GaN Power Density Offers Unlimited Design Potential


This article is part of Power management Series: Dive Into Power Density

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What you will learn:

  • How GaN Helps Improve Data Center Efficiency.
  • What is LLC Resonant Converter?
  • Use of AC-DC-AC converter for transformerless inverters.

Gallium nitride (GaN) FETs have inherently superior performance than traditional silicon FETs. This performance advantage enables design engineers to push the boundaries of power designs to new levels of power density and efficiency. Applications range from AC-DC power supplies to three-phase multi-kilowatt converters.

GaN power density in the data center

GaN technology enables energy efficient power supplies for data center servers. GaN power transistors reduce the weight, size and cost of data center power designs while reducing power consumption. GaN’s high-speed switching enables market trends such as ultra-thin power supplies, integrated motor-driven robotics, and the ability to achieve power density above 200% in next-generation data centers.

High performance telecommunications and server applications require high efficiency, high power density isolated DC-DC converters. As such, LLC resonant converters appear to be an optimal choice for this use case. This converter architecture has zero voltage switching capability (ZVS) at full load, as well as low cut-off current for the primary side switches. In the case of switching frequencies lower than the resonant frequency, the synchronous rectifier (SR) devices are deactivated with zero current switching (ZCS).

Integrating the driver and protection circuits with GaN FETs will provide excellent power density for space-constrained data center boards.

GaN in power modules for the data center

In today’s 48V-based input power architecture in the data center, LLC resonant converters provide good power architecture for power supplies. The footprint of the data center card should be reduced, especially for power supplies. Power density is critical here, and where GaN power devices come in, as they offer dramatically improved power density over silicon.

48V to 12V DC-DC converter using GaN power devices offers higher frequency, better efficiencies and excellent power density. The server industry has seen the benefits of 48V power distribution, which are:

  • Much easier dispensing and delivery for higher horsepower.
  • An improved solution to powering the processor is to use a non-isolated / regulated converter to convert 48V to 1V or less for the data center processors. The best way to achieve maximum efficiency is a two-stage design with 48V to 12V, then 12V to 1V or less (or 5V to 1V or less), which powers the central processing unit ( CPU).

Why is an LLC resonant converter the best in this design to achieve optimum power density?

A quasi-resonant floating buck converter is an ideal topology to use in this application. (Fig. 1). It reduces switching losses by switching the valley FET resonantly1 without any additional components.

This design has a transformer, full bridge, or half bridge on the primary side. Synchronous rectification takes place on the secondary side. It is a relatively simple architecture and has a very high power density. The topology features multi-phase operation for higher output currents. Using the optimal transformer with high efficiency FETs leads to an efficiency of 98%, even up to 1 kW.

Changing the transformer ratio makes it easily adaptable for higher output voltages. This LLC solution is the best in size and efficiency for over 600W voltage with 48V to 12V or 48V to 5V, making it perfect for a power module (smaller than a 1/8 brick). GaN FETs in this LLC design architecture allow for a smaller and more efficient converter architecture with lower losses and a switching frequency of 1 MHz, which leads to smaller magnetism. Power density exceeds 1500 W / in.3 without the need for a heat sink.

GaN FETs are also ideally suited for soft-switched LLC resonant converters.

GaN powered LED driver

LEDs are revolutionizing the lighting industry. They are efficient, last much longer than incandescent or fluorescent lamps, and are dimmable. LEDs need DC power, but since traditional lighting is AC powered, a rectifier needs to convert AC power to DC. Power converters for LED lighting generally have to be in a small form factor and often have to withstand relatively high temperatures in the housings.

Input the GaN power transistors into a high switching frequency switching power supply, which leads to smaller magnetism. Soft switching is a bonus for this kind of designs. One design idea is to use a 600V, 1 GaN FET for the power transistor. Since GaN is capable of switching at very high megahertz rates, the sizes of the passive filter components will be much smaller, greatly improving the power density.

