BSNPC Inverters In Photovoltaic Applications Using SiC And GaN

In spite of silicon's popularity as a semiconductor material, recent developments have revealed it to lack a number of advantages in terms of temperature range and frequency. New studies have shown how Silicon Carbide (SiC) and Gallium Nitride (GaN) are the ideal substitutes for conventional silicon-based semiconductors in multiple applications including temperature sensors, 3-D printing, LED bulbs, etc. Aside from that, SiC and GaN manufacturing has proven to reduce production and assembly costs. In terms of manufacturing, this is a win-win situation for both manufacturers, implying lower costs for users as well.

In the field of renewable energy, it is crucial that power generated from any renewable source reaches the appropriate end user. The energy being distributed must be effective enough to allow for optimal utilisation at minimal initial system costs. For the reasons outlined above, the efficiency of the power electronic converter is extremely important to the system's efficiency.

Ⅰ. Properties Of SiC And GaN In Photovoltaic-Based Applications

For such applications, 1200V SiC MOSFETs and 600V GaN Gate Injection Transistors (GITs) are a potential future in photovoltaic applications in the renewable resource sector. SiC converters can achieve 98.8% peak efficiency while their overall losses can be halved when compared to Si IGBTs with SiC JFETs. For the same switching frequency, replacing Si IGBTs with SiC MOSFETs can result in a 1% efficiency gain. Moreover, the synchronous

 rectification capability of SiC MOSFETs is utilised for 3-level ANPC inverters which have successfully operated at a  grid connection up to 80kHz. At a switching frequency of 16 kHz, all SiC inverters achieve a peak efficiency of 98.3% under different ambient temperatures.

Apart from this, GaN GITs were also used in experiments for implementations in a DC/DC converter for maximum power point tracking for PV applications. This converter had a peak efficiency of 98.59% at a switching frequency of 48kHz. Furthermore, devices made of GaN were capable of operating at a switching frequency of 1MHz with 96.4% efficiency at an output power rate of 1kW.

Ⅱ. Switched Neutral Point Clamped Inverter

The Bi-directional switched neutral point clamped (BSNPC) is an inverter which is derived from the half-bridge inverter topology with three different output voltage levels. BSNPCs are used in high-power transformerless applications as well as commercial products.

Fig 1(a) BSNPC switching pattern  Fig 1(b) BSNPC topology

The schematic architecture of the BPNC converter and switching mechanism is shown in figures 1(a) and 1(b). A half-bridge consists of switches S1 and S4 that are rated for VDC, and a bidirectional consists of switches S2 and S3. Positive half-wave commutation takes place between S1 and S2, which is followed by negative half-wave commutation between S3 and S4. In order to use the synchronous rectification capability of GaN GITs, S3 and S2 are completely on during the positive and negative halves of the output current. Due to the intrinsic body diode and bidirectional current capability of SiC MOSFETs, as well as the bidirectional current capability and freewheeling capability of GaN GITs, anti-parallel diodes between each device are optional. Conduction losses across bidirectional switches should be minimized by minimizing the dead time between S1, S2 and S3, S4 switches. In comparison to S1 and S4 with unity power factor operation, S2 and S3 have a critical reverse conduction performance, which significantly impacts overall conduction losses. During unity power factor operation, current flows from drain to source terminals through S1 and S4, so body diodes of devices do not conduct in normal operation.

Ⅲ. Switching Performance

For examining the switching performance of different WBG device technologies, 600V GaN GITs and 1200V SiC MOSFETs at 3kW output power were studied. Fig. 2 and 3 illustrate the turn-off and turn-on drain-source voltages (Vds), source currents (Ids), and converter output currents (Iout) of GaN GITs and SiC MOSFETs.

Fig 2 (a) Switching in S2 GaN turn-off.          Fig 2 (b) Switching in S1 SiC turn-on.  

It was observed that commutation occurs between S1 and S2 during the positive half, and soft commutation occurs between S2 and S1. As shown in Fig 6 (a), there was a hard commutation under 50ns during turn-off with an overshoot of 160V. Similarly, as shown in Fig 6 (b), the drain-source current (at the source terminal) in the turn-on region of SiC MOSFETs overshoots 24A. Hence, in comparison to SiC MOSFETs, GaN-made GITs had reverse conduction significantly lower. This makes it a great substitute to SiC MOSFETs in terms of achieving a 1200V WBG switch

Ⅳ. Analyzing Output Filter Volumes

Figure 3 compares converter power loss to the volume of the output inductor and capacitor for switching frequencies that range between 16 and 160 kHz.

Fig 3 Output LC filter volume and power loss comparison.

With a 20% ripple current and the necessary attenuation of 0.01, the inductor volume predominates the output filter volume. After around 96 kHz, the filter volume drop becomes less noticeable, and an increase in switching frequency has little effect on the filter volume. The primary causes are an increase in switching frequency. This in turn increases core volume as well as winding volume while decreasing Bmax and window utilisation factor ku.

Ⅴ. Conclusion

Inverters based on SiC and GaN were studied along with their impact on passive photovoltaic systems was analyzed. At full load, the inverter can achieve 99.2% peak efficiency and 97.33% efficiency at 160kHz. These devices have the capability to operate at high efficiency under a wide range of heat sink temperatures. Due to high heat sink temperatures and high-frequency operations, the heat sink volume can be reduced by 74% and the output filter size by a further 57%. Low inductance packages with efficient cooling options are crucial for GaN devices to improve switching speed and minimize voltage overshoots, following switching losses.

In terms of broader forthcoming research, other aspects of grid connections in photovoltaic inverters can be thoroughly researched, which includes a common-mode filters for high-frequency operation and grid-connected control of high-frequency inverters with reduced output filter inductance.