Generally, low frequency is dominated by differential mode caused by the
current demand of the switcher being imprinted across the impedances of the
components; then in the midrange its anybody's guess, but usually common
mode, because the capacitors and series impedance elements start to take
effect; at high frequency rarely have I seen anything but common mode
because the series impedances really start to dominate and even the
parasitic capacitances (ie. between cables) dominate.
At the low end you can tell which is which by adding components to the line
filter: If the source is differential the signal drops dramatically when
series elements are added just before the last line to line capacitor. If
the source is common mode, the signal drops dramatically as you increase the
size of the common mode choke. Note: increasing Y-caps affect both modes
sometimes depending on the topology of your filter.
Without the experimentation of changing the unit there is a way to quantify
common mode versus differential mode emissions by using a LISN which gives
you access to both ports simultaneously and use a scope in differential mode
(carefully balanced) then take the output of the scope into the spectrum
analyzer. Then differential and common mode contributions can be
quantified.
- Robert -
-----Original Message-----
From: [email protected] <[email protected]>
To: [email protected] <[email protected]>
List-Post: [email protected]
Date: Tuesday, January 04, 2000 3:10 AM
Subject: Common Mode or Differential Mode
>
>
>
>Are there "rules of thumb" or a "quick and dirty" means of determining
>whether conducted emission noise (or radiated) is common mode or
>differential mode?
>
>Thanks and have a great new year.
>
>Bob Heller
>3M Company
>
>
>
>---------
>This message is coming from the emc-pstc discussion list.
>To cancel your subscription, send mail to [email protected]
>with the single line: "unsubscribe emc-pstc" (without the
>quotes). For help, send mail to [email protected],
>[email protected], [email protected], or
>[email protected] (the list administrators).
>
>
---------
This message is coming from the emc-pstc discussion list.
To cancel your subscription, send mail to [email protected]
with the single line: "unsubscribe emc-pstc" (without the
quotes). For help, send mail to [email protected],
[email protected], [email protected], or
[email protected] (the list administrators).
