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What is the easiest implementation to do a closed loop wireless power transfer system

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bhl777

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Hi All, I have an open loop wireless power transfer system. I use a class E power amplifier to drive the Tx coil. In the Rx side I have a rectifier to generate an output DC voltage.
PTE.png
Would anyone teach me how to implement an easiest closed loop regulation of this WPT system? Ideally I want to use some analog components (such as the op-ampm, instead of the microcontroller or any other programming logic circuits) to achieve a ceratin level of the V_REC voltage regulation. The regulation is not necessarly perfect, but the feedback loop must use the same Tx and Rx coils. Is it possible to design such a loop?
Thank you!
 

Your degrees of freedom are the following ones:
- VDD -> by varying the VDD you can adjust the output VDC.
- Duty cycle of the gate driver -> by varying the duty cycle of the gate driver of the NMOSFET surely varies the output voltage
- Frequency of the gate driver -> This option is out because you have tuned the coils to work in resonance at that frequency
- full bridge rectifier -> could be a controlled full bridge rectifier and hence control the output DC voltage.

I am not aware of the linearity of the above mentioned degrees of freedom. Voltage transfer function could tell e.g. how does the duty cycle influence the output voltage ... however, drawbacks of working with other than 50% duty cycle in class E must be regarded as well.

Check the above and see whether or not it is possible for your application.
 

Your degrees of freedom are the following ones:
- VDD -> by varying the VDD you can adjust the output VDC.
- Duty cycle of the gate driver -> by varying the duty cycle of the gate driver of the NMOSFET surely varies the output voltage
- Frequency of the gate driver -> This option is out because you have tuned the coils to work in resonance at that frequency
- full bridge rectifier -> could be a controlled full bridge rectifier and hence control the output DC voltage.

I am not aware of the linearity of the above mentioned degrees of freedom. Voltage transfer function could tell e.g. how does the duty cycle influence the output voltage ... however, drawbacks of working with other than 50% duty cycle in class E must be regarded as well.

Check the above and see whether or not it is possible for your application.

Hi CataM, thank you so much for your help! Would you briefly explain to me why changing the duty cycle of the SWITCH can help? My questions are:
(1) For example, if in the nominal condition, I used 50% to get a 5V rectified DC voltage. In the misalignment condition, the rectified DC voltage will be reduced. How am I supposed to change the duty cycle?
(2) If I can only have a fixed duty cycle, which one should I use to maintain the highest power under the misalignment conditions?
Thank you!
 

Closed loop means you want an either analog or digital reverse communication channel.

I believe that some kind of modulation is the best way to achieve it. Look e.g. at the Qi wireless charger standard which is implementing a "backscatter" modulation with 2 kHz to send device information (e.g. power demand, device present, device fully charged etc.) to the charger.

Similarly, you can use low frequent amplitude, frequency or pulse width modulation to transmit an analog quantity like the intended voltage feedback.
 
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    bhl777

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If you just want a regulated output voltage, simplest thing to do is put a wide range regulator on Vrec, and run the transmitter at full power all the time. Though you have to make sure that when coupling is maximum you don't exceed any voltage ratings on the receiver. This certainly isn't the most efficient way, but it's much simpler than varying the transmitter, which requires communication between tx and rx.

If that's not good enough you'll need some sort of wireless communication. Probably best to go with an existing chipset as FvM suggests.
 
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    bhl777

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(1) For example, if in the nominal condition, I used 50% to get a 5V rectified DC voltage. In the misalignment condition, the rectified DC voltage will be reduced. How am I supposed to change the duty cycle?
You want to get 5 V DC at the output no matter whether the coils work in the designed condition or in misalignment, right ?
You could very well go with the method posted by mtwieg or figure out the voltage transfer function as a function of the coupling and figure out the expected DC level at low coupling and how you can increase it (by varying VDD or duty cycle which you need to look up those formulas in a book).

The duty cycle depends on how do you create the square wave that goes to the gate driver.
(2) If I can only have a fixed duty cycle, which one should I use to maintain the highest power under the misalignment conditions?
Calculate the output voltage as a function of the duty cycle and figure out what duty cycle achieves that (there could be no duty cycle to achieve that).
 
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    bhl777

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You want to get 5 V DC at the output no matter whether the coils work in the designed condition or in misalignment, right ?
You could very well go with the method posted by mtwieg or figure out the voltage transfer function as a function of the coupling and figure out the expected DC level at low coupling and how you can increase it (by varying VDD or duty cycle which you need to look up those formulas in a book).

The duty cycle depends on how do you create the square wave that goes to the gate driver.

Calculate the output voltage as a function of the duty cycle and figure out what duty cycle achieves that (there could be no duty cycle to achieve that).

Thank you CataM! I have an additional question to you regarding this WPT system. Do you know how can I measure the AC-AC power transfer efficiency of this system? I guess that is just related to the AC voltages and AC currents of the two coils, but I am not sure how to measure and calculate it. Thank you!
 

Do you know how can I measure the AC-AC power transfer efficiency of this system?
Method 1:
Using a Wattmeter you can measure active power. Use the wattmeter to measure the input and the output power (if output is DC or the load is resistance, you would not need this one to calculate the output power). Ask in your university about this one. It is very likely to have (at least) one.

Mehtod 2:
1)Measure the RMS voltage at the input using an oscilloscope or a multimeter which has the label "True RMS".
2)Measure the RMS current at the input using an oscilloscope or a multimeter which has the label "True RMS".
3)Measure the phase difference between the input voltage and the input current.
4) Active Input Power = 1) * 2) * cos(3))
5) Do the same thing in order to get the Active Output Power unless the load is resistive. Besides, if DC at output, simply VDC^2/Rload. If output is AC, VRMS^2/Rload.

Last step: Power Transfer efficiency (eta) = Pout/Pin

...but I am not sure how to measure and calculate it.
In order to calculate it, you can do the same as above but mathematically.
Probably it is easier to compute the Pin as Pin=Pout + Power lost in resistive elements along the circuit

Power lost in a resistance is simply found as : P=IRMS^2 * R = VRMS^2/R = VRMS*IRMS
 
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