Residential Energy Storage Solutions (ESS) are not only applied in industrial and power generation settings but have also become crucial in the residential sector, reflecting current applications and market trends. While residential ESS solutions require lower power, the demands for efficiency and safety remain comparable to industrial applications. This article will introduce you to the market trends in residential ESS solutions and the functional features of the SiC-related solutions introduced by Arrow and Rohm.
Residential ESS applications for storing and managing electrical energy
Residential ESS is an energy storage solution designed for use in residential settings. Its purpose is to store and manage electrical energy, aiming to improve energy efficiency, reduce energy costs, and enhance energy supply stability. Residential ESS applications typically involve solar power generation systems (photovoltaic systems), where solar photovoltaic panels are commonly installed on rooftops or other suitable locations to convert sunlight into direct current (DC) electrical energy.
The ESS also requires a charging controller responsible for monitoring the output of the solar power generation system and controlling the flow of electrical energy to the energy storage system. It ensures that the electrical energy generated by solar power is stored in the battery. The battery is the core component of the ESS, used to store the electrical energy generated by solar power during the day for supply during the night or on cloudy days. Common battery technologies include Lithium-ion batteries (Li-ion) and lead-acid batteries.
The ESS also requires an inverter to convert the direct current (DC) stored in the battery into alternating current (AC) for powering household appliances and lighting. Additionally, an Energy Management System (EMS) is used to monitor the home's energy consumption, weather forecasts, electricity prices, and other information. This system optimizes the use and storage of energy, automatically controlling the charging and discharging processes to ensure optimal energy efficiency.
Residential ESS can also be connected to the electrical grid, allowing households to purchase electricity when needed or sell excess energy back to the grid when there is an abundance of energy. This bidirectional flow of energy is known as "bidirectional metering." Through monitoring systems, homeowners can monitor the real-time operational status of the energy system, track energy generation and consumption, and make operational adjustments remotely. This includes changing the operational mode of the energy storage system or setting charging and discharging times.
The architecture of residential ESS can be adjusted based on specific requirements and technologies to ensure optimal performance and efficiency. This system helps achieve energy self-sufficiency, energy conservation, and emission reduction. Additionally, it provides a backup power source during grid outages.
The requirements for residential ESS applications differ from industrial applications, primarily in the lower power demand of residential ESS, typically requiring power less than 10 kW. It must support bidirectional power conversion and often utilizes high-efficiency AC/DC topologies with high electromagnetic compatibility characteristics, along with high-efficiency and high safety specification DC/DC topologies. Residential ESS must support a wide range of busbar voltage (360V - 550V) and usually places the battery on the DC side. The system efficiency is typically required to exceed 90%, and reliable system stability is essential. There is an emphasis on achieving high power density to meet size and weight reduction goals. Additionally, cost reduction is a key consideration, and there are high demands for safety standards, EMC, and noise characteristics.
SiC devices exhibit superior performance compared to silicon devices
To meet the aforementioned requirements, it is common to use Silicon Carbide (SiC) for power conversion. This is because SiC devices offer significant advantages, enhancing system efficiency under high current and high-temperature conditions. The high breakdown field of SiC material allows SiC devices to operate at higher voltages, providing higher voltage tolerance compared to silicon devices. This makes SiC devices particularly useful in power conversion applications.
In addition, SiC devices exhibit a higher electron mobility, making them superior in high-frequency applications. For applications like high-frequency converters and power amplifiers, SiC devices offer better performance. The thermal conductivity of SiC is three times that of silicon devices, enabling smaller size and weight, thereby increasing power density and optimizing system costs. With a decrease in cost per unit volume, energy can be bidirectionally converted safely and reliably. This leads to achieving goals of reducing volume by 50% and lowering the cost per watt unit, implying that for the same power level, SiC devices have a smaller volume and lighter weight.
SiC material is chemically stable, exhibiting minimal susceptibility to corrosion from corrosive substances. This property makes SiC devices more suitable for applications in extreme environments. The high carrier mobility of SiC devices results in faster switching speeds. This is beneficial for reducing switching losses, improving conversion efficiency, and enhancing the dynamic characteristics of the devices.
Adopting SiC energy storage solutions allow for smaller product dimensions and reduced weight. It enables higher switching frequencies, and due to the use of smaller magnetic devices, smaller transformers/inductors can be employed. This results in lower losses and better heat dissipation. The same power can be accommodated in a smaller enclosure compared to silicon IGBTs. In comparison to silicon IGBTs, SiC offers a doubled power density (W/kg), achieving high power density. It can utilize simple bidirectional converter topologies with fewer loop controls, resulting in higher efficiency.
SiC devices feature lower on-resistance per unit volume, resulting in reduced conduction losses. They exhibit low on-state losses during turn-off, eliminating the phenomenon of current tailing, leading to low switching losses. The recovery losses of the body diode are very small, and SiC devices enable a reduction in the bill of materials (BOM). The system is robust, durable, and provides higher reliability.
