It is known that DC/DC converters are the principle culprits in generating conducted Electromagnetic noise interference due to the switching action of their MOSFET. Passive filters have been a straightforward solution to overcome conducted EMI problems at the expense of space, weight and power losses. The objective of this work is to find an EMI filtering method which does not depend on the input parameters of the power supply such as current, voltage, and load impedance which has direct effect on the size and the weight of the product. An active EMI filtering method is sought to be the the candidate. This technique is based on the superposition of two signals with the same magnitude and opposite phase, traveling in the same direction on the same medium. However, the combination of the active and the passive filters shows a substantial attenuation of the conducted EMI emissions as compared to the passive filter only. Also, this combination contributes to an important reduction of the size and weight of the bulky passive filter. A case-study to demonstrate the performance and the effectiveness of the active EMI filter in DC/DC converters is presented in this book.