Flyback power supply component capacitor life

　　Although measuring and calculating first is a good start for optimizing component selection, for long-term reliability, the most important parameter is the temperature rise of the components in the working environment, which can be measured in the finished product.

　　Temperature rise is a function of component stress, heat conduction design, airflow, and the proximity of surrounding components. Radiation and thermal convection from adjacent components cause a greater temperature rise on the component than the internal resistance loss. This is especially true for electronic capacitors.

　　Due to the influence of ripple current and peak operating temperature, the maximum allowable temperature rise of a capacitor varies with the type of capacitor and the manufacturer. For the component used in this example, the maximum allowable temperature rise for ripple current in the air is 8°C, and the manufacturer uses this to determine the ripple current rating. This rating can be used when the ambient air is up to 85°C or the maximum temperature is 93°C.

　　Regardless of the effect of temperature rise, the absolute limit of temperature (in this example, 93°C) needs to be used to determine the limit of unit operation. This should be measured under the normal environment and the maximum rated temperature and load. The maximum temperature affects the life of the capacitor. Therefore, it is recommended that the capacitor work at a lower temperature. If you question the temperature rise caused by the internal ripple current (which is difficult to calculate under a complex flyback waveform), proceed as follows.

　　(1) Measure the temperature rise of the capacitor under normal working conditions, and keep it away from other heat sources (if possible, place the capacitor on a short winding wire, insert a heat shield between the device and the capacitor), and measure it by the ripple component alone The temperature rise caused is compared with the manufacturer's limit value. The allowable temperature rise is not always obtained from the data sheet, but can also be obtained from the manufacturer's test and QA department. The typical value of allowable temperature rise is between 5~10℃.

　　(2) If the temperature rise caused by the ripple is acceptable, install the capacitor in a normal position, let it withstand the highest temperature impact and load condition of the power adapter, measure the surface temperature of the capacitor to ensure that it is in the manufacturer's Within the rated value, this method must ensure to avoid heat dissipation, which is possible with electrolytic capacitors.

　　The power components of the flyback system have been discussed in considerable detail. To get good performance and reliable work, the most basic thing is to pay attention to the ratings and working conditions of each component. For engineers of power adapter manufacturers, this will become the second feature.

　　Component selection is a laborious process, this process is essential to get the most economical and reliable parts. The calculation only takes a small part of the designer's time. For these choices, a large amount of important information cannot be obtained without proper measurement.

　　The leakage inductance, wiring layout and size of the transformer, ESR and ESL values of output components, component layout and cooling arrangement all have a considerable influence on the impact and rating of the components. These effects cannot be reliably predicted. When no real measurement is performed, a larger safety margin should be taken for the calculated value when selecting the component rating.

　　Most optimization and verification experiments are easier to implement during the design approval phase, and they will be limited to those prototypes designated as final products.

　　If a reliable and economical design is required for long-term work, the professional power adapter engineer should ensure that the design optimization is performed before the product is completed, and all required verification experiments are implemented.

　　Select the power components of the flyback converter

　　Generally, under the same power, the component level required by the flyback converter is higher than that of the forward converter of the same power. In particular, the ripple current of the output diode, output capacitor, transformer, and switching transistor is required to be greater. However, the circuit is simplified, no output inductor is required, and each output power adapter has only one rectifier diode, which can offset the cost increase caused by larger components.

　　In flyback applications, the selection of components should meet the special needs of the voltage and current of each unit circuit. However, the designer needs to keep in mind that even for the same rated output power, different working methods have different stress conditions on the components. The selection of power components described in the following chapters is particularly suitable for the ones given in Chapters 1 and 2 of Part II. Flyback converters, but usually also used in all flyback converters.

　　The figures and components shown are only examples, not to say that they are the most suitable necessary components. Many manufacturers have similar suitable components available.

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