by Carl Schramm, Reliability Laboratory Manager / RECOM Development & Trading GmbH & Co KG.

Smaller, more powerful, better performance…are the buzzwords in the area of DC/DC module power supplies. Good thermal management of the heat generated has become an important part of the design-process. But what needs to be done?

An indisputable fact is that the efficiency of any energy conversion process is always less than 100%. That means that a part of the energy being changed goes astray, in other words, is converted into heat and that ultimately this waste heat must be removed. The laws of thermodynamics state that heat energy can only flow from a warmer to a colder environment. So, for DC/DC converters, this means that if the internal heat is to be dissipated out of the module, that the ambient temperature must always be lower than the maximum allowable internal temperature. The smaller this difference is, the less heat energy will be lost and thus the converter will warm up even more.

The case (surface) temperature of DC/DC modules are typically given as +100°C or +105°C. This value appears at first to be very high; however this figure includes not only the self-warming through internal losses but also the ambient temperature itself. Remember: The smaller the difference between case surface and ambient, so the smaller amount of heat can be lost to the surroundings. If a converter has higher internal losses, then it will be more affected by a smaller temperature difference than a converter with lower internal losses. The internal losses occur mainly through switching losses in the transistors, rectification losses, core losses in the transformer and resistive losses in the windings and tracks. The maximum allowable internal temperature is determined by the Curie temperature of the transformer core material, the maximum junction temperature in the switching transistors and rectification diodes and the maximum operating temperature of the capacitors. In order to achieve an optimal internal thermal environment, RECOM converters are fully potted with a thermally conductive epoxy so that the concentrated sources of heat are conducted evenly throughout the converter. The epoxy has a low thermal conductivity of 400mWmK-1 to ensure an effective transfer of internally generated heat to the case outside surface. It is possible to further lower the thermal resistance to ambient by fitting an external heat sink (see later).

RECOM states the minimum and maximum ambient operating temperature because this is easiest for the end user to measure and to monitor. The advantage is that true ambient temperature can be measured in the actual application and it need not be theoretically calculated, plus the results are valid for both sealed and open constructions with a through-flow of cooling air. Additionally, the measured case temperature can be used to decide on a suitably dimensioned heat sink so that the maximum case temperature is not exceeded at the maximum ambient temperature.
Nevertheless, the internal losses and thermal resistances can also be derived mathematically. For the calculations, Ohm's Law of R=V/I can be modified so that R becomes thermal resistance, V becomes temperature and I becomes power dissipation. The following equations can thus be derived:

With help of the above formulae, the maximum allowable ambient temperature for a given set of operating conditions can be calculated, but it is important to remember that efficiency is dependent on both the output load and the input voltage (refer to graphs below). The formulae also demonstrate that case temperature is not the same as operating temperature, as is so often falsely claimed.

If the thermal dissipation calculations reveal that the DC/DC Module will overheat at the desired ambient operating temperature, then there are still a number of options available to reach a solution.

One option is to derate the converter, i.e, use a higher power converter running at less than full load. The derating diagrams in the datasheets essentially define the maximum load at any given temperature within the operating temperature range. The derating curves are in reality not so linear as they are declared in most datasheets. However, reliable manufacturers will always err on the safe side so that the values given can be safely relied on in practice.

If the converter has a plastic case, then the next largest case size with the same power rating could be chosen to increase the available surface area. However, care must be taken not to compromise on efficiency otherwise no net gain will be made.

If the converter has a metal case, then adding a heat sink is can be very effective, particularly in conjunction with a forced-air cooling system. If a heat sink is used with fan cooling, then the thermal resistance equation becomes:

The value of R_THHS-ambient incorporates the thermal resistance of the heat sink as well as the thermal resistance of any thermally conductive paste or silicon pads used for a better thermal contact to the case. If these aids are not applied, then a value of approximately 0.2 K/W must be added to the thermal resistance of the heat sink alone. When establishing of the value of R_THHS-ambient it is also necessary to know how much air is being blown across the heat sink fins. These values are most often given in lfm (linear feet per minute) and declared by the fan manufacturer. The conversion to m/s is 100lfm = 0.5 m/s.

This has been a basic overview for all users confronted with a thermal management issue, but it has covered the main points that need to be considered. If however, the results of your calculations or measurements are border-line, then the issue must be examined in more depth. So, for example, there is a difference in thermal performance between vertically and horizontally mounted modules, between static air and freely convecting air and with air at low atmospheric pressures. However, investigating all of these factors would take up too much space in this article.