Hello,
I am currently designing a flyback regulator which uses the LM2585-ADJ. My needs are to start from +24V and obtain +60V and -60V. In order to do so, I have selected the 750311889 by Wurth Electronics, which has two seocondary windings at 2.5:1 ratio with the primary winding, and a primary inductance of 37uH. It works all right up to a certain point, let's say +/-40V: above that the device becomes unstable, the trasformer begins to hiss and click and pop and current draw increases dramatically. After a while, the device blows. :-(
According to my calculations, transformer choice should be ok, except for this note I found on page 23:
STABILITY All current-mode controlled regulators can suffer from an instability, known as subharmonic oscillation, if they operate with a duty cycle above 50%. To eliminate subharmonic oscillations, a minimum value of inductance is required to ensure stability for all boost and flyback regulators. The minimum inductance is given by:
Lmin=2.92((VINmin-Vsat)x(2Dmax-1))/(1-Dmax)
where • VSAT is the switch saturation voltage and can be found in the Characteristic Curves.
If my calculations are correct:
Lmin = 2.92 ((24-0.5)x(1.32-1))/(1-0.66)
Lmin = 2.92x23.5x0.32/0.34
Lmin = 65uH
The diode is STPS20S100CT and the primary inductance of the 750311889 is 37uH, so almost half of what is required by your formula. Could this be the reason for the malfunctioning?
However, what puzzles me is that even lowering the input voltage does not help. If I put 12v, I should get:
Lmin = 2.92 ((12-0.5)x(1.32-1))/(1-0.66)
Lmin = 32uH
which should be well within specs of my inductance.
Can you please enlighten me and explain why my regulator is oscillating and how to solve the issue? And also how you calculate values for the compensation network? There is nothing about it in the datasheet...
I am attaching the schematic:
