Electronic – Coupled inductor/pulse transformer for very tiny power supply

flybackinductortransformer

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?

Best Answer

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.

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