[Elecraft] switcher filtering

[Rick Shindley]

>>I have had noise problems with switchers as well.  What I want to know is
"why
>>is it so hard to filter/bypass these supplies to get rid of the noise?"

>>I'm old enough to remember the advent of TVI and the development of TVI
>>suppression techniques for homebrew and commercial rigs.  Why can't we
take a
>>similar approach to switching power supplies?

>>73,
>>Vic, K2VCO
>>Fresno CA

The problem with many switchers is with limiting their common mode noise at
the power line side. Common mode currents can turn into differential
currents easily enough, depending on the application.  These rf currents
will appear on your radio ground wires thus making the chassis potential
vary with the switcher noise.  Add a grounded antenna and you'll have a mess
on your hands!

If the radio chassis is jumping around at some millivolt rf level caused by
poor common mode noise limiting of a line-powered switcher, any stray
capacitance to ground in your antenna or lead-in system will result in noise
currents at the radio's antenna terminal!  ARRGGGHHH!

For safety reasons, UL and other agencies place limits on the amount of
"leakage current" the supply represents from its line ("hot") input to
ground. Filter designers must respect that limit so they are practically
constrained to using filter caps from line to ground totalling less than
0.01 uF.  They try make up for it by using large amounts of series
inductance.  The use of inductors adds cost and increases size. High-volume
power supply makers avoid adding cost!

Common mode filtering is more difficult to achieve than ordinary
differential filtering due to the safety-limited C to ground.  Designers use
common mode-wound toroids to restrict those currents, but such inductors add
size and cost, and they often aren't enough anyway. Switchers are not much
more than a small box of compromises!

The real-world solution is to restrict the use of switchers in sensitive
radio environments, or use switchers designed to operate at frequencies far
removed from the radio operating frequencies. Better to just to avoid them
if you are having interference problems.

Rick  KC0OV
K2 1360