Power Module Thermal Derating, Part II
Author: Jim Beauchamp, Regional Power Specialist, Avnet EM
Suppliers typically provide thermal derating curves based upon a test methodology that is conducted at their facility. The test methods utilized can vary as there is no official industry standard method for thermal modeling of power modules. The most common approach is the use of a wind tunnel, a test board that consists of the device under test soldered in place, either thermocouples or a thermal camera, and a data logger to capture test results based upon various input voltages, output load, air flow/direction, component orientation, and shadowing of adjacent PCBs and/or components. To properly interpret the manufacturer’s results, the system designer should determine if the supplier used a thermal camera or thermocouples and if the measurements were made at a single location on the PCB or at critical “hot spot” components. Due to other thermal complexities in the end system, the supplier specified thermal de-rating curve will only allow the engineer to arrive at a ballpark power solution prior to final design verification.
Thermal testing within the final system is critical to verify design performance. Supplier evaluations do not take into account such system variables as PCB layout including number of layers and copper thickness, air flow obstructions, air flow turbulence, adjacent heat generating devices, PCB spacing within the system, failure modes, and a myriad of other variables. As thermal imaging cameras probably will not fit into the system under test, thermocouples may be required for system test.
The most critical parameters to measure are the PCB temperatures adjacent to the module and any hot spot locations which reside on the module itself. These measurements must be performed at the worst case maximum ambient temperature of the end system. Hot spot analysis is especially critical to determine semiconductor and magnetic maximum operating temperatures. FETs in particular are susceptible to degradation if operated at or near its maximum junction temperature. As stated above, this can result in degradation of the overall module and system MTBF.
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