Find the 'crux' of power supply reliability design to 'prescribe medicine'

by:MOSO     2019-12-28

There are many power supply manufacturers in China now, but there are no world-class brands. Why? The products of many domestic power supply manufacturers have good functions and low prices, but they are still not competitive. Why? The main reason is that the gap between domestic electronic products and foreign leading enterprises' products is no longer the gap in functional performance, but the gap in quality and reliability.

The main reasons for poor quality and reliability are: low design level and lack of reliable hardware application experience. According to the statistical analysis of failure analysis data of electronic products, the failure rate of power supply occupies a very high proportion and is often among the best, which shows that the reliability of power supply is relatively low, another aspect also shows that as long as the reliability of the power supply is improved, the reliability of the whole product can be greatly improved.

everything should be traced back from the source. Let's first look at the main factors that affect the reliability design of power supply. There are many main factors, such as layout in high noise power supply environment, transformer design, component selection, electromagnetic compatibility and electromagnetic interference design, overheat protection, etc. are all key factors affecting power supply reliability. Since we have found the crux of the problem, we must begin to prescribe the right medicine. How to prescribe it? A few days ago, at the Innovation Seminar on home appliance power supply and frequency conversion Control Technology held in Shunde, PI senior field application engineer Xie Houlin gave a detailed explanation on this issue.

02 Transformer Design

This paper mainly expounds the transformer design from two points, one point is to avoid transformer saturation, I . e. high magnetic flux density, one thing is to ensure good coupling between windings. In steady state operation, the magnetic flux density is limited below 3800 Gauss. A certain margin should be provided in typical design, and the influence of temperature factor should be considered. At the same time, the current waveform under the worst conditions should be checked. The worst conditions include these conditions: full load, minimum input voltage, maximum operating temperature, various input voltages, etc. How to ensure good coupling between windings? First, try to reduce the leakage, which can reduce electromagnetic interference (EMI)And improve efficiency; If the lower primary distributed capacitance can be realized in the design of the second transformer, the switching loss can be reduced, the efficiency can be improved, and the temperature rise of the power supply can be improved.

03 component selection

because components directly determine the reliability of the power supply, the selection of components is very important. The failure of components mainly focuses on the following four points: manufacturing quality problems, device reliability problems, design problems and loss problems. This should be paid enough attention in use.

The voltage stress of various devices in the power supply is an important factor affecting the reliability of the power supply. In the power supply, many devices have specified maximum withstand voltage values, such as Vds and Vgs of FET, reverse withstand voltage of diode, maximum VCC voltage of IC and maximum withstand voltage of input and output capacitance. Therefore, we must consider the maximum voltage to be borne by the device when designing. Then select the appropriate device according to the voltage, and finally carry out actual tests to verify it. The current stress of each device in the power supply is often closely related to the thermal stress. Therefore, we must meet the current stress and thermal stress of the device at the same time when selecting the device; On the premise of satisfying the thermal stress of the device, the device with appropriate rated current value can ensure the reliability requirement of the power supply.

It is not difficult to design the power supply itself, but it is definitely not easy to design a stable and reliable power supply. We only have to fully consider all the factors that affect it, and continue to debug and verify, we can design a higher quality, more stable power supply.

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