# AC Sweep not working for LLC Resonant Converter

**URL:** <https://forum.plexim.com/t/ac-sweep-not-working-for-llc-resonant-converter/731>\
**Category:** PLECS\
**Tags:** ac-sweep\
**Created:** [January 13, 2020, 12:40pm UTC](https://forum.plexim.com/t/ac-sweep-not-working-for-llc-resonant-converter/731 "2020-01-13T12:40:24Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Maida](https://avatars.discourse-cdn.com/v4/letter/m/8797f3/32.png) [@Maida](https://forum.plexim.com/u/Maida)\
**Post date:** [January 13, 2020, 12:40pm UTC](https://forum.plexim.com/t/ac-sweep-not-working-for-llc-resonant-converter/731/1 "2020-01-13T12:40:24Z")

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Hi all,

I am trying to run AC sweep for LLC converter but it is not recognizing the system period. Is there any way to go about it?

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**Author:** ![Kris\_Eberle](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/kris_eberle/32/6_2.png) [@Kris\_Eberle](https://forum.plexim.com/u/Kris_Eberle)\
**Post date:** [January 14, 2020, 3:25pm UTC](https://forum.plexim.com/t/ac-sweep-not-working-for-llc-resonant-converter/731/2 "2020-01-14T15:25:49Z")

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Hello,

For converters with variable-frequency control a Multitone Analysis should be used for generating the frequency response. The AC Sweep tool employs the Steady-State Analysis that needs the information of the system period, which for variable-frequency controlled converters, is not known prior to the simulation. Please also see the PLECS demo model “Half-Bridge LLC Converter with Analysis Tools” for an example. Good luck!

Regards,

Kris