High operating temperature range DC/DC converters

RECOM’s recently released series of isolated DC/DC converters, the PowerlinePlus, use the company’s ICE (Innovation in Converter Excellence) Technology to minimize internal heat dissipation and maximize the heat transfer to offer high-end performance. One improvement is in the ambient operating temperature range: -45°C to 96°C for the PowerlinePlus series, compared to the -40°C to 60°C range for industry standard 30W converter. This article will explore some of the techniques that have led to this dramatic improvement in performance.

But first, let us go back to basics. The difference between the input power and the output power is the internal power dissipation, which manifests itself as heat. If the heat generated within the converter is not lost to its surroundings, the converter will overheat. Adding heat sinks, base plates or fans increases the rate of heat flow out of the converter, but if the converter itself is inefficient at converting power, then adding these external remedies just cure the symptoms rather than addressing the basic problem.

All this can be expressed mathematically as:



The internal power dissipation, P_dissipation, is the difference between the power consumed by the converter and the power available at the outputs. The higher the efficiency, the more output power that can be delivered for the same internal dissipation. The thermal impedance, R_{THcase-ambient}, is a measure of the ease at which heat can be lost to the surroundings. At room temperatures, the waste heat can easily be convected away to the air around the converter but as the ambient temperature approaches the maximum allowed case temperature, it is again the power dissipation that is the deciding factor.

So, first and foremost, the converter must have the highest possible efficiency over the widest possible input voltage range and load conditions. Most power converters are designed to be most efficient under standardised conditions of 25°C, full load and nominal input voltage, and thus offer a compromise performance when lightly loaded or operated at the maximum ambient temperature.

The PowerlinePlus series uses state-of-the-art techniques to improve power conversion efficiency by approximately 2 percent compared to standard converters over the whole input voltage range and load range. A two percent improvement may not sound very much, but the difference between a converter with 88 percent efficiency and one with 90 percent efficiency is a 16 percent reduction in the dissipated power. The increase in conversion efficiency results from a combination of factors including careful choice of components, optimum layout of the internal PCB with very low impedance interconnections and the use of synchronous rectification.

Thermal management
The rate of heat transfer between a hot body and its cooler surroundings is given by Fourier’s Law:

q=-kΔT

where the rate of heat transfer, q, is proportional to the temperature difference, ΔT, and the thermal conductivity, k. If the thermal conductivity can be made larger, then the rate of heat transfer can still match or exceed the rate of heat generation at lower temperature differences ΔT and the converter will have an extended operating temperature range.

ICE Technology splits the thermal conductivity problem into two areas and attacks each area separately. First, the internal heat transfer to the case is maximized by a combination of novel converter construction and clever thermal design. PowerlinePlus converters use a construction where the hottest components—the switching FET, the transformer and the synchronous rectification FETs) are placed closest to the case wall. This construction makes the manufacture of the converter more difficult, but this lack of compromise greatly reduces the internal thermal impedance.

Secondly, the rate of heat transfer to the surroundings is improved by a novel case construction, which incorporates a built-in heat sink. The case is also made from thick aircraft grade aluminium to provide a better thermal junction between the case and the high thermal conductivity silicone potting material used inside the converter.

The operating temperature of the ferrite core used in the transformer is also an important limiting factor. If the temperature exceeds a certain limit (the Curie temperature), the thermal energy within the ferrite core disrupts the magnetic domains and the transformer rapidly loses magnetism. Selecting the correct ferrite material is thus critical to the overall high temperature performance of the converter.

The final technique is to use high-temperature rated internal components. PowerlinePlus converters use high temperature grade components throughout to permit a maximum case temperature of 115°C, some 10°C to 15°C higher than the industry norm. This allows full power operation at up to 100°C ambient without the need for fans to force cool the converter.

Low temperature operation
At very low temperatures, two characteristics start to take effect. First, the H_FE gain of bipolar transistors has a temperature dependence of approximately 1 percent/°C, so at -40°C, the gain will be 60 percent lower than at room temperatures. Although FET transistors can be commonly found in the power output and rectification stages, bipolar transistors are used in the optocoupler and current monitoring circuits. This H_FE gain reduction thus primarily affects the feedback and oscillator start-up circuits within a DC/DC converter so that at very low temperatures, the converter refuses to start up. The bias currents through the bipolar transistors can be increased to compensate, but then efficiency drops at high operating temperatures. Therefore, there is always a compromise required between low and high temperature operation.

The second effect is mechanical. At very low temperatures, the different coefficients of thermal expansion between the different materials used in a converter generate mechanical stresses in the components. A typical FR4 material PCB has a temperature coefficient of around 16ppm/°C in the x-y plane, whereas a ceramic capacitor can be as low 6ppm/°C. Ceramic capacitors are also very susceptible to mechanical stress and they have been known to physically snap at low temperatures. In addition to the expansion mismatch problem, lead-free solder contains a high percentage of tin which becomes brittle at low temperatures. Thus a combination of low operating temperature and mechanical shock or vibration can also prove fatal for a converter.

Finally, although a wide operating temperature performance is a significant feature of ICE Technology design, it also addresses the need for electromagnetic compatibility by incorporating a built-in EN55022 Class B grade filter inside the converter. There is little point designing a high efficiency converter if it has to be used with an inefficient input filter stage to meet the regulatory requirements. Furthermore, the converters have been designed from the ground up to meet EMC requirements rather than a conventional design process where first the converter is optimized for performance and then an external filter is added to combat the conducted interference.


Author information
Steve Roberts is a Technical Support Manager for RECOM Development & Trading GmbH & Co KG.