Reducing noise from power supplies

Fan size, construction and speed all affect the amount of noise produced.  Smaller fans have to spin faster to generate the same airflow and so are noisier.

Mechanical noise produced by the bearings can cause vibration, and if the frequency of that vibration matches any resonant frequencies of the enclosure, the amplified noise may reach an intolerable level.  Although a sleeve type bearing is quieter than a ball bearing, it has a shorter lifetime. Power supply manufacturers, therefore, tend to use ball bearing in their fans to enhance field reliability, as well as endure higher operating temperatures and help counter variations in mounting orientation.

The stream of air from the fan blades causes turbulence in the airflow and contributes to the acoustic noise level. Ideally, there should be no obstructions in the airflow path, but internal to the power supply are heatsinks and electronic components. Turbulent air in contact with a physical body will generate noise, often in the form of a discrete tone, raising the noise level by as much as 10 dBA. To put this into context, a 10 dBA increase equates to a doubling of the perceived loudness.

Because of these issues, the use of variable speed fans is gaining in popularity.  Here, a thermal sensor is either mounted in the airflow, or attached to a hot component. As the power supply gets hotter, due to changes in ambient temperature or output loading, the fan will speed up accordingly to achieve the desired temperature. The hysteresis of the fan control circuit is also important, as continually changing fan noise can become equally annoying. TDK-Lambda’s QM modular power supplies sense the incoming air temperature. This keeps the fan speed relatively constant, allowing quiet operation in a human environment, yet still providing sufficient cooling.

The efficiency of the power supply has a big impact on wasted power, which dissipates as heat. Figure 1 shows how a 600W output power supply with an efficiency of 90% will generate 67W of wasted power, compared to 106W for an 85% efficient product.

Figure 1: Power supply efficiency versus wasted power

Less wasted heat allows the use of slower speed fans, reducing the amount of acoustic noise generated. For higher output power supplies, 1000W and above, the use of two fans running at slower speeds can reduce audible noise further. TDK-Lambda’s 91% efficient QM modular power supplies used two 60mm fans for cooling. In order to benchmark the QM unit against similar products currently available on the market, the design engineering team performed extensive acoustic noise studies, according to the BS EN ISO 3744:2010 standard (Acoustics: Determination of sound power levels and sound energy levels of noise sources using sound pressure).

Results were impressive with the 1500W rated QM7 measuring as low as 44.3 dBA compared to competitive products with levels as high as 58 dBA. Note again, a 10 dBA increase equates to doubling of the perceived loudness. In a ‘blind’ acoustic noise study, a range of 21 volunteers of differing ages rated the same products on their relative loudness by assigning an ‘annoyance factor’. Interestingly, the results for annoyance level correlated closely with the results from the first test.