Design Article
Power Tip #4: Damping an Input Filter " Part 2 of 2
Robert Kollman, Texas Instruments
9/29/2008 10:09 AM EDT
Choosing damping resistance
Interestingly, there is an optimum choice of damping resistance once the four other circuit elements are picked. Figure 2 shows the output impedance of this type of filter as the damping resistor is varied. The red curve shows a damping resistor that is too large. Consider an extreme case. If the damping resistor was open, then the peak would be quite high and would be set only by CO and LO. The blue curve shows a damping resistor that is too low. If the resistor was made a short, the resonance would be set by the parallel combination of the two capacitors and the inductor. The green curve represents the optimum value of damping. This value is easily found by numerical methods (Reference 1) that contain the closed form solution.
Figure 2: For a given CD to CO ratio, there is an optimum damping resistor.
(Click this image to view a larger, more detailed version)
Next: Selecting components
Interestingly, there is an optimum choice of damping resistance once the four other circuit elements are picked. Figure 2 shows the output impedance of this type of filter as the damping resistor is varied. The red curve shows a damping resistor that is too large. Consider an extreme case. If the damping resistor was open, then the peak would be quite high and would be set only by CO and LO. The blue curve shows a damping resistor that is too low. If the resistor was made a short, the resonance would be set by the parallel combination of the two capacitors and the inductor. The green curve represents the optimum value of damping. This value is easily found by numerical methods (Reference 1) that contain the closed form solution.
Figure 2: For a given CD to CO ratio, there is an optimum damping resistor.
(Click this image to view a larger, more detailed version)
Next: Selecting components
|
|
|
|
|
Navigate to related information
NOPROBLEM
10/2/2008 12:08 PM EDT
Interesting. I believe there are three basic topologies that dampen the input filter. This extra capacitor has the disadvantage that for instant connection of a high input voltage, the resistor needs to absorb 0.5CU^2. I experienced this as a problem. Another solution, used by computer products is a LC-series combination in parallel with LO. The math behind it is the same. The theme has been discussed since the eighties. I remember three articles. Only one of them uses the calculation of pole location to find an optimum. This should also work for two stage input filters. I could not see in the article presented here wether or not the NEGATIVE input impedance has been used in calculating the frequency spectrum. The second chart I find very interesting, but I wonder what kind of mathematical model has been used to derive it. Is this 50% approach a rule of a thumb thing or does it give the optimum ? Apart from the optimum value of R, there are also a minimum and a maximum value beyond which there are no stable solutions for C.
Sign in to Reply
AJW in OR
8/6/2012 12:37 PM EDT
I'm getting "404 File Not Found" errors now when I try to view the Figures. Help! Where did they go?
Sign in to Reply
donschmitt
2/10/2013 8:16 PM EST
Robert,
The figures for Power Tip #4 are missing.
Don
Sign in to Reply