Unloading Problem in A Secondary Winding of a SMPS

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moataz fouad

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Dear all,
I've an AC-DC SMPS with 2 outputs, one +12V regulated, stable output & another +15V unregulated stable output; the problem is when the +15V output is unloaded, the output goes up to 30V !!
Any clue why this happens & how to solve this problem ??
Thanks in advance.
 

Check what happens when 1kohm 2W resistor is connected as load resistor.
Can you post schematic from PS.
 

This is normal. I think it is caused by voltage drop in secondary diode and copper loss. When +12V is loaded, the voltage drop in parasite components increases.
And 15V output with no load will be too high.
 

i think 12v output is having both current and voltage feedback for over current protection and votage regulation, there may be a problem in current sence feed back. if you post circuit is more helpfull
 

As "moataz fouad" mentioned, 15V output is a open loop.
It should be out of accurate control.
murali_dece said:
i think 12v output is having both current and voltage feedback for over current protection and votage regulation, there may be a problem in current sence feed back. if you post circuit is more helpfull
Click to expand...
 

Apart from stating that the effect is "normal operation", It should be mentioned, that it depends on the converter operation mode respectively secondary rectifier configuration and also on the leakage inductance between secondary windings.
 

Dear all,
The design is in the attached pic.
Thanks in advance.
 

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  • Flyback SMPS.png
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The flyback configuration should basically achieve linked output voltages. Leakage inductance is the main reason for voltage rise of unloaded outputs. If the transformer design can't be improved, adding a base load to all outputs is the only way.
 

i am suspecting output capacitor c18, there maybe any minute leakage or output choke dry soldering
 

@ FvM:
could you tell me from where comes the leakage inductance & how to fix it ?
Thanks in advance.
 

Careful transformer design is needed here to get close coupling of the primaries to all the secondaries - to avoid uncontrolled winding outputs going too high when the controlled outputs are loaded - Regards, Orson Cart.
 

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