5 V 1.2 A SMPS with TNY255

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Hi
I’m trying to design a 5V/1.2A (6W) switching mode power supply using TNY 255. The schematic is below. I use EE19 core with Np= 273 turns, Ns=15 turns Lg=245 micron and Lp=6mH . But it doesn’t work properly. Some results are here :
Load (ohm) Vo (V) I (mA) P (W)
No load 5.48 - -
22 5.29 240 1.26
18.9 5.23 276 1.44
15.7 5.10 324 1.65
10.5 4.98 474 2.36
7.2 4.92 683 3.36
6.1 4.86 796 3.86
5.1 4.80 941 4.51
4.1 4.73 1153 5.45

What is the problem?
Thanks!

https://obrazki.elektroda.pl/9693394400_1455600472.png
 

Your system is showing poor load regulation. Possible reasons:

1. Transformer under powered.
2. Incorrect number of turns.

Are you using one half of a center tap transformer?
Try to reverse the secondary connections.
 
Use TL431 or similar secondary voltage regulator instead of Z-diode.
 
I used TL431 too, but it didn't work much better!!
Click to expand...
Could be, if the problem is insufficient output power rather than bad regulation. Can't be answered from your results, unfortunately.
 
I looked up the datasheet:

The recommended size for the transformer is EE13 or EE16. Yours is oversized. Can it drive a bigger transformer?

In your circuit, there will be 0.7V (across R2) to driver the LED in the optocoupler. Once the voltage is down to 4.7V, this will become 0.4V. Barely sufficient (??) to drive an IR LED?

You need to pay attention to the dots indicated on the transformer winding.
 
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