GaN HEMT’s For Medium Range Power Supplies

Power Supply Transistors

As switching frequency increases, the size of passive components like transformers and inductors decreases, enabling more compact designs. However, higher frequencies also result in greater switching losses, necessitating careful selection of transistor technology.

Image used courtesy of Adobe Stock

In power supplies operating in the 1kVA to 100kVA range, switching frequencies are typically below 100 kHz. These systems are commonly used in applications such as electric vehicle (EV) inverters, solar and wind power systems, and railroads, where the focus is on delivering high power output rather than compactness. At the lower end of the power spectrum, where power supplies operate below 1kVA, switching frequencies can range from 10 kHz to over 1 MHz. In these cases, efficiency and compact design become paramount, and GaN high electron mobility transistors (HEMTs) are increasingly the preferred choice. These transistors are particularly suitable for server power supplies, automotive DC/DC converters, and consumer power adapters, where operating voltages typically range from 100 to 600V.

Figure 1. Transistor power vs frequency application space.

GaN HEMT Overview

HEMTs represent a significant advancement in power switching devices. Introduced in the late 1990s and early 2000s, these transistors leverage a unique band structure that creates a two-dimensional electron gas (2DEG) with minimal scattering. This structural advantage allows for extremely high switching speeds and low on-state resistance compared to silicon MOSFETs. In traditional silicon MOSFETs, current flows vertically through doped silicon, leading to higher resistance. GaN HEMTs, on the other hand, utilize lateral current flow through the 2DEG as shown in the figure below. This significantly reduces resistance and enables efficient high-speed operation.

Figure 2. Vertical Si transistor structure compared to HEMT lateral structure. (https://techweb.rohm.com/trend/engineer/16113/)

GaN, as a material, is classified as a wide-bandgap (WBG) semiconductor and offers several advantages. Its wider bandgap and higher thermal conductivity compared to silicon allow for thinner layers to achieve the same breakdown voltage. Additionally, its high dielectric breakdown strength supports compact, high-voltage devices. ROHM’s GaN devices are fabricated on silicon substrates, balancing performance and cost-effectiveness to enable broad market adoption.

GaN HEMTs excel in several performance areas, including voltage handling up to 650V, industry-leading Rds × C efficiency metrics, and minimal reverse recovery time. As shown in the figure below, their switching speed is exceptionally high, and they demonstrate superior resistance stability during switching cycles. Furthermore, the compact molded packages incorporate integrated ESD protection, making them well-suited for demanding applications. While GaN devices outperform many alternatives, challenges such as oscillations, increased electromagnetic interference (EMI), and package limitations for high-current operations remain, especially at high frequencies. ROHM has addressed these issues through innovative designs, advanced material solutions, and supporting IC's.

Figure 3. Comparison of GaN HEMT to its silicon counterpart.

ROHM’s GNP20XX GaN HEMT

The ability to switch high voltages is critical for both physical size and power supply efficiency. For the same power output, higher voltage tolerance will yield lower current, in turn reducing on-state losses and minimizing physical device size. Overall, the higher voltage HEMT will have greater application across a wider variety of power supply topologies and offer competitive advantages in size and performance. To this end, ROHM has further developed its HEMT voltage capability to include discrete GaN devices up to 650V, and plans to roll out a broad portfolio over the next 4-5 years. ROHM’s GNP20XX series is a state-of-the-art lineup of GaN HEMT devices optimized for medium-power applications. These devices offer a voltage handling capacity of up to 650V and are housed in a TOLL-4A SMT package with built-in ESD protection, shown below. They are available in a range of current ratings from 26A to 69A. Their compact design and robust features make them ideal for applications such as flyback converters, power factor correction (PFC), and inverters.

Figure 4. GNP20XX package TOLL-4A and inner structure.

As shown in the comparison below, the human body model susceptibility of this package to ESD is 3.5kV, yielding a rating higher than class 2 performance. 

Figure 5. ESD performance of GNP20XX TOLL-4A package.

One of the standout features of the GNP20XX series is its dynamic Rdson performance, as shown in the figure below. Dynamic Rdson measures resistance changes during switching cycles, and ROHM’s devices boast the industry’s lowest Rdson increase within 200ns of switching, maintaining this advantage for up to 600ns. This characteristic significantly reduces losses, enhancing overall efficiency.

Figure 6. Dynamic Rdson comparison.

A common figure of merit for switching transistors is the product of the nominal on-state resistance and the input (Ciss) or output (Coss) parasitic capacitance. As shown below, ROHM’s GNP20XX devices outperform the nearest competitors by a significant margin.

Figure 7. HEMT figure of merit comparison for Ron x C.

Conclusion

ROHM’s EcoGaN™ lineup exemplifies its commitment to advancing GaN technology. By combining cutting-edge performance with sustainable designs, these devices enable greater energy savings and system miniaturization. ROHM’s leadership in GaN technology ensures best-in-class devices for high-frequency, medium-power switching applications. With a GaN lineup that ranges from 26A to 69A current capability, the company continues to set benchmarks for performance and reliability.

Figure 8. ROHM’s GaN products in TOLL package.

Beyond GaN HEMTs, ROHM offers a comprehensive portfolio of power products, including silicon and silicon carbide devices, shunt resistors for precision current monitoring, power management ICs, and integrated power modules. This extensive range supports diverse applications and enables customers to achieve optimal performance in their designs.

Figure 9. ROHM power product portfolio.

For more information, please visit:

https://www.rohm.com/products/gan-power-devices

This Industry White Paper was written by Ming Su, ROHM Semiconductor.