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    uc3842 current limmiting without current sense

    hello;iwant to design a buck converter using uc3845 controller;and i want to limit the peak current without current sensing...can any one help me and introduce a config to me?

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    Re: uc3842 current limmiting without current sense

    Hi

    difficult. It's like you want to keep the speed limit without knowing your actual speed.

    What's your idea behind this requirement?

    Klaus
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    Re: uc3842 current limmiting without current sense

    Quote Originally Posted by omid2006 View Post
    hello;iwant to design a buck converter using uc3845 controller;and i want to limit the peak current without current sensing...can any one help me and introduce a config to me?
    Why do you not want current sensing? Without current sense, you cannot tell the PWM to switch in realtime and you do not want that.
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    Re: uc3842 current limmiting without current sense

    Do you mean current sensing on the output (short circuit protection), or do you mean current sensing in the switch element on the primary side?

    The whole point with uc3842 is to sense the current in the switch element on the primary side, so it is meaningless to use the uc3842 without it.

    If you mean current limiting for short circuit protection, you don't need an extra current sense for that.



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    Re: uc3842 current limmiting without current sense

    The question tells me that the OP has not bothered to read the datasheet.

    The UC384x-series, by its own datasheet description, are Current-mode PWM controllers.

    Without the current feedback, the controller will not have the necessary waveform information to properly control the PWM cycle.
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    Re: uc3842 current limmiting without current sense

    I suspect that the OP wants to do a buck with a hi-side fet and yet doesn’t want the hassle of hi-side current sense……..but I would say, possibly use a current sense transformer to do your current sense.

    Ive actually seen solar bucks with no current sense…….they charge batteries….and as long as the battery voltage is high enough, you can actually do it without current sense, as long as you obviously limit the duty cycle…….the problem happens.. if the output suddenly goes short circuit and then of course your buck inductor current staircases up into saturation and bang goes the fet……unless of course, your microcontroller was able to sense the output short circuit quick enough and turn off the fet pronto.
    But yes, if using the ucc384x then you would have to drive its oscillator with a micro because otherwise it needs the current sense ramp to know when to turn off the fet.

    - - - Updated - - -

    here is a nice uc384x buck, to show how it works..in ltspice



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    Re: uc3842 current limmiting without current sense

    To sense current without sensing current you need a Xlax hybrid flux capacitor model # 37 56779980 21 from Newark, this part has globular crystal technology that predicts the current so that the turn off time can be pre-programmed - hope this helps ...



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