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tonyc Junior Member Offline Posts: 10	Where does the skin effect in inductors cause the most pain? Aug 07^th, 2011, 5:12pm I know it's an issue at RF, but does anyone know if there are other applications where it imposes practical limitations? PS: This is a great site, Kens work is impressive.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Where does the skin effect in inductors cause the most pain? Reply #1 - Aug 8^th, 2011, 1:04am be clear man while asking...''Where does mean''. In what frequency regime or in which application or...etc Thanks.
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Lex Senior Member Offline Posts: 201 Eindhoven, Holland	Re: Where does the skin effect in inductors cause the most pain? Reply #2 - Aug 8^th, 2011, 2:20am Skin effects are also quite important in the design of mains 50/60 Hz electrical power distribution. I suggest you at least read the wiki page or use google?
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tonyc

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Re: Where does the skin effect in inductors cause the most pain?
Reply #3 - Aug 8^th, 2011, 1:55pm

Thanks for the replies and my apologies for not being clearer.

Yes, I am looking for applications where skin effect imposes a practical limit on circuit performance. What do I mean by "practical"?

As a hypothetical example, let's imagine the reduction in the Q of an inductor (due to skin effect) sets a maximum frequency of an oscillator due to increased phase noise. This "problem" is solved by using a different type of oscillator.

Why do I care about such things? Well, I have a solution for reducing the skin effect. It increases the complexity of a circuit of course. This increase in complexity means it's only an attractive solution (commercially) when it presents more benefit than the alternative does to circuit designers.

�

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aaron_do

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Re: Where does the skin effect in inductors cause the most pain?
Reply #4 - Aug 8^th, 2011, 6:51pm

Hi tonyc,

skin effect is a huge problem. It directly translates to power loss, which degrades efficiency and output power in transmitter PAs, and leads to poorer achievable NF in receivers.

In terms of practical limits, I'm don't know of any, but they definitely exist. They may be very structure/technology dependent, and so maybe you need to treat them on a case by case basis.

Good luck with your idea.

regards,
Aaron

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there is no energy in matter other than that received from the environment - Nikola Tesla

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RFICDUDE

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Re: Where does the skin effect in inductors cause the most pain?
Reply #5 - Aug 10^th, 2011, 5:04pm

Skin effect is important to different applications. As mentioned, any AC power application where the conductor thickness is thicker than the skin depth will suffer from skin effect losses.

In integrated RF circuits, the conductor thicknesses tend to be on the order of the skin depth, so it is debatable whether or not it is worthwhile to mitigate skin effect losses on chip.

Off-chip applications/components with thick conductors may benefit the most from a technique that mitigates skin effect losses.

Coincidentally, I have been doing some hobby work with crystal radios lately and it is very interesting to note that skin effect dominates Q factor even at 1MHz! The solution at these frequencies is to use bundled insulated wires (Litz wire) where the diameter of each strand is less than the skin depth. This is a very good solution for fixed frequency applications, but the downside is that Litz wire is expensive.

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tonyc

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Re: Where does the skin effect in inductors cause the most pain?
Reply #6 - Aug 11^th, 2011, 6:01pm

Thanks RFICDUDE.

AC power apps are out, technical incompatibility.
RF power/lna IC apps are out for the reason you give.

YES to off-chip apps.

I googled Litz wire. The "induction cooker" app is very interesting, could be a very good fit.

I'd appreciate if you could explain in a little more detail why/how/where you use Litz wire in your crystal radio app. I don't know much about it. Is it a significant cost component to the radio? If no, why is it worth getting rid of?

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RFICDUDE

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Re: Where does the skin effect in inductors cause the most pain?
Reply #7 - Aug 12^th, 2011, 3:50am

Crystal radios directly convert RF power into intelligible audio power without use of amplifiers. Any power wasted in the RF components will directly reduce the available audio power of a received station.

Litz wire minimizes the losses in the wires used to make the inductor coils and sometimes the antennas.

Selectivity of the crystal radio is limited by the loaded Q of the tuning circuits. For broadcast band AM radio the stations are 10kHz apart in a band that that spans 0.5MHz to 1.6MHz. At 1.6MHz 10kHz 3dB bandwidth is a Q = 160. So you also want to maximize Q in the long wire inductors to prevent them from limiting the loaded Q of the tuning/matching circuits.

Also, Litz wire is widely used for cheap AM radio antennas. Almost any cheap AM radio has a ferrite bar antenna mounted inside the radio. There is usually some wire that looks like it has a reddish or pinkish coating wrapped around the bar. This wire is typically Litz because they can use a thinner Litz wire for the same loss as a thicker solid wire.

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tonyc

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Re: Where does the skin effect in inductors cause the most pain?
Reply #8 - Aug 12^th, 2011, 4:37pm

Crystal radios sound like fun but since they don't use power my solution isn't compatible. I wouldn't have stumbled across induction cookers if not for your reply RFICDUDE, so thanks! I'm not sure that app has enough "pain" with Litz wire for its developers to consider something different but I'll dig into it.

My solution was originally developed for optimizing high rate charge and discharge of batteries (also works great at low temps). It turned out that the perceived tradeoffs were too great so it never got adopted. Looking into it further led me here (this forum) as I began researching dielectric absorption in capacitors. Ken has a great �paper on that. But that "app" (high-res ADCs limited by DA in sampling caps) has already found alternative solutions that are now mature.

After reading another of Ken's papers, I learned my technique actually also applies to the skin effect (or more generally, any kind of phenomena that must be modeled by frac-poles). This is really nice on-line community for experienced designers to share ideas. I hope to contribute on the answer side in the future.

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