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Experimenting with Magnetic Components
Blog Experimenting with Magnetic Components - Common Mode Choke
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  • Author Author: Jan Cumps
  • Date Created: 16 Oct 2021 6:31 PM Date Created
  • Views 2078 views
  • Likes 1 like
  • Comments 5 comments
  • common_mode_rejection
  • experimenting_with_magnetic_components
  • magnetic_components
  • bourns
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Experimenting with Magnetic Components - Common Mode Choke

Jan Cumps
Jan Cumps
16 Oct 2021

I'm reviewing a set of inductors for the Experimenting with Magnetic Components design challenge.

In this post: investigating the behaviour of common mode filtering, with inductor #11 of the kit: SRF9045A-701Y Choke, Common Mode, AEC-Q200, 7.5 µH, 5ASRF9045A-701Y Choke, Common Mode, AEC-Q200, 7.5 µH, 5A.

image

 

Common Mode Noise

 

simplified description: Common noise is an unwanted signal that exists on two wires.

If you would probe each of these wires relative to ground, you 'd see the same noise.

 

Because these signals are the same, it's possible to use both to cancel each other. A common mode choke is one of the techniques to do that.

Its purpose is to pass anything that is different on both inputs (called the differential signal because it's different on both inductor inputs). For these signals, it's as if this device is just two wires.

Any signal that's the same on each of the inputs (the common mode noise) should be rejected.

 

The wanted signal enters via one wire and returns via the other (differential current).

Because they are opposite, their generated magnetic fields cancel each other out, resulting in no magnetic field and no impedance. A wire.

The unwanted common mode noise enters via the two wires in the same direction. Their magnetic fields accumulate, resulting in a high impedance. A blocker for that common noise.

 

image

 

The Common Mode Choke

 

The kit has three devices of this type. I'm reviewing the SRF9045A-701Y Common Mode Choke 7.5 µHSRF9045A-701Y Common Mode Choke 7.5 µH.

 

image

image

images source: Bourns datasheet

 

 

Measurements with the LCR meter (100 kHz, except DCR 0 Hz):

 

attributeinductor ainductor b
L6.6 µH6.6 µH
Q2229
ESR0.12 Ω0.15 Ω
ϕ87°86°
D0.0510.042
RS0.150 Ω0.151 Ω
DCR0.03 Ω0.02 Ω

 

image

 

Differential Signal Acceptance and Common Signal Rejection

 

I've put a differential signal on the input. It should pass through without attenuation.

image

 

image

 

I'm using a 100 kHz, 4 VPP AC square wave signal.

Output measured with a differential probe.

image

 

Result:

image

 

Now common mode. I don't have a good high-frequency generator.

Instead, I have put the output of a power supply on both inputs. This should inject the same noise in both inductors. And then probed the two outputs of the choke, referenced to power supply ground.

image

All three signals are with the oscilloscope set in AC mode, BW filter off, identical probes x10.

 

image

 

read:

An investigation of TI on some common noise sources.

An HP application note on common mode noise sources and solutions.

 

 

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4. LCR meter experiments
5. SMD transformers experiment gizmo part 1: Build
6. SMD transformers experiment gizmo part 2: Measure
7. Common Mode Choke
8. Make your own Inductor
9. Calculate your own Inductor
10. Boost Converter part 4: Efficiency
11. DIY Inductance Meter
Planar PCB Transformer: GaN Point of Load converter 48V to 1V 50A
Measure Unknown Inductor Value with Function Generator and Oscilloscope
Experimenting with Magnetic Components: About the Competition
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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to Jan Cumps

    That's it for today:

    image

    If you look closely at the photos, you may be able to see that it's not just two sets of windings, each at one half of the core.

    Halfway the windings, after winding 7, the wire jumps two quadrants and goes in the inverse direction for the next 7 windings.

    You can see the two wires going through the middle of the air gap, meanwhile crossing from the upper rim to the lower.

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to Jan Cumps

    better:

    image

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to Jan Cumps

    Attempt #2:

    image

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to Jan Cumps

    The path ain't smooth:

    image

    I'll need smaller wire

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  • Jan Cumps
    Jan Cumps over 3 years ago

    In my application I promised to roll my own inductor.

    image

     

    I'm preparing for that now:

    image

     

    The plan is to make a balanced twisted windings custom common mode choke.

    Check my next post, or wikipedia, for info.

    All ingredients come from a Project14 shopping cart. I love that program. It's an opportunity to design small projects, and get some components you'd usually not purchase.

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