Design of load factor for switching power supply reliability

Since the load rate has a significant impact on reliability, an important aspect of reliability design is the design of the load rate. According to the characteristics of components and practical experience, when the load rate of components is within the following values, the reliability and cost of the power supply system are better.

(1) Semiconductor components. The voltage derating factor of semiconductor components should be less than 0.6, and the current derating factor should be less than 0.5. In addition to the load rate, semiconductor components also have tolerance design. When designing a switching power supply, the allowable variation range of semiconductor component parameters should be appropriately relaxed, including manufacturing tolerance, temperature drift, time drift, drift caused by radiation, etc. This ensures that when the parameters of semiconductor components change within a certain range, the switching power supply can still work normally.

(2) Capacitors. The load rate (ratio of operating voltage to rated voltage) of capacitors should preferably be around 0.5, and generally should not exceed 0.8, and non-polar capacitors should be used as much as possible. Moreover, under high-frequency application conditions, the voltage derating should be further increased, especially for electrolytic capacitors. Special attention should be paid to the fact that capacitors have the problem of low-voltage failure. The voltage derating factor for ordinary aluminum electrolytic capacitors and non-polar capacitors should not be less than 0.3, but the voltage derating factor for tantalum capacitors should be below 0.3. The voltage derating should not be too much, otherwise the failure rate of the capacitor will increase.

(3) Resistors and potentiometers. The load rate of resistors and potentiometers should be less than 0.5, which is the upper limit of resistor design. However, a large number of tests have shown that when the resistor derating factor is lower than 0.1, the expected effect will not be achieved and the failure rate will increase. The resistor derating factor is 0.1 as the lower limit of reliability derating design.

In short, the load rate of various components should be kept at around 0.3 whenever possible, and preferably not more than 0.5. With such a load rate, the probability of causing unreliability in the power system is very small.