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.
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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.
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.
images source: Bourns datasheet
Measurements with the LCR meter (100 kHz, except DCR 0 Hz):
| attribute | inductor a | inductor b |
|---|---|---|
| L | 6.6 µH | 6.6 µH |
| Q | 22 | 29 |
| ESR | 0.12 Ω | 0.15 Ω |
| ϕ | 87° | 86° |
| D | 0.051 | 0.042 |
| RS | 0.150 Ω | 0.151 Ω |
| DCR | 0.03 Ω | 0.02 Ω |
Differential Signal Acceptance and Common Signal Rejection
I've put a differential signal on the input. It should pass through without attenuation.
I'm using a 100 kHz, 4 VPP AC square wave signal.
Output measured with a differential probe.
Result:
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.
All three signals are with the oscilloscope set in AC mode, BW filter off, identical probes x10.
read:
An investigation of TI on some common noise sources.
An HP application note on common mode noise sources and solutions.
