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I am designing a SEPIC power supply using the LT3758. I have simulated the circuit in LTSpice and all the waveforms are as expected except the L1 current. Here I see massive overshoots when the MOSFET switches on (eg normal current is about 3A pk-pk and overshoot goes up to over 20A). The spike is extremely narrow (< 1nS).

I am new to LTSpice, so might be missing something obvious.

Is this spike real or a simulator aboration? What could be causing it and what can I do about it?

Overshoot

SEPIC schematic

Input +28V, Output +40V 1A.

[Edit] I have posted a follow on question here LTSpice SEPIC design low freq ringing

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    \$\begingroup\$ Are you using realistic values for parasitics? \$\endgroup\$
    – PlasmaHH
    Jan 1, 2016 at 10:59
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    \$\begingroup\$ If you put 0.01 ohms in series with the inductor what happens. \$\endgroup\$
    – Andy aka
    Jan 1, 2016 at 11:21
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    \$\begingroup\$ I see you are using coupled inductors; if you do not use a realistic model (preferably from the manufacturer), your results can contain 'interesting' artefacts. I have had this issue and it mostly disappeared when I used the manufacturers model. \$\endgroup\$ Jan 1, 2016 at 16:53
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    \$\begingroup\$ The V1 source also has 0 resistance. Add the expected ESR of your power source to that. \$\endgroup\$
    – rdtsc
    Jan 1, 2016 at 17:35
  • \$\begingroup\$ And why V=50 on C5? \$\endgroup\$
    – rdtsc
    Jan 1, 2016 at 17:37

1 Answer 1

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The only place this can come from is stored charge in D1 (or gate drive on Q1, but that's not the case).

Your currents look non-linear with time -- likely your circuit is not very efficient.

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  • \$\begingroup\$ LTSpice claims the circuit is 88% efficient. I had some suspicions about the diode specs but changing the inductor coupling seems to have sorted that problem - see comment above. \$\endgroup\$
    – David G
    Jan 2, 2016 at 0:14

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