cupoftea
Advanced Member level 5
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Are common mode chokes (and their usefulness in SMPS) a bluff?
Because common mode noise is due to the circuit strays, and exact calculation of an exact common mode choke is
not possible (not in the normal timescales of a typical project).
So why don't we prefer an inductor in both Line and Neutral?...That gives a common mode impedance.
OK, you don't have the advantage of field cancellation and thus small size that a common mode choke has...but for <200W
SMPS, the currents aren't that high anyway, so the alternative inductor solution wouldn't be that big.
We are all told that because of mains RCD currents, we must have small Y caps,
and then we need the common mode choke, with its enormous common mode inductance
despite small size.
..But common mode currents are very high frequency, and are likely to couple between the coils of a common mode choke anyway
, and couple through the coil capacitance too. This degree of coupling would be less with separate inductors in line and neutral.
Surely common mode chokes are a devil to wind for assembly staff. So this means westerners keep relying on the Chinese
to do it for us.....but aren't we led to believe that we need common mode chokes, and so must keep buying SMPS from the Chinese
(because we couldn't possibly do the fiddley common mode choke winding ourselves).
I appreciate their are torroid winding machines, but these are too expensive, and don't tend to get used for common mode chokes.
How much of the "common mode chokery" is bluff?..and how much is real?
I mean....take a look in a <100W offline SMPS...take out the common mode choke...look at the coils...they are messily wound with multiple layers
right over each other and the inter, and intra winding capacitance must be relatively enormous......which most certainly does not help
with high frequency common mode noise filtering.
Because common mode noise is due to the circuit strays, and exact calculation of an exact common mode choke is
not possible (not in the normal timescales of a typical project).
So why don't we prefer an inductor in both Line and Neutral?...That gives a common mode impedance.
OK, you don't have the advantage of field cancellation and thus small size that a common mode choke has...but for <200W
SMPS, the currents aren't that high anyway, so the alternative inductor solution wouldn't be that big.
We are all told that because of mains RCD currents, we must have small Y caps,
and then we need the common mode choke, with its enormous common mode inductance
despite small size.
..But common mode currents are very high frequency, and are likely to couple between the coils of a common mode choke anyway
, and couple through the coil capacitance too. This degree of coupling would be less with separate inductors in line and neutral.
Surely common mode chokes are a devil to wind for assembly staff. So this means westerners keep relying on the Chinese
to do it for us.....but aren't we led to believe that we need common mode chokes, and so must keep buying SMPS from the Chinese
(because we couldn't possibly do the fiddley common mode choke winding ourselves).
I appreciate their are torroid winding machines, but these are too expensive, and don't tend to get used for common mode chokes.
How much of the "common mode chokery" is bluff?..and how much is real?
I mean....take a look in a <100W offline SMPS...take out the common mode choke...look at the coils...they are messily wound with multiple layers
right over each other and the inter, and intra winding capacitance must be relatively enormous......which most certainly does not help
with high frequency common mode noise filtering.