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  1. #21
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    Re: current-mode boost converter UC3845

    Hi,
    Datasheet says:
    PCB Layout Considerations
    To achieve best performance from an LT3782 circuit, the
    PC board layout must be carefully done. For lower power
    applications, a two-layer PC board is suffi cient. However,
    at higher power levels, a multiplayer PC board is recom-
    mended. Using a solid ground plane under the circuit is
    the easiest way to ensure that switching noise does not
    affect the operation.

    In order to help dissipate the power from MOSFETs and
    diodes, keep the ground plane on the layers closest to the
    layers where power components are mounted. Use power
    planes for MOSFETs and diodes in order to improve the
    spreading of the heat from these components into the
    PCB.
    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  2. #22
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    Re: current-mode boost converter UC3845

    turn on your fets slower to reduce radiated noise - try 33 ohm for starters ...



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  3. #23
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    Re: current-mode boost converter UC3845

    pcb design as attached



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  4. #24
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    Re: current-mode boost converter UC3845

    thank you all
    can i use pcb trace as shunt resistor?i need two 0/008 ohm resistor for current sensing.can i use 10mm length and 0/8mm width trace(0/008 ohm resistance and 0/4 watt losses) as current sense resistor?



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  5. #25
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    Re: current-mode boost converter UC3845

    Sorry no its temperature dependent.



  6. #26
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    Re: current-mode boost converter UC3845

    Click image for larger version. 

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    i made this circuit with all the components used instead of the current sense resistor

    at the source pin of the the mosfet. wound the inductor choke myself. the circuit input is 9vdc

    and output is 48vdc but the trimmer indicated in the diagram when connected to pin # 2-Vfb of uc3843

    the oscillator stops. when disconnect from pin # 2 start oscillating give output of 48vdc.

    i don't know the reason why the oscillator stops when trimmer connect with Vfb with no output voltage.

    - - - Updated - - -

    Click image for larger version. 

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    this is the PCB layout of the boost converter circuit. with trimmer not working.



  7. #27
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    Re: current-mode boost converter UC3845

    too few volts into pin 2 perhaps? it is designed to see 2v5 from the output divider at all times ....

    a soft start ckt would be a good idea too ...



  8. #28
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    Re: current-mode boost converter UC3845

    with Vout = 48vdc. if the trimmer is set to the bottom then the voltage at Vfb = 3.1 volt.

    for Vfb = 2.5v by setting the trimmer at the bottom mean 10k + 4.7k = 14.7k in series with 0.8k.

    but i want to change the output voltage to 14vdc using the trimmer which is not working.



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  9. #29
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    Re: current-mode boost converter UC3845

    you cannot have Vout < Vin due to the diode - you need to have a load on the o/p to test properly ...



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