How to select bypadd capacitor to filter 1.4MHz?

Thread Starter

tprofits

Joined Mar 25, 2013
6
I have a SMPS that has a 1.4MHz transient I'm trying to shunt to ground. How do I select the correct capacitor to filter 1.4MHz?
 

crutschow

Joined Mar 14, 2008
25,269
Capacitors filter a broad range of frequencies with their impedance inversely proportional to frequency and capacitor size. You want a capacitor with a low ESR and inductance, such as ceramic types, perhaps in parallel with a larger electrolytic capacitor.

Such transient noise from an SMPS is very hard to filter and may require a inductor along with the capacitor or perhaps a feedthrough filter or common-mode choke. Without knowing your configuration, it's difficult to make better recommendations.

Where is the noise appearing?
 

Thread Starter

tprofits

Joined Mar 25, 2013
6
I have the following images. One is a screen shot of the transient. The second is the circuit. I'm thinking a capacitor in parallel with the 68uF might help. But, I'm not sure how to calculate the correct capacitor?


 

crutschow

Joined Mar 14, 2008
25,269
There is no "correct" value. You just need a very low impedance capacitor to high frequencies which is typically provided by small ceramic types of 0.1μF. The problem generally is the inductance of the connections to the capacitor. Even a mm of wire has a significant inductive impedance to high frequencies, thus the best way to connect the capacitor is a surface-mount type connected directly to the ground plane with the trace being filtered running through (in and out of) the other capacitor pad.

Alternately some type of feed-through capacitor connected directly to a grounded chassis can help, or an added inductor in series with the output with another capacitor to ground.
 

MrChips

Joined Oct 2, 2009
21,686
Putting an inductor in series with a capacitor is not a good idea. This is a resonant circuit.
100μH in series with 68μF will resonate at 2kHz.
 

Thread Starter

tprofits

Joined Mar 25, 2013
6
crutschow - I was using 4" leads of wire soldered to my PCB. I'm going to add a 0.1μF cap in series with the 68μF (with a short lead wire). I'll record the results. Thanks!
 

crutschow

Joined Mar 14, 2008
25,269
crutschow - I was using 4" leads of wire soldered to my PCB. I'm going to add a 0.1μF cap in series with the 68μF (with a short lead wire). I'll record the results. Thanks!
4" wires are a no-no.
And you want the 0.1μF cap in parallel not in series.
How short is "short"?
 

Thread Starter

tprofits

Joined Mar 25, 2013
6
Thanks for the assistance! I tried a 0.22μF, then a 0.1μF and then a 0.01μF ceramic capacitor in parallel with the 68μF capacitor. It didn't fix the transient. Would you mind double checking my scope calculation. Am I targeting in on the right frequency (1.33MHz)? The capacitor wires measured 4.5mm.

 

kubeek

Joined Sep 20, 2005
5,731
How long are the ground and power traces to the diode, induictor and output cap?
Can you post a picture of the physical layout?
 
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