Technology
(FB) node (high impedance) and away from the switch node
or any dv/dt source. The parasitic capacitive coupling from the
switch node can induce a current and depending on the value
of the resistors, offset the voltage at the output.
Ÿ Use an input filter at switching converter to prevent the
Conducted EMI generated (from the switching converter) to
couple back to the source. Detailed design technique can be
found in Application note AN-2162Simple Success With
Conducted EMI From DC-DC converters.
Radiated EMI in a power Tree
A typical system will have a single power source (example a
Battery) and multiple point-of-load conversions. Each converter
will have an LC filter or a bypass capacitor which will provide the
least impedance path for the high frequency currents to the
system ground. However each input filter (or capacitor
combinations) will be designed as per the switching converters
operating frequency and will likely be different. Figure 6b shows
the net (Ideal) impedance of all the filters on the Vdd line. Figure
6c shows the actual impedance. There are few points in the graph
where the impedance is very high (peaking) which simply means
that those frequencies will not be bypassed to the ground; hence
they will have longer return paths and hence radiate.
The basic solution is to use moderate ESR capacitors or input
filters. Choosing electrolytic capacitors is often the best solution.
Figure 6d shows the profile due to added ESR.
Figure 6c: Actual Impedance profile
Figure 6d: Impedance profile due to added ESR
Author: Akshat Garg is an Analog Applications Engineer in Texas
Instruments Incorporated. His areas of interest include Power
design, Signal chain for Data acquisition systems, communication
interfaces and thermal management.
Figure 6a: Real system
Figure 6b: Ideal Impedance profile
ELE Times | 54 | November, 2016