Nowadays, the size of power module is getting smaller and smaller, and the power density is getting higher and higher, and the working environment of power module is getting worse and worse. The problems of high and low temperature design, thermal design and stress have gradually attracted the attention of engineers. What is the secret of reliability design of power module? This article reveals for you.
For a power supply module, the input and output characteristics such as input voltage range, rated power, isolation withstand voltage, efficiency, ripple-noise and so on should be satisfied to meet the application requirements. After that, the most common parameter that engineers pay attention to is its high and low temperature performance.
High and Low Temperature Testing
Generally, in different application areas, the requirements for the working temperature range of power module are different.
High and low temperature tests are used to determine the adaptability and consistency of products under extreme climatic conditions of low and high temperatures. Because the characteristics of components will change under the conditions of low temperature and high temperature, the performance parameters have the characteristics of temperature drift. So often many power modules are not in trouble at room temperature, but when they are tested in high and low temperature environment, they will find that their work is abnormal or their performance parameters are obviously reduced.
Low temperature and high temperature operation of power supply module often results in the following phenomena:
Operating oscillation, output voltage ripple and noise become larger, frequency changes, serious or even output voltage jump, module scream;
Poor start-up, such as the output voltage rise waveform has obvious trench drop, output voltage instability, or even module start-up failure;
The capacity of capacitive load is weakened and can not start with maximum capacitive load.
When starting, the output voltage overshoot range becomes larger, which exceeds the prescribed range.
The output voltage decreases significantly when the load is heavy or full.
High temperature aging damage, module output;
The thermal design of power supply module, in short, is to minimize the heat generated in the module, reduce the thermal resistance and choose a reasonable cooling mode by thermal design on the premise of meeting the performance requirements. The heating components should be distributed as far as possible. When designing PCB board, the current carrying capacity of printed line must be guaranteed, and the width of printed line must be suitable for current conduction. For high-power patch components, large area copper foil can be used to increase the heat dissipation area of PCB. The temperature rise of components in power module can be reduced by filling thermal conductive silica gel and resin. For larger power modules, heat sinks can be used to heat up, increasing the surface area of convection and radiation, thus greatly improving the heat dissipation effect of electronic devices.
The figure shows a power supply module that has not been encapsulated. After working at room temperature for a long time, the surface temperature of the power module is measured by infrared thermal imager. Among them, the maximum temperature of the MOS tube without encapsulation at room temperature is 85.5 C, and then the temperature of the product filled with encapsulation glue is tested at high temperature with thermocouple and data acquisition instrument. The maximum temperature is 97.2 C. For the MOS tube with the highest temperature of 175 C, the temperature drop meets the first grade reduction, and its performance is excellent.
The so-called reduction design is a design method to make the service stress of components lower than its rated stress. Reducing the quota of components makes the working stress of electronic components properly lower than the rated value. The specific reduction grade can be referred to the National Military Standard - Component Reduction Criterion GJB/Z35-93, which can be generally divided into three reduction grades:
For the stress design of power supply module, we focus on the field effect transistor (MOS), diode, transformer, power inductance, electrolytic capacitor, current limiting resistance and so on. Ensure that there is enough reduction in all kinds of limit conditions such as steady state, transient, short circuit, etc. within the full voltage range to ensure the reliability of the product. For example, for a MOS transistor with a maximum voltage of 100V, as the main power switch of the power supply module, its various states under the highest input voltage are measured (as shown in Fig. 1-3). The maximum Vds is 67.2V, and the reduction factor is 0.672, which satisfies the level I reduction. The balance is sufficient.
As the power module tends to be smaller and smaller, the power density is higher and higher, and the thermal design problems of the power module are particularly prominent. Especially for power module with electrolytic capacitor, high temperature will accelerate the consumption of electrolyte of electrolytic capacitor and greatly reduce the life of electrolytic capacitor. High temperature will accelerate the aging of components, such as reducing the insulation characteristics of transformer enameled wires, resulting in poor insulation voltage withstand and even inter-turn short circuit. Good thermal design can not only prolong the service life of power module and its surrounding components, but also make the whole product heat uniformly and reduce the occurrence of faults.