SMPS modification

[ATIKU]

Pls
I modified a SMPS (19v 3A PC charger) by changing the output voltage feedback to get 5v

And it does output my 5v as required, but with 0 current, that can't even be used to power a 5v fan
And with some tiki tik sound on the transformer.

Pls, how could I solve this or have to rewind the transformer base on my target

 

[scopeprobe]

Pls
I modified a SMPS (19v 3A PC charger) by changing the output voltage feedback to get 5v

And it does output my 5v as required, but with 0 current, that can't even be used to power a 5v fan
And with some tiki tik sound on the transformer.

Pls, how could I solve this or have to rewind the transformer base on my target

What did you modify to get your 5V?

 

[andre_luis]

Did you check the design tips on the UC3843 datasheet?

Also, did you consider making a functional software simulation ?
I believe this part is quite standard on LTSpice.

--- Updated Feb 7, 2025 ---

BTW:

LTspice model equivalent to UC384x series

Hi. I want to make an offline Flyback converter simulation in LTspice. Can anybody suggest a PWM IC model that is equivalent to or as near as possible to TI UC384x series in it?

www.edaboard.com

 

[cupoftea]

Your bias coil is coupled to the secondary, so if the sec is only 5V...then your bias coil will not give enough voltage to run the uc3843. So yes you need to rewind the transformer....and maybe some other things too need changing.

Also, with 5vout, you may well not get enough current through your opto feedback diode.

With a rewound transformer you have to also think what voltage you are now reflecting to the primary side.

The uc3843 is old and primitive with no output overload protection built in so i am amazed to hear its built into PC chargers....maybe it was a home brew one/