Reduce ringing using EMI filter block

Thread Starter

gdylp2004

Joined Dec 2, 2011
101
Some portion of that ringing is due to your scope probe ground. You should always measure the output of a SMPS with a VERY short ground probe. The typical method is to remove the ground clip and wind some bus wire around the ground ring on the probe and leave a little bit sticking out to use as a ground lead. You want to measure that ringing directly across the output cap.

I would bet that almost half of your ringing is caused by the probe. The rest is mostly circuit layout. Only so much you cn do with a breadboard.

Have you thought about adding a second stage low pass filter on the output? Just a simple LC filter?
I've like added a muRata filter at its output. This filter is a comprehensive
package which inludes ferite bead, 3-terminal cap. But the results seem s
little useless.

Do u think adding on another LPF will work?
 

Thread Starter

gdylp2004

Joined Dec 2, 2011
101
The scope ground is not causing the ringing, it is simply an antenna which can pick it up.

I spent many years fighting this type of ringing in switchers, and you can usually kill or reduce it with a suitable R=C snubber applied across the offending component(s). The switch transistor usually needs a snubber, the switching transformer/inductor does as well.

The leads of the R=C snubber must be as short as possible. Wire length over an inch negates the snubber because of lead inductance.

Snubbers burn power as given by:

ps = c V (sq) x Freq

You can experiment with values and see what works best
Hi,

Thanks for the suggestion. I've suppress quite an amout of noise by adding
an additional low ESR, ESL ceramic cap (100nF) across +Vin.

The output noise occurring at rising edge was greatly reduced!

But the noise occurring @ the falling edge now seems "bigger".

Do you've any suggestion to tackle this specifically?

My guess is getting the right RC snubber across the FW diode is the key,
because during turn-off, the FW diode, main L, output C is the
Resonant circuit.

What do you think?
 
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