Improving Residential Solar System Designs

­The deployment of residential solar systems, which is expected to proliferate over the next five years, will provide many benefits to users, including providing a reliable, clean, green energy source to power home appliances, charge electric vehicles and transfer power back to the grid. This article provides an overview of the main components of a residential solar system. It also discusses the benefits of a range of power solutions from onsemi, which can help to make solar systems more efficient, reliable and cost-effective.

Residential Solar Inverter Systems
A residential solar inverter system consists of an array of photovoltaic (PV) panels which generate a variable DC voltage. A DC-DC boost converter increases this voltage to a DC link operating level using a technique called maximum power point tracking (MPPT), which optimizes the captured energy depending on the strength and direction of sunlight during the day. Finally, a single−phase DC-AC inverter converts the DC link voltage (typically < 600 VDC) – from the solar panel array to an AC voltage (120 to 240 V) which connects either to a load or the power grid.

There are several residential solar inverters types, but the two most common are micro and string inverters. A micro inverter solar system uses multiple DC-AC inverters, each connected to a single photovoltaic (PV) panel, typically producing up to 1kW of output power. Since each panel’s voltage level is tracked individually, which is an efficient approach. Furthermore, micro inverter systems are easy to scale to meet the required energy capacity. On the other hand, a string inverter system combines inputs from multiple series−connecting PV to provide hundreds of volts. However, connecting several solar panels can be less efficient than a microinverter system. For example, if one panel receives less light than the others in the series, the entire system output is impacted. String inverter systems are typically less expensive than microinverter systems, which require an inverter for each panel.

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Figure 2: Micro Inverter System (left) and String Inverter System Block Diagram (right)

A power optimizer (a DC−DC converter with integrated MPPT) can increase the efficiency of string inverter systems. This converts the variable DC voltage from PV panels to a fixed DC voltage, meaning low PV output from an individual panel doesn’t impact the overall system efficiency.

Battery Energy Storage System

The battery energy storage system (BESS) is another essential component in a residential solar system. In most cases, energy is captured when it is least required such as during daylight hours when occupants are at work. Using a lithium-ion or lead acid battery to store energy provides the flexibility to use power when needed – in the evening when the family is home. A bi−directional converter connects the BESS to the solar system. When PV panels produce power, the converter charges the battery array. At night, when panels are not generating energy, the bidirectional converter releases the stored energy from the batteries to drive loads. Storing energy locally also provides extra reassurance of having a backup power source during an electricity shortage or grid outage. In addition, modular energy storage systems can be easily added without significantly modifying the existing system.

DC−DC Boost Converter

The single boost DC-DC converter is the most common non−isolated topology for residential systems, while the flyback converter is popular when isolation is required. Both topologies are low-cost and have a narrow form factor.

DC-AC Inverter

Inverters used in residential systems can be constructed using a variety of topologies, for example, the HERIC H6.5 converter featuring the onsemi NXH75M65L4Q1 IGBT module. This design does not require a transformer, reducing overall system weight, size, and cost. In addition, the topology helps mitigate leakage currents caused by the common mode (CM) voltage acting on the parasitic capacitances of the PV array. Furthermore, it provides higher efficiency than an H−bridge−based approach. Generally, a 3−level topology like this one is recommended for both single and three−phase applications to minimize distortion and provide a smoother output voltage.

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Figure 3: The H6.5 topology is suitable for residential solar inverters

Bidirectional DC−DC converter

The bidirectional DC−DC converter charges and discharges the battery in the energy storage system. This typically uses a resonant CLLC or dual active bridge buck−boost isolated topology. It supports a wide input and output voltage range and uses zero voltage switching (ZVS) to improve efficiency. It also has the added benefit of providing safety by isolating the battery pack from the PV panel.

IGBTs for Residential Solar Systems

onsemi’s 600 V and 650 V rated silicon IGBTs are suitable for residential solar systems. These IGBTs incorporate a narrow mesa, wide trench width Field Stop 4 (FS4) technology, providing latch immunity and smaller gate capacitances. The field stop layer increases the blocking capability and reduces drift layer thickness, lowering conduction and switching energy losses to less than 30 μJ/A. Thinner IGBT chip helps lower thermal resistance, and narrow mesa increases power density, enabling the same current IGBT into a smaller package. The FS4 IGBT design provides better light load power efficiency in a 4kW boost converter than a converter using Field Stop 3 (FS3) designs, and its performance equals that of other competing devices.

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Figure 4: Field Stop 4 (FS4) Efficiency in 4 kW Boost Converter

Further Improving Performance
Silicon Carbide (SiC) devices enable smaller inverters in residential solar systems while providing better performance than silicon-based devices. Compared to silicon IGBTs, Eon and Eoff losses are significantly reduced during high-frequency switching. Furthermore, SiC exhibits superior stability than IGBT over wider temperature ranges, making them more reliable. SiC devices also generate less EMI than Super Junction MOSFETs at fast switching frequencies. Better thermal performance and lower switching losses during high-frequency operation reduce the overall system footprint, enabling a lighter inverter design. onsemi 650 V SiC discrete MOSFETs have low R_DS(ON) across both V_GS and temperature and can be driven using a negative gate voltage. This improves noise immunity and avoids false turn-ons when used in bridge topologies.

Accelerating Residential Solar System Design

onsemi provides an extensive portfolio of products and tools to simplify component selection for a solar system, including reference designs like the SECO−HVDCDC1362−40 W−GEVB 40 W SiC high voltage auxiliary power supply. This includes all documentation (user manual, bill of materials, Gerber files, etc.) required to speed up product development. The company also provides SPICE models for system designers wishing to perform more advanced system evaluations and development. These SPICE models can assist in investigating switching devices' reverse-recovery behaviour and parasitic effects at the circuit, module, and die levels. Furthermore, these models also support thermal simulations for exploring self−heating effects.

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