> High frequency switching in the diodes was my thinking also. When
> the=20
> diodes in the rectifier turns on (capacitor voltage is lower than
> two=20
> diode drops, the circuit starts to draw current from the transformer
> which results in slightly higher voltage drop for the diodes and=20
> lower output voltage from the transformer. This will produce short
> time, high frequency, relatively high current on/off switching in
> the=20
> diodes until the transformer voltage has rised high enough to keep
> the diodes on. This can produce lots of EMI. Another way to reduce
> this is to put small capacitors in parallel with each rectifier=20
> diodes.
>=20
> Since this switching is much higher than the 100Hz ripple (but at=20
> 100Hz intervals) it should be much easier to filter out than the=20
> 100Hz ripple in itself.
>=20
> Note that only using a capacitor as the filter element for the power
> supply will reduce the maximum output DC current.
>=20
> The inductor in series with the supply filter capacitor will reduce
> ripple current in the capacitor, lower the peak output voltage and
> increase the maximum output DC current after the filter.
>=20
> See <http://www.hammondmfg.com/pdf/5c007.pdf>
>=20
> /Ruben

Adding to this: The inducuctor needs to be fairly large to have any=20
significant impact on the 100Hz ripple which is probably why it isn't=20
used very often.

/Ruben

>=20
> > stages. I
> > would also look at possibly using an inductor plus capacitors on
> the
> > input
> > and increasing the filtering capacitance. If you increase the
> > capacitance
> > without adding inductance, then the current drawn from the input
> > supply
> > will become ultra-short, very high current pulses only right
> around
> > the
> > peaks of the AC waveform, which is bad for thermal reasons and
> > possibly for
> > hum production, too, since it may introduce high-frequency
> > components to
> > the ripple which are not well rejected by the regulator. The
> > inductors not
> > only perform additional filtering but they also smooth out the
> diode
> > bridge
> > conduction.
> >=20
>=20
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