I am confused about the Transformer dot polarity. I got design from ST's edesign Suite. Here is schematic for that :-

enter image description here

Now I need to order the design for transformer and the transformer design centre asking for dot polarity/Start-End point of pins. According to circuit, there are only 4 pins used on AC side and 2 pins on DC Side. The ST recommends EE-10 size Bobbin. Here is layout for that :-

enter image description here

What does 2 Pin in above figure donates? Is it start point or end point? Where to connect this pin in circuit?

I only need 6 pins for my circuit, but the layout for Transformer shows 10 Pins. I am also confused about the polarity.

The Transformer specs :-

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Design Specs :-

enter image description here

  • \$\begingroup\$ I'm confused as to why you convert the basic 6 pin transformer in the schematic to the one that uses pin 2 as a centre tap to the primary. Why do that? \$\endgroup\$ – Andy aka Feb 25 '17 at 17:50
  • \$\begingroup\$ @Andyaka I didn't do that. The ST's recommends wurth EE10 size bobbin and according to wurth they recommed about transformer layout. Here is the link :-katalog.we-online.de/pcd/datasheet/070-5248.pdf \$\endgroup\$ – Embedded Geek Feb 25 '17 at 17:53
  • \$\begingroup\$ listen, it needs three windings with no centre taps so stick to what the schematic suggests and don't get drawn into choosing something that might be a compromise. You also need to establish the ferrite core type used and if any mapping is implemented. \$\endgroup\$ – Andy aka Feb 25 '17 at 17:59
  • \$\begingroup\$ @Andyaka Okay, I understand. So is it necessary to go with 10 Pin transformer, I can even go with smaller size transformer with just 8 pins. \$\endgroup\$ – Embedded Geek Feb 25 '17 at 18:06
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    \$\begingroup\$ The core size of the transformer has to be "big enough". Having unused pins on the bobbin is not an issue. \$\endgroup\$ – Andy aka Feb 25 '17 at 19:23

2 is Wurth's number for centre tap. It's needed in push-pull configurations. Actually there are 2 primaries in push-pull systems and the center tap is only a drawing habit due their serial connection. The halves are used in turns. You need only single primary, no need to make the centre tap.

"The parallelled 3" should NOT be ignored. That can be 3 identical windings in parallel to make the losses smaller. 3 wires is better than a thicker single wire due the skin effect. Refer the documentation.

Wurth has thought of 2 secondaries. Low loss full wave rectifier needs them. You need single secondary, but again check, if it must be 3 identical windings in parallel.

Wind all windings to the same direction. The dot is the starting point of the winding. Fail in this => wrong pulse polarities => smoke.

Unused pins are harmful only because they need space.

  • \$\begingroup\$ What are the end point of windings? For primary ? For Aux? For Secondary? \$\endgroup\$ – Embedded Geek Feb 26 '17 at 9:53
  • \$\begingroup\$ @AnujMattóõ If this transformer was mine and I winded it by hand then the main primary is from pin 1 to pin 2, the aux(=to diode Dvdo) is from 4 to 5 and the secondary is from 7 to 9. Let me see what your design system demands on them - give a link or at least some screenshots to reveal the possible limitations there. \$\endgroup\$ – user287001 Feb 26 '17 at 10:07
  • \$\begingroup\$ Sure! I have added the useful information and edited the question and posted screenshots as well. \$\endgroup\$ – Embedded Geek Feb 26 '17 at 10:29
  • \$\begingroup\$ @AnujMattóõ The new data confirms: 3 parallelled means "wind 3 parallel wires at the same time, solder the ends to the same two pins. Do you make it by hand? If not, the supplier's ordering form is interesting . do it allow this? You really can choose the pinout freely if you can make the circuit board as you want. Only keep the output pins at the other side for the best insulation. \$\endgroup\$ – user287001 Feb 26 '17 at 10:43
  • \$\begingroup\$ @AnujMattóõ BTW have you any idea how to make the proper insulation between the mains voltage and the DC output into the transformer? It's urgent! It's your life and your home. \$\endgroup\$ – user287001 Feb 26 '17 at 10:54

It's just a termination halfway through the primary winding to "pause" it, wind the secondary and then resume the rest of the primary. This is to reduce the leakage inductance, hence lower losses and better regulation. Google interleaved transformer.

As for the polarity, each dot denotes the start of each winding, assuming you wind all of them in the same direction. You can see it as the end of each winding too but this is an uncommon denotion.

  • \$\begingroup\$ So you mean Pin 2 is centre tapped? and also the winding starts from Pin 1 to Pin3 for primary and from Pin 3 to Pin4 for aux and from Pin 7,8 to Pin 9,10 for Sec, Am I right? \$\endgroup\$ – Embedded Geek Feb 25 '17 at 17:56
  • \$\begingroup\$ You could call it center tapped. For a 50/60 Hz this doesn't matter but for a flyback it makes a big difference at no/very low extra cost. No 4-5 is aux. \$\endgroup\$ – winny Feb 25 '17 at 18:07
  • \$\begingroup\$ According to schematic do I require centre tapped/interleaved transformer? \$\endgroup\$ – Embedded Geek Feb 25 '17 at 18:20
  • \$\begingroup\$ "Require" is a difficult word since it depends on your requirements. You might require it if you go for a very sloppy single layer design and don't have enough voltage margin to take the inductive kick from the leakage inductance. I always go for interleaving as my first try and you should too. \$\endgroup\$ – winny Feb 25 '17 at 18:29

If you PUSH current INTO a transformer lead with a DOT, then current will flow OUT OF, any other winding with a DOT. So, for example, if you PUSH current into lead 1, current will flow OUT OF, leads: 4 and 7 and 8.


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