Introduction

As global demand for photovoltaic (PV) systems grows, manufacturers face mounting pressure to reduce costs without compromising reliability. Innovative technologies are critical to achieving these objectives, especially for commercial and residential PV systems. These systems generally fall into two main configurations: micro-inverters and string inverters.

Micro-inverters, which use an inverter for each panel, enable each panel to operate at its peak energy potential. However, their cost can be prohibitive due to the need for an AC converter in every module. On the other hand, string inverters connect multiple panels to a central inverter. While cost-effective, string inverters suffer from reduced energy harvesting capability when shading or other factors impact individual panels.

To bridge this gap, optimizers have emerged as a cost-efficient solution that rivals the energy harvesting capabilities of micro-inverters. These power modules optimize energy harvesting for each panel while maintaining compatibility with existing string inverters. This post explores the role of optimizers, the advantages of GaN FETs in their design, and the potential of solutions like the EPC9178 reference design.

String Inverter Energy Harvesting Challenges

A typical string inverter system connects the DC outputs of multiple PV panels in series, feeding the combined current into a central inverter. This setup converts the DC voltage energy to AC current for delivery into the grid. However, shading or uneven solar insolation can significantly reduce overall energy harvesting efficiency.

The challenges become evident when examining individual panel voltage-current-power characteristics, as shown in Figure 1. Variations in shading result in reduced current output, leading to a mismatch in power contribution among panels. This mismatch translates into substantial energy losses, highlighting the need for optimizers to enhance system-level energy harvesting.

Optimizer Overview

An optimizer is a DC power converter installed between a PV panel and the series string connection to the central string inverter. Its primary functions are:

1. Maximum Power Point Tracking (MPPT): ): Ensuring the attached panel operates at its peak energy output.
2. Constant Power Delivery: Maintain maximum power delivery by adjusting the output voltage while matching the inverter’s current demand.

The most popular topology for an optimizer is the back-to-back buck-boost converter (Figure 2). This topology demonstrates high efficiency regardless of the desired voltage conversion – whether stepping up, stepping down, or maintaining a voltage level close to the input.

The optimizer dynamically adjusts panel voltage and current to align with the string inverter's MPPT algorithm. It operates in three modes:

1. Constant Power Mode: Optimizer’s normal operating mode in the absence of output voltage or current constraints, yielding the maximum power output.
2. Constant Current Mode: Activated when the string inverter draws maximum current.
3. Constant Voltage Mode: Engaged when the inverter draws minimal current.

This interplay ensures optimal energy harvesting while adhering to the string inverter's operational constraints.

The EPC9178 Demonstration Board: A Case Study

The EPC9178 is a versatile back-to-back buck-boost converter specifically designed for PV optimizers. It operates within an input voltage range of 30 V to 60 V and offers selectable output voltage settings of 30 V, 45 V, and 60 V. Key features include:

• Compact Design: The EPC9178's high-frequency operation (450 kHz) minimizes the size of passive components, such as inductors and capacitors, resulting in a lightweight and compact form factor.
• High Efficiency: With a peak efficiency of 98%, the EPC9178 demonstrates industry-leading performance for solar applications.
• Advanced GaN Technology: The EPC9178 employs 100 V rated EPC2306 eGaN FETs with a very low 3.8 mΩ on-resistance, enabling low power loss operation with improved thermal management compared to silicon MOSFETs.
• Simplified Control: The inclusion of the LM5177 controller from Texas Instruments integrates gate drivers and control logic, streamlining the design and reducing component count.

Performance Results

The EPC9178 was experimentally evaluated across typical PV panel voltage ranges. The input of 30 V, 45 V and 60 V with a fixed output of 30 V were chosen to demonstrate the EPC9178’s efficiency and power losses. As illustrated in Figure 3, the converter achieved a peak efficiency of 98%. However, for the 60 V input, thermal and current limitations were encountered.

The Role of GaN FETs in PV Optimizers

Gallium nitride (GaN) FETs are revolutionizing power electronics, offering advantages such as:

• Lower Conduction Losses: Due to low on-resistance.
• Reduced Switching Losses: Enabled by low output capacitance.
• Thermal Management: Simplified by reduced power dissipation.
• Reduction in cost: By operating at higher frequency, the size and cost of magnetic components can be reduced.

These characteristics make GaN FETs an ideal choice for hard-switching applications like PV optimizers. Their integration into designs such as the EPC9178 demonstrates the potential to achieve higher performance while reducing system size and cost.

Conclusion

By leveraging eGaN® FETs and a dedicated ASIC controller, the EPC9178 achieves exceptional efficiency, compactness, and reliability. These attributes are vital for PV systems as the industry moves toward more cost-effective and efficient renewable energy solutions.

As the renewable energy sector evolves, GaN technology is poised to remain at the forefront of innovation, delivering solutions that balance performance, reliability, and affordability for a brighter, greener future.

For more information including design files for the EPC9178 visit: www.epc-co.com/epc/products/evaluation-boards/epc9178.

For inquiries or to discuss how our solutions can transform your PV projects, contact our team today at Ask a GaN Expert.

Parinda Chantarasereekul, Application Engineer