LLC resonant converter with air core

Question

Is it possible to do an LLC resonant converter with an air core transformer (wireless power transfer)? I've read basics about it but I've all read that they use some type of ferrite cores. I think that it should be possible at first but I am having second thoughts based on what I've read but I just want to know from more experienced ones.

I was thinking for an air-core based, the inductances (and coupling factor k) simply change depending on the 'state' of the two coils (primary and secondary). By state, I mean it depends on how far are the two coils (distance) and how axially oriented are they. I believe it goes like this: 1.) Lm (magnetizing inductance) represents how much flux is coupled from the primary to secondary. Thus, if the two coils are further, less flux is coupled from primary to secondary coil, and this changes Lm. 2.) Lk (leakage inductance) represents how much flux is "wasted", meaning how much flux isn't coupled from the primary to secondary coil. So in a air-core one, since permeability of air is not high, Lk would be high compared to ones with core. Moreover, Lk also changes when distance between two coils are varied since as distance becomes larger, more flux is "wasted".

For an LLC resonant converter, the tank consists of Lm (magnetizing inductance), Lres (resonant inductor, external?), and Cres (resonant capacitor, external). I do know that resonant controller ICs use variable frequency control to regulate the output instead of PWM. So I was thinking, in an air-core transformer, distance can change which means Lm (part of resonant tank) changes. This also changes the resonant frequencies. And I believe in the design part I've read, resonant frequencies are part of the design so with changing resonant frequency in an air-core, is implementing LLC actually possible?

Thanks

Answer 1

Is it possible to do an LLC resonant converter with an air core transformer (wireless power transfer)?

Sure it is and I've actually done several in coupling power to electronics mounted on a spinning shaft.

I've read basics about it but I've all read that they use some type of ferrite cores.

If you can use ferrite cores you get benefits in that you need fewer turns to generate the same magnetic field and you can use ferrite to prevent your generated field being "killed off" by excessive eddy currents in conducting metal that might surround the coils.

Your middle paragraph is correct as far as I can tell but there is an easier way to get to a solution that also considers how the magnetic field attenuates with distance.

Consider this: -

The above does not consider tuning but tuning with capacitors has huge benefits. Firstly, the driven coil can be parallel tuned to produce a much bigger magnetic field for a given excitation current. The receiver coil also benefits from tuning too.

I was thinking, in an air-core transformer, distance can change which means Lm (part of resonant tank) changes.

Yes, as distance gets bigger the coupling is smaller and the resonant frequency becomes wholly dictated by the transmit coil and its resonant capacitor. As distance shortens you get more coupling and if you plotted the frequency response (such as on a sim) you'd find two peaks emerging either side of the original resonance.

Changing the driving frequency can offer a means of regulation but remember you need a feedback mechanism from the receiver to tell the primary controller what the receiver voltage is.