Catch here
modules under different input/output voltage, output current and temperature conditions.
An alternative approach eliminates unknowns
Figure 2: Derating curves generated from the test setup in Figure 1. Source: Flex Power Modules part PKU4213D, open frame arrangement
Figure 3: A brick converter with a baseplate fitted shows significantly reduced hot-spot temperatures. Source: Flex Power Modules, part PKU4213D
heat path is through the DC/DC module pins to the circuit board, particularly with open frame parts. As an example, high values of load current through the resistance of these pins makes them thermal generators, effectively increasing their thermal resistance to heat flow from the DC/DC converter to the board. Of course, the end-user’s circuit board also sees the higher current and dissipates more power, adding to local temperature and thermal resistance from DC/DC module to the wider environment. For these reasons, a single value for individual element thermal resistance can only be an approximation. Another variable is whether the DC/ DC converter has a baseplate fitted. With forced air cooling, there is often the option to use an ‘open frame’ part, or one fitted with a baseplate. Although there may be no conduction cooling, the baseplate has a heat-spreading effect and provides a flat surface for heat to be eliminated from, so its effect is beneficial. As an example, Flex Power Modules showed that their PKU4213D product hotspots reduce by up to around 15°C at high load and airspeed, with the baseplate fitted compared with no baseplate (Figure 3).
Figure 4: A 3-axis plot of pin and baseplate temperature of a DC/DC module versus load available is more accurate than simple airflow derating data. Source: Flex Power Modules, part BMR491
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Despite this, DC/DC module data sheets can include thermal resistance figures and power dissipation curves that can be used to estimate temperature rises and as input to thermal models at system level. Editable thermal models for the latest Flex Power Modules products are also available, compatible with Siemens’ Simcenter Flotherm software. In addition, the free- to-use ‘Flex Power Designer’ software can also provide power loss data for selected
An alternative approach to specifying what power is available from a DC/DC is being promoted by Flex Power Modules by giving data that more closely relates to the limits of the module itself. In practice, if maximum pin and baseplate temperatures are specified, maximum power available is more accurately known from data Flex Power Modules gathers on the DC/DC internal thermal resistances and physical measurements. This eliminates assumptions about the user’s PCB construction and its thermal resistance, and the airflow characteristics. Variations in losses at high temperature are automatically included as the indicated maximum power available is calculated by taking the critical components hot-spots to their maximums by increasing the load.
The data is best presented in a three- axis plot such as in Figure 4 representing the Flex Power Modules BMR491. The end- user can simply measure pin and baseplate temperature in the application and see what power is available. This contrasts with just providing airflow according to a derating graph, which may either be overly conservative or perhaps marginally insufficient to keep junction temperatures in the DC/DC converter within safe limits. At worst, temperatures will not be high enough to trip protection but still over recommended values, resulting in lower reliability and shorter lifetime.
A combination of approaches gives an optimum result
A comprehensive solution to providing sufficient cooling to a brick DC/DC module will perhaps start with an approximation from airflow derating curves and thermal simulation, but this can be supplemented by pin and baseplate temperature measurements in the end application. In combination with the 3-axis plot provided by Flex Power Modules, power available can then be determined accurately, to ensure the maximum performance is extracted from the DC/DC with the best reliability.
References [1]Flex design note 019 – ‘Thermal Characterization of Flex Power Modules’
https://flexpowermodules.com/ Components in Electronics February 2022 37
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