Cost-effective micro-inverter design based on integrated MCU

Micro-inverter provides an effective solution at the various panel levels by providing power conversion to harvest solar energy. The advent of highly integrated MCUs provides an attractive approach to the design of micro-inverters, offering an option that reduces the complexity of the cost which limits the widespread adoption of micro-inverters in the past. Designers can now leverage existing MCU semiconductor manufacturers, including Freescale Semiconductor, Infineon, Microchip's technology, Spansion and Texas Instruments, as well as other designs to build efficient micro-inverters. Solar collection systems continue to evolve away from traditional centralized solutions (Figure 1). Unlike single inverters based on a single central inverter or even multiple inverter systems, the micro-inverter converts power from a single panel. In turn, the AC power supply pair generated on each panel by the micro-inverter is output to the load.


Figure 1: The solar energy harvesting method continues from the central inverter system (left) to the evolution to more distributed string inverter systems (middle), and the final microinverter (right) is built into a separate panel. (provided by Freescale Semiconductor) by using each solar panel that produces AC power, the micro-inverting method reduces or eliminates expensive wiring, cooling, and other associated with a central- or even string inverter system The cost associated with the facility requirements. At the facility level, the total conversion efficiency is greatly reduced due to age differences in lighting, shading, dirt, or panels. While the use of micro-inverters increases the cost of individual panels, solar devices as a whole typically have lower cost and higher conversion efficiencies.

Cost-effective solution

For all the advantages, the micro-inverter has not appeared yet, with a cost-effective alternative to the traditional inverter system. In the past, the complex functionality of solar inverter designs has increased the cost of these systems to offset the shortcomings of concentrated-solar energy solutions. In fact, it is necessary to respond to changing environmental conditions to maximize solar energy conversion requiring complex system designs that are difficult to achieve in terms of cost and effectiveness in the use of a single solar panel. Ideally, the solar panel produces its maximum power output as determined by the characteristics of the environment at a particular point in its IV curve and the panel itself (Figure 2, left). In practice, this maximum power point (MPP) can be elusive, shifting to a different point power curve for shadowing across the panel from the cloud or debris collected on the panel (Figure 2, right). Complex solar systems use the Maximum Power Point Tracking (MPPT) method to modify the panel's operating voltage to ensure that the panel produces its maximum power output despite changing conditions.


Figure 2: From a sufficient sunlight (left) and a partially shaded (right) 72-cell 180 watt solar panel output power to a specific point on the maximum power curve, but found that the maximum power point can represent a significant challenge. (The Linear Technology offers) Popular MPPT methods, such as the disturbing and observation (P&O) use of the periodic steps of the periodic adjustment panel of the working voltage, seeking any increase or decrease of the panel's operating voltage, can improve the panel power supply simple expedient Meter output. If the gradual increase in operating voltage results in a low power output, the P&O algorithm will gradually reduce the operating voltage in the next adjustment step - repeat this process until the change in operating voltage increment results in lower panel output power in both directions. . In practice, however, the appearance of a local maximum in the power output curve (see Figure 2, right) typically requires a more sophisticated approach than simple step increments.

MCU-based solution

In the past, building an effective MPPT system was a complex task that could quickly increase costs and extend the schedule of engineers to handle corners such as local maxima or other factors. Today, engineers can find a wide variety of equipment that can provide a complete solution with minimal additional components. In fact, the integrated MCU provides the measurement and analysis requirements that the on-chip function can handle, often requiring only the input and power adjustment of the output on the voltage and current sense of the complementary analog circuit (Figure 3). Microcontroller manufacturers typically provide an associated software library, including the MPPT algorithm to be used, further simplifying the design process for microinverter designers.


Figure 3: An integrated microcontroller such as Microchip's PIC16F690 provides a complete set of functions and on-chip peripherals that require digital control of the micro-inverter. (Provided by Microchip's technology) For MPPT implementations, suitable MCUs such as Microchip's PIC 16F series, Spansion's FM3 MB9B520M series, and the Infineon XMC4000 series combine a processor core with a comprehensive complement of memory analog peripherals. At a minimum, these MCUs provide an analog-to-digital converter (ADC) for measuring panel voltage and current, voltage reference and analog comparators for accurate analog processing, and pulse width modulation (PWM) outputs for voltage conversion. Level. For example, Microchip Technology's PIC MCU 16F family of devices combines an 8-bit CMOS microcontroller core with flash memory and the required set of analog peripherals. The PIC16F690 MCU integrates a 12-channel 10-bit ADC, two analog comparators, programmable on-chip voltage reference, and capture/compare/PWM, providing 16-bit capture with resolution down to 12.5 NS and 16-bit resolution compared to 200 nanoseconds. Extending these features to other members of the PIC MCU 16F family and the need to provide additional peripherals that further enhance the microinverter design. For example, a liquid crystal control module is placed around Microchip's PIC16F913 MCU docking station, allowing designers to provide user feedback directly on the panel. Freescale Semiconductor's MC56F82xx Digital Signal Controller (DSC) family offers 32-bit DSP cores and on-chip peripherals for microinverter designs. It features the MC56F82xx family based on a 60 MIPS 32-bit 56800E core. The Harvard architecture of the series incorporates three execution units for parallel operation, allowing up to six operations per instruction cycle. At the same time, these devices provide an MCU-style programming model and an optimized instruction set. In their peripherals, these devices include two 12-bit 8-channel ADCs, three analog comparators, an integrated ADC and Freescale's enhanced FlexPWM (eFlexPWM) with contrast, side-mounted, and triggering capabilities. Texas Instruments addresses the real-time performance of a family of C2000 MCUs, including 32-bit Piccolo MCUs, 32-bit multicore Delphino MCUs, 32-bit fixed-point DSCs, and 16-bit DSP microcontrollers. Designed for real-time applications, the C2000 MCU family offers a wide range of price/performance points for highly integrated devices that can quickly acquire analog data, perform the required calculations, and adjust the PWM output in a single clock cycle. In addition to the need for on-chip peripherals and real-time performance for MCUs, engineers developing solar energy harvesting can also find their own safety requirements, such as the necessary supplements for high voltage applications of the IEC 61508 SIL-3 safety standard. To meet this emerging demand, Texas Instruments (TI) offers the Hercules RM Series of secure microcontrollers. Surrounded by a pair of lock-step ARM Cortex-R4F cores, these devices combine ADCs with PWM design to constantly monitor their operation and provide near real-time fault detection without compromising performance.

in conclusion

By placing each individual solar panel, micro-inverter can provide high efficiency solar conversion and reduce costs. In the past, however, there was a need for a design that was able to extract maximum power from a solar array to eliminate the widespread use of such dispersion. Today, engineers can take advantage of the extensive MCU array integration to perform the peripherals needed for efficient energy harvesting. In this way, engineers can quickly realize the design of cost-effective micro-inverters based on MCUs that can meet a wide range of application performance requirements and functional capabilities.