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          Thermal design rules for power adapters you don't know
          Add time:2019-08-28    Click:948
          In addition to electrical stress, temperature is the most important factor affecting the reliability of power supply equipment, which will lead to the failure of components. When the temperature exceeds a certain value, the failure rate will increase exponentially. According to one of the design rules, the failure rate doubles for every 10 C rise in temperature when the components work at 65 C or above. This common rule is based on the assumption that the products used for comparison are made using similar design and manufacturing principles, and that the components work under similar conditions (for example, in a given peripheral environment, the temperature of the chip is the same). In fact, different design conditions will affect the overall performance and reliability of the power adapter.
          According to another design rule, if the power adapter works at 70%-80% of its rated maximum junction temperature (Tjmax), it will enjoy high reliability. For power devices in power adapters, Tjmax is usually guaranteed to be +150 C or +175 C. The junction temperatures of power devices in power adapters should be kept below +120 C or +135 C, respectively. According to this design rule, keeping the junction temperature of power devices in the power adapter at a low level will greatly improve the reliability of the whole system.
          Power adapter manufacturers develop thermal pointers or reduction curves for power adapters through in-house tests, usually with the assistance of wind tunnel systems, to determine the thermal performance of power adapters under different convective conditions. Because power adapter manufacturers test according to their own internal standards, these standards are often affected by existing test equipment, test costs and many other factors. These effects mean that the power adapter deduction curve can be misleading, so designers should take into account the impact of the results of these internal tests on the design.

          1) Fever image
          Another way to determine thermal performance by comparing module tests individually in special application systems is to use heating images. Another way to determine thermal performance is to use thermal images, even using infrared cameras to measure temperature. This is very effective for determining the correct temperature, but the designer must study the direction, type and stability time of the air flow in the power adapter. The best way to compare thermal data is to have different power adapters side by side for infrared scanning (including different directions and test boards).
          When comparing the reliability indicators of power adapters, we should first make clear what assumptions and conditions these indicators are obtained. Reliability is closely related to thermal performance and working temperature. The failure rate doubles with the increase of working temperature by 10 C. In typical applications, the calculation of MTBF is very meaningful, but due to the influence of heat generated by other components in the system, the temperature of air near the power adapter is generally about 55 C. This requires that the power adapter selected in the design must be able to provide the highest efficiency and minimum heat dissipation when the temperature rises, and the minimum temperature rise in the base plate (substrate).
          2) Deduction curve
          The reduction curve can show the pullable current or power at different ambient temperatures, while maintaining the power module within its temperature specifications. Wind tunnels have many different shapes and sizes, and power adapters can be placed in different locations of the wind tunnel, which will affect the test results. For example, consider whether the wind tunnel forces air to flow through the power adapter or whether the air flows freely through the power adapter. If the air flow system is large, then it is enough to let the air flow around the power adapter. Unlike funnel wind tunnel, funnel wind tunnel forces air directly to the power adapter. Since most applications do not use funnel or forced air flow, non-funnel testing procedures will yield the most robust results.
          The measurement of air flow is also very important. Hot wire anemometer should be used to directly measure the air flow in front of the power adapter to ensure the accuracy of flow. The laminar flow is a more conservative and robust method for airflow system, and better test results will be obtained. The drawdown curve is in the worst direction to ensure that the power adapter is not affected in any direction.
          In the process of testing, the longer the time of temperature stabilization, the more accurate the measurement results are. Based on this method, although the actual test time will be longer, but the measurement results are enough to ensure the stability of temperature, and accuracy is the most important part of the preparation of heat drop curve. In special applications, the test of power adapter should be compared individually.
          Reliability is closely related to thermal performance and working temperature. In fact, different design conditions will affect the overall performance and reliability of power modules.

                Disclaimer: This article comes from individual, KRECO has the right of final interpretation.