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Transient response of LLC converter


Advanced Member level 5
Jun 13, 2021
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Do you know why the shown HHC method of LLC control never took off (brought out 7 years ago)

It has no downsides, makes control easier, and can make transient response faster, and is cheap (UCC25640), and reduces BOM cost since smaller output caps are needed.
...But nobody uses it..why?

One problem that i can think of with LLC + HHC is that if the PFC output voltage falls, then the LLC has problems because LLC's are not conducive to operating with a wide input voltage it seems a waste of time improving the transient response when the transient is likely to also result in the PFC output voltage (the LLC's input voltage) getting pulled down so low that the TTF cannot properly operate anyway.
Last edited:
24 AUG 2023

Thanks, i am seekign a fuller report of the charge controlled LLC or the HHC LLC.

Its hard to believe that LLC can be operated any other way than just kind of voltage mode control, where the frequency is adjusted in correspondance with the Vout.
LLC has a big LC power stage, and any other control method seems likely to instigate dreadful oscillations.

The LLC cannot compete with the 2 tran forward for fast transient response hysteretic mode control.....the attached shows the 2 tran forward in burst LLC can compete with this for excellent transient response....resonant converters are just not capable of this kind of sharp on/off control....they would ring like mad.

LTspice and PNG shown


  • 2TF burst mode.png
    2TF burst mode.png
    72.7 KB · Views: 54
  • Two tran forward
    3.1 KB · Views: 43
correct - LLC and similar resonant converters need a few cycles to move up to full output ( or the reverse )

any type of forward converter, can have better dynamics - as long as Vin is high enough - pure hysteretic control is the fastest form of control for forward based converters.

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