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LTC4120 not charging when in a wireless system

mtx4
on Jan 16, 2021

Hello, 

I made a simple wireless power transfer system based on LTC4120 and a simple oscillator, described in the datasheet.

LTC4120 as battery charger works very good, when powered by a Lab supply at 15V.

The simple oscillator itself works well, with 30V p-p sinewave.

For first coupling, I didn't use the DHC control, so there is only a series LC resonator, rectifier and input capacitor on the system. Well, when I couple the receiver with the trasmitter, charge LED goes ON without a battery connected and this is strange (the LED should be off without a battery), but anyway if I connect a battery the LED is still ON but no charging at all.

I can see the input voltage goes from 12 to 25V moving down and up the receiver but no charging at all. How can be possible??

With a power supply charges well and with a voltage rectified from the resonator doesn't charge, it's a mistery for me now.

    •  Analog Employees 
    on Jan 18, 2021 2:44 PM 1 month ago

    Hello, 

    It sounds like the receiver side is not getting enough power when charged from a transmitter. What type of the transmitter are you using in your test ? 

    Thanks,

    Wenwei. 

  • on Jan 20, 2021 11:47 AM 1 month ago in reply to Wenwei

    Hello,

    I used the simple trasmitter taken from the datasheet with parallel resonant tank.The receiver side get enough power, I'm sure, the LED is still ON, should be OFF. Anyway with DHC enabled the system works.

    Now I find a behaviour that I don't like: when the DHC starts to change the resonant frequency according to load, at certain air gap, I can hear a whistle from the IC. I found that corresponds to the activity of the DHC. This is a bad noise, and I don't know if is normal, or maybe I have to change C2S and C2P to move this frequency away.

    •  Analog Employees 
    on Jan 20, 2021 3:41 PM 1 month ago in reply to mtx4

    Yes that is possible to hear some noise on the capacitors for a certain frequency range. Should not be from the IC. One way to locate the noise source is to use a tip of the tweezers to touch the component and see if that change the noise. 

    Moving to another frequency should help in lowering down the noise. 

    Thanks,

    Wenwei. 

  • on Jan 21, 2021 1:58 PM 1 month ago in reply to Wenwei

    Yes it comes from the input Capacitor, and it starts when DCH stable at 0V begins to change resonant frequency, maybe under 8 mm of air gap, with 10-12 mm is fine. From the image the acivity frequency of DHC is about 10 kHz.

    The ripple on the input voltage increase enough, not a good thing. Actually frequency are so set:

    f_Tx = 119 kHz

    f_tuned = 116 kHz

    f_detuned = 127 kHz

    What do you think can I do to lower this noise? Change input capacitor should help? 

    Maybe I can increase detuned frequency

  • on Jan 26, 2021 4:46 PM 1 month ago in reply to Wenwei

    Hello, sorry for an other question, but I found that when the noise starts, the charge current decreases. When the noise go away but the receiver is pretty near the trasmitter coil it increas until 380 mA. This happen only playing with the two coild distance and aligment. With more distance but no noise I get more current. This is definitely strange! Why ?