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Old 19th September 2014, 04:28 AM   #1
rcbuck is offline rcbuck  United States
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Default Common mode choke core type

For the AC input side of a SMPS what is the ferrite core type that is typically used to make a common mode choke?

For example, I have one that measures 7.5 mH. It is about 18mm OD and 7mm ID. The best I can tell, it has 32 turns on each side of the core.

Using my handy mini Ring Core Calculator, I find that a Ferroxcube TN 20/10/7-3C90 core will yield 7.5 mH with 37 turns. I this the type of core that they use? I can't tell by looking if it is wound with TIW.
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Old 19th September 2014, 10:09 AM   #2
AndrewT is offline AndrewT  Scotland
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32 Turns means a small diameter wire is used. That wire sets the current capability of the CM choke.

Normally a current rating of 3.1A/sq mm is used, but the CM choke allows good cooling of the single layer wire. Allowing ~6A/sqmm is OK and maybe a bit more.

Assuming 0.3mm diam copper with 0.071sqmm gives a current rating of 0.42Aac when 6A/sqmm is used. On 110/120Vac that amounts to <50VA.

If you quadrupled the current rating to 12A/sqmm you are still less than 100VA.

Check your wire diameter.
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Old 19th September 2014, 04:12 PM   #3
rcbuck is offline rcbuck  United States
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This is a choke I pulled out of a PC power supply. The wire is .65 mm diameter.

I was more interested in finding out what type of cores are used for common mode chokes than current capacity.

Also, my first post where I said 3C90 material was used is wrong. I actually meant to say 3E25 material is what I entered into the mini Calculator program.

Do they use cores similar to the TN20/10/7-3E25 for these chokes?
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Old 19th September 2014, 07:48 PM   #4
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Ferrite Cores - Ferrite Toroids,Ferrite Pot Cores and Ferrite Shapes (E,I,U,EFD,ETD,EER,EC,UR,ER) and Hardware| Magnetics®

Scroll down about half way to common mode choke, recommended types J and W
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Old 20th September 2014, 03:24 AM   #5
rcbuck is offline rcbuck  United States
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Thanks powerbob. That is the information I was looking for. I will study the Magnetics parts catalog and see what I can learn.
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Old 20th September 2014, 08:28 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by rcbuck View Post
.........................
For example, I have one that measures 7.5 mH. It is about 18mm OD and 7mm ID. The best I can tell, it has 32 turns on each side of the core..................
Quote:
Originally Posted by rcbuck View Post
.................The wire is .65 mm diameter.
........................
How do they get 32T on each half of a core with an I.D.=7mm?

If the core was already used as a CM choke, then it pretty certain the manufacturer chose a material that suited this duty.

Double the diameter will give ~four times the current. Probably 2A to 3A for that 0.65mm diam wire.
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Last edited by AndrewT; 20th September 2014 at 08:33 AM.
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Old Yesterday, 08:29 PM   #7
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Quote:
Originally Posted by AndrewT View Post
How do they get 32T on each half of a core with an I.D.=7mm?
have you ever taken one apart?
a 7mm diameter core might have a window area of 14 by 8 mm
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Old Today, 09:03 AM   #8
AndrewT is offline AndrewT  Scotland
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the wire is laid up in a single layer.
You cannot get 32Turns of 0.65mm diameter wire in a single layer in the half circumference of a 7mm bore and keep the two coils separated.
The reason for the single layer is to keep the wire turn to turn voltage very low. A two layer wind would place a high voltage wire next to a low voltage wire. This risks insulation breakdown.

It's not a problem at 50/60Hz. The total voltage drop along the whole coil is very low, maybe 1 to 10mVac at the fundamental mains frequency.

At HF where the filter is working to oppose easy passage of pulses, the voltage drop could be tens or even hundreds of volts. Expect voltage pulses on the mains approaching and even exceeding 1kVpk. Much of that voltage could appear across the coil if the filter is working well at the frequency of the pulse.
A common mode choke must keep the two coils separeted.
It must also keep the ends of each coil separated from each other AND spread the voltage drop evenly along the whole coil such that turn to turn voltage does not burn through the insulation.

Look up a pic of a common mode choke.
It has two single layer windings.
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Old Today, 09:07 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
a 7mm diameter core might have a window area of 14 by 8 mm
?
a 7mm diameter hole has an area of 38.4845mm˛
Half for each winding equates to <19.25mm˛ per winding

14 by 8mm = 112mm˛
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