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

schematic

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

V2 setup

This is the simulation output:

simulation

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

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  • \$\begingroup\$ Are you certain your calculations are correct? What does the inductor current look like? \$\endgroup\$ – Hearth Oct 7 '19 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 '19 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 '19 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 '19 at 23:33
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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)

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  • \$\begingroup\$ This worked, thank you! \$\endgroup\$ – w00t Oct 7 '19 at 23:33

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