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Advantages of using a current transformer (CT) for current sensing in switched mode power converters are that galvanic isolation is inherent, losses are low, the bandwidth is high and a high-amplitude output signal may be obtained. However, 'droop' occurs as a result of the magnetising current drawn. Peak current droop is defined as the instantaneous per-unit shortfall in sensed current at the end of a rectangular pulse. Average current droop is defined as the per-unit shortfall in the average current sensed. The CT inherently operates in a resonant mode when sensing a unidirectional current pulse. This is advantageous as some of the current-time product lost to the magnetising branch may be recovered thereby alleviating the average current droop. Average current droop is investigated when diode rectification is used. Three operating modes are identified and described. These are designated the discontinuous magnetising current, continuous magnetising current and discontinuous secondary current modes (DSCM). It is shown that the CT's core losses may predominately influence the average current droop. Provided the DSCM is avoided, simple correction factors are shown to be appropriate for substantially correcting the sensed current to allow for these losses.
Date of Publication: April 2008