The design can be a 20W power converter, which is measured to have a 1.87 times reduction in size of the power stage compared to an existing commercial product while achieving an efficiency of 91.2%, a total harmonic distortion (THD) of the input current of 15.9% and power factor of 0.976.2

GaN based AC-DC-AC converter for transformerless UPS

This GaN AC-DC-AC converter The power supply design does not need a transformer and can be used in an uninterruptible power supply (UPS). The design has a common neutral between the input and output AC ports.

In each topology shown in Figure 2, the presence of a transformer designed for peak AC power limits their efficiency (typically less than 90%) and negatively affects power density. All three examples exhibit distinct characteristics in terms of switchover time between normal mode and emergency mode (also called transfer time), as well as their ability to reject voltage disturbances at the input voltage.

In an offline UPS (Fig. 2a), there will be a high change time. It offers the least protection against disturbances of the input voltage during normal operation, which is due to the fact that the inverter only works in emergency mode. This architecture is best for home use where the input voltage will only see short term disturbances without affecting the load. In this case, the grid voltage is usually of high quality and normally available.

In large industrial areas, where the power grid can be subject to quite frequent and high power, quality is important. UPS line-interactive (Fig. 2b) will be better than an offline inverter.

And finally, in areas where uninterrupted input voltage is paramount for the safety and operation of critical loads, the online UPS is best suited. (Fig. 2c). The best choice of topology for this UPS architecture is the online UPS offered with a common neutral between input and output. (Fig. 3). It has a front power factor correction (PFC) rectification stage which is followed by a voltage mode reversal stage with a single energy buffer capacitor (CBUS) connected across the intermediate DC bus. .

The in-line inverter design will provide up to 1kVA of power to the load with a PFC ranging from -0.5 to +0.5. It has the ability to operate with a 120 Vrms at a line frequency of 50/60 Hz with an AC output voltage of 120 Vrms at 60Hz.

Now, with a reasonable compromise between power density and efficiency, a design option is chosen using two GaN transistors in parallel. Figure 4 shows designs using a lower number of transistors in parallel, resulting in better power density. This is because the reduced board area occupied by the transistors and associated gate driver circuits will result in lower efficiencies as the designs are dominated by conduction loss.

Thus, to achieve a reasonable compromise between power density and efficiency, a design option with two GaN transistors in parallel was chosen. A prototype 1 kVA GaN-based, non-electrolysis in-line inverter was designed, built and tested. The prototype inverter achieved a peak efficiency of 95.2% and maintained a high efficiency of over 92.3% over the entire output power range. The prototype non-electrolysis in-line inverter achieved a power density of 26.4 W / in3.

Summary

This article has only presented a few power design architectures using GaN power FETs. A plethora of examples of GaN power density designs exist in the references, as well as online, for the reader to consider. New articles and application notes are created almost daily regarding power density and GaN.

Read more articles in the Power management Series: Dive Into Power Density

The references

1. GaN in practical applications.

2. “GaN based high efficiency and power density LED driver”, Texas Instruments

3. “GaN-based high density AC-DC-AC converter for transformerless single-phase in-line uninterruptible power supply” IEEE transactions on power electronics, Flight. 36, n ° 12, December 2021.

The quest for GaN power density continues ”, Summary of microwave products (mpdigest.com).

“TI brings the power of GaN technology to Delta Electronics’ energy efficient server power supplies for data centers,” Yahoo Finance.

“Design of a 1 kW GaN PFC stage with more than 99% efficiency and 155 W / in3 Power density ”, IEEE 2017.

“A high efficiency, high power density, 1 MHz LLC converter with GaN devices and integrated transformer,” IEEE 2018.

“A 6-level flying capacitor multi-level converter for single phase buck type power factor correction”, IEEE 2019.

“An E-Mode Monolithic GaN Offline Hysteretic Buck Converter, 15W 400V, with 95.6% Efficiency,” 2020 IEEE International Solid-State Circuits Conference, IEEE 2020.


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