negative rail for low side current sense?

stick a diode clamp on the sense resistor

If you know the +15 supply comes up before motor current, and the sense resistor (s) are near ground, you can power a few current sources (PNP transistor, emitters thru matched current-program resistors to +15, bases tied together and driven by an op amp for half milliamp each). Those, will pull up end B of a few level-translation resistors with their ends A connected to a current-sense resistor terminal.

At half a milliamp, a + 2.5V offset (into a high impedance) happens when the resistors are 5000 ohms. It's accurate enough if the sense resistors are much under 5000 ohms, of course.

1% resistors means that's 2.5 +/- .07V or so, it'll have to be calibrated for 'zero'. 15V gives you enough leeway that the PNPs can just get their bases connected to (filtered) +5 if you don't need to regulate better than the +15/+5 rail difference voltage.

You add small offset to the non inverting input (VCC/(gain*2)

Like seen in page 6 here:

Cheers

Klaus

Flybuck requires additional windings and this alone might suffice to cause panic. ;)

Best regards, Piotr

The DRQ74 family of dual-winding inductors are great, and cheap.

An autotransformer flyback makes twice the voltage as a single inductor version. Add in the input voltage too!

Yes, I am using these for 20W SEPICs. Another benefit is that you can one more diode footprint and one capacitor to the PCB and populate it for SEPIC mode or a plain boost mode at the P&P stage. But when you need more insulation strength or just more windings, you quickly run out of options and a custom design is needed.

Best regards, Piotr