Why do solar panels require bypass diodes?

一, Hot spot effect: the 'invisible killer' of photovoltaic systems
1. Formation mechanism of hot spot effect
When one or a group of solar cells in a solar panel cannot generate electricity due to obstruction (such as leaves, bird droppings, building shadows), contamination, or damage, its internal resistance will sharply increase, becoming a "load" in the series circuit. At this point, the current generated by other normally functioning battery cells will continue to pass through the faulty area, causing the local temperature to rapidly rise above 200 ℃, forming a "hot spot". This high temperature not only accelerates the aging of battery cell materials, but may also cause components such as junction boxes and backplates to burn out, and even lead to fires.
2. Chain reaction of hot spot effect
Power loss: The power generation efficiency of the solar cells in the hot spot area is reduced to zero, and it will consume the energy of other normal solar cells, resulting in a 10% -30% decrease in the output power of the entire component.
Material degradation: High temperatures cause the decomposition of EVA film, backplate, and other materials, releasing harmful gases and shortening the lifespan of components.
System crash risk: In large photovoltaic power plants, the hot spot effect may cause cascading failures, leading to the shutdown of the entire array.
二, Bypass diode: the ultimate solution for hot spot effect
1. Working principle: "Intelligent shunt" for current
The bypass diode is usually connected in reverse parallel at both ends of the battery string, and its core function is to achieve intelligent switching of the current path through dynamic conduction and cutoff:
Normal working state: When all battery cells are generating electricity normally, the diode is in reverse cutoff state and has no impact on the circuit.
Fault state: When a series of battery cells are obstructed or damaged, causing the reverse bias voltage to exceed the threshold, the diode conducts in the forward direction, short circuiting the fault area and causing current to bypass the faulty battery cells and flow to the load through the diode.
Recovery state: After the obstruction is removed or the fault is eliminated, the diode automatically returns to the cutoff state, and the component resumes normal power generation.
2. Key technical parameters
Forward conduction voltage: Due to the gold half contact characteristic, the conduction voltage of Schottky diodes is reduced to 0.2-0.4V, much lower than the 0.6-0.8V of PN junction diodes, which can significantly reduce self heating.
Reverse breakdown voltage: It needs to be greater than 1.2 times the open circuit voltage of the battery string to prevent high-voltage breakdown.
Thermal resistance coefficient: Low thermal resistance design (such as ceramic packaging) can accelerate heat dissipation and avoid diode failure due to high temperature.
Response speed: The switching response time of Schottky diodes is less than 10ns, which can quickly respond to thermal spot transient impacts.
3. Typical application scenarios
Roof photovoltaic system: frequent obstruction caused by leaves, snow, etc., bypass diodes can prevent local obstruction from causing the entire string of batteries to fail.
Agricultural photovoltaic power station: Crop growth may obstruct the solar panels, and diodes can maintain power generation continuity.
Desert photovoltaic power station: Dust accumulation can easily cause hot spots, and diodes can protect components from high temperature damage.
三, Industry standards and testing standards: Ensuring the reliability of bypass diodes
1. 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 ℃.
2. Failure modes and protective measures
Failure reasons: diode breakdown caused by high temperature and high current, thermal runaway caused by reverse leakage current, and solder joint detachment caused by mechanical stress.
Protection plan:
Redundant design: Parallel backup diodes are connected in the junction box, which automatically switch when the main diode fails.
Intelligent monitoring: Real time monitoring of diode junction temperature through temperature sensors, triggering warnings or automatic power outages.
Material upgrade: Using silicon carbide (SiC) diodes, the temperature resistance has been improved to over 200 ℃, and the lifespan has been extended to 20 years.
四, Market Trend: From Passive Protection to Active Optimization
1. Explosive growth in demand
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.
2. Technical iteration direction
Intelligent reconstruction diode: Controlled by MCU, dynamically adjust the conduction threshold of the diode to optimize the power generation efficiency under shielding conditions.
Integrated design: Integrating diodes with junction boxes and connectors to reduce component volume and cost.
Lead free process: compliant with RoHS standards, reducing environmental pollution risks.
3. Cost benefit analysis
Taking a 100MW photovoltaic power station as an example, configuring bypass diodes can reduce the power loss caused by thermal spots from 15% to below 3%, increase annual power generation by about 12 million kWh, and have a payback period of only 2-3 years.