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    Flyback controller explain

    Hello,
    I'm working on flyback converter LTC3805 and i have situation when sometimes (one in ten cases) i get overcurrent protection triggerd and "hiccup" mode statring. there is no short-circuit, when i replace controller the circuit works fine.

    When i look on the primary current at the Oscilloscope i see every TSS time (200ms) sequence of climb current until reach OC protection. but my question is, if someone could explain me:

    1. why the current climibg each pulse (if you could send me to equation)
    2. how the contoller take part at the begining of the start-up? because if output voltage not build yet it doesnt have loop.

    link to the current primary (green) picture:

    Click image for larger version. 

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    link to the circuit picture:

    Click image for larger version. 

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    thank you very much for your help.

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    Re: Flyback controller explain

    Hi,

    The essence of soft start is to make the converter start slowly with very low duty cycle and then rise gently to steady state.

    With low duty cycle, the primary current (and of course the secondary current) can only rise so much. As duty cycle is increased, the current has more time to increase more.

    - - - Updated - - -

    You have softstart circuitry built into your converter and I believe that is the reason why your have that behaviour with the current.
    -------------
    --Akanimo.



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    Re: Flyback controller explain

    thank you for the reply,
    yes I know that soft start slowly increases the output voltage, but it makes that at large time (TSS) and I ask
    specifically on the small sequence(as you can see duty cycle stay the same). why is the current continues to rise in each pulse.? what causes that? how the controller manage the start-up operation of the converter?



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    Re: Flyback controller explain

    Please show the MOSFET gate voltage.
    -------------
    --Akanimo.



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    Re: Flyback controller explain

    Mosfet gate voltage:
    Click image for larger version. 

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    Re: Flyback controller explain

    is the diode/cap the right way round on the sec side - i.e. Tx phasing ... ?

    - - - Updated - - -

    also try a 22 ohm gate resistor - for starters - reduces the noise ...

    - - - Updated - - -

    also 150pF on pin 7 to gnd...

    - - - Updated - - -

    lastly - Tx design is critical to a good result ...



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    Re: Flyback controller explain

    the diode/cap in the right way on the sec side. -most of the time converter works fine ( doesn't work about one in ten cases).
    I try 100-ohm gate resistor, which decreases the current peak and the signal looks better (fewer noises), but the MOSFET gets very heat (about 100 degrees). can anyone tell me why it happened? (formula of that?)

    and again if someone could tell me why in the first situation the current goes up?



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    Re: Flyback controller explain

    The shape of the green waveform seems like your 'transformer' is saturating.

    By noise in this case, did you mean audible noise?
    -------------
    --Akanimo.



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    Re: Flyback controller explain

    impossible to assist without seeing the real ckt and the output side - is you diode & mosfet well heatsunk ...?



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    Re: Flyback controller explain

    Hi,

    I'm surprised that you haven't gotten help till now. I just hurriedly looked at the datasheet for the IC but I have to go to work.

    Take a look at the datasheet pages 15 and 16.

    - - - Updated - - -

    If after you look at the datasheet you can't still find a solution, then please indicate that you haven't been able to find a solution and I'll find time in my busy evening to help you out.
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    Re: Flyback controller explain

    Hi, I understand how the ic works according to the datasheet but it describes how it works when its close-loop, not at the beginning transient ( and its process in the beginning and how the ic know how much pulse to give to ramp voltage)



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    Re: Flyback controller explain

    Startup behavior is established by soft start ramp, error amplifier compensation, output load. The basic mechanism is described in the datasheet.

    As mentioned by Easy peasy, you didn't yet reveal your complete circuit, it's impossible to analyze the feedback operation. A good way to evaluate the IC behavior is to use the LTC 3805 test fixture in Ltspice.


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    Re: Flyback controller explain

    Quote Originally Posted by benjal View Post
    Hi, I understand how the ic works according to the datasheet but it describes how it works when its close-loop, not at the beginning transient ( and its process in the beginning and how the ic know how much pulse to give to ramp voltage)
    Okay.

    Let me also add that one known cause of hiccup in smps is overload.

    Please explain to us what you're referring to by saying that the circuit works fine when you replace controller.
    -------------
    --Akanimo.


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    Re: Flyback controller explain

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



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    Re: Flyback controller explain

    First thank you very much for your help.
    from some reason my reply dont upload so i reply again.

    the circuit is a flyback converter with 12 secondary (3.3V,5V,-5V,10V).
    Click image for larger version. 

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    please tell me if i understand how the IC works at the beginning:
    the IC create sequences every time (according to soft start capacitor) to ramp the output voltage
    every sequence built from switch pulses that grow until IC gets satisfied voltage(according to load) and stop the sequence.
    the grow is because voltage not build yet so there isn't supply to load so the flux is remain as before and charge again and grow, am i correct?
    please tell me if and where i wrong so i could understand how the ic works so i can understand the problem and then fix it.

    thank you,



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    Re: Flyback controller explain

    given the total capacitive loading on the sec's - you need a much longer soft start ...

    - - - Updated - - -

    however - transformer construction is also vital to success



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    Re: Flyback controller explain

    thanks,
    there is a soft-start capacitor of 330nF that give 85ms, is not enough? could you write how to calculate the appropriate soft-start time according to the capacitive loading?
    in addtion, could you tell me if my Assumption about IC works at beginning is correct?



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    Re: Flyback controller explain

    soft start cap needs to be bigger for a longer soft start - say 1uF and 470ohm current limit R in series

    how do you know that the phasing is correct for the transformer and all its outputs?



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    Re: Flyback controller explain

    I dont, how i know if there is something wrong with the phasing?



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    Re: Flyback controller explain

    Quote Originally Posted by benjal View Post
    I dont, how i know if there is something wrong with the phasing?
    The output voltage will probably be wrong, and it will change when the input voltage changes.
    You can check with a scope that the diodes on the secondary side conduct when the switch on the primary side is off.



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