Advanced digital isolation technology improves solar inverter reliability

Fossil fuel power generation facilities have proven to be a robust and reliable source of energy for more than a century, but these tried-and-true power facilities are large, complex, and increasingly expensive to build. Operating them cleanly with minimal carbon footprint and environmental impact is also challenging and costly. In contrast, modern photovoltaic (PV) power systems offer a sustainable alternative to fossil fuel power plants, offering lower long-term operating costs, modular scalability, and lower long-term operating costs than centralized power generation facilities. flexibility, greater efficiency and a significantly lower carbon footprint.

Photovoltaic power systems are made up of multiple components, such as photovoltaic panels that convert sunlight into electricity, mechanical and electrical connections and installation, and solar inverters, which are critical to delivering solar power to the grid.

What is a photovoltaic solar inverter?

Photovoltaic panels convert sunlight into DC voltage, which must be converted into high-voltage AC to reduce line losses and enable longer power transmission distances. Photovoltaic solar inverters perform this DC-to-AC conversion and are the most critical component in any photovoltaic power generation system. However, this is only one key feature provided by PV inverters.

PV inverters also provide grid disconnection capability to prevent the PV system from supplying power to a disconnected utility; that is, an inverter can cause the PV system to fail when the grid is disconnected or power is supplied through an unreliable connection. Feedback to local utility transformers generated thousands of volts on utility poles, endangering utility workers. Safety standard specifications IEEE1547 and UL1741 stipulate that when the AC line voltage or frequency is not within the specified range, all grid-connected inverters must be disconnected and must be shut down if the grid no longer exists. On reconnection, the inverter cannot provide power until the inverter detects rated utility voltage and frequency for more than a 5-minute period. However, this is not the end of the inverter’s duties.

The inverter also compensates for environmental conditions that affect power output. For example, the output voltage and current of a photovoltaic panel are very susceptible to changes in temperature and light intensity per unit area of ​​each unit (called "irradiance"). The battery output voltage is inversely proportional to the battery temperature, and the battery current is directly proportional to the irradiance.

Extensive changes in these and other critical parameters cause the optimal inverter voltage/current operating point to shift significantly. Inverters solve this problem by using closed-loop control to maintain what is called the maximum power point (MPP), where the product of voltage and current is at its highest value. In addition to these tasks, the inverter supports service operation with manual and automatic input/output disconnection, EMI/RFI conducted and radiated suppression, ground fault interruption, PC-compatible communication interface, and more. Encased in a rugged package, the inverter is capable of maintaining full power for outdoor operation.In business for over 25 years. No small feat!

Observe carefully

The single-phase PV inverter example uses a digital power controller and a pair of high-side/low-side gate drivers to drive a pulse-width modulated (PWM) full-bridge converter. The full-bridge topology is commonly used in inverter applications because it has the highest power-carrying capabilities of any switch-mode topology. The PWM voltage switching action synthesizes a discrete (albeit noisy) 60Hz current waveform at the full-bridge output. Perform inductance filtering on high-frequency noise components to generate a 60Hz sine wave with medium and low amplitude.

figure 1. Single-stage, single-phase inverter block diagram

The filtered waveform passes through an output transformer that performs three functions: first, it further smoothes the AC waveform; second, it corrects the voltage amplitude values ​​to meet specified grid requirements; and third, it electrically isolates the inverter’s DC input from High voltage AC grid.

PV inverter design is full of design compromises that can cause designers grief if the wrong trade-offs are made. For example, photovoltaic systems are expected to operate reliably and at full rated output for at least 25 years, but they need to be priced competitively, forcing designers to make difficult cost/reliability trade-offs. Photovoltaic systems require efficient inverters because more efficient inverters run cooler and last longer than less efficient inverters, and they save cash for PV system manufacturers and users.

The ongoing pursuit of high inverter efficiency creates more design trade-offs that can affect component selection (primarily gate drivers, power switches, and magnetic components such as transformers), PCB structure, and thermal requirements of the inverter packaging. The output voltage of PV panels also varies severely with changes in sunlight exposure; therefore, it is beneficial to match the input voltage range of the inverter to the output voltage range of the PV panels.

This creates more design trade-offs that further impact system complexity, cost, and efficiency, and that's just for the hardware. Now let's look at the control side of this problem.

