# Filter Cutoff Frequency

1. ## Filter Cutoff Frequency

Hi all,
How can I calculate the CM mode cutoff and DM cutoff frequency of the filter below?  Reply With Quote

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2. ## Re: Filter Cutoff Frequency

Hi,

Imagine Line and Neutral are shorted ... at both sides of L_CM.
Then L_CM formes a series impedance
And C_Y1 and C_Y2 are shorted, too, giving a total paeallel capacitance of (C_Y1 + C_Y2).

Now you have L and C. It forms a resonant circuit.
(I don't like undamped resonance circuits, thus I recommend to add some damping circuit)

Usual formulas for LC filters can be applied to calculate cutoff frequency

Klaus  Reply With Quote

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3. ## Re: Filter Cutoff Frequency

Ok then, the CM cut-off frequency can be caluculated as this formula below, (I assumed that C_Y1=C_Y2)

Fc_cm=1/(2pi*SQRT(L_CM*2*C_Y) am I right?

Now I want to calculate the differential mode cut-off frequency. How can calculate it?
I read somewhere that an LC filter is formed between the leakage inductance of the common mode choke and the differential mode capacitors. Is this true?

PS: Damping circuit is not shown in this picture. But I added on my real circuit.  Reply With Quote

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4. ## Re: Filter Cutoff Frequency

the noise flows from right to left - ideally the CM caps need to be on the LHS ( left had side ) in order to get a CM LC filter - this can be undone if the primary mosfets are heatsunk to an earthed heatsink as the CM choke is then bypassed by having the CM caps on the HS ...  Reply With Quote

5. ## Re: Filter Cutoff Frequency

Presumably also, if its an input stage to an isolated SMPS, and the oputput of the SMPS is earthed, then that also bypasses the input filter common chokes/capacitors.

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You cannot so easily calculate the common mode cut-off frequency, because an important cut-off frequency will involve the stray inductance and interwinding capacitance of the common mode choke.  Reply With Quote

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