I am designing a 311V to 48V buck converter in LTSpice using ideal components for now.


I am using a 48V/311V = 15.434% duty cycle in V2 which controls S1:

V2 setup

This is the simulation output:


Below is the inductor current and downstream switch voltage at steady-state:

enter image description here

I don't understand why the output voltage is ~52.1V instead of 48V. I tried different SPICE directives for the switch from various tutorials online but I get the same result. I also tried decreasing the max time step to 1u, to no avail.

Any idea why this is happening? Thanks

  • \$\begingroup\$ Are you certain your calculations are correct? What does the inductor current look like? \$\endgroup\$
    – Hearth
    Oct 7, 2019 at 22:18
  • \$\begingroup\$ Your calculations look correct, and with such a large inductor you will be in CCM mode where your equation applies. Post a picture of the switch node and inductor current in steady-state and we may get some clues from there. \$\endgroup\$
    – John D
    Oct 7, 2019 at 22:34
  • \$\begingroup\$ One issue, though I think it would cause lower rather than higher output, is that inductors in LTSpice default to nonzero resistance. Setting the inductor resistance to 0 explicitly might be worth trying. \$\endgroup\$
    – The Photon
    Oct 7, 2019 at 22:51
  • \$\begingroup\$ Setting the inductor resistance to 0 didn't work :( I added a picture of the switch node and inductor current in steady state. The issue was solved by adding explicit rise and fall times for the pulsed voltage source! \$\endgroup\$
    – w00t
    Oct 7, 2019 at 23:33

1 Answer 1


Insert a rise time and fall time for the pulsed voltage source. There are default values used when you input 0 that are realistic but >0, if you want to see more ideal behavior use a value like 1n.

PULSE(0 5 10u 1n 1n 1.5434u 10u)

  • \$\begingroup\$ This worked, thank you! \$\endgroup\$
    – w00t
    Oct 7, 2019 at 23:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.