while switch nodes determine the electric field intensity. Engineers may need to use thermal vias on the PCB to reduce the switch node.

Snubber circuits can be introduced to reduce the overshoot and ringing. There are software approaches to reduce the EMI, such as the spread spectrum technique, which is often used in a power factor correction application. When designing the power switching converter topology, using a soft switching-based control method is preferred when using faster switching devices.

This short article does not cover the EMC design considerations in detail. But readers are welcome to get in touch regarding EMC design topics.

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