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CM choke as signal transformer
CM choke as signal transformer
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Old 18th September 2018, 10:46 AM   #1
SemperFi is offline SemperFi  Wake Island
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Default CM choke as signal transformer

I need to isolate some test signals in the 100k to 1MHz range. Can I use a CM choke as transformer for this? I have heard of such usage but cannot find any good info on pros & cons. Perhaps some of you transformer gurus can spill some? Thanks.
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Old 18th September 2018, 07:49 PM   #2
Elvee is offline Elvee  Belgium
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Yes, it does work rather well, I have used such an expedient more than once.
You have to be aware of the the limitations though: you have to choose the inductance according to your needs (impedance level/frequency range/ isolation voltage).

For example, you could be tempted to use a large value, like 47mH and a large isolation voltage, like 250VAC, just to be safe and universal, but it would be a bad idea if you need to pass high frequencies: such inductors have their windings completely separate and therefore a large leakage inductance. You thus you need to keep the magnetizing inductance to the lowest reasonably tolerable value.

If you only need a frequency range of one decade, the requirements should be easy to satisfy, provided you make sensible choices.

Of course, CM chokes used as transformers need to take into account all the parameters relevant to a transformer. In particular, the V*s product tolerated by the winding/core combination. This might be problematic, because the datasheet does normally not make mention of this parameter or any alternative like the core area or turn number, susceptible to help in computing the product.

If your lowest frequency of interest is 100kHz, this is unlikely to pose a problem, unless you need large amplitudes or use a very small component.

Anyway, I recommend you carefully verify in reality your chosen inductor under all possible conditions before you proceed to actual testing: many of the important parameters for transformer operation are in fact missing from the datasheet.

One more (obvious) thing: such "transformers" are only good for pure AC: the slightest DC component will be problematic.
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Old 18th September 2018, 10:22 PM   #3
SemperFi is offline SemperFi  Wake Island
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Thanks a lot for good info! The DC component and high leakeage L are things I didn't think about.
Also they are not shielded so how about noise sensitivity...?
They are to be driven by an opamp and sec terminated with 100ohms. Max level ca 1Vrms.
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Old 19th September 2018, 02:37 PM   #4
Elvee is offline Elvee  Belgium
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With a volt or so and low impedance, noise sensitivity shouldn't be an issue.

Directly driving the primary from an opamp makes the inductance almost irrelevant (within reason), but it exacerbates DC problems: with 1mV offset and 0.1 ohm winding resistance, the DC current will reach 10mA, already problematic.
Possible remedies include ultra-accurate offset adjustment, inclusion of a series capacitor, or transferring 1 or 2 ohm of the 100ohm secondary to the primary.
Each method has its pro and con
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Old 19th September 2018, 05:40 PM   #5
SemperFi is offline SemperFi  Wake Island
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Thanks. I will include series capacitor to block DC offset.
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Old 19th September 2018, 07:47 PM   #6
Elvee is offline Elvee  Belgium
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A cap is fine, but be aware that it will add a resonance, normally at a relatively low frequency, below the intended range.

Even so, this resonance can sometimes show some influence within the useful band if it is exceptionally strong.

Sometimes, there are nastier interactions between the opamp and the primary: as the reluctance of the core is near zero, it takes very little to drive it into saturation, and since the output of the opamp also becomes non-linear when pushed too far, this can lead to relaxation oscillations or ferro-resonance-like phenomenons when the capacitor is added.
All of this is rather unlikely, but it sometimes happens, and can be puzzling, because the circuit seems to work properly most of the time, but breaks into violent oscillations after a transient, or even for no apparent reason.
If this happens, you know where to look, and the remedy is generally as simple as adding damping resistors to the cap, either parallel or series.
They will have minimal influence (parallel case), or their influence can be calculated (series case)
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Old 19th September 2018, 08:19 PM   #7
SemperFi is offline SemperFi  Wake Island
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Thanks again for tip. Will test with square waves and look out for excessive ringing or odd behaviour. I generally have 'stopper resistors' on inputs and outputs depending on how reactive the ckt is. Often I include them as a safety feature even if I suspect they are overkill. This bugs some folks during the design review but then my designs usually finish with first to second revision.
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Old 19th September 2018, 08:57 PM   #8
voltwide is offline voltwide  Ireland
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Quote:
Originally Posted by SemperFi View Post
Will test with square waves and look out for excessive ringing or odd behaviour.
generally this is a good idea.
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Old 20th September 2018, 12:15 AM   #9
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To maximize the chance of avoiding problems with leakage inductance I'd choose the CMchoke with the fewest turns. That would normally be one with the highest current rating.
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Old 20th September 2018, 03:16 AM   #10
chris719 is offline chris719  United States
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CM choke as signal transformer
If the part is advertised for power supply use, check how it is wound. There are some parts with purposely high leakage inductance. If it is advertised for data use it probably has low leakage inductance.
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