The "brains" behind an inverter is its controller, usually a digital power controller (DPC) or digital signal processor (DSP). Typically, the controller's firmware is implemented in a state machine format to allow for the most efficient execution using non-blocking (falling) code, thus preventing execution from inadvertently entering an endless loop.

Firmware execution is hierarchical, typically servicing lower-order functions more frequently than the highest-priority functions. In the case of PV inverters, isolated feedback loop compensation and power switching modulation are usually the highest priority, followed by critical protection functions supporting UL1741 and IEEE1547 safety standards, and finally efficiency control (MPP). The remaining firmware tasks mainly involve optimizing operation of the current operating point, monitoring system operation and supporting system communications.

Photovoltaic inverters are exposed to high and/or cold temperatures for 25 years, causing people to pause when considering the components used in the inverter. Obviously, components such as electrolytic capacitors to filter ripple and electrical couplers to provide galvanic isolation have no chance of "walking this distance." The electrolytic capacitor dries out and dies, the optocoupler's LED brightness gradually weakens to a weak glow and stops working. Solutions for these precision components include replacing electrolytic capacitors with high-value film capacitors (more reliable, but significantly more expensive). The best long-term solution is to move away from optocouplers in favor of modern CMOS isolation components.

CMOS process technology offers advantages such as high reliability, cost-effectiveness, high-speed operation, small feature size, low operating power, operational stability at extreme voltages and temperatures, and many other desirable characteristics. Additionally, unlike the gallium arsenide (gallium arsenide) process technology used in optocouplers, devices fabricated in CMOS have no inherent wear mechanisms. The underlying CMOS isolation cells are capacitive, fully differential, and highly optimized for tight timing performance, low-power operation, and high immunity to data errors caused by external fields and fast common-mode transients.

In fact, the advantages of CMOS process technology combined with proprietary silicon product designs make robust, "near-ideal" isolation devices possible for the first time. These devices offer greater comprehensive feature integration, higher reliability (60+ year isolation barrier life), maximum VDD continuous operation from 40°C to +125°C, and significant improvements in performance, power consumption, board space savings and Significant improvements in ease of use.

Photovoltaic inverter component solutions for the 21st century

Photovoltaic inverter architecture does not end with the single-phase transformer inverter shown in Figure 1. Other common types include high-frequency, bipolar, three-phase, transformerless and battery-powered inverters. Although these topologies differ from each other, they generally require the same component solutions. The block diagram in Figure 2 shows several CMOS isolation devices used in transformer-based three-phase phase inverters.

This is a classic closed-loop architecture in which a digital controller modulates the duty cycle of a power switch to force the amplitude and phase of the PV system output voltage to exactly match that of the grid. These isolation gate drivers combine safety-certified electrical isolation (rated at 1kV, 2.5kV or 5kV) and high-side level shifting functionality in a single package, eliminating the need for external isolation equipment. Each driver output is isolated from the next, allowing positive voltages to be used without latch-up.

Current feedback to the controller is provided by a single 4mmx4mmx1mm1mm CMOS isolated AC current sensor (its 1kV isolation rating is limited by package size - larger package versions are rated up to 5kVrms). These monolithic sensors offer a wider temperature range, higher accuracy, and higher reliability than current sensing sensors. The sensor is controlled using the inverter gate control signal generated by the digital controller.

Cycling reset eliminates the need for external reset circuitry. Grid feedback is an important part of the system feedback control mechanism. Resistive attenuators are used to reduce the grid voltage to a range compatible with PWM modulators, which convert sine wave inputs into discrete PWM waveforms, safely isolated by CMOS digital isolators.

in conclusion

Photovoltaic systems are a relative newcomer to the world of energy production. Like other emerging technologies, photovoltaic systems will change rapidly as the technology matures. Therefore, photovoltaic systems will undoubtedly continue to evolve to meet market demands for higher capacity, lower costs and greater reliability. As this happens, the capabilities of PV inverters will expand and designers will require more integrated, application-specific component-level devices to further leverage and drive innovation in CMOS isolation. As these events unfold, photovoltaic power systems will become more widespread and eventually become a viable part of the utility mainstream, significantly reducing our dependence on fossil fuels.