LTspice: efficiency calculation

Discussion in 'General Electronics Chat' started by tom66, Nov 9, 2010.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
    How to do an efficiency calculation in LTspice IV? The menu has "Mark Start" and "Mark End" options, but these are greyed out. Does anyone have any experience with this?
  2. SgtWookie


    Jul 17, 2007
    It's covered in the LTSpice Help files, but basically...
    Your circuit must have:
    1) Exactly ONE voltage source.
    2) Exactly ONE current source set up as a load for the output.
    3) One or more of Linear Technology's provided IC models (I haven't figured a way around this yet).
    4) On "Edit Simulation Command", you must check the "Stop simulating if steady state is detected". Optionally, check the "Don't reset T=0 when steady state is detected" box; I find this useful.

    Check out the attached simulation; it's a 1.5v to 30v buck supply using an LT1171 2.5A 100kHz switcher. Tile the panes vertically. After the simulation finishes running, click on View -> Efficiency Report -> Show on Schematic.
    Then click View -> Zoom to fit.
    Last edited: Nov 9, 2010
  3. tom66

    Thread Starter Senior Member

    May 9, 2009
    Thanks. So it doesn't work with resistive loads? I just tested it out by using a calculator and tried different loads of 11 ohms, 25 ohms and 1.1k ohms, it worked but it was pretty arduous. I used RMS power calculations to get the readings.

    I'm simulating a SEPIC and it's working quite well. Output 12V @ 1A with 6V in gave me 94% efficiency, which seems very high.

    I don't have my laptop connected to the 'net right now (typing during break), so I can't download the schematic, but I will try it later today. I'm running LTspice in a virtual machine (VirtualBox) and it works well.