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Faust | The use of SiC and GaN in bidirectional power applications

Bidirectional switches are particularly useful in power applications such as electric vehicles, renewable energy generation, vehicle-to-vehicle communication, and energy storage. These switches efficiently control the bidirectional flow of energy, ensuring reliable and safe operation under a wide range of operating conditions. Monolithic bidirectional switches based on these technologies achieve high power conversion efficiency and are increasingly recognized as the industry standard for power electronics applications.

Two-way power flow

In the power device market, which is currently dominated by silicon devices, GaN and SIC are gradually increasing their market share. Dr. Victor Veliadis, Executive Director and CTO of Power America, said, "Both the GaN and SiC power device markets are growing rapidly, and the combination of SiC and GaN is expected to account for approximately 50% of the overall power device market by 2029, with silicon accounting for the remaining 50%. ”

Next, we will see how wide bandgap semiconductors can be successfully applied to bidirectional power applications. Bi-directional flow enables a variety of applications, including:

· Electric vehicles (Vehicle-to-Grid G2V, Grid-to-Vehicle V2G, Vehicle-to-Home V2H, Vehicle-to-Vehicle V2V)

· Distributed and grid-connected power systems using renewable energy and/or energy storage components

· Data center grid services (frequency regulation, demand management, peak shifting) and bi-directional UPS

· Solid state circuit breaker protection

· Bidirectional DC-DC converters

· Charging and discharging of energy storage systems

· Regenerative electricity (brakes, elevators, conveyor belts)

Landscape and portrait configurations

Above 900V, high-power devices are typically configured in a longitudinal orientation. The existing GaN switches on the market are transverse devices, and the spacing between the drain and gate determines their breakdown voltage. As shown in Figure 1-a, the larger the pitch, the higher the voltage that the device can block.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 1a

"This also limits the use of GaN transverse devices for high voltages. When the gate-to-drain spacing becomes very large to accommodate high voltages, the device starts to take up too much space on the wafer, reducing the yield," Veliadis said.

Existing or demonstrated SiC power devices are available in portrait orientation. The doping and thickness of the longitudinal drift layer determine the breakdown voltage of the device. A 600V device drift layer thickness is about 4 microns; Increasing the thickness to 100 μm yields a 12KV rated device (as shown in Figure 1-b). This is a great advantage when manufacturing high-voltage devices, as there is no increase in the corresponding device area.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 1b

By default, GaN power devices are typically normally open. However, for some applications, normally closed devices are preferred under fail-safe operating conditions. As shown in Figure 2, a common way to obtain normally closed GaN devices is to insert a P-doped GaN layer below the gate. It is very important that the fabrication of transverse GaN devices is compatible with the large-scale fabrication of silicon.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 2

How to make a GaN bidirectional switch

"Regarding current flow, GaN transverse devices are inherently bidirectional. It is the same regardless of whether the current is from source to drain or drain to source, because no body diode is involved. However, the blocking of this configuration is unidirectional because it is determined by the gate-to-drain spacing," Veliadis said.

One way to make a device bidirectional in terms of blocking is to make the gate-to-drain spacing equal to the source-to-gate spacing. This way, you can maintain the same voltage from either side, regardless of whether the high voltage is coming from the source or drain, because your source-gate and gate-drain spacing are the same. The disadvantage is that the cell spacing of the device has increased.

An interesting way to solve this problem and keep the cell spacing to a minimum is a double-gate structure as shown in Figure 3. Gate one is used when high voltage enters from source two, and a common drain area blocks high voltage, keeping the source-to-gate spacing small. Similarly, if a high voltage enters from source one, gate two will control the device.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 3

"The dual-gate bidirectional switch utilizes a common drain area to keep the cell spacing of the device as small as possible," Veliadis said.

Panasonic demonstrated this concept by using a normally closed double-gate monolithic GaN bidirectional switch to achieve a symmetrical 100A conductive and 1,100V blocking voltage.

How to make a SiC bidirectional switch

SiC power transistors, whether planar MOSFETs, trench MOSFETs, or JFETs, are primarily longitudinally arranged and have an internal diode that sets the conditions for symmetrical bidirectional current flow. Although symmetrical bidirectional flow is possible, blocking longitudinal devices in terms of bidirectional voltage becomes a problem.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 4a

One possible solution is to connect the two devices back-to-back in a common source configuration, as shown in Figure 4-a. This configuration allows you to control bidirectional blocking and conduction with a single gate driver. Alternatively, you can connect the devices in a common drain configuration, as shown in Figure 4-b. In this case, two independent gate drivers are required. You can have a single device with a common drift layer, as shown in Figure 4-c.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 4b

"The common source connection configuration can be implemented monolithically, meaning that two chips are connected back-to-back on the same wafer. This solution simplifies packaging and helps reduce the inductance that is critical in wide bandgap devices," Veliadis said.

Another method is to connect two SiC MOSFETs in a common-drain configuration to obtain a four-terminal switch with two different gates controlling the current flow.

Common-source inverter applications

A voltage source inverter (VSI) topology (shown in Figure 5) built from conventional silicon switches (MOSFETs, IGBTs, and diodes) has some limitations. Capacitors tend to be quite fragile and limited in temperature. In addition, high dV/dt puts pressure on the motor insulation, resulting in a common-mode phenomenon in electromagnetic interference (EMI) noise.

The Common Source Electrode Configuration (CSI) uses an inductor, a very rugged component that can withstand high temperatures. Figure 5 shows that the CSI configuration achieves very low harmonic distortion, reducing common-mode EMI and bearing currents.

Faust | The use of SiC and GaN in bidirectional power applications

Figure 5

Wide bandgap monolithic bidirectional switching enables common source inverters to overcome the limitations of VSI topologies and reduce the number of components because the diodes used in VSI topologies are no longer needed. This also results in lower conduction losses and higher efficiency and power density.

Faust Technology is deeply engaged in the field of power devices, providing customers with IGBT, IPM modules and other power devices, as well as single-chip microcomputers (MCUs) and touch chips, and is an electronic component supplier and solution provider with core technology.

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Faust | The use of SiC and GaN in bidirectional power applications