What is the key role of diodes in series connection of photovoltaic arrays?

1, Technical principle: Unidirectional conductivity lays the functional foundation
The core characteristic of a diode is unidirectional conductivity, which means forward conduction and reverse cutoff. This characteristic is determined by the semiconductor physical structure of the PN junction: when a forward voltage is applied to the PN junction, carrier diffusion forms a current; Under reverse voltage, the depletion layer width increases and the current is almost zero. In photovoltaic arrays, diodes achieve three major functions through this characteristic:
Anti backflow protection
In low light conditions such as night or cloudy days, photovoltaic cells stop generating electricity. If the system is not equipped with anti backflow diodes, the current from the battery or grid may flow back into the photovoltaic array, causing the cells to heat up or even burn out. For example, in an energy storage system, a blocking diode is connected in series between the photovoltaic string and the battery. When the photovoltaic voltage is lower than the battery voltage, the diode automatically cuts off, blocking reverse current and protecting component safety.
Inhibition of hot spot effect
When a certain cell in the photovoltaic array is obstructed or damaged, its internal resistance sharply increases, becoming a "load" in the series circuit, consuming the energy generated by other normal cells, causing the local temperature to soar above 200 ℃, forming a hot spot. Hot spots not only accelerate the aging of battery cell materials, but may also cause components such as junction boxes and backplates to burn out. The bypass diode is connected in parallel to both ends of the battery string. When the polarity of the voltage in the hot spot area reverses, the diode conducts in the forward direction, providing a low resistance bypass path for current to avoid overheating in the fault area while maintaining the remaining power generation function.
Fault branch isolation
In large photovoltaic power plants, the array is usually composed of multiple series of battery packs connected in parallel. If a certain string of battery packs experiences a decrease in output voltage due to obstruction or malfunction, the current of other normal branches may form a loop through the low-voltage branch, causing energy loss. Isolation diodes are connected in series to the output terminals of each battery pack. When the voltage of a certain branch is abnormal, the diode will cut off in reverse to prevent current backflow and ensure the normal operation of other branches.
2, Application scenario: Full chain protection from component level to system level
The application of diodes runs through the entire life cycle of photovoltaic array design, installation, and operation, and its value is particularly prominent in the following scenarios:
Roof mounted photovoltaic system
Rooftop photovoltaic arrays are susceptible to obstruction from leaves, snow, building shadows, and other factors, leading to a sharp drop in local cell power generation efficiency. Taking a 10kW rooftop photovoltaic system as an example, if bypass diodes are not configured, blocking a single cell may result in a power loss of over 30% for the entire module; After adopting bypass diodes, the power loss can be controlled within 5%, significantly improving the system's power generation.
Agricultural photovoltaic power station
In the "Agricultural Photovoltaic Complementary" project, crop growth may obstruct photovoltaic panels, and agricultural activities such as irrigation and fertilization can easily cause module fouling. Bypass diodes can quickly respond to voltage anomalies caused by obstruction or fouling, avoiding long-term damage to components caused by thermal spot effects. For example, a 50MW agricultural photovoltaic power station optimized the layout of bypass diodes, reducing component failure rates by 40% and increasing annual power generation by approximately 8 million kWh.
Desert Photovoltaic Power Station
Frequent accumulation of sand and dust in desert environments can lead to the formation of localized dirt layers on component surfaces, causing hot spots. In addition, the large temperature difference between day and night can cause thermal expansion and contraction of battery cells, which may lead to hidden cracks. The synergistic use of isolation diodes and bypass diodes can isolate faulty branches and divert hot spot currents, reducing the risk of component burnout by more than 90%.
3, Industry Practice: Evolution from Standard Specifications to Technological Innovation
With the large-scale development of the photovoltaic industry, the application of diodes has evolved from a single function to intelligence and integration, and industry norms and technical standards continue to improve
International Standard System
IEC 62979:2017: defines the "thermal runaway test" for bypass diodes, which requires the diode to withstand 1.25 times the short-circuit current for 1 hour in a high temperature environment of 90 ℃, and then instantly switch to the reverse bias state to ensure that the junction temperature does not continue to rise.
IEC 61215: It is stipulated that diodes must undergo environmental adaptability tests such as "wet freezing test" and "thermal cycling test" to verify their reliability at extreme temperatures ranging from -40 ℃ to+85 ℃.
Innovation in Materials and Processes
Schottky diode: adopting a gold half contact characteristic, the conduction voltage is reduced to 0.2-0.4V, reducing self heating by more than 50% compared to traditional PN junction diodes (0.6-0.8V), suitable for high-density packaging scenarios.
Silicon carbide (SiC) diodes: their temperature resistance has been improved to over 200 ℃, and their lifespan has been extended to 20 years, which can meet the needs of extreme environments such as deserts and plateaus.
Intelligent reconstruction diode: dynamically adjusting the conduction threshold through MCU control, optimizing the power generation efficiency under shielding conditions, such as automatically reducing the conduction voltage and reducing power loss when partially shielded.
System Integration Trends
Integration of junction boxes: Integrating bypass diodes with junction boxes and connectors to reduce component volume and cost. For example, a smart junction box launched by a certain enterprise integrates temperature sensors and diodes, which can monitor junction temperature in real time and trigger warnings to avoid thermal runaway.
Lead free process: Complies with RoHS standards, reduces environmental pollution risks, and promotes the green transformation of the photovoltaic industry.
4, Economic benefits and market prospects
The application of diodes not only enhances the safety of photovoltaic systems, but also brings significant economic benefits. Taking a 100MW photovoltaic power station as an example:
Power generation increase: Installing bypass diodes can reduce power loss caused by hot spots from 15% to below 3%, increasing annual power generation by approximately 12 million kWh.
Reduced operation and maintenance costs: The fault branch isolation function reduces the frequency of component replacement, resulting in a 20% -30% decrease in operation and maintenance costs.
Shortened investment payback period: With the comprehensive increase in power generation and cost savings, the investment payback period only takes 2-3 years.
According to industry predictions, the global demand for photovoltaic bypass diodes is expected to reach 3.6 billion units by 2025 and exceed 4 billion units by 2026. As the world's largest producer of photovoltaic modules, China's export volume reached 238.8GW in 2024, driving the continuous expansion of the bypass diode market. In the future, with the advancement of materials science and intelligent control technology, diodes will evolve towards higher reliability, lower losses, and greater intelligence, providing solid support for the global energy transition.