Hi Tony Ref: your response to Jerry Fix's mail..
> I thought power line harmonics from power supplies were > due to the non-sinusoidal waveshapes of the input current > waveforms from switchers. > > Wouldn't a linear power supply create only sinusoidal > waveshapes at the line frequency such that there would > be essentially no harmonics from the input current waveform > (i.e. no special design treatment required for linear designs)? > > At least this would seem to be true to the extent there is no distortion > of the input waveform due to lienar circuitry. The problem with Linear supplies is the charging current of the main smoothing/storage capacitors. In each mains half cycle, the diodes in the main rectifier will only conduct when the instantaneous supply voltage exceeds the capacitor voltage by app. 0.7V. As the capacitor is probably charged up to just about Peak supply volts, the only time this happens is right at the top of the sine wave. This often means that the capacitor must be recharged in something like 1mS if the capacitors are over sized (as they often are). If the supply output is 10A, this means that the rectifier has to provide 100A (10mS/1mS @ 50Hz) for the 1mS right in the middle of the half cycle. This very high current at the peak of the sine wave is the odd harmonic that causes all the problems. It has the effect of squaring the supply sinewave. Cures include placing large inductances in series with the feed to the main smoothing capacitors (remember the 'old fashioned' valve radio supplies and their choke input filters..), reducing the size of the main capacitors (increasing ripple, but reducing harmonics) or using HF non synchronous PWM switched rectifiers with suitable input filtering. A better solution, not available to domestic, light industrial, is to rectify three phase, the resultant output hardly needs any capacitive filtering if regulation is used downline. That should make things as clear as mud... Chris Dupres EMC Specialist. VG Microtech. [email protected] tel +44 (0) 1825 761077 fax +44 (0) 1825 768343 'Opinions expressed are personal, not necessarily Corporate'
