I am trying to implement the following resonant oscillator for wireless power transfer application. There will be a secondary coil coupled with Lp. enter image description here.

Reference - WPT Application Note

One of the pitfalls of this circuit is that the power consumption can be high when the secondary coil is absent.

Is there a way to design an auxiliary circuit to detect when the receiver is not present and to reduce the idle power consumption (possibly, by introducing a high series impedance and decrease resonant current)? However, it should also be able to detect when the receiver is coupled and activate the normal operation. Can we use something like tickler coil for this purpose?


I want to exclude obvious approaches which require the installation of additional sensing layer, for example, proximity sensors to detect receiver or additional communication link.

Next, as some of the answers suggested, using a simple micro to detect the changes in primary phase or define a current threshold to cut off the supply can be implemented. However, such approaches are highly dependent on the circuit parameters (frequency, power, etc) and most importantly, these sensing become challenging if we want coupling to be weak. When coupling is small, reflected impedance may be very small to make significant changes in primary side.

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    \$\begingroup\$ A proximity sensor maybe? \$\endgroup\$ – Dorian Jun 27 '18 at 20:59

I think the first step to a circuit like this would be to measure the current through the Lp coil. A current measurement typically consists of a low value resistor and a current amp. The current would change if a receiving coil were placed near the transmitting coil. A threshold could be set on the current measurement (with a comparator)to allow the circuit to determine if the coil were there or not.

The circuit could be turned on periodically to determine if there was a device there and go into high power mode.

Another way the detection of a coil could happen is by monitoring the phase of the current and voltage phase, the receiving coil would change the relationship between them, this is how it is done on some wireless power IC's.

You don't have to do this with a microprocessor, you could use a 555 timer for the pulse to check the circuit, after you did so you could 'and' the result together with the current detection signal, if it's high then keep the power active, if it's low, then shut the circuit down.

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  • \$\begingroup\$ Thank you @laptop2d, However, for such implementation, we need some digital processing. I was wondering if we can implement an analog circuit for this. \$\endgroup\$ – Pojj Jun 27 '18 at 19:58
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    \$\begingroup\$ You don't have to have a microprocessor circuit to implement this, it would be much cheaper and simpler to do it with a microprocessor, There is probably not a good way to do this with analog electronics, although you could use an oscillating opamp circuit for the pulse, a summer to check the circuit and an time constant decay circuit to keep the power active if it decided it needed to source current, all of these things will be more expensive than digital electronics. A micro can be had for a few dollars. Every single wireless transfer IC has digital logic in it \$\endgroup\$ – Voltage Spike Jun 27 '18 at 20:11
  • \$\begingroup\$ meta.stackexchange.com/questions/126180/… \$\endgroup\$ – Voltage Spike Jun 27 '18 at 20:17
  • \$\begingroup\$ I didn't mean the implementation of the logic by an analog circuit. It will be very easy to implement with a micro. However, when coupling is small, reflected impedance may be very small to make significant changes in primary side. If we want to let the circuit operate at weak coupling, it will challenging to sense current magnitude or phase changes. \$\endgroup\$ – Pojj Jun 28 '18 at 10:02

First of all do something about the lossey oscillator circuit .Adress your coil ,your gate drive and look carefully at the DC filter inductor configuration.Your losses will be lower now but there is still power being wasted.Measure the osc frequency and note the difference between active and idle states .The frequency difference can complement current measurements and is easy to implement using a micro these days .

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  • \$\begingroup\$ Thanks, The referenced application note itself has some optimized circuits with gate drives etc. The difference in the oscillation frequency is negligible when the coupling is weak. \$\endgroup\$ – Pojj Jun 28 '18 at 9:58

I have such circuit for wireless power, there are many small coils which generate power for a secondary circuit. In that circuit, some section of the circuit may stop when the current is below a threshold.

this circuit works well when the efficiency is high. when the secondary coil place farther and the efficiency drop the threshold current has a mistake.

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  • \$\begingroup\$ Can you share how the circuit look like? \$\endgroup\$ – Pojj Jun 28 '18 at 9:58
  • \$\begingroup\$ imgur.com/BgoXCPi I have a dynamic rotating load, in the picture as receiver coils. when the power to load below a minimum we can turn off some section of sender coil. this reduces the static power and heat. \$\endgroup\$ – M KS Jun 28 '18 at 10:37

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