Abstract:
As an indispensable component in electronic circuits, the failure rate of electrolytic capacitors accounts for more than 30% of circuit failures. This article deeply analyzes the five typical failure modes of electrolytic capacitors: capacity attenuation, chain damage of rectifier circuits, hidden dangers of false soldering, overvoltage and reverse polarity, proposes solutions based on application scenarios, and cites authoritative industry data and cases to provide engineers with a systematic maintenance guide.
I. Capacity attenuation: the “invisible killer” of switching power supply vibration
1.Failure mechanism: In switching power supplies, electrolytic capacitors are usually connected in parallel to the power supply end of the PWM chip (such as UC3842). The attenuation of their capacity will cause the chip power supply voltage to be unstable, causing the power supply to fail to start or repeatedly protect.
2.Data support: According to statistics from the China Supercapacitor Industry Network, about 45% of switching power supply failures are related to the decrease in filter capacitor capacity. For example, a certain brand of inverter has a 50% surge in output ripple and triggers overcurrent protection because the capacity of the 100μF/50V electrolytic capacitor drops below 30μF.
3.Solution:
- Replacement principle: Give priority to 105℃ high temperature capacitors, which can increase the capacity by 20%-50% (such as the original 10μF can be replaced with 22μF).
- Detection tool: Use an LCR meter to measure the ESR value. If it exceeds 2 times the [KEMET capacitor ESR standard], it needs to be replaced.
II. Chain damage of rectifier circuit: “symbiotic crisis” of capacitors and rectifier bridges
1.Fault scenario: If the filter electrolytic capacitor (such as 400V/220μF) at the rear end of the rectifier bridge breaks down due to surge, it will directly cause the rectifier tube to burn out due to overcurrent. Capacitor failure can increase the failure rate of the rectifier bridge by 70%.
2.Experimental verification: In an AC 220V input circuit, if the electrolytic capacitor leakage current is greater than 1mA (the standard should be less than 0.01CV), the temperature rise of the rectifier bridge will exceed 85℃, accelerating aging.
3.Maintenance strategy:
- Synchronous replacement: The rectifier bridge and capacitor must be replaced in pairs to avoid residual damage causing secondary failures.
- Reinforcement design: Parallel TVS diodes (such as P6KE440A) absorb spike voltages to reduce the risk of breakdown.
III. Hidden solder joint hazards: the “hidden threat” of large-capacity capacitors
1.Risk analysis: Large-capacity electrolytic capacitors (such as 10000μF/450V) are prone to mechanical stress and cause solder joints due to thick pins and large pad areas. Statistics from an industrial power supply manufacturer show that solder joint problems account for 18% of its after-sales failures.
2.Detection technology:
- Thermal imaging detection: The temperature of a solder joint is usually 10-15℃ higher than that of a normal solder joint.
- Vibration test: Apply 5G acceleration vibration to the capacitor, monitor the change of pin impedance, and screen potential defects.
- Safety regulations: Before maintenance, use a bleeder resistor (such as 10kΩ/5W) to discharge the high-voltage capacitor to avoid the risk of electric shock.
IV. Overvoltage: The ignored “peak voltage trap”
- Calculation error: The DC voltage after rectification is not simply 1.414 times the AC effective value. For example, after 18V AC is rectified by full bridge, the no-load peak value can reach 25V (theoretical value 18×√2≈25.45V). If a 25V withstand voltage capacitor is selected, the actual margin is less than 10%, which is very easy to fail.
- Industry standards: According to [TDK Electrolytic Capacitor Selection Manual]: The operating voltage should be ≤80% of the rated value. For example, a 24V circuit needs to use 35V and above specifications.
Design optimization:
- Redundant design: Add a varistor (such as 14D471K) on the AC side to limit input surges.
- Multiple capacitors in series: For high voltage scenarios (such as ≥450V), use two capacitors in series and a voltage-equalizing resistor (100kΩ/2W).
V. Reverse polarity: an irreversible “fatal error”
1.Failure mechanism: When the electrolytic capacitor is reversed, the oxide film is reduced, and hydrogen is generated inside, causing bulging or even explosion. Experiments show that when the reverse voltage exceeds 1V, the 1000-hour life will be shortened to less than 10 hours.
2.Protection plan:
- PCB error-proofing design: Use special-shaped packaging or silk-screen polarity identification (in accordance with [IPC-7351 standard]).
- Automatic detection circuit: Integrated polarity detection IC (such as LTC4412), automatically cut off power supply when reversed.
Summary
The five major faults of electrolytic capacitors (capacity attenuation, chain damage, cold soldering, insufficient withstand voltage, and reverse polarity) directly affect equipment reliability. Through quantitative testing (such as ESR value monitoring), redundant design (voltage resistance increased by 20%) and standardized maintenance process (synchronous replacement strategy), the failure rate can be reduced by more than 80%. Engineers need to combine theoretical analysis with actual cases to establish a systematic fault diagnosis framework in order to achieve efficient maintenance and long-term stable operation.