A good example of a distributed energy storage device (DESD) incorporates an isolated bidirectional DC-DC converter with 650V GaN transistors. The DESD integrates a 13.2V low-voltage Li-ion battery pack, an embedded bi-directional DC-DC converter and wireless communication system. These three parts can be packaged together, enabling it to be directly connected to high-voltage (380V) DC grid. This enables a modular approach for battery energy storage systems. Two 650V enhancement mode GaN transistors are used at the high voltage side. A 400V to 12V DC (for example, auxiliary power), 1kW converter for 1kWh DESD is shown in Figure 2, at right.
GaN power devices enable a marked improvement over a Si device. GaN transistors make performance improvements by expanding the operation range to light load, reducing switching loss and EMI, increasing the total efficiency of charging and discharging operation.
A half bridge center-tap design, with an active clamp power stage, can be designed to provide considerable benefits in low voltage and high current bidirectional power conversion. See Figure 3, at right.
GaN power switches will also extend the safe operation area (SOA) and increase efficiency over Si devices. GaN devices achieve this improved performance over Si devices due to the smaller COSS capacitance which enables lower turn-off losses; this can allow the converter to operate in a hard switching mode. Also, there is near zero reverse recovery time and reverse recovery charge of the body diode in a GaN device that shortens the turn-off time when the high voltage side switches operate in the rectifying mode.
Silicon carbide (SiC) technology provides another excellent power improvement over Si at the core of ESS solutions. SiC power semiconductor solutions can enable at least 50% more efficiency than Si and will easily handle higher grid-scale voltages. The system-level gains added performance from the efficiency, power density, and fast switching speed that SiC power devices provide.
Using a SiC module: In order to reach high switching speeds with low switching losses, a package can be designed to achieve low stray inductance for both the module and the system-level busbar design.
The current loops within Wolfspeed modules are wide, low profile, and yield even distribution between the devices, resulting in equivalent impedances across a switch position. The power terminals on the module are also vertically offset. This enables design of simple bussing between the DC link capacitors and the module, laminated all the way up to the module without requiring bends, coining, standoffs or any complex isolation. The result is a power loop stray inductance of just 6.7 nH at 10 MHz — as demonstrated in the XM3 inverter reference design. See Figure 4, at right.
With half the weight and volume of a standard 62 mm module, Wolfspeed’s XM3 power module platform maximizes power density (up to 450A) while minimizing loop inductance and enabling simple power bussing. See Figure 5, at right.
The XM3’s SiC-optimized packaging enables 175 °C continuous junction operation.
Energy storage systems are critical for the fast-growing renewable energy market worldwide. The use of Silicon power devices in such a design does no justice to the size, weight, and power of these systems as compared to using WBG devices which will also greatly improve efficiency and operate at higher voltages.