Interpreting Complex Data Sheet Specifications—Part III

For the third installment of our ongoing series on interpreting power supply data sheets, we turn our attention to thermal specifications. Despite demands for higher power and wider input ranges, customers still want their power supplies delivered in small packages. As a result of this demand, engineers have to come up with creative ways to avoid overheating.

Here’s how the addition of information on thermal management solutions, such as heat sinks, adds to the length and complexity of data sheets:

Current Limiting Versus Heat Sinking. Current limiting the output and heat sinking are the two most common ways engineers avoid overheating in power supplies. Deciding which one to use often boils down to operating environment:

• Current limiting is a feature that can be built into the circuitry of a power supply, whereby resistors on the PC board limit the amount of current emitted by the transformer. Because this method requires free air convection, however, it cannot be used in enclosed environments.

• Heat sinking. This option eliminates heat from inside the power supply by dispersing heat and improving energy use. Adding the heat sink to the outside surface allows for a more direct airflow to reach the direct source of heat. This method radiates heat so it doesn’t get trapped inside and overheat the power supply. While heat sinking also relies on free air convection, there are special cases where the power supply can be made with a base plate that functions as the heat sink. This allows the unit to be used in enclosed environments with limited to no airflow, including oil refineries or other applications that involve toxic material or explosive gas.

Effect On Data Sheets. Data sheets need to reflect all the thermal specifications you need to properly design your power supply. For one, power supply manufacturers often have to test for different thermal environments—the results of which must be listed on the data sheet.

Each new thermal management solution—whether current limiting or heat sinking—must also be listed alongside a mechanical drawing. Bear in mind too that each solution can be delivered a number of ways. For example, manufacturers can provide heat sinks with clamps or without, or sometimes heat sinks can be built into the bottom of the package. Listing all of these variations, along with the drawings and test results, takes up a significant amount of real estate on the data sheet.