# 3 Wire Common Mode Choke

• posted

I have a 3 wire power supply input which are + 15V, -15V, and ground. There is a need to place a common mode choke to filter these lines. Is there a topology by which all three lines can be filtered with with two or more two coil common mode chokes such as this one:

? If not what is the correct part to do this with?

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• posted

Sure, just use two duals and connect the grounds in parallel.

Now, note this breaks the assumption of compensated current -- in equilibrium, the ground current will split evenly (according to DCR), while the +/-15V currents won't. Also, since the supplies are opposite, the ground current will average towards zero (or more exactly, whatever average the loads are actually drawing), which is even more awkward for this arrangement.

You want the unbalanced current to be as near zero as possible; typical CMCs saturate at 10s of mA, so you lose the benefit rapidly as imbalance goes up.

If the supplies are equally loaded (so that ground current is nearly zero), swap windings around so +/-15V goes through one part, and GND goes through another (or just any regular FB of similar value).

As for the linked part, it can probably handle up to an amp of imbalance before saturating too much (evidently it's not very "typical"; "typical" ones also have a peak impedance in the low kohms, whereas this is more like... a very pricey ferrite bead :-) ).

If you can't keep imbalance within range, then you may be better off using inductors (proper), rated for load current, and value large enough to get the desired impedance. Namely, 1-2uH is enough here, maybe in parallel with a resistor (only one R is needed if you've got bypasses linking the supplies together on either side of the inductors). The resistor being about 200 ohms to get roughly similar performance to the part shown. Or use much larger values if you actually need more impedance. Don't forget to adequately dampen the inductors and bypasses (use a bypass C, then a bulk cap around 3*C with ESR = sqrt(L/C)).

Tim

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Electrical Engineering Consultation and Contract Design ```
• posted

if the current in the +15 and -15 are equal and the current in the ground is ~=0 then just use and ordinary CM choke for the +/- 15 and and a standard single choke for the ground. m

• posted

I object: the 'single choke for ground' means you've raised the ground wire impedance, which is a no-no. It ruins the regulation, for one thing.

A 'normal' kind of circuit topology can take output from an op amp in the form of positive rail and negative rail current imbalance, created by loading the output pin to ground. A choke on the ground would become part of the circuit behavior, and a second CM core that impedes mismatch of +/- currents would also intrude on normal operation.

• posted

I am looking at using this three coil choke:

For +15V, -15V, and ground.

I also have a lot of 2 coil common mode chocks on hand. Suppose I use three in parallel. One for +15V, and -15V, the second for +15V and ground, and the third for ground and -15V?

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• posted

What {else do you think a CMC is supposed to do | horrible chokes have you been using}? :-)

Note that impedance != resistance!

Tim

```--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design ```
• posted

the AC current will automatically balance with only two common mode chokes. but the DC currents will balance according to Ohm's law and that may leave all three chokes saturated.

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• posted

I am well below the rated current for the choke.

There is significant current being returned on the ground line, and it must be accounted for in common mode.

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