Even the best power conversion system, efficiency will never reach 100%. A small amount of unrealized conversion energy will become heat, which is the challenge of system reliability. Without efficient thermal management, power transistors A heat-generating component such as a resistor may overheat during operation, resulting in an early failure or, in extreme cases, exceeding its maximum rated temperature, resulting in rapid component damage.
Reliability follows Arrhenius's law, which encourages cooling to improve reliability: Lowering the operating temperature of a component by 10 °C can double its life. In addition, measures are taken to ensure a lower junction temperature to increase power capacity. And allows the power supply to operate safely over a wider range of ambient temperatures.
A small amount of electrical energy, after entering the power transistor, is not transmitted to the load, but is dissipated as heat at the junction of each device. The relationship between junction temperature and power consumption is as follows:
Tjmax = (PDmax x Rθja) + Ta
Where Tjmax is the junction temperature PDmax is the maximum power consumed Rθja is the thermal resistance of the junction to the surrounding environment Ta is the ambient temperature
When designing a power supply, the goal is to design the junction temperature not only to protect the device, but also to ensure the required reliability. The device's data sheet efficiency curve can be used to estimate the maximum power consumption, as well as the heat from the surrounding environment. The resistance Rθja can be found in the data sheet curve, which also takes into account other cooling effects such as PCB metallization and airflow.
When the required power is supplied to the load, if the acceptable junction temperature is not achieved, the design effort must focus on reducing Rθja. There are several technical ways to achieve this, including:
Choice of package. Take advantage of the inherent enhancements of the package, such as the more thermally efficient clamps to replace the traditional lead connections, or the expanded metallization area on the underside or upper side of the chip, or double-sided cooling on both sides. The area is directly connected to the heat sink or exposed metal pad and can be soldered to the PCB metallization layer or to the heat sink. Board design. Includes increased copper thickness, or directly connected to the heat sink (eg heavier metal) Adding thermal vias under the heat sink. If you need very high heat dissipation, consider insulating metal substrates. More direct thermal management techniques, such as heat sinks or heat sinks, may be used with cooling fans.
The question now is which of these technologies can achieve the best results, such as acceptable size and weight, and minimal impact on bill of materials (BOM) costs, etc. Although there is no clear answer, it is very obvious Excessive or insufficient thermal solutions have unfavorable potential consequences.
It is not feasible to study different thermal designs by building multiple prototype designs. On the other hand, if you find that the selected solution is not suitable at the end of the project, redesign the board to add additional vents or switch to other The type of package may also be impractical.
Fortunately, we now have the tools at hand to help with the design. This thermal simulation software helps engineers look at thermal behavior from a system perspective and identify problem areas before implementing the first prototype.
Some of the thermal simulation online tools are even available for free, and TI's WebTHERMTM is one example of a thermal analysis of power designs created using the WEBENCH® online environment. Using selected controllers or DC/DC converter ICs, In addition to the power input and output voltage range requirements, the power supply can be originally designed as a WEBENCH project.
After the basic design is completed, WEBENCH can estimate the bill of materials and calculate parameters such as power loss and Rθja, which can be used to calculate Tjmax manually with known ambient temperature data. By running webTHERM simulation, the user can graphically view thermal performance, and It can also display some auxiliary effects, such as the common thermal effect of components that may be difficult to visualize. The result of the simulation is a color temperature curve that helps to quickly determine the area of interest.
When running the simulation, the user needs to input the load current, the top and bottom ambient temperature and the device case temperature and other parameter settings. The thermal simulation can be completed in a few minutes, and the results can be graphically analyzed through a color temperature curve. The design can be changed in WEBENCH, and the thermal performance can be further optimized by changing the board size or copper material characteristics on any PCB layer, or by adding and adjusting thermal vias.
Thermal simulations can be run multiple times and the results compared to determine an acceptable temperature design. If the appropriate Tjmax cannot be ensured, additional thermal management functions such as heat sinks or heat sinks can be used for faster access from the system. Remove heat. Temperature curves can help focus on areas that require attention.
Add heat sink
The heatsink is easy to understand and very reliable, has no moving parts, no trouble-free mode, and does not require any operating costs. The heat sinks are usually made of materials such as aluminum or copper, ranging from simple stamped metal wings for individual transistors to milling or Extrusion parts, wherein the fins are designed to intercept convective airflow for maximum heat transfer. As the hot air flow rises, convection naturally occurs, which allows the airflow to persist. Care must be taken to ensure unobstructed gas from the inlet to the outlet. Flow and ensure that the airflow inlet is below the radiator level and airflow outlet. This helps prevent the hot airflow from stagnating above the heat sink to avoid possible junction temperature rise.
Although heat sinks have many advantages, they can be bulky, bulky, and costly if large amounts of heat are to be dissipated. The location of the heat sink that achieves optimal airflow can affect board layout, and the heat sink can be dusty or dirty. Blocking, thus affecting the cooling effect. Using the clamps or screws or a layer of thermal interface material (TIM) to properly connect the heat sink to the assembly will also increase assembly time.
Manufacturers such as Aavid Thermalloy or Wakefield-Vette offer a wide range of heat sinks, including heat sinks optimized to fit specific components such as processors or FPGAs. On the other hand, heat sinks can be selected based on calculations, due to These heat sinks reduce the overall thermal impedance Rθja of the air from the die junction to the heat sink, thus enabling a lower junction temperature at a specific power dissipation.
Figure 1 shows a power transistor in a thermally enhanced package designed for top-mounted PCB heatsinks with efficient double-sided heat dissipation. The system is shown between the running junction and the top and bottom of the PCB. Thermal impedance Rth network. The thermal resistance of the heat sink Rth indicates the efficiency of heat transfer from the heat sink substrate to the surrounding environment.
Extend design freedom with heat pipes
In some designs, due to overall size, board layout or airflow impediment limitations, it may not be suitable to connect a desired size of heat sink directly to a converter IC or power transistor. The heat sink shown in Figure 2 can provide a A practical alternative that allows heat to be transferred from the source to another location where a suitable heat sink can be placed to provide greater airflow for heat dissipation. The Wakefield-Vette model 120231 can withstand up to 25W Thermal load, but its size is only 6mm in diameter × 100mm in length.
The heat pipe itself is not a radiator, but a sealed pipe that transfers heat from the hot end to the cold end by using the phase change principle. At the hot end, the heat is absorbed and the working fluid in the tube is evaporated, after which the steam flows to the cold end and condenses into The liquid, which releases heat during this process. The liquid then returns to the hot end of the pipe and repeats the process. One of the advantages of the heat pipe is that no power is required to maintain the phase change mechanism, and the designer is free to place the heat pipe The cold end is selected in the most suitable position.
Forced air cooling
If passive thermal management using a heat sink or heat sink does not achieve the desired junction temperature, consider using a high quality fan from a manufacturer such as Delta Electronics for forced air cooling. Increase the size by selecting the fan size and adjusting the fan speed. Reduced airflow (cubic feet per minute (CFM)) for optimized and flexible cooling.
in conclusion
Proper thermal management is critical to maximizing the performance and reliability of PCB power supplies or DC/DC converters. Designers have a large number of tools that can be used, but over-design must be avoided to prevent excessive volume. High BOM costs or more complex assembly challenges. Accurate thermal simulation tools are available for free, and these tools provide a visual guide to meeting thermal management challenges before starting to build hardware. Such as custom-designed heat sinks, cooling ducts or cooling Other technologies, such as fans, can help overcome system limitations such as board layout or airflow.