I am using ST's eDesign Suite to design a power supply. The suite helps with circuit, BOM and transformer design specifications.

I contacted the transformer design firm to get my transformer designed. I gave them all the information regarding the number of turns and length of wire etc.

They are asking me the polarity of the transformer and also the start and end pin connection of the winding.

Being an amateur in transformer design, I am unable to find any relevant information for the above query.

I need help with that.

Here are my power supply specifications:

  • Input: 100 VAC - 265 VAC - 50 Hz
  • Operating VDC [98.7 V - 374.8 V]
  • Output Power: 14.96 W
  • Out 1: 17 VDC (35 mVpp ripple) - 880 mA maximum - 14.96 W
  • Regulated
  • Switching Frequency (fsw): 60 kHz

Transformer information:

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  • \$\begingroup\$ You can see the dots in the schematic. That's the start of the winding. As for pinout, that depends on your layout, creepage requirements and so on. \$\endgroup\$
    – winny
    Jan 13, 2017 at 21:07

1 Answer 1


You define for them where you want the start and end pins. They will wind the transformer and connect the wires to the appropriate pins accordingly.

It's one of those tedious things where you'd perhaps prefer to say 'whatever' and have them draw what they've decided and done, but as the commissioning designer, it's really down to you to choose the transformer pinout, and draw what you want.

You wouldn't go into a restaurant and say 'feed me'. You'd say 'I want the steak, bloody, with new potatoes'. Similarly they are waiting for you to draw a little insulating plate with 6 pins spaced 'yay' apart, labelled primary start, primary end, secondary ...


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