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    the output load is oscillating in flyback smps

    I designed a SMPS with 12v and 1 amps output. When i connect a dc bulb of load 12v 10W. The smps output oscillates (turns on and off) frequently for some time and then the output goes stable. what causes this problem. The pwm ic used here is TNY 278PN and feedback circuit with optocoupler and TL431.

    How to overcome this? so that my smps output remains stable once it turned on without oscillating.

    Schematic is attached here.SMPS Schematic.pdf

    •   Alt26th November 2016, 13:04

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    Re: the output load is oscillating in flyback smps

    Hi,

    i assume
    * bad layout (a breadboard is useless here)
    * weak power input
    * wrongly dimensionated transformer (saturating)

    Klaus



    •   Alt26th November 2016, 13:17

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    Re: the output load is oscillating in flyback smps

    Hi Klaus
    Thanks for your reply.

    Am new to power electronics..

    With input above 230v Ac the there is no oscillation. If the input voltage drops even 5v there is oscillation.

    But i want to make it work from 150v input..



    •   Alt26th November 2016, 15:14

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    Re: the output load is oscillating in flyback smps

    You may have some issue with the input filter inductors (L2/3). Try to dampen it more with lower values (100 Ω) for R21/24 to see if it is stable.



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    Re: the output load is oscillating in flyback smps

    I guess the problem won't occur with resistor load and is most likely caused by incandescent lamp increased inrush current triggering TNY278 current limit.

    I'm not sure if the device is well suited for this kind of load. If you don't expect permanent overload, you might try increasing the current limit by using a 10 µF C10 capacitor as described in datasheet. You can also try if you achieve stable operation when the output voltage is slowly increased to the final value, in this case a RC combination parallel to R12 could provide a voltage ramp.



    •   Alt26th November 2016, 17:16

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    Re: the output load is oscillating in flyback smps

    Quote Originally Posted by E-design View Post
    You may have some issue with the input filter inductors (L2/3). Try to dampen it more with lower values (100 Ω) for R21/24 to see if it is stable.
    will try ur suggestion and come back to u... But i removed the inductor and shorted the points at the negative side.. The condition remains same at that state.



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    Re: the output load is oscillating in flyback smps

    Quote Originally Posted by FvM View Post
    I guess the problem won't occur with resistor load and is most likely caused by incandescent lamp increased inrush current triggering TNY278 current limit.

    I'm not sure if the device is well suited for this kind of load. If you don't expect permanent overload, you might try increasing the current limit by using a 10 µF C10 capacitor as described in datasheet. You can also try if you achieve stable operation when the output voltage is slowly increased to the final value, in this case a RC combination parallel to R12 could provide a voltage ramp.
    yes u r right.. If i connect the load after powering up the SMPS i found no issues. But when it is powered on with the load connected it goes oscillating.

    About increasing the capacitor to 10uF, will look into dat.



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