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problem with load regulation and auto-restart

Posted by: Adel on Mon, 09/14/2009

hi PI engineers,
how can i design the quantity of red elements in "cir main.jpg" which attached to this post?
in practice they can determine the time of auto-restart and the margin of that for fault condition
and can affect on load regulation but i don't know how can i design the best value for them to be effective and suitable.
the software design the capacitor (in this design it uses c=680nF),only but in practice when i want drawn current about 400mA or
500mA the topswitch resets again and again and load regulation is very bad, i put a resistor in series with
capacitor and the auto-restart and load regulation go well and well but i don't know how can
i determine the best value for capacitor and resistor. please help me to design them. please
pay attention to my circuit in "cir main.jpg" .


Attachment Size
cir main.jpg 331.6 KB


Submitted by John Carpenter on Tue, 09/15/2009

Multiple output power supplies can be difficult to stabilize. The best way to compensate the circuit is to use a power supply loop analyzer. This will give you the feedback loop phase and gain.
Since you have stopped the power supply from oscillating, you can monitor the feedback voltage at the TL431 with an oscilloscope and apply a step load to the output. Adjust the feedback compensation components to eliminate or minimize the ringing on the feedback signal. Verify that the power supply operates properly under all line and load conditions.

I entered your design in PI Expert and found that it did not show C17. Was this required to stabilize your circuit? Also the value for R7 is 61.9 Ohms. It is better to use the lower value resistor to make sure that the TL431 does not saturate.

Z. Cochrane

Thanks of your help, i used C17 for soft start and soft finish of power supply, and i used R9 for discharge of C17 at turn off power supply. i used R7=100ohm because software said that the CTR of OptoCoupler must be 80% and it is PC817A, But i used PC817B with CTR 130% so i changed the value of R7 to 100ohm of course the software told me the value of 100ohm,
today i want go to university so i will test your opinion and will tell you about that,thank you for your help.