How to design a common mode filter?

[Chris Carlen]

Greetings:

For my PWM amp design that has been beaten about in thread "PWM Amp
Design" I am now interested in implementing a common mode output filter,
per the valuable suggestion of Nico Coesel.

Trouble is, I need to know what load impedance is presented to the
output of a filter in order to get the right LC values.

What to figure for the common mode load Z for the output of a PWM amp?
The load is isolated from ground, of course, since the H-bridge output
is differential and doesn't mean much with respect to ground.

Should the common mode filter be just a common mode choke or a common
mode choke followed by a capacitor on each output leg to ground?

How does one typically chose a common mode output filter?

Thanks for input.


Good day!



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_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Chris Carlen]

bill.sloman@ieee.org wrote:

How does one typically chose a common mode output filter?

My millidegree Peltier controller used a common filter to clean up the
output from the PWM output stage into the Peltier junction.

E-mail me if you want me to send you a copy of the paper, which
contains a brief discussion of the common mode filter as well as a
circuit diagram (figure 8), but you can also get it from "Measurement
Science and Technology" volume 7, pages 1653-64 (1996). Sandia should
have access to the U.K. Institute of Physics web site

http://www.iop.org/EJ/journal/MST

where you could pick up the .pdf file from the digital archives.

We used a bought-in balun as well as four power chokes, four ferrite
beads, and three 1.0uF film capacitors - two going to ground and the
third across the input to the balun. It seemed to work.

Component values were largely chosen on the basis of the space
available for the parts we could buy from distributors - we had enough
control of the PWM frequencies that we could avoid the filter
resonances, rather than damping them.

Bill, if you could please send me a .pdf I'd appreciate it.

Send to my email below (after fixing it, of course.)


Thanks!


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_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
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