How to determine if the diodes in a solar energy system are damaged?

一, The core role and fault risk of diodes in solar energy systems
Types and functions of diodes
Bypass diode: parallel connected at both ends of the battery string, providing an alternative path for current when some battery cells are obstructed or fail, avoiding thermal spot effect (permanent damage to battery cells caused by local overheating).
Anti reverse charging diode: Connected in series between the photovoltaic array and the controller, it prevents the battery pack from discharging in reverse through the photovoltaic panel at night or on rainy days, protecting the battery life.
Blocking diode: used in multiple series parallel systems to prevent current from flowing back into low-power series, ensuring independent operation of each series.
Typical consequences of diode damage
Heat spot effect intensifies: After the failure of the bypass diode, the blocked battery cells continue to withstand reverse voltage, and the temperature may rise to over 200 ℃, leading to aging of the packaging material and even ignition.
System power generation decrease: Short circuit of anti reverse charging diode can cause the battery pack to self discharge at night, resulting in a daily loss of up to 5% -10% of electricity.
Risk of equipment damage: Blocking diode breakdown may cause inter string circulation, burning connectors or cables.
二, Visual manifestation and preliminary judgment of diode damage
1. Appearance abnormality detection
Erosion marks: The surface of a normal diode is smooth, but after damage, black burn marks, cracks, or pin melting may appear (as shown in Figure 1). For example, a 5kW photovoltaic system experienced carbonization marks at the connector due to the breakdown of the bypass diode.
Packaging deformation: High temperature causes expansion of epoxy resin or plastic packaging, commonly seen in long-term overload or short circuit scenarios.
Color change: Silicon material diodes will change from black to gray white at high temperatures, which can be identified by comparing components from the same batch.
2 System performance abnormalities
Unbalanced string voltage: Use a multimeter to measure the open circuit voltage of each string. If the voltage of one string is significantly lower than that of other strings (such as more than 10% lower than the rated value), it may be due to the failure of the bypass diode conduction.
Nighttime current backflow: In the absence of light, use a clamp ammeter to detect the output terminal of the photovoltaic array. If there is a reverse current (>0.1A), it indicates that the anti reverse charging diode is short circuited.
Abnormal heating: The infrared thermal imaging device detects the surface temperature of the diode. The normal operating temperature should be below 85 ℃. If the local temperature exceeds 120 ℃, the machine should be stopped immediately for inspection.
三, Professional detection methods for diode damage
1 Offline detection (system power failure)
Positive pressure drop test:
Set the multimeter to diode mode (or 2V DC mode).
Connect the positive pole to the anode of the diode and the negative pole to the cathode, and record the forward voltage drop (VF).
Normal silicon diode VF should be 0.5-0.7V, and Schottky diode VF should be 0.2-0.4V. If VF>1V or displays OL (open circuit), it indicates that the diode is open circuit; If VF<0.1V, there may be a short circuit caused by breakdown.
Reverse leakage current test:
Use a high-precision multimeter (such as Fluke 87V) in the μ A range.
Reverse connect the diode (with the positive pole connected to the cathode and the negative pole connected to the anode) and apply a reverse voltage (such as 20V).
The normal reverse leakage current of a diode should be less than 1 μ A. If it is greater than 10 μ A, it indicates a decrease in insulation performance.
2 Online detection (system energized)
Dynamic current monitoring:
Use an oscilloscope to capture the voltage waveform across the diode.
The normal bypass diode is in a reverse cutoff state when the string is normal, and the voltage is close to the open circuit voltage of the string; When the string is obstructed, the diode conducts in the forward direction and the voltage drops to around VF.
If the waveform shows continuous forward conduction or reverse leakage current fluctuations, it indicates diode parameter drift.
Infrared thermal imaging positioning:
Scan the diode area with an infrared thermal imager when there is sufficient sunlight.
The normal diode temperature should be close to the surrounding components. If the local temperature is higher than 20 ℃, it may cause overheating due to increased conduction loss.
四, Root cause analysis and preventive measures of diode damage
1 Common causes of damage
Overvoltage surge: Lightning strikes or grid fluctuations cause the reverse voltage of the diode to exceed the rated value (such as when the VRRM of 1N4007 is 1000V, a 20% margin is required for actual use).
Overcurrent and overheating: Improper string matching causes some diodes to carry excessive current for a long time (such as when the design current is 10A, but it actually flows 15A).
Manufacturing defects: virtual soldering or chip cracks in the internal metallization layer of the diode, which are difficult to detect in the initial stage and gradually fail after operation.
Environmental corrosion: In coastal or high humidity areas, oxidation of diode pins leads to increased contact resistance and accelerated aging due to local temperature rise.
2 Prevention and maintenance strategies
Selection optimization:
The rated current of the bypass diode should be ≥ 1.25 times the short-circuit current of the string, and the reverse voltage should be ≥ 1.5 times the maximum voltage of the system.
The anti reverse charging diode should be selected as a low VF type (such as SB5100, VF=0.45V) to reduce nighttime power consumption.
Installation specifications:
The length of the connecting wire between the diode and the battery string should be less than 30cm to reduce parasitic inductance.
The installation surface of the heat sink should be flat and coated with thermal conductive silicone grease (thermal conductivity>2W/m · K) to ensure thermal resistance<1 ℃/W.
Regular testing:
Scan key diodes with an infrared thermal imager every quarter and establish temperature records.
Conduct offline testing once a year to replace components with VF deviation>10% or leakage current exceeding the standard.
五, Case study: Troubleshooting of diode faults in a 10kW photovoltaic system
1. Fault phenomenon
After 3 years of system operation, the power generation decreased by 15% compared to the same period, and the nighttime battery pack voltage dropped from 51.2V to 49.8V.

2. Investigation process
Appearance inspection: It was found that the packaging of the bypass diode in string 2 was slightly bulging, and there were oxidation marks on the pins.
Offline testing:
The forward voltage drop test shows VF=1.2V (normal value 0.6V), indicating an increase in on resistance.
The reverse leakage current test is 5 μ A (normal value<1 μ A), and the insulation performance decreases.
System level verification:
After replacing the faulty diode, the voltage of string 2 returned to the same level as the other strings (36.5V).
At night, the voltage of the battery pack remained stable at 51.0V, and the power generation increased to 98% of the design value.
Root cause analysis
Improper selection: The original rated current of the diode was 8A, but the actual short-circuit current of the string reached 10A, which accelerated aging due to long-term overload.
Insufficient heat dissipation: No heat sink installed, diode junction temperature exceeds 125 ℃ for a long time.