I am working on a design that would use the LM5160 to generate an isolated rail. The design parameters are as follows:
- Vin = 24V (typ.).
- Vin_min = 10V.
- Vin_max = 30V.
- Vout_iso = 3.3V.
- Iout_iso = 0.4A.
- L_prim = 47uH (primary winding of transformer).
- L_sec = 47uH.
- R_prim = R_sec = 0.15ohm.
- Transformer: Wurth 744870470 (https://www.we-online.de/katalog/datasheet/744870470.pdf).
Based on these and following the design guidelines in section 8.2.1 of the datasheet (https://www.ti.com/lit/ds/symlink/lm5160.pdf?ts=1616703058199), I set the primary output voltage to 4.1V (should yield ~3.3V on isolated output allowing for transformer losses and rectifier forward voltage). I selected switching frequency to be 600kHz, which satisfies the equations for maximum switching frequency. The transformer was chosen by calculating the minimum inductance as 38uH. The steady state of the isolated output should be a 0.4A load at 3.3V.
The problem I am having is that the isolated output is varying drastically with load changes in the attached simulation, anywhere between 1.2V - 5V depending on where the load resistor value falls in the range of 8.25ohm (to give 0.4A in nominal state) and 2kohm (nominal open state). Working with the LM5160 eval board has given similar results so I know I'm making a mistake in my design/setup but I can't nail down what that is.
What am I missing about this design? How can I modify it to create a stable isolated output? I'm following all design rules in the datasheet to the best of my knowledge, are there additional guidelines I need to satisfy?