Basic rules for correctly performing thermal tests for electronics
Thermal tests are one of the most frequently performed tests at the stage of creating electronic devices. The main purpose of such tests is to determine whether the device will not only break down at extreme temperatures, but also whether the derating values for individual components, especially semiconductors, will not be exceeded. All types of diodes, transistors, microprocessors, LDO regulators, integrated voltage conversion circuits, and integrated Greatz bridges have a permissible junction temperature Tj (temperature junction) specified by the manufacturer, which in most cases is 150°C. Exceeding this value results in the destruction of the semiconductor junction structure, therefore it is necessary to perform appropriate tests and analyzes to prevent such situations.
What is worth knowing before performing thermal tests?
Below are some rules.
It is necessary to determine which components and under what conditions will heat up the most on the board.
In order to determine which components will be most exposed to high temperatures, preliminary worst-case calculations and analyzes must be performed.
When writing about test conditions, I mean not only the setting of temperature limits in the thermal chamber, but primarily such conditions as: the value of the main supply voltage, or, as e.g. in LED applications, the selection of the appropriate LED voltage bin.
In the case of switching power supplies powered from the 80V – 320V/50Hz network, equipped with a PFC (Power Factor Corrector) stage, the input components, such as the filter choke, Greatz bridge and PFC choke, get hot the most at the minimum value of the main supply voltage, so for 80V AC.
In the case of LED applications powered by integrated current sources, the most power losses on this current source will occur if LEDs with the lowest voltage bin are connected to its output. Once we have determined which components will be critical in terms of thermal testing, it is good practice to test such a board using an infrared camera to verify previously performed calculations and analyses.
We attach thermocouples to component housings using appropriate glue with as little as possible to eliminate the influence of glue on the measurement. The length of thermocouples also matters and it is worth keeping them as short as possible.
Please note where exactly the thermocouple point should be attached. This is important for tall components, such as electrolytic capacitors, as the heat from the components escapes upwards, i.e. the temperature of these components is the highest at the upper sphere of their surface.
Thermal testing time in the chamber
The time constant at which the temperature values will be determined must be determined in advance. If, for example, we have applications that use various types of chokes with ferromagnetic cores, it is worth remembering that these cores heat up the longest. This is due to their high thermal capacity values. For this reason, even if the temperature of the remaining components has long been established at a constant level, it is worth extending the tests to check what value the above-mentioned choke cores can reach. To sum up, thermal tests, from initial calculations and analyzes to the testing itself, require a lot of work to ensure the high quality of the finished product implemented for mass production.
AUTOR: Łukasz Liput, Senior Hardware Design Engineer, ALTEN Polska