SG3525, SMPS need help

[jegandren]

Hi all,

Attached is a Voltage mode control SMPS. When the circuit is switched on without giving a feedback to the SG3525 controller the output voltage that i got is +/- 45V...

Can anyone advise me on how am i to get +/- 25V could it be done by adjusting the zener values of the feedback circuit ?

After 3 times switching the circuit on and off for testing purpose, my mosfet and the controller IC were toasted...later found that that one out of 4 of the mur1520 (switching diode) is not working. perhaps i guess that would be reason to short the circuit and burn the mosfet.

[sawreyrw]

To begin you need LC filters in both the plus and minus output circuits. The transformer turns ratio needs to be selected to be compatible with the desired output voltages. For plus and minus 25 volts output the total zener voltage should be in the range of 49 volts.

I didn't look much at the rest of the circuit, but the 1K resistor at -In seems too low. 100uF on the comp pin is way too large.

[jegandren]

sorry comp is 100nF.

I never connect the LC filter 1st, because this is for testing purpose, just to test the circuit. later i will connect the LC filter...but i have connected the output to a load..... hope the circuit mosfets dun burn again.

[jegandren]

attached is the transformer that im using perhaps instead of +/- 25V at the secondary im getting +/- 48 V ...when a +/- 160Vdc is applied at the primary.... can anyone suggest on the transformer windings that need to be ammended.

[darkfenriz]

Primary 'ground' is connected on the wrong side of the NTC.

[jegandren]

Dear Darkfenriz,

i have uploaded the corrected version ...where the ground have been change to the other end of the NTC..please check.

[luka]

Hi

Why do you all use so small values of caps with IR, it won't kill you to put 10uF or even 100u to be sure it will hold that charge for fet drive...and it only needs to be 16v, no need for 100v...

Also you don't need 1000u on primary side, 330u will be enough, since output voltage is only +/-25v so I don't think this will push a lot of power....

Gate resistors would be better to be 22R, no need to go lower

Use 470u instead of 33u with 7812, again it won't kill you to use bigger values...

And with this FB you will get +/-30v...

Again since I don't think this will push any big powers, you can use 1A fuse, 3.15 if you really need...

And those 33k resistors on primary side can be 2w only...

[Eva]

High bootstrap capacitor values cause trouble at startup due to the long high-current charging peaks and due to main gate drive supply straving. They should never be employed unless strictly necessary.

[jegandren]

hi

Looking at my transformer data each secondary is designed for 54 Volts when the input is 240.

the value i am getting +- 48 is by considering dead time and secondary diode drops.. this is without connecting the feedback.

when i connect feedback, of zener voltage of 50Vz ..is it possible for me to get +/- 25V as my output...

[Tahmid]

Jegandren,
Why don't you use line filter (two inductors) at the input side to limit peak inrush current?
Another question: Why did you use a large 24A bridge rectifier at the input? What is your required power output?
Tahmid.