Analysis of the Failure of Flyback Power Adapter

　　Analysis of early failure problems

　　A large proportion of the failure of the flyback 6V power adapter is the early failure of the output rectifier filter capacitor. The main reasons are: the capacitor is unknown, the capacitor rated current is significantly less than the actual current, and the actual effectiveness of the electrolytic capacitor. The current value is significantly higher than the expected current value and the hydration reaction.

　　1. The source of the power adapter capacitor is unknown

　　The unknown source of the capacitor is the main problem that causes the failure of the electrolytic capacitor. The reason is very simple. Some electrolytic capacitors in the electronic market are the offline products of electrolytic capacitor manufacturers. The main problem is the excessive leakage current, and the final result is the electrolysis of the power adapter. The life of the capacitor is significantly shortened.

　　Electrolytic capacitors of unknown origin may also be manufactured by informal electrolytic capacitor manufacturers, and some are not even high-temperature aging before leaving the factory. Because they are directly powered on in the hands of the user, the defects generated during the manufacturing process are repaired. If there is no current limiting resistor, it will be generated. The high current and high heat will eventually lead to the "explosion" of the electrolytic capacitor.

　　Unknown electrolytic capacitors may also be disassembled capacitors, or even re-coated, changing the lower rated voltage to a higher rated voltage product. This will lead to the early failure of the electrolytic capacitor.

　　2. The capacitance is too low

　　Some low-end flyback power adapters choose low-capacity output rectifier filter capacitors in order to reduce costs.

　　A practical case: a 5V/1A output flyback power adapter, the output rectifier filter capacitor chooses a 330F/16V electrolytic capacitor, which leads to early failure. The power supply manufacturer considers it to be a problem with the electrolytic capacitor. The electrolytic capacitor data all show that the rated ripple current of the electrolytic capacitor under this capacitance and rated voltage is only 500-600mA, and the actual current can definitely exceed 1200mA, which obviously exceeds the rated current, which will inevitably lead to the early failure of the capacitor.

　　3. The rated current of the capacitor is significantly smaller than the actual current

　　If the design of the power adapter transformer is unreasonable, it will cause the ripple current of the output rectifier filter capacitor to be higher than the expected current, making the capacitor work in an overcurrent state, which will lead to early failure.

　　4. Hydration

　　Since the simplest and cheapest way to reduce the ESR of an electrolytic capacitor is to increase the moisture content of the electrolyte, and increasing the moisture content will aggravate the "hydration reaction" of the electrolytic capacitor under high temperature conditions, which will shorten the life of the electrolytic capacitor, and even lead to the early stage of the electrolytic capacitor. Invalid.

　　In order to prevent the "hydration reaction" of electrolytic capacitors with long and low ESR life, electrolytic capacitor manufacturers have spared no effort to come up with various anti-"hydration reaction" methods, such as adding anti-"hydration reaction" additives and other measures, if anti-"hydration reaction" If the additive purity is not enough, there will be early failure due to "hydration reaction". Not only that, the manufacturing process of high water content electrolyte capacitors is strict, and a slight deviation will lead to early failure of the product. Therefore, the manufacturing of electrolytic capacitors strives to avoid using high water content. High-speed, long-life and high-temperature electrolytic capacitors, and switching to e-commerce with low or no water content is also solving the problem.

　　The best way to avoid early failure due to "hydration reaction" is to avoid electrolytic capacitors working at high temperatures and try not to use electrolytic capacitors with high moisture content.

　　Do not expect the temperature conversion factor and frequency conversion factor to increase the operating current of the electrolytic capacitor

　　In order to reduce the cost of power adapters, sometimes the temperature compensation coefficient and frequency compensation coefficient of electrolytic capacitors are thought of to reduce the capacitance and rated current requirements. In fact, this is a relatively unrealistic idea. The reason is that, first of all, the low ESR rated ripple The wave current is under the condition of 100kH, and there is no room for improvement; secondly, trying to use the temperature compensation coefficient to reduce the rated ripple current of the capacitor, the result is a corresponding shortening of the life, which is not disputed.

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