Current-Sense Xfmr in SMPS?

Power Transformers
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Why can’t i use a current-sense transformer such as this one, in a gate-driven SMPS similar to this? (aside from missing center-tap)

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Welcome boom,

Well, the center-tap is critical to allow such a topology to work, and since the 749252020 does not have one, it cannot be used. Additionally, as the 749252020 is a current-sense transformer, it is not really designed for such applications. Furthermore, in such a configuration, it would be very difficult to regulate the output voltage. Switch-mode power supply transformers such as these are better suited for such applications.

I would recommend that you determine your Vin and Vout requirements as well as your load current requirements, and then find an appropriate DC-DC Switching Regulator (with integrated switching transistors) or Switching Controller (without integrated switching transistors) which meets those requirements. Then, read the datasheet and see what they recommend for an appropriate transformer.

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Hi David

“the center-tap is critical to allow such a topology to work"

  • Why can’t a bridge rectifier work in place of a center tap?

“as the 749252020 is a current-sense transformer, it is not really designed for such applications."

  • Why? Can you say what specific transformer properties are the issue?

“in such a configuration, it would be very difficult to regulate the output voltage"

  • In my application, I don’t need regulation. It’s fine if the output voltage varies with input voltage, temperature, and load.

“Switch-mode power supply transformers such as these are better suited for such applications.”

  • How can i filter on current-rating? That parametric search doesn’t seem to have current-rating.

Many Thx

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Hi boom,

Answers:

“the center-tap is critical to allow such a topology to work"

  • Q:* Why can’t a bridge rectifier work in place of a center tap?
  • A: If you used a full bridge rectifier, it could, but not the half bridge you show. That requires the center tap for the return current. Without the center tap, the second diode would block the return path.

“as the 749252020 is a current-sense transformer, it is not really designed for such applications."

  • Q:* Why? Can you say what specific transformer properties are the issue?
  • A: No I can’t. I would expect that physical and electrical characteristics are likely to be optimized for certain applications, and since Wurth specifically labelled this as a current transformer, I would think that they intend it to be used primarily for that application. I am not an inductor design expert, so I can’t say what characteristics they may vary to get optimal performance for any given application. That being said, I would not say that it wouldn’t work at all (using with a full bridge rectifier, at least); merely that this isn’t its intended use, and therefore, may not give optimal results.

.“in such a configuration, it would be very difficult to regulate the output voltage"

  • Statement: In my application, I don’t need regulation. It’s fine if the output voltage varies with input voltage, temperature, and load.
  • Response: That improves likelihood for success.

“Switch-mode power supply transformers such as these are better suited for such applications.”

  • Q:* How can i filter on current-rating? That parametric search doesn’t seem to have current-rating.
  • A: Unfortunately, you cannot. You can either try to ascertain their ratings by looking in their respective datasheets or the datasheets of the IC’s they are designed to work with (when listed). Obviously, this is not optimal. That is why I recommended starting with the IC rather than the transformer.

May I ask why you posed such an odd original question? Do you have one on hand? Do you have a particular set of requirements and came across this part while trying to find a solution?

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“May I ask why you posed such an odd original question? Do you have one on hand? Do you have a particular set of requirements and came across this part while trying to find a solution?"

  • I need the shortest transformer I can find, which can handle 2A+. This is shorter than anything I can find from any mfgr. It’s rated for 7A @ 3.55mm tall.
  • I don’t care about Length or Width, just Height.
  • Perhaps an inductor expert will comment :blush:

thx

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Aaah, that IS low-profile.

Two things:

  • Do you need 7A on the low turns (N1) side or the high turns (N2) side? This can handle 7A on the N1 side, but not on the N2 side. It would be closer to 350mA.

  • I am not sure this part is readily available at this time. I see it on their website, listed as “New”, but did not see it in anyone’s inventory. You may be able to get a sample from them.

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