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About 5 years ago the input choke gained interest again, also for low voltage equipment, because of the European and Japanese mains harmonic requirements (IEC 6). By using this it is possible to increase the efficiency of switched mode power supplies at low currrent demands, like in the stand by mode of TVs and VCRs.įor some decades this choke approach was hardly used because of high cost and the availability of cheap high current diodes and high value electrolytics.
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Interestingly, the swinging choke re-appears in switched mode power supplies as stepped choke with high inductance at low currents an low inductance at high currents. When Si-diodes, able to handle large current peaks, appeared, the input choke lost some of its value. At small DC currents the inductance was high, causing some voltage drop, at higher currents the core came slowly into saturation and the inductance dropped, causing a smaller voltage drop. This was done with a so-called swinging choke. These chokes were even used to stabilize the output voltage under changing load conditions. This used to be done in the past mainly for tube amplifiers because tube rectifiers could not handle big current peaks and it helped in smoothing DC with the relatively small value electrolytic caps. I suppose these coils are not used as common mode choke, to suppress common mode interferences, but really as input chokes to stretch the active charging current pulse through the rectifier to charge the capacitors.