Baseplate Cooling in AC/DC Converters: An Innovative Fanless Solution

Fans additionally present other problems; they have a limited lifetime, typically 30,000 hours at 50°C for sleeve types (sensitive to orientation) and about double that for more expensive ball-bearing versions. At higher temperatures, the expected lifetime drops dramatically; Figure 2 shows a typical life expectancy curve for a 90% survival rate in a population of installed ball-bearing type fans.

Acoustic noise can also be an issue, increasing as components age, and in some applications, such as medical applications and broadcasting, any background noise is undesirable. Fans also introduce dust and dirt contamination into electronics unless the input filtering is very effective, in which case the air flow is reduced anyway.

More expensive temperature-controlled variable-flow fans mitigate these problems to an extent, but at a minimum, applications using fans require a program of performance monitoring and regular replacement of filters and the fan itself, with all the attendant costs involved.

One solution to mechanical fan cooling problems is to specify a part with a higher power rating under forced air conditions, then use it at lower power with just convection cooling. Unfortunately, products designed for forced air cooling often derate heavily without any forced airflow.

Figure 3 is a typical example that needs 21CFM for its rated 500W power, but in natural convection delivers only 125W up to 50°C.

An obvious disadvantage is the extra cost for an over-specified converter, but there are other considerations; a 500W converter run at 125W is often not optimized for efficiency, and the application may need additional protection to withstand much higher fault currents under overload and short-circuit conditions that the power supply can deliver, again adding to cost.

A good example of a product series that achieves this is the RACM230-G and RACM550-G from RECOM, which employs advanced techniques to minimize the thermal resistance between critical components, such as the power switches and transformer, and their baseplates for effective conduction cooling.

The resulting performance of the RACM550-G is shown in Figure 4, allowing a full 300W up to 50°C, 50% extra compared with the example in Figure 3, and more than double the power, 225W, at 70°C ambient. Interestingly, the convection cooling performance is also much better, as the closely thermally-coupled baseplate forms a good surface for dissipation in still air, compared with the localized component hot spots in the Figure 3 example.

In some applications, a high peak load or start-up current requirement may create an opportunity to draw more instantaneous power and then use conduction cooling or natural convection cooling for the average load, depending on the peak load's duty cycle. In the RACM550-G datasheet, RECOM provides a useful guide to calculating the maximum average load, depending on the cooling conditions, given the peak load and duty cycle.

An example calculation in the product datasheet shows that a load with 550W peak and 81W ‘recovery’ value with a duty cycle of 10 seconds peak to 40 seconds recovery, sets the average output power to 245W with some safety margin. From Figure 4, this is handled by the product with natural convection up to 40°C ambient or up to nearly 65°C with baseplate cooling.

The RACM230-G offers peak output power of 230W in a compact 4”x2” footprint and can also supply up to 135W of continuous power up to 50°C ambient with just baseplate and natural convection cooling. A further important specification is the maximum output power at lower input voltage; all AC power supplies deliver more power with 230VAC mains than with 115VAC supplies, because for the same load the input current is halved for the higher input voltage. Both the RACM230-G and the RACM550-G are universal input power supplies. With forced air cooling, the load rating is independent of the supply voltage as long as the input voltage does not fall below 110VAC. However, with baseplate cooling only, there is no difference in the load rating. For the RACM230-G, for example, it is up to 160W at 230VAC and 115VAC as well.