in this PSU schematic, you can see on the right, close to the two schottkey diodes, a coupled inductor L15, marked as SQR1257.

You can also read the note close to it: "2_1: 25uH / 4_3: 21uH"

the only datasheet that I could find is this: http://inductor.com/media/catalog_pdf/product/sqr1242-sqr1257-sqr1277.pdf

The marking on the actual inductor is 275 (I only got a picture of it, so I cannot measure it). Link to the pic: http://cdm.link/app/uploads/2014/03/ms20kit1.jpg

But I can't find on the datasheet this marking, and 21uH or 25uH is not an option either.

So what do you think this inductor is?

also, as bonus question, what's the purpose of the MOSFET on this circuit? to me looks like it acts as switch to switch off the PSU (from the KILL signal by the CPU or by the mechanical switch), am I right?


I found those infos about that inductor, and it is a custom wound one. I could not find on Mouser a coupled inductor with 25uH + 21uH, do you think I could just use one with 22uH on both coils? for example this: https://www.vishay.com/inductors/list/product-34359/ enter image description here

  • \$\begingroup\$ I think they're not zener diodes, but schottkey diodes. The symbols look quite the same until you get more familiar with them. A schottkey diode is a diode with low voltage drop and fast recovery. \$\endgroup\$
    – Oskar Skog
    Commented Jul 12, 2017 at 17:54
  • \$\begingroup\$ Try read this ti.com/lit/an/slva369a/slva369a.pdf \$\endgroup\$
    – G36
    Commented Jul 12, 2017 at 19:31

1 Answer 1


This is a variation on the "fly-buck" topology:

An isolated buck converter, also known as flybuck converter, is created by replacing the output filter inductor (L1) in a synchronous buck converter with a coupled inductor (X1) or flyback-type transformer, and rectifying the secondary winding voltage using a diode (D1) and a capacitor (COUT2). The topology can be extended to any number of isolated secondary outputs. It also can be used to generate one or more inverting outputs.

You can read all the details here: Flybuck Topology

The caveat for your schematic is that the flybuck will not work without continuous inductor current.

Your topology is using a buck converter as an inverting buck-boost for the primary load (your -15V), so that load current will have to be high enough to keep the inductor current continuous or the +15V rail will lose regulation. [Edit: This may not be true according to the app note linked by G36 (Good find!), since in buck-boost mode the duty cycle will be different from a straight buck. You would have to run the calculations to be sure.]

The MOSFET is exactly as you described, a switch to turn the converter on and off based on the KILL signal.

  • \$\begingroup\$ Thanks, but how that answers the question about the unidentified inductor? \$\endgroup\$ Commented Jul 12, 2017 at 21:03
  • \$\begingroup\$ I took your question "what do you think this inductor is?" more as "what do you think the purpose of this inductor is and why is it a coupled inductor" than "what is its value and specs or where can I buy one"?. If this is from a mass produced product it's very likely it's a custom wound coupled inductor and you won't find a spec sheet on it. (I can't view your picture, my work blocks the hosting site.) Using the links provided you can come up with a replacement design. \$\endgroup\$
    – John D
    Commented Jul 12, 2017 at 21:15
  • \$\begingroup\$ Thanks for your support. Unfortunately I'm not an engineer and I found difficult to understand how to substitute that inductor. After a quick search on Mouser, I could not find a coupled inductor with 21uH + 25uH as the one used on that circuit (you're right, that one looks like a cusyom wound one). \$\endgroup\$ Commented Jul 12, 2017 at 21:46

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