How to replace faulty diodes in energy equipment on site?

一, Fault diagnosis and localization
1. Phenomenon identification and preliminary judgment
Appearance inspection: Observe whether the diode package is cracked, the pins are oxidized or burned (such as black pins in TO-220 package). In a wind power converter case, the oxidation of diode pins caused an increase in contact resistance, leading to local overheating.
Odor and sound: The faulty diode may emit a burnt smell or be accompanied by a slight arc sound (such as the "crackling" sound produced when reverse breakdown occurs).
Abnormal temperature: Using an infrared thermal imager for detection, the junction temperature of the faulty diode may be 20-50 ℃ higher than that of normal devices. In a certain photovoltaic inverter case, the diode junction temperature reached 140 ℃ (normal value ≤ 110 ℃), triggering overheating protection.
2. Electrical parameter testing
Offline testing: Use LCR tester to measure static parameters, with a focus on:
Forward voltage drop (VF): The VF of Schottky diodes should be ≤ 0.5V (such as 1N5819). If the measured value is greater than 0.7V, it indicates device aging.
Reverse leakage current (IR): When VR=800V, the IR of a 1000V diode should be ≤ 10 μ A. If it exceeds 50 μ A, it needs to be replaced.
Reverse recovery time (Trr): The Trr of the fast recovery diode should be ≤ 50ns (such as MUR860). If it exceeds 100ns, it will affect the switching efficiency.
Online testing: Capture the voltage waveform at both ends of the diode through an oscilloscope. When reverse breakdown occurs, a negative peak (such as -10V) will appear, while the normal waveform should be a smooth rectified waveform.
3. Root cause analysis of faults
Overvoltage breakdown: Check if the driving circuit generates a spike voltage (such as dv/dt>5kV/μ s when IGBT is turned off).
Overcurrent burnout: Verify whether the current protection threshold is reasonable (such as triggering the protection within 10 μ s at 1.2 times the rated current).
Thermal runaway: Check if the cooling system is blocked (such as dust accumulation in the air duct causing a 30% increase in thermal resistance).
二, Device selection and verification
1. Key parameter matching
Voltage level: The voltage withstand value of the replacement device should be ≥ 1.2 times that of the original device (if a 600V diode is used, a 700V model can be selected).
Current capacity: The rated current should be ≥ 1.5 times the maximum operating current of the system (for example, if the maximum current of the system is 30A, a 45A diode should be selected).
Switching frequency: High frequency applications (such as>50kHz) require the use of fast recovery diodes with Trr<35ns (such as ESD5B series).
2. Packaging and installation compatibility
Physical dimensions: The pin spacing and thickness of the replacement device must be consistent with the original device (such as TO-247 package pin spacing of 2.54mm).
Installation method: Screw fixed type requires verification of torque (such as M3 screw torque of 0.6-0.8N · m), welding type requires control of welding point temperature (≤ 260 ℃).
Heat dissipation matching: If the original device uses heat sinks, it is necessary to ensure that the thermal resistance (R θ JA) of the new device is ≤ the original device (such as reducing from 5 ℃/W to 4 ℃/W).
3. Verification of alternative solutions
Reduced usage: In light load scenarios, higher voltage resistant devices can be used as substitutes (such as using 1200V diodes instead of 600V models).
Parallel expansion: If the current of a single tube is insufficient, devices of the same model can be connected in parallel (with VF dispersion controlled to be ≤ 5%).
Upgrade and replacement: Replacing silicon diodes with SiC diodes can reduce VF by 30% (such as from 1.2V to 0.8V) and improve efficiency by 2%.
三, On site replacement operation specifications
1. Safety preparation
Power off operation: Disconnect the DC side fuse and use a multimeter to verify that there is no voltage (residual voltage<36V).
Personal protection: Wear insulated gloves (withstand voltage ≥ 1000V) and anti-static wristbands (resistance<1M Ω).
Tool preparation: Use an electric soldering iron with ESD protection (temperature adjustable to 350 ℃), tin absorber, and torque screwdriver.
2. Disassembly process
Welded components:
Heat the solder joint to 240-260 ℃ and remove the solder with a tin absorber.
Gently shake the device to detach it from the PCB, avoiding violent pulling that may cause the solder pads to detach.
Clean the solder pad residue (using anhydrous ethanol and cotton swabs).
Screw fixed components:
Use a torque screwdriver to loosen the screws in diagonal order (rotating 45 ° each time).
Record the screw position to avoid confusion (such as marking "1" and "2").
Pay attention to the bending direction of the pins when removing the device.
3. Installation of new components
Welded installation:
Apply lead-free solder paste (Sn96.5Ag3Cu0.5) on the solder pads.
Align the pins and solder pads, heat to 250 ℃ to melt the solder.
Check if the solder joints are full (without virtual soldering or bridging).
Screw fixed installation:
Apply thermal grease (thickness 0.1-0.2mm) between the device and the heat sink.
Tighten the screws in diagonal order, with a final torque of 0.6-0.8N · m.
Verify that the distance between the pins and the PCB is greater than 0.5mm to prevent short circuits.
四, Verification testing after replacement
1. Static testing
Positive conduction test: Apply 0.5V DC voltage, and the measured current should be ≥ rated value (such as 1A diode current>1.2A).
Reverse blocking test: Apply 80% of the rated reverse voltage (such as applying 480V to a 600V diode), and the leakage current should be less than 1 μ A.
2. Dynamic testing
Light load test: Input 10% rated current and verify that the output waveform is distortion free (THD<3%).
Full load test: Input rated current, run continuously for 2 hours, monitor junction temperature (≤ 110 ℃).
Transient testing: Simulate switch actions (such as IGBT turning on and off 1000 times per second) to verify that the diode has no overvoltage spikes.
3. System integration debugging
Control parameter calibration: Adjust the driving resistance (such as from 10 Ω to 8 Ω) to optimize the switching speed.
Protection threshold verification: Trigger overcurrent protection (such as 1.2 times rated current) and record the action time (should be<10 μ s).
EMC testing: Compliant with IEC 61000-4-5 standard, capable of withstanding 8kV/5kA surge impact.
五, Typical Case Analysis
Case 1: Replacement of DC side diodes in photovoltaic inverters
Fault phenomenon: The inverter reports a "DC Link Overvoltage" fault, and upon inspection, it was found that the DC side anti reverse diode had broken down.
Replacement process:

Choose the same model of 1000V/20A fast recovery diode (MUR2010CT).
During welding, control the temperature of the soldering iron to 250 ℃ and the welding time to be less than 3 seconds.
After replacement and full load testing, the efficiency increased from 97.2% to 97.5%.
Case 2: Replacement of embedded diodes in IGBT modules of wind power converters
Fault phenomenon: The inverter reports an "IGBT Overheat" fault, and detection shows that the embedded diode VF has risen to 1.4V (normal value ≤ 1.1V).
Replacement process:

Choose SiC diode (C3D10060H) instead of silicon diode, with a withstand voltage of 600V and VF=1.2V.
Adjust the driving resistance from 15 Ω to 12 Ω and optimize the switching speed.
After replacement, the system efficiency increased by 1.8% and the junction temperature decreased by 15 ℃.