I'm a bit new to SMPS in general but i decided to make a simple self excited flyback converter for a Li-Ion battery charger.
The flyback converter is based on this schematic:
http://www.pocketmagic.net/wp-content/uploads/2012/05/simple_smps_3.pngI need three isolated 5V outputs at 1A for a lithium battery charger so i changed the transformer abit.
The primary is 38 turns of litz wire while the three 5V secondaries are 5 turns of litz and the feedback is 5 turns of thin magner wire.
While i was at it i also added a 12v output with 12 turns of litz.
At first i made the opto feedback to work off of the 12 V secondary and it worked out well, it gave me 13.3 volts (12v zener + 1.3V opto) and the voltage kept constant with different loads, i then took a look at the 5V outputs, which were at 4.8V no load but would not regulate at all as the voltage would drop instantly even on light loads, the voltage would also increase if i put load on the regulated output.
I then connected the opto to the 5V output with a 5V1 zener and the output regulated well at 6.4V but now the 12V output was at 34V and higher if i put load on the regulated output, which damaged the capacitor on the 12V output.
Also the other two 5V outputs were not regulating at all and would drop with a light load and also while the voltage was at 6.3V no load the output 220uF 16V caps ( generic SMD caps from a LG plasma TV ) were getting very hot.
But that's not all, after i put the opto back on the 12V output and put a 470uF 16V cap ( ESR was 0.13 Ohm ), which was fine at no load but would get mighty hot under light load and completely blew apart at a moderate 200mA load.
More info on my SMPS:
The core used was taken from an old CRT monitor ( E-E core ) and is pretty decent in size.
The switching MOSFET is a generic one designed for switching ( came from a LG plasma TV ).
At no load the switching side stays ice cold.
The SMPS operates at about 30kHz.
Output diodes are SS110 shottky diodes.
The is no snubber network on the primary ( not enough space on proto-board ).
The transformer was wound on the original bakelite bobbin.
I took care of isolation between windings.
Would be used with three TP4056 chargers to charge an internal 3S Li-Ion pack on a portable speaker.
The 12V output was intended to power the amp while the batteries were charging.
When i was making the SMPS i thought that all the voltages would come from a common magnetic field thus all the outputs should be the same and if one is regulated then all of them are but i guess i was wrong.
Then again when i check out other flyback SMPSs that have multiple outputs of them only one is tapped off for feedback which does make me wonder if i should consider winding a new transformer and maybe try to calculate the number of windings instead of eye-balling it.
Also i don't know if 30kHz switching frequency is acceptable for a flyback SMPS, most switching IC i see operate at 100-200kHz, i guess the only way to increase the frequency would be to decrease the number of turns on the primary to saturate the core quicker.
I would greatly appreciate if anyone who has a bit more experience/knowledge in this could help me out.
Thanks.