1. Introduction
A question often asked of RECOM technical support is, “What is the maximum operating temperature?” for a particular converter. Although there are several ways to determine this, it is still not an easy question to answer precisely. This is not because we don’t know how to characterise our products, but because the final answer depends on many different factors, many of which have nothing to do with the converter itself.
All operating power converters generate heat (caused by the internal power dissipation), which needs to escape the package to the ambient surroundings if the converter should not be overheating. Typical “hot spot” sources of heat are the power semiconductors such as transistors and diodes, which have switching junction losses, inductive components such as transformers and inductors that have both DC and AC losses and power resistors used in snubbers and filters with I²R losses. The design engineer has to minimise these losses to maximise the efficiency of the converter, but they are unavoidable.
How the internal heat escapes from a hot junction inside a semiconductor package is also dependent on many factors. The shortest ‘path-of-least-resistance’ is typically from the junction to the top surface of the case or housing, but from there, the heat energy can escape to the surroundings by either convection, conduction or radiation. Furthermore, there could also be thermal conduction paths to the surroundings via a baseplate using heatsinks or gap-pads to the housing, through the mounting pins to the PCB itself or the sides or bottom surfaces of the converter case.
Depending on the setup, even the results from thermal measurements may differ significantly. For example, what is ‘free air convection’ cooling? Is the cooling flow of air completed unrestricted? Or could there be a local build-up of heat caused by closely positioned components, adjacent boards blocking the airflow or poorly ventilated cases? Additionally, are the thermal measurements done with ‘still air’ with zero LFM or with free-air movement (20 LFM) or with forced-air cooling (100 LFM)?
For board-mount converters, there may be other issues such as whether the board is mounted horizontally or vertically if the board is made from glass fibre (FR4) or uses a different substrate or what copper thickness is used for the tracks. All of these factors can affect the thermal performance of the parts mounted on the board.
All operating power converters generate heat (caused by the internal power dissipation), which needs to escape the package to the ambient surroundings if the converter should not be overheating. Typical “hot spot” sources of heat are the power semiconductors such as transistors and diodes, which have switching junction losses, inductive components such as transformers and inductors that have both DC and AC losses and power resistors used in snubbers and filters with I²R losses. The design engineer has to minimise these losses to maximise the efficiency of the converter, but they are unavoidable.
How the internal heat escapes from a hot junction inside a semiconductor package is also dependent on many factors. The shortest ‘path-of-least-resistance’ is typically from the junction to the top surface of the case or housing, but from there, the heat energy can escape to the surroundings by either convection, conduction or radiation. Furthermore, there could also be thermal conduction paths to the surroundings via a baseplate using heatsinks or gap-pads to the housing, through the mounting pins to the PCB itself or the sides or bottom surfaces of the converter case.
Depending on the setup, even the results from thermal measurements may differ significantly. For example, what is ‘free air convection’ cooling? Is the cooling flow of air completed unrestricted? Or could there be a local build-up of heat caused by closely positioned components, adjacent boards blocking the airflow or poorly ventilated cases? Additionally, are the thermal measurements done with ‘still air’ with zero LFM or with free-air movement (20 LFM) or with forced-air cooling (100 LFM)?
For board-mount converters, there may be other issues such as whether the board is mounted horizontally or vertically if the board is made from glass fibre (FR4) or uses a different substrate or what copper thickness is used for the tracks. All of these factors can affect the thermal performance of the parts mounted on the board.
