In message , Steven Briggs
writes
The same non-linear, high rms input current, factors apply to (small)
switch mode power supplies. Depending upon equipment class, larger
switch mode power units (75W for most) must have some form of power
factor correction, either simple passive to slug off the current input
peak, or clever electronics to force a near-sinusoidal input current.
Good designs will often achieve a PF of 0.99.



For anyone who wants to know more,
http://www.reo.co.uk/files/kbase/handbook%20en%2061000-3-2.pdf
is a readable introduction to the subject, pages 4-5 for the basic
explanation.

--
steve