What is the impact of diode breakdown on the inverter?

一, Types and physical mechanisms of diode breakdown
Diode breakdown can be divided into two types: electrical breakdown and thermal breakdown, and their physical mechanisms are closely related to material properties, doping concentration, temperature, and other factors.

1. Electric breakdown: a reversible physical process
Electric breakdown includes two mechanisms: Zener breakdown and avalanche breakdown

Zener breakdown: occurs in highly doped PN junctions (such as voltage regulators), where the depletion layer width is extremely narrow (<1 μ m). Under the action of reverse voltage, a strong electric field directly pulls out the valence electrons in covalent bonds, forming electron hole pairs, resulting in a sharp increase in reverse current. Zener breakdown voltage is usually below 4V and has a negative temperature coefficient (breakdown voltage decreases with increasing temperature).
Avalanche breakdown: commonly seen in low doped PN junctions, with a wide depletion layer (>10 μ m). Reverse voltage accelerates minority carriers, causing them to collide with the lattice and generate new carriers, forming an avalanche chain reaction. The avalanche breakdown voltage is generally higher than 6V and has a positive temperature coefficient (the breakdown voltage increases with temperature).
Electric breakdown is essentially a reversible physical process.

2. Thermal breakdown: irreversible catastrophic failure
When the reverse current continues to increase after electrical breakdown, or when infinite current measures are taken in the circuit, the power consumption of the PN junction exceeds the limit value, resulting in a sharp rise in junction temperature. At this point, the valence electrons in the covalent bond gain sufficient energy to break free from atomic constraints, forming a large number of free electron hole pairs, further exacerbating the current growth and forming a positive feedback loop. Eventually, the PN junction melts due to overheating, forming a permanent short circuit, known as thermal breakdown. Thermal breakdown is irreversible, and the diode will completely lose its function.

二, Direct harm of diode breakdown to inverters
Diodes in inverters are mainly used for rectification, freewheeling, and clamping, and their breakdown can cause fault propagation in different paths.

1. Rectifying diode breakdown: power short circuit and capacitor explosion
In photovoltaic inverters or industrial power sources, the rectifier bridge consists of 6 diodes (3 common cathode and 3 common anode). If a single diode is thermally broken down to form a short circuit, it will cause the positive and negative poles of the DC bus to conduct directly, leading to a power short circuit. At this point, the filtering capacitor rapidly heats up due to overcurrent, causing the electrolyte to vaporize and expand, which may lead to an explosion. For example, in a certain photovoltaic power station, the breakdown of the rectifier diode caused the DC side capacitor to explode, resulting in the scrapping of the entire inverter module and a direct economic loss of over 100000 yuan.
2. Clamp diode breakdown: Bus voltage out of control
In multi-level inverters, clamp diodes are used to limit DC bus voltage fluctuations. If the clamping diode breaks down, the bus voltage may exceed the IGBT withstand voltage range, causing chain breakdown. For example, a medium voltage frequency converter experienced a breakdown of the clamp diode, causing the DC bus voltage to surge from 600V to 900V, resulting in damage to all 12 IGBT modules and a system shutdown time of up to 72 hours.

三, System level effects of diode breakdown
1. Electromagnetic interference (EMI) and signal distortion
When a diode breaks down, the rapid change in short-circuit current will generate high-frequency electromagnetic interference, which is coupled to the control circuit through parasitic capacitance and causes distortion of the IGBT drive signal. In a wind power converter case, the EMI interference caused by the breakdown of the freewheeling diode resulted in a 10 μ s pulse loss of the IGBT drive signal, causing the motor torque to fluctuate beyond 20% and triggering a mechanical vibration alarm.
2. Protection circuit misoperation and system paralysis
Modern inverters are usually equipped with overcurrent, overvoltage, and over temperature protection functions. However, diode breakdown may lead to misjudgment of the protection circuit:
Overcurrent protection misoperation: Short circuit current may be mistaken for a sudden change in load, triggering current limiting protection and causing system derating operation;
Overvoltage protection failure: If the clamp diode breaks down, the bus voltage monitoring point fails, and the overvoltage protection cannot be activated;
Overtemperature protection delay: The temperature at the diode breakdown point may be higher than the temperature at the sensor monitoring point, causing a delay in triggering the overtemperature protection.
In a case of traction inverter in a certain rail transit, the breakdown of rectifier diode caused overcurrent protection misoperation, resulting in frequent derating operation of the system. Eventually, due to heat accumulation, the IGBT module exploded and the train was stopped for 12 hours.
四, Protection Strategy and Reliability Design
1. Circuit design: redundancy and current limiting
Redundant design: In the rectifier bridge, an "N+1" redundant configuration is adopted, which means additional diodes are connected in parallel. When a single diode breaks down, the system can still operate at reduced capacity;
Current limiting resistor: Connect small resistance resistors (such as 0.1 Ω/5W) in series across the diode to limit the peak short-circuit current;
RC buffer circuit: Add an RC buffer circuit (such as C=0.1 μ F, R=10 Ω) to the IGBT diode parallel circuit to absorb the turn off overvoltage and reduce the reverse stress of the diode.
2. System monitoring: real-time diagnosis and predictive maintenance
Infrared thermal imaging detection: Real time monitoring of diode shell temperature through an infrared thermal imager, triggering an alarm when the temperature exceeds the rated value of 15 ℃;
Electrical parameter monitoring: Real time monitoring of diode current through current sensors (such as Hall sensors), and protection is activated when the current exceeds 1.2 times the rated value;
AI fault prediction: Train machine learning models based on historical data to predict the remaining life (RUL) of diodes and replace high-risk components in advance.