Enabling High-Density Multi-Kilowatt Grid Converters w/ GaN

Grid converters manage the flow of power between the electric grid and loads, as well as energy storage and generation sources. Their efficient operation and physical size have become more important recently for a couple of reasons. First, any inefficiency has a direct impact on operating costs, as well as cooling and thermal management of the installation. Second, as these converters become more mainstream, their physical size and form factor plays an important role in purchasing decisions. After all, no one wants a bulky converter taking up room in their garage or facilities.

For decades, converters designed with insulated gate bipolar transistors (IGBTs) and silicon metal-oxide semiconductor field-effect transistors (MOSFETs) served the industry well. These converters have reached a performance plateau, however. When the first generation of silicon carbide (SiC) FETs entered the market a number of years ago, they provided a much-needed boost to traditional converters by improving their efficiency and power density. Now, with gallium nitride (GaN) as a mainstream contender, many designers are further exploring ways to achieve new levels of performance and system cost never possible before. Let’s find out how.

Market trends and challenges

As shown in Figure 1, grid-tied converters are found in many applications and end equipment such as solar, electric vehicle charging infrastructures, energy storage, and industrial and telecommunication power supplies.

There are three common trends among these applications:

· The need for higher efficiency and power density. Most applications target >99% efficiency to reduce the installation footprint and energy costs. Also, due to the high adoption rate in consumer and smaller industrial applications, physical size (that is, power density) is a key factor.

· Simplified cooling. In most applications, convection cooling is the best approach. Alternative or traditional methods such as fans or liquid cooling increase both installation and maintenance costs. Convection cooling also makes these solutions more consumer-friendly by eliminating fan noise in the environment.

· Lower manufacturing costs. Traditional designs with large or through-hole power modules and passives can only reduce solution costs so much. Designers needs smaller and surface-mount active and passive components to simplify production and lower manufacturing costs.

Traditional grid converters

Traditional converters, as shown in Figure 2, typically employ high-voltage IGBTs in a standard three-phase half-bridge topology with switching frequencies at 20 kHz or lower. Given the low switching frequency, these designs require fairly large magnetic components. Consequently, the open-frame power density tends to be low – typically around 70 W/in³ (open frame number).

Click image to enlarge

Figure 2: A typical traditional grid converter

Multilevel converters to the rescue

Many designers have opted for multilevel topologies to improve efficiency and power density. Multilevel inverters enable the use of lower-voltage power devices with better switching characteristics, therefore achieving better overall system performance. Figure 3 shows a high-performance multilevel converter.

Click image to enlarge

Figure 3: A flying capacitor four-level (FC4L) bidirectional grid converter with GaN

Multilevel topologies bring four key benefits to grid converters:

· Better system switching figures of merit (FOMs): These converters enable designers to use lower-voltage devices with superior FOMs, such as GaN or SiC, versus traditional devices such as IGBTs.

· Lower system costs: This includes both manufacturing costs, reduced through the use of surface-mount devices, and also significantly reduced electromagnetic interference components, magnetic filter size, and cooling.

· Better thermal distribution: This is particularly important in applications using convection cooling.

· Higher system density: Due to the higher switching frequency of these converters, there is a significant savings in the size of passives as well as heat sinks throughout the system.

Using GaN in multilevel converters

In 2018, Siemens and Texas Instruments (TI) jointly demonstrated the first 10-kW cloud-enabled grid link with GaN (Figure 4). TI will also demonstrate its own convection-cooled 5-kW platform at the Applied Power Electronics Conference (APEC) March 15-19, 2020, as shown in Figure 5.

Click image to enlarge

Figure 4: A 10-kW grid converter demonstrated by Siemens and TI in 2018

Click image to enlarge

Figure 5: TI will demonstrate this 5-kW convection-cooled bidirectional grid converter at APEC 2020

Both solutions demonstrate how GaN-based multilevel converters can surpass not only traditional IGBTs but also SiC in efficiency, power density and solution cost. GaN’s superior switching performance enables designers to increase the switching frequency while minimizing overall losses in the system. Table 1 compares the three power technologies.

Click image to enlarge

Table 1: Comparing power devices in grid converters

As I mentioned earlier, GaN not only enables designers to achieve higher levels of efficiency and power density, but also provides the lowest solution cost compared to alternative topologies based on SiC.

Figure 6 provides a detailed cost breakdown for using SiC and GaN to achieve 99% in efficiency at 5kW. As evident here, inductor cost is one of the major factors in deciding between topologies. Due to its higher switching frequency and smaller voltage steps, GaN enables 5x reduction in magnetics and their associated cost in grid converters.

Click image to enlarge

Figure 6: Grid converter relative cost breakdown

Figure 7 illustrates the overall measured efficiency of the 5-kW convection-cooled grid converter that TI will show at APEC 2020. This three-phase bidirectional multilevel converter is designed using TI’s 50-mΩ 600-V LMG3410R070 GaN FET with integrated driver and protection and its C2000 real-time control microcontroller.

Click image to enlarge

Figure 7: Measured efficiency of TI’s 5-kW grid converter

Summary

In the past, power engineers had limited device and topology choices when designing grid converters. However, with the wide availability of GaN devices on the market at very competitive prices, new and viable options have emerged. GaN-based converters enable solutions with 300% more power density of IGBTs and 125% more power density of SiC devices.

Furthermore, TI GaN devices are backed by over 3 million hours of device reliability testing and well over 3 gigawatt hours of power conversion.

Texas Instruments