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I am designing a power supply based on ST Viper 37. The supply is isolated and outputs 17V with CT transformer.

I got the transformer designed from the design center. One with Centre tap and another without center tap.

But upon using CT transformer, the output of the power supply is 17V.(As needed)


Using NON CT transformer, the output of the supply is 7~9v, it keeps changing.

Here is the schematic:

enter image description here

Here is the drawing pattern with center tap :-

enter image description here

Without Center tap :- enter image description here

Transformer specs :

enter image description here

The PCB is same for the both transformer. The schematic suggest that CT is not needed. So, I was confused and thought of disassembling the transformer.

Upon disassembling, I found non-similarity in the design.

Flow of winding of CT Transformer:-

  • Primary Winding with 55 turns
  • Taping
  • Secondary Winding with 22 turns
  • Taping
  • Auxiliary Winding with 16 turns
  • Taping
  • Primary Winding with 50 turns

Flow of winding of Non CT Transformer :-

  • Primary Winding with 105 turns
  • Taping
  • Auxiliary Winding with 16 turns
  • Taping
  • Secondary Winding with 22 turns

The Auxiliary Winding is before the secondary winding in Non CT transformer and in CT Transformer the secondary winding is before auxiliary winding.

Where is issue? Why I am not getting same output with the Non CT transformer?

EDIT

This question is not duplicate but inspired from that question and it is result of that question, I faced this issue.

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closed as unclear what you're asking by Wesley Lee, Voltage Spike, Dmitry Grigoryev, brhans, JonRB Apr 1 '17 at 9:36

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Your question is contradictory. At the top you say it outputs 17 volts but show a transformer with no centre tap. Later on you contradict yourself - you say it is 7 to 9 volts. Then you don't explain how you connected the split primary transformer? Very confusing. \$\endgroup\$ – Andy aka Mar 14 '17 at 21:53
  • \$\begingroup\$ Measure the leakage inductance of both transformers and you will notice something magical. \$\endgroup\$ – winny Mar 14 '17 at 21:55
  • \$\begingroup\$ @Andyaka I am also confused why it is happening. The schematic suggests Non CT transformer, and Non CT transformer doesn't output the desired voltage instead 7-9V. Upon going with wurth's drawing design I got CT transformer and connected. The Pin 2 in CT transformer is not connected to any component. It is not connected. \$\endgroup\$ – Embedded Geek Mar 14 '17 at 21:56
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    \$\begingroup\$ Possible duplicate of Confused about polarity of Transformer \$\endgroup\$ – winny Mar 14 '17 at 21:57
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    \$\begingroup\$ If it's too high that you run out of duty cycle at load, your sense winding is very loose/high leakage. Measure the leakage inductance and post the results along with oscillograms. \$\endgroup\$ – winny Mar 15 '17 at 6:12
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If you connect the two 'halves' of the CT primary in series to make the same total primary as the non-CT version, then both transformers have a total number of primary turns of 105, secondary 22 turns, and aux 16 turns.

However, as the CT version interleaves the primary about the secondaries, you could expect the CT version to have lower leakage inductance.

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  • \$\begingroup\$ I understand that, but why their is difference in output of the supply? \$\endgroup\$ – Embedded Geek Mar 14 '17 at 21:42
  • \$\begingroup\$ Does having auxiliary winding before secondary winding or secondary winding before auxiliary windings has any effect on the output? \$\endgroup\$ – Embedded Geek Mar 14 '17 at 21:43
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    \$\begingroup\$ I suspect you've connected them differently, or they aren't wired/wound as you say. If faced with a choice between believing that two transformers with the same number of primary turns produce two outputs different in magnitude by a factor of 2, or that someone's misocnnected something, I'd pick the latter. \$\endgroup\$ – Neil_UK Mar 15 '17 at 5:52
  • \$\begingroup\$ so does having CT in transformer might be the reason for the correct output voltage due low leakage inductance? Or in Non CT transformer output is less due to high leakage inductance.? \$\endgroup\$ – Embedded Geek Mar 15 '17 at 8:22
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    \$\begingroup\$ The difference in leakage inductance will not be enough to explain a 2:1 difference in output voltage, you have connected them differently. Post pictures of how they're connected and we might be able to see what you've done. \$\endgroup\$ – Neil_UK Mar 15 '17 at 8:59

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