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Using LTspice to check Current sense transformer reset?

cupoftea

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Hi

Trying to see from LTspice whether the CST is getting reset in this circuit.
It looks like there is a slowly rising voltage across the secondary of the CST , meaning it doesn’t reset…but the Zener voltage is high enough to reset it, so whats going wrong?

LTspice sim and jpeg scm attached
 

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  • CST reset.jpg
    CST reset.jpg
    150.6 KB · Views: 28
  • current sense transformer reset_1.zip
    3 KB · Views: 29

cupoftea

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Sorry, here is the sim with reset zener connected...
 

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  • current sense transformer reset_2.zip
    3.1 KB · Views: 24

dick_freebird

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Are you confident that your transformer model would show core reset, or a real enough terminal electrical response, that figuring volt seconds or probing current to get an idea is anything more than GIGO?
 

cupoftea

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I see your point, ...though its just to kind of get a ball-park idea of how close the reset zener value is to what's required...i mean, i dont want to overdo the zener voltage, even though it would still work.

For this UCC28070A PFC application, the idea is to set the UCC28070A max duty cycle to the max level that could still always ensure reset of the CST....but its practically impossible, over the entire mains half cycle, ,to calc what is the exact reset zener voltage required at whatever exact max duty cycle setting.....i mean, you can overdo it and get it working, but we want to maximise duty cycle and minimise zener reset voltage....we were hoping the sim could help in this optimisation process?

woops sorry, attached is the LTspice sim with reset zener in the right place.

Its amazing UCC28070A even bothers to offer max duty cycle setting facility...i mean, in the above sim, even if you set the zener reset to 4V7, (ridiculously too low), the txfmr still resets fine........if you view the magnetising current ( I(L9)+I(L8)/48 ) then it never builds up high at all.....the CST i am using uses E20/10/6 and Isat is over 150mA....and imag, even with 4v7 zener, only goes up to 10mA.......i mean, is resetting a CST the easiest job in power electronics?...almost whatever you do and it resets fine.
 

Attachments

  • current sense transformer reset_3.zip
    3 KB · Views: 22
Last edited:

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