# SMPS voltage drop in 230v ac to 5v dc flyback

I am building a SMPS circuit from a switching IC. This IC has feedback and enable pins for timing and current calculations respectively.

Transformer Data: (10 pin 1-5, 6-10) primary: 1-3 - 60 turns, 3-2 25 turns (1-2 is total coil with 1.5 mH) secondary: 6-10 - 7 turns aux: 4-5 - 21 turns

When this circuit is operated in open mode (without load), it gives 5.2V (which is what i need). But when we start connecting load, voltage drops a little up to around 4.8V for some 1.4 A or 1.5 A need (10 ohm 10 W resisters in parallel to make load). If i take total parallel load up to 2.5 ohm, it should ideally give 2A at 5V. But happens is voltage start decreasing to 3 V as current raises to 2 A.

Why is that?

(I have tried using 2 1R0 or 2R2 instead of 2 1R5 for cs/enable but with no result, Also using 10 ohm in feedback after diode instead of 1 ohm)

IC: CT5503s

EDIT: Updated circuit Image

• can you rewrite the second paragraph is very difficult to understand what is happening.And please attach the IC datasheet – Navaro May 8 '19 at 9:10
• corrected few details – Akshit May 8 '19 at 9:32
• Probably the core approaches to the saturation point as the load current increases. Does the core of the transformer (actually, it's a coupled inductor) have air gap? – Rohat Kılıç May 8 '19 at 9:38
• I do not know how to know that. We built transformer from data given by supplier. We used ferrite core EE 16/8/5. – Akshit May 8 '19 at 9:44
• Where in the device data sheet does it tell you that you can expect a perfectly regulated 5 volts with a 2 amp load? Try adding another 10 uF capacitor after the bridge - the example circuit in the data sheet does have two. – Andy aka May 8 '19 at 10:14