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How to decide input filter specifications for linear power supply design

Farukh Khan

Jun 12, 2015
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Hello,

I am trying to design a precision linear power supply and I have already chose a transformer which will meet my output requirements. Now, I am not sure how to figure out what parts should I use for EMC/EMI filters for the input of the transformer from main's AC. I have basic knowledge about X and Y rated caps and common mode chokes but I am not sure what specification or ratings parts should I choose to go with a simple ac filter input stage before hitting the primary winding of the transformer. The transformer I am using is: https://my.mouser.com/ProductDetail/Triad-Magnetics/VPM18-2780?qs=bdQtRzgkM2RlDm/tY2Kgow==

Also, planning to use HRC Fuses for both the live and neutral mains input but not sure how to select the HRC fuses. What specifications I need in terms of trip current and time delay etc. Any help would be really appreciated. Thank You.
 

Harald Kapp

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Designing an EMC filter can be tricky.
Even more complex is measuring the effectivity of such a filter as it requires expensive equipment.
I suggest you use a commercially available filter.
One form is built into an IEC inlet, possibly the least space consuming method and in the ideal position next to the inlet. No long wires that can pick up EMC and radiate it to the outside.
cropthumb_IEC%20Inlet%20Filters%20Oct13.jpg

Another form is sealed modules that can be placed anywhere convenient. Here you'll have at least some additional wiring to do which can act as an antenna.
cropthumb_SIFI%20Filters%20Oct13.jpg


planning to use HRC Fuses
Why HRC fuses? If your power supply isn't an extremely high power unit, standard glas tube fuses will suffice. These can be used with conveniently placed fuse holders for easy repair.
upload_2021-8-10_6-46-45.png

Fuses for both the live and neutral mains input
Fuses on both live and neutral are only required if reversal of the two is possible.
 

Farukh Khan

Jun 12, 2015
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The goal is to learn how to design EMC filter for specific power supply use case and that's why buying a premade one is not an option here. @Harald Kapp Also, I was planning to put HRC fuses to avoid the arching problem of normal glass tube fuse. Want to use the best quality items possible for the supply.

@crutschow Yes the supply does have a regulator. But I am trying to design the primary side first and then move on towards the secondary part of the supply.
 

Harald Kapp

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The goal is to learn how to design EMC filter for specific power supply use case
Do you have access to the necessary EMV testing tools to verify your design is doing what it's supposed to do? Without these tools your effort is imho wastedas you can't verify the effectivity of your design.

Anyway, this website offers some webinars on EMC filter design, see under the heading "EMC Components".
 
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