Nobuhira Tanaka

Cooling performance of piezoelectric fan in notebook system

Thin form factor and high computing performance are the inevitable trend of notebook industry. Both vectors impose more stringent challenge on thermal solution design to maintain internal components and external chassis surfaces within temperature requirements. Cooling industry contributed great effort in miniaturizing conventional blower fan to thin form factor for supporting thin notebook cooling. However, due to the physical size constraints such as rotary blade, magnetic coil, bearing mechanism & circuit board, the effective flowrate of thin form factor fan is relative low. Multiple past researches have been focusing on characterization of thermal performance at component level. However, investigation of practical cooling application of piezofan in thin notebook has not been widely studied. This paper investigates the feasibility of implementing piezoelectric fan (referred as piezofan from now) as an alternative cooling technology in thin notebook. The piezofan design is constrained by volumetric size not greater than blower fan used for cooling thin notebook. The selected notebook is low power thin & light 13″ notebook, with processor TDP (Thermal Design Power) of 18W. Firstly, the cooling impact of heat exchanger design is studied in terms of position, fin gap & inlet shape. The optimum cooling performance is observed when heat exchanger design is with 1.5mm fin gap, additional recesses at top & bottom of inlet plane and positioned at in front (∼0mm) of piezofan vibration tip. Secondly, cooling characteristic of piezofan is studied in terms of blade length (l), thickness (t), operating frequency (ƒ) and amplitude (A). First resonance frequency is observed to be linearly dependent on t/l2. The cooling performance of piezofan is concluded inversely power to A׃. Better cooling performance is obtained at higher resonance frequency and larger vibration amplitude. Piezofan with shorter vibration blade generates higher A׃ value (better cooling performance). The sound pressure level of piezofans are measured as well, and found to be below typical notebook acoustic limit of 40dB(A). It is observed that higher resonance frequency generates higher acoustic in general. Finally, a piezofan operating at 138Hz frequency and 30Vpp voltage is installed in selected thin & light notebook system for thermal test. The temperature of processor, palm rest, touch pad and bottom skin of notebook are monitored. Test data indicated that piezofan perform better cooling for processor and bottom skin when compared with blower fan. Other components are less than 10% temperature difference. This data shows the thermal feasibility of piezofan to be implemented in thin & light notebook with processor TDP of 18W (or other low power mobile devices), with low acoustic noise and low power consumption.