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

SMPS Circuit

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

  • \$\begingroup\$ can you rewrite the second paragraph is very difficult to understand what is happening.And please attach the IC datasheet \$\endgroup\$
    – Navaro
    May 8, 2019 at 9:10
  • \$\begingroup\$ corrected few details \$\endgroup\$
    – Akshit
    May 8, 2019 at 9:32
  • 1
    \$\begingroup\$ 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? \$\endgroup\$ May 8, 2019 at 9:38
  • \$\begingroup\$ I do not know how to know that. We built transformer from data given by supplier. We used ferrite core EE 16/8/5. \$\endgroup\$
    – Akshit
    May 8, 2019 at 9:44
  • \$\begingroup\$ 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. \$\endgroup\$
    – Andy aka
    May 8, 2019 at 10:14

1 Answer 1


You have the output (secondary) of the transformer's phase reversed.

  • \$\begingroup\$ Thats in diagram i guess, in real it's 180 degree. Any way to recheck that? \$\endgroup\$
    – Akshit
    May 8, 2019 at 16:26
  • 1
    \$\begingroup\$ @Akshit That ”diagram” and “guess” is a problem to combine in one sentence. If you don’t show 100 % correlation between your schematic diagram with your built circuit, you will have a bad time getting help here, or anywhere else. EEs talk in the language of schematics. \$\endgroup\$
    – winny
    May 8, 2019 at 19:25

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