page_title: Active EMI reduction
section_title: Feed forward
hierarchical_section_title: Active EMI reduction / Filter topologies / Feed forward
caption_attribution_description: English: Feed forward configurations for active filtering
context_page_description: In the field of EMC, active EMI reduction refers to techniques aimed to reduce or to filter electromagnetic noise making use of active electronic components. Active EMI reduction contrasts with passive filtering techniques, such as RC filters, LC filters RLC filters, which includes only passive electrical components. Hybrid solutions including both active and passive elements exist.
  Standards concerning conducted and radiated emissions published by IEC
  and FCC
  set the maximum noise level allowed for different classes of electrical devices. The frequency range of interest spans from 150 kHz to 30 MHz for conducted emissions and from 30 MHz to 40 GHz for radiated emissions. Meeting these requirements and guaranteeing the functionality of an electrical apparatus subject to electromagnetic interference are the main reason to include an EMI filter. In an electrical system, power converters, i.e. DC/DC converters, inverters and rectifiers, are the major sources of conducted EMI, due to their high-frequency switching ratio which gives rise to unwanted fast current and voltage transients.
context_section_description: While feedback filters register the noise at load side and inject the compensation signal at source side, the feed forward devices do the opposite: the sensing is at source end and the compensation at load port. For this reason, there cannot be feedforward-type implementation for type (b) and (c). Type (a) (current sensing and injecting) and type (d) (voltage sensing and injecting) can be implemented and the calculated ILs result to be:
  Considering also in these two cases the condition for maximum noise reduction, i.e. maximum IL, it can be achieved when the filter's gain is equal to one. If , it follows that . It can also be noted that, if or, generally speaking, , the insertion loss becomes negative and thus the active filter amplifies the noise instead of reducing it.
caption: Fig. 3: Schematics of the possible configurations for feed forward topology.
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