Taking the example of a DC-DC high-side design with a busbar voltage of 500V, one can use a combination of 1200V SiC and IGBT on the high-voltage side. The drive voltage is 15V/-2.5V, and the switching frequency is 30kHz. On the other side of the circuit, 650V SiC and IGBT can be employed with a drive voltage of 15V/-2.5V and a switching frequency of 76kHz. The efficiency is higher when using SiC devices on the high-voltage side. SiC power devices operate with a 15V drive and are compatible with IGBT power device solutions.
Design challenges and solutions for bidirectional DC/DC power converters
When designing bidirectional DC/DC power converters for ESS, there are numerous challenges to address. For example, in discharge mode, solving steady-state operation and low-side MOS Vds stress under no-load conditions is crucial. One solution is to increase the inductance to 200µH at the primary side of the transformer. This approach can reduce voltage stress by 25% and improve efficiency by 6% to 7%.
Additionally, there is a need to address Vds voltage stress issues during discharge mode and startup. The solution is to use PWM+PFM hybrid control at the input port. This can reduce voltage stress by 27%, with Vmax reaching 124V at 80V. Similarly, in discharge mode, there may be issues with excessively high temperatures (96°C@2100W) in the resonant capacitor. Changing the capacitor model to mkp21224/400VDC can lower the resonant capacitor temperature to 65°C@3000W.
On the other hand, in discharge mode, the operating frequency may suddenly change to around 180kHz, causing instability in the gain curve. To address this issue, the fixed conduction time frequency point of SRMOS can be tuned to be lower than 180kHz, ensuring stability in the gain curve.
SiC MOSFET products meet the requirements for DC-DC designs
The 6600V 48V bidirectional high-frequency isolated DC-DC reference design, supported by Shenzhen Winchen Electronics and Arrow, provides an example. In the charging section, it supports a DC busbar charging range of 380-480 VDC, charging current ≤16A, output voltage of 40-60 VDC, output current ≤140A, and a maximum output power of 6.6 kW. The charging efficiency can reach 95% at 420V, and the charging current ripple coefficient is 1%. In the discharge section, the battery-side voltage range is 40-60 VDC, battery-side current ≤140A, DC busbar voltage range is 380-480 VDC, maximum output power is 6.6 kW, discharge efficiency can reach 94% at 54V, and the busbar voltage ripple coefficient is 1%.
In this reference design, without the Buck_Boost regulator, the operating range on the low-voltage side is 43V-57V, the full-power operating range is 49V-57V, the maximum stable output current is 142A, and the maximum short-term output current is 150A (Vin = 420V, resistive load). With the Buck_Boost regulator, the operating range on the low-voltage side is 43V-57V, the full-power operating range is 49V-60V, the maximum stable output current is 145A, and the maximum short-term output current is 150A (Vin = 420V, resistive load). This reference design uses 8 pieces of Rohm's SCT3030AR TO-247 packaged SiC MOSFETs, along with the BM61S41RFV-C gate driver and RJ1P12BBDTLL power MOSFET.
Rohm's SCT3030AR is a 650V Nch 4-pin packaged SiC MOSFET, well-suited for applications demanding high efficiency such as servers, solar power inverters, and electric vehicle charging stations. It features a trench gate structure SiC MOSFET with separate power source pin and driver source pin in a 4-pin package, maximizing high-speed switching performance, especially significantly improving conduction loss. Compared to traditional 3-pin packages (TO-247N), the total conduction and switching losses can be reduced by approximately 35%.
Rohm's SCT3030AR features low on-resistance, fast switching speed, quick reverse recovery, ease of parallel connection, and simple drive. It is packaged in a Pb-free plating, compliant with RoHS standards, making it suitable for a wide range of applications including solar power inverters, DC/DC converters, switch-mode power supplies, induction heating, and motor drives.
BM61S41RFV-C is a gate driver with an isolation voltage of 3750 Vrms, a maximum gate drive voltage of 24V, a maximum I/O delay time of 65 ns, a minimum input pulse width of 60 ns, and an output current of 4A. It features undervoltage lockout (UVLO) and active Miller clamp functions, complies with AEC-Q100 standards, and is packaged in SSOP-B10W. RJ1P12BBD is an Nch 100V 120A power MOSFET with low on-resistance, high power in a small mold package. It uses Pb-free plating, is compliant with RoHS standards, halogen-free, and passing UIS tested.
Conclusion
As green energy receives increasing attention from the international community, it is driving the rapid development of residential ESS applications. This involves a considerable number of electronic components and solutions, representing a vast market opportunity. Arrow can assist customers in developing DC-DC solutions for ESS applications. Rohm's SiC MOSFET and related products can meet the application requirements for DC-DC. For more detailed information, please contact Arrow directly.