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Why skin effect is respected when it comes to transformers but not to filters, even when they are dealing with same high frequencies? For example, in full bridge SMPS, with more than 100 kHz, we found that the wire thickness is too small in transformer and too thick in the output filter.

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  • \$\begingroup\$ "Filters" generally implies signals which are usually low current so resistive losses such as skin effect don't matter as much. A powerful signal would be something like a large radar or antenna and in that case it would matter. Also, your example is an SMPS and if it is stepping down voltage, the output current would be higher than the input current. \$\endgroup\$
    – DKNguyen
    Commented Jan 14 at 7:45
  • \$\begingroup\$ Yes the output current is higher, why don't use multiple thin wires to deal with high current, this current is the same in secondary side transformer which is using multiple wires instead of one thick wire \$\endgroup\$
    – NBK SOFT
    Commented Jan 14 at 8:33
  • \$\begingroup\$ If you estimate the length of wire and diameter in that toroidal inductor and calculate the difference in effective resistance between 100Hz and 100kHz (assuming all the current is at 100kHz, an assumption you should also question), what difference do you get? \$\endgroup\$
    – Spehro Pefhany
    Commented Jan 14 at 9:02
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    \$\begingroup\$ If the inductor is a series element in a filter to suppress high frequencies, additional resistance for high frequencies just adds a bit of dissipation. If it was part of a series resonance circuit as a parallel filter element, the increased resistance would hurt. (Don't remember that done in power handling.) \$\endgroup\$
    – greybeard
    Commented Jan 14 at 9:30
  • \$\begingroup\$ @NBKSOFT Costs more to wind and the extra performance is not required. If you actually wanted to use that approach you would use Litz wire which is multiple insulated strands, which again costs more. \$\endgroup\$
    – DKNguyen
    Commented Jan 14 at 20:06

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The product photo shown in your question is an 800W (400V in, 12V out) ZVS full bridge converter. The inductor you marked is the output choke. The answer below applies to DC-DC converters in general and buck-derived topologies specifically.

Why skin effect is respected when it comes to transformers but not to filters, even when they are dealing with same high frequencies?

The current flowing through the output choke has two components:

  • The DC component, which is the output current that flows to the load
  • The AC component, which is the ripple current that flows through the output capacitor(s)

For DC-DC converter applications, the amplitude of this ripple is generally much lower than the DC component (usually between 10% and 40% of the DC current but depends on the application). Therefore, the copper loss component due to the AC current and the skin effect is generally much lower compared to the core loss or the DC copper loss (If the operation mode is DCM the amplitude drops further therefore the loss becomes much less).

enter image description here

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The inductor shown in the product photo has a DC resistance of 1 mOhm. Since the output current is 67 ADC, the copper loss due to the DC current will be 4.5 W. If we assume the ripple current is 5 App (peak-to-peak), the RMS current of the ripple will be ~1.5 A. Even if we assume the AC resistance peaks up to 100 mOhm which is a very pessimistic guess, the AC copper loss becomes approx 0.23 W.

But for the main transformer, especially for the primary winding, the entire copper loss comes from the AC current and the skin effect. So the designer should be more careful when designing the transformer windings.

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  • \$\begingroup\$ Note the core also appears to be #52 powdered iron (blue/green coloration), which also suffers highly from skin effect on its particles. The same reasoning applies: core loss is low because the ripple fraction is low. \$\endgroup\$
    – Tim Williams
    Commented Jan 14 at 13:47
  • \$\begingroup\$ Wow thank you alot for this deep explanation, I was searching on internet for so long and it seems no one is talking about this issue, I always use multiple wires in my filters and buck converters designs , but when looking at big companies design, they really don't care about wire thickness as long it can handle the needed current. \$\endgroup\$
    – NBK SOFT
    Commented Jan 14 at 16:25
  • \$\begingroup\$ @NBKSOFT At 100 kHz, proximity effect is a larger loss factor than skin effect in magnetics. With proximity effect, you'll find that large diameter conductors suffer higher AC losses than with smaller diameter conductors. At a fixed frequency, there is an optimal diameter which takes the combined effects of DC and AC losses in to consideration. \$\endgroup\$
    – qrk
    Commented Jan 14 at 18:41

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