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There's a variety of snap-on wireless charging solutions on the market. For example, this one includes a charging mat and a plastic case that is put onto iPhone.

I suppose the winding is in the middle of the case back and occupies a rather huge area. For comparison, MIFARE Ultralight smart cards are size of a credit card and wires can be seen if the card is lit through with a bright light source - they run right along the card perimeter. This question contains a scan of a stripped card.

Now when the phone is placed onto the mat the charging system air core transformer completes and I suppose the magnetic field is rather strong there. How does that field not induce currents in the phone circuits and not interfere with the phone functioning?

I mean, if the phone was shipped with inductive charging the manufacturer would have to design the phone appropriately from the beginning. Yet the described system is shipped by a third party and clearly it was designed separately.

How do such third party snap-on systems not interfere with the device functioning?

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2 Answers 2

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For maximum sensitivity, an antenna would be sized to the wavelength of the incoming signal. The sizes of most wires (traces) in a phone's circuit board are too small by maybe 2-3 orders of magnitude to be a good antenna for the wireless power signal. The size of wires inside the chips would be orders of magnitude smaller than that. The wireless power receiver wire loop is sized to resonate with the transmitter, and has circuitry to actively guide resonance. The frequencies involved in wireless power are very low compared to most of what's going on in a phone - kHz compared to GHz.

Any digital processing would be immune to noise so long as it remains well below the threshold of creating bit errors. The more sensitive parts of the phone to wireless power would be the analog areas, such as RF receiver and audio. Audio would be especially susceptible as the signal frequencies are closer together, both in the kHz range, although possibly still one or two orders of magnitude apart (4kHz for voice vs 100?kHz for power).

Most people don't use their phone while it's on the charger, so whatever problems are caused wouldn't usually be noticed, so long as no permanent damage is caused. Worst case scenario might be some missed calls, or bad-sounding audio if charged while connected to the stereo.

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Inductive equipment charging

Summary

  • A matching receiver is required.

  • The receiver uses a resonant circuit to greatly increase the voltage levels experienced. See below and Wikipedia resonance page re what resonance does.
    Voltage increases of 100+ times may occur with resonance.

    Big voltages with resonance -> good power transfer.
    Low voltages when non -resonant = low interference.

  • The field is near field magnetic.
    Some induction into audio circuits may be noticed if the equipment is used while charging.

They say:

  • Now charging your iPhone 4 is as simple as setting it down! Complete solutions are pre-pack bundles pairing a Powermat one position charging mat and a receiver to provide a seamless upgrade to wireless charging. Charge your iPhone 4 with the Powermat 1Xi and Receiver Case for iPhone 4.

AND

  • Wireless Receiver Door for iPhone 4

    The wireless Powermat Receiver case for iPhone 4 allows your iPhone 4 to charge wirelessly on any Powermat wireless charging mat Powermat Receiver Case seamlessly upgrades your iPhone 4 to wireless charging

    One time replacement of your phone's case with a slim, sleek, formfitting design that mirrors your iPhone's original design Simply place on any Powermat Mat to charge wirelessly, no plugging and unplugging

Image: Phone held above charging base.

enter image description here

The key point to note there is "and a receiver" - implemented in this case in the replacement case unit.

The receiver has an inductive pickup which is tuned to resonate at the transmit frequency, which very greatly increases the available voltage levels.

enter image description here

Graph: Increase of amplitude as damping decreases and frequency appro aches resonant frequency of a damped simple harmonic oscillator. From Wikipedia resonance page

The fields generated are significant and MAY cause problems in some equipment but in most cases will not have any effect on untuned circuitry. The most likely effect would be audio pickup in audio circuits if the equipment is used while charging - but in almost all cases the equipment will not be used when on the charging "mat".

Image: Phone with "receiver" attached.

enter image description here

Wireless Power Consortium

  • The Wireless Power Consortium is a cooperation of independent companies. The cooperation is governed by a Consortium Charter that defines rules for confidentiality, intellectual property and decision making,

From here on their site

Resonant Coupling

enter image description here

  • by: Eberhard Waffenschmidt, Philips Research

    From the beginning of inductive power transmission, resonant circuits are used to enhance the inductive power transmission. Already Nicola Tesla used resonances in his first experiments about inductive power transmission more than hundred years ago. Especially for systems with a low coupling factor, a resonant receiver can improve the power transfer. Resonant power transmission is a special, but widely used method of inductive power transmission and is limited by the same constraints of magnetic fields emissions and efficiency.

    To understand the effect, it can be compared to mechanical resonances. Consider a string tuned to a certain tone as mechanical resonator. Even a far away and low level sound generator can excite the string to vibration, if the tone pitch is matched.

    Here, the resonator in the receiver consists of the receiver inductance and a capacitor. Also the transmitter can have a resonator. The general arrangement is illustrated in Figure 6a. The transmitter and receiver coils LTx and LRx can be considered as weakly coupled transformer. For this, an equivalent circuit diagram consisting of magnetizing and stray inductance can be derived, as shown in 6b. In this diagram, also the resistances of the windings are shown. The diagram shows clearly, that the resonant capacitors cancel out the stray inductance in the receiver and the magnetizing inductance in the transmitter. Now, the only remaining limit for the power transmission is the winding resistances of the coils, which impedance is one or two orders of magnitude lower than that of the inductances. Therefore, for a given generator source, much more power can be received.

Related - interest only:

I hold one US patent (now expired). It is for inductive power transfer between a long loop and "stations" which both receive power from the loop and which bidirectionally signal over the loop. Application was a warehouse picking system. Available power per station was a Watt or few. From memory the signalling rate was in the 100 kbps range. Quite a challenge overall. The system worked but the client did not proceed commercially with the overall system.

The reason why:

Some apparently competent readers have queried WHY the description above describes what happens. Taken from the above text, the following summarises the key points. Sections in bold are comments/explanations on the text from above. :

  • The receiver uses resonant circuit to greatly increrase the voltage levels experienced.

    It's generally found that big voltages have more effect than little voltages.

  • The receiver uses a resonant ciruit to greatly increrase the voltage levels experienced.

    Resonance causes voltages at the selected frequency to be much larger. This is a fundamental principle of tuned circuits. In fact, this is essentially what "tuned circuit" and "resonance" MEAN.

  • Some induction into audio circuits may be noticed if the equipment is used while charging.

    Even the little voltages **MAY cause some effects - ie the induction system MAY in fact have SOME effect.**

  • The receiver has an inductive pickup which is tuned to resonate at the transmit frequency, which very greatly increases the available voltage levels.

    What it said.

  • The fields generated are significant and MAY cause problems in some equipment but in most cases will not have any effect on untuned circuitry.

    And again

  • The most likely effect would be audio pickup in audio circuits if the equipment is used while charging - but in almost all cases the equipment will not be used when on the charging "mat".

    If you put your head against the phone and listen while it's on the charging mat you MAY hear some effects from the induced voltages - but that may just be the effect on your in-ear bluetooth earpiece :-).

  • The diagram shows clearly, that the resonant capacitors... which impedance is one or two orders of magnitude lower ... Therefore, for a given generator source, much more power can be received.

    As shown in the diagram - the capacitors tune out the parts of the ircuit which are high impedance so you can get big voltages at the selected frequency when you use resonance. Without resonance you may get 100 times less current flow. 100 is a lot.

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    \$\begingroup\$ Yes, that's all very interesting - but how do they stop the magnetic field from interfering with the phone's operation? Specifically the communications...? All those magnetic waves around the mat must surely affect the incoming and outgoing radio signals. \$\endgroup\$
    – Majenko
    Commented Oct 6, 2011 at 13:05
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    \$\begingroup\$ @Majenko - see above. It says: "The receiver has an inductive pickup which is tuned to resonate at the transmit frequency, which very greatly increases the available voltage levels. The fields generated are significant and MAY cause problems in some equipment but in most cases will not have any effect on untuned circuitry. The most likely effect would be audio pickup in audio circuits if the equipment is used while charging - but in almost all cases the equipment will not be used when on the charging "mat".". YMMV :-) \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 6, 2011 at 13:45
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    \$\begingroup\$ @RussellMcMahon - I think that our understanding of tuned vs. untuned circuits is the essential part of this answer; you need to make why this is important more explicit. You provided a lot of information, but don't explain why it matters to the questioner. \$\endgroup\$
    – Kevin Vermeer
    Commented Oct 6, 2011 at 14:35
  • \$\begingroup\$ How do you know that these charging mats are resonant and not just regular transformers? Resonant transfer actually works better with a gap, no? \$\endgroup\$
    – endolith
    Commented Oct 6, 2011 at 21:10
  • \$\begingroup\$ How do I know any thing? // ... wireless consortium ... // US patent ... // bought an example recently just to pull apart ... // .... two orders of magnitude ... /// Doing it without resonance and with a normal magnetic circuit is possible but vanisingly unlikely, both for power transfer capability reasons* and because resonance helps remove the very issues the questioner asks abou and because ... . // Using standard electromagnetic transformer theory, look at the effect of an airgap of a fw mm. // No resonant transfer does not work "better" with increased air gap. \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 7, 2011 at 2:21

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