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Questions about IEC inlet filters, again...
Questions about IEC inlet filters, again...
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Old 14th June 2018, 01:41 PM   #1
MagicBus is offline MagicBus  Greece
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Default Questions about IEC inlet filters, again...

Hello,
I was looking at some commercial IEC filters with an intention to clone them. My understanding is that even if the schematics are available, the lay out and actuall construction might be critical for performance so this would be a good reason to buy the ready products. But my little research left me with more questions about how to choose them. In particular, among the models for different current consumption, the only thing that change is the inductor's value: as the current goes up inductance goes down. I suppose it has to do with inductor's saturation but anyway how do we choose the proper filter for a given device? If, for example, the current consumption is at 1A then filters rated at 3A and 10A would perform differently?

I also found datasheets stating inductance at a certain frequency. What would that mean? Does inductance change with frequency?
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Old 14th June 2018, 04:24 PM   #2
infinia is offline infinia  United States
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Quote:
Originally Posted by MagicBus View Post
Hello,
In particular, among the models for different current consumption, the only thing that change is the inductor's value: as the current goes up inductance goes down. I suppose it has to do with inductor's saturation but anyway how do we choose the proper filter for a given device? If, for example, the current consumption is at 1A then filters rated at 3A and 10A would perform differently?
look here 1st https://www.schaffner.com/fileadmin/...on_chockes.pdf

Remember the key is essentially they are all LCR low pass filters. Most of the attenuation is common mode, but they do have a differential measurement as well . Within the same filter series, the magnetic core usually remains the same size, so a 1 Amp part will allow more wire turns, more turns of wire = higher inductance & I^2*R heat losses, (but sometimes at the risk of lower self resonance frequency) than their higher rated current brothers. Performance is compared by looking at the stop band attenuation for the frequencies of interest for your device. Besides CM stop band attenuation there is , insertion loss, good high frequency atten. most require shielding cans and proper component lead dressing, don't forget safety is very important ( fire and shock ).

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I also found datasheets stating inductance at a certain frequency. What would that mean? Does inductance change with frequency?
Common mode inductance is fairly constant over current and frequency, unlike the differential mode.
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Last edited by infinia; 14th June 2018 at 04:35 PM.
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Old 14th June 2018, 05:43 PM   #3
MagicBus is offline MagicBus  Greece
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The link is great! A first read answered almost all my questions. It also puts me back to the diy approach. I'll take the time to read it more thoroughly. Many thanks Infinia!
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Old 28th August 2018, 06:27 PM   #4
MagicBus is offline MagicBus  Greece
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Hello again!
I spent some time reading the paper linked above and it was quite helpful to clear some things in my mind. I understand that for a given noise target - of known frequency and magnitude measured or just estimated - that needs to be attenuated to a level that complies with the IEC norms, and knowing that the filter's attenuation is rising 40dB per decade, we have to set the corner frequency of the filter (-3dB) so that it will have the "room" to reach the target. All that provided that the inductor will have a self resonance frequency higher than the noise and it will not saturate, so its behavior will be predictable. Trying to find some real life examples, I used the equations to do a reverse engineering to some commercial products. I see some of them setting a corner frequency well within the audio spectrum while others keep it much higher at 60kHz and above. So, I need to ask if is there any problem to set the corner in audio frequencies or should we keep it above 20kHz? (or more?)
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Old 29th August 2018, 12:39 AM   #5
infinia is offline infinia  United States
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Originally Posted by MagicBus View Post
. So, I need to ask if is there any problem to set the corner in audio frequencies or should we keep it above 20kHz? (or more?)
Hi
Remember undesirable RF or HF interferences can combine or be converted in-band (Eg. baseband or audio ) by any circuits non linearity. So mixing 'spurs at RF frequencies or those close enough so the difference is within 20,000 or less can be processed same as the desired audio signals. ..eg reduction of SN.

Choosing cutoff frequencies? I'm pretty sure its like anything else.. a trade-off mostly by constraint of size , cost rather than by looking at attenuation charts then arbitrarily choosing a too low of cutoff to which then cannot be reached by anything practicable (>50dB forget it). Also consider the testing source and load impedance don't scale precisely anyway.

Id do a few design cuts on some magnetic cores you want to use, Get a practicable handle on the magnetics designs. then most folks choose to cascade 2 pole LPF stages as you can only get 40-50 dB per stage (unless extra shielding) for the more pessimistic applications.
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Last edited by infinia; 29th August 2018 at 01:03 AM.
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Old 29th August 2018, 09:08 AM   #6
MagicBus is offline MagicBus  Greece
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Hi
Thanks for reply


Quote:
Originally Posted by infinia View Post
Hi

Choosing cutoff frequencies? I'm pretty sure its like anything else.. a trade-off mostly by constraint of size , cost rather than by looking at attenuation charts then arbitrarily choosing a too low of cutoff to which then cannot be reached by anything practicable (>50dB forget it). Also consider the testing source and load impedance don't scale precisely anyway.

Id do a few design cuts on some magnetic cores you want to use, Get a practicable handle on the magnetics designs. then most folks choose to cascade 2 pole LPF stages as you can only get 40-50 dB per stage (unless extra shielding) for the more pessimistic applications.

Yes, that's something I wanted to ask too. The conclusion is that even if we start from too low there isn't enough bandwidth for high attenuation. I just don't know how much attenuation would be sufficient. (For usual kinds of noise in an ordinary environment if this does make any sense) If we need to attenuate significant noise level at say 30-70kHz, then cascaded filters seem the only way. But do we need to reach that low? This is supposed to be noise coming from SMPS that incorporate their own properly designed filters, or at least they should. The same for digital circuits. I was thinking about the noise sources that are not being filtered at the first place, RF picked up from the mains rails, and the not so smart devices such as refrigerators, ovens, hair dryers and the alike, but I need to gather more information about that.





Quote:
Remember undesirable RF or HF interferences can combine or be converted in-band (Eg. baseband or audio ) by any circuits non linearity. So mixing 'spurs at RF frequencies or those close enough so the difference is within 20,000 or less can be processed same as the desired audio signals. ..eg reduction of SN.

I'm not sure I can understand this. If it is about IMD then I get the meaning. The lower harmonics of the noise will be processed as audio signal by the amp.
But the upper frequencies of the audio signal could be attenuated by the filter's (low) cut off?
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Old 29th August 2018, 10:02 AM   #7
DF96 is offline DF96  England
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You set the filter bandwidth to reduce whatever it is that you need to reduce. For a passive filter of a finite size there is a tradeoff between attenuation and bandwidth. Each filter section will do a certain amount of each. You then cascade sections to get either greater bandwidth (sections differ) or greater attenuation (sections the same).
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Old 29th August 2018, 01:50 PM   #8
MagicBus is offline MagicBus  Greece
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Thanks DF96! Is it OK if we cascade filters for a sharper roll off always keeping in mind audio applications? I have a rough understanding of how things work in speakers crossovers but here I can't understand if phase would be an issue. Of course, I'm asking about what theory could predict.
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Old 29th August 2018, 03:40 PM   #9
DF96 is offline DF96  England
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Yes, you can cascade filters. It is not the sharper rolloff which matters, but the deeper attenuation (if the filters cover the same frequency band). Phase is not an issue; you are filtering the power supply input, not the audio signal.
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Old 29th August 2018, 05:13 PM   #10
MagicBus is offline MagicBus  Greece
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First time I feel I'm not going blind into this. Thank you Infinia and DF96! There is a schematic that is slowly taking shape in my mind. I'll report progress.
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