A soft start technique to minimize EMI implications in dual voltage vehicle electrical system

In this paper the Electromagnetic Compatibility of the new automotive 14/42 V dual voltage electrical systems has been examined. From this point of view EMI (Electromagnetic Interference) must be minimized since the design stage. The attention has been particularly focused on the 42 V engine cooling fan drive. In order to reduce its inrush current a soft start circuit has been proposed. First a model of the electrical drive has been developed, then the model has been experimentally validated and finally the possibility for a further optimisation of the proposed soft start circuit has been studied. Near to the inrush current reduction, the proposed circuit produces, also: a reduction of the high frequency harmonics of the current transients (due to a reduction of the slew rate of the current surge) and the absence radiated EMI as it is switched at low frequencies. Moreover the proposed soft start circuit can be simply realised by using Smart Power technology and can be used to regulate the speed of the drive, according to engine temperature's variations.