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.

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

Like seen in page 6 here:

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