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Testing LLC Resonant Tank Current, different test methods make different results

Jankel
Intellectual 731 points

Sch is as follows

Test result is as follows ( adding a series resistor in the resonant network, and test this resistor's voltage. R34 in the sch is this series resistor, with the value of 0.2R )

Note:First waveform is the voltage across R34.

We can see that the resonant current(voltage across R34) is very strange, is there someone else having the same experience?

And could somebody tell me why?

But if i use the Current Transformer made by myself using a ferrite toroid core, test result is as usual.

  • 0
    Intellectual 731 points

    I found that when i remove the earth connection of my oscilloscope, the voltage across R34 becomes right.

    Why there's such a huge difference between these two conditions ???

    One condition is connecting the oscilloscope to the earth, and the other is disconnecting the oscilloscope to the earth.

  • 0 in reply to Jankel
    TI__Intellectual 2230 points
    HI Jeakel,

    Did you use differential probe when you test the voltage on R34? If not, you should not connect the negative terminal of probe to the R34 directly, becasuse this will short the switching node to ground and may affect the converter operation. The better way to test resonant tank current will be using a current probe or current transformer.

    Thanks
    Oliver
  • 0 in reply to Oliver Pan91
    Intellectual 731 points

    Hi Oliver,

    Of course i didn't use the differential probe because i cannot afford.
    As you said, the better way to test resonant tank current will be using a current probe or current transformer and it works well.
    Although i only can use a CT, also because i cannot afford a current probe.

  • 0 in reply to Jankel
    TI__Mastermind 25570 points

    Hi Jankel

    You could try to put the resistor between GND and CBB - Red X below - and measure the current there instead using a single ended probe ('scope probe).

    This has the advantage that there is no large common mode voltage signal like that at the position of R34 so you should be able to get a clean signal. The currents at the two positions will be the same - less that drawn off by the OC network, C11 etc...

    Regards
    Colin

  • 0 in reply to Colin Gillmor
    Intellectual 731 points
    Hi Colin,

    Thanks, it is a quite good method, and I will take your suggestion in my next design.