Power


Challenges and alternative approaches in the specification of DC/DC converter power derating


By Oscar Persson, specialist R&D engineer, Flex Power Modules M


odern electronic systems pack vastly more functionality into ever smaller sizes. However, power consumption


also typically increases, so to match the trend, DC/DC converters providing system voltage rails must provide more power in smaller packages, that is, have higher ‘power density’.


While current converter designs can have very high efficiency, significant heat must still be extracted to keep critical components below their maximum temperature ratings. With the common use of ‘brick’ converters, it is quite normal that with no attached heatsink and no forced air, the part heavily derates with local ambient temperature and will often only supply full power from around 20 Celsius, an unrealistic temperature inside a housing that might have kWs of local loads such as in server racks. To achieve operation at typical local environments at 50°C or more, forced air or conduction cooling must be applied, and now the thermal characterization of the converter, the effect of airflow, and the other heat paths to the surroundings become even more important considerations.


Assumptions must be made about the DC-DC environment There are many variables when considering the effectiveness of airflow and other heat flow paths on a DC/DC converter. These include: n Local ambient temperature n Airflow velocity n Direction of airflow n Orientation of the converter n Size of the printed circuit board (PCB) n Number and thickness of the conductive layers within the PCB


n Layout of the PCB n Other component power dissipation on the PCB


36 February 2022 Components in Electronics Figure 1: A test setup to characterize DC-DC converter temperature derating. Source: Flex Power Modules www.cieonline.co.uk


n ‘Shadowing’ of airflow by other components


n Turbulence of the airflow


Converter manufacturers cannot predict most of these effects in an end-system so they can only make assumptions in the characterization of their parts with a defined test set-up such as the one in Figure 1 used by Flex Power Modules. This is used to generate derating curves such as Figure 2, shown for their part PKU4213D, a 12V, 15/17A output, isolated converter with 36-75V input.


The test arrangement is intended to be a reproducible representative case using a test board of defined area, with a specified number and thickness of copper layers, depending on the power level of the converter being evaluated. The ‘opposite’ board shown is important to try to more closely represent a real rack environment and affects airflow direction and turbulence close to the power module. For rapid results, measurements


are made at local ambient temperature, typically a few degrees higher than the room, then hot spot temperatures of defined critical components on the converter are monitored, along with defined airflow close to the test board. At a chosen airflow rate, nominal input voltage and load current, the difference between hot-spot and local ambient temperature is measured to give a temperature rise, and then this is extrapolated to give a maximum local ambient before the critical component temperatures are exceeded, resulting in the derating curves.


Airflow derating leaves some unknowns


The arrangement and measurements described allow some comparison between alternative DC/DC converters (although not all manufacturers use the same test setup), but will always be different to the end-customer’s environment, so it can only be the starting


point for evaluation of power available from the DC/DC converter. When this power is extrapolated from temperature rise limits, it is assumed that efficiency remains the same at higher ambient. This is not the case however, due to variation in semiconductor switch voltage drops, increase in conductor resistance and increased magnetic core losses, all of which work to decrease efficiency. The extra losses themselves contribute to higher temperatures so the effect is cumulative. Some components, such as MOSFETs, have on- resistance and conduction loss that strongly increase with temperature, whereas diodes, for example, drop less as temperature rises. This means that the thermal ‘footprint’ or gradient across the DC/DC components varies in a non-linear way with temperature. At higher load current not only does efficiency typically worsen because of component and copper losses but thermal resistances can change as well. An important


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