Using SiC and GaN to reduce the thickness of AC-AC converters

Recently, there have been a number of prototypes using next-generation power devices that can significantly reduce power loss. This time, Yaskawa Electric introduced the technologies and results achieved for AC-AC converters using SiC power devices and power conditioners using GaN power devices.

AC-AC converters need to be thinner in particular

The above is an introduction to the application background of SiC and GaN power devices. Next, we will introduce the details of the AC-AC converter using SiC power devices and the power conditioner using GaN power devices mentioned at the beginning.

The thin AC-AC converter with an output power of 45kW has an overall dimension of 358mm deep x 282mm wide x 29mm thick (Figure 1). The output power density is relatively high, about 15kW/L.

Figure 1: Using SiC power components to reduce form factor

The AC-AC converter has an external dimension of only 358mm×282mm×29mm (a). The circuit is mainly composed of an AC-DC converter and an inverter (b). It is actually composed of a "power substrate" on which the SiC power elements used in the AC-DC converter and inverter are installed, a "gate substrate" that drives the elements, and a reactor module and other components (c).

The AC-AC converter is composed of an AC-DC converter and an inverter (DC-AC conversion). Specifically, it is composed of a "power substrate" on which the SiC power elements constituting the AC-DC converter and inverter are installed, a "gate substrate" on which the gate circuit that drives the element is installed, a "control substrate" responsible for the overall drive of the AC-AC converter, a module equipped with multiple reactors (reactor module), a power module, and a cooler. The power substrate is equipped with a copper plate for transmitting electrical signals and dissipating heat. The back of the power substrate is installed with a power module and a smoothing capacitor that accommodates and releases the SiC power element. The cooler adopts water cooling, and is used to cool not only the power module but also the reactor module.

The power module is manufactured by ROHM and has a reduced thickness. The SiC power element is equipped with a Schottky barrier diode (SBD) and a dual-channel MOSFET. The dual-channel type has a channel structure at the gate and source. This alleviates the electric field concentration and improves the withstand voltage of the gate insulation film compared to ordinary channel-type SiC MOSFETs.

The power module adopts a "two-in-one" package, equipped with a single-arm inverter circuit, with a rated voltage of 600V and a rated current of 260A. The on-resistance is only 3mΩ, and it can switch at a high frequency of 100kHz. Compared with the two-in-one package of ordinary IGBT modules, the size is only about 1/8. The power loss of the power module is also only about 1/3 of that of the IGBT module.

Achieved an output density of 128kW/L

We use this SiC power module in AC-DC converters and inverters for AC-AC converters. In AC-DC converters, it is used to increase the switching frequency and reduce the size of passive components, and in inverters, it is used to reduce the loss of power components and reduce the size of coolers. The output density of the inverter unit is 128kW/L, which is very high.

This time, we set the switching frequency of the AC-DC converter to 100kHz, which is about 10 times the original frequency, and reduced the size of the reactor. As a result, the size of the reactor module is 186mm×126mm×27mm, which is about 1/10 of the size when the original AC-DC converter was used (Figure 2).

Figure 2: Using a small low-loss reactor module

The AC-AC converter uses a reactor module with an outer dimension of only 186 mm × 126 mm × 27 mm (a). Magnetic field analysis results show that a high magnetic field density can also form a reactor (b).

However, simply increasing the switching frequency would increase the iron loss of the magnetic material used in the reactor core, which would reduce the efficiency of the power conversion device. Therefore, the core uses the low-loss magnetic material "Liqualloy" manufactured by Alps Green Devices Note 1.

Note 1: Liqualloy is a type of amorphous alloy that has a high saturation magnetic flux density comparable to that of electromagnetic steel sheets and has low loss in high-frequency components.

GaN-based power conditioners are ready for commercialization

As mentioned at the beginning of this article, the AC-AC converter using the SiC power device is positioned as a flagship product, so there is no plan to put it into production as is. However, the power conditioner using the GaN power device is planned to be commercialized and put into production in 2014.

We have developed a household power conditioner with an output of 4.5kW. We adopted high-efficiency production technology as a prerequisite for productization, and pursued high efficiency and miniaturization of the power conditioner through GaN power elements. Therefore, although it is a research and development product, compared with Yaskawa Electric's existing products using Si IGBT, the power loss is reduced by about half, the volume is reduced by 40%, and the weight is reduced by about 27% (Figure 3). In terms of efficiency, the maximum efficiency of Yaskawa Electric's existing products is about 96.5%, while the development product has reached more than 98%.

Figure 3: GaN power devices enable miniaturization and high efficiency

The use of GaN power elements has achieved miniaturization and high efficiency of the power conditioner. The external dimensions are 350mm×200mm×150mm, which is 40% smaller than the existing products using Si IGBTs (a). The efficiency has been increased from the maximum of about 96.5% of the existing products to more than 98% (b).

There are many ways to improve efficiency, but most of them will make the main circuit more complicated and increase the number of parts. Therefore, we have adopted a simple main circuit structure as much as possible, and increased the switching frequency to 50kHz by using GaN power devices and improving the control method.

The main circuit consists of a boost chopper circuit, a single-phase output bridge circuit, and a sine wave filter. GaN power elements are used in the chopper circuit and bridge circuit. High-frequency switching reduces the size of reactors and sine wave filters. In addition, the use of GaN power elements reduces losses, thereby improving efficiency and reducing the size of the cooler, contributing to the miniaturization of the prototype.

Use products with a withstand voltage of 600V

The GaN power element used is a GaN power transistor with a withstand voltage of 600 V. GaN was used this time because it has advantages in cost and performance when used in power conditioners compared to SiC MOSFETs with the same withstand voltage.

In terms of cost, GaN power transistors, which can be manufactured on cheap 6-inch Si substrates, are more promising than SiC MOSFETs. In terms of performance, in applications with a withstand voltage of 600V, the switching loss and input capacity are smaller than those of SiC MOSFETs.

The GaN power transistor is manufactured by Transphorm in the United States. The transistor uses a two-in-one power module and is equipped with a single-leg inverter circuit. The rated voltage is 600V, the output current is 65A, and the on-resistance is 34mΩ.

Since the input capacity is less than 1/10 of that of a superjunction Si MOSFET with the same rated voltage, the scale of the gate circuit is very small. The gate circuit is also simple, mainly consisting of a gate driver IC and a gate resistor. Since the gate circuit is simple, the wiring inductance is small, and stable high-speed switching is achieved.

In the future, the company plans to reduce radiated electromagnetic noise, improve the reliability of power modules, verify system connections, and reduce losses in reactors in order to realize commercialization.