I need an isolated 24V 3mA supply to activate some optocoupler that will be read by some microcontroller. Its power supply will be the microcontroller's 5V. My first instinct was to use a flyback transformer, but these guys seem too start out at 500mA, so they are way too bulky.

I then noticed parts listed as "coupled inductor" or "pulse transformer" seem to have values in the right ballpark. However, I am not sure if these parts are a good fit because the datasheets for inductive parts don't provide much info.

So, two questions:

  1. Are the datasheets really bad or am I missing something? I want maximum input voltage, frequency response, LI² values, this sort of stuff. Can I figure out these somehow from the datasheets? How do the people who use these parts know these parameters are ok?

  2. Is there any fundamental difference between parts listed as coupled inductors, pulse transformers and flyback transformers? How about audio transformers and signal transformers?

  • 1
    \$\begingroup\$ Looks par for the course to me. I only saw maximum voltage listed once and it was 55V. If you're really lucky you'll get Q-factor and inductance vs frequency graphs. If you're less lucky you'll get just the self-resonant frequency. If you're not lucky, as you are here, you'll just get vague notions of the frequency range you're meant to use this thing at. 10kHz is the only number that appears in this case. I think flyback and pulse transformers usually don't have 1:1 ratio while coupled inductors do. Pretty sure coupled inductors don't aren't isolated from each other as well too. \$\endgroup\$
    – DKNguyen
    Commented Mar 1, 2019 at 22:03
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    \$\begingroup\$ BTW, you might want to check out the PWB series from coilcraft. \$\endgroup\$
    – DKNguyen
    Commented Mar 1, 2019 at 22:15
  • \$\begingroup\$ And have you considered these: digikey.ca/products/en/power-supplies-board-mount/… \$\endgroup\$
    – DKNguyen
    Commented Mar 1, 2019 at 22:29
  • \$\begingroup\$ Inductor data sheets tend toward sketchy, particularly things like pulse transformers that have been used for ever and ever. Those look pretty much par for the course. If you only need 72mW, a 12mm square inductor seems to be overkill. \$\endgroup\$
    – TimWescott
    Commented Mar 2, 2019 at 0:33

2 Answers 2


There is no fundamental difference between all ferrite-core inductive devices. But each series usually tailored/optimized for specific application. For example, for signal conditioning such parameters as impedance, frequency response, linearity are of value. On the other side, for power conversion, DC resistance, max (saturation) current are important. So, datasheets usually provide data relative to application field.
Let's go back to your questions.
"Мaximum input voltage" – which voltage do you mean: peak voltage across single winding terminals? Or between different windings? Latter parameter is provided in both mentioned datasheets as 'isolation voltage'. Former one rarely mentioned in specs, but it can be estimated to be the same order as 'isolation'.
"Frequency response" – in my opinion, it's weird parameter for power conversion applications.
"LI² values" – may be estimated either from L and Imax or from L and Vus values. For your specific application (small power converter) there is no need to use inductive device close to the limits, so even ballpark estimations of needed parameters would be quite enough, I think.


You could consider something like this


5V in, 24V out, 42mA, $7.89


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