Micro Inverters Achieving an optimized design for a 400W solar micro inverter

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This article summarises the findings of a paper presented by CEA Tech at the PCIM Digital Days conference, 2020; these describe an evaluation of different wide-band-gap semiconductor-based designs for compact solar micro inverters.

Standard photovoltaic (PV) power arrays comprise PV modules connected in series. However, the power of such a series chain can be significantly reduced as the modules become unbalanced in terms of their operating conditions. This is due to factors including radiation, shade, orientation, ageing, dirt, and temperature.

These factors can be mitigated, and the solar farm’s yield improved, by tracking the maximum power point at each PV module, rather than at string or multi-string level. This approach also has built-in redundancy; as such, it can ensure continued power availability even if a module’s power electronics fails – an improvement over using a single centralized inverter.

Accordingly, there is a trend to integrate the energy conversion function (DC-DC or DC-AC) into the PV module junction box. This trend, along with the emergence of bifacial cell technologies – where both sides of a solar panel perform PV energy collection – is creating new challenges for compact micro-inverter design. These challenges can be addressed by using designs based on high performance wide-band-gap (WBG) semiconductors such as gallium nitride (GaN) or silicon carbide (SiC).

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Micro-inverter evaluation

One practical example of a micro-inverter using WBG devices has been identified by a research team at CEA Tech in France; it comprises a 400W WBG-based grid tie micro-inverter equipped with a current fed full bridge DC-DC and a full bridge DC-AC inverter. The evaluation that led to this identification is fully described in CEA Tech’s paper ‘A compact high-efficiency GaN based 400W solar micro inverter in ZVS operation’ presented at PCIM’s Europe Digital Days conference in July 2020, while the key points are discussed below.

Several DC/DC topologies (Interleaved flyback topology, push-pull topology, single switch forward topology, interleaved forward with series connected output topology, half-bridge LLC resonant converter topology, full-bridge LLC resonant converter topology, current fed half bridge converter topology and current fed full bridge converter with active clamp topology) were considered in selecting the most suitable topology for the GaN based micro-inverter design.

The comparisons were based on efficiency, cost, compactness, robustness, and complexity of the system. As a result, the three most suitable topologies for further study were identified as:

• FED: Micro-inverter with a current fed full bridge DC/DC converter + H4 topology DC/AC

• FLY: Micro-inverter with a Fly-back converter with active clamp (single stage)

• LLC: Micro-inverter with full bridge LLC resonant DC/DC converter + H4 topology DC/AC converter

Estimates showed that the efficiencies of the FED, FLY and LLC designs are 95.5 per cent, 95.8 per cent, and 95.6 per cent, respectively, while the Bill of Materials (BOM) of the FED is the lowest, and that of the FLY is highest. LLC and FLY were estimated as the most and least compact solutions respectively.

The team then considered the technologies’ figures of merit for complexity, as applicable to various aspects of their design. The results are shown in Fig.1, where the smaller the area, the lower the complexity.

Note that FLY and LLC are resonant topologies that require precise inductor and capacitor components, and a more complex control strategy accordingly.

Fig.1 also shows that LLC is the most complex topology, while the difference between FLY and FED is relatively small and insignificant. However, the comparison also shows that FED is the least complex topology in terms of control strategy and the precision of the capacitor and inductor power components required to ensure resonant performance. This is critical as the micro-inverter works at high temperature.

Next, the risk of failure, or robustness, was assessed by calculating various voltages, currents and frequency tolerance for each candidate. Fig. 2 shows the results. It reveals that FLY has more risks than LLC and FED. Additionally, LLC has more risk of failure than FED due to its required precision of frequency.

Finally, the team made a comparison between the three selected topologies based on efficiency, cost, compactness, robustness, and complexity, as shown in Fig.3. In this figure of merit, FED and FLY topologies are better than LLC. Among these three selected topologies, the interleaved flyback design is the most widespread within solar inverter industrial products.

Results

Results show that the three structures are close in terms of their chosen comparison criteria. However, having weighed the advantages and disadvantages of each of these three converters, the FED structure was finally chosen, as it shows more benefit with the largest margin for progression for the GaN technology, particularly in terms of compactness, reliability, and the possibility of increasing the switching frequency. The selected micro-inverter combines a current fed full bridge DC/DC converter for the first stage and a full bridge DC/AC inverter for the second stage.

The operating parameters selected for the micro-inverter were:

• 25V to 65V input voltage, based on a 72 cell PV module

• 400W power

• 230V_RMS 50Hz output

• EU ponderated target efficiency of 95.5 per cent

The paper also presents a simulation of the grid-tie micro-inverter, and details of an experimental validation.

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Conclusion

Today, the solar micro-converter is a mature product. It has enjoyed continuous development for about 20 years, with most of the technologies implementing a flyback converter for the DC-DC high step up stage.

The current fed full bridge DC/DC topology with a full bridge DC/AC inverter combined with WBG semiconductors, as evaluated in the CEA Tech paper, is an interesting and technologically differentiated way of shrinking the size of the power converter